The Block Universe – Refuting a Common Argument
The “block universe” interpretation of SR has come up repeatedly in threads here on PF. Rather than link to them, I want to summarize a common argument that is made for the “block universe” being necessary, and then summarize the arguments I made in those threads to show why the common argument is not valid.
The common argument for the “block universe” comes in several forms, all logically equivalent; the one I’ll use here is the “Andromeda paradox”:
http://en.wikipedia.org/wiki/Rietdijk%E2%80%93Putnam_argument
That Wikipedia entry quotes from Roger Penrose in The Emperor’s New Mind, which is where I first encountered the argument many years ago. Here’s what Penrose says:
[P]eople pass each other on the street [on Earth]; and according to one of the two people, an Andromedean space fleet has already set off on its journey, while to the other, the decision as to whether or not the journey will actually take place has not yet been made. How can there still be some uncertainty as to the outcome of that decision? If to either person the decision has already been made, then surely there cannot be any uncertainty. The launching of the space fleet is an inevitability. In fact neither of the people can yet know of the launching of the space fleet. They can know only later, when telescopic observations from earth reveal that the fleet is indeed on its way. Then they can hark back to that chance encounter, and come to the conclusion that at that time, according to one of them, the decision lay in the uncertain future, while to the other, it lay in the certain past. Was there then any uncertainty about that future? Or was the future of both people already ‘fixed’?
We might summarize this argument in a single sentence as: relativity of simultaneity implies the block universe (i.e., it implies that all of 4-D spacetime must be fixed).
However, as just stated, the argument is not complete; we need an additional premise. Penrose gets at it indirectly when he says “according to one of them, the decision lay in the uncertain future, while to the other, it lay in the certain past”. In other words, Penrose is implicitly claiming that every observer, at a given event, divides the universe into the “uncertain future” and the “certain past”, based on his “surface of simultaneity” through that event. “Surface of simultaneity” is a long-phrase to type, but several posters in the PF threads I referred to above have used a shorter term, “3D world”, which I’ll use henceforth. The added premise then becomes: events to the past of an observer’s “3D world” at a given event are fixed and certain.
With the added premise, we can now see the argument more clearly. At any given event, there can be observers moving on any timelike worldline through that event. The 3D worlds of all these observers are “tilted” with respect to each other because of the relativity of simultaneity. But if events to the past of any observer’s 3D world are fixed and certain, then the entire region of spacetime which is spacelike separated from the given event must be fixed and certain.
As you can probably see, the above argument at a single event is not enough to show us that *all* of 4D spacetime is fixed and certain; it’s only enough to show us that the region of spacetime which is spacelike separated from our chosen event is fixed and certain. In order to extend that to all of 4D spacetime, we need an additional premise: that the above argument holds at *any* event. 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.
So we can summarize the Andromeda paradox argument as follows:
(1) Relativity of simultaneity + all observers’ 3D worlds are real at every event = block universe
(I’ve used the word “real” here because that’s the word that block universe proponents often use, but note that it’s really shorthand for “events to the past of any observer’s 3D world at any event are fixed and certain”.)
Now, in those PF threads I referred to on this topic, a lot of electronic ink was spilled in arguing for proposition (1). However, all of that was really a waste of time, because I already agree with the proposition (1)! (And so, I suspect, do others who posted in those threads expressing similar objections to mine.) Proposition (1), in itself, is not the problem. Nobody needs to be convinced that, given its premises, the conclusion of proposition (1) is true. The problem is the premises, specifically the second one.
Most block universe proponents spend no time at all on the second premise, apparently because they think it’s so obvious that it doesn’t need justification or argument. But, as the above shows, simply assuming the second premise is tantamount to assuming the conclusion! (Strictly speaking, you still need the first premise as well, but everybody also agrees on the first premise; relativity of simultaneity is an accepted fact. So the second premise is the one that’s doing all the work.) 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.
In a couple of those PF threads, when challenged on the second premise, block universe proponents did offer two proposed justifications for it:
(2-1) The only alternative to the second premise is solipsism (only my present event is real).
(2-2) 3D worlds can be directly observed.
Proposition (2-1) is false because there is another alternative to the second premise that accounts for all of our observations:
(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.
The reason this accounts for all of our observations is that information can’t travel faster than light, so anything we observe at a given event can only give information about the past light cone of that event. (More on this below.) Also, note that proposition (3) is obviously consistent with relativistic causality, whereas Penrose’s assumption that 3D worlds are what divide the universe into “the uncertain future” and “the certain past” is not. (It’s rather ironic, btw, that Penrose himself gives a good explanation of relativistic causality in the same book in which the above argument appears: he even has a diagram showing the division of spacetime into causal past, causal future, and “elsewhere”, the spacelike separated region. Our intuitions don’t really know how to deal with “elsewhere”; it’s neither fixed and certain, since we can’t predict what happens there with certainty based only on the data in our past light cone, nor changeable since we can’t causally affect what happens there; we can only causally affect events in our future light cone. Yet, even though Penrose explains all this, he appears to forget it when making the Andromeda paradox argument. This is an instructive example of why arguments from authority should not be given weight; you should always check up.)
Given the above, proposition (2-2) is obviously false as well; we can’t directly observe a 3D world because of the finite speed of light. (As a side note, this proposition has been agreed to be false repeatedly by block universe proponents; but in later threads, they forget they agreed and again present this invalid argument.) What we directly observe is our past light cone; 3D worlds are constructions from the data in our past light cones. But there is nothing requiring us to accept constructions from our data as fixed and certain, and there is at least one good reason not to: our constructions may end up being wrong because our information is incomplete.
When I raised this last objection in PF threads, (that 3D worlds are constructions from the data, which may turn out to be wrong), an argument against that was advanced:
(4) People can communicate the experimental results that show the relativity of simultaneity (as Penrose has the two people on Earth doing when they later make telescopic observations of the Andromedan fleet); this amounts to communicating their knowledge of 3D worlds, which therefore must be real.
However, this is also false, because, as I noted above, the information in our observations can only travel at the speed of light; and furthermore, in order to communicate, people must exchange information about their observations, which of course can only be done at the speed of light. So by the time the people have information about all the events that show the relativity of simultaneity, all of those events are in their past light cones, so there is no need to postulate entire 3D worlds in order to explain the observations; simply accepting their past light cones as real is enough.
So in summary: a common argument for the “block universe” view, based on a proposition (1), is invalid; but it’s invalid not because the conclusion doesn’t follow from the premises (it does), but because the second premise is not established. Block universe proponents gloss over this by simply assuming the second premise; but when challenged, they are unable to give any cogent justification for doing so. So the block universe view is not established by this argument, and one should not take at face value pop-science books and TV shows that imply that it is.
- Completed Educational Background: MIT Master’s
- Favorite Area of Science: Relativity
[QUOTE=”PeterDonis, post: 5291452, member: 197831″]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.[/QUOTE]
Predictions of what? Observations of what? Physics is the study of [i]what[/i] exactly? You’re being purposely non-committal and vague for unknown reasons.
[QUOTE=”rjbeery, post: 5291414, member: 171140″]A state or quality having a physical structure associated with it.[/QUOTE]
Evidently we have different definitions of “rigorous”.
[QUOTE=”rjbeery, post: 5291414, member: 171140″]this is Physics Forums, and reality is the subject at hand, right?[/QUOTE]
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.
[QUOTE=”rjbeery, post: 5291414, member: 171140″]what predictions can SR give that don’t necessitate the presumption of space-like separated entities?[/QUOTE]
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.
[QUOTE=”meviccar, post: 5291423, member: 578014″]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.'[/QUOTE]
Very well summarized.
[QUOTE=”meviccar, post: 5291423, member: 578014″]I understand the surface of simultaneity as being *just* Event B.[/QUOTE]
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.
[QUOTE=”meviccar, post: 5291423, member: 578014″]Either you are conflating ‘plane of simultaneity’ with ‘surface of simultaneity’, or they are the same thing in jargon-world[/QUOTE]
They are the same thing.
[QUOTE=”meviccar, post: 5291423, member: 578014″]the act of being an observer is an observation that exists outside of one’s past light cone[/QUOTE]
What does this even mean?
[QUOTE=”meviccar, post: 5291423, member: 578014″]this is much too philosophical to be considered to be a rigorous definition[/QUOTE]
Definitely. Which means it’s off topic here.
[QUOTE=”meviccar, post: 5291423, member: 578014″]your arguments are simply establishing that it is still possible remain silent on the issue[/QUOTE]
Remain silent on what issue?
[QUOTE=”meviccar, post: 5291423, member: 578014″]pop-science people are equally as justified when they get up on the tele to say, ‘our best understanding results in BU.'[/QUOTE]
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”.
[QUOTE=”rjbeery, post: 5291414, member: 171140″]Because this is Physics Forums, and reality is the subject at hand, right?[/QUOTE]
Not necessarily. Some would rather say that physics is just about the description and prediction of observations.
[QUOTE=”nikkkom, post: 5291415, member: 339375″]Haha, someone needs to throw a quantum-mechanically entangled wrench into this discussion to make it even more convoluted :)[/QUOTE]
Actually I have been trying to fit many worlds into the block universe for quite some time now.
Haha, someone needs to throw a quantum-mechanically entangled wrench into this discussion to make it even more convoluted :)
[QUOTE=”PeterDonis, post: 5291395, member: 197831″]Do you have a rigorous definition of “reality”?[/quote]
Sure: A state or quality having a physical structure associated with it.
[quote=”PeterDonis”]
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?[/QUOTE]
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?
[QUOTE=”rjbeery, post: 5291389, member: 171140″]So past events are certain events, but beyond that you aren’t really saying anything about reality at all.[/QUOTE]
Do you have a rigorous definition of “reality”?
[QUOTE=”rjbeery, post: 5291389, member: 171140″]Is the absence of a stance a stance in itself?[/QUOTE]
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?
[QUOTE=”PeterDonis, post: 5291379, member: 197831″]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”.[/QUOTE]
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?
[QUOTE=”meviccar, post: 5291378, member: 578014″]is it unreasonable to say that the very language of ‘observers’ and ‘past light cones’ necessarily implies a ‘surface of simultaneity’?[/QUOTE]
It depends on what you think is “necessarily implied”. See below.
[QUOTE=”meviccar, post: 5291378, member: 578014″]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.[/QUOTE]
Yes.
[QUOTE=”meviccar, post: 5291378, member: 578014″]Event B, in this case, is the present[/QUOTE]
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.
[QUOTE=”rjbeery, post: 5291363, member: 171140″]so for a given event all of reality is encapsulated in its past light cone[/QUOTE]
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.
[QUOTE=”rjbeery, post: 5291363, member: 171140″]that doesn’t consider new experiences though.[/QUOTE]
New experiences mean the given event has changed, which means the past light cone has changed.
[QUOTE=”rjbeery, post: 5291363, member: 171140″]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?[/QUOTE]
By not equating “reality” with “what is certain”.
[QUOTE=”PeterDonis, post: 5291315, member: 197831″]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.[/QUOTE]
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.
[QUOTE=”Smattering, post: 5291325, member: 576347″]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.[/QUOTE]
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.
[QUOTE=”PeterDonis, post: 5291315, member: 197831″]
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?
[/QUOTE]
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”.
[QUOTE=”rjbeery, post: 5291198, member: 171140″]There are three options that I see:[/QUOTE]
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.
[QUOTE=”rjbeery, post: 5291243, member: 171140″]Perhaps Peter is claiming
D) Due to Relativity we can make no non-local claims about Reality.[/QUOTE]
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.
[QUOTE=”rjbeery, post: 5291294, member: 171140″]What does existence mean?[/QUOTE]
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?
[QUOTE=”rjbeery, post: 5291294, member: 171140″]Kind of seems like we need more definitive statements. Simply accepting a Block Universe is an extremely definitive statement,[/QUOTE]
So is “which events are certain depends on which event’s past light cone you are considering”. It’s just a different definitive statement.
[QUOTE=”rjbeery, post: 5291294, member: 171140″]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.[/QUOTE]
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?]
[QUOTE=”rjbeery, post: 5291254, member: 171140″]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”.
[/QUOTE]
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.
[QUOTE]
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.[/QUOTE]
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.
[QUOTE=”PAllen, post: 5291280, member: 275028″]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.[/QUOTE]
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.
[QUOTE=”rjbeery, post: 5291243, member: 171140″]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. ;)[/QUOTE]
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.
[QUOTE=”Smattering, post: 5291248, member: 576347″]Did he actually agree that relativity is problematic for B and C?
Anyway, I am still struggeling to understand [B]why[/B] relativity should be problematic to either B or C. Maybe you can explain this to me.[/QUOTE]
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.
[QUOTE=”rjbeery, post: 5291243, member: 171140″]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.[/QUOTE]
Did he actually agree that relativity is problematic for B and C?
Anyway, I am still struggeling to understand [B]why[/B] relativity should be problematic to either B or C. Maybe you can explain this to me.
[QUOTE=”Smattering, post: 5291207, member: 576347″]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.[/QUOTE]
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. ;)
[QUOTE=”rjbeery, post: 5291198, member: 171140″]
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 [I]certain[/I] 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.
[/QUOTE]
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.
[QUOTE=”PeterDonis, post: 5291128, member: 197831″]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.[/QUOTE]
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 [i]certain[/i] 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 [i]physical[/i] difference between the past, the now, and the future for a given event?
[QUOTE=”PAllen, post: 5291129, member: 275028″]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.[/QUOTE]
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
[INDENT]None, as far as I’m concerned.[/INDENT]
to RUTA:
[INDENT]What difference does it make to your physics?[/INDENT]
Regards,
Buzz
[QUOTE=”PeterDonis, post: 5291139, member: 197831″]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.[/QUOTE]
Hi Peter:
Thanks for correcting my oversight. When I responded to Smattering’s reply
[INDENT]For example because two distant observers are [B]not[/B] at rest to each other when each of them observes isotropic CMB. Instead, they are moving away from each other.[/INDENT]
to my question
[INDENT]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?[/INDENT]
I thought his point was that the Doppler would be in general different when “observers are [B]not[/B] 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
[QUOTE=”RUTA, post: 5291143, member: 181686″]As Peter points out, it follows from relativity of simultaneity + no preferred frame.[/QUOTE]
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.
[QUOTE=”RUTA, post: 5291143, member: 181686″]If you deny relativity of simultaneity (as Peter does by denying simultaneity at all, for example)[/QUOTE]
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.
[QUOTE=”RUTA, post: 5291143, member: 181686″]What difference does it make to your physics?[/QUOTE]
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.
[QUOTE=”Buzz Bloom, post: 5291124, member: 547865″]The Doppler of CBR measurements effect for each of the two observers would be different[/QUOTE]
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.
[QUOTE=”Ilja, post: 5291100, member: 4997″]I prefer presentism[/QUOTE]
In what sense? Is the “present” just one event (whichever event I am at “now”), or is it more than that?
[QUOTE=”Buzz Bloom, post: 5291112, member: 547865″]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[/QUOTE]
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.
[QUOTE=”rjbeery, post: 5291077, member: 171140″]Then I go back to requesting your definition of a Block Universe, because *mine* would basically be a 4D unchanging block of spacetime.[/QUOTE]
Sure, that will work.
[QUOTE=”rjbeery, post: 5291077, member: 171140″]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.[/QUOTE]
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.
[QUOTE=”Smattering, post: 5291013, member: 576347″]For example because two distant observers are [B]not[/B] at rest to each other when each of them observes isotropic CMB. Instead, they are moving away from each other.[/QUOTE]
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
[QUOTE=”PAllen, post: 5291039, member: 275028″]Since both are useful for different purposes, and both are purely conventional, how do you know which is the ‘real’ definition of ‘present’.[/QUOTE]
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
[QUOTE=”Ilja, post: 5291100, member: 4997″]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?[/QUOTE]
Ilja, does Presentism apply to non-local events?
[QUOTE=”andrewkirk, post: 5288403, member: 265790″]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 [I]implies[/I] an ultimate fixed block, and [I]requires[/I] 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.[/QUOTE]
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?
[QUOTE=”PeterDonis, post: 5291072, member: 197831″]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.[/QUOTE]
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 [i]against[/i] exactly?
[QUOTE=”rjbeery, post: 5291067, member: 171140″]if there is no special physicality associated with “now” then how do you justify a differentiation between a certain past and an uncertain future?[/QUOTE]
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.
[QUOTE=”PeterDonis, post: 5291062, member: 197831″]I don’t believe in a “flowing now” either, so why should I try to provide a definition of it?[/QUOTE]
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?
[QUOTE=”rjbeery, post: 5291061, member: 171140″]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. :)[/QUOTE]
I don’t believe in a “flowing now” either, so why should I try to provide a definition of it?
[QUOTE=”PeterDonis, post: 5291057, member: 197831″]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.[/QUOTE]
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. :)
[QUOTE=”rjbeery, post: 5290993, member: 171140″]I believe the essence of the issue boils down to the concept of a “flowing now”.[/QUOTE]
Okay, so what is your rigorous definition of this concept?
[QUOTE=”rjbeery, post: 5290993, member: 171140″]Your argument is basically dodging the question[/QUOTE]
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.
[QUOTE=”Smattering, post: 5290987, member: 576347″]I guess I just cannot realize the argument you are refuting as an argument at all.[/QUOTE]
Believe me, I sympathize. :wink: 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. :wink:
[QUOTE=”Smattering, post: 5291013, member: 576347″]For example because two distant observers are [B]not[/B] at rest to each other when each of them observes isotropic CMB. Instead, they are moving away from each other.[/QUOTE]
Good, much simpler answer than mine.
[QUOTE=”Buzz Bloom, post: 5290990, member: 547865″]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[/QUOTE]
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, [I]disagrees[/I] 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’.
[QUOTE=”Buzz Bloom, post: 5290990, member: 547865″]
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?[/QUOTE]
For example because two distant observers are [B]not[/B] at rest to each other when each of them observes isotropic CMB. Instead, they are moving away from each other.
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
[QUOTE=”PAllen, post: 5288607, member: 275028″]Also note that there is no general way to carry simultaneity boundary of certainty to GR, since there is no generally preferred simultaneity possible.[/QUOTE]
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
[QUOTE=”PeterDonis, post: 5290964, member: 197831″]Which post are you referring to now?[/QUOTE]
This one:
[URL]https://www.physicsforums.com/threads/the-block-universe-refuting-a-common-argument-comments.843000/page-2#post-5290428[/URL]
Edit: I guess I just cannot realize the argument you are refuting as an argument at all.
[QUOTE=”Smattering, post: 5290926, member: 576347″]what were you referring to when you reminded me of the fact that information cannot be transmitted faster than light?[/QUOTE]
Which post are you referring to now?