The Block Universe - Refuting a Common Argument - Comments

[Smattering]

Do you mean in this thread, or in the article?

I was referring to this post of yours:

https://www.physicsforums.com/threa...-argument-comments.843000/page-2#post-5289354

What I do not understand: When those third-party observations are part of my past light cone, then the observers should have been able to report their observations to me, shouldn't they? I thought being part of my past light cone means exactly this: Being able to send me a light signal.

 

[maline]

I'd just like to point out that Roger Penrose is absolutely not a Blockworld proponent. He does believe strongly in realism, and that the question of what is "fixed" is meaningful. The Andromeda argument was made to raise the question, to refute classical presentism, and in particular, to show that treating the EPR nonlocality as an influence that propagates faster than light (but forward in time) is inconsistent.

 

[PAllen]

Peter,

If, hypothetically, you could prove that the present (surface of simultaneity/3D world) could be directly observed, wouldn't that negate your claim that your alternative premise 'accounts for all observations'?

I just can't shake the feeling that you're argument basically hinges on the technicality that laws of physics prevent one from proving that the present exists. Sure, reality is unintuitive, but that feels like a stretch. And yes, I recognize the irony in my statement.

In GR, there is no well defined notion of present at all. How do you account for that? Of course, it is easy and useful to use a BU interpretation with GR, but the motivating argument of 'present must be real' breaks down if present can't even be defined.

 

[PeterDonis]

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.)

 

[PeterDonis]

Roger Penrose is absolutely not a Blockworld proponent.

It's been a while since I actually went back and looked at The Emperor's New Mind, but my memory is that, at least in the particular argument quoted in the article, Penrose was taking the "blockworld" position. I definitely don't remember him pointing out in the book the logical flaw that I discuss in the article. It seems to me that discussing that argument without mentioning the logical flaw is leaving out something crucial.

 

[PeterDonis]

When those third-party observations are part of my past light cone, then the observers should have been able to report their observations to me, shouldn't they?

Yes, of course. I said the same thing in the article. I still don't understand what the problem is.

 

[Smattering]

Yes, of course. I said the same thing in the article. I still don't understand what the problem is.

So what were you referring to when you reminded me of the fact that information cannot be transmitted faster than light? Did you try to point out to me that the reports from the third-party observers will not arrive at me before I can make the observation myself?

 

[PeterDonis]

what were you referring to when you reminded me of the fact that information cannot be transmitted faster than light?

Which post are you referring to now?

 

[Smattering]

Which post are you referring to now?

This one:

https://www.physicsforums.com/threa...-argument-comments.843000/page-2#post-5290428

Edit: I guess I just cannot realize the argument you are refuting as an argument at all.

 

[Buzz Bloom]

Also note that there is no general way to carry simultaneity boundary of certainty to GR, since there is no generally preferred simultaneity possible.

Hi Paul:

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?

Regards,
Buzz

 

[rjbeery]

PeterDonis, I read your synopsis and I respect what you're saying but I think Block Universe (the subject being debated) needs a rigid definition. I believe the essence of the issue boils down to the concept of a "flowing now". If we assign a special physicality to our local concept of now, what do we declare about such special physicality which is non-local? If special physicality exists, then events which are non-local to us either share this special physicality or they do not.

The fact of the matter is that when we are forced to rigidly define such a non-local special physicality when describing events, regardless of any definition I can conceive, we are forced to assign such special physicality to the entire Block Universe. Your argument is basically dodging the question by exposing that such a rigid special physicality definition is ambiguous and of course practically unverifiable; in any event my personal feeling is that to DENY the existence of a special physicality leads us to the same (Block Universe) conclusion.

-R

 

[Smattering]

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?

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.

 

[PAllen]

Hi Paul:

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?

Regards,
Buzz

That's not based on any GR law. That's analogous to saying we can choose to define absolute motion within the solar system by referring it to the sun. CMB based simultaneity is simply choosing a (useful) reference based on the state of matter in a particular solution. It also does not address finer grained simultaneity - e.g. the region near a binary neutron star, where dynamic spacetime geometry makes it impossible to have any well defined notion of simultaneity.

Finally, note that CMB simultaneity, as useful as it is for cosmological question, disagrees with a regional simultaneity that would result from building as large as possible 'near inertial frame' as you could. Since both are useful for different purposes, and both are purely conventional, how do you know which is the 'real' definition of 'present'.

 

[PAllen]

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.

Good, much simpler answer than mine.

 

[PeterDonis]

I guess I just cannot realize the argument you are refuting as an argument at all.

Believe me, I sympathize. But evidently a lot of people, including a lot of well-known physicists (for example, Brian Greene, who was quoted in a number of the previous threads I referred to in the article), think it is a valid argument. So I thought it was worth taking the trouble to point out the obvious flaw in it. That way, the next time someone tries to use it here on PF, we can just point at my article instead of having to refute it again.

 

[PeterDonis]

I believe the essence of the issue boils down to the concept of a "flowing now".

Okay, so what is your rigorous definition of this concept?

Your argument is basically dodging the question

No, your post is dodging the question, by failing to give a rigorous definition of the concept that you said was the essence of the issue, after you had just said that rigorous definitions were needed. If you try to give your concept of a "flowing now" a rigorous definition, you will run right up against the issues I identified in the article.

 

[rjbeery]

Okay, so what is your rigorous definition of this concept?
No, your post is dodging the question, by failing to give a rigorous definition of the concept that you said was the essence of the issue, after you had just said that rigorous definitions were needed. If you try to give your concept of a "flowing now" a rigorous definition, you will run right up against the issues I identified in the article.

I don't believe in any sort of "flowing now", for the very reasons given. I was inviting you to provide a definition of such if you disagree. :)

 

[PeterDonis]

I don't believe in any sort of "flowing now", for the very reasons given. I was inviting you to provide a definition of such if you disagree. :)

I don't believe in a "flowing now" either, so why should I try to provide a definition of it?

 

[rjbeery]

I don't believe in a "flowing now" either, so why should I try to provide a definition of it?

Agreed! But if there is no special physicality associated with "now" then how do you justify a differentiation between a certain past and an uncertain future?

 

[PeterDonis]

if there is no special physicality associated with "now" then how do you justify a differentiation between a certain past and an uncertain future?

First, you are incorrectly assuming that "certain past" and "uncertain future" are the only two categories. They aren't; there's a third, "spacelike separated", which I called "elsewhere" in the article.

Second, the boundaries of these regions do not require any "now"; they are the past and future light cones of a given event, which are invariant geometric properties of spacetime and require no arbitrary choice of "now". So the concept of "now", or any "special physicality" associated with it, is not needed at all.

 

[rjbeery]

First, you are incorrectly assuming that "certain past" and "uncertain future" are the only two categories. They aren't; there's a third, "spacelike separated", which I called "elsewhere" in the article.

Second, the boundaries of these regions do not require any "now"; they are the past and future light cones of a given event, which are invariant geometric properties of spacetime and require no arbitrary choice of "now". So the concept of "now", or any "special physicality" associated with it, is not needed at all.

Then I go back to requesting your definition of a Block Universe, because *mine* would basically be a 4D unchanging block of spacetime. 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. If you don't recognize any "flowing of now" then I don't understand how you can come to any other conclusion.

In other words, what are you arguing against exactly?

 

[Ilja]

Personally, I find the B Theory preferable on the grounds of Occam's Razor, based on the following argument. If we imagine the world as a growing block then we can always take the union of all such blocks, over all time, to get a fixed block, in which everything is 'certain'. A pan-dimensional being that could see that fixed block could also see all the growing blocks arrayed before them in their different stages of development. Further, one needs the 'current universe' to extend at least a little into the future (ie for every point in it to have a neighbourhood that overlaps with the future) in order for the Riemann, metric and stress-energy tensors to be defined.

Given then, that the growing block implies an ultimate fixed block, and requires a block that extends at least a little into the future, why not just discard the intermediate stage from one's picture and a have a simpler model in which we only imagine the fixed block?

But to repeat, I see this as entirely a matter of taste.

I prefer presentism because of Occam's razor, it minimizes what we have to assume to exits (now). As long as it is possible to have such a hypothesis, it should be preferable.

An even more serious point is the violation of Bell's inequality. To explain it with presentism is easy - one needs, of course, a hidden preferred frame, but this is anyway a cost of presentism, and not a big one, because the only argument against the existence of hidden information is - hm, what? - wishful thinking that humans are somehow able to get all relevant information from Nature, because of ... our belief.

The alternative is to give up realism as well as causality. Ok, if we accept a blockworld, we accept fatalism, thus, anyway causality becomes bogus and nonsensical. So, the argument with causality is not decisive.

But there is also a metaargument in favor of causality: Would there be any hope of successful science in a fatalist world?

 

[rjbeery]

I prefer presentism because of Occam's razor, it minimizes what we have to assume to exits (now). As long as it is possible to have such a hypothesis, it should be preferable.

An even more serious point is the violation of Bell's inequality. To explain it with presentism is easy - one needs, of course, a hidden preferred frame, but this is anyway a cost of presentism, and not a big one, because the only argument against the existence of hidden information is - hm, what? - wishful thinking that humans are somehow able to get all relevant information from Nature, because of ... our belief.

The alternative is to give up realism as well as causality. Ok, if we accept a blockworld, we accept fatalism, thus, anyway causality becomes bogus and nonsensical. So, the argument with causality is not decisive.

But there is also a metaargument in favor of causality: Would there be any hope of successful science in a fatalist world?

Ilja, does Presentism apply to non-local events?

 

[Buzz Bloom]

Since both are useful for different purposes, and both are purely conventional, how do you know which is the 'real' definition of 'present'.

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

 

[Buzz Bloom]

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.

Hi Smattering:

I do not understand why not being at rest w/r/t each other is relevant. The Doppler of CBR measurements effect for each of the two observers would be different, but by averaging averaging over the entire sphere of directions, that difference should cancel out. Or am I wrong about this?

Regards,
Buzz

 

[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.