Insights The Block Universe - Refuting a Common Argument - Comments

[PeterDonis]

Then I go back to requesting your definition of a Block Universe, because *mine* would basically be a 4D unchanging block of spacetime.

Sure, that will work.

If you accept that a certain past is applicable to a given event, bounded by a past light cone, and that no event has any special physicality (including those in the future) then all events are certain -- period.

Now you are assuming that "certain" is absolute, not relative. If "certain" is relative--in other words, if which events are "certain" is different for different events--then your argument here is not valid.

I'm wondering if you even read my article, since it discusses exactly this point.

 

[PAllen]

Hi Paul:

I don't mean to be flippant, but I would answer your question as follows. If the definition of a concept, like "the present", depends on a context, then one should use the definition appropriate to the context that is relevant to the usage. I get that the fact that there are different definitions of "present" means that in an absolute sense, there is no absolute "present".

Here is an example. Imagine two people, A and B, who are the last surviving members of a tontine, are living on different planets, say A in on Earth, and B is on a planet revolving around Proxima Centauri. Both A and B die, and the trustees of the tontine need to know who has died last, so the tontine assets can be given to the appropriate estate. It may well be that this issue cannot be resolved by "building as large as possible 'near inertial frame'". However, the CBR clock might be used for this context.

Regards,
Buzz

But that all supports my point. Picking a convention for different purposes is useful. But as soon as you recognize that, it becomes difficult to accept the reality of an objective present. Note, that this is all orthogonal to whether or not BU is true. Disbelieving BU does not require a belief in a flowing present.

 

[PeterDonis]

I prefer presentism

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

 

[PeterDonis]

The Doppler of CBR measurements effect for each of the two observers would be different

No, it wouldn't; at least, not if both observers are "comoving". Two comoving observers at different spatial locations in the universe will both see no variation in the CMB redshift with direction--it will look perfectly isotropic to both of them. But they are not at rest relative to each other.

 

[RUTA]

I'm always surprised by the amount of energy people invest in arguing for and against blockworld (BW). As Peter points out, it follows from relativity of simultaneity + no preferred frame. If you believe there is a preferred frame, it doesn't follow. If you deny relativity of simultaneity (as Peter does by denying simultaneity at all, for example), then it doesn't follow. Of course, that doesn't mean it's *not* a BW either. It could still be a BW even if you deny the premises of this argument. Newtonian spacetime with absolute simultaneity could be a BW. What difference does it make to your physics? That's the meaningful question for physicists.

 

[PeterDonis]

As Peter points out, it follows from relativity of simultaneity + no preferred frame.

No, it doesn't. It follows from relativity of simultaneity + no preferred frame + all events to the past of any observer's surface of simultaneity are fixed and certain. In the article, I used "relativity of simultaneity" to mean what you are calling "relativity of simultaneity + no preferred frame". But the article makes clear that that by itself is not sufficient; you also need the additional premise I just gave.

If you deny relativity of simultaneity (as Peter does by denying simultaneity at all, for example)

I have done no such thing. Saying that events to the past of a given observer's surface of simultaneity, but not in that observer's past light cone, are not fixed and certain, is not at all the same as "denying simultaneity". Unless, of course, when you say "simultaneity" you are implicitly smuggling in the additional premise I referred to above. But that additional premise is there, whether you want to admit it or not.

What difference does it make to your physics?

None, as far as I'm concerned. But you posted 5 articles about "blockworld", which would seem to indicate that it does make a difference to you.

 

[Buzz Bloom]

No, it wouldn't; at least, not if both observers are "comoving". Two comoving observers at different spatial locations in the universe will both see no variation in the CMB redshift with direction--it will look perfectly isotropic to both of them. But they are not at rest relative to each other.

Hi Peter:

Thanks for correcting my oversight. When I responded to Smattering's reply

For example because two distant observers are not at rest to each other when each of them observes isotropic CMB. Instead, they are moving away from each other.​

to my question

I am seeking a physical explanation of: Why the average temperature of the CBR in all directions can not be used as a common clock that establishes a basis for absolute simultaneity?​

I thought his point was that the Doppler would be in general different when "observers are not at rest to each other". I has overlooked that the Doppler effect was not always present, as you pointed out w/r/t co-moving observers.

Regards,
Buzz

 

[Buzz Bloom]

Picking a convention for different purposes is useful. But as soon as you recognize that, it becomes difficult to accept the reality of an objective present.

Hi Paul:

I guess I am mostly indifferent to the difficulty regarding the objectivity of "present". I am a pragmatist at heart. I particularly liked Peter's response

None, as far as I'm concerned.​

to RUTA:

What difference does it make to your physics?​

Regards,
Buzz

 

[rjbeery]

Sure, that will work.
Now you are assuming that "certain" is absolute, not relative. If "certain" is relative--in other words, if which events are "certain" is different for different events--then your argument here is not valid.

I'm wondering if you even read my article, since it discusses exactly this point.

Then I don't appreciate the subtlety here. There are three options that I see:

A) A Block Universe in which all events share a physical representation on equal grounds in an unchanging object of reality
B) A "growing" Block Universe in which all certain events share a physical representation, bounded by "now" which bifurcates the past from the future
C) Some form of Presentism in which only only "now" has a physical representation.

I'm having a problem understanding your stance. You mentioned misinterpreting received data in the article, but I don't see why that has any bearing on physical reality - we shouldn't be worried about perceived reality when we are discussing reality itself. Do you believe there's a physical difference between the past, the now, and the future for a given event?

 

[Smattering]

A) A Block Universe in which all events share a physical representation on equal grounds in an unchanging object of reality
B) A "growing" Block Universe in which all certain events share a physical representation, bounded by "now" which bifurcates the past from the future
C) Some form of Presentism in which only only "now" has a physical representation.

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.

 

[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.