A Case for the 4-D Space-Space Block Universe

[bobc2]

 

[PeterDonis]

I don't think people in these forums object to discussing the idea of a "block universe" per se. I don't. The only think I would object to in your post above is your use of the term "space-space" to describe the block universe; I would use the term "spacetime". Using that term does not require adopting any concept of observers "moving" along worldlines; it simply recognizes the fact, which you mention, that the metric of spacetime is not positive definite. The term "space" unqualified implies a positive definite metric. Or at least, using the term "space" instead of "spacetime" for a manifold that does not have a positive definite metric is a great way to invite confusion, IMO. But that's more a question of terminology than physics.

There is one technical correction I would make: it is not correct to say that the X4 vector for a moving observer "slants" while the three spatial vectors remain perpendicular. The space vector in the direction of motion "slants" as well. Your diagrams show this, or at least a consequence of it (the tilting of the planes of simultaneity).

With the above corrections/clarifications, I personally have no problem with the concept of a block universe, because I don't view it as a final statement about "reality". I view it as a model. As a model, it simplifies a lot of calculations and makes a lot of relativistic physics easier to visualize.

 

[goodabouthood]

What exactly is the fourth dimension?

Are you saying the fourth dimension is just another spatial dimension? But how can these be viewed in reality?

 

[bobc2]

What exactly is the fourth dimension?

Are you saying the fourth dimension is just another spatial dimension?

Yes. To us it is different for a few reasons, among which are 1) Our consciousness "sees" just a continuous sequence of 3-D cross-sections of the 4-D space as our consciousness moves at the speed of light along our 4th dimension world line and 2) Four dimensional objects are vastly longer along the 4th dimension, X4, (perhaps billions or trillions of miles long) as compared to the lengths along X1, X2, and X3.

But how can these be viewed in reality?

First, the 4th spatial dimension is implied by considerations depicted in the side sketches of my post. The fact that special relativity describes a situation in which different observers "live in different planes of simultaneity", each one including different cross-sections of 4-D objects, implies a spatial 4th dimension. Beyond that, we actually have views of the 4th dimension in this sense: We experience a sequence of cross-sections of the 4th dimension. If you merge those cross-sections together you have the 4-dimensional picture. So, the 4th dimension is as real as our normal X1, X2, and X3. (I'm avoiding any discussion of philosopher's viewpoints that deny the existence of an external reality.)

As you travel a 1000 mile interstate cross country you experience a piece of it at a time. You have no problem understanding that the whole stretch of interstate actually exists "out there", even though you cannot see it all at once. After traveling it, you can merge the whole path together to form the mental picture of the highway as it stretched for 1000 miles.

Remembering the continuous historical past sequence of experienced 3-D images, you can merge them together to get a kind of 4-dimensional image in your mind. However, the brain does not seem to be equiped with the tools for actually visualizing a 4-dimensional object. This makes your question difficult to deal with.

 

[goodabouthood]

You say that I am moving through the fourth dimension at the speed of light. I certainly don't feel like I'm moving at the speed of light through any dimension.

 

[bobc2]

I don't think people in these forums object to discussing the idea of a "block universe" per se. I don't.

Thanks for jumping in. I didn't do your comments justice at the end of the other thread where we worked on this subject. I felt like I was unfairly hi-jacking the topic from the original poster. Also, you had me boxed into a corner, and I couldn't quite figure out the appropriate response. Not to say anything of my pushing the boundaries of our forum rules.

The only think I would object to in your post above is your use of the term "space-space" to describe the block universe; I would use the term "spacetime". Using that term does not require adopting any concept of observers "moving" along worldlines; ...

I have to agree with you here. I was uncomfortable using a term that is not standard in the literature. I have no business offering a new terminology here. I was of course wanting to emphasize the character of the 4th dimension, X4, as not that of time, but rather having the same quality of X1, X2, and X3.

...it simply recognizes the fact, which you mention, that the metric of spacetime is not positive definite. The term "space" unqualified implies a positive definite metric. Or at least, using the term "space" instead of "spacetime" for a manifold that does not have a positive definite metric is a great way to invite confusion, IMO. But that's more a question of terminology than physics.

The only reason I would hesitate to agree here is that again I wanted to make it clear that an indefinite metric can be associated with four spatial dimensions. I agree that folks hearing the term "space" unqualified would be assuming that one is referring to a manifold having a space defined by a positive definite metric. But, I agree that in discussions among physicists about special relativity it is best to use the "spacetime" terminology while adding the qualifier that with L4 space we can have four spatial dimensions.

There is one technical correction I would make: it is not correct to say that the X4 vector for a moving observer "slants" while the three spatial vectors remain perpendicular. The space vector in the direction of motion "slants" as well. Your diagrams show this, or at least a consequence of it (the tilting of the planes of simultaneity).

My introductory sentence about the X4' slant may have left that impression, but I intended the following statement referring to both X4' and X1' slanting to make clear that it is the combined slanting of X4' and X1' with Lorentz boosts that accounts for the notion that the 4th dimension is "different." And of course my sketches make it explicitly clear.

With the above corrections/clarifications, I personally have no problem with the concept of a block universe, because I don't view it as a final statement about "reality". I view it as a model. As a model, it simplifies a lot of calculations and makes a lot of relativistic physics easier to visualize.

I agree physicists most likely can never prove any model for external reality (many philosphers would not even permit any external reality--much less the exact nature of such a reality). We can attempt to identify models that are consistent with established physical theory.

The block universe seems at least to be a model that is consistent with special and general relativity and can be comprehended. A model that describes the 4th dimension as time or a mixture of time and space is quite vague and difficult to describe in an explicit way, although it easily fits a Max Tegmark approach if one wishes to regard reality as some sort of mathematical reality.

 

[PeterDonis]

The only reason I would hesitate to agree here is that again I wanted to make it clear that an indefinite metric can be associated with four spatial dimensions.

Same comment here as with the term "space" instead of "spacetime". Under what definition of the term "spatial dimension" do you claim that X4 is a spatial dimension? The only generally accepted definition of the term "spatial" that I'm aware of requires the dimension in question to be part of a positive definite metric. So you're going to need to clarify what you mean by "spatial".

The block universe seems at least to be a model that is consistent with special and general relativity and can be comprehended. A model that describes the 4th dimension as time or a mixture of time and space is quite vague and difficult to describe in an explicit way, although it easily fits a Max Tegmark approach if one wishes to regard reality as some sort of mathematical reality.

This is another area of apparent confusion. The "block universe" model *is* a model that describes the 4th dimension as "time"; although a better way of putting it, IMO, would be that it is a model that describes what we call "time" as a fourth dimension in a manifold with a non-positive-definite metric, which we call "spacetime". There's no need to repudiate the term "time"; you just have to clarify that "time" does not require a "flow" of time; that's a separate idea that is not required to understand spacetime (though it may help one to visualize some problems).

 

[bobc2]

You say that I am moving through the fourth dimension at the speed of light. I certainly don't feel like I'm moving at the speed of light through any dimension.

It's kind of like watching a movie. You are caught up in the continuous evolving of the succesion of movie scenes without any thought of a long strip of movie film moving through the projector. We would have a more direct analogy of course if the movie film were strung out in space along a straight line and you were doing the moving along the length of the strip watching that same movie, experiencing the same movie-going experience without any thought of your motion through space.

So, a bundle of 4-dimensional neurons is analogous to the long strip of movie film. But, the neurons extend for billions or trillions of miles along ones 4-dimensional world line with consciousness flying along at the speed of light watching the movie.

 

[PeterDonis]

But, the neurons extend for billions or trillions of miles along ones 4-dimensional world line with consciousness flying along at the speed of light watching the movie.

Hmm, I didn't spot this part before. The idea of "your consciousness moving along your worldline at the speed of light" is, as I said in a previous post, an *addition* to the basic idea of the block universe; it is not necessary to the basic model. In the basic model, the speed of light is just a conversion factor that comes in because, for historical reasons, we measure the X4 dimension in different units than the X1, X2, X3 dimensions. Interpreting this conversion factor as a "speed" implicitly depends on interpreting X4 as "time" in the common sense of that term, instead of just as another dimension of the manifold. The same goes for the idea of "moving" along your worldline; in the basic block universe model, there is no motion.

Also, adding the idea of "motion" is not necessary to understand our conscious experience, as Julian Barbour has pointed out in a number of papers:

http://platonia.com/papers.html

You can capture what we experience simply by saying that there is a collection of "snapshots", each one capturing an "instant" of our experience, and that the collection of these snapshots has a structure: some snapshots are "earlier" or "later" than others, and some snapshots that are "later" contain data that is correlated with others that are "earlier" (this is what a "memory of a past event" is). What we normally think of as "time" emerges as a derived concept from all this; there is no need for it as a fundamental concept.

 

[bobc2]

Hmm, I didn't spot this part before. The idea of "your consciousness moving along your worldline at the speed of light" is, as I said in a previous post, an *addition* to the basic idea of the block universe; it is not necessary to the basic model.

Yes. You are right. It is not necessary to the basic 4-dimensional spatial universe populated by 4-dimensional objects to bring in time. However, since the obvious question arises about how time relates to such a model, it seemed fruitful to pose a possible explanation for the perception of time flowing.

I have Barbour's book along with others on the concept of "block time" and am well aware of what you bring in here. I only avoided further discussion along those lines to avoid a more extended tortuous discussion. There have been two approaches to bringing time into the picture of the 4-D spatial universe. The first is the traveling of consciousness along the 4th dimension at light speed (this leads to zombies and solipsism, which Einstein cautioned against).

The other concept puts consciousness simultaneously along the entire world line of an observer. This was the concept of which Einstein seemed to refer. I have not seen the book, but evidently Fred Hoyle wrote a novel in which observers existing with their 4-dimensional consciousnesses accompanying their entire 4-D material structure were at the mercy of a devious super hyperspatial being who was at a console of buttons allowing him to stimulate the consciousness arbitrarily at one point along a world line, then another. He could fiendishly cause the observer's focus of attention to jump from one point to another, randomly, up and down the world lines. The observers had no awareness at all about what was going on. At any given station along a world line the observer is only aware of what information is presented at that point, i.e., the normal memories, hopes and desires, etc., at that point.

In the basic model, the speed of light is just a conversion factor that comes in because, for historical reasons, we measure the X4 dimension in different units than the X1, X2, X3 dimensions. Interpreting this conversion factor as a "speed" implicitly depends on interpreting X4 as "time" in the common sense of that term, instead of just as another dimension of the manifold. The same goes for the idea of "moving" along your worldline; in the basic block universe model, there is no motion.

That is contrary to the concept I have described in the original post. I thought I had made it clear with the example of calibrating distance along the interstate with time markers. So, it is exactly the reverse of what you've said here. The 4th dimension is spatial in the same sense as X1, X2, and X3. We are able to calibrate physical distance along a world line using the conversion factor, t = X4/c. It is actually just for historical reasons that time has come into usage as a 4th dimension.

And of course there is no physical motion of 4-D material objects in a spatial block universe. However, there is the psychological impression of motion arising from the interaction of consciousness with the 4-dimensional object. The impression of time flowing results. This impression of the flow of time can be represented mathematically as the focus of consciousness moving along the world line at the speed of light.

Also, adding the idea of "motion" is not necessary to understand our conscious experience, as Julian Barbour has pointed out in a number of papers:

http://platonia.com/papers.html

You can capture what we experience simply by saying that there is a collection of "snapshots", each one capturing an "instant" of our experience, and that the collection of these snapshots has a structure: some snapshots are "earlier" or "later" than others, and some snapshots that are "later" contain data that is correlated with others that are "earlier" (this is what a "memory of a past event" is). What we normally think of as "time" emerges as a derived concept from all this; there is no need for it as a fundamental concept.

I don't argue with that concept. I was quite familiar with it and acknowledge that is consistent with the 4-D spatial block universe. Again, I hesitated to expand my post with extended discussions that might account for many of the implications of the interaction of consciousness with the 4-spatial dimension block universe. If the use of the term "block universe" causes confusion, given the different versions of this concept, I suppose we could emphasize the 4th spatial dimension version by using something like "spatial block universe." But, I don't want to be the author of new terminology.

 

[PeterDonis]

I have not seen the book, but evidently Fred Hoyle wrote a novel in which observers existing with their 4-dimensional consciousnesses accompanying their entire 4-D material structure were at the mercy of a devious super hyperspatial being who was at a console of buttons allowing him to stimulate the consciousness arbitrarily at one point along a world line, then another. He could fiendishly cause the observer's focus of attention to jump from one point to another, randomly, up and down the world lines. The observers had no awareness at all about what was going on. At any given station along a world line the observer is only aware of what information is presented at that point, i.e., the normal memories, hopes and desires, etc., at that point.

Hmm, interesting, I hadn't heard about this book. If you can find a link to a review, summary, etc., please post.

That is contrary to the concept I have described in the original post. I thought I had made it clear with the example of calibrating distance along the interstate with time markers. So, it is exactly the reverse of what you've said here. The 4th dimension is spatial in the same sense as X1, X2, and X3. We are able to calibrate physical distance along a world line using the conversion factor, t = X4/c. It is actually just for historical reasons that time has come into usage as a 4th dimension.

Again, you are using the term "spatial" here without an adequate definition. You need to explain how X4 can be a "spatial" dimension when the metric is not positive definite. (The question does not arise for X1, X2, X3 because on any spacelike slice there is an induced metric involving just X1, X2, X3 on that slice which *is* positive definite, so there is a clear definition of how those dimensions are "spatial". You can't do this with a 3-surface that has X4 as one of its dimensions.)

And of course there is no physical motion of 4-D material objects in a spatial block universe. However, there is the psychological impression of motion arising from the interaction of consciousness with the 4-dimensional object. The impression of time flowing results. This impression of the flow of time can be represented mathematically as the focus of consciousness moving along the world line at the speed of light.

Or it can be represented, as Barbour does, as an ordering relation on the "snapshots" that does not require "flow" at all. This has the advantage of not requiring the concept of the "speed" at which consciousness is "moving" along a worldline. Again, interpreting the conversion factor c as a "speed" requires you to already have a concept of "time", so you can't use speed to define the concept of time as we experience it.

 

[bobc2]

Hmm, interesting, I hadn't heard about this book. If you can find a link to a review, summary, etc., please post.

I finally found the reference in Paul Davies's book "About Time" (paper back pg. 41). Hoyle's book is "October First Is Too Late." It has been quite some while since I read the Davies book and my memory of it mixed the initial musings of Davies with Hoyle's theme. Davies had similar thoughts of jumping in time, even in his youth. He would muse over pushing magic buttons that would transport him randomly to different times: "...I would have no subjective impression of randomness, because at each stage the state of my brain would encode a consistent sequence of events." He continued, "It is but a small step from this wild fantasy to the suspicion that maybe someone else--a demon or fundamentalist-style deity perhaps--is pressing those buttons on my behalf, and I, poor fool, am totally oblivious to the trickery..."

Davies then describes Hoyle's book: "Hoyle also imagined some sort of cosmic button-pusher, but one who fouled things up and got different bits of the world out of temporal kilter." ..."Hoyle's fictional scientist caught up in this nightmare has no truck with the notion of time as an 'ever-rolling stream', dismissing it as a grotesque and absurd illusion."

I googled "October First Is Too Late" and found reviews on Amazon. Also, you can find Chapter 14 in pdf format. The sub script to the Chapter 14 title is a quote by Hoyle:

The 'science' in this book is mostly scaffolding for the story, story-telling in the
traditional sense. However, the discussions of the significance of time and of the
meaning of consciousness are intended to be quite serious, as also are the contents of
chapter fourteen. Fred Hoyle, 14 July 1965

Again, you are using the term "spatial" here without an adequate definition. You need to explain how X4 can be a "spatial" dimension when the metric is not positive definite. (The question does not arise for X1, X2, X3 because on any spacelike slice there is an induced metric involving just X1, X2, X3 on that slice which *is* positive definite, so there is a clear definition of how those dimensions are "spatial". You can't do this with a 3-surface that has X4 as one of its dimensions.)

Again, I maintain that the indefinite metric is related to the slanting of the X4' and X1' axes. It is not at all related uniquely to time. We can have slanted spatial axes without any need for time.

How do you define or characterize the quality of X4? Well, how do you characterize the quality of X1, X2, and X3? The existential quality of the physical space along different directions is not dependent on the geometric orientation of the axes. You don't turn a coordinate axis into time by rotating it. The thing that associates X4 with time is that 4-D objects are vastly longer in X4 as compared to X1, and consciousness interacts along the X4 world line in a manner that results in psychological illusions of a flow of time. And that flow is associated with a continuous sequence of 3-D images having a quality of change and movement. Certain 4-D objects, like mechanical clocks, have a periodic repetitive geometry that provides a measure of distance along X4 (which is particulary useful when calibrating the 4-D positions as t = X4/c).

Further, when considering the arbitrary different cross-sections of 4-D space, there can be no comprehensible meaning given to various vague kinds of mixtures of space and time as the cross-sections for different observers cut through the same 4-dimensional space at different angles.

Or it can be represented, as Barbour does, as an ordering relation on the "snapshots" that does not require "flow" at all. This has the advantage of not requiring the concept of the "speed" at which consciousness is "moving" along a worldline.

That is certainly a concept that is consistent with the spatial 4-dimension universe. Nevertheless, we as thinking observers do have a psychological sense of time passing, notwithstanding the illusion aspect you refer to. The idea of consciousness moving along the world line at speed c aids in the description of that psychological phenomena.

Again, interpreting the conversion factor c as a "speed" requires you to already have a concept of "time", so you can't use speed to define the concept of time as we experience it.

The notion of time and time passing was anchored in the psyche of man centuries before special relativity. Likewise the concept of motion and speed. Special relativity and the geometry of Lorentzian space provided, perhaps for the first time, a logical and easily comprehended world view that encompases space and time. It leads logically to the understanding that one must focus attention on the understanding of consciousness in order to understand time, rather than looking to the 4th spatial dimension.

 

[PeterDonis]

Again, I maintain that the indefinite metric is related to the slanting of the X4' and X1' axes. It is not at all related uniquely to time. We can have slanted spatial axes without any need for time.

You can have non-orthogonal spatial coordinates, yes, but that's not what the "slanting" of X1' and X4' relative to X1 and X4 is doing. X1' is still orthogonal to X4', just as X1 is orthogonal to X4. The only reason X1' and X4' look slanted is that they are drawn in the X1-X4 frame; if you drew everything in the X1'-X4' frame then X1 and X4 would look slanted.

In any case, you still haven't justified the term "spatial" when used in reference to a non-positive-definite metric. That's the key issue with that terminology, and nothing in the rest of your post appears to me to address it.

The idea of consciousness moving along the world line at speed c aids in the description of that psychological phenomena.

Agreed. I didn't mean to imply that I thought the idea of consciousness moving along the worldline was not useful, just that it was not fundamental to the "block universe" idea.

 

[bobc2]

You can have non-orthogonal spatial coordinates, yes, but that's not what the "slanting" of X1' and X4' relative to X1 and X4 is doing. X1' is still orthogonal to X4', just as X1 is orthogonal to X4. The only reason X1' and X4' look slanted is that they are drawn in the X1-X4 frame; if you drew everything in the X1'-X4' frame then X1 and X4 would look slanted.

That is correct, of course. I'm not suggesting anything at all contrary to that. Nevertheless those Lorentz boosts require a metric resulting in the invariances of special relativity theory. And some new quality, such as "time", is not at all necessary to arrive at the successful metric, one that results in, X4^2 - X1^2 = X4'^2 - X1'^2. Why the need to insert the quality of time to achieve relationships that are all about space?

In any case, you still haven't justified the term "spatial" when used in reference to a non-positive-definite metric. That's the key issue with that terminology, and nothing in the rest of your post appears to me to address it.

I think the situation is just the opposite. I think the burden is heavily on the mathematician to demonstrate that when you have a space with indefinite metric the space then fails to maintain the same existential quality in all directions. Just having a different sign in the signature does not automatically signal a change in the quality of space. There is especially no rationale for assigning time as a new quality for the spatial 4th dimension.

However, you've brought a very useful discussion to the table. You've crystallized this issue of how the 4th dimension should be chacterized. I know my comments do not resolve the issue. We may need to start again, looking at this issue from the ground up. Perhaps begin carefully building up the mathematical machinery step-by-step, considering the quality of space in each direction as we go. Look carefully at what (if any) quality can be ascribed to the manifold, the topology, what spaces are available, and what linear vector spaces are available and what can be said about the quality of the spaces in each of the directions.

Agreed. I didn't mean to imply that I thought the idea of consciousness moving along the worldline was not useful, just that it was not fundamental to the "block universe" idea.

Sure. And again, the consciousness moving along the world lines at c can lead to bizarre implications that hardly anyone would like.

 

[PeterDonis]

And some new quality, such as "time", is not at all necessary to arrive at the successful metric, one that results in, X4^2 - X1^2 = X4'^2 - X1'^2. Why the need to insert the quality of time to achieve relationships that are all about space?

You're correct that one does not need to add a new quality like "time" to X4 to justify the indefinite metric. And there is a use of the term "space" in mathematics that is general enough to encompass the way you are using it here. But AFAIK that usage is not common in relativity physics; the more precise technical term, "manifold", is used instead to convey what I think you are trying to convey (that spacetime has a certain structure, dimensionality, topology, etc., without reference to the metric).

It is true that in quantum physics, the term "space" is used much more generally, as it is in mathematics; it basically means any larger structure that objects of interest "live" in. For example, quantum state vectors are elements of a Hilbert space. But this usage of "space" is highly abstract and certainly does not imply any easily comprehended relationship with what we commonly think of as space; most quantum Hilbert spaces, for example, have an infinite number of dimensions.

I think the burden is heavily on the mathematician to demonstrate that when you have a space with indefinite metric the space then fails to maintain the same existential quality in all directions. Just having a different sign in the signature does not automatically signal a change in the quality of space.

In some ways it does. For example, in a manifold with positive definite metric, the only way for a vector to have zero norm is to be the zero vector. In a manifold with non-positive-definite metric, there is an infinite number of vectors with zero norm (the null vectors). This leads to some properties that may seem counterintuitive: for example, every null vector is orthogonal to itself.

Perhaps begin carefully building up the mathematical machinery step-by-step, considering the quality of space in each direction as we go. Look carefully at what (if any) quality can be ascribed to the manifold, the topology, what spaces are available, and what linear vector spaces are available and what can be said about the quality of the spaces in each of the directions.

This has already been done; it's called "differential geometry".

http://en.wikipedia.org/wiki/Differential_geometry

 

[bobc2]

I've drawn on Paul Davies's cosmic button-pusher to present a fanciful example that may get across the point I've been trying to make. I'm not offering this as a proof for the spatial 4th dimension, but rather, hopefully, a clarification of the concept.

 

[PeterDonis]

I've drawn on Paul Davies's cosmic button-pusher to present a fanciful example that may get across the point I've been trying to make. I'm not offering this as a proof for the spatial 4th dimension, but rather, hopefully, a clarification of the concept.

I see two questions regarding this:

(1) The cosmic button-pusher starts out by setting up a Euclidean space with a positive definite metric. Then, somehow, the mere act of granting consciousness to the blue observer changes the metric. How does that happen? Of course we know what the actual metric of spacetime is, so we know in advance what the answer is supposed to be, but how does that answer arise logically, within the scenario, as a result of granting the blue observer consciousness? I don't see any logical connection there. Put another way, since we already know that the metric of spacetime is not positive definite, your introduction of a hypothetical Euclidean space whose metric magically gets changed when an observer is granted consciousness is superfluous; it can be eliminated without changing anything else.

(2) You've used something like the second picture before in another thread, and I pointed out an issue there which, IIRC, you never responded to. Yes, for the particular set of values you chose, you can write down a "Pythagorean theorem" that appears to apply, but it only applies to particularly chosen sets of values; it does not apply generally. For example, draw a similar right triangle with sides blue X1, blue X4, and red X1 (instead of red X4); in other words, drop a perpendicular from the blue X1 axis to the red X1 axis (instead of from the blue X4 axis to the red X4 axis). You can still write down a "Pythagorean-style" equation: (red X1)^2 = (blue X1)^2 - (blue X4)^2. The problem is, now the LHS is a *side* of the right triangle, *not* the hypotenuse! So the cosmic button-pusher has *not* restored a Euclidean metric; he has merely picked out a particular set of values that can be rearranged in a certain way.

 

[Snip3r]

We would have a more direct analogy of course if the movie film were strung out in space along a straight line and you were doing the moving along the length of the strip watching that same movie, experiencing the same movie-going experience without any thought of your motion through space.

i just jumped into say wow that someone sharing my thoughts. i was always a bit unhappy that 4th dimension had different units. sometimes i think light as 2d structures in 3d space (may be that's why wave-particle duality... just kidding) but what i thought was all the 3d objects that we know to move at C in 4-d space not just consciousness.

 

[bobc2]

i just jumped into say wow that someone sharing my thoughts. i was always a bit unhappy that 4th dimension had different units. sometimes i think light as 2d structures in 3d space (may be that's why wave-particle duality... just kidding) but what i thought was all the 3d objects that we know to move at C in 4-d space not just consciousness.

Good to have you jumping in here, to have one who can relate to the issue at hand. Don't be a stranger.

 

[bobc2]

I see two questions regarding this:

(1) The cosmic button-pusher starts out by setting up a Euclidean space with a positive definite metric. Then, somehow, the mere act of granting consciousness to the blue observer changes the metric. How does that happen? Of course we know what the actual metric of spacetime is, so we know in advance what the answer is supposed to be, but how does that answer arise logically, within the scenario, as a result of granting the blue observer consciousness? I don't see any logical connection there. Put another way, since we already know that the metric of spacetime is not positive definite, your introduction of a hypothetical Euclidean space whose metric magically gets changed when an observer is granted consciousness is superfluous; it can be eliminated without changing anything else.

I knew I was not giving an accounting of how the slanted X1' pops out. Just didn't want to make the post so long and involved. The other part of the story would involve the cosmic guy's apprentice. Before adding in the consciousness the cosmic guy tells the apprentice to add in some other 4-dimensional objects to the positive definite manifold, then cosmic guy walks away leaving the apprentice to play with the toy universe. But, the apprentice is actually quite ingenious and puts objects in with very special orientations. He has developed an elaborate set of rules about how all of the objects should be arranged on the positive definite manifold.

To picture the kinds of geometric patterns the apprentice used in placing the objects, just think things like Feynman diagrams, processes involving conservation laws, etc.

Thus, when the consciousness is turned on and sent on a trip along the blue guy's world line, the only way the consciousness could acquire any comprehension of the continuous sequence of 3-D cross-section views of the 4-dimensional manifold with embedded objects would be to psychologically adjust his X1 cross-section view so as to be in sync with a Lorentz boosted view of the universe. That's just because the apprentice formed the patterns in just the right way to produce the unique invariances that are normally associated with Lorentz boosts. If the blue guy did not view the universe across a Lorentz boost view there would not be the kind of correlation in the sequence of events unfolding around him that could produce a comprehensible experience. It's kind of analogous to the difference between listening to random noise and music. If you are in an environment of a loud audible random noise, yet there is a lone violin playing a melody somewhere in the background, your brain has a way of filtering the violin melody so that you comprehend the sound in spite of the noise. The symmetry of geometric patterns present in the 4-D spatial universe makes possible some kind of correlation within the brain that plays some kind of role in the ability of the consciousness to recognize and comrehend. For the blue guy it was a matter of psychologically adjusting his cross-section view to the proper Lorentz boost that makes for an intelligible continuous sequence.

The cosmic button-pusher initially did not realize what his apprentice had done, but he quickly discovered the benefit of switching to a new metric so he could recognize the invariances and appreciate the local physics that resulted. He was quick to realize why the blue guy's consciousness automatically began scanning 4-D space in the slanted X1' direction. But, it is important to note that it was not necessary for the cosmic button-pusher to change metrics for his bird's eye view of the universe. The cosmic button-pusher could happily muse over the varied patterns of objects placed on the positive definite metric space. He just wouldn't see the physics that the apprentice had built into the patterns--which are only apparent if you change the metric so as to recognize the invariances associated with the Lorentz boost. Arranging objects does not change the intrinsic mathematical properties of the manifold--it's topology, etc.

The whole point of the story is to try to explain in what sense you can start with a positive definite manifold, yet then orient objects in a way that leads to the selection of an indefinite metric to make the orientation of objects intelligible. The quality of the four spatial dimensions did not change at all in that process. And there was certainly no rationale for regarding the 4th dimension as "time."

(2) You've used something like the second picture before in another thread, and I pointed out an issue there which, IIRC, you never responded to. Yes, for the particular set of values you chose, you can write down a "Pythagorean theorem" that appears to apply, but it only applies to particularly chosen sets of values; it does not apply generally.

I don't agree with that. You give me any pair of observers moving relative to each other at any speed you wish. There is always a rest frame for which both observers are moving in opposite directions at the same speed. So, a symmetric space-time diagram can always be found that works in general for any pair of observers. And for example the Lorentz time dilation equation may be derived directly from the Pythagorean Theorem.

And yes, you're right; it's the differential geometry and associated mathematical machinery. However, all through the physics Master's and PhD curriculum, in all of the functional analysis, tensor analysis, group theory, set theory, QM courses, classical field theory, special relativity, general relativity, and cosmology courses, none of my professors ever discussed manifolds in this context. I tried only two or three times to discuss this with my doctoral relativity advisor, but he was quite annoyed that I would allow myself to get so distracted from doing real physics. And he was right in terms of how I should have spent my time in that phase of education.

 

[PeterDonis]

The whole point of the story is to try to explain in what sense you can start with a positive definite manifold, yet then orient objects in a way that leads to the selection of an indefinite metric to make the orientation of objects intelligible.

In which case the manifold is no longer positive definite. Put another way, your claim that the manifold started out positive definite is not justified: "positive definite" is supposed to describe the actual physical metric that describes actual physical intervals, not an unobservable starting point that you then throw away and that plays no part in predicting any actual measurements.

The quality of the four spatial dimensions did not change at all in that process.

As dimensions in a topological manifold, no. As dimensions in a metrical space, yes, X4 *did* change; you started out saying the metric was positive definite but as soon as any actual physical measurements were made it changed to non positive definite. You can't just handwave away that change in metric structure.

I don't agree with that. You give me any pair of observers moving relative to each other at any speed you wish. There is always a rest frame for which both observers are moving in opposite directions at the same speed. So, a symmetric space-time diagram can always be found that works in general for any pair of observers. And for example the Lorentz time dilation equation may be derived directly from the Pythagorean Theorem.

For *that particular pair of observers*, in *that particular frame*. A real metric is not like that; it gives the right answer for *all* pairs of observers, in any frame and any state of motion, without any special setup required each time.

However, all through the physics Master's and PhD curriculum, in all of the functional analysis, tensor analysis, group theory, set theory, QM courses, classical field theory, special relativity, general relativity, and cosmology courses, none of my professors ever discussed manifolds in this context. I tried only two or three times to discuss this with my doctoral relativity advisor, but he was quite annoyed that I would allow myself to get so distracted from doing real physics. And he was right in terms of how I should have spent my time in that phase of education.

I'm sorry you went through that kind of experience. I avoided it because I didn't study differential geometry at all in school, which may have been a good strategy for actually being able to learn something about it.

But that doesn't mean the subject can't be learned. I learned it mainly from Misner, Thorne, & Wheeler, which I also had the advantage of not having to learn in school. But that may not be the best up to date source. Others here at PF could give better advice than I on where to look.

 

[bobc2]

In which case the manifold is no longer positive definite.

Correct. But the whole point is that the quality of the X4 did not change. The apprentice simply added objects to the space without changing the fundamental quality of the space--it remains characterized as four spatial dimensions.

Put another way, your claim that the manifold started out positive definite is not justified: "positive definite" is supposed to describe the actual physical metric that describes actual physical intervals, not an unobservable starting point that you then throw away and that plays no part in predicting any actual measurements.

Wait. The cosmic button-pusher established a displacement vector, V, and established its magnitude as invariant with respect to both orthogonal coordinates using the positive definite metric.

As dimensions in a topological manifold, no. As dimensions in a metrical space, yes, X4 *did* change; you started out saying the metric was positive definite but as soon as any actual physical measurements were made it changed to non positive definite. You can't just handwave away that change in metric structure.

Of course the measure of X4 changed with the selection of a new metric that could account for the kinds of symmetries present in the new geometry associated with the objects added into the space by the apprentice. But the whole point is that the character of the space itself did not change. We started with four spatial dimensions and did nothing to change the spatial character of the 4th dimension. Now, if we could not rely on a one-to-one mapping, then things might be different.

For *that particular pair of observers*, in *that particular frame*. A real metric is not like that; it gives the right answer for *all* pairs of observers, in any frame and any state of motion, without any special setup required each time.

I didn't say that the derived equation was a derivation of the metric (you boxed me in on that one once before--good job, too). I just said that the cosmic button-pusher knew that he had the right form for the metric after checking that result. After all, he knew that he had reciprocal coordinate systems, i.e., the red was the dual of the blue. He understood the implications of the contravariant-covariant relationship that was manifest. And that observation was the inspiration for the apprentice's choice of rules for positioning the new objects--he wanted to utilize the metric,

I'm sorry you went through that kind of experience. I avoided it because I didn't study differential geometry at all in school, which may have been a good strategy for actually being able to learn something about it.

But that doesn't mean the subject can't be learned. I learned it mainly from Misner, Thorne, & Wheeler, which I also had the advantage of not having to learn in school. But that may not be the best up to date source. Others here at PF could give better advice than I on where to look.

You are to be highly commended for your accomplishments. There's a lot to be said for learning in your own way at your own pace and having the ability to dig deep when something really interests you and picking up the pace as you wish. You seem to be more knowlegeable than I (and keep boxing me into corners with your insight), especially for all the course work I've had. But, I have all the necessary resources at hand with just about all of my textbooks, notes and other literature. I enjoy reading new books from time to time as well, such as Naber's "The Geometry of Minkowski Spacetime", Penrose's "The Road To Reality" and B. Crowell's "General Relativity" (excellent formal approach by Ben--that guy knows what he is doing--you should check it out on the internet).

 

[PeterDonis]

Correct. But the whole point is that the quality of the X4 did not change. The apprentice simply added objects to the space without changing the fundamental quality of the space--it remains characterized as four spatial dimensions.

...

Wait. The cosmic button-pusher established a displacement vector, V, and established its magnitude as invariant with respect to both orthogonal coordinates using the positive definite metric.

And as soon as the metric is changed to respect the Lorentz symmetry instead of the Euclidean symmetry, it is impossible to maintain those supposedly established magnitudes for all displacements. You can, by carefully choosing only certain displacements, make it seem as though the magnitude is invariant for those particular displacements. But there's no way to do it for *all* displacements. It's not possible; it would amount to equating a positive definite metric with a non positive definite metric. It can't be done. So your claim that X4 remains a "spatial" dimension when the metric changes simply can't be sustained.

Of course the measure of X4 changed with the selection of a new metric that could account for the kinds of symmetries present in the new geometry associated with the objects added into the space by the apprentice. But the whole point is that the character of the space itself did not change.

So you don't think that the metric is part of "the character of the space itself". That viewpoint is not inconsistent, but it's also not very common, and as I said earlier, trying to describe things this way will increase confusion, not reduce it. The standard viewpoint in relativity views the metric as part of "the character of the space itself", because you can't describe all of the physics without it. You can describe *some* properties without it, as you note: for example, the topology of the manifold. But you can't describe *all* properties that are needed for physics.

One property in particular that you can't describe without the metric is causality: without the metric there is no way to tell whether a given pair of events is timelike, null, or spacelike separated, so you don't know what causal relationships are possible or forbidden between them. This is one big reason why the standard viewpoint considers the metric to be "part of the character of the space itself".

B. Crowell's "General Relativity" (excellent formal approach by Ben--that guy knows what he is doing--you should check it out on the internet).

I have, I agree it's a great site and pedagogical resource. It gets linked to fairly frequently around here.

 

[bobc2]

And as soon as the metric is changed to respect the Lorentz symmetry instead of the Euclidean symmetry, it is impossible to maintain those supposedly established magnitudes for all displacements. You can, by carefully choosing only certain displacements, make it seem as though the magnitude is invariant for those particular displacements. But there's no way to do it for *all* displacements. It's not possible; it would amount to equating a positive definite metric with a non positive definite metric. It can't be done. So your claim that X4 remains a "spatial" dimension when the metric changes simply can't be sustained.

No. That has never been my point. I've not implied that you can have the same measurement of distance going from one metric to another. All you have to do is to start with the Lorentz transformations, compute the products, and there's no way the physical distances from one point to another come out the same.

So you don't think that the metric is part of "the character of the space itself". That viewpoint is not inconsistent, but it's also not very common, and as I said earlier, trying to describe things this way will increase confusion, not reduce it. The standard viewpoint in relativity views the metric as part of "the character of the space itself", because you can't describe all of the physics without it. You can describe *some* properties without it, as you note: for example, the topology of the manifold. But you can't describe *all* properties that are needed for physics.

The mathematicians have given us the mathematical machinery that describes mathematical objects and relationships. They don't a priori give us the physical reality. The physicist takes the manifold, topology, set theory, group theory, linear vector spaces, etc., and uses them to describe reality as he envisions it, with the requirement that any models developed will be consistent with established theories of physics (unless new analysis can prove otherwise after experimental confirmation). So, the metrics of the mathematician do not automatically give us the physical character and quality of different directions in physical space. We have the abstract mathematical space of the mathematician, and we have the physical space envisioned by the physicist. Some physicists envision the 4th dimension as some kind of physical time. Other physicists say that is not comprehensible; there is no basis to assume any different physical character and quality to the 4th dimension that would make it any different than the normal X1, X2, and X3.

One property in particular that you can't describe without the metric is causality: without the metric there is no way to tell whether a given pair of events is timelike, null, or spacelike separated, so you don't know what causal relationships are possible or forbidden between them. This is one big reason why the standard viewpoint considers the metric to be "part of the character of the space itself".

Of course we need the L4 space with its metric to make intelligible the physics hiding in the manifold. That's why the cosmic button-pusher was initially confused with the arrangement of 4-dimensional objects placed by the apprentice. He initially viewed the assortment of 4-D objects in the context of his original Euclidean metric induced on the manifold. But once he switched over to the relevant L4 metric all of the invariances came into play manifesting the illusion of physical laws (resulting from the apprentice's ingenious placement of the 4-dimensional objects).

In spite of this situation, it is not correct (in the view of the initial post here) to say that the metric accounts for the X4 as being either physical time or physical space. But yes, the metric is intimately associated with the revelation of the physics manifest on the manifold. But: It is the physics first (the very special arrangement of the 4-D objects) that prompts for a successful selection of a metric. The L4 metric has been revealed to us, but only because the 4-D objects have been arranged in that very special way.

I have, I agree it's a great site and pedagogical resource. It gets linked to fairly frequently around here.

 

[bobc2]

A couple more comments: The metric does not place a preference on the quality of the dimensions. The traditional view among physicists is the one PeterDonis has been advocating. Although the mathematical system applied in desribing special relativity theory does not force this view, Minkowski himself embraced it. Most physicists embraced the idea of time as the 4th dimension. However, that did not mean that they did not embrace the idea of the block universe. Weyl wrote: "The world does not happen, it simply is." Einstein apparently subscribed to this view as well (everyone always references Einstein's letter to the wife of his close friend, Besso, at the time of Besso's passing).

Typical of sentiments in the early years of special relativity is the commentary from the writings of Sir James Jeans on Space-Time unity (book "Physics and Philosophy): "The physical theory of relativity suggests, although without absolutely conclusive proof, that physical space and physical time have no separate and independent existences; they seem more likely to be abstractions or selections from something more complex, namely a blend of space and time which comprises both.

This is exactly the view that I've tried to refute with the beginning post of this thread.

So, the two versions of block time: 1) A four-dimensional universe all there at once with physical time as the 4th dimension and 2) A four-dimensional universe all there at once with the 4th dimension as just another "physically spatial" dimension (we use "physically spatial" to avoid the confusion of the meaning of the mathematical abstract space that implies particular metrics--metrics that really do not force X4 to be either physical time or physical space). This physically spatial 4th dimension could be accompanied either by a 3-D consciousness moving along a world line at speed c, or it could be accompanied by a consciousness that exists simultaneously all along the world line.

By the way, a point made by Einstein ("Albert Einstein - Philosopher-Scientist", Library of Living Philosophers, Edited by Paul Schilpp": "First a remark concerning the relation of the theory to 'four-dimensional space.' It is a wide-spread error that the special theory of relativity is supposed to have, to a certain extent, first discovered, or at any rate, newly introduced, the four-dimensionality of the physical continuum. This, of course, is not the case. Classical mechanics, too, is based on the four-dimensional continuum of space and time. But in the four-dimensional continuum of classical physics the subspaces with constant time value have an absolute reality, independent of the choice of the reference system. Because of this [fact], the four-dimensional continuum falls naturally into a three-dimensional and a one-dimensional (time), so that the four-dimensional point of view does not force itself upon one as necessary. The special theory of relativity, on the other hand, creates a formal dependence between the way in which the spatial co-ordinates, on the one hand, and the temporal coordinates, on the other, have to enter into the natural laws."

And again, he is expressing a concept we've tried to refute in this thread. He could just as easily regarded the 4th dimension of Newton as a physically spatial dimension with time as a parameter. Nevertheless, his view gives me an opening to make a point of rebuttle to the notion that the metric detemines the physical nature of a dimension.

In Einstein's example here we have a positive definite metrice (the X4' and X1' axes are not slanted symmetrically in accordance with the Lorentz boost at all. If a positive metric is associated with physically spatial coordinates only, then Einstein could not have his "time" as a 4th dimension in the Newtonian Euclidean 4-D space.

 

[PeterDonis]

So, the metrics of the mathematician do not automatically give us the physical character and quality of different directions in physical space.

The actual metric we use in physics is not "of the mathematician". Mathematicians can come up with zillions of different metrics to put on a 4-D manifold. It's physicists who tell us that the metric that actually applies to our actual universe is the Lorentz metric.

Some physicists envision the 4th dimension as some kind of physical time. Other physicists say that is not comprehensible; there is no basis to assume any different physical character and quality to the 4th dimension that would make it any different than the normal X1, X2, and X3.

Except that it has opposite sign in the metric. Do you know of any physicists who deny that, other than those who multiply it by i, which just moves the difference from one place (the sign of the term in the metric) to another (the kind of number used for the coordinate)? This is a physical difference, not a mathematical difference. Without it you don't have causality as we observe it, as I said before.

 

[bobc2]

The actual metric we use in physics is not "of the mathematician".

Minkowski was Einstein's professor of mathematics.

Mathematicians can come up with zillions of different metrics to put on a 4-D manifold. It's physicists who tell us that the metric that actually applies to our actual universe is the Lorentz metric.

That's exactly what I said in a previous post. And some physicists choose to let X4 represent physical time and others choose to let X4 represent a physically spatial dimension. This is more natural because it is easily comprehensible. The notion of a "mixture of time and space" is not a concept that can be physically comprehensible. We have our common sense notion of what physical space is from everyday experience. We also have a fairly definite psychological feeling about time. But picturing time as a physical dimension and then space-time as a mixture of space and time is not nearly as natural to handle conceptually as 4 physically spatial dimensions. It is not conceptually difficult to envision time as closely associated with consciousness.

Except that it has opposite sign in the metric.

Again, the metric associated with the Lorentz boosts we see diagrammed in the space-time diagrams is directly related to the orientation of the four coordinates of the 4-D physical space. There is nothing that implies the metric demands a physical time for X4.

Do you know of any physicists who deny that, other than those who multiply it by i, which just moves the difference from one place (the sign of the term in the metric) to another (the kind of number used for the coordinate)?

I did not invent the block universe, nor the consciousness moving along world lines. Of course the ideas were present in the Davies and Hoyle references mentioned earlier. The earliest notion of a block universe without time of which I'm familiar was offered by Einstein's Princeton colleague, Kurt Godel (one of the foremost among mathematicians and logicians who gave us the Incompleteness Theorem). He solved Einstein's with world lines curving back on themselves and used the example to demonstrate a block universe without time.

This is a physical difference, not a mathematical difference. Without it you don't have causality as we observe it, as I said before.

Yes. Dramatic physical phenomena are manifest which are intimately related to the use of the L4 metric. But, again, the physical quality (other than the obvious geometric characteristics) of the 4th dimension are not among these. A concept of a block universe with four physical spatial dimensions is consistent with the L4 metric and special relativity.

 

[PeterDonis]

Minkowski was Einstein's professor of mathematics.

Yes. So what?

And some physicists choose to let X4 represent physical time and others choose to let X4 represent a physically spatial dimension.

References, please? I'm particularly curious to see in what, if any, contexts the word "spatial" is used to refer to X4 or its equivalent. This discussion is mostly about terminology, so the actual usage of actual physicists is important.

This is more natural because it is easily comprehensible. The notion of a "mixture of time and space" is not a concept that can be physically comprehensible.

Really? I seem to have no trouble comprehending it. Nor, I suspect, do lots of other relativity physicists who use the Lorentz metric all the time.

Again, the metric associated with the Lorentz boosts we see diagrammed in the space-time diagrams is directly related to the orientation of the four coordinates of the 4-D physical space. There is nothing that implies the metric demands a physical time for X4.

I could ask here, what does "physical time" mean? But that's really beside the point. I am not objecting to the usage of the word "space" as in "4-D manifold" per se; I am only objecting to the specific usage of the word "spatial" to imply that there is no physical difference between X4 and the other three dimensions. There is. It's a metrical difference, not a topological difference, but it's still a genuine physical difference.

If all you are saying is that spacetime diagrams help to visualize relativity problems, I have no objection to that. But spacetime diagrams are tools; you don't have to adopt any particular position on whether or not X4 is "spatial" in order to use spacetime diagrams.

I did not invent the block universe, nor the consciousness moving along world lines. Of course the ideas were present in the Davies and Hoyle references mentioned earlier. The earliest notion of a block universe without time I'm familiar with is Einstein's Princeton colleague, Kurt Godel (one of the foremost mathematicians and logician who gave us the Incompleteness Theorem). He solved Einstein's with world lines curving back on themselves and used the example to demonstrate a block universe without time.

No, he found a solution of Einstein's field equation that had closed timelike curves through every point. That's not the same thing as a "block universe without time". In a purely "spatial" universe (i.e., one with a positive definite metric), there are no such things as timelike curves, and there are no potential causal paradoxes associated with closed curves in any dimension (because there's no causality to begin with). Do you think Godel's solution would have been such a big deal if all it showed was that we can have spatial circles through every point? It was the fact that there were closed *timelike* curves through every point that was the big deal. But you can't even form the concept of timelike curves as distinct from spacelike curves at all, let alone closed ones, without admitting that there is a physical difference associated with one of the dimensions.

 

[PeterDonis]

Dramatic physical phenomena are manifest which are intimately related to the use of the L4 metric. But, again, the physical quality (other than the obvious geometric characteristics) of the 4th dimension are not among these. A concept of a block universe with four physical spatial dimensions is consistent with the L4 metric and special relativity.

So you think that causality is not a "physical quality" or "physical phenomenon" of any consequence, so there's nothing wrong with calling a timelike dimension "spatial". As I've said before, this terminology of yours is highly nonstandard and is likely to cause a lot of confusion if you try to use it. There's a reason physicists pay close attention to the difference between timelike and spacelike (and null) curves; your suggested terminology basically ignores it.

 

[bobc2]

So you think that causality is not a "physical quality" or "physical phenomenon" of any consequence, so there's nothing wrong with calling a timelike dimension "spatial".

Of course these are physical qualities. In the block universe model these characteristics arise from the geometry and orientations of the 4-dimensional objects that populate the 4-D space. The problem here is that a complete geometry-based physical theory is not yet available. Einstein was unable to complete his program. Kaluza tried. The string theorists are working on it. What I'm saying is that the causality is inherent in the geometry. And yes, we need the L4 metric to represent this mathematically. Still, just because we have some physical concepts implied by the L4 metric, that does not require we ascribe a physical time quality as representing the 4th dimension.

As I've said before, this terminology of yours is highly nonstandard and is likely to cause a lot of confusion if you try to use it.

There's far worse confusion about the 4th dimension with the continuing representation that it is time. And the idea that you are reading time directly when you look at a clock is one of the confusion factors commonly sustained.

There's a reason physicists pay close attention to the difference between timelike and spacelike (and null) curves; your suggested terminology basically ignores it.

No. I just recommend qualifiers when using the standard terminology. Time-like, Space-like and null cone have specific mathematical and physical meaning that does not need to be compromised. And there is no damgage done when clarifying the distinctions among the competing concepts related to the 4th dimension in special relativity theory.

 

[bobc2]

Really? I seem to have no trouble comprehending it. Nor, I suspect, do lots of other relativity physicists who use the Lorentz metric all the time.

I'll respond to your other comments later. In the meantime please discuss what you mean by

a) Time as the 4th dimension.

b) What is meant by "a mixture of time and space?" We know what that means mathematically, and we can easily see it mathematically when it is represented on a space-time diagram--its just a mathematical cross-section of a mathematical space-time.

But please offer some kind of description that would give us a concept of the mixture of space and time for which we would have no trouble comprehending. Discuss the quality or character of that mixture. I can comprehend in some sense the quality of space based on experience with X1, X2, X3. I can comprehend the notion of time from my direct psychological experience with time passing. How do you mix those two concepts and come up with a comprehensible concept? They are so different. The mathematical description is useful and necessary, but that alone does not provide a comprehensible concept.

If I had been deaf, blind and without the sense of touch all my life, I may not be capable of having a comprehensible concept of space. My care giver could tell me when he is moving me from place to place, but the concept of space should be much more difficult to comprehend for someone in that state.

I could comprehend a notion of time in that state. But, the notion of a mixture of space and time would be hopeless, even if my care giver could teach me mathematics, i.e., differential geometry and special relativity.

 

[PeterDonis]

a) Time as the 4th dimension.

In standard Minkowski coordinates on spacetime, one dimension has a different sign in the metric. That's the one we call "time".

b) What is meant by "a mixture of time and space?" We know what that means mathematically, and we can easily see it mathematically when it is represented on a space-time diagram--its just a mathematical cross-section of a mathematical space-time.

Yes, that's what it means.

But please offer some kind of description that would give us a concept of the mixture of space and time for which we would have no trouble comprehending.

I may not be able to. There is no guarantee that every concept that can be described mathematically can be described in a way your intuition will comprehend. That's why we have mathematics.

I can comprehend in some sense the quality of space based on experience with X1, X2, X3. I can comprehend the notion of time from my direct psychological experience with time passing. How do you mix those two concepts and come up with a comprehensible concept?

Why do you need to? The mathematics provides enough of a description to predict the results of experiments. Why must there be a way of comprehending it that fits with your intuition?

If I had been deaf, blind and without the sense of touch all my life, I may not be capable of having a comprehensible concept of space. My care giver could tell me when he is moving me from place to place, but the concept of space should be much more difficult to comprehend for someone in that state.

For "comprehend" in the sense of "visualize", yes. But that's not the only way you could approach the concept.

I could comprehend a notion of time in that state. But, the notion of a mixture of space and time would be hopeless, even if my care giver could teach me mathematics, i.e., differential geometry and special relativity.

But if you knew the mathematics, you would have a way of dealing with the concept that did not require you to "comprehend" it in the sense you mean.

 

[bobc2]

References, please? I'm particularly curious to see in what, if any, contexts the word "spatial" is used to refer to X4 or its equivalent. This discussion is mostly about terminology, so the actual usage of actual physicists is important.

For now, here is one reference: "A World Without Time - The Forgotten Legacy Of Godel and Einstein" (a Discover magazine best seller).

Here is the writeup from the back cover of the book: In 1942, the logician Kurt Godel and Albert Einstein became close friends; they walked to and from their offices every day, exchanging ideas about science, philosophy, politics, and the lost world of German science in which both men had grown up. By 1949, Godel had produced a remarkable proof: In any universe described by the Theory of Relativity, time cannot exist. Einstein endorsed this result reluctantly, but he could find no way refute it, and in the half-century since then, neither has anyone else. Even more remarkable was what happened afterward: nothing. Cosmologists and philosophers alike have proceeded as if this discovery was never made. In "A World Without Time," Palle Yourgrau sets out to restore Godel to his rightful place in history, telling the story of two magnificent minds put on the shelf by the scientific fashions of their day, and attempts to rescue from undeserved obscurity the brilliant work they did together.

I have another reference, a paper written by Godel for inclusion in a book dedicated to Einstein, the one I mentioned earlier edited by Arthur Schilpp. Schilpp mangaged to collect writings from a number of physicists, mathematicians and philosophers, all dedicated to commenting on Einstein's life and work, which formed one volume in Schilpp's series on Living Philosophers. I'll have to dig it out again and locate some pertinent excerpts from which Godel is describing his solutions to Einstein's equations of general relativity in the context of "destroying time."

 

[bobc2]

References, please? I'm particularly curious to see in what, if any, contexts the word "spatial" is used to refer to X4 or its equivalent.

Here is an excerpt from a review of the Godel-Einstein book. I give it just because it explicitly refers to the "spatial dimension."

Yourgrau manages to convey fairly clearly what exactly Gödel demonstrated in his short paper, taking Einstein's theory of relativity and focussing on the knotty time-issue, presenting a world model in which he could show "that t, the temporal component of space-time, was in fact another spatial dimension". The implications and consequences are profound and far-reaching...

 

[PeterDonis]

Here is an excerpt from a review of the Godel-Einstein book.

Was there supposed to be a link here? I'm not seeing one.

Edit: Ok, I see that the last paragraph of the post is supposed to be the excerpt. It would still be nice to have a link to the entire review.

 

[PeterDonis]

For now, here is one reference: "A World Without Time - The Forgotten Legacy Of Godel and Einstein" (a Discover magazine best seller).

You do realize that that last parenthetical phrase by itself is a minus for credibility...

The wording of the description of Godel's "remarkable proof" is already a red flag: the author is being way too dogmatic about what Godel's proof "means". The author appears to be a philosopher, not a physicist, which IMO is another reason to take what he says about the application of such proofs to physics with a grain of salt. John Stachel, a physicist, reviewed the book, and his review is online here:

http://www.ams.org/notices/200707/tx070700861p.pdf

He appears to have similar misgivings about the author's flat assertions about what Godel actually proved. This review, btw, also has an interesting quote from Einstein:

"Time and space are fused in one and the same continuum, but the continuum is not isotropic. The element of spatial distance and the element of duration remain distinct in
nature…"

 

[bobc2]

John Stachel, a physicist, reviewed the book, and his review is online here:

http://www.ams.org/notices/200707/tx070700861p.pdf

He appears to have similar misgivings about the author's flat assertions about what Godel actually proved.

Yes. I was just getting ready to post Stachel's reference when I saw your post. Stachel (Boston University physicist?) is definitely in the strongest disagreement with Godel's logic on the role of time in relativity, as are many physicists--particularly those of the logical positivist bent. And Stachel has no regard at all for Yourgrau (yes, he is definitely a philospher). I think that Stachel's view is representative of the majority of physicists today.

However, I have a hunch that more and more physicists--at least among those interested in the subject at all--are beginning to have second thoughts about the concept of time and the 4th dimension in special relativity. Probably most physicists are too involved in their own specialty to give much thought to these questions. My advisor certainly thought it a waste of my time to worry about it. He had no time for those kinds of discussions, and he devoted all of his time to relativity and published often in Phys Rev.

This review, btw, also has an interesting quote from Einstein:

"Time and space are fused in one and the same continuum, but the continuum is not isotropic. The element of spatial distance and the element of duration remain distinct in
nature…"

You would need more detail from Einstein to be sure exactly what he meant here. He could be referring to 4-dimensional physical world lines that are accompanied by conscious over the entire world line in a way that results in a local psychological flow of time experience. By the way, Stachel was in agreement with Godel that there is no global time in Godel's universe model. Stachel thought that was irrelevant, because the flow of time is local--it is associated with local world lines.

One should not lose track of the requirement that material objects must extend into the 4th dimension if different observers simultaneously exist in different cross-sections of 4-D, each including different instantaneous 3-D volumes of the same 4-D object. And it is not comprehensible that those different 3-D volumes are made of some kind of mixture of space and time--they must indeed be cross-sections cutting across at different angles. (see my sketch illustrating this in the very first post of this thread)

There is no argument about whether the cross-sections are spatial--they are of course--X1', X2', and X3'. How you could get those different 3-D volumes to all come out as spatial in all three dimensions when they were cut at totally different angles from a 4-D object without a spatial quality in the 4th dimension defies logic. I don't think you can cite a reference where Einstein dismissed the notion of 4-D objects. What would it mean to have a 4-dimensional object whose 4th dimension is not spatial?

The reason the continuum is not isotropic is because of the way 4-D objects extend into the spatial 4th dimension (X4)--extending billions and trillions of miles--as compared to the relatively miniscule extent along X1, X2, and X3. And then the consideration of the different relationships of X4 and X1 with respect to the light cone.

 

[bobc2]

PeterDonis, You never did offer an explanation as to how you could have simultaneous different cross-section views (by two different observers) of the same object unless that object was actually a 4-dimensional object.

 

[PeterDonis]

You would need more detail from Einstein to be sure exactly what he meant here. He could be referring to 4-dimensional physical world lines that are accompanied by conscious over the entire world line in a way that results in a local psychological flow of time experience.

I agree we can't be sure without more context (there is a bit more context in Stachel's review), but I don't see anything to indicate that Einstein was talking about the psychological flow of time at all. It looks to me like he was making a direct reference to the fact that "time" as a dimension of the 4-D manifold is different, physically, from space.

By the way, Stachel was in agreement with Godel that there is no global time in Godel's universe model.

Yes, in the sense that Godel's spacetime can't be sliced up into a "stack" of spacelike hypersurfaces of "constant time" that each worldline crosses only once.

Stachel thought that was irrelevant, because the flow of time is local--it is associated with local world lines.

Basically, yes. I think he was also referring to causality and the limitation of signal speed to the speed of light. See below.

One should not lose track of the requirement that material objects must extend into the 4th dimension if different observers simultaneously exist in different cross-sections of 4-D, each including different instantaneous 3-D volumes of the same 4-D object.

This is how things are usually described, but strictly speaking, it's not true. These "instantaneous 3-D volumes" are not direct physical observables; nobody actually observes a full 3-D volume at an "instant of time", because of the finite speed of light. The instantaneous 3-D volumes are mathematical constructs, and as useful as they are as part of a model, one needs to be careful about what they might or might not imply about the nature of "reality".

What we actually observe is what is in our past light cone, and in so far as objects we interact with must "extend into the 4th dimension", all we can directly observe is that they extend into our past light cone. As we look further into the past, we can consider intersections of the interior of our past light cone with surfaces of "constant time" as being snapshots of a portion of "space" at that time in the past, but it's still only a portion, and the "space" is still a mathematical construct. The same goes for the different "spaces" that are formed by slicing through our past light cone at different "angles", as would be done by someone just passing by us now at some appreciable relative velocity.

This sort of thing is what I think Stachel was getting at when he talked about "time" being local. Once you have the mental model of spacetime as a "stack" of 3-D spaces formed into a single 4-D manifold, it's natural to ask questions like "what is happening in the Andromeda galaxy *right now*"? But these questions have no well-defined physical meaning. Once you realize that different states of motion correspond, in the model, to "slicing" the 4-D manifold into a stack of 3-D spaces at different angles, it's natural to ask further questions like "since what time 'now' is in the Andromeda galaxy can change depending on how I'm moving, doesn't that mean that the whole, entire worldline of the Andromeda galaxy must already 'exist', so the Andromeda galaxy really extends through the 4th dimension as well as the other three"? But those questions don't have any well-defined physical meaning either. As I said in an early post in this thread, I view the "block universe" as a *model*, not as a claim about "reality". As a model, it clearly distinguishes between X4 and the other three dimensions.

 

[bobc2]

I agree we can't be sure without more context (there is a bit more context in Stachel's review), but I don't see anything to indicate that Einstein was talking about the psychological flow of time at all. It looks to me like he was making a direct reference to the fact that "time" as a dimension of the 4-D manifold is different, physically, from space.

You are probably right about that.

Yes, in the sense that Godel's spacetime can't be sliced up into a "stack" of spacelike hypersurfaces of "constant time" that each worldline crosses only once.

And that was the fundamental basis for Godel’s thesis. His string of logic supposedly deduces that if you don’t have global time, then you don’t have time at all, that is as a global 4th dimension. Any notion of a 4th dimension (either time or spatial) must exist in that form over the entire 4-dimensional space. Yes, you can have a local time concept having to do with consciousness associated with the local world line. But, that is not a description of time as a 4th dimension. That is a description of consciousness coupled to a 4-dimensional object that extends into a spatial 4th dimension.

I don’t think Stachel thought carefully through Godel’s position.

Basically, yes. I think he was also referring to causality and the limitation of signal speed to the speed of light. See below.

This is how things are usually described, but strictly speaking, it's not true.

I wouldn’t say it is not true. It could be true, because it is consistent with the special theory of relativity. We already have our 3-D prototype for space, and we can easily comprehend an extension of that concept to include the 4th dimension along with the general idea of a close link between time and consciousness as the consciousness couples to the 4th dimension (either moving along world lines at light speed or else coupling simultaneously all along the entire world line).

Of course we must acknowledge that it is possible that it is not true, i.e., the assertion that objects exist as 4-dimensional. I have no problem with that.

It’s just that a spatial 4th dimension seems more natural than a time 4th dimension. For the realists, the thrust of physics is comprehending what is “out there” as something physically real, existing as a space populated by “real” objects. It is not necessary to observe those objects to know they are there if measurements along with supporting analysis sufficiently imply an existence. And that extence concept must at the very least be compatible with special and general relativity.

This is in contrast to the idealistic notion of “things” existing in the mind (a concept dangerously close to solipsism). My graduate philosophy of physics professor told our class that the class room next door did not exist until he entered that room and observed it (nothing exists unless it is being observed). He had PhDs in physics and philosophy and seemed to be a disgruntled ex-physicist. He was always in attendance at our physics department Friday colloquium.

These "instantaneous 3-D volumes" are not direct physical observables; nobody actually observes a full 3-D volume at an "instant of time", because of the finite speed of light.

This has never been an absolute criterion for implying the existence of real objects. We have a scientific method capable of inferring objects that are not directly observable. If I began enumerating those objects it would make for a very long list. And physicists routinely take into account the speed of light in constructing configurations of objects throughout space. A physicist friend at The University of Texas who works on the RHIC Star project at Brookhaven told me that they had data that showed that the gold nucleons in their collision were flattened into a near-disk just before the instant of collision with another gold nucleus. (did the disk of gold nuclei really exist in the reference frame of the lab?) It reminded me of George Gamow's limmerick: There once was a man named Fisk; Whose fencing was exceedingly brisk; So fast was his action; The Fitzgerald Contraction; Reduced his rapier to a disk.

The instantaneous 3-D volumes are mathematical constructs, and as useful as they are as part of a model, one needs to be careful about what they might or might not imply about the nature of "reality".

Of course we must always be careful when describing a key component of external objective reality. It would appear that much more care would be warranted when considering notions of time as a 4th dimension.

What we actually observe is what is in our past light cone, and in so far as objects we interact with must "extend into the 4th dimension", all we can directly observe is that they extend into our past light cone.

I don’t think physicists depend on that as a criterion for the existence of real objects in the world “out there.” That would mean that as I sit here, at a given instant, only objects that existed in my past really existed (but right now they don’t exist), because I get the evidence belatedly. This implies that objects do not exist in my present volume of simultaneity, because a finite amount of time will elapse before the light arrives. The sun does not exist right now because it will be another 8.3 minutes before the light arrives.

So it seems that you reject the concept of the simultaneous volumes (“planes”) of simultaneity described by special relativity. That would seem to imply that you would reject the concept of time dilation and length contraction. Particularly length contraction, since both ends of the beam in the simultaneous space of special relativity could not be observed simultaneously.

When starting this thread, I probably should have stipulated that we are only interested in discussing the relative merits of a 4th dimension having the essence of space as compared to the 4th dimension having the essence of time--and discussing this with the stipulation that, either way, we base our discussion on the assumption that we have accepted the concept of a real objective external world. That means we would agree that every observer experiences existence in an extended real external world of 3 dimensions.

As we look further into the past, we can consider intersections of the interior of our past light cone with surfaces of "constant time" as being snapshots of a portion of "space" at that time in the past, but it's still only a portion, and the "space" is still a mathematical construct. The same goes for the different "spaces" that are formed by slicing through our past light cone at different "angles", as would be done by someone just passing by us now at some appreciable relative velocity.

I don’t consider that a valid criterion for the existence of objects, whether those objects exist in angled slices of space-time or not.

This sort of thing is what I think Stachel was getting at when he talked about "time" being local. Once you have the mental model of spacetime as a "stack" of 3-D spaces formed into a single 4-D manifold, it's natural to ask questions like "what is happening in the Andromeda galaxy *right now*"? But these questions have no well-defined physical meaning. Once you realize that different states of motion correspond, in the model, to "slicing" the 4-D manifold into a stack of 3-D spaces at different angles, it's natural to ask further questions like "since what time 'now' is in the Andromeda galaxy can change depending on how I'm moving, doesn't that mean that the whole, entire worldline of the Andromeda galaxy must already 'exist', so the Andromeda galaxy really extends through the 4th dimension as well as the other three"? But those questions don't have any well-defined physical meaning either.

Within the context of special relativity, and with the criterion that all observers occupy a 3-dimensional space as described by the theory of special relativity, those questions have very well defined meaning. I think the ideas you’ve drawn on come out of the legacy of the old Vienna Circle of logical positivists, the philosophy of which seems to have been largely discredited—even by current philosophers. The logical positivists denied reality for anything not directly observed. They denied metaphysical concepts, generally.

The point I’ve been trying to make is that, given that every observer occupies an external 3-dimensional world, and given the theory of relativity, then a spatial 4th dimension is implied.

I agree with you that if you cannot assume a real external world for a given observer, then the kind of analysis we've been doing here is not meaningful.

As I said in an early post in this thread, I view the "block universe" as a *model*, not as a claim about "reality". As a model, it clearly distinguishes between X4 and the other three dimensions.

Yes. But, it is a model that is not prominent at all as compared to the model with time as the 4th dimension. My point has been that the 4th spatial dimension concept is far more natural and consistent with special and general relativity.

 

[PeterDonis]

PeterDonis, You never did offer an explanation as to how you could have simultaneous different cross-section views (by two different observers) of the same object unless that object was actually a 4-dimensional object.

I did respond indirectly to this when I talked about the finite speed of light. Neither blue nor brown directly observe red at R1 or R2; they *reconstruct* that those were the respective positions of red in their frames simultaneous with the event of their crossing, based on light signals they receive later on. Normally this reconstruction is glossed over in SR because there's no need to go into it when solving practical problems; but if we're going to talk about fundamentals then the fact that the reconstruction is necessary is crucial, because it reveals an extra assumption that must hold for the reconstruction to happen as you illustrate: nothing must happen to red between R1 and R2 that alters his trajectory.

For example, suppose red fires a rocket between R1 and R2. Blue has been keeping track of red's motion and, when blue and brown pass, blue sends brown a message saying that red is at R1 simultaneous with their crossing, in blue's frame. Brown knows his velocity relative to blue, so he can calculate that red is at R2 simultaneous with the crossing in brown's frame. But when brown receives light signals that red emits after R1, he will see that his calculation was wrong: red's worldline never actually goes through the event R2, and if brown reconstructs his surface of simultaneity at the event where he passed blue, he will find, on the basis of later light signals, that red actually crossed that surface at some other event, R3.

The point of all this is that the view that red's worldline between R1 and R2 must "already exist" when blue and brown cross is a *model*, not reality; and the model may need to be updated as more data comes in. In theoretical problems, when we're just trying to study the logical implications of a model, we can stipulate that things like red firing his rocket don't happen; but in the real world, we can't do that. We have to recognize that there is always more data that hasn't come in yet about what is happening "now" outside our past light cone; and until we receive that data, we can't treat those events as having "happened".

 

[PeterDonis]

His string of logic supposedly deduces that if you don’t have global time, then you don’t have time at all, that is as a global 4th dimension. Any notion of a 4th dimension (either time or spatial) must exist in that form over the entire 4-dimensional space.

Not true. The concept of timelike vs. spacelike (vs. null) worldlines is local. Godel's universe still has local timelike worldlines and local light cones and local causality; it just so happens that in this universe the topology of the "time" dimension is closed, like a circle, instead of open, like the real line.

Yes, you can have a local time concept having to do with consciousness associated with the local world line. But, that is not a description of time as a 4th dimension. That is a description of consciousness coupled to a 4-dimensional object that extends into a spatial 4th dimension.

The local concept of timelike vs. spacelike vs. null worldlines does not require a concept of consciousness.

I don’t think Stachel thought carefully through Godel’s position.

I think he understood it quite well; he just also understood its limitations.

It’s just that a spatial 4th dimension seems more natural than a time 4th dimension.

Again, you keep on using the word "spatial" but you have never justified it, at least not as you are using it here. Nobody is disagreeing that we can treat the universe as a 4-dimensional manifold, and nobody is disagreeing that, in every respect except the non-positive-definite metric, this manifold has the properties of a "space". But you are trying to go further and claim that the X4 dimension is *no* different from the others, even with the metric taken into account; and that is simply not justified. Unless you can justify it, you should not be using "spatial" in this sense.

For the realists, the thrust of physics is comprehending what is “out there” as something physically real, existing as a space populated by “real” objects. It is not necessary to observe those objects to know they are there if measurements along with supporting analysis sufficiently imply an existence. And that extence concept must at the very least be compatible with special and general relativity.

If you are just talking about a theoretical model, sure, this is fine. But if you are trying to talk about the actual universe, then no, you cannot make this claim as you are making it. See my previous post about red and blue and brown; until blue and brown receive red's light signals from R2, they can't know whether red's worldline actually passes through R2. They can't even know for sure that red still *exists* at R2--his rocket may have exploded, or he may have fallen into a black hole. You can't just ignore these possibilities if you are talking about fundamentals.

This is in contrast to the idealistic notion of “things” existing in the mind (a concept dangerously close to solipsism). My graduate philosophy of physics professor told our class that the class room next door did not exist until he entered that room and observed it (nothing exists unless it is being observed).

I am not making any such claims. I am merely pointing out the limitations of what any given observer can know at a given point on his worldline. I am not contesting the fact that other objects exist and that we can know about them by observing them, and that we can *assume*, for practical purposes, that they exist even when we don't observe them; but such assumptions are always subject to falsification if new data comes in (for example, if blue and brown see light signals from the explosion of red's rocket before he reached R2).

I don’t think physicists depend on that as a criterion for the existence of real objects in the world “out there.” That would mean that as I sit here, at a given instant, only objects that existed in my past really existed (but right now they don’t exist), because I get the evidence belatedly. This implies that objects do not exist in my present volume of simultaneity, because a finite amount of time will elapse before the light arrives. The sun does not exist right now because it will be another 8.3 minutes before the light arrives.

Once again, I am not making any such claims. I am merely claming that, until we get light rays from the sun 8.3 minutes from "now", we do not *know* for certain what the sun's state was "now". The sun could have exploded just 10 seconds ago as I sit here "now" writing this; we won't find out about it for another 8 minutes, so we are going on the best evidence we have right now if we say the sun exists "now", but that doesn't change the fact that we would be wrong if the sun had exploded 10 seconds ago. I am not trying to say anything mysterious; I am merely pointing out that, when talking about fundamentals, we must be careful to recognize the limitations of what we can know.

So it seems that you reject the concept of the simultaneous volumes (“planes”) of simultaneity described by special relativity.

When have I said any such thing? I am only saying that those planes of simultaneity are *constructs* in a model, and the model only gets validated (or changed) progressively as we receive light signals from new regions of spacetime that were previously outside our past light cone.

That would seem to imply that you would reject the concept of time dilation and length contraction. Particularly length contraction, since both ends of the beam in the simultaneous space of special relativity could not be observed simultaneously.

Observing time dilation and length contraction can be done purely by means of light signals; there is no requirement to observe things instantaneously. Read Einstein's original thought experiment about this; he carefully states everything in terms of light signals.

When starting this thread, I probably should have stipulated that we are only interested in discussing the relative merits of a 4th dimension having the essence of space as compared to the 4th dimension having the essence of time--and discussing this with the stipulation that, either way, we base our discussion on the assumption that we have accepted the concept of a real objective external world. That means we would agree that every observer experiences existence in an extended real external world of 3 dimensions.

I am not questioning that a real external world exists. See above.

I don’t consider that a valid criterion for the existence of objects, whether those objects exist in angled slices of space-time or not.

It is a valid criterion for asserting *knowledge* of whether something exists; otherwise your model is asserting things which might later be falsified.

Within the context of special relativity, and with the criterion that all observers occupy a 3-dimensional space as described by the theory of special relativity, those questions have very well defined meaning.

Good, then define it. As soon as you do, I will be able to propose a different definition, and there will be no experiment we can possibly run that says either one of us is right or wrong. Of course we can arbitrarily adopt any simultaneity convention we want to; but the convention will be just that, arbitrary. It will have no physical meaning, in the sense that no experimental result can possibly be affected by choosing one convention over another.

I think the ideas you’ve drawn on come out of the legacy of the old Vienna Circle of logical positivists, the philosophy of which seems to have been largely discredited—even by current philosophers. The logical positivists denied reality for anything not directly observed. They denied metaphysical concepts, generally.

I may sound like a Vienna Circle positivist to you, but that's because you're not reading carefully enough. I am not saying that it is never justified to talk about things that can't be directly observed. Blue and brown can certainly talk about what red is doing between R1 and R2, even though they haven't seen any light signals from that portion of red's worldline yet. Who's to stop them? But until they *do* receive those light signals, they don't *know* for certain whether what they were saying was right or wrong.

The point I’ve been trying to make is that, given that every observer occupies an external 3-dimensional world, and given the theory of relativity, then a spatial 4th dimension is implied.

4th dimension, yes. "Spatial", no; see above.

 

[bobc2]

It seems that there is this concept that one cannot know that he is existing in a volume (plane) of simultaneity, because the only things experienced (observed) directly in the mind are thoughts based on information arriving from the past. There is no external existence within ones instantaneous Lorentz space that can be directly observed until sometime later—then it is too late—it is already in your past by then. This idea is certainly compatible with the Vienna logical positivist school and gave strength to the idealists like Berkeley who maintained that reality existed within the mind (ultimately, the mind of God). The idea is favored by solipsists as well (Einstein warned about falling into the trap from which there would be “…no escape from solipsism”).

Here, we will appeal to Hawking’s more flexible approach he refers to as “model-dependent realism: the idea that a physical theory or world picture is a model (generally of a mathematical nature) and a set of rules that connect the elements of the model to observations. This provides a framework with which to interpret modern science.” This is from “The Grand Design” by Stephen Hawking and Leonard Mlodinow. Actually, PeterDonis, I believe the ideas you are advancing here are generally much more in line with Hawking. So, I should not be accusing you of advocating logical positivism or solipsism. (Perhaps I owe you and apology on that count.)

So, here is a space-time diagram with a sequence of events. The diagram represents a sort of sequence of thought experiments in which observers in a Lorentz space send messages back and forth as they move through 4-dimensional space. At the end of the sequence of experiments, they get back together and compare notes to see if there is a basis for knowing that each other existed in any of the Lorentz space simultaneous external world volumes.

In the sketch below the brown, light brown, and blue observers are together. They are initially together in the brown rest frame, synchronizing space-time markers, displays of actual distance traveled in 4-dimensional space, referenced from a point assigned as zero distance. The distances displayed (and photographed at selected points) will correspond to distances traveled along the respective world lines.

All three are together at event 1 (events in the space-time diagram are brown, light brown, or blue circles) where they synchronize their distance markers). It is planned that the light brown guy and the blue guy will move to a new position that puts them in brown’s instantaneous plane (3-D volume) of simultaneity at event 8. Light brown and blue have used Lorentz transformations to assure that their distance markers display the same values at event 8 as the brown guy’s markers display at event 2. Light brown and blue both transmit pictures of their displayed values so that brown can validate their numbers when he (brown) arrives at event 3 in the space-time diagram (brown calculates how far he has traveled along the 4th dimension since leaving event 2.

At event 9 the light brown guy transmits a photo of his distance display, which is received by blue at event 14 and received by brown at event 4. The blue and brown guys do calculations that demonstrate that events are still occurring in agreement with theoretical physics. At this point the brown guy is able to confirm that the light brown guy existed in his plane of simultaneity back when he (the brown guy) was at event 3). From the data received from the blue guy at event 4 he is also able to determine that the blue guy was also in his (brown’s) plane of simultaneity back at event 3.

Just one experiment doesn’t seem enough, so they continue acquiring data.
The blue guy arrives at the brown guy’s position at event 5. So, here the blue guy and the brown guy simultaneously occupy the same position at the intersection of their X4 axes. Special relativity tells them that if the light brown guy really still exists, then the light brown guy must exist at event 11 in blue’s instantaneous 3-D space volume, while simultaneously existing at event 12 in brown’s simultaneous space. However, as PeterDonis points out, they can’t really be sure, because they have no way of getting information from those events instantly while they are at event 5. They must wait until later for confirmation from the light brown guy.

Brown gets his confirmation when he arrives at event 6, receiving the picture of light brown’s event 12 photo of his distance traveled along the 4th dimension, which is exactly the same distance that the brown guy recorded for his own trip when at event 5. Thus, brown concludes that the light brown guy must have been in his simultaneous space which included both event 5 and event 12 simultaneously.

The blue guy has to wait until his event 16 for confirmation that light brown was at event 12 simultaneously with event 5 (when both blue and brown guys were simultaneously at event 5). Of course the blue guy saved a copy of brown’s distance position along brown’s X4 dimension when they were together at event 5. So, now he had confirmation that light brown was in brown’s simultaneous space, i.e., both brown and light brown were in the simultaneous space of events 5 and 12.

But, now, blue asks whether the light brown guy was in his (blue’s) simultaneous space when blue was at event 5. Fortunately, light brown included a photo of his X4 position corresponding to event 11. Light brown and blue both used Lorentz transformations to figure out what each other’s positions should be along their respective X4 axes when blue arrived at brown’s position, event 5. Light brown transmitted his computations that he had made about what blue’s X4 position should be when light brown was at event 11. And blue computed the X4 reading that light brown should have when he (blue) was at event 5.

Light brown and blue wanted to be sure science was working right, so they took photos of their respective X4 distances corresponding to blue’s simultaneous space at blue’s event 16. Light brown did calculations (by prearranged agreements) at event 13. At event 17 blue found that light brown was in his (blue’s) simultaneous space when blue was at event 16 and light brown was at event 13.

Without including it in the space-time diagram we have all three observers get together at the end of the experiments and review all of their data. They conclude that sure enough, when the brown guy and blue guy were at event 5, the light brown guy simultaneously existed at event 12 (in brown’s simultaneous space) and event 11 (in blue’s simultaneous space). They then conclude that the light brown guy is actually a 4-dimensional object, and-- by extension--they all are. Thus, we have a model in which objects are 4-dimensional extending into a 4th spatial dimension.

Further, once it is recognized that we have a 4-dimensional spatial universe populated by 4-dimensional objects, it is obvious that the objects do not move. There is a perception of motion related to 3-dimensional cross-sections of the 4-dimensional objects, and this is compatible with the notion of consciousness moving along the 4-dimensional world line of an observer, the consciousness having a deep connection to the perception of the "flow of time." Finally, it implies two possible models of consciousness: 1) A 3-D consciousness that moves at light speed along the observer's world line, or 2) A 4-D consciousness that is coupled to the 4-dimensional material structure of the observer over the full extent of the observer world line.

These experiments (albeit thought experiments) would satisfy the criteria for a good model fit, at least in agreement with Hawking’s criteria:

1. Is elegant
2. Contains few arbitrary or adjustable elements
3. Agrees with and explains all existing observations
4. Makes detailed predictions about future observations that can disprove or
falsify the model if they are not borne out.

To satisfy number 4 in a more convincing manner, The three observers could have sat down together and planned the entire set of experiments in advance, predicting all of the readings that would be observed at each of the events in the scenario.

 

[PeterDonis]

Actually, PeterDonis, I believe the ideas you are advancing here are generally much more in line with Hawking.

In general, yes, what I've been saying is consistent with Hawking's model-dependent realism. As I've said before, I'm certainly not trying to claim that the only things that "exist" are those things we directly observe.

I have no problem in general with your scenario; as you've specified it, all three observers could, as you say, plan the whole sequence of experiments in advance, and (provided nothing external interfered) collect the data to confirm that their model was correct.

The only thing I would add is to take note of that parenthetical statement I put in about something external interfering. In the real world, there is always the possibility of something external interfering, and when that happens, we can't know about it until light signals from the external event reach us. For example, suppose that, when brown, light brown, and blue actually run the experiments as you specify, something unexpected happens: a powerful laser pulse comes in from the negative x direction and hits light brown at event 12. The impulse of the laser pushes light brown into a new trajectory that is moving to the right (i.e., in the positive x direction) with the same velocity as blue, relative to brown.

Light brown knows immediately at event 12 that his trajectory from event 12 on will not match what they all predicted before the experiments were begun. But brown will not know this, and will not be able to base any predictions on it, until event 6, when he sees the light from event 12 and realizes he was hit by the laser pulse. And blue will not know it until event 16, when the light from event 12 reaches him (and of course brown can't send him a signal any faster).

Now, how will brown and blue reconstruct the effect of the laser pulse? Brown, reconstructing events after he sees the light pulse from event 12, will say that light brown was hit by the laser simultaneously with event 5, i.e., at the same time as brown and blue were passing each other. But to blue, event 5 happens *before* event 12, so even though blue and brown are co-located at event 5, they will not agree on whether or not light brown had been hit by the laser pulse "at the same time" as they were passing each other. This is why statements like "event A and event B happened at the same time" can't have any physical meaning unless they are qualified (by referring them to a specific observer's frame); two observers can disagree on them without having any way of settling their disagreement by making any further observations.

 

[bobc2]

...Now, how will brown and blue reconstruct the effect of the laser pulse? Brown, reconstructing events after he sees the light pulse from event 12, will say that light brown was hit by the laser simultaneously with event 5, i.e., at the same time as brown and blue were passing each other. But to blue, event 5 happens *before* event 12, so even though blue and brown are co-located at event 5, they will not agree on whether or not light brown had been hit by the laser pulse "at the same time" as they were passing each other. This is why statements like "event A and event B happened at the same time" can't have any physical meaning unless they are qualified (by referring them to a specific observer's frame); two observers can disagree on them without having any way of settling their disagreement by making any further observations.

Exactly. And that is the whole point behind the understanding the 4-dimensional universe. After the laser intervenes, Blue and Brown guys will both recognize (later -- after the fact) that the laser impulse at event 12 could not possibly occur in each of their planes of simultaneity corresponding to those planes in which their X4 axes intersect. They would do the special relativity calculations (later, after the fact) and, in retrospect, know that blue did not yet have a laser event in his 3-dimensional world.

One has to be careful with the language. I agree that when we say that the two observers, blue and brown, are together at "the same time," different people could come away with different interpretations about the meaning of that phrase. In this instance it certainly does not mean that the blue guy and red guy would each be reading the same proper time on their world line clocks--we can't use the phrase "same time" in that sense (just as we have different "age times" in the reunion of the twin paradox).

But, there is no confusion in understanding that there are two different events, 11 and 12, and that as blue and brown exist together (next to each other), event 11 is in brown's world but it is not in blue's world; and event 12 is in blue's world but it is not in brown's world. And it would appear that we can only have such a circumstance if the light brown object is 4-dimensional (allowing different 3-D cross-section views) and extends into the 4th dimension.

 

[PeterDonis]

But, there is no confusion in understanding that there are two different events, 11 and 12, and that as blue and brown exist together (next to each other), event 11 is in brown's world but it is not in blue's world; and event 12 is in blue's world but it is not in brown's world.

I do not agree with this way of stating it. I would state it as follows: until blue or brown receive light signals from events 11 or 12, they don't know what happened at those events, so they can't say anything about them as *actual events*. They can *predict* what they think will happen at those events, based on what they know prior to receiving light signals from them; but those predictions are only tentative, and do not require any belief that the events are "real" or "actually happened" prior to receiving light signals from them.

Blue and brown can build *models* of their "worlds", and in those *models*, yes, event 11 is "in blue's world at event 5" but event 12 is not, while event 12 is "in brown's world at event 5" but event 11 is not. But those models are just that, models; the "worlds" in them do not have to be "real worlds" in order to serve their function as models. There are certainly "real events" 11 and 12, but blue and brown, at event 5, *don't know* what those actual, real events are; they only know what their models say about them.

It may well seem that I'm belaboring this, but I'm only doing so to make what I think is a fundamental point about what the "block universe" model does or does not claim. As I understand it, the "block universe" model does *not* claim that, for example, events 11 or 12 have to be "real" or have "actually happened" from blue or brown's point of view at event 5. All that the "block universe" model requires is that blue and brown have a *model* of events other than those along their own worldlines, such as events 11 and 12; in other words, blue and brown have a *model* of light brown traveling along his worldline, and that model will recognize that different events on light brown's worldline (11 and 12) are seen as "simultaneous" to blue and brown at event 5. But the model must also recognize that no physical experience blue or brown can have at event 5 can possibly be affected by what happens at events 11 or 12, so there is no need or reason for blue or brown to claim that events 11 or 12 have "already happened" or "are happening *now*" at event 5.

Similar remarks apply to what blue or brown might say after receiving light signals from event 12 showing the laser pulse hitting light brown and changing his motion. Blue or brown can certainly reconstruct events and show that event 12 was simultaneous with event 5 in brown's frame, but happened *after* event 5 in blue's frame. But what, if anything, should they conclude from this? My answer is, nothing of any consequence. The fact still remains that no physical experience blue or brown could have had at event 5 could possibly have been affected by what happened at event 12.

In other words, these 3-D "worlds" you are talking about, as tempting as they are conceptually to our intuitions, are actually not the right things to concentrate on, even in the "block universe" model. The crucial "boundaries" in spacetime are not surfaces of simultaneity, but light cones; and the crucial relationships between events are not simultaneity relationships but causal relationships--timelike, spacelike, or null separation. Events 11 and 12 are both spacelike separated from event 5, and *that* is the crucial fact about them. The "block universe" model is not supposed to change that.

 

[bobc2]

...Blue and brown can build *models* of their "worlds", and in those *models*, yes, event 11 is "in blue's world at event 5" but event 12 is not, while event 12 is "in brown's world at event 5" but event 11 is not. But those models are just that, models; the "worlds" in them do not have to be "real worlds" in order to serve their function as models. There are certainly "real events" 11 and 12, but blue and brown, at event 5, *don't know* what those actual, real events are; they only know what their models say about them.

But, the whole point of the three observers planning out the experiments, with predictions about the outcomes, was to show that the light brown guy actually existed, in a real physical sense, at events 11 and 12, in the simultaneous planes of blue and brown while blue and brown were next to each other. The implication of the light brown guy existing as a 4-dimensional object is profound and cannot be dismissed as a mere mathematical example--dismissed as a model without relevance to a real world.

When blue and brown are next to each other, either the light brown guy exists as a real world object or he doesn't. Then, if he does exist in the real external world of blue and brown when they are next to each other, one must reconcile the data that shows light brown existing in the real world of blue as well as the real world of brown. All of the data they analyzed after the fact leads to the conclusion that the light brown guy existed at events 11 and 12, even though it was impossible for blue and brown to confirm that at the time when blue and brown were next to each other and in their respective planes of simultaneity.

It may well seem that I'm belaboring this, but I'm only doing so to make what I think is a fundamental point about what the "block universe" model does or does not claim. As I understand it, the "block universe" model does *not* claim that, for example, events 11 or 12 have to be "real" or have "actually happened" from blue or brown's point of view at event 5. All that the "block universe" model requires is that blue and brown have a *model* of events other than those along their own worldlines, such as events 11 and 12; in other words, blue and brown have a *model* of light brown traveling along his worldline, and that model will recognize that different events on light brown's worldline (11 and 12) are seen as "simultaneous" to blue and brown at event 5. But the model must also recognize that no physical experience blue or brown can have at event 5 can possibly be affected by what happens at events 11 or 12, so there is no need or reason for blue or brown to claim that events 11 or 12 have "already happened" or "are happening *now*" at event 5.

Given the difficulty of the concepts that we have been wrestling with, I don't think you are belaboring this at all. We have clearly been representing two different assessments of the significance of the scenario outlined with my sketch above. I believe the issue of the existence of physical objects in a real external world is crucial to the foundations of physics. And if those objects exist as 4-dimensional objects occupying 4-dimensional space, this is indeed profound and is certainly worth discussion.

 

[Dale]

bobc2, I have noticed references by you to the idea that it is possible to nave a non-positive definite metric in four spatial dimensions. Do you have a mainstream scientific reference for that.

I always thought that a non-positive definite metric implied at least one spatial dimension (positive signature) and one temporal dimension (negative signature). I don't see how you can possibly get a non-positive definite metric if they are all spatial and therefore all have a positive signature.

 

[PeterDonis]

When blue and brown are next to each other, either the light brown guy exists as a real world object or he doesn't.

See, here is where you appear to me to be going beyond what the model justifies. You use the word "when". This implies a simulaneity convention. Which one, blue's or brown's? In other words, in the sentence just quoted above, are you talking about light brown at event 11, or light brown at event 12? Or both? Or neither? Or the entire length of light brown's worldline from the past light cone of event 5 (where blue and brown cross) onward? As you appear to be viewing things, the statement quoted above has no well-defined meaning until you have specified exactly one of the above possibilities.

Whereas my answer is, it doesn't matter. The latest event on light brown's worldline that blue and brown can have direct evidence of at event 5 is the event where light brown passes through event 5's past light cone. *All* of light brown's worldline beyond that is unknown to blue and brown at event 5. That's the physical fact. That does not mean that portion of light brown's worldline does not "exist"; it simply means that blue and brown, at event 5, don't *know* what that portion of light brown's worldline consists of. That means that there are things that our normal, conventional ways of speaking would lead blue and brown to say, at event 5, that from the standpoint of physics, simply can't be said from their actual knowledge at that event.

For example, suppose the laser pulse I postulated hits light brown at event 12 and destroys him, instead of just changing his motion. Brown and blue, when they later find out about this, would have to disagree, if they follow your prescription, about whether light brown was killed "at the same time" as they passed each other. Brown would say yes, he was killed at that time; blue would say no, he wasn't killed until "later", so at that event, light brown was "still alive". But if you were blue or brown, would you care? The primary fact about event 12 as it relates to event 5, as I said before, is that the two are spacelike separated; nothing blue or brown could do at event 5 would have kept light brown from getting killed at event 12. Given that inescapable fact, if you were blue or brown, would you bother arguing about whether light brown was killed "at the same time" as you passed each other, or not?

The point of all this is that when you assert what you asserted in the quote above, you are asserting something whose truth might depend on events outside blue or brown's control at event 5. Brown, on your view, would have to say that no, light brown did *not* exist as a real-world object when brown and blue passed; he had just been killed that instant. Whereas blue, on your view, would have to disagree, and say that no, light brown *did* exist as a real-world object when brown and blue passed; he wasn't killed until "later". But on my view, if they are arguing about this at all, they are missing the whole point; they are arguing about something for which there simply is no invariant "fact of the matter"; whether light brown still "existed" at event 5 is frame-dependent.

All of the data they analyzed after the fact leads to the conclusion that the light brown guy existed at events 11 and 12, even though it was impossible for blue and brown to confirm that at the time when blue and brown were next to each other and in their respective planes of simultaneity.

Yes, when they receive light signals from the appropriate events, blue and brown can now know for sure what happened to light brown at events 11 and 12, including light brown being killed by the laser pulse at event 12. So their model of that portion of spacetime is now in full correspondence with what actually happened in that portion of spacetime. And even then, whether or not light brown "still existed" at event 5 will be frame-dependent; brown and blue will reconstruct different answers to that question, if they insist on treating their frame-dependent "viewpoints" and surfaces of simultaneity as being "real" instead of just features of the model. And they will therefore be arguing about something which, in my view, is pointless to argue about.

Given the difficulty of the concepts that we have been wrestling with, I don't think you are belaboring this at all. We have clearly been representing two different assessments of the significance of the scenario outlined with my sketch above. I believe the issue of the existence of physical objects in a real external world is crucial to the foundations of physics. And if those objects exist as 4-dimensional objects occupying 4-dimensional space, this is indeed profound and is certainly worth discussion.

And, once again, I am not saying and have never said that the real external world, and the objects in it, do not exist. I have merely been trying to draw a critical distinction between those real, actual objects, and what really, actually happens to them, and our *knowledge* of those objects and happenings when we are spacelike separated from them.

 

[bobc2]

bobc2, I have noticed references by you to the idea that it is possible to nave a non-positive definite metric in four spatial dimensions. Do you have a mainstream scientific reference for that.

I always thought that a non-positive definite metric implied at least one spatial dimension (positive signature) and one temporal dimension (negative signature). I don't see how you can possibly get a non-positive definite metric if they are all spatial and therefore all have a positive signature.

I'm afraid I cannot cite a reference that specifically discusses spatial coordinates with an indefinite metric--other than those in the context of a purely abstract mathematical application. I've used an indefinite metric quite often in mechanics applications, but, again, that is in the context of an N-dimensional abstract space--although the coordinates do relate to physical translational displacements and rotations on a real structure such as a space shuttle subsystem (a space with around 20,000 to 100,000 or more coordinates is not unusual). In fact slanted coordinates are used, having some similarity to our L4 space, but my coordinates were not resticted to the symmetric angular positions about a 45-degree line ("photon worldline"). However, a dual space is utilized (leading to a covariant-contravariant-like situation similar to coordinates associate with Lorentz boosts and the indefinite metric).

It is interesting to me that of the large number of references I've encountered on the subject of block universe, four-dimensionalism (a philosopher's favorite), presentism, eternalism, etc., it is quite rare to see an author explicitly refer to the 4th dimension as "spatial." You often find comments about 3-dimensional objects physically extending into the 4th dimension, but that 4th dimension is usually identified only as time (if identified explicitly at all).

It has always seemed elemental to me that if you have a material object extending into a 4th dimension, maintaining its material character with the extension, that 4th dimension would necessarily be spatial. That does not mean that you could not have time associated with that dimension in some manner (consciousness coupling, etc.), but how do you have a material 4-D objects existing without the 4-dimensional space?

But back to your question. I don't think the indefinite metric specifies the essence of the 4th dimension. That signature is there because it is required to give you the orientation of the X1 coordinate and X4 coordinate with the constraint that they must always be rotated symmetrically about the worldline of the photon. That orientation has nothing to do with whether X4 is time or spatial.

The other geometric aspect of that is that nature has populated 4-dimensional space with 4-dimensional objects that are billions and trillions of miles long, always oriented in a 4-dimensional direction confined to the inside of a light cone. So, it's these geometric aspects of the 4-dimensional world that account for the special nature of the 4th dimension.

So, the signature of the metric is required to account for the geometry, and our measure of 4-dimensional distance (interlinked with our physics, the conservation laws) in this unusual 4-D world.