I Is Time Just a Stubborn Illusion in the Block Universe Theory?

[tophatphysicist]

Einstein said, "The distinction between past, present and future is only an illusion, even if a stubborn one."

That's my problem--that illusion is so vivid and stubborn.

Paul Davies said in his book "About Time": There is only one rational conclusion to draw from the relative nature of simultaneity; events in the past and future have to be every bit as real as events in the present.

 

[jbriggs444]

Paul Davies said in his book "About Time": There is only one rational conclusion to draw from the relative nature of simultaneity; events in the past and future have to be every bit as real as events in the present.

If you decide that by the force of pure reason you can tell how things must be, that is a clue that you have departed from science into philosophy.

 

[tophatphysicist]

If you decide that by the force of pure reason you can tell how things must be, that is a clue that you have departed from science into philosophy.

But, he didn't decide just by the force of pure reason. He decided based on measurements and observations culminating in the relative nature of simultaneity.

I like your comments, jbriggs444. And your avatar is by far my favorite on the forum.

 

[PeterDonis]

Paul Davies said in his book "About Time": There is only one rational conclusion to draw from the relative nature of simultaneity; events in the past and future have to be every bit as real as events in the present.

This is a common unjustified claim in pop science books. Read this Insights article:

https://www.physicsforums.com/insights/block-universe-refuting-common-argument/

 

[tophatphysicist]

This is a common unjustified claim in pop science books. Read this Insights article:

https://www.physicsforums.com/insights/block-universe-refuting-common-argument/

Peter, does an expert such as Einstein, Roger Penrose or Paul Davies lose his stature and authority in physics as soon as he writes a book for the general public? Or, maybe you're suggesting that he alters the true description of the physics for public consumption? I was quite impressed with Roger Penrose's book (also read it years ago). I still look forward to reading your article. Thanks for your contributions to the forum.

 

[Ebeb]

Peter, does an expert such as Einstein, Roger Penrose or Paul Davies lose his stature and authority in physics as soon as he writes a book for the general public? Or, maybe you're suggesting that he alters the true description of the physics for public consumption? I was quite impressed with Roger Penrose's book (also read it years ago). I still look forward to reading your article. Thanks for your contributions to the forum.

I found this interesting quote about Einstein and block universe:
Karl Popper about Einstein:
<< The main topic of our conversation was indeterminism. I tried to persuade him to give up his determinism, which amounted to the view that the world was a four-dimensional Parmenidean block universe in which change was a human illusion, or very nearly so. (He agreed that this had been his view, and while discussing it I called him "Parmenides".)... >> (Karl Popper, Unended Quest: An Intellectual Autobiography.Routledge Classics. Routledge. pp.148–150).

 

[PeterDonis]

does an expert such as Einstein, Roger Penrose or Paul Davies lose his stature and authority in physics as soon as he writes a book for the general public?

No, because they never had any in the first place in the sense you mean. Science does not work on "stature and authority". Nobody should be believed or their arguments accepted without examination simply because they are a famous physicist. That goes just as much for their peer-reviewed science as for their pop science. Even famous physicists sometimes get things wrong, so you have to check what they say. That was part of the reason I wrote that Insights article--to show that even someone as smart as Roger Penrose can make mistakes.

maybe you're suggesting that he alters the true description of the physics for public consumption?

I don't think that's the case with the particular argument I refuted in the Insights article. I think it was an honest mistake. But an honest mistake is still a mistake.

I do think a lot of pop science books leave things out, and the authors aren't always very careful to say what they are leaving out, or why, or how it affects what they keep in. Feynman, in one of his pop science books (I think it was QED, but I'm not sure), at least said that he was being careful to mention all the stuff he was leaving out (and did a pretty good job of keeping that promise). But many, if not most, authors aren't as careful as that.

 

[Ebeb]

This is a common unjustified claim in pop science books. Read this Insights article:

https://www.physicsforums.com/insights/block-universe-refuting-common-argument/

Peter, I gave it another go. Correct me if I'm wrong, but you don't seem to accept that special relativity is about 4D objects and 4D universe. In previous posts in this thread I agreed with Dale , JBriggs ... that special relativity is all about 4D objects and 4D universe. 4D means units that include past, present and future. And I found it very interesting and agreed with them that "3D section" (3D world of simultaneous events) should be replaced by 4D slice out of the full 4D object/universe.

Obviously, if you don't accept special relativity is about 4D objects/4D universe, then you can never understand, nor accept Block Universe, nor slices through 4D Block Universe... Is this what I read in your article?

 

[jbriggs444]

And I found it very interesting and agreed with them that "3D section" (3D world of simultaneous events) should be replaced by 4D slice out of the full 4D object/universe.

Please do not put words into our collective mouths.

 

[Dale]

@Ebeb note that I do not "accept Block Universe". I use the Block Universe interpretation when it suits my needs and I use LET instead when it suits my needs. I don't accept or reject either.

Instead, I pay attention to which parts of each interpretation are physical (experimentally measurable) and which parts are philosophical. I use the most convenient philosophical tools as needed, but work hard to keep the experimentally measurable stuff in the forefront. I am not 100% successful, but I work at it.

 

[PeterDonis]

Correct me if I'm wrong, but you don't seem to accept that special relativity is about 4D objects and 4D universe.

I made no such general statement in the article; I specifically said that I was only refuting one particular argument, the argument that relativity of simultaneity implies that the 4D "block universe" model must be true.

Is this what I read in your article?

No. See above.

 

[Dale]

I have no problem calling the universe an object and therefore it is physically 4D. Certainly, it has been measured to have nonzero extension in the time dimension.

However, the Block Universe goes further and asserts that the future is also fixed. I have no physical measurements that support that claim. I don't object to it, but in the absence of measurements of the future, it is in my "not physical" category.

 

[Ebeb]

I have no problem calling the universe an object and therefore it is physically 4D. Certainly, it has been measured to have nonzero extension in the time dimension.

However, the Block Universe goes further and asserts that the future is also fixed. I have no physical measurements that support that claim. I don't object to it, but in the absence of measurements of the future, it is in my "not physical" category.

Allow me to try once more understand what you guys have in mind.

Do you have physical measurement that a present world exists?
You cannot measure a present world, because 'present world' is based on simultaneity, and because simultaneity is a mathematical relative thing, you have to consider 'present world' non-existent. This means there is nothing existing 'now' at a distance from an observer. And because you don't accept the full life of a remote object exists out there, there is simply nothing out there to be observed/measured.
Only the present 'now' event of an observer does definitely 'exist now' for him.
This is even more confusing, because you would definitely reject we are solipsists.
Yes, I guess you will find the above a total mess, but the posts I read in this thread won't help either.
It's sad, but there seems to be no way we can or will understand each other... Very weird because for decades now I do understand perfectly what Einstein, Penrose, Davies, ... tell me in their writings. But that too I got wrong, isn't it?

 

[PeterDonis]

Do you have physical measurement that a present world exists?

No. Our physical measurements can only tell us about our past light cone. We don't physically measure 3-D worlds. (This is all discussed in my Insights article.)

 

[PeterDonis]

This is even more confusing, because you would definitely reject we are solipsists

That's right--because our direct observations tell us that all the events in our past light cone exist, so what exists is certainly not limited to our present moment, i.e., to a single event. And those events in the past light cone include all kinds of evidence of the existence of objects other than ourselves. (This is also discussed in my Insights article.)

 

[PAllen]

Allow me to try once more understand what you guys have in mind.

Do you have physical measurement that a present world exists?
You cannot measure a present world, because 'present world' is based on simultaneity, and because simultaneity is a mathematical relative thing, you have to consider 'present world' non-existent. This means there is nothing existing 'now' at a distance from an observer. And because you don't accept the full life of a remote object exists out there, there is simply nothing out there to be observed/measured.
Only the present 'now' event of an observer does definitely 'exist now' for him.
This is even more confusing, because you would definitely reject we are solipsists.
Yes, I guess you will find the above a total mess, but the posts I read in this thread won't help either.
It's sad, but there seems to be no way we can or will understand each other... Very weird because for decades now I do understand perfectly what Einstein, Penrose, Davies, ... tell me in their writings. But that too I got wrong, isn't it?

There are things far away from me that I detect from my past light cone. I readily infer their history continues beyond my last observation. However, which point in such unobserved future of the object corresponds to my present moment is not part of physics, it is the composition of convention and extrapolation. Even in my past light cone, the correspondence between events in the distant object's history and events in my past is a matter of convention, not physics. Even in SR, there is more than one convention in common use, while in GR, one hardly even bothers with the notion of distant now because of the plethora of choices with no basis for preferring any.

 

[tophatphysicist]

PeterDonis, just trying to make sure I understand the Premise 2 that you are concerned about in your article on Block Universe. Is it that the Block Universe should not be accepted because it has not been verified with observations not available within our light cones? Should it be accepted if events outside of our light cone could demonstrate the same conclusions as presented by Penrose in his paradox?

 

[PeterDonis]

Is it that the Block Universe should not be accepted because it has not been verified with observations not available within our light cones?

My argument is not that the block universe "should not be accepted". It is that the specific argument I described in the article is incorrect, because its second premise is not required by SR.

if events outside of our light cone could demonstrate

It is impossible for us to have knowledge of events outside our past light cone, so this suggestion is meaningless.

 

[tophatphysicist]

It is that the specific argument I described in the article is incorrect, because its second premise is not required by SR.

Sorry to belabor the point, but what do you mean by "...its second premise is not required by SR"? Do you mean that SR does not require relativity of simultaneity for events outside of our light cones, therefore the Block Universe theory is also not required by SR outside of our light cones? Would you then be saying that SR theory is not validated for events outside of our light cones?

 

[PeterDonis]

what do you mean by "...its second premise is not required by SR"?

The second premise is "3D worlds are real at every event". That premise is not required by SR.

Would you then be saying that SR theory is not validated for events outside of our light cones?

How can it be, since, as I said before, it is impossible for us to have knowledge of events outside our past light cone?

 

[tophatphysicist]

How can it be, since, as I said before, it is impossible for us to have knowledge of events outside our past light cone?

O.K. I think I see your position -- a fundamental tenet of operationalism. This is the concept that none of the laws of physics can be known to apply in the future (outside of our past light cones). I had not anticipated that the Physics Forums would operate from that stance. You can certainly make the point that this would certainly be the case particularly for the 4-D object universe. Particularly, since the Block Universe discards causality, so that now the orientation of future segments of the 4-D fiber bundles are not constrained to follow patterns dictated by causal events involving forces, etc. In that sense they are ever free to take arbitrary paths, without the laws of physics to order their configurations. Sometimes operationalism gets us wandering too far into philosophy. I don't think most physicists work within that kind of context.

We amass data from the past light cone and infer future conformance of test results to past results.

 

[PeterDonis]

This is the concept that none of the laws of physics can be known to apply in the future (outside of our past light cones).

I didn't say anything about the laws of physics. I said it is impossible for us to have knowledge of events outside our past light cone. As far as the laws of physics, that means that we can't confirm by evidence that the laws of physics apply outside our past light cone. But it says nothing at all about our ability to use the laws of physics to predict events that we have not yet observed--which is, of course, what we do with them all the time. We just have to bear in mind that those are predictions, and that there is a difference between predictions and actual observations.

the Block Universe discards causality,

It does no such thing. The block universe is perfectly consistent with SR, which is a causal theory.

the orientation of future segments of the 4-D fiber bundles are not constrained to follow patterns dictated by causal events involving forces

I have no idea where you are getting this from.

Sometimes operationalism gets us wandering too far into philosophy.

It seems to me that you are wandering too far away from actual physics.

We amass data from the past light cone and infer future conformance of test results to past results.

Yes, we infer. Inferring is not the same as observing. Once we observe that a prediction was confirmed, the events that confirmed the prediction are in our past light cone.

 

[Ebeb]

The second premise is "3D worlds are real at every event".

That cannot be the premise, because "3D worlds are real at every event" is the conclusion: block universe.The premise you probably introduce and refute is: "a 3D world is real at an event". Correct? I.o.w. you don't accept simultaneity of events in the spacelike/elsewhere zone? In that case let me first make sure what you precisely mean. Do you only refute simultaneity in the spacelike/elsewhere zone, but still accept there are/occurring/existing events in your spacelike/elsewere zone? Or are you saying that at the apex you don't know anything about spacelike/elsewhere zone, hence there are no events at tall occurring/existing in that zone?

 

[PeterDonis]

That cannot be the premise, because "3D worlds are real at every event" is the conclusion: block universe.

The block universe conclusion is not "3D worlds are real at every event". It is "all of 4D spacetime is fixed and certain". That's the claim that is being argued for in the argument I refute in the article. The claim "3D worlds are real at every event" translates to "at any given event, all 3D worlds that contain that event are fixed and certain".

The claim about 3D worlds certainly implies the block universe claim: I discuss that in the article. But that doesn't make it the same claim.

 

[PeterDonis]

let me first make sure what you precisely mean.

None of your attempts capture what I meant. Go back and read the article again, and this time read the actual words I wrote, without filling in anything from your own assumptions.

 

[Ebeb]

None of your attempts capture what I meant. Go back and read the article again, and this time read the actual words I wrote, without filling in anything from your own assumptions.

Seems we cannot even agree what Block Universe means. This is getting hilarious. (I must definitely be an idiot. Let me think about it.)

 

[PeterDonis]

Seems we cannot even agree what Block Universe means.

If you have trouble understanding what I said in the article, go back to the sources I referenced (in particular the Wikipedia article I linked to and the book by Penrose that it quotes from). If you think I'm misstating what they mean, feel free to post here explaining why.

 

[PeterDonis]

Also, for reference, here is the previous discussion on the Insights article:

https://www.physicsforums.com/threads/the-block-universe-refuting-a-common-argument-comments.843000/

 

[Ibix]

Draw a picture of the universe as it is now on a piece of paper. Draw a picture of the universe as it is a second later and stack it on top of the first piece. Draw another picture of the universe another second later and stack it on top of the other two. Repeat until you've drawn the entire future and past of the universe (warning: this may require infinite time). Now find an acid that erodes paper but doesn't touch your ink. Remove the paper, leaving towers of ink that are the 2+1 dimensional history of all the objects in the universe. Smooth out the jags in the towers caused by the original choice of finite time steps..

This is the block universe.

When we pick a reference frame we choose a definition of "time" and hence a definition of "the universe now", and we think of inserting paper into our ink-only model. But there's nothing to tell us how to do that insertion. We could choose any set of planes, not even necessarily flat. And we don't actually insert the paper and there was never any paper to begin with - just the ink. Imagining a stack of paper one sheet at a time is just something we do because that's how our poor monkey brains visualise the universe. And since we're not actually doing anything and there's no reason to prefer one choice of planes over another, we say that "now" isn't a physical thing. We aren't denying that stuff outside our light cone exists, just saying that anything that's a free choice with no effect on the results (like your choice of how to insert the paper, which you can't actually do) can't have physical significance.

 

[Ebeb]

Also, for reference, here is the previous discussion on the Insights article:
https://www.physicsforums.com/threads/the-block-universe-refuting-a-common-argument-comments.843000/

I see I'm in the same boat of the participants of that thread. I'm in fact very glad to see I'm not the only one having trouble understanding your article.

None of your attempts capture what I meant.

O.K. Knowing this helps.

Go back and read the article again, and this time read the actual words I wrote, without filling in anything from your own assumptions.

I'll give it another shot.

 

[Ebeb]

@Ebeb note that I do not "accept Block Universe". I use the Block Universe interpretation when it suits my needs and I use LET instead when it suits my needs. I don't accept or reject either.

Wouln't this be against the forum rules?

Generally, in the forums we do not allow the following:

• Attempts to promote or resuscitate theories that have been discredited or superseded (e.g. Lorentz ether theory); this does not exclude discussion of those theories in a purely historical context

 

[PeterDonis]

Wouln't this be against the forum rules?

Using it as an interpretation for calculational convenience is not the same as claiming it is an actual theory, i.e., that the world "really is" the way LET or the Block Universe says it is. The latter is what is not allowed.

 

[tophatphysicist]

Dale said: ↑
@Ebeb note that I do not "accept Block Universe". I use the Block Universe interpretation when it suits my needs and I use LET instead when it suits my needs. I don't accept or reject either.

Dale, I'm not trying to be contentious here. I'm just trying to get clarification on your basis for not accepting the Block Universe theory. I think I understand your point that even though the block description appears to be valid within your past light cone, it cannot be accepted as valid in the future, since it is not possible to know the future, i.e., not possible to make measurements outside of your light cone. Do I have that part correct?

 

[PeterDonis]

even though the block description appears to be valid within your past light cone

This doesn't make sense. The block universe view is that all of 4D spacetime is fixed and certain, not just the portion within the past light cone of some chosen event.

 

[Dale]

I'm just trying to get clarification on your basis for not accepting the Block Universe theory

The Block universe is not a theory, it is an interpretation. It makes no new experimental predictions. It makes the same prediction for every experimental measurement as LET, the other major philosophical interpretation of SR. So they are physically indistinguishable (again with "physical" meaning "experimentally measurable")

Since nature does not prefer one over the other, I see no need to do so either.

I think I understand your point that even though the block description appears to be valid within your past light cone, it cannot be accepted as valid in the future, since it is not possible to know the future, i.e., not possible to make measurements outside of your light cone. Do I have that part correct?

This is essentially correct, but it is not my motivation. My motivation is that I want to use models that make accurate predictions of experimental measurements. Thus my interest is in whether or not something is "physical".

Interpretations go beyond the experimental predictions and make claims about "reality" and "existence". These sound like science terms, but they are philosophical terms. Specifically, they are from the branch of philosophy called metaphysics.

So I accept or reject theories based on physical evidence compared to the physical predictions. I remain neutral about interpretations, neither accepting nor rejecting them, on principle.

For me, interpretations merely serve as a mental aid to help organize my thoughts as I solve a problem. Different mental organizations may be useful for different problems, so I learn multiple interpretations and use whichever seems convenient for the problem at hand. So for me, the proper way to evaluate an interpretation is useful/useless rather than accept/reject.

 

[Ebeb]

The Block universe is not a theory, it is an interpretation. It makes no new experimental predictions. It makes the same prediction for every experimental measurement as LET, the other major philosophical interpretation of SR. So they are physically indistinguishable (again with "physical" meaning "experimentally measurable")

Since nature does not prefer one over the other, I see no need to do so either.

You call LET a "major philosophical interpretation", instead of a "theory"? But the forum rule correctly stipultates it's a theory (see forum quote below) . And LET means ... Lorenz Ether Theory, not Lorentz Ether philosophy.

Attempts to promote or resuscitate theories that have been discredited or superseded (e.g. Lorentz ether theory); this does not exclude discussion of those theories in a purely historical context.
(my Bold)

If LET is a philosophical interpretation but not a theory, then why calling it a theory?

Just my personal view on this issue.
LET gives another explanation for the phenomena than SR does. That makes it a different theory. LET uses ether, SR not.
If LET is only a philosophical interpretation and no theory, then I would call SR also only interpretation, both being a different interpretation of the Lorentz Transformations: LET interprets with ether, SR without.

Thinking about it LET as interpretation of SR, can we turn it around and call SR an interpretation of LET ? (Never thought physics it can be that much fun;-)

Anyway, don't worry, I won't dig any further into this.

 

[PeterDonis]

If LET is a philosophical interpretation but not a theory, then why calling it a theory?

You're getting hung up on words instead of looking at the substance. The point is that LET and block universe both make all of the same predictions for all experimental results.

I would call SR also only interpretation

"SR" as we are using the term here means the mathematical machinery that makes the predictions, and that's it. No "explanations" beyond that. LET and block universe are two different "explanations" (in your sense of the term) of why that mathematical machinery works. But both use exactly the same mathematical machinery and so make exactly the same predictions.

 

[jbriggs444]

You call LET a "major philosophical interpretation", instead of a "theory"? But the forum rule correctly stipultates it's a theory (see forum quote below) . And LET means ... Lorenz Ether Theory, not Lorentz Ether philosophy.

The Bard:

What's in a name? that which we call a rose. By any other name would smell as sweet

 

[Dale]

If LET is a philosophical interpretation but not a theory, then why calling it a theory?

It is a name that has stuck for historical reasons. It is not a separate theory in the modern usage, but that usage was not fixed at the time it was named.

If LET is only a philosophical interpretation and no theory, then I would call SR also only interpretation, both being a different interpretation of the Lorentz Transformations:

I am fine with that. I assume that in your usage "SR" is synonymous with the "Block Universe". So I think you are saying the same thing I am, but using different words. I don't know that there is an official standard usage.

LET gives another explanation for the phenomena than SR does. That makes it a different theory.

The explanation doesn't define a theory, the experimentally measurable predictions do. Or, at a minimum, the explanation is not physical, which is what most interests me.

 

[SlowThinker]

Draw a picture of the universe as it is now on a piece of paper. Draw a picture of the universe as it is a second later and stack it on top of the first piece. Draw another picture of the universe another second later and stack it on top of the other two. Repeat until you've drawn the entire future and past of the universe (warning: this may require infinite time).
...
This is the block universe.

You start with the assumption that the future is fixed now, and conclude that the future is fixed now.
Anyway, even if it was, the radioactive atoms still were free to decide when they want to decay, and the photons in double slit experiment were free to decide where they want to land, when the block universe was being built. These decisions might as well be happening only "now" as our consciousness reaches them.

 

[PAllen]

You start with the assumption that the future is fixed now, and conclude that the future is fixed now.
Anyway, even if it was, the radioactive atoms still were free to decide when they want to decay, and the photons in double slit experiment were free to decide where they want to land, when the block universe was being built. These decisions might as well be happening only "now" as our consciousness reaches them.

Ibix is not making any attempt to derive or justify the BU interpretation (over any other), he is just trying to describe it clearly.

 

[Ibix]

You start with the assumption that the future is fixed now, and conclude that the future is fixed now.

As @PAllen says, I'm not justifying the model, merely describing it and setting out why "now" doesn’t fit into it well. The whole point is that there isn't any reason to prefer one interpretation over another except convenience for any given application.

An analogous description of the Lorentz ether theory would be a tablet, possibly with a curved screen, displaying an animated picture of the universe at one time (a series of screen captures could be used to build up the block universe model as I described in the post you quoted). In this model there is a now, but it turns out to be undetectable. Perhaps the best way to explain that is to point out that there are an infinite number of possible tablets with the same shape of screen and, for each choice of screen shape, an infinite number of choices of origin and orientation. So "the" Lorentz ether theory is a bit of a misnomer - there are actually an infinite set of experimentally indistinguishable Lorentz ether theories. So "now" only means something if you adopt one particular Lorentz ether theory - which is a choice on top of choosing Lorentz ether theory, just as it is a choice on top of choosing the block universe.

These decisions might as well be happening only "now" as our consciousness reaches them.

It's worth noting that you don't reach the vast majority of the universe. You are only a tiny little worldline lost in the vastness of space (by any choice of space). So you have to ask: which "now"? And you can't answer by experiment, not even in principle, which is why you can choose either interpretation.

 

[Ebeb]

I am fine with that.

I start feeling better now.

I assume that in your usage "SR" is synonymous with the "Block Universe".

Not necessarily. In that quote I said <<... , both being a different interpretation of the Lorentz Transformations: LET interprets with ether, SR without.>>
In that quote, whether SR means BU is irrelevant. Yes I'm nitpicking here, but let me phrase it differently:
If you start f.ex. with different postulates for explaining phenomena -whether you use different mathematical formulae or not is irrelevant- , then you have a different theory, or not?
But If we get agreement calling SR and LET both "philosophical interpretation" of the LT, I'll settle with that. But it actually means from now on I don't have to call SR a "theory". I'll have to get used to that. ;-)

So I think you are saying the same thing I am, but using different words. I don't know that there is an official standard usage.

You say you "don't know"... I would appreciate you get back to me when you do know whether there is a difference between theory and philosophical interpretation. Just to make sure for future reference on PF discussions.

The explanation doesn't define a theory, the experimentally measurable predictions do.

Well, I'm not so sure about this. It's not what I learned at school.

Or, at a minimum, the explanation is not physical, which is what most interests me.

What interests me is whether they are theories or not. Because, it's important for usagage of specific words on PF forum. And understanding what that PF rule about theories exactly means. I think all participants would appreciate.

Back to BlockUniverse now.
Suppose one day we develop a set of specific BU mathematical formulae (other than LT), producing same experimental (prediction) results as SR or LET does. Would you call it a different theory than SR?

 

[Ebeb]

About 4D terminology.
I still have to sort out a lot more about terminology used -or allowed to use (?)- on PF.
I still don't have a clear view on how the term 4D object is used here.
4D object means we consider the object as one thing, a block. Block Universe (BU) is a 4D object.
Hence, when one would say that SR uses 4D objects/units -I agree-, but refute BU, then I'm lost. Completely lost.

I get the impression that on this forum "spacetime" is considered to be 4D, but not a 4D unit/object/block as a whole?
If spacetime is not 4D unit/object/block, then Newton 'space and time' is also called 4D.
"4D object", will then be used as -I agree- a block, hence BU is a block where past, present and future exist.
Can we stick to this?
I'll then read through the last pages of that other thread (Proper (and coordinate) times re the Twin paradox) and see whether it helps me to understand better the post content.

You have to understand that if you now would get back to me telling there are different 'interpretations' of what 4D means, -so be it- then I need specific guidelines how to differentiate between them, so that all participants understand what is being said in the posts. Because It would be sad such a "miscommunication" issue -and I guess that happens a lot on fora- would be interpreted as not following the PF rules. That's also why I wanted to know whether PF rules considers LET being a theory or not (see above).

 

[SlowThinker]

It's worth noting that you don't reach the vast majority of the universe. You are only a tiny little worldline lost in the vastness of space (by any choice of space). So you have to ask: which "now"?

The events could be happening once their past light cone has been built. I don't think you need some global definition of "now" for that.
Is this what you were trying to picture with those curved tablet screens?

 

[Ibix]

The events could be happening once their past light cone has been built. I don't think you need some global definition of "now" for that.
Is this what you were trying to picture with those curved tablet screens?

I've no idea what you mean here. Lorentz ether theory means that there is a global now, but it's undetectable. It can certainly be any now corresponding to a global inertial frame, but I don't believe it has to be an inertial frame. A non-inertial spatial slice is curved, but it's still a "global now".

 

[Boing3000]

Draw a picture of the universe as it is now on a piece of paper.

It is by far the best way of picturing things, but to fix the misconception about the future being fixed, I prefer to see it the following way:

The stack of "now" should be pictured as a pyramid/cone whose tip/circumference will be imprinted by a local 3D/picture snapshot of the universe (actually the light signals there and then). Then cone grows with time because each circumference 'slice' is your past whose own volume growth at light speed to meet other past.

I may have missed it on this thread, but isn't a 4D map of spacetime entirely frame dependent ? Only those portion of different past cones that intersects (causally linked) will be reconciled by special relativity.

 

[jbriggs444]

I may have missed it on this thread, but isn't a 4D map of spacetime entirely frame dependent ? Only those portion of different past cones that intersects (causally linked) will be reconciled by special relativity.

4D spacetime is entirely frame independent. Special relativity only allows causal influences within light cones. Light cones are not frame dependent.

The coordinate chart(s) that you lay onto the 4D spacetime can vary. And in this sense, yes, a 4D "map" is frame dependent. But none of the physics depends on the map that is chosen. All the physics is frame independent.

 

[PeterDonis]

If you start f.ex. with different postulates for explaining phenomena -whether you use different mathematical formulae or not is irrelevant- , then you have a different theory, or not?

This is a question about words, not about physics.

I get the impression that on this forum "spacetime" is considered to be 4D, but not a 4D unit/object/block as a whole?

The term "spacetime" generally refers to a model using a 4D locally Lorentzian manifold. The model might cover the entire history of a universe, past and future, or it might not--it might only cover a local region (for example, near the Earth during an experiment that has already been run). It depends on the specific scenario being considered.

it's important for usagage of specific words on PF forum.

What's important isn't words, it's physics. As long as you are clear about what physical model your words refer to, that's fine. But you should not be getting hung up on words; the physical models and the predictions they make are what's important.

I need specific guidelines how to differentiate between them

Why? They all use the same model--the same mathematical machinery--and make the same predictions. The model and the predictions are what's important. See above.

That's also why I wanted to know whether PF rules considers LET being a theory or not (see above).

It depends on what you mean by the word "theory". But once again, don't get hung up on words. What we're trying to do here at PF is to focus on the physics--models and the predictions they make. If you find yourself wanting to talk about two thingies that seem different to you, but both use the same mathematical model and make the same predictions, then you should take a step back and reconsider, because whatever differences you see are not differences in the physics.

 

[Boing3000]

4D spacetime is entirely frame independent. Special relativity only allows causal influences within light cones. Light cones are not frame dependent.

The coordinate chart(s) that you lay onto the 4D spacetime can vary. And in this sense, yes, a 4D "map" is frame dependent. But none of the physics depends on the map that is chosen. All the physics is frame independent.

I clearly specified map in my first sentence. Collection of events may be another. The problem is those map are collections of quadruplet of numbers, and all those number are entirely frame dependent.

I never implied the universe is frame depended. I implied clearly that map are frame depended, and the only thing that special relativity allows you is to compare those patch of DIFFERENT map/block that intersects. Only then you always obverse that those map represent the same exact "fix/block" events (once both numbers have been transformed using SR formulas).

I find this way of picturing things much more sane than to speak of future light cone that exist nowhere in any frame of reference. There is a past block universe, only accessible trough patch of individual frame dependent block map.

The "absolute" 4D spacetime (if such a thing exist) is nowhere to be observed directly. But I do agree it is independent. It is so independent that there are patch of it/them that are not even connected (if the expansion of the universe keeps accelerating).