# The Block Universe

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1. Sep 4, 2014

### Staff: Mentor

[Reposted from my PF blog]

The "block universe" interpretation of SR has come up repeatedly in threads here on PF. Rather than link to them, I want to summarize the key argument that is made for the "block universe" being necessary, and then summarize the arguments I made in those threads to show why I don't agree.

The key argument comes in several forms, all logically equivalent; the one I'll use here is the "Andromeda paradox":

http://en.wikipedia.org/wiki/Rietdijk–Putnam_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:

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, 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: the key 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, and one should not take at face value pop science books and TV shows that imply that it is.

2. Sep 5, 2014

### atyy

How about GR? We get all spacetime and matter in one go as a solution of the Einstein Field Equations.

3. Sep 5, 2014

### PAllen

Such a solution is a construction in Peter's sense. You only know the validity of such a construction within your past light cone. A moment later you might see evidence that your initial (or boundary) conditions weren't quite right and need to be adjusted to account for both what you knew before and what you know now.

4. Sep 5, 2014

### atyy

The argument seems too powerful. If we accept it, then even the validity of GR in the future is unknown.

5. Sep 5, 2014

### atyy

While I find it hard to argue against the block universe in any deterministic theory, there are some fascinating coments by Haag on this issue in quantum theory in his book "Local Quatum Physics" https://www.amazon.com/Local-Quantum-Physics-Theoretical-Mathematical/dp/3540610499 p313

"Our question is rather: why should there be any difference between quantum physics and classical physics with respect to the status of irreversibility? A short answer is that quantum physics introduces an element of discreteness manifested in the existence of stable structures and the "indivisibility of a quantum process". This is closely tied to indeterminacy. The future is open, not precisely determined by the past. Though some remnant of time-reversal symmetry persists in quantum mechanics and quantum field theory, there is the asymmetry of the basic statements discussed at the end of section VII.1. In Bohr's view this "reminds us of the essential irreversibility inherent in the very concept of observation". In other words it is tied to the psychological arrow of time. But if we do not want to place the concept of observation into the center of physics we must ask ourselves: what would be the natural picture if we claim that there are discrete, real events, i.e., random, irreversible choices in nature?

Starting from this question we come almost unavoidably to an evolutionary picture of physics. There is an evolving pattern of events. At any stage the past consists of the part which has been realized, the future is open and allows possibilities for new events. Altogether we have a growing graph or, using aanother mathematical language, a growing category whose objects are the events and whose (directed) arrows are the causal links. ..."

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6. Sep 5, 2014

### bahamagreen

Interesting, but I can't tell quite what you are posing as the problem... I must be missing something about the nature of light cones?

I guess I've always thought that the block universe idea came from abstracting each event's past light cone of "certain past" to the geometric logical implication that every event in spacetime is within the past light cones of some other events' "certain and consistent pasts"... so the 2nd premise acts as the base case in a proof by mathematical induction...

If the second premise is "events to the past of any observer's "3D world" at a given event are fixed and certain", I'm not seeing why additional premises would be required.

Maybe more explanitory detail, but not additional premises...

"every event is in the past light cone of some other event(s)"
"an event in the past light cone of another event, being in the certain past of that event, must have some certainty in its future light cone"
"all events within an event's future light cone are also those for some other events for which the events in that future light cone fall within the past light cones of these other events"...

Are all those details true?

If so, it seems like these are just restating the geometry of spacetime that already contains these by construction. Or maybe I'm missing something about spacetime...?

I can't tell by your discussion if you are suggesting that the 2nd premise doesn't geometrically imply that all events are certain by virtue of all events being in the past lightcones of other events or not.

It does seems to me that bringing up observers, communication, data construction, and light speed confuses the question - the block universe implications look like they emerge beyond what's observable at single events and rather are abstracted through applying the 2nd premise to multiple events.

7. Sep 5, 2014

### stevendaryl

Staff Emeritus
The interplay between quantum mechanics and SR is interesting, but to me, it doesn't conclusively point one way or the other. An argument from QM that sort of supports the idea of the block world is the question of the ontology of the wave function. Consider an EPR-type experiment where a twin pair of spin-1/2 particles is produced. Alice on Earth measures the spin of one of the particles, while Bob on Alpha Centauri measures the spin of the other. For simplicity, assume that they both measure spins in the same direction, so they should get opposite results. At the moment that Alice measures spin-up, she knows instantly that Bob will measure spin-down. But obviously (according to SR), her measurement result can't possibly affect Bob's measurement result. So it seems to me that the fact that Bob would measure spin-down must have been true (although unknown to Alice) BEFORE Alice made her measurement. To me, this suggests that Bob's result was pre-determined. And from Bob's point of view, Alice's result was pre-determined. So both results were pre-determined.

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8. Sep 5, 2014

### Staff: Mentor

It is true that the future validity of any scientific theory is unknown. However, that is not the point of the argument.

The point of the argument is that if you assume the validity of GR you cannot know the validity of the solution you derive for the EFE at any event outside of your past light cone. The solution depends on the distribution of matter (stress energy), and you do not know the distribution of matter outside of your past light cone.

9. Sep 5, 2014

### Geometry_dude

You have to be very careful in these kind of arguments what exactly you mean. I have found it helpful to draw Minkowski diagrams to look at these questions, but other's might not agree.

I think we can all agree on the statement that if you are an observer (pointlike mass moving on a curve $\gamma$, whose influence on spacetime geometry can be neglected) at a spacetime point $\gamma(\tau) = q \in \mathcal Q$ at proper time $\tau$, then everything you see is your past lightcone. Since the measurement of how long a process takes or how long an object is, might differ for another observer not having the same spacetime position and (4-)velocity as you, it is convenient to define 'the present' at proper time $\tau$ as everyting you see at point $q=\gamma(\tau)$, that is everything on the past lightcone. Now this definition of the present strongly depends on the event $q$ or on the observer and due to the twin effect it is physically not meaningful to define a global notion of simultaneity that conceptually resembles the one you have in Newtonian mechanics (there do exist other ones).

Accordingly, we define the past as the collection of events in the past lightcone (not on it) and the future consisting at least of the collection of events in the future lightcone. I think we can also agree that everything in the past is 'already determined'.

Now, if you believe that all spacelike seperated events from $q$ are 'already determined' in an ontological sense (of course, they come after our definition of present), then you can take a second such observer at $q'$, say lying almost on the future lightcone, and because you are just as "special" as him or her (here we reject solipsism), everything that is spacelike seperated from $q'$ is 'already determined', that means also some events lying in the future lightcone of #q#. If you proceed with this reasoning you will find that "the entire universe" is 'already determined', thus leading you to the block universe picture, which by this argument must include determinism contrary to what others might say. However, there is no to contradiction to quantum theory on this level as quantum theory alone does not contradict determinism, also contrary to what some others might say (it depends on how you resolve the measurement problem).

Now, it's true that you can escape this argument by rejecting that all spacelike seperated events from your current event are 'already determined', I mean why should you believe that, right?
But then you are again either faced with solipsism, that means in this case your 'current' spacetime position defines the global present as all the events on the past lightcone (note that you don't need determinism here), or you define a "global now", in mathematical terms that would be a foliation of a globally hyperbolic spacetime into spacelike $3$-leaves. But then you introduce something into your worldview that is unobservable in principle and defines what is "now" and "real". In addition, if you accept the existence of such a global, non-solipsisitic now, then you might still have trouble getting rid of determinism - and if you don't there's no reason to introduce such a global now to begin with. :-)

It is true though that if you stay within usual GR, then there's no global simultaneity, everything is deterministic and you get the block universe.

Last edited: Sep 5, 2014
10. Sep 5, 2014

### Staff: Mentor

While this is a valid criticism, it is not a valid argument because the criticism applies equally to both positions being argued.

Also, the identification of a past light cone with a global present or even a local present is a little strange to say the least. I don't recall anyone else make that statement.

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11. Sep 5, 2014

### Geometry_dude

I fully agree. Solipsism it is then?

I always thought we moved past that point as a society. :P

12. Sep 5, 2014

### Staff: Mentor

I don't know why block universe advocates are so stuck on solipsism. Nobody ever brings it up but them. It is a nonsense connection.

As long as I believe that the events on my past light cone are external to me and not merely figments of my imagination then I am not a solipsist, regardless of what I may choose to assert or not assert about the structure of the universe outside of my past light cone.

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13. Sep 5, 2014

### atyy

I would like to understand this position better, so here is a paraphrase for those who don't support the block universe to comment on.

1) In a theory with a Cauchy surface, the past and future are determined by the state at any one point in time. Here, not assuming the block universe means we accept determinism, but we don't accept the reality of the certain future. In special relativity, this is equivalent to postulating an aether, since there is a privileged time with which time evolution occurs.

2a) In a theory without a Cauchy surface, then the future or some large section of the manifold is not determined by any smaller subset of the manifold, and the undetermined section is therefore considered not real.

2b) Does the LCDM model have a Cauchy surface? What are the physically important solutions of GR that do not have a Cauchy surface?

14. Sep 5, 2014

### atyy

It seems close to solipsism, because it means that in an EPR experiment, if I am Alice and Bob is at spacelike separation, then Bob is not in my past when I get a result. So Bob does not exist, if I understand Peter Donis's position correctly. This is one of the ways of denying that a loophole free experimental violation of the Bell inequality implies nonlocality - denying the independent reality of distant results. It is not accepted that this implies solipsism, but it is close enough that views that are close to this usually try to explicitly defend against it. For example, http://arxiv.org/abs/1301.3274 (preprint version of Rev. Mod. Phys. 85, 1693)

15. Sep 5, 2014

### Staff: Mentor

It is nowhere near solipsism. I can believe that I am some "chosen" being and that my past light cone uniquely defines what "really actually exists" and still believe that those events are external to me. (note, I am not claiming that I am a "chosen" being nor do I believe that Peter Donis is claiming that, but I am trying to show how far you can push this idea without being a solipsist)

It is nothing more than philosophical name calling or at least trying to imply some sort of "guilt by association". Solipsism is a very specific philosophical position, not merely a broad category which includes any philosophical position with a privileged observer.

16. Sep 5, 2014

### atyy

Yes. I think it's a bit more plausible in QM if we take the Copenhagen interpretation, because although the entire universe may be determined, it isn't in our theory, since we don't have a wave function of the universe. I think what you are saying is that superdeterminism could be true, which we all agree on, but it is not useful to us. On the other hand, in GR, even if it were true that the future is undetermined, and maybe GR will even fail in the future, within the theory itself, no such failure is predicted.

17. Sep 5, 2014

### martinbn

I don't understand this implication. The existence of a Cauchy surface and a time function is not unique. Why would there be a privileged time?

Why would it be considered not real?

LCDM has.

If you think strong cosmic censorship is true and that most of the solutions are globally hyperbolic (i.e. have a Cauchy surface) then I guess the answer is none.

18. Sep 5, 2014

### atyy

But is correct to say that the position advocated in the OP is that events at spacelike separation from me now do not exist?

19. Sep 5, 2014

### Staff: Mentor

Does GR have any Cauchy surface? GR doesn't have a theory of matter, so I cannot see how it would have any Cauchy surfaces ever.

20. Sep 5, 2014

### Staff: Mentor

I don't think that the OP advocated any position, merely showed the error in the argument of the block universe advocates.