The Block Universe – Refuting a Common Argument

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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”:

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 any 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 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 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 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 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 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.

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  1. dl58
    dl58 says:

    It is a reductio ad absurdum.1.  Assume the future is uncertain.2. Show the contradiction "according to one of them, the decision lay in the uncertain future, while to the other, it lay in the certain past."3. Conclude the future is certain (no destinction between the uncertain future and certain past)

  2. dl58
    dl58 says:

    A. Your scenario was explicitly forbidden in my scenario.B. Alice still would not see anything that contradicts what Bob saw.  C. You have only pointed out that the explosion may be unknown to Alice in a alternative scenario. If Bobs fixed past is not fixed for Alice then it would be possible for Alice to observe something which contradicts Bobs observation.My scenario shows Alice will observe what Bob sees or she will remain outside of the light cone. She will not observe anything that contradicts what Bob saw.There is no evidence showing that if Bob sees something then Alice will see that which contradicts Bobs observation.It can be experimentally observed and repeated that what Alice sees will either agree with Bob or the light may never reach her.  Your counter argument seems to be "anything's possible" That argument works on anything.You said:"There is no warrent for claiming that any one of them is "fixed and certain"This is not a preffered event. 100 years later somebody may observe that only one of the infinite possible futures have come to pass.

  3. eltodesukane
    eltodesukane says:

    "I see this as entirely a matter of taste.""As far as "interpretations" in general go (not just of SR but any physical theory), I entirely agree."Interpretations are irrelevant if they have no consequences.But interpretations with consequences are.Newton interpreted gravity as an instantaneous action at distance, while Einstein interpreted it as local spacetime curvature propagating at the speed of light. Those two models, or interpretations, have different consequences, so we are able to choose the better one according to experimental results.

  4. Torbjorn_L
    Torbjorn_L says:

    Oy. I saw comment boxes on Mark Stuckey's "blockworld" series and prepared a lengthy comment in response to his ideas. But those boxes were not functional, comments were turned off for the whole series. Since the comment is related to the topic of this article and indeed refer to it, I post the comment here instead. If it is too irrelevant, feel welcome to delete it.****I have some problems with [Mark Stuckey's] article series. No doubt the block universe works in some generic schemes. But until it can be shown that static models can be as rich as dynamic models (doubtful, see e.g. differential equations of dynamics vs equations of statics), it seems to me to be a problematic proposition.And how would we test it? It is easy to observe and test time (clocks) as well as GR. To point to GR as a test of a "spatiotemporal global constraint" is all well and good, but the present [sic!] theory of time predicts more.I am also wary of the prodigious use of philosophy. For the problems with claiming "real" in general, I refer to Peter Donis's excellent article. [ ] For the specifics, first the discussion of 'real' in relativity of simultaneity. The latter says simply that you would expect only an agreement on physical laws. The existence of time is such a law.Then as for for 'local realism' it is more precisely a tested hypothesis of no hidden variables. (Tested locally in Bell tests but holds globally over the light cone in gauge theories.) All that it says is that some quantum systems aren't local in the sense of correlations, and that the philosophic idea of 'real' – as a superfluous add on on nature – doesn't hold for quantum properties between observations. Or if you insist they do, you have to adopt theories like Many Worlds that makes correlations _really_ non-local (holds over many universes).The discussion of cosmology and CTCs seems confused, with all respect for that the author has a PhD in general relativistic cosmology and I have not. Shrinking the cosmological scale factor to zero makes no sense, and it wouldn't be "one point". (I don't think that is what the author wanted to say however.) Inflation makes the era before the Hot Big Bang of indefinite duration, which accords with the problem of deriving a zero scale factor or a singularity out of semiclassical worldlines. [If you like the latest Planck data release, it may be that we are probing the inflation era so successfully that we can say inflation had a finite duration. The more or less model less fits of potentials sees them being steep.]CTSs are generally invalid solutions to GR, are they not? No one claims that GR solutions always are physical, no more than they claim quantization of fields are automatically such. They have to be tested for relevancy. Specifically here CTSs implies time travel, which seems to be forbidden. I refer to CS Scott Aaronson's paper on this: if time travel was physical, the universe would have such systems already and/or we could make time travel computers. Then the complexity measures of CS falls, and all physics is simple. But we observe it is not, hence no time travel – or CTCs – exist.

  5. meviccar
    meviccar says:

    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.

  6. Mark Stuckey
    Mark Stuckey says:

    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.

  7. meviccar
    meviccar says:

    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.

  8. meviccar
    meviccar says:

    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.'

  9. greswd
    greswd says:

    This has probably been addressed before, but so far I can’t find anything that captures it as I think it is.

    The article says: “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.”

    It also says: “Penrose’s assumption that 3D worlds are what divide the universe into “the uncertain future” and “the certain past” is not”

    Why is the assumption not consistent?

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