The Block Universe - Refuting a Common Argument - Comments

[rjbeery]

But as far as I understand, Peter did not aim to refute any of these possibilities. What he refutes is a specific argument in favor of A and against B and C.

Perhaps I was confusing Peter's criticism of a specific argument with having a counter argument for another position, but I can't seem to actually find his stance on the subject. If A, B and C are the only possibilities considered, and Relativity is problematic for B and C, then I don't see where else there is to go. Perhaps Peter is claiming

D) Due to Relativity we can make no non-local claims about Reality.

To me, that's the only other alternative, either everything exists or nothing does. ;)

 

[Smattering]

If A, B and C are the only possibilities considered, and Relativity is problematic for B and C, then I don't see where else there is to go.

Did he actually agree that relativity is problematic for B and C?

Anyway, I am still struggeling to understand why relativity should be problematic to either B or C. Maybe you can explain this to me.

 

[rjbeery]

Did he actually agree that relativity is problematic for B and C?

Anyway, I am still struggeling to understand why relativity should be problematic to either B or C. Maybe you can explain this to me.

They both share the same problem which is the "3D world" that Peter mentions. Different observers in relative motion, even in close proximity, would be assigning distant time-like separated events as having this physical attribute of "now". If "now" is to have any sort of non-local special physical manifestation and we can show that it applies to time-like separated events then it equally applies to ALL time-like separated events.

 

[PAllen]

Perhaps I was confusing Peter's criticism of a specific argument with having a counter argument for another position, but I can't seem to actually find his stance on the subject. If A, B and C are the only possibilities considered, and Relativity is problematic for B and C, then I don't see where else there is to go. Perhaps Peter is claiming

D) Due to Relativity we can make no non-local claims about Reality.

To me, that's the only other alternative, either everything exists or nothing does. ;)

Why do you say that? How about the alternative presented in the article: reality is a meaningless term. However, the division between what is certain and what is not is the past light cone, and you just have to accept that this is different at different events. There is no global notion of what is certain, but what is certain is much more than local.

"Either everything exists or nothing does" simply rejects, on grounds of your philosophy, numerous other alternative.

 

[rjbeery]

Why do you say that? How about the alternative presented in the article: reality is a meaningless term. However, the division between what is certain and what is not is the past light cone, and you just have to accept that this is different at different events. There is no global notion of what is certain, but what is certain is much more than local.

"Either everything exists or nothing does" simply rejects, on grounds of your philosophy, numerous other alternative.

Perhaps I don't understand what is being claimed if we say that reality is a meaningless term. Does non-local reality have a physical representation or not? What does existence mean? Does the past have a physicality which is different from the future? What about the local "now"?

Kind of seems like we need more definitive statements. Simply accepting a Block Universe is an extremely definitive statement, btw, and in my opinion attempts to refute it are rooted in fear of Fatalism. Even the name FATALISM is scary, like adherents to it are busy killing off Hope, Free Will and The Human Spirit.

 

[Smattering]

They both share the same problem which is the "3D world" that Peter mentions. Different observers in relative motion, even in close proximity, would be assigning distant time-like separated events as having this physical attribute of "now".

O.k., until now I can follow, although it is unclear how do you define "close proximity", and by how much time these events would be separated depending on he observers' relative velocity.

If "now" is to have any sort of non-local special physical manifestation and we can show that it applies to time-like separated events then it equally applies to ALL time-like separated events.

Why do you insist on the non-locality?

Are you familiar with dynamic programming? For example, look at how the Floyd–Warshall algorithm fills up its result matrix. There is absolute no need for absolute simultaneity in order to allow the universe to grow successively. The universe can just grow anywhere as long as the past light cone of the growing event already exists. At least in this growing case I cannot see any issues at all.

 

[PAllen]

Perhaps I don't understand what is being claimed if we say that reality is a meaningless term. Does non-local reality have a physical representation or not? What does existence mean? Does the past have a physicality which is different from the future? What about the local "now"?

Kind of seems like we need more definitive statements. Simply accepting a Block Universe is an extremely definitive statement, btw, and in my opinion attempts to refute it are rooted in fear of Fatalism. Even the name FATALISM is scary, like adherents to it are busy killing off Hope, Free Will and The Human Spirit.

No one is arguing BU doesn't work or isn't simple, or is wrong. The only argument, and IMO, comes only from "BU only" proponents, is that either BU or correct or "wild false caricature of any alternative".

[As to the difficulties with reality - start debating this and you get into pure philosophy conundrums - how do we know we are not the dream of an intelligent machine?]

 

[PeterDonis]

There are three options that I see:

They don't exhaust the possibilities; none of them captures the alternative I described in the article, that events in the past light cone of a given event are "certain" relative to that event.

Perhaps Peter is claiming

D) Due to Relativity we can make no non-local claims about Reality.

If events in your past light cone are "real", then that seems to me to be a "non-local claim about Reality". It's just a different "non-local claim" than the one you were considering.

What does existence mean?

Well, you're the one who seems concerned about the term, so what do you think it means? Can you give "existence" a rigorous definition?

Kind of seems like we need more definitive statements. Simply accepting a Block Universe is an extremely definitive statement,

So is "which events are certain depends on which event's past light cone you are considering". It's just a different definitive statement.

 

[Smattering]

Well, you're the one who seems concerned about the term, so what do you think it means? Can you give "existence" a rigorous definition?

For the "growing" block universe to be possible, it should be entirely sufficient if there is some kind of ordering such that the past light cone of every currently "growing" event has already "grown" before. I do not understand why that thing would require absolute simultaneity to be able to "grow".

 

[PeterDonis]

it should be entirely sufficient if there is some kind of ordering such that the past light cone of every currently "growing" event has already "grown" before.

I agree that this is sufficient for a "growing" interpretation, and such an ordering certainly exists; the time ordering of any pair of timelike or null separated events is invariant, and the past light cone of any event consists entirely of events that are timelike or null separated from it and are "earlier" according to the invariant ordering of such event pairs.

 

[rjbeery]

They don't exhaust the possibilities; none of them captures the alternative I described in the article, that events in the past light cone of a given event are "certain" relative to that event.
If events in your past light cone are "real", then that seems to me to be a "non-local claim about Reality". It's just a different "non-local claim" than the one you were considering.
Well, you're the one who seems concerned about the term, so what do you think it means? Can you give "existence" a rigorous definition?
So is "which events are certain depends on which event's past light cone you are considering". It's just a different definitive statement.

I see, so for a given event all of reality is encapsulated in its past light cone; that doesn't consider new experiences though. How do I account for all new events continuously arriving in my growing light cone without postulating that there is something else beyond my experiences? From my perspective, reality would be apparently growing, randomly, from nothing. This seems incomplete at best.

 

[meviccar]

hypothetically, you could prove that the present (surface of simultaneity/3D world) could be directly observed

You can't; that contradicts the laws of SR, which say that information can't travel faster than light. It's pointless to make hypotheticals that contradict the laws of physics. (Also, the "block universe" interpretation explicitly says it accepts the laws of SR, so any hypothesis that contradicts the laws of SR also contradicts the "block universe" interpretation anyway.)

Peter,

Instead of a hypothetical, let me ask: is it unreasonable to say that the very language of 'observers' and 'past light cones' necessarily implies a 'surface of simultaneity'? By saying that it takes time for information regarding an event to get to an observer, we are necessarily stating that the observer of event A, is actually at event B. Event B, in this case, is the present, and though it is not being directly observed, its existence is implied simply because event A exists and is being observed.

I do agree with your main thesis, that technically, one is not justified in saying that a block universe is *necessarily* implied by SR, but I'm playing devil's advocate here because it is not yet clear to me why the block universe interpretation is not, far and away, the most reasonable.

Pardon my thickness if all this seems trivial.

 

[PeterDonis]

so for a given event all of reality is encapsulated in its past light cone

No, what is "certain" at a given event is what is in its past light cone. I am not claiming that "reality" is the same as "what is certain". Only you are.

that doesn't consider new experiences though.

New experiences mean the given event has changed, which means the past light cone has changed.

How do I account for all new events continuously arriving in my growing light cone without postulating that there is something else beyond my experiences?

By not equating "reality" with "what is certain".

 

[PeterDonis]

is it unreasonable to say that the very language of 'observers' and 'past light cones' necessarily implies a 'surface of simultaneity'?

It depends on what you think is "necessarily implied". See below.

By saying that it takes time for information regarding an event to get to an observer, we are necessarily stating that the observer of event A, is actually at event B.

Yes.

Event B, in this case, is the present

Not if "the present" means an entire surface of simultaneity. Event B is just one event--just one point in spacetime. If I am at Event B, observing Event A, which must be in my past light cone at Event B, that says nothing about any other events outside my past light cone at Event B--which includes all other events in any hypothetical "surface of simultaneity" containing Event B.

 

[rjbeery]

No, what is "certain" at a given event is what is in its past light cone. I am not claiming that "reality" is the same as "what is certain". Only you are.
New experiences mean the given event has changed, which means the past light cone has changed.
By not equating "reality" with "what is certain".

So past events are certain events, but beyond that you aren't really saying anything about reality at all. Is the absence of a stance a stance in itself?

 

[PeterDonis]

So past events are certain events, but beyond that you aren't really saying anything about reality at all.

Do you have a rigorous definition of "reality"?

Is the absence of a stance a stance in itself?

Why is a stance on "reality" required for a scientific theory? Or, to keep focused more on the topic of this thread, why is a stance on "reality" necessary based on the postulates of SR?

 

[rjbeery]

Do you have a rigorous definition of "reality"?

Sure: A state or quality having a physical structure associated with it.

Why is a stance on "reality" required for a scientific theory? Or, to keep focused more on the topic of this thread, why is a stance on "reality" necessary based on the postulates of SR?

Because this is Physics Forums, and reality is the subject at hand, right? As I said, if an observer restricts himself to a reality that lies in his past light cone then SR is demonstrably incomplete. Actually, I'm not even sure it's utilizable at all -- what predictions can SR give that don't necessitate the presumption of space-like separated entities?

 

[nikkkom]

Haha, someone needs to throw a quantum-mechanically entangled wrench into this discussion to make it even more convoluted :)

 

[meviccar]

is it unreasonable to say that the very language of 'observers' and 'past light cones' necessarily implies a 'surface of simultaneity'?

It depends on what you think is "necessarily implied". See below.

By saying that it takes time for information regarding an event to get to an observer, we are necessarily stating that the observer of event A, is actually at event B.

Yes.

Event B, in this case, is the present

Not if "the present" means an entire surface of simultaneity. Event B is just one event--just one point in spacetime. If I am at Event B, observing Event A, which must be in my past light cone at Event B, that says nothing about any other events outside my past light cone at Event B--which includes all other events in any hypothetical "surface of simultaneity" containing Event B.

I understand the surface of simultaneity as being *just* Event B. Either you are conflating 'plane of simultaneity' with 'surface of simultaneity', or they are the same thing in jargon-world and I'm inventing things for myself. At any rate, I think my point still stands: the act of being an observer is an observation that exists outside of one's past light cone, and so your alternative premise does not account for all observations. Yes, this is much too philosophical to be considered to be a rigorous definition, and so again, I do agree with your general point. I just want to stress that your arguments are simply establishing that it is still possible remain silent on the issue. The argument is simply moving BU from being "the only possible interpretation", to, "the only possible interpretation unless you want to remain silent." You are completely justified in doing that, but pop-science people are equally as justified when they get up on the tele to say, 'our best understanding results in BU.'

 

[Smattering]

Haha, someone needs to throw a quantum-mechanically entangled wrench into this discussion to make it even more convoluted :)

Actually I have been trying to fit many worlds into the block universe for quite some time now.

 

[Smattering]

Because this is Physics Forums, and reality is the subject at hand, right?

Not necessarily. Some would rather say that physics is just about the description and prediction of observations.

 

[PeterDonis]

I understand the surface of simultaneity as being *just* Event B.

An event is a point, not a surface. A surface of simultaneity at Event B is an entire 3-D spacelike surface containing Event B.

Either you are conflating 'plane of simultaneity' with 'surface of simultaneity', or they are the same thing in jargon-world

They are the same thing.

the act of being an observer is an observation that exists outside of one's past light cone

What does this even mean?

this is much too philosophical to be considered to be a rigorous definition

Definitely. Which means it's off topic here.

your arguments are simply establishing that it is still possible remain silent on the issue

Remain silent on what issue?

pop-science people are equally as justified when they get up on the tele to say, 'our best understanding results in BU.'

I disagree. Saying that "our best understanding results in BU" is saying that BU is somehow logically required by SR. It isn't. "Our best understanding" is not somebody's preferred interpretation; it's what's actually been confirmed by experiments. Until somebody can devise an experiment that distinguishes the BU interpretation from alternative interpretations, none of them are "our best understanding".

 

[rjbeery]

I just want to stress that your arguments are simply establishing that it is still possible remain silent on the issue. The argument is simply moving BU from being "the only possible interpretation", to, "the only possible interpretation unless you want to remain silent." You are completely justified in doing that, but pop-science people are equally as justified when they get up on the tele to say, 'our best understanding results in BU.'

Very well summarized.

 

[PeterDonis]

A state or quality having a physical structure associated with it.

Evidently we have different definitions of "rigorous".

this is Physics Forums, and reality is the subject at hand, right?

No. PF is for discussing mainstream science, not philosophy. Unless you can tell me how to determine, experimentally, what is "real" and what is not, "reality" is a matter of philosophy, not science.

what predictions can SR give that don't necessitate the presumption of space-like separated entities?

Predictions are not "reality". Predictions are predictions. I discussed the difference in the article; once again, you're making me wonder if you read it.

 

[rjbeery]

Evidently we have different definitions of "rigorous".
No. PF is for discussing mainstream science, not philosophy. Unless you can tell me how to determine, experimentally, what is "real" and what is not, "reality" is a matter of philosophy, not science.
Predictions are not "reality". Predictions are predictions. I discussed the difference in the article; once again, you're making me wonder if you read it.

Predictions of what? Observations of what? Physics is the study of what exactly? You're being purposely non-committal and vague for unknown reasons.

 

[PeterDonis]

You're being purposely non-committal and vague for unknown reasons.

No, I'm pointing out that you are throwing around terms that don't have rigorous definitions.

If you want my personal view, it is that we don't know "what reality is" in any scientific sense. We have theories that allow us to make predictions of the form "if we do X, we will observe Y", which are confirmed by experiments--when we do X, we do indeed observe Y. That's what our knowledge of physics consists of. Claims that some theory of physics means "reality" must be a certain way are not scientific claims, because there's no way to test them; they're someone's personal opinion.

For purposes of this thread, though, neither my personal view nor your personal view are relevant. The topic of discussion is the specific argument that I refuted in the article.

 

[rjbeery]

No, I'm pointing out that you are throwing around terms that don't have rigorous definitions.

If you want my personal view, it is that we don't know "what reality is" in any scientific sense. We have theories that allow us to make predictions of the form "if we do X, we will observe Y", which are confirmed by experiments--when we do X, we do indeed observe Y. That's what our knowledge of physics consists of. Claims that some theory of physics means "reality" must be a certain way are not scientific claims, because there's no way to test them; they're someone's personal opinion.

For purposes of this thread, though, neither my personal view nor your personal view are relevant. The topic of discussion is the specific argument that I refuted in the article.

Fine, if I can't get you to commit to anything, define anything, or even voice your opinion on what is lacking in my attempts at a definition of "reality", then I'll stick to your article. Specifically, your problem with what you refer to as the "added premise" that a fixed and certain past should apply to *all* events leads us to this conclusion:

In the Andromeda paradox, for example, we could run the argument from the Andromedan’s perspective: two Andromedans passing each other on the street will have 3D worlds passing through events on Earth’s worldline that may be separated by years. By the above argument, all events on Earth’s worldline that are spacelike separated from the chosen event on Andromeda’s worldline must be fixed and certain. But, since those events include events in the future light cone of the event we originally chose on Earth (where two people passing on the street disagree on whether the Andromedan space fleet has been launched), we can see that extending the argument to any event forces us to conclude that all of 4D spacetime, including our causal future as well as the region spacelike separated from us, is fixed and certain.

(Bolding is mine)
That's it. So your refutation of this added premise is that it would require a Block Universe because it would force us to conclude 4D spacetime...you cannot do this. You can't reject a premise simply because it forces you to a conclusion. You can certainly reject a premise on other grounds, but your article gives no such grounds. If you think that having a fixed and certain causal future should force us to reject an otherwise reasonable premise then you are guilty of doing the very thing that you later accuse Block Universe proponents of doing:

But, as the above shows, simply assuming the second premise is tantamount to assuming the conclusion! ... In other words, if your argument for the block universe basically consists of helping yourself to the second premise, you’ve avoided the real issue.

This is all philosophy, of course, but we should choose our premises for reasons other than where they lead us, unless that is to a logical contradiction.

 

[PeterDonis]

(Bolding is mine)

Are you saying that what you bolded is what I called the "added premise"? It isn't. The "added premise" is this: "all observers’ 3D worlds are real at every event". The text you quoted is just describing how relativity of simultaneity plus that added premise implies the "block universe" as a conclusion.

your refutation of this added premise is that it would require a Block Universe because it would force us to conclude 4D spacetime

I'm not refuting the added premise; I'm just pointing out that it's an added premise, and that added premise is not logically required by the postulates of SR. Therefore, the argument that the "block universe" interpretation is logically required by SR is not valid, because it depends on an added premise that is not logically required by SR.

 

[SlowThinker]

You can't reject a premise simply because it forces you to a conclusion.
...
This is all philosophy, of course, but we should choose our premises for reasons other than where they lead us, unless that is to a logical contradiction.

The same can be said about BU proponents. They are accepting the premise for the sole reason of liking where it leads them.

 

[SlowThinker]

I still don't quite understand this discussion.
An event can only be affected by events in its past light cone, and can only affect events in its future light cone.
Where does the "reality" and "fixation" come into play?

 

[rjbeery]

I'm not refuting the added premise; I'm just pointing out that it's an added premise, and that added premise is not logically required by the postulates of SR. Therefore, the argument that the "block universe" interpretation is logically required by SR is not valid, because it depends on an added premise that is not logically required by SR.

I suppose we could point to time symmetry to appeal to a fixed and certain past and future, but neither of those concepts are logically required by the postulates of SR either.

 

[nikkkom]

There is an argument against BU, one flavor of it, which says that "present" is just a quirk of how we, humans, perceive the physical world. That we think that future doesn't exist merely because we don't feel it.

The counter-argument is that human/animal perception mechanism was not created randomly. Actually, it was under immense selection pressure to evolve to give as accurate predictions about future as possible. You slip from a tree, you need to realize that you can die, and need to grab a branch to save yourself. You need to throw a rock to kill your prey and not die of starvation, for this you need to predict how rocks fly.

If future "already exists" (whatever that means), it is definitely not easily predictable from the current event. Otherwise animals would evolve mechanisms to use "already existing" information from the future.

 

[zonde]

There is an error in the article:

(3) All events in the past light cone of a given event are real (i.e., fixed and certain) for an observer at that event.

Things can't be real for an observer. This is self contradictory statement. Things are either real for all observers or subjective by definition (as much as we have got definitions in philosophy).

 

[PeterDonis]

Things can't be real for an observer. This is self contradictory statement.

Only on a particular definition of "real". But the whole point of the article is that you can have different definitions.

Also, you're ignoring the parenthesis right after "real", which makes it clear that I'm defining it as "fixed and certain". So I'm not talking about a philosophical definition of "real".

 

[zonde]

So I'm not talking about a philosophical definition of "real".

You are providing alternative to "solipsism". You are not free to redefine "real" as "subjective" in this context.