Insights The Block Universe - Refuting a Common Argument - Comments

[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.

 

[PAllen]

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

Observer dependent does not mean subjective. The latter implies it depends on state of mind (e.g. QM interpretations that attribute a key role to consciousness). Observer dependent simply means that the scope of what is fixed depends on the event of observation or measurement - emphasis on event, not state of mind.

Turning it around, suggesting that something unobservable and inherently unverifiable (BU and reality of simultaneity surface) must be considered real seems seems inherently subjective to me. If it is in principle not subject to verification, it is subjective.

 

[PeterDonis]

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

I'm not. I'm redefining it as "dependent on which event you pick." Events are objective; they are points in spacetime, and all observables at an event are invariants. There's no subjectivity.

 

[zonde]

I'm not. I'm redefining it as "dependent on which event you pick." Events are objective; they are points in spacetime, and all observables at an event are invariants. There's no subjectivity.

I don't know. I still think your argument is flawed. You take philosophical statement and provide an argument against it. There is generally recognized philosophical idea of "realism" that is alternative to "solipsism". If you want to provide valid argument in this context you have to stick to proper meanings of philosophical terms (in particular meaning of "real"). And for anybody reading the article there is no reason to expect that you would use "real" with some different meaning.

Maybe you can provide valid argument based on your idea but you have not done this in the article IMO.

 

[Ilja]

Ilja, does Presentism apply to non-local events?

Hm. Presentism means that what exists, exists now. The future does not yet exist, the past is past, and no longer exists, what exists, exists now. What exists is, of course, something global - the whole world. How this world is structured is an independent question.

 

[Ilja]

In what sense? Is the "present" just one event (whichever event I am at "now"), or is it more than that?

What is present is the whole world. A preferred space-like hypersurface of spacetime, the one which exists, while the other parts of the spacetime exist only in memories and hopes, but not actually in reality.

 

[Smattering]

There is generally recognized philosophical idea of "realism" that is alternative to "solipsism".

But this discussion is not about realism vs. solipsism, but rather about presentism vs. eternalism.

In particular, the growing block universe interpretation does not conflict with realism. The events just have to grow in a particular order such that the past light cone of every currently growing event has already grown before. But this does not imply that only my own past light cone is real. It's rather the other way around: My past light cone must exist or at least must have existed as a prerequisite for my own existence.

 

[PWiz]

Just curious - is there an argument which combines the Block Universe interpretation with the Many-Worlds interpretation? (I don't know if this should be in a separate thread or not. If any mentor feels it should be somewhere else, feel free to move it.)

 

[Smattering]

Just curious - is there an argument which combines the Block Universe interpretation with the Many-Worlds interpretation? (I don't know if this should be in a separate thread or not. If any mentor feels it should be somewhere else, feel free to move it.)

From my point of view, believing in the many worlds interpretation of QM would be an argument against the block universe - at least against a "classical" block universe that really resembles some kind of block. After all, an eternal many worlds universe would have a tree-like structure.

 

[rjbeery]

I have a general question for PeterDonis, but anyone else is welcome to give their opinion: do you think a "certain past" but an "uncertain future" implies time-asymmetry? It's difficult for me to accept the idea that the rules of Physics differ depending where in spacetime an observer is located.

 

[Smattering]

I have a general question for PeterDonis, but anyone else is welcome to give their opinion: do you think a "certain past" but an "uncertain future" implies time-asymmetry? It's difficult for me to accept the idea that the rules of Physics differ depending where in spacetime an observer is located.

Why do you think that a certain past and an uncertain future imply different physical laws for different observers?

 

[PAllen]

I have a general question for PeterDonis, but anyone else is welcome to give their opinion: do you think a "certain past" but an "uncertain future" implies time-asymmetry? It's difficult for me to accept the idea that the rules of Physics differ depending where in spacetime an observer is located.

All fundamental classical physical laws have time symmetry (Standard Model does not, even at the fundamental level). However, emergent laws, e.g. thermodynamics are time asymmetric, leading to a common view (no idea of percentages) that the universe has an objective time arrow, and that evolution is viable interpretation (for geometric theories, it is treated under the umbrella of EBU - evolving block universe; George F.R. Ellis of "Hawking and Ellis" fame has written a series of papers and essays motivating this point of view.)

 

[rjbeery]

Why do you think that a certain past and an uncertain future imply different physical laws for different observers?

Because we can follow a causal string of events in both directions, right? "Unfrying an egg" is practically impossible but not unphysical; we could record an egg being cooked and reverse the video: there is no uncertainty as we unfry it, no physical laws are being broken, no stochastic processes are coming into play, and there are no options other than the resulting uncooked egg...so why do we believe that things must be different when they reside in our future light cones? Before QM came along there was no reason to question a clockwork universe, and I get the feeling that some people believe free will has been "saved" by the apparent statistical nature of QM.

Basically I'm trying to identify the resistance to a Block Universe.

 

[PAllen]

Because we can follow a causal string of events in both directions, right? "Unfrying an egg" is practically impossible but not unphysical; we could record an egg being cooked and reverse the video: there is no uncertainty as we unfry it, no physical laws are being broken, no stochastic processes are coming into play, and there are no options other than the resulting uncooked egg...so why do we believe that things must be different when they reside in our future light cones? Before QM came along there was no reason to question a clockwork universe, and I get the feeling that some people believe free will has been "saved" by the apparent statistical nature of QM.

Basically I'm trying to identify the resistance to a Block Universe.

I don't believe there is resistance. What I see here and elsewhere is that one side says BU is not a required interpretation, but a personal philosophic choice; while BU proponents argue that it is strongly preferred and engage in tendentious critiques of other possiblities.

[edit: note, that in classical physics, there was already the mystery of why all thermodynamic processes, everywhere/when seem to run in the same direction. This already suggested to many physicists an objective arrow of time, with an objective asymmetry for the universe as a whole]

 

[rjbeery]

I don't believe there is resistance. What I see here and elsewhere is that one side says BU is not a required interpretation, but a personal philosophic choice; while BU proponents argue that it is strongly preferred and engage in tendentious critiques of other possiblities.

Fair point. I'd like to point out that in this very thread there are some Presentists (which is basically incompatible with SR), so the only reason I might ever argue that BU is strongly preferred is because (in my experience) people have not fully grasped the consequences of Relativity.

 

[PAllen]

Fair point. I'd like to point out that in this very thread there are some Presentists (which is basically incompatible with SR), so the only reason I might ever argue that BU is strongly preferred is because (in my experience) people have not fully grasped the consequences of Relativity.

Presentism is not incompatible with SR. Preferred frame LET is an interpretation of SR that cannot be ruled out by any experiment or observation, and it trivially supports strict presentism. This is yet another example of insisting that personal choice is a requirement. Note that, aesthetically, I find preferred frame repugnant, but I would never argue that it is an invalid point of view.