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

In summary: Universe...were equally real. This was a departure from the Newtonian view, where the past and future events were seen as "imaginary", or not real in the same sense as the present.In SummaryJust 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.In relativity, there is no global concept of "now". From what I learned, relativity
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Lynch101
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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.
PeterDonis said:
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?
 
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  • #2
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.
 
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PAllen said:
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.
 
  • #4
Lynch101 said:
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.
 
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Lynch101 said:
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.
 
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  • #6
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.
 
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  • #7
Lynch101 said:
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.

Lynch101 said:
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.
 
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  • #9
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.
 
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vanhees71 said:
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
 
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  • #11
Nugatory said:
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.

Nugatory said:
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"?
 
  • #12
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.
 
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Dale said:
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.

Dale said:
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.

Dale said:
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.
 
  • #14
Lynch101 said:
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.
 
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  • #15
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!
 
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PAllen said:
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?
 
  • #17
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?
 
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  • #18
Lynch101 said:
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.
 
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  • #19
Lynch101 said:
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.

Lynch101 said:
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
 
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  • #20
PeterDonis said:
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.
:biggrin: 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.

PeterDonis said:
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.
 
  • #21
PeroK said:
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.
 
  • #22
vanhees71 said:
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.
 
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  • #23
Lynch101 said:
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"
 
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  • #24
vanhees71 said:
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
 
  • #25
PeroK said:
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.
 
  • #26
vanhees71 said:
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.
vanhees71 said:
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.

vanhees71 said:
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.
 
  • #27
vanhees71 said:
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.
 
  • #28
Dale said:
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.

Dale said:
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.
 
  • #29
Dale said:
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".
 
  • #30
Lynch101 said:
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.

Lynch101 said:
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.

Lynch101 said:
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.

Lynch101 said:
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).

Lynch101 said:
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.

Lynch101 said:
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.

Lynch101 said:
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. :wink:

Lynch101 said:
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.
 
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  • #31
Lynch101 said:
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.
 
  • #32
Lynch101 said:
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.
 
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  • #33
Lynch101 said:
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.
 
  • #34
Lynch101 said:
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.
 
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  • #35
Lynch101 said:
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”.

Lynch101 said:
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
 
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