B Unlearning the Block Universe: How Relativity Challenges Our Perception of Time

[Lynch101]

TL;DR Summary

I've learned about relativity through pop-science and so I had come to associate it with the Block Universe. Now, it seems I need to try and unlearn it, so I have lots of questions.

Just a heads up, this post is quite long. I've tried to be as detailed as possible from the outset because I find it can help avoid the need to clarify things later, or helps when clarifying things later. There is only one question posed at the end, but I think it might be useful to read the body of the post to give the full context.

Where I'm coming from

Firstly, I just want to outline where I'm coming from. The route, so to speak, of how I've come to learn (what little I know) about relativity. It's somewhat of a disclaimer, but I think it might help to give an idea of where I am at now.

My first encounter with relativity was through pop-science videos and books. That prompted me to do further reading and discussing of the subject. To cut a long story short, most of my learning was in the context of the Block Universe, so I have come to associate relativity with the Block Universe.

I believe that, although having struggled with the main concepts [of relativity of simultaneity, time dilation, and length contraction] I now have a relatively (no pun in 10 did) decent grasp on them, for a person without a background in mathematics or physics. That is, I understand them conceptually but cannot necessarily calculate solutions to mathematical problems. Where I kept tripping up and thinking there must be some contradiction, I am now able to reason through issues and understand how the different aspects intertwine to "resolve" apparent paradoxes.

Just as I thought I had a decent grasp of it, I come across this article by @PeterDonis telling me that everything I thought I knew was wrong! Well, not quite everything, thankfully, rather that the Block Universe is not a necessity of relativity. It had been presented as such in the materials I had encountered and the discussions I have had, to the extent that I had come to assimiltate this into my own understanding. Now, I must try and unlearn it, or at least see why it isn't a necessity.

I will probably be referencing statements from this thread, related to Peter's article as well as Peter's article itself. I will try to outline the understanding that I have arrived at, through my engagement with different material and discussions. This will probably involve putting forward the arguments that have been put to me, which helped me develop my understanding. This will probably appear as though I am trying to defend the Block Universe but that is not my intention. By putting forward the arguments as I understand them and having the issues pointed out, I find I can develop a deeper understanding than when I simply read something and try to assimilate it myself.

Right! That was long-winded enough. So, if you're still here...thank you...and I'll get to it.Start From Now
The best place to start is probably where I started myself, with an idea that is captured by this comment from Peter in this thread.

This also means there is no global concept of "now" in relativity.

When I approached relativity I had a more Newtonian view, so to speak. The notion I had was that of a deterministic Universe where everything existed in a single, universal present moment, a global "now". A more accurate picture would perhaps be that the configuration of all the particles in the Universe constituted this "now", even if they couldn't all be observed. This now was the only moment in which things happened, or could happen. Again, even if we couldn't say what all those events were, the Universal present ensured that there was only one moment in which they could happen, and that was the moment we experience as "now". A feature, or consequence, of this universal present moment was that past or future events, or configurations of the Universe which we consider to be the past or future, were imaginary, while only the present was real - in any sense of the word.

When I encountered relativity, I found that Einstein's theory overturned this limited view of the world. As Peter succinctly put it, there is no global concept of "now" in relativity. From what I learned, relativity necessitated that past and future events be as "real" as those of our present. I know "real" can prove to be a difficult term to pin down, but I was told that it simply meant that the present, which we experience, has the same ontological status as the past and the future.

This was explained in conjunction with the concept of world lines in spacetime, where the locus of all events that make up our history (and that of every object) extend through spacetime as world lines. This meant that, within the structure of the Universe, all the events in our history or the history of any object co-existed in a 4D Minkowski spacetime structure. In this picture our past, present, and future events are all equally "real". Contrast this with the Newtonian picture which says that only our present event is real i.e. us in the present moment.

An analogy that I often encountered was that of a reel of film, the likes that would be used in old-school projectors. If we imagine the film of our lives stretching out from our birth to our death, then this represents our world line, which would exist in spacetime. While our experience is more akin to watching the movie play out on screen - or perhaps more like the original filming of the movie - this is just an illusion. In the block structure, our world line stretches out like this, where each frame enjoys the same ontological status of every other frame. That is they are all equally "real" (no pun intended). That term "real" might be somewhat nebulous, but whatever it is, it applies equally to all the frames on the reel of film, or to all the events that make up our world line.

I struggled for a long time to get my head around many of the consequences of relativity, but over time it began to make more and more sense, including this Block Universe picture, to the point where I now struggle to see how it isn't necessitated by relativity.First Question
So, finally! To the first question.

If the relativity of simultaneity overturned the Newtonian idea of a universal, or global "now", how is it possible to have such a relativistic Universe where past and future states or configurations aren't as "real" as the present?

I would be inclined to think that, any departure from this Newtonian idea of a universal present moment, must necessitate the existence of past and future configurations of the Universe, in such a way that those configurations are equally as "real" as the configuration of the present. In terms of the analogy of the reel of film, while the content of each frame might be different, each frame is made from the same kind of "stuff".

If such past and future states don't exist, then wouldn't we be left with a global "now" or universal present, by way of necessity? A global "now" similar to (but not necessarily the same as) the universal present of Newtonian mechanics?

 

[PAllen]

Just because the block universe is certainly not a forced consequence of relativity is no reason you have to abandon it as a perfectly valid interpretation.

 

[Lynch101]

Just because the block universe is certainly not a forced consequence of relativity is no reason you have to abandon it as a perfectly valid interpretation.

I won't completely abandon it, but I am interested to see how it's not a forced consequence of relativity.

The way I have come to learn it, it has been presented as a forced consequence and unfortunately that is how I have come to view it, so that is what I need to unlearn.

 

[Nugatory]

A more accurate picture would perhaps be that the configuration of all the particles in the Universe constituted this "now", even if they couldn't all be observed.

You can choose to think about it that way, and if you want to retain the notion of "now" (a word that you're tossing around rather freely, considering that you haven't defined it) you will be driven to that conclusion.

But there is an another way of thinking about it. Given an event, you are sorting all the other events in the universe into three categories: happened after our event; happened before our event; and happened at same time as our event (that is, "now"). You could instead sort all the other events in the universe into the categories: in future light cone; in past light cone; spacelike-separated so outside of both light cones.

 

[Dale]

Summary:: I've learned about relativity through pop-science and so I had come to associate it with the Block Universe. Now, it seems I need to try and unlearn it, so I have lots of questions.

If the relativity of simultaneity overturned the Newtonian idea of a universal, or global "now", how is it possible to have such a relativistic Universe where past and future states or configurations aren't as "real" as the present?

I think that you may need to unlearn even more than you think you need to unlearn. For instance, Newtonian physics is also perfectly compatible with the block universe concept. You can have presentism in special relativity and you can have eternalism (the block universe) in Newtonian physics.

Presentism and eternalism are both purely philosophical concepts. They have no experimental consequences. From a scientific perspective they are both equally vacuous because they are both unfalsifiable.

 

[PAllen]

As a trivial example of presentism in SR, nothing in SR precludes the existence of an unknowable evolving global spacelike boundary between present and future. The only difference between SR and Newtonian physics is that in Newtonian it seems there is an obvious choice, but, in practice, you cannot, in principle, determine that such an evolution boundary exists even in Newtonian physics. Nor can you prove it doesn’t exist in relativity.

 

[PeterDonis]

how is it possible to have such a relativistic Universe where past and future states or configurations aren't as "real" as the present?

The term "real" is not a scientific term. It's a philosophical term. Philosophy is off topic here. The reason it's not a scientific term is that there is no way to tell by experiment whether or not something is "real". Suppose I tell you that whatever event in spacetime you will be at tomorrow at noon by your clock is "real" right now. How could we possibly test by experiment whether or not I am right? Sure, if you wait until tomorrow noon by your clock, you will experience some event, and it will seem real to you then; but I didn't say it would be real to you then, I said it was real now. There's no way to test that latter statement by waiting until tomorrow.

So we have to come up with a better term than "real" to even formulate a scientific question at all. The term I suggested and used in my Insights article was "fixed and certain". Using that term, your question becomes: "how is it possible to have a relativistic Universe where past and future states or configurations aren't as fixed and certain as the present?" And the answer then becomes obvious: in relativity, your past at a given event--the past light cone of that event--is fixed and certain, as fixed and certain as the present event you are experiencing right now. So your question is based on a false premise: it is not possible to have a relativistic Universe in which no event other than your present event is as fixed and certain as your present event: at a minimum, events in your past light cone are just as fixed and certain.

The question that remains, then, is whether any other events besides your present event, and its past light cone, are as fixed and certain as those events are. And as I made clear in my article, relativity by itself does not require that any other events are. It allows a "block universe" interpretation in which all events, in the entire 4-d spacetime, are fixed and certain, but it does not require one. Nor does it require an interpretation in which an entire "present", in the sense of an entire spacelike 3-surface containing your present event, is fixed and certain. All relativity requires is that your present event and its past light cone are fixed and certain.

If such past and future states don't exist, then wouldn't we be left with a global "now" or universal present, by way of necessity?

No. See above.

 

[PeroK]

Summary:: I've learned about relativity through pop-science and so I had come to associate it with the Block Universe. Now, it seems I need to try and unlearn it, so I have lots of questions.

I'd start here:

https://scholar.harvard.edu/files/david-morin/files/relativity_chap_1.pdf

 

[vanhees71]

Another good source is the electromagnetism volume of Susskind's Theoretical Minimum series. I've no clue what this "block universe" idea should be about. I always thought the damage philosophy does to physics students is over for quite some time now, because the philosophers are even more effective in confusing students of quantum (field) theory ;-)) and thus gave up about relativity (aka spacetime models) now.

 

[Demystifier]

I've no clue what this "block universe" idea should be about.

It's an interpretation of relativity theory. According to this interpretation, time and space should be treated on an equal footing (up to the opposite sign in the metric) and the 4-dimensional perspective should be taken seriously. In particular, since there is no such thing as "flow of space", there should also be no "flow of time". In other words, the past and presence are not more real than the future, just as left and here are not more real than the right.

Clearly, such a block universe interpretation contradicts our intuitive common sense experience of time. See also my https://fqxi.org/community/forum/topic/259

 

[Lynch101]

You can choose to think about it that way, and if you want to retain the notion of "now" (a word that you're tossing around rather freely, considering that you haven't defined it) you will be driven to that conclusion.

Thanks Nugatory. Unfortuately, I wouldn't be sure how to go about rigorously defining the term "now". I would need help with that. I guess I wouldn't be sure how to go about rigorously defining most terms really. I would be more able to describe what it is that I am talking about in such a way that the other person knows what I am referring to.

For example, if I were to describe something edible that grows on trees, that often comes in the colours green or red, can be used to make cider, and is, in folklore, the object that fell on Newton's head that prompted his theory of gravity. I'm sure most people would know what I was talking about. But, I wouldn't be confident that I could define the term "apple" rigorously.

But there is an another way of thinking about it. Given an event, you are sorting all the other events in the universe into three categories: happened after our event; happened before our event; and happened at same time as our event (that is, "now"). You could instead sort all the other events in the universe into the categories: in future light cone; in past light cone; spacelike-separated so outside of both light cones.

Yes, this is what I need to try and get my head around. I guess I'm starting from a picture of the Block Universe and am trying to deconstruct it somewhat.

I am familiar with the concept of future and past light cones, but it was explained to me in the context of the Block Universe, so I'm not yet sure how to interpret it in such a way that is distinguished from the Block Universe.

I usually find that discussing these things back and forth something gets said along the way that helps me to interpret things differently.

New way of looking at it
For example, you mention that we can sort all events into 3 categories, happened after our event; happened before our event; and happened at same time as our event , which in this case is what we refer to as "now".

If we take "our event" as being our 30th birthday, then we say that there are events which happen at the same time as this, and this is what we call now. We can then categorize certain events happening before this event, such as our 10th birthday, as "the past" and events happening after this, such as our 50th birthday , as "the future".

In the Newtonian picture, if we were to pause the universe and every event in it right at the moment of our 30th birthday (any moment in that event), we would have a snapshot of the Universe and everything in it. The structure of the Universe would only include those events which happen at the same time as our 30th birthday. This would represent the idea of a global "now".

Relativity tells us, however, that there is no such global "now". The relativity of simultaneity necessitates that there be no global "now".

If we depart from this idea of there being no global "now" then it means that this snapshot of the Universe on our 30th birthday does not represent the overall structure of the Universe. It would seem to necessitate that either past and/or future events also make up the structure of the Universe. This is essentially the picture that the block Universe paints. Past events, such as our 10th birthday - or more precisely, the snapshot of the Universe on our tenth Birhtday, with all the particles in the positions they were in - form part of the overall structure of the Universe, together with the present and future snapshots - whatever those snapshots happen to be, even if we cannot define them.

If past and future events do not form part of the structure of the Universe, the are we not left with a global "present"?

 

[vanhees71]

Well, sure one should take the 4D spacetime description seriously, but the signature of the fundamental form (1,3) also allows for a "causal structure", and we always use the "causal arrow of time" to, e.g., in classical field theories to choose the retarded solutions in favor of any other possible solution, because it describes the phenomena. So in addition to what seems to be called "block universe" we always also conjecture this "causal arrow of time".

As I said before, I thought this (in my opinion non-sensical) debate about the physical description of time, causality and all that vs. the subjective apprehension of time is well decided in favor of the physical notion of time.

Nevertheless recently I've seen some semi-physical philosophical papers about the old infamous "hole argument", which is somewhat related to this arguments about time, but which was based on a pre-GR spacetime model by Einstein and Grossmann and which is observationally wrong.

The resolution of this problem by GR is in my opinion that indeed all that is objectively observable are coincidences of events in spacetime.

 

[Lynch101]

I think that you may need to unlearn even more than you think you need to unlearn. For instance, Newtonian physics is also perfectly compatible with the block universe concept.

Ah yes, thank you Dale. I tend to ignore this because it seems that a block structure in Newtonian physics would have the continued existence of past events and the already in existence future events as ad hoc assumptions that have no explanatory power. When relativity and the Block Universe was being explained to me, I found it helpful when the non-block formation of Newtonian physics was put in juxtaposition with relativity.

You can have presentism in special relativity and you can have eternalism (the block universe) in Newtonian physics.

Presentism as I have understood it involves the idea of a global "now". From how it has been explained to me, that is not compatible with relativity. The quote from @PeterDonis in the OP would seem to necessitate this.

Presentism and eternalism are both purely philosophical concepts. They have no experimental consequences. From a scientific perspective they are both equally vacuous because they are both unfalsifiable.

I think a Newtonian Block Universe would have certain experimental consequences, if I remember correctly. I'd have to try and refresh my memory, but I think that might be a subject for a different thread.

 

[PeroK]

In the Newtonian picture, if we were to pause the universe and every event in it right at the moment of our 30th birthday (any moment in that event), we would have a snapshot of the Universe and everything in it. The structure of the Universe would only include those events which happen at the same time as our 30th birthday. This would represent the idea of a global "now".

Relativity tells us, however, that there is no such global "now". The relativity of simultaneity necessitates that there be no global "now".

If we depart from this idea of there being no global "now" then it means that this snapshot of the Universe on our 30th birthday does not represent the overall structure of the Universe. It would seem to necessitate that either past and/or future events also make up the structure of the Universe. This is essentially the picture that the block Universe paints. Past events, such as our 10th birthday - or more precisely, the snapshot of the Universe on our tenth Birhtday, with all the particles in the positions they were in - form part of the overall structure of the Universe, together with the present and future snapshots - whatever those snapshots happen to be, even if we cannot define them.

If past and future events do not form part of the structure of the Universe, the are we not left with a global "present"?

Relativity tells us that there is no absolute or unique global "now". In SR, every inertial reference frame has a global now, but this set of simultaneous events does not form a global now for other reference frames.

But, relativity also tells us that each of these frames is a valid description of the "structure of the universe", in terms of a subset of events. Specifically, if we use our inertial reference frame and apply the laws of physics we'll be okay. And the whole concept of simultaneity that once seemed so fundamental turns out to be not important at all.

I guess there are two things about the relativity of simultaneity: 1) understanding it; and 2) understanding that the loss of absolute simultaneity does not destroy the laws of physics.

 

[vanhees71]

The only model where a "global now" seems to make sense is of course Newtonian mechanics with its absolute time and absolute space. It's simply a continuous number of copies of Euclidean space ordered along a time axis (a socalled fiber bundle). The absolute time defines a "global now" by construction.

There's no such thing as that already in special relativity, because there's no absolute time, but I still do not get what sense this idea of a "global now" should make to begin with. All you need in physics as we know it today is a causality structure, and this is implemented in relativistic spacetime descriptions in the signature, (1,3) or equivalently (3,1), of the fundamental form (pseudo-Euclidean in SR, pseudo-Riemannian in GR).

The obvious point is that if you really want to understand physics you have to learn the only known adequate language to talk and think about it, which is math!

 

[Lynch101]

As a trivial example of presentism in SR, nothing in SR precludes the existence of an unknowable evolving global spacelike boundary between present and future. The only difference between SR and Newtonian physics is that in Newtonian it seems there is an obvious choice, but, in practice, you cannot, in principle, determine that such an evolution boundary exists even in Newtonian physics. Nor can you prove it doesn’t exist in relativity.

Ah, OK. This is certainly new to me, thank you!

Does this then mean that relativity permits a global "now", although observers will disagree about what the content of that global "now" is?

 

[vanhees71]

Well, you have to define what you mean by "global now" in a clear (i.e., mathematical!) way using the given spacetime structure of SR (Minkowski space). I have no clue, how to define such a thing, and I'm not very motivated to think about it, because I don't see what it is good for, i.e., what does it help in describing the observable facts, if it exists?

 

[PeroK]

Ah, OK. This is certainly new to me, thank you!

Does this then mean that relativity permits a global "now", although observers will disagree about what the content of that global "now" is?

One of the tragedies of this thread (and many like it on PF previously), is that the debate centres on either the twin paradox, time dilation or - in this case - the relativity of simultaneity. And philosophical ideas like the block universe enter centre stage.

Whereas, the real deal as far as SR is concerned is energy-momentum. Every hour spent thinking about the block universe (instead of learning about relativistic energy-momentum) is an hour wasted, IMHO.

 

[Dale]

I think a Newtonian Block Universe would have certain experimental consequences, if I remember correctly.

This is incorrect. There is no experimental consequence to eternalism or presentism in either relativity or Newtonian physics. If you disagree then I challenge you to find such an experiment.

Presentism as I have understood it involves the idea of a global "now". From how it has been explained to me, that is not compatible with relativity.

This is also not correct. Lorentz aether theory is an interpretation of relativity that is favored by some people precisely because it is easy to associate with a presentism-based philosophy. It is perfectly compatible with all observations and equations of relativity

 

[Lynch101]

The term "real" is not a scientific term. It's a philosophical term. Philosophy is off topic here. The reason it's not a scientific term is that there is no way to tell by experiment whether or not something is "real". Suppose I tell you that whatever event in spacetime you will be at tomorrow at noon by your clock is "real" right now. How could we possibly test by experiment whether or not I am right? Sure, if you wait until tomorrow noon by your clock, you will experience some event, and it will seem real to you then; but I didn't say it would be real to you then, I said it was real now. There's no way to test that latter statement by waiting until tomorrow.

Thanks Peter. I'm smiling here because I posed similar questions in the discussions I was having when I was first learning this. I am mindful of not getting into a philosophical discussion about what the term "real" actually means, but the answer I was given at the time is that we don't need to know what the term "real" means, we just need to know that it applies equally to past, present, and future events. Whatever it means, it is the same for all events on the world line of an object. The event on the world line of an object that corresponds to the present moment, is no different to events that we consider to be in the past or in the future. All are equally "real" - whatever that term might be defined as.

The Newtonian picture would say that only the event which corresponds to the present is "real". In this picture, past events do not continue to "exist"* and form part of the overall strcuture of the Universe. If we think in terms of the reel of film. The Newtonian picture could be likened to the movie set, where the scene is constantly changing, while the Block Universe with world lines in spacetime would be like the reel of film with all the frames laid out from start to finish. On this reel of film, no particular frame is singled out over another, they are all equal in stature and they all co-exist in the structure of the film. Even when we see the movie in the theater and only one frame is visible on the screen at a time, all the other frames are there and they are just like the frame being shown on the screen. The same wold be true for our world lines and the world lines of all objects. In this way we can avoid the need for lengthy expositions on the nature of reality, instead we can say that no single event on the world line is preferred over another.One of the reasons I struggle to see why the Block Universe is not a necessity of relativity, is because if past and future events do not exist, then I can't see how we are left with anything other than a global "now".

*exist is another one of those terms that can lead to endless philosophical debate, but as with the term "real" we don't necessarily need to define it, as long as we say it applies equally to all events.

So we have to come up with a better term than "real" to even formulate a scientific question at all. The term I suggested and used in my Insights article was "fixed and certain". Using that term, your question becomes: "how is it possible to have a relativistic Universe where past and future states or configurations aren't as fixed and certain as the present?" And the answer then becomes obvious: in relativity, your past at a given event--the past light cone of that event--is fixed and certain, as fixed and certain as the present event you are experiencing right now. So your question is based on a false premise: it is not possible to have a relativistic Universe in which no event other than your present event is as fixed and certain as your present event: at a minimum, events in your past light cone are just as fixed and certain.

The question that remains, then, is whether any other events besides your present event, and its past light cone, are as fixed and certain as those events are. And as I made clear in my article, relativity by itself does not require that any other events are. It allows a "block universe" interpretation in which all events, in the entire 4-d spacetime, are fixed and certain, but it does not require one. Nor does it require an interpretation in which an entire "present", in the sense of an entire spacelike 3-surface containing your present event, is fixed and certain. All relativity requires is that your present event and its past light cone are fixed and certain.

I'm glad you raise this. I was going to ask a question about this in the OP but I wanted to try and keep the focus narrow, and take it one step at a time, but I had a few questions on this.

In what sense do you mean events are "fixed and certain"? The Block Universe also appears to incorporate the light cone structure but this light cone structure applies to all events, past, present, and future - from Big Bang to Big Crunch, or heat death, or whatever events are in our future - so all events in the past light cone of the Big Crunch would be fixed and certain. The Block Universe has past events, or past configurations of the Universe, continuing to exist* and future configurations already in existence. All events are equally "fixed and certain".

Under your picture, do events in our past light cone still "exist"*, or perhaps does the structure of the universe (or arrangement of particles) that constituted past event(s) still form part of the structure of the Universe? If so, then would it be fair to say that your proposal is similar to the growing Block conceptualisation?

If past (and/or future) events do not co-exist with the present, in the overall structure of the Universe, then what we are left with appears to be a global "now".

That is a paraphrasing of how it was explained to me.

 

[Lynch101]

I'd start here:

https://scholar.harvard.edu/files/david-morin/files/relativity_chap_1.pdf

Thanks PeroK. That looks similar to a lot of the other stuff I've read, but I certainly like the title. I'll give it a read because I often find that when I read back over material like this, I pick up new pieces of information that I didn't fully catch the first time.

 

[Dale]

Well, you have to define what you mean by "global now" in a clear (i.e., mathematical!) way using the given spacetime structure of SR (Minkowski space). I have no clue, how to define such a thing, and I'm not very motivated to think about it, because I don't see what it is good for, i.e., what does it help in describing the observable facts, if it exists?

Exactly. Once you have defined the mathematical/theoretical "global now" then you need to find the experimental consequences. If it has no experimental consequences then it is as scientifically vacuous as the Lorentz aether. I have never seen any such proposal with any experimental consequences.

 

[Dale]

we don't need to know what the term "real" means, we just need to know that it applies equally to past, present, and future events.

With that definition then the block universe is necessary in Newtonian physics. I.e. it is not the physics that forces the acceptance of the block universe, it is this assumed property of "real"

 

[Lynch101]

Another good source is the electromagnetism volume of Susskind's Theoretical Minimum series. I've no clue what this "block universe" idea should be about. I always thought the damage philosophy does to physics students is over for quite some time now, because the philosophers are even more effective in confusing students of quantum (field) theory ;-)) and thus gave up about relativity (aka spacetime models) now.

It's the version of relativity that is probably most prominent in the pop-science genre.
This youtube video is a good example of how it is usually characterised in pop-science:
Nova (Brian Greene) - The Block Universe

 

[Lynch101]

Relativity tells us that there is no absolute or unique global "now". In SR, every inertial reference frame has a global now, but this set of simultaneous events does not form a global now for other reference frames.

But, relativity also tells us that each of these frames is a valid description of the "structure of the universe", in terms of a subset of events. Specifically, if we use our inertial reference frame and apply the laws of physics we'll be okay. And the whole concept of simultaneity that once seemed so fundamental turns out to be not important at all.

I guess there are two things about the relativity of simultaneity: 1) understanding it; and 2) understanding that the loss of absolute simultaneity does not destroy the laws of physics.

Thanks PeroK, I am familiar with the ideas you mention here. It did take me a looooong time to get my head around them, but I can understand them now. I can understand and, in some limited cases, give examples of the relativity of simultaneity and I can understand how it works to ensure that observers agree on measurements, even if their relatively moving counterpart observes their clocks to tick slower and be out of sync, and their measuring sticks to be contracted. Without being able to probe the inner workings of the mathematics, I understand it conceptually and that the loss of absolute simultaneity does not destroy the laws of physics. The issue I am having at the moment is seeing how/why relativity does not necessitate a Block Universe structure. As I learned relativity through reading and discussing, I was given the impression that relativity = the Block Universe. This is how I have come to assimilate it. Now I'm trying to understand why the relativity does not necessitate the Block Universe.When I came to relativity first, I came to it with a more Newtonian picture in my mind, of a Universe with a global "now". I was taught that relativity overturned this idea and that the relativity of simultaneity necessitated the Block Universe. So, it is from this perspective that I am trying to learn why it isn't the case that the Block Universe = relativity.

If we start with a personal example, focusing on ourselves at this moment. That would represent the global now of our inertial reference frame, but not the global now for all other relatively moving reference frames. If we link our present moment to an event, say our 30th birthday, just to make it a little less abstract. If our now was a global "now", as in the Newtonian picture, it would mean that the Universe only contained or was only comprised of those events which are happening during our 30th birthday - this would include events in distant parts of the universe, even if we did not know what they were. If there was a [Newtonian] global "now" events could only happen during our 30th birthday.

In this picture, the universe no longer comprises events which we consider to be "in the past" e.g. our 10th birthday. Also the Universe does not comprise those events which we consider to be "in the future" e.g. our 50th birthday. In the Newtonian picture, the Universe only comprises our 30th birthday and those events that are ongoing during it, including in distant parts of the Universe.

The Block Universe picture says that the Universe comprises all events that we consider to be in the past and in the future. So, in this picture, the Universe comprises our 10th Birthday, our 30th birthday, and our 50th birthday together.

If we have a Universe which doesn't comprise past, present, and future events, then it seems as though we are left with a global "now" of the Newtonian variety.

If relativity overturns the Newtonian notion of a global "now", then it would seem to necessitate a Universe comprising present and/or past and/or future events hence, the Block Universe.This is kind of how the argument was put to me and how I have come to understand relativity. Which is why I have come to associate the Block Universe as a necessary consequence of the relativity of simultaneity.

 

[Lynch101]

The only model where a "global now" seems to make sense is of course Newtonian mechanics with its absolute time and absolute space. It's simply a continuous number of copies of Euclidean space ordered along a time axis (a socalled fiber bundle). The absolute time defines a "global now" by construction.

I think this is the view that the majority of people hold before they encounter relativity. This appears to be the more intuitive understanding because we obviously don't experience the world at speeds where the effects of relativity are noticeable. Most people perceive the world through the eyes of their "now" being the global, universal "now". I think this is why relativity can prove so difficult to those who don't study it, because it goes against our intuitive understanding.

It was from the Newtonian picture that I started and I found it helpful when it was juxtaposed with relativity, to learn about the consequences of relativity.

There's no such thing as that already in special relativity, because there's no absolute time, but I still do not get what sense this idea of a "global now" should make to begin with. All you need in physics as we know it today is a causality structure, and this is implemented in relativistic spacetime descriptions in the signature, (1,3) or equivalently (3,1), of the fundamental form (pseudo-Euclidean in SR, pseudo-Riemannian in GR).

I think for those who haven't studied relativity, the "global now" is an intuitive idea. I know I had a more Newtonian view when I first encountered relativity and it took quite a bit to break through that barrier of intuition. To try and bring it back to a real world example, we can think of an event, such as our 30th birthday. The Newtonian picture would say that the Universe comprises only those events which are happening during our 30th birthday.

If we contrast this with the Block Universe picture, the Block Universe says that the Universe comprises all past, present, and future events, so instead of comprising only our 30th birthday, it also comprises our 10th and 50th birthdays also.

In my personal study and discussions, as I have tried to better understand relativity, I have been taught that the Block Universe and relativity are one and the same i.e. that relativity necessitates the Block Universe. This is an understanding I have assimilated over time. Now, I find it difficult to see how we can have the relativity of simultaneity without invoking a block universe. This is because, if the Universe doesn't comprise past and/or future events, along with present events, then we seem to be left with a Universe comprising only present events which is more in line with the Newtonian picture.

The obvious point is that if you really want to understand physics you have to learn the only known adequate language to talk and think about it, which is math!

Thanks vanhees, I have ordered a math book recently so will be cracking on with that. I know it can only deepen my understanding. I do still think it is possible understand the theory conceptually, in the absence of mathematics. The thought experiments, the likes of which Einstein used really help to make the concepts less abstract and mathematical. It allows us to put ourselves into theoretical experimental situations which are otherwise not practical and allows us to get a sense of what we might observe in those situations. It can help translate the mathematics into real world examples.

 

[Lynch101]

Well, you have to define what you mean by "global now" in a clear (i.e., mathematical!) way using the given spacetime structure of SR (Minkowski space). I have no clue, how to define such a thing, and I'm not very motivated to think about it, because I don't see what it is good for, i.e., what does it help in describing the observable facts, if it exists?

Unfortunately, I am a long way off being able to do that. I can only try to describe what it is that I am talking about and hope that those who I am discussing with get a clearer idea. For example, I wouldn't be able to define an edible fruit that grows on trees in a clear mathematical way, but I hope that by providing enough detail, such as they are usually green or red, they grow in orchards, and, in folklore, one of them falling on Newton's head lead to him developing his theory of gravity. I would be relying on the person I'm discussing with to interpret that according to their own knowledge and experience and hopefully understand what I mean. Obviously, "apple" is a little easier to understand but I think the idea of "now" is intuitive, and then it would mean describing what is meant by the term global.

 

[Lynch101]

This is incorrect. There is no experimental consequence to eternalism or presentism in either relativity or Newtonian physics. If you disagree then I challenge you to find such an experiment.

I don't want to sidetrack the discussion here, but when I was suggesting the possibility of a block universe based on the Newtonian picture - I think I was trying to cling on to my intuitive notion of time - an argument was outlined to me outlining issues with such a Newtonian block. As I say, I don't want to go off on that tangent in this thread, but it could be the subject of a separate thread.

This is also not correct. Lorentz aether theory is an interpretation of relativity that is favored by some people precisely because it is easy to associate with a presentism-based philosophy. It is perfectly compatible with all observations and equations of relativity

Yes, but Lorentz aether theory utilises a different conceptualisation of time. It retains Newtonian absolute time and the "relativity of simultaneity" is explained by changes in the internal dynamics of "local" clocks, which tell "local" time but not global time. It also requires the ad hoc assumption of an undetectable absolute rest frame, whose clock is the only clock in the whole universe which tells the "true" global time. It also rests on some mysterious dynamics to explain length contraction, which relativity, as a kinematical theory doesn't.

The key point, however, is that Lorentz aether theory does not incorporate the relativity of simultnaeity in truth, because it retains the absolute simultaneity of Newtonian mechanics.

 

[Lynch101]

With that definition then the block universe is necessary in Newtonian physics. I.e. it is not the physics that forces the acceptance of the block universe, it is this assumed property of "real"

Apologies, Dale, I might not have said that in the clearest terms. That isn't the argument for why relativity necessitates the Block Universe, that addresses the nebulous nature of the term "real".

Past, present, and future events are not being defined as real. To paraphrase, the question was "what does it mean to say past, present, and future events are "real"? The answer I got to this at the time, and I see how it makes sense, is that we don't need to define what it means to be "real", its sufficient that whatever criteria apply to events in the present moment, equally apply to past and future events. They are as real, or as unreal as each other. What the actual meaning of the term "real" is, can be left to the philosophers.

I found the analogy of the film reel helpful when struggling with this point. If we imagine all the frames on a reel of film. When the movie is being projected onto the screen we only see the image from one frame at a time and it seems to be constantly changing in a one direction i.e. from present to future. Now, even though we only see the image from one frame at a time, the entire real of film is there. Some frames are in "the past", some are in "the future", while one frame is constantly changing on the screen. We don't need to talk about whether or not the frames on the film are "real" we just need to know that the reel comprises all of them and no single frame is preferred over another, even though we only ever see one frame at a time on the screen.That is simply a description of the Block Universe picture and why we don't need to get bogged down in the word real, we can simply talk about what the Universe comprises. The Block Universe says it comprises past, present, and future events, with none of those events being more special than any other.

The alternative Newtonian picture is more like the original shooting of the movie, where the actors play out the roles in an ever changing scene. In this picture, the Universe does not comprise past and future events, only present ones. This is a Universe with a global "now".

The argument for why relativity necessitates the Block Universe is that, if the Universe doesn't comprise past and future events i.e. it isn't a Block Universe, then we are left with the Newtonian global "now".

 

[PeterDonis]

the answer I was given at the time is that we don't need to know what the term "real" means, we just need to know that it applies equally to past, present, and future events

That doesn't help because it doesn't tell me how to test whether it applies equally to past, present, and future events. Basically this looks to me like a way of dodging a question one doesn't have a good answer to.

The Newtonian picture would say that only the event which corresponds to the present is "real".

No, it doesn't. Newtonian physics is perfectly consistent with viewing the past as fixed and certain. It is also consistent, as has already been pointed out, with a "block universe" view in which the future is fixed and certain as well as the present and past (this works because Newtonian physics is deterministic). So it seems like you were given an incorrect view of Newtonian physics as well as relativity.

if past and future events do not exist

You keep talking as if this is a viable alternative. It's not. As I've already explained, at a minimum, the events in the past light cone of your present event are fixed and certain, which would seem to mean they, at least, must "exist".

I also note that you keep switching terminology. First it was "real", now it's "exist", and in another post you used "form part of the structure of the Universe". None of those are really scientific terms; I've already explained why "real" isn't, and the explanation for "exist" or "form part of the structure of the Universe" would be similar. I suggested "fixed and certain" as a better alternative; see further comments below.

In what sense do you mean events are "fixed and certain"?

That what happens at them is fixed and will not change. Or, to put it another way, if we consider all possible 4-d spacetime models that could be realized, given what you know at your present event, all of them will have the same set of events (things that happen) in the past light cone of your present event. But not all of them will have the same set of events (things that happen) outside of the past light cone of your present event.

As I've pointed out, block universe proponents ignore this obvious fact because they look at models that they have constructed, in which they declare by fiat what events happen everywhere in the 4-d spacetime of their model. But the real world doesn't work like that. You can't dictate by fiat what happens outside your past light cone. You can try to predict what will happen outside your past light cone, but those predictions can never be perfect, because you don't have sufficient data in your past light cone to determine for certain what will happen at any event outside your past light cone.

Note that this is true even if the fundamental physical laws are deterministic; even in a deterministic system, in order to have sufficient data to fix all events everywhere in the spacetime, you need to have initial data on an entire spacelike 3-surface. But no past light cone contains such data, and nobody ever has or ever will have such data. I discuss this in my Insights article (and IIRC there was more discussion of it in the comment thread on it).

Under your picture, do events in our past light cone still "exist"*, or perhaps does the structure of the universe (or arrangement of particles) that constituted past event(s) still form part of the structure of the Universe?

Neither of these are scientific questions. See above.

would it be fair to say that your proposal is similar to the growing Block conceptualisation?

My proposal is consistent with this view, as far as I can see, but my proposal has the advantage, from a scientific point of view, of not making claims that are not scientific.

It's the version of relativity that is probably most prominent in the pop-science genre.

The fix for this is simple: don't try to learn actual science from pop science sources. Brian Greene in particular is a frequent offender--if I had a dollar for every PF thread where we've had to correct someone's misconceptions based on one of his pop science books or shows or videos, I'd be retired now.

The argument for why relativity necessitates the Block Universe is that, if the Universe doesn't comprise past and future events i.e. it isn't a Block Universe, then we are left with the Newtonian global "now".

This argument is simply wrong. It's not even as good as the argument I refuted in my Insights article, which at least made some pretense of using concepts specifically from relativity. This argument could just as well be made for Newtonian physics, and is just as wrong when applied to that. The obvious flaw is the smuggling in of "past and future events" as though they had to go together, when they obviously don't.

 

[jbriggs444]

Past, present, and future events are not being defined as real. To paraphrase, the question was "what does it mean to say past, present, and future events are "real"? The answer I got to this at the time, and I see how it makes sense, is that we don't need to define what it means to be "real", its sufficient that whatever criteria apply to events in the present moment, equally apply to past and future events. They are as real, or as unreal as each other. What the actual meaning of the term "real" is, can be left to the philosophers.

If you do want to avoid constraining "real", that includes not specifying whether it does or does not apply equally to events in the "past" and "future", whatever those terms might mean.

 

[PAllen]

Ah, OK. This is certainly new to me, thank you!

Does this then mean that relativity permits a global "now", although observers will disagree about what the content of that global "now" is?

Not quite. I am proposing that you may believe there exists a unique global now, but you will never be able to find out what it is except that your local present is part of it. Such a belief in something you can't verify isn't really different from belief in the block universe. Both beliefs are compatible with relativity, neither can be verified.

 

[PAllen]

I think a Newtonian Block Universe would have certain experimental consequences, if I remember correctly. I'd have to try and refresh my memory, but I think that might be a subject for a different thread.

It could not possibly have any experimental consequences. Don't know what you might be thinking of.

 

[Nugatory]

Unfortuately, I wouldn't be sure how to go about rigorously defining the term "now".

Here it is then: The events that are happening now are the ones whose time coordinate is the same as the time coordinate of the event “I just said ‘now’”. That can be extended into the more general notion of “at the same time”: all events that have the same time coordinate are said to have happened at the same time. An important special case is when an event has the same time coordinate as the event “my wristwatch reads X”; I will say that the event happened at time X.

These terms ("now", "happened when", ...) tell us more about how we assign time coordinates to events than the relationships between events.

 

[Dale]

Yes, but Lorentz aether theory utilises a different conceptualisation of time.

Of course. If you compare presentism and eternalism you will clearly have a different conceptualization of time.

The point is that both conceptualizations are compatible with Newtonian physics and both conceptualizations are compatible with relativistic physics. Neither conceptualization is forced on us by the physics.

By “the physics” I mean the experimental evidence and the mathematical framework used to make experimental predictions. It seems like you may be adding some philosophy to the physics in what you understand relativity to be. So you are already assuming a specific conceptualization of time which is not part of “the physics”.

The key point, however, is that Lorentz aether theory does not incorporate the relativity of simultnaeity in truth, because it retains the absolute simultaneity of Newtonian mechanics.

So LET shares all of the mathematical framework and all of the experimental predictions of the block universe. The physics of both is the physics of SR. They differ in their conceptualization of time. This shows that the conceptualization of time is not part of the physics

 

[Lynch101]

That doesn't help because it doesn't tell me how to test whether it applies equally to past, present, and future events. Basically this looks to me like a way of dodging a question one doesn't have a good answer to.

You keep talking as if this is a viable alternative. It's not. As I've already explained, at a minimum, the events in the past light cone of your present event are fixed and certain, which would seem to mean they, at least, must "exist".

I also note that you keep switching terminology. First it was "real", now it's "exist", and in another post you used "form part of the structure of the Universe". None of those are really scientific terms; I've already explained why "real" isn't, and the explanation for "exist" or "form part of the structure of the Universe" would be similar. I suggested "fixed and certain" as a better alternative; see further comments below.

I've grouped these comments together because I think they all speak to the same point. I acknowledge that I keep changing terminology. That is an attempt to find a more suitable way of describing what the Block Universe says. That is what I am trying to do first, simply establish what the BU says.

===============================================
I'm returning to the top of this reply just to mention that I have been replying to your proposal of the term "fixed and certain" below. It certainly a preferable term to that of "real" but I think it needs some further clarification before it can be used in this context. Perhaps, more accurately, I need further clarification before I will be able to start using it in this context.
===============================================

What does the fox BU say?
You are obviously well familiar with the Block Universe, given your writings on it; and hopefully I'm not confusing you with another poster, but I think you've said that the Block Universe is a possible interpretation of relativity. But , you argue that it is not an absolute necessity, as often tends to be presented. Am I correct in that?

To start, we're just trying to establish what the familiar picture of the Block Universe says - aside from the claim that it is a necessary conclusion. Prior to that, we just need to be on the same page about what the BU says about past and future events. You might be able to state that more rigorously than I can but the BU definitely says something about past, present, and future events. It says something about past and future events that makes it different to a universe that is based only on a global "now".

Can we say past and future events have the same status?/ontology?/constitution?/[insert the most precise possible term here] as present events.

Ontology
Instead can we talk about the ontology of events that constitute a world line. The BU says that all points on a world line are ontologically equal. No single event is preferred over another. The observations we make in our scientific experiments might correspond to single events on our world line but that doesn't single it out as special.

To give a real world example, again, of our 10/30/50th birthdays. If now is our 30th birthday then we consider our 10th birthday to be in the past and our 50th birthday to be in the future. A universe with a global present comprises only the event of our 30th birthday, only a single event on our world line. The Block Universe, however comprises all of those events 10/30/50th birthdays. None of the events is singled out over the others.Dodging the question
I don't want to get bogged down on this particular point because I know how endless these philosophical rabbit holes can be, so I am keen to avoid such a debate. I think it's more important to just get on the same page as to what the BU says about past, present, and future events. I don't want to dodge the point above, however.

The way it is used here, the term "real" isn't a set of criteria which something must fulfil, it is instead simply a label that we can try to apply in our discussion. If we apply the label "real" to present events, say our 30th birthday, then the Block Universe says that label applies equally to past events, our 10th birthday, as well as future events, such as our 50th birthday. We don't need to know the true nature of reality to use the term in this way, instead, if we say that our 30th birthday is real, then we must say the same thing about our 10th and 50th birthdays.

We could use a different label, but "real" has the benefits of our preconceptions of what it means. It is a double edged sword however, for the reason you raise above, but any other term would probably cause more confusion.

No, it doesn't. Newtonian physics is perfectly consistent with viewing the past as fixed and certain. It is also consistent, as has already been pointed out, with a "block universe" view in which the future is fixed and certain as well as the present and past (this works because Newtonian physics is deterministic). So it seems like you were given an incorrect view of Newtonian physics as well as relativity.

Thank you Peter, yes. My apologies, when I refer to the Newtonian picture I am referring to the conception of it that is best juxtaposed with the Block Universe to offer a contrast. From here on, if I say the Newtonian picture, I am referring to the conception that is based on a global "now" not on a Newtonian Block Universe.

That what happens at them is fixed and will not change. Or, to put it another way, if we consider all possible 4-d spacetime models that could be realized, given what you know at your present event, all of them will have the same set of events (things that happen) in the past light cone of your present event. But not all of them will have the same set of events (things that happen) outside of the past light cone of your present event.

As you have mentioned above, the term "fixed and certain" can be applied equally to a Newtonian universe, either with a single global "now" or a Newtonian block universe, to a relativistic block universe, or in another way that isn't equivalent to the BU.

These represent very different pictures of the Universe, so the idea of "fixed and certain" would need some additional clarification. You mentioned above that the BU considers future events to be "fixed and certain", so, according to your application, we only consider events in the past light cone to be fixed and certain, it sounds very much like the conceptualisation usually referred to as "the growing block universe".

You say

My proposal is consistent with this view, as far as I can see, but my proposal has the advantage, from a scientific point of view, of not making claims that are not scientific.

As you have said, your proposal is consistent with all the different views, but all of the different views present very different pictures of the Universe, so I think it is necessary to clarify what we are referring to when we use it.

Is it intended to be more of a catch all phrase?

You say

As I've pointed out, block universe proponents ignore this obvious fact because they look at models that they have constructed, in which they declare by fiat what events happen everywhere in the 4-d spacetime of their model. But the real world doesn't work like that. You can't dictate by fiat what happens outside your past light cone. You can try to predict what will happen outside your past light cone, but those predictions can never be perfect, because you don't have sufficient data in your past light cone to determine for certain what will happen at any event outside your past light cone.

Note that this is true even if the fundamental physical laws are deterministic; even in a deterministic system, in order to have sufficient data to fix all events everywhere in the spacetime, you need to have initial data on an entire spacelike 3-surface. But no past light cone contains such data, and nobody ever has or ever will have such data. I discuss this in my Insights article (and IIRC there was more discussion of it in the comment thread on it).

Absolutely agree on this point. We cannot define what happens outside our past light cone.

I don't think this affects our conclusion though. We can talk strictly about the events that constitute our own world line and reason from there.
You mentioned

The fix for this is simple: don't try to learn actual science from pop science sources. Brian Greene in particular is a frequent offender--if I had a dollar for every PF thread where we've had to correct someone's misconceptions based on one of his pop science books or shows or videos, I'd be retired now.

If I could go back and choose again (and if free will really exists) I would choose to study physics. Unfortunately, my past is fixed and certain so that isn't an option.
I am grateful for pop-science though because it has allowed me to at least engage with physics, where otherwise I would not have. My understanding of physics is certainly better because of pop-science material, because without it, it would be non-existent. Even if it means the arduous task of unlearning some things, but I feel like it's at least given me a foundation from which to start.You say

This argument is simply wrong. It's not even as good as the argument I refuted in my Insights article, which at least made some pretense of using concepts specifically from relativity. This argument could just as well be made for Newtonian physics, and is just as wrong when applied to that. The obvious flaw is the smuggling in of "past and future events" as though they had to go together, when they obviously don't.

I'm not sure what you mean by smuggling in "past and future" events (with emphasis on and). Are you implying that relativity necessitates a block universe which comprises past and present events, but not necessarily future events?

Or are you saying that neither past nor future events can be "smuggled in"?I'm not actually sure how I'm smuggling anything in. I'm simply juxtaposing two alternatives and saying if not this, then that.

 

[Lynch101]

If you do want to avoid constraining "real", that includes not specifying whether it does or does not apply equally to events in the "past" and "future", whatever those terms might mean.

If now is your 30th birthday, then your 10th birthday would be a "past event", while your 50th birthday would be a "future event".

 

[Lynch101]

Not quite. I am proposing that you may believe there exists a unique global now, but you will never be able to find out what it is except that your local present is part of it. Such a belief in something you can't verify isn't really different from belief in the block universe. Both beliefs are compatible with relativity, neither can be verified.

That's true, but we can specify that either a global now exists, or it doesn't. If a global now does exist, then it would necessitate absolute simultaneity. Relativity tells us that this is not the case, doesn't it?

 

[Lynch101]

Here it is then: The events that are happening now are the ones whose time coordinate is the same as the time coordinate of the event “I just said ‘now’”. That can be extended into the more general notion of “at the same time”: all events that have the same time coordinate are said to have happened at the same time. An important special case is when an event has the same time coordinate as the event “my wristwatch reads X”; I will say that the event happened at time X.

These terms ("now", "happened when", ...) tell us more about how we assign time coordinates to events than the relationships between events.

Thanks Nugatory. That's similar to how Einstein defines simultaneity in his paper on relativity, isn't it?

I was thinking more along the lines of trying to define it without reference to clocks or simultaneity, but it might not be possible to do that rigorously.

 

[Lynch101]

Of course. If you compare presentism and eternalism you will clearly have a different conceptualization of time.

The point is that both conceptualizations are compatible with Newtonian physics and both conceptualizations are compatible with relativistic physics. Neither conceptualization is forced on us by the physics.

By “the physics” I mean the experimental evidence and the mathematical framework used to make experimental predictions. It seems like you may be adding some philosophy to the physics in what you understand relativity to be. So you are already assuming a specific conceptualization of time which is not part of “the physics”.

So LET shares all of the mathematical framework and all of the experimental predictions of the block universe. The physics of both is the physics of SR. They differ in their conceptualization of time. This shows that the conceptualization of time is not part of the physics

Presentism would necessitate absolute simultaneity though, which the relativity of simultaneity tells us isn't correct. So presentism can't be compatible with Einstein's relativity.

 

[Dale]

From here on, if I say the Newtonian picture, I am referring to the conception that is based on a global "now" not on a Newtonian Block Universe.

I would prefer if you not do that.

The correct term is “presentism”. If you use the term “Newtonian” to refer to presentism then the terminology makes the whole discussion more cumbersome. We know that relativistic picture of physics is different from the Newtonian picture of physics in many ways. So if you label “presentism” as Newtonian then it becomes difficult to disentangle the presentism philosophy from the Newtonian physics, the mathematical framework and experimental predictions. The philosophy and the physics are separate things.

Please use the correct philosophical term “presentism”.

If a global now does exist, then it would necessitate absolute simultaneity. Relativity tells us that this is not the case, doesn't it?

“The physics” (experimental predictions and mathematical framework) of relativity is perfectly compatible with an undetectable global now, just as it is compatible with an undetectable aether frame.

 

[PAllen]

That's true, but we can specify that either a global now exists, or it doesn't. If a global now does exist, then it would necessitate absolute simultaneity. Relativity tells us that this is not the case, doesn't it?

No, it would not. The relativity of simultaneity is what would make a global past / future surface undetectable. It doesn't prove its nonexistence.

 

[PeterDonis]

That is what I am trying to do first, simply establish what the BU says.

Then, instead of trying to make up your own terminology, why don't you read what the proponents of the BU interpretation actually say? For example, you could read the very statements I quoted in my Insights article, or the ones made in the references I gave there.

I have been considering your proposal of the term "fixed and certain". It certainly a preferable term to that of "real" but I think it needs some further clarification before it can be used in this context. Perhaps, more accurately, I need further clarification before I will be able to start using it in this context.

If you will read my Insights article, you will see that I chose that term because Roger Penrose, in the quote from The Emperor's New Mind that I gave, uses "certain" and "fixed" in his own description of the argument that I refute in the article.

I really wonder whether you have actually read my Insights article with proper attention, since you keep bringing up issues that I explicitly addressed there, precisely in order to try and forestall the kind of rehashing of them that we seem to be doing here.

I think you've said that the Block Universe is a possible interpretation of relativity. But , you argue that it is not an absolute necessity, as often tends to be presented. Am I correct in that?

Yes.

The way it is used here, the term "real" isn't a set of criteria which something must fulfil, it is instead simply a label that we can try to apply in our discussion.

And a useless one, as far as science is concerned, since it has no experimental consequences. Science is not concerned with applying labels to things that have no experimental consequences. Science is concerned with things we can actually test by experiment.

Of course people like to label things in ways that science doesn't address; that's fine. But it's not science. And to the extent the BU interpretation involves such labels, it's not science either. It's philosophy, or metaphysics, or whatever you want to call it other than "science", but it isn't science.

We could use a different label, but "real" has the benefits of our preconceptions of what it means.

What you see as a benefit, I see as a hindrance. Preconceptions are not useful if they lead you to make claims that are not valid, such as BU proponents claiming that BU is required by relativity.

any other term would probably cause more confusion.

"Fixed and certain", as I noted above, was the term used by Penrose, and it seems pretty clear what he means by it. It certainly works a lot better in his presentation of the argument than "real" would have.

These represent very different pictures of the Universe, so the idea of "fixed and certain" would need some additional clarification.

These represent very different pictures of the part of the Universe we don't know about yet, i.e., the part of the Universe that we haven't yet observed because no information from that part has reached us, because of the finite speed of light. But all of these very different pictures of the Universe share the belief that the things we already know about won't change. What you ate for breakfast yesterday isn't going to change depending on what you observe tomorrow. Where you were born, where your parents went to school, etc., aren't going to change depending on whether or not it rains tomorrow. And so on. All of these very different views of the Universe agree on simple mundane things like that.

That common property that all of these very different pictures of the Universe agree that the things we already know about have, is what I mean by "fixed and certain". The question is whether any other parts of the Universe, besides the part we already know about, have this property. The BU view is the view that all of the events everywhere in 4-d spacetime have this property. The argument I refute in the Insights article claims that that view is required by relativity. It's not.

according to your application, we only consider events in the past light cone to be fixed and certain, it sounds very much like the conceptualisation usually referred to as "the growing block universe".

I've already said that my proposed view is consistent with the "growing block universe" view. But your calling it the "growing block universe" view does not mean it's the same as the block universe view I discuss in my Insights article. It's not. The term "growing block universe" is thus a very unfortunate term to describe that view.

We can talk strictly about the events that constitute our own world line

Not all of them, since only the events on your worldline that are either your present event, or to the past of your present event, are in your past light cone. Events on your worldline to the future of your present event, such as you eating breakfast tomorrow, are not.

I'm not sure what you mean by smuggling in "past and future" events (with emphasis on and).

You are talking as if past events and future events must have the same "ontological status" (or whatever term you want to use). That's obviously false. There is nothing at all inconsistent about saying that past events are fixed and certain but future events are not. So there is nothing that requires past and future events to always be treated on the same footing.

But the argument you were quoting assumes that that is required: it claims that the only alternative to a block universe view, i.e., to having both past and future be "as real as" (or whatever term you want to use) the present, is to have only the present be "real" (or whatever term you want to use). If it's not required to treat the past and future on exactly the same footing, then the obvious other alternative is to treat the present and the past as "real" (or whatever term you want to use), but not the future. (And with the proper relativistic definition of "present" and "past", i.e., the "present" is your present event, and the "past" is its past light cone, that is exactly why my proposed alternative view does.)

The fact that this obvious alternative is not even mentioned is why I say the argument you were quoting here is not even as good as the one I refuted in my Insights article.

Are you implying that relativity necessitates a block universe which comprises past and present events, but not necessarily future events?

Not quite, no. First, "present" is only one event--your present event. Second, "past" is just your past light cone, which not everyone is clear about. Third, "future" is not all of the rest of spacetime; it's just your future light cone. There is also the region I called "elsewhere" in my Insights article (another useful term from Roger Penrose, who uses it in The Emperor's New Mind and, IIRC, other books as well), all of the events that are spacelike separated from your present event.

I'm simply juxtaposing two alternatives and saying if not this, then that.

Which is invalid reasoning if there are any other alternatives that you have not considered. Which there are in this case. See above. This logical fallacy is so common that it has a name: "False dichotomy".

 

[Nugatory]

Thanks Nugatory. That's similar to how Einstein defines simultaneity in his paper on relativity, isn't it?

It is the definition that he used, but it’s not his definition - it‘s the only definition that has ever been used (although usually not so explicitly) by anyone. Einstein’s contribution was to show the limitations of this generally accepted definition of “now”.

 

[Lynch101]

I would prefer if you not do that.

The correct term is “presentism”. If you use the term “Newtonian” to refer to presentism then the terminology makes the whole discussion more cumbersome. We know that relativistic picture of physics is different from the Newtonian picture of physics in many ways. So if you label “presentism” as Newtonian then it becomes difficult to disentangle the presentism philosophy from the Newtonian physics, the mathematical framework and experimental predictions. The philosophy and the physics are separate things.

Please use the correct philosophical term “presentism”.

Thank you Dale. I know that philosophy is off topic here, so I wanted to ground it in the context of scientific theory. Apologies if this caused confusion.

“The physics” (experimental predictions and mathematical framework) of relativity is perfectly compatible with an undetectable global now, just as it is compatible with an undetectable aether frame.

Maybe I'm interpreting this incorrectly then, but it would seem to suggest that relativity is compatible with absolute simultaneity, because presentism would necessitate absolute simultaneity. Is it compatible with absolute [undetectable] simultaneity?

 

[Lynch101]

No, it would not. The relativity of simultaneity is what would make a global past / future surface undetectable. It doesn't prove its nonexistence.

Ah I see. I was told that the relativity of simultaneity ruled out absolute simultaneity.

Or is it compatible on the basis of how simultaneity is defined, as opposed to on the basis of an underlying metaphysics?

 

[Dale]

it would seem to suggest that relativity is compatible with absolute simultaneity, because presentism would necessitate absolute simultaneity. Is it compatible with absolute [undetectable] simultaneity?

Yes. It is compatible with LET and from there presentism is easy to see.

I know that philosophy is off topic here, so I wanted to ground it in the context of scientific theory. Apologies if this caused confusion.

This is inherently a purely philosophical discussion. Using incorrect terminology doesn’t suddenly ground the discussion in science. “Block universe” is a standard synonym for “eternalism”. But “Newtonian view” is not a standard synonym for “presentism”, and I object to its use.

 

[PAllen]

Ah I see. I was told that the relativity of simultaneity ruled out absolute simultaneity.

Or is it compatible on the basis of how simultaneity is defined, as opposed to on the basis of an underlying metaphysics?

Not sure why we keep talking in circles. Try this: if Newtonian physics were true, there would be a procedure that could actually be performed to synchronize separated clocks such that any pair of clocks synchronized in one frame would be synchronized when compared to relatively moving clocks synchronized using the same procedure by someone comoving with those clocks. This is what absolute simultaneity means.

In relativity, this is false - it cannot be done.

None of this determines what is the boundary between what is fixed and certain versus not is. A block universe interpretation of either Newtonian physics or relativity simply posits that all that ever will be is as fixed as all that ever was. In neither theory is it possible to test this belief. Similarly either theory is compatible with the idea that there is a unique boundary defining a global now that separates what is fixed and certain from what is not. In neither theory is it possible to experimentally address this belief. The only difference is that in Newtonian physics it would be natural to identify this boundary with the absolutely simultaneous times as described above. However, you would still have no way to prove this over a block universe belief.

 

[Lynch101]

I just want to start this post by saying that I think I see the point you were trying to make, that we can't assume that the universe comprises events which are in our future light cone, we can't say that those events are fixed and certain. There are still one or two things I think I'm not be clear on. I'll reply below, but I just wanted to put this in at the start.

I may have confused the issue by using the term Block Universe because I have also been saying if past and/or future events, so I haven't necessarily been making an assumption about future events, although my use of the term Block Universe has clouded the issue.

Then, instead of trying to make up your own terminology, why don't you read what the proponents of the BU interpretation actually say? For example, you could read the very statements I quoted in my Insights article, or the ones made in the references I gave there.

If you will read my Insights article, you will see that I chose that term because Roger Penrose, in the quote from The Emperor's New Mind that I gave, uses "certain" and "fixed" in his own description of the argument that I refute in the article.

I really wonder whether you have actually read my Insights article with proper attention, since you keep bringing up issues that I explicitly addressed there, precisely in order to try and forestall the kind of rehashing of them that we seem to be doing here.

"Fixed and certain", as I noted above, was the term used by Penrose, and it seems pretty clear what he means by it. It certainly works a lot better in his presentation of the argument than "real" would have.

I wasn't familiar with the terminology you used in the article so it didn't stick in my mind. I understood what was meant as I read it, but the terminology I have become accustomed to is speaking about past and future as "real", or about the ontological nature of events on a world line. This seemed to be a major stumbling block, so I was trying to throw terms out there to see if I could illustrate the point I was trying to make, so that we would be on the same page. I was unfamiliar with your use of "fixed and certain", so I wasn't confident of using them in such a way as to convey my own understanding.

These represent very different pictures of the part of the Universe we don't know about yet, i.e., the part of the Universe that we haven't yet observed because no information from that part has reached us, because of the finite speed of light. But all of these very different pictures of the Universe share the belief that the things we already know about won't change. What you ate for breakfast yesterday isn't going to change depending on what you observe tomorrow. Where you were born, where your parents went to school, etc., aren't going to change depending on whether or not it rains tomorrow. And so on. All of these very different views of the Universe agree on simple mundane things like that.

That common property that all of these very different pictures of the Universe agree that the things we already know about have, is what I mean by "fixed and certain". The question is whether any other parts of the Universe, besides the part we already know about, have this property. The BU view is the view that all of the events everywhere in 4-d spacetime have this property. The argument I refute in the Insights article claims that that view is required by relativity. It's not.

If I understand correctly, your argument refutes the idea that events in our future light cone are fixed and certain, but allows for differing views about events in our past light cone.

It seems to be compatible with a presentist universe, but also with what is often colloquially referred to as the growing block, as you mention below.

I've already said that my proposed view is consistent with the "growing block universe" view. But your calling it the "growing block universe" view does not mean it's the same as the block universe view I discuss in my Insights article. It's not. The term "growing block universe" is thus a very unfortunate term to describe that view.

I understand, yes, the growing block is a different idea form the fully formed block.

This would mean that the "fixed and certain" definition applies to either a presentist or "growing block" type universe, am I correct in that?

Not all of them, since only the events on your worldline that are either your present event, or to the past of your present event, are in your past light cone. Events on your worldline to the future of your present event, such as you eating breakfast tomorrow, are not.

I understand your point now, I believe. This would mean that the Universe comprises either our present event, or our past and present events, but not necessarily our future events.

I was under the impression that when we add in a relatively moving observer, and their calculations about when events must have happened, based on when light from an event reaches them, leads us to the idea that the universe must also comprise events in our future light cone.

As in the example of the relatively moving observers on the train and platform, and the lightning strikes.

You are talking as if past events and future events must have the same "ontological status" (or whatever term you want to use). That's obviously false. There is nothing at all inconsistent about saying that past events are fixed and certain but future events are not. So there is nothing that requires past and future events to always be treated on the same footing.

I see how your criterion about events in our past light cone necessitates this. But my understanding was that, when we have two relatively moving observers, one of them either observes or calculates that an event which is part of their present (the flash of light to the front of the train) is in the future light cone of the other observer. Similarly, the observer on the platform can calculate, after the fact, that an event in their future light cone must have already happened when the other observer passed them.

Even if the observer on the platform can't say, in the present, that the event in their future light cone is fixed and certain, their calculations lead them to the conclusion that the future event must have been fixed and certain.

If I have understood the explanation correctly.

But the argument you were quoting assumes that that is required: it claims that the only alternative to a block universe view, i.e., to having both past and future be "as real as" (or whatever term you want to use) the present, is to have only the present be "real" (or whatever term you want to use). If it's not required to treat the past and future on exactly the same footing, then the obvious other alternative is to treat the present and the past as "real" (or whatever term you want to use), but not the future. (And with the proper relativistic definition of "present" and "past", i.e., the "present" is your present event, and the "past" is its past light cone, that is exactly why my proposed alternative view does.)

The fact that this obvious alternative is not even mentioned is why I say the argument you were quoting here is not even as good as the one I refuted in my Insights article.

I see the assumption I was making now, but, if my understanding of what was explained to me is correct, it means that the picture of the Universe that relativity allows us to calculate, isn't just dependent on what is in our own past light cone. We can calculate what a relatively moving observer would observe and when events happen in the relatively moving frame, cross reference with the location of the observer in our frame, and then make further deductions about events that would have been in our future light cone.

This is all calculated after the fact, but it allows us to build a more complete picture...I think.

Not quite, no. First, "present" is only one event--your present event. Second, "past" is just your past light cone, which not everyone is clear about. Third, "future" is not all of the rest of spacetime; it's just your future light cone. There is also the region I called "elsewhere" in my Insights article (another useful term from Roger Penrose, who uses it in The Emperor's New Mind and, IIRC, other books as well), all of the events that are spacelike separated from your present event.

OK, maybe I haven't fully understood your point then. Am I right in saying that we can't consider events in our future light cone as fixed and certain, at the very least?

If relativity doesn't necessitate that the Universe comprises

Is the above description talking about one single observer and what they can say is fixed and certain at any given moment?

My understanding is that, after the fact, observers could apply relativity theory and calculate a picture of the Universe that tells them that when a relatively moving observer passed them, an event they considered to be in their future light cone had already happened in the frame of the relatively moving observer. So, although they cannot say in advance that events in their future light cone are fixed and certain, they can calculate afterwards that such events must have been fixed and certain.

I might be misinterpreting that, but that is how I have understood it. It might be easier to explain by way of a thought experiment.

Which is invalid reasoning if there are any other alternatives that you have not considered. Which there are in this case. See above. This logical fallacy is so common that it has a name: "False dichotomy".

I have been talking about past and/or future events, so I haven't really been making an assumption about future events, although I can see how the Block Universe is the idea that past and future events "exist".

 

[PeterDonis]

If I understand correctly, your argument refutes the idea that events in our future light cone are fixed and certain, but allows for differing views about events in our past light cone.

No. I have said repeatedly that events in our past light cone are fixed and certain on all views (although I did not previously mention presentism, which is a possible exception--but see below). Certainly events in our past light cone are fixed and certain on the alternative view I proposed in the article. The question is whether any events other than our present event and its past light cone are fixed and certain.

In any case, my argument did not refute the claim that events in our future light cone are fixed and certain. It only refuted a particular argument that purports to claim that relativity requires events in our future light cone to be fixed and certain. I have explained this several times now.

It seems to be compatible with a presentist universe

It depends on what you mean by "presentism", and discussions of that view are too contaminated with vague terms like "real" for me to know what they are really saying. Also, the term "present" is ambiguous; on the alternative view I gave in the article, it means "your present event", but on many "presentist" views, it seems to mean "some spacelike 3-surface that contains your present event".

This would mean that the "fixed and certain" definition applies to either a presentist or "growing block" type universe, am I correct in that?

I am using the term "fixed and certain" as a general term that can be used by any viewpoint; what each viewpoint does is specify exactly which events are fixed and certain.

This would mean that the Universe comprises either our present event, or our past and present events, but not necessarily our future events.

This describes two possible viewpoints, yes.

I was under the impression that when we add in a relatively moving observer, and their calculations about when events must have happened, based on when light from an event reaches them, leads us to the idea that the universe must also comprise events in our future light cone.

Sorry to shout, but THIS IS EXACTLY THE ARGUMENT I REFUTED IN MY ARTICLE. Have you really read it? Really? Are you sure you don't need to go back and read it again?

Am I right in saying that we can't consider events in our future light cone as fixed and certain, at the very least?

No. You have repeatedly failed to understand what I actually said in the article.

Here is what I said in the article: Relativity does not require all events in 4-d spacetime to be fixed and certain.

Here are some things I explicitly did not say in the article:

I did not say that events in our future light cone are definitely not fixed and certain. (I only said relativity does not require that they are.)

I did not say that only our present event is fixed and certain. Nor did I say that it isn't.

I did not say that only some spacelike 3-surface containing our present event is fixed and certain. Nor did I say that it isn't. (I did propose an alternative that would say it isn't, except for our present event itself, but that's a different thing.)

I did not even say that relativity requires events in our past light cone to be fixed and certain. (I only said that this view is an obvious alternative that (a) makes sense, and (b) was not even proposed or considered by all the many people who have argued about the block universe.)

I hope this helps.

My understanding is that, after the fact, observers could apply relativity theory and calculate a picture of the Universe that tells them that when a relatively moving observer passed them, an event they considered to be in their future light cone had already happened in the frame of the relatively moving observer.

No, that's not what they can calculate after the fact. No calculation can tell you "when" some distant event happened according to some observer. That is something you assign by choosing coordinates. It has no physical meaning whatsoever.

What observers can calculate after the fact are the causal relationships between events: which events are in which other events' past or future light cones, which events are spacelike separated from which other events. Those things are invariants and have physical meaning. But in order to do those calculations, all of the events the calculations apply to must already be in the past light cone of the person calculating them. Which means nobody ever has to treat any events other than those that are in their past light cone as fixed and certain in order to do calculations. Which I pointed out in my article.