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
“Meaningless question because ‘now’ is only a convention in SR and undefinable in GR. The question itself is wrongheaded. IMO, SR partly, and GR fully, removed ‘now’ from having any possible objective meaning. This does not imply any unreality of the universe, only unreality of an obsolete Newtonian concept.”
Most people don’t believe it’s meaningless at all. What is my brother doing right Now in Ohio? Where is that probe in its orbit about Jupiter right Now? You want to find your friend in the store and text them, “Where are you?” Most people believe it’s meaningful to assume those things all coexist with them Now. But, if you don’t, that’s solipsism. Again, it’s very simple.
“Does any thing else coexist with you right Now? If yes, then that collection of coexisting entities occupying that particular Now constitutes a SoS and the argument follows. If not, solipsism. You can claim you don’t know, that’s not an ontological claim, that’s an epistemological claim. BW is an ontological claim.”
Meaningless question because ‘now’ is only a convention in SR and undefinable in GR. The question itself is wrongheaded. IMO, SR partly, and GR fully, removed ‘now’ from having any possible objective meaning. This does not imply any unreality of the universe, only unreality of an obsolete Newtonian concept. The universe exists, “now” does not exist (in my current philosophical position).
“Nope, that is false logic. I can believe other people are known to have existed up to the surface of my past light cone instead of an SoS (which, in any case, has no preferred definition at all in GR, in general), and can predict their extremely high likelihood of existing beyond my past light cone. That is not solipsism unless you adopt the utterly absurd definition that anything other than BW (and not just BW – your particular flavor of it – see later) is solipsism. IMO, irrespective of BW (which I rather like, actually), the concept of ‘now’ at a distance has no objective meaning in SR or GR. In fact, I would argue that BW makes ‘now’ completely superfluous, since all foliations, and all history exists ‘at once’.”
Does any thing else coexist with you right Now? If yes, then that collection of coexisting entities occupying that particular Now constitutes a SoS and the argument follows. If not, solipsism. You can claim you don’t know, that’s not an ontological claim, that’s an epistemological claim. BW is an ontological claim.
“BW has to do with coexistence. If you believe other people coexist with you at any instant, then that constitutes a SoS and the argument for BW (coexistence of future, past and present) follows. If not, that’s solipsism. It’s that simple.”
Nope, that is false logic. I can believe other people are known to have existed up to the surface of my past light cone instead of an SoS (which, in any case, has no preferred definition at all in GR, in general), and can predict their extremely high likelihood of existing beyond my past light cone. That is not solipsism unless you adopt the utterly absurd definition that anything other than BW (and not just BW – your particular flavor of it – see later) is solipsism. IMO, irrespective of BW (which I rather like, actually), the concept of ‘now’ at a distance has no objective meaning in SR or GR. In fact, I would argue that BW makes ‘now’ completely superfluous, since all foliations, and all history exists ‘at once’.
“You are bundling things that need not be bundled. Relativity of simultaneity, to me, means NOTHING more than if two observers in relative motion synchronize clocks using the same procedure, each will think the other’s clocks to be out of synch. It has nothing to do with a surface of simultaneity, nor with the FURTHER assumption that such surface has anything to do with what is fixed and certain. Further, you keep stating that believing ‘past light cone is fixed’ means only current event is real. I completely disagree with this coupling as well.”
BW has to do with coexistence. If you believe other people coexist with you at any instant, then that constitutes a SoS and the argument for BW (coexistence of future, past and present) follows. If not, that’s solipsism. It’s that simple.
“You are arguing against an argument that one has to choose between statements 1. and 2. as we can choose statement 3. instead (I tried to write it similar to 1.)”
Yes, this is one way of summarizing what I was saying in the article, taken as a whole.
[USER=181686]@RUTA[/USER] nowhere have I ever seen Peter Donis advocate for solipsism. Nor have I ever seen anyone else on PF do so during any block universe discussion.
“No, I’m not. I’m taking a particular argument for a “philosophical” statement and showing that it’s not valid. That’s not an “argument against” the statement; I’m not proving that the statement is wrong. I’m just proving that a particular argument that claims to prove the statement is true, is not valid. That’s all I’m doing. Nothing else. No more than that.”
It’s hard to decipher your post but I wrote the argument as I understand it.
So there are three statements:
1. Events to the past of any observer’s “3D world” at a given event are fixed and certain.
2. Solipsism (only my present event is real).
3. Events in any observer’s past light cone at a given event are fixed and certain.
You are arguing against an argument that one has to choose between statements 1. and 2. as we can choose statement 3. instead (I tried to write it similar to 1.)
Seems fine that way.
“Your “additional premise” is only required if you first get rid of the reality (co-existence) of the events on your surface of simultaneity (SoS), i.e., you abandon the first premise because you get rid of the notion of a SoS. You can certainly disassemble M4 into individual perspectives. You just don’t have anything to say about what’s happening Now anywhere other than your location on your worldline at any given instant. That’s essentially solipsism.”
That’s your personal, extreme, and IMO false definition of solopsism.
[edit: More: It is patently false that ‘past light cone certain’ means you can’t say anything about what is happening beyond the past light cone elsewhere. It simply means that such a statement is prediction, based on information in the past light cone; and such statement may have exceedingly high probability of being verified. Finally, you wouldn’t be inclined to make such a prediction and expect it to be born out if you thought there was no ‘reality’ outside the past light cone. I continue to be amazed at how you bundle logically separate concepts in the ONE WAY YOU PREFER that you define as THE ONE WAY POSSIBLE.]
“Your “additional premise” is only required if you first get rid of the reality (co-existence) of the events on your surface of simultaneity (SoS),”
I’m not “getting rid of” it; you’re assuming it. It’s not logically required by the laws of SR, so it’s an additional premise you have to assume, not something I have to “get rid of”.
“you abandon the first premise because you get rid of the notion of a SoS”
No, I just say that events on the SoS are not “fixed and certain” with respect to an observer, at a particular event, whose SoS it is. Your argument implicitly assumes that events on the SoS are “fixed and certain”; but that is not logically required by the laws of SR. It is just as consistent with the laws of SR to say that only events in the past light cone of a given event are “fixed and certain” with respect to an observer at that event.
“If you don’t assume the reality of all points on a surface of simultaneity, but only the point where you exist”
The past light cone is not the same as a single event.
Please, take the time to read what I’m actually saying. You have not responded at all to what I actually said in the article.
“You can avoid the BW conclusion of RoS + no preferred frame by getting rid of either premise. If you don’t assume the reality of all points on a surface of simultaneity, but only the point where you exist, then you’ve gotten rid of the notion of a surface of simultaneity and dispensed with the first premise.”
You are bundling things that need not be bundled. Relativity of simultaneity, to me, means NOTHING more than if two observers in relative motion synchronize clocks using the same procedure, each will think the other’s clocks to be out of synch. It has nothing to do with a surface of simultaneity, nor with the FURTHER assumption that such surface has anything to do with what is fixed and certain. Further, you keep stating that believing ‘past light cone is fixed’ means only current event is real. I completely disagree with this coupling as well.
“You are assuming that “past” and “future” are the only two categories. Did you read my article? It discusses this exact error. There is a third category, which I called “elsewhere” in the article. You can’t just help yourself to the assumption that all events to the past of any surface of simultaneity are fixed; that’s precisely the additional premise that is not logically required by SR.
In other words, your statement quoted above is just another way of stating the additional premise; it doesn’t help establish it at all.”
Your “additional premise” is only required if you first get rid of the reality (co-existence) of the events on your surface of simultaneity (SoS), i.e., you abandon the first premise because you get rid of the notion of a SoS. You can certainly disassemble M4 into individual perspectives. You just don’t have anything to say about what’s happening Now anywhere other than your location on your worldline at any given instant. That’s essentially solipsism.
“Because on the microscopic level the physical laws are being applied in a time-symmetric manner.”
Then why is frying observed so often, but unfrying almost never?
Edit: PeterDonis has already answered this.
“but that does not mean that physical laws are time-asymmetric.”
I never claimed that physical laws are time-asymmetric.
“How is the alternative presented in the article solipsism – that instead of “all events to the past of any observer’s surface of simultaneity are fixed and certain” you adopt “all events in the past light cone of any observer are fixed and certain”?”
You can avoid the BW conclusion of RoS + no preferred frame by getting rid of either premise. If you don’t assume the reality of all points on a surface of simultaneity, but only the point where you exist, then you’ve gotten rid of the notion of a surface of simultaneity and dispensed with the first premise.
“given the fact, that frying is many magnitudes more likely than unfrying, how can you seriously consider this an example for symmetry?”
To put rjbeery’s response in somewhat different words, as I said in a previous post, the asymmetry is in the initial conditions, not the laws–i.e., it’s in the particular solution of the laws that is realized in the universe we live in. The symmetry of the laws shows up in the complete set of all possible solutions; it does not have to show up in a particular solution taken by itself.
“O.k., but given the fact, that frying is many magnitudes more likely than unfrying, how can you seriously consider this an example for symmetry?”
Because on the microscopic level the physical laws are being applied in a time-symmetric manner. The actual difference between past and future is purely epistemological (meaning what we [i]know[/i], as opposed to what actually [i]is[/i]). This is a consequence of an entropy gradient in spacetime. This gradient gives us an asymmetry in information available but that does not mean that physical laws are time-asymmetric.
“”Observing” such a phenomenon is unlikely to say the least but it is not unphysical in any way.”
O.k., but given the fact, that frying is many magnitudes more likely than unfrying, how can you seriously consider this an example for symmetry?
“Simply look at the surfaces of simultaneity for observers in motion along your surface of simultaneity. No preferred frame then fixes your past (and future).”
You are assuming that “past” and “future” are the only two categories. Did you read my article? It discusses this exact error. There is a third category, which I called “elsewhere” in the article. You can’t just help yourself to the assumption that all events to the past of any surface of simultaneity are fixed; that’s precisely the additional premise that is not logically required by SR.
In other words, your statement quoted above is just another way of stating the additional premise; it doesn’t help establish it at all.
“BW follows deductively from the two premises I listed”
No, it doesn’t. See above.
“do you think a “certain past” but an “uncertain future” implies time-asymmetry?”
Distinguishing “past” and “future” in itself already implies time asymmetry. We do it because that’s what we observe: the two “directions” of time don’t work the same. We can remember the past but we can’t remember the future; entropy increases towards the future but not the past; etc.
“It’s difficult for me to accept the idea that the rules of Physics differ depending where in spacetime an observer is located.”
Time asymmetry doesn’t mean the laws are different at different events. It just means the laws, which are the same at every event, are time asymmetric; the “future” direction works differently from the “past” direction.
However, it’s not necessarily true that time asymmetry in our observations is due to time asymmetry in the laws; it could also be due to time asymmetry in the initial conditions. See below.
“”Unfrying an egg” is practically impossible but not unphysical; we could record an egg being cooked and reverse the video: there is no uncertainty as we unfry it, no physical laws are being broken, no stochastic processes are coming into play, and there are no options other than the resulting uncooked egg…so why do we believe that things must be different when they reside in our future light cones?”
Because of the initial conditions–or more precisely the conditions at the instant of time where the egg has just been fried, which aren’t really “initial”, but we’re assuming we can “back-predict” as well as “forward predict” here so that doesn’t matter. The conditions at the instant of time where the egg has just been fried are asymmetric: the microscopic motions of all the particles in the egg back-predict that it was unfried in the past, but they do not forward-predict that it will unfry itself in the future; they forward-predict that it will stay fried, at least until it gets digested. :wink:
So the asymmetry here isn’t in the laws; it’s in the particular solution of the laws that is being realized. The time symmetry of the laws (assuming for the moment that we know the laws are in fact time-symmetric, which isn’t actually true in all cases–weak interactions, at least, aren’t) shows up in the complete set of solutions, not in any particular solution. If there is a solution with the egg unfried in the past and fried in the future, there is also a solution that is the exact time reverse of that one, with the egg fried in the past and unfried in the future. Observation just tells us which particular solution we happen to be living in.
“Your “additional premise” is subsumed by the other two, since you’re not using solipsism. Simply look at the surfaces of simultaneity for observers in motion along your surface of simultaneity. No preferred frame then fixes your past (and future).
”
How is the alternative presented in the article solipsism – that instead of “all events to the past of any observer’s surface of simultaneity are fixed and certain” you adopt “all events in the past light cone of any observer are fixed and certain”?
“You take philosophical statement and provide an argument against it.”
No, I’m not. I’m taking a particular argument for a “philosophical” statement and showing that it’s not valid. That’s not an “argument against” the statement; I’m not proving that the statement is wrong. I’m just proving that a particular argument that claims to prove the statement is true, is not valid. That’s all I’m doing. Nothing else. No more than that.
(I feel the need to keep repeating that because people continue to say I claimed something in the article that I didn’t claim, that the article said I didn’t claim, and that I’ve already said several times in this thread I didn’t claim.)
“I doubt that. Physics is all about observations. As the process if unfrying an egg has never been observed, and there is no theory that predicts how we could practically unfry an egg, I’d say that this is indeed unphysical.
But if you reverse the video, it does not represent anything that can be observed anymore.
No physical laws are being broken? Then please explain to me how to observe the process of unfrying an egg.”
Unfrying an egg is not unphysical. No physicist would say otherwise.
If the molecules in the walls and ceiling surrounding the stove emit energy (in the form of heat and sound waves) in just the right way, directed toward the fried egg sitting in the pan, then that energy can break the egg white proteins which have bound together, freeing the individual proteins to reattach to themselves rather than their neighbors. The result is an uncooked egg. While we’re discussing this “conspiracy of molecules” we could also go in to how the stove top is coordinating its heat energy, directing what will eventually be electrical energy back into the city grid.
“Observing” such a phenomenon is unlikely to say the least but it is not unphysical in any way.
“Presentism is not incompatible with SR. Preferred frame LET is an interpretation of SR that cannot be ruled out by any experiment or observation, and it trivially supports strict presentism. This is yet another example of insisting that personal choice is a requirement. Note that, aesthetically, I find preferred frame repugnant, but I would never argue that it is an invalid point of view.”
Ahh, of course you’re right! I think of Presentism as being ambiguous by design but there’s nothing preventing someone from simply claiming a preferred frame. Not very…elegant…but not technically wrong.
“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.”
Your “additional premise” is subsumed by the other two, since you’re not using solipsism. Simply look at the surfaces of simultaneity for observers in motion along your surface of simultaneity. No preferred frame then fixes your past (and future).
“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.”
BW follows deductively from the two premises I listed, so you’re either denying the first or the second and clearly it’s not the second.
“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.”
It certainly does. See for example: “Modified Regge Calculus as an Explanation of Dark Energy,” W.M. Stuckey, Timothy McDevitt & Michael Silberstein,Classical & Quantum Gravity 29 055015 (2012). [URL=’http://arxiv.org/abs/1110.3973′][U]http://arxiv.org/abs/1110.3973[/U][/URL].
”
Because we can follow a causal string of events in both directions, right? “Unfrying an egg” is practically impossible but not unphysical;
”
I doubt that. Physics is all about observations. As the process if unfrying an egg has never been observed, and there is no theory that predicts how we could practically unfry an egg, I’d say that this is indeed unphysical.
”
we could record an egg being cooked and reverse the video:
”
But if you reverse the video, it does not represent anything that can be observed anymore.
”
there is no uncertainty as we unfry it, no physical laws are being broken,
”
No physical laws are being broken? Then please explain to me how to observe the process of unfrying an egg.
“Fair point. I’d like to point out that in this very thread there are some Presentists (which is basically incompatible with SR), so the only reason I might ever argue that BU is strongly preferred is because (in my experience) people have not fully grasped the consequences of Relativity.”
Presentism is not incompatible with SR. Preferred frame LET is an interpretation of SR that cannot be ruled out by any experiment or observation, and it trivially supports strict presentism. This is yet another example of insisting that personal choice is a requirement. Note that, aesthetically, I find preferred frame repugnant, but I would never argue that it is an invalid point of view.
“I don’t believe there is resistance. What I see here and elsewhere is that one side says BU is not a required interpretation, but a personal philosophic choice; while BU proponents argue that it is strongly preferred and engage in tendentious critiques of other possiblities.”
Fair point. I’d like to point out that in this very thread there are some Presentists (which is basically incompatible with SR), so the only reason I might ever argue that BU is strongly preferred is because (in my experience) people have not fully grasped the consequences of Relativity.
“Because we can follow a causal string of events in both directions, right? “Unfrying an egg” is practically impossible but not unphysical; we could record an egg being cooked and reverse the video: there is no uncertainty as we unfry it, no physical laws are being broken, no stochastic processes are coming into play, and there are no options other than the resulting uncooked egg…so why do we believe that things must be different when they reside in our future light cones? Before QM came along there was no reason to question a clockwork universe, and I get the feeling that some people believe free will has been “saved” by the apparent statistical nature of QM.
Basically I’m trying to identify the resistance to a Block Universe.”
I don’t believe there is resistance. What I see here and elsewhere is that one side says BU is not a required interpretation, but a personal philosophic choice; while BU proponents argue that it is strongly preferred and engage in tendentious critiques of other possiblities.
[edit: note, that in classical physics, there was already the mystery of why all thermodynamic processes, everywhere/when seem to run in the same direction. This already suggested to many physicists an objective arrow of time, with an objective asymmetry for the universe as a whole]
“Why do you think that a certain past and an uncertain future imply different physical laws for different observers?”
Because we can follow a causal string of events in both directions, right? “Unfrying an egg” is practically impossible but not unphysical; we could record an egg being cooked and reverse the video: there is no uncertainty as we unfry it, no physical laws are being broken, no stochastic processes are coming into play, and there are no options other than the resulting uncooked egg…so why do we believe that things must be different when they reside in our future light cones? Before QM came along there was no reason to question a clockwork universe, and I get the feeling that some people believe free will has been “saved” by the apparent statistical nature of QM.
Basically I’m trying to identify the resistance to a Block Universe.
“I have a general question for PeterDonis, but anyone else is welcome to give their opinion: do you think a “certain past” but an “uncertain future” implies time-asymmetry? It’s difficult for me to accept the idea that the rules of Physics differ depending where in spacetime an observer is located.”
All fundamental classical physical laws have time symmetry (Standard Model does not, even at the fundamental level). However, emergent laws, e.g. thermodynamics are time asymmetric, leading to a common view (no idea of percentages) that the universe has an objective time arrow, and that evolution is viable interpretation (for geometric theories, it is treated under the umbrella of EBU – evolving block universe; George F.R. Ellis of “Hawking and Ellis” fame has written a series of papers and essays motivating this point of view.)
“I have a general question for PeterDonis, but anyone else is welcome to give their opinion: do you think a “certain past” but an “uncertain future” implies time-asymmetry? It’s difficult for me to accept the idea that the rules of Physics differ depending where in spacetime an observer is located.”
Why do you think that a certain past and an uncertain future imply different physical laws for different observers?
I have a general question for PeterDonis, but anyone else is welcome to give their opinion: do you think a “certain past” but an “uncertain future” implies time-asymmetry? It’s difficult for me to accept the idea that the rules of Physics differ depending where in spacetime an observer is located.
“Just curious – is there an argument which combines the Block Universe interpretation with the Many-Worlds interpretation? (I don’t know if this should be in a separate thread or not. If any mentor feels it should be somewhere else, feel free to move it.)”
From my point of view, believing in the many worlds interpretation of QM would be an argument against the block universe – at least against a “classical” block universe that really resembles some kind of block. After all, an eternal many worlds universe would have a tree-like structure.
Just curious – is there an argument which combines the Block Universe interpretation with the Many-Worlds interpretation? (I don’t know if this should be in a separate thread or not. If any mentor feels it should be somewhere else, feel free to move it.)
“There is generally recognized philosophical idea of “realism” that is alternative to “solipsism”.”
But this discussion is not about realism vs. solipsism, but rather about presentism vs. eternalism.
In particular, the growing block universe interpretation does not conflict with realism. The events just have to grow in a particular order such that the past light cone of every currently growing event has already grown before. But this does not imply that only my own past light cone is real. It’s rather the other way around: My past light cone must exist or at least must have existed as a prerequisite for my own existence.
“In what sense? Is the “present” just one event (whichever event I am at “now”), or is it more than that?”
What is present is the whole world. A preferred space-like hypersurface of spacetime, the one which exists, while the other parts of the spacetime exist only in memories and hopes, but not actually in reality.
“Ilja, does Presentism apply to non-local events?”
Hm. Presentism means that what exists, exists now. The future does not yet exist, the past is past, and no longer exists, what exists, exists now. What exists is, of course, something global – the whole world. How this world is structured is an independent question.
“I’m not. I’m redefining it as “dependent on which event you pick.” Events are objective; they are points in spacetime, and all observables at an event are invariants. There’s no subjectivity.”
I don’t know. I still think your argument is flawed. You take philosophical statement and provide an argument against it. There is generally recognized philosophical idea of “realism” that is alternative to “solipsism”. If you want to provide valid argument in this context you have to stick to proper meanings of philosophical terms (in particular meaning of “real”). And for anybody reading the article there is no reason to expect that you would use “real” with some different meaning.
Maybe you can provide valid argument based on your idea but you have not done this in the article IMO.
“You are not free to redefine “real” as “subjective” in this context.”
I’m not. I’m redefining it as “dependent on which event you pick.” Events are objective; they are points in spacetime, and all observables at an event are invariants. There’s no subjectivity.
“You are providing alternative to “solipsism”. You are not free to redefine “real” as “subjective” in this context.”
Observer dependent does not mean subjective. The latter implies it depends on state of mind (e.g. QM interpretations that attribute a key role to consciousness). Observer dependent simply means that the scope of what is fixed depends on the event of observation or measurement – emphasis on event, not state of mind.
Turning it around, suggesting that something unobservable and inherently unverifiable (BU and reality of simultaneity surface) must be considered real seems seems inherently subjective to me. If it is in principle not subject to verification, it is subjective.
“So I’m not talking about a philosophical definition of “real”.”
You are providing alternative to “solipsism”. You are not free to redefine “real” as “subjective” in this context.
“Things can’t be real for an observer. This is self contradictory statement.”
Only on a particular definition of “real”. But the whole point of the article is that you can have different definitions.
Also, you’re ignoring the parenthesis right after “real”, which makes it clear that I’m defining it as “fixed and certain”. So I’m not talking about a philosophical definition of “real”.
There is an error in the article:
[quote](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.[/quote]
Things can’t be real for an observer. This is self contradictory statement. Things are either real for all observers or subjective by definition (as much as we have got definitions in philosophy).
There is an argument against BU, one flavor of it, which says that “present” is just a quirk of how we, humans, perceive the physical world. That we think that future doesn’t exist merely because we don’t feel it.
The counter-argument is that human/animal perception mechanism was not created randomly. Actually, it was under immense selection pressure to evolve to give as accurate predictions about future as possible. You slip from a tree, you need to realize that you can die, and need to grab a branch to save yourself. You need to throw a rock to kill your prey and not die of starvation, for this you need to predict how rocks fly.
If future “already exists” (whatever that means), it is definitely not easily predictable from the current event. Otherwise animals would evolve mechanisms to use “already existing” information from the future.
”
I’m not refuting the added premise; I’m just pointing out that it’s an added premise, and that added premise is not logically required by the postulates of SR. Therefore, the argument that the “block universe” interpretation is logically required by SR is not valid, because it depends on an added premise that is not logically required by SR.”
I suppose we could point to time symmetry to appeal to a fixed and certain past [i]and[/i] future, but neither of those concepts are logically required by the postulates of SR either.
I still don’t quite understand this discussion.
An event can only be affected by events in its past light cone, and can only affect events in its future light cone.
Where does the “reality” and “fixation” come into play?
“You can’t reject a premise simply because it forces you to a conclusion.
…
This is all philosophy, of course, but we should choose our premises for reasons other than where they lead us, unless that is to a logical contradiction.”
The same can be said about BU proponents. They are accepting the premise for the sole reason of liking where it leads them.
“(Bolding is mine)”
Are you saying that what you bolded is what I called the “added premise”? It isn’t. The “added premise” is this: “all observers’ 3D worlds are real at every event”. The text you quoted is just describing how relativity of simultaneity plus that added premise implies the “block universe” as a conclusion.
“your refutation of this added premise is that it would require a Block Universe because it would force us to conclude 4D spacetime”
I’m not refuting the added premise; I’m just pointing out that it’s an added premise, and that added premise is not logically required by the postulates of SR. Therefore, the argument that the “block universe” interpretation is logically required by SR is not valid, because it depends on an added premise that is not logically required by SR.
“No, I’m pointing out that you are throwing around terms that don’t have rigorous definitions.
If you want my personal view, it is that we don’t know “what reality is” in any scientific sense. We have theories that allow us to make predictions of the form “if we do X, we will observe Y”, which are confirmed by experiments–when we do X, we do indeed observe Y. That’s what our knowledge of physics consists of. Claims that some theory of physics means “reality” must be a certain way are not scientific claims, because there’s no way to test them; they’re someone’s personal opinion.
For purposes of this thread, though, neither my personal view nor your personal view are relevant. The topic of discussion is the specific argument that I refuted in the article.”
Fine, if I can’t get you to commit to anything, define anything, or even voice your opinion on what is lacking in my attempts at a definition of “reality”, then I’ll stick to your article. Specifically, your problem with what you refer to as the “added premise” that a fixed and certain past should apply to *all* events leads us to this conclusion:
[quote]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), [b]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.[/b][/quote]
(Bolding is mine)
That’s it. So your refutation of this added premise is that it would require a Block Universe because it would force us to conclude 4D spacetime…you cannot do this. You can’t reject a premise simply [b]because[/b] it forces you to a conclusion. You can certainly reject a premise on other grounds, but your article gives no such grounds. If you think that having a fixed and certain causal future should force us to reject an otherwise reasonable premise then you are guilty of doing the very thing that you later accuse Block Universe proponents of doing:
[quote]But, as the above shows, simply assuming the second premise is tantamount to assuming the conclusion! … 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.[/quote]
This is all philosophy, of course, but we should choose our premises for reasons other than where they lead us, unless that is to a logical contradiction.
“You’re being purposely non-committal and vague for unknown reasons.”
No, I’m pointing out that you are throwing around terms that don’t have rigorous definitions.
If you want my personal view, it is that we don’t know “what reality is” in any scientific sense. We have theories that allow us to make predictions of the form “if we do X, we will observe Y”, which are confirmed by experiments–when we do X, we do indeed observe Y. That’s what our knowledge of physics consists of. Claims that some theory of physics means “reality” must be a certain way are not scientific claims, because there’s no way to test them; they’re someone’s personal opinion.
For purposes of this thread, though, neither my personal view nor your personal view are relevant. The topic of discussion is the specific argument that I refuted in the article.
“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.”
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.
“A state or quality having a physical structure associated with it.”
Evidently we have different definitions of “rigorous”.
“this is Physics Forums, and reality is the subject at hand, right?”
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.
“what predictions can SR give that don’t necessitate the presumption of space-like separated entities?”
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.
“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.'”
Very well summarized.
“I understand the surface of simultaneity as being *just* Event B.”
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.
“Either you are conflating ‘plane of simultaneity’ with ‘surface of simultaneity’, or they are the same thing in jargon-world”
They are the same thing.
“the act of being an observer is an observation that exists outside of one’s past light cone”
What does this even mean?
“this is much too philosophical to be considered to be a rigorous definition”
Definitely. Which means it’s off topic here.
“your arguments are simply establishing that it is still possible remain silent on the issue”
Remain silent on what issue?
“pop-science people are equally as justified when they get up on the tele to say, ‘our best understanding results in BU.'”
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”.
“Because this is Physics Forums, and reality is the subject at hand, right?”
Not necessarily. Some would rather say that physics is just about the description and prediction of observations.
“Haha, someone needs to throw a quantum-mechanically entangled wrench into this discussion to make it even more convoluted :)”
Actually I have been trying to fit many worlds into the block universe for quite some time now.
Haha, someone needs to throw a quantum-mechanically entangled wrench into this discussion to make it even more convoluted :)
“Do you have a rigorous definition of “reality”?[/quote]
Sure: A state or quality having a physical structure associated with it.
”
Why is a stance on “reality” required for a scientific theory? Or, to keep focused more on the topic of this thread, why is a stance on “reality” necessary based on the postulates of SR?”
Because this is Physics Forums, and reality is the subject at hand, right? As I said, if an observer restricts himself to a reality that lies in his past light cone then SR is demonstrably incomplete. Actually, I’m not even sure it’s utilizable at all — what predictions can SR give that don’t necessitate the presumption of space-like separated entities?
“So past events are certain events, but beyond that you aren’t really saying anything about reality at all.”
Do you have a rigorous definition of “reality”?
“Is the absence of a stance a stance in itself?”
Why is a stance on “reality” required for a scientific theory? Or, to keep focused more on the topic of this thread, why is a stance on “reality” necessary based on the postulates of SR?
“No, what is “certain” at a given event is what is in its past light cone. I am not claiming that “reality” is the same as “what is certain”. Only you are.
New experiences mean the given event has changed, which means the past light cone has changed.
By not equating “reality” with “what is certain”.”
So past events are certain events, but beyond that you aren’t really saying anything about reality at all. Is the absence of a stance a stance in itself?
“is it unreasonable to say that the very language of ‘observers’ and ‘past light cones’ necessarily implies a ‘surface of simultaneity’?”
It depends on what you think is “necessarily implied”. See below.
“By saying that it takes time for information regarding an event to get to an observer, we are necessarily stating that the observer of event A, is actually at event B.”
Yes.
“Event B, in this case, is the present”
Not if “the present” means an entire surface of simultaneity. Event B is just one event–just one point in spacetime. If I am at Event B, observing Event A, which must be in my past light cone at Event B, that says nothing about any other events outside my past light cone at Event B–which includes all other events in any hypothetical “surface of simultaneity” containing Event B.
“so for a given event all of reality is encapsulated in its past light cone”
No, what is “certain” at a given event is what is in its past light cone. I am not claiming that “reality” is the same as “what is certain”. Only you are.
“that doesn’t consider new experiences though.”
New experiences mean the given event has changed, which means the past light cone has changed.
“How do I account for all new events continuously arriving in my growing light cone without postulating that there is something else beyond my experiences?”
By not equating “reality” with “what is certain”.
“They don’t exhaust the possibilities; none of them captures the alternative I described in the article, that events in the past light cone of a given event are “certain” relative to that event.
If events in your past light cone are “real”, then that seems to me to be a “non-local claim about Reality”. It’s just a different “non-local claim” than the one you were considering.
Well, you’re the one who seems concerned about the term, so what do you think it means? Can you give “existence” a rigorous definition?
So is “which events are certain depends on which event’s past light cone you are considering”. It’s just a different definitive statement.”
I see, so for a given event all of reality is encapsulated in its past light cone; that doesn’t consider new experiences though. How do I account for all new events continuously arriving in my growing light cone without postulating that there is something else beyond my experiences? From my perspective, reality would be apparently growing, randomly, from nothing. This seems incomplete at best.
“it should be entirely sufficient if there is some kind of ordering such that the past light cone of every currently “growing” event has already “grown” before.”
I agree that this is sufficient for a “growing” interpretation, and such an ordering certainly exists; the time ordering of any pair of timelike or null separated events is invariant, and the past light cone of any event consists entirely of events that are timelike or null separated from it and are “earlier” according to the invariant ordering of such event pairs.
”
Well, you’re the one who seems concerned about the term, so what do you think it means? Can you give “existence” a rigorous definition?
”
For the “growing” block universe to be possible, it should be entirely sufficient if there is some kind of ordering such that the past light cone of every currently “growing” event has already “grown” before. I do not understand why that thing would require absolute simultaneity to be able to “grow”.
“There are three options that I see:”
They don’t exhaust the possibilities; none of them captures the alternative I described in the article, that events in the past light cone of a given event are “certain” relative to that event.
“Perhaps Peter is claiming
D) Due to Relativity we can make no non-local claims about Reality.”
If events in your past light cone are “real”, then that seems to me to be a “non-local claim about Reality”. It’s just a different “non-local claim” than the one you were considering.
“What does existence mean?”
Well, you’re the one who seems concerned about the term, so what do you think it means? Can you give “existence” a rigorous definition?
“Kind of seems like we need more definitive statements. Simply accepting a Block Universe is an extremely definitive statement,”
So is “which events are certain depends on which event’s past light cone you are considering”. It’s just a different definitive statement.
“Perhaps I don’t understand what is being claimed if we say that reality is a meaningless term. Does non-local reality have a physical representation or not? What does existence mean? Does the past have a physicality which is different from the future? What about the local “now”?
Kind of seems like we need more definitive statements. Simply accepting a Block Universe is an extremely definitive statement, btw, and in my opinion attempts to refute it are rooted in fear of Fatalism. Even the name FATALISM is scary, like adherents to it are busy killing off Hope, Free Will and The Human Spirit.”
No one is arguing BU doesn’t work or isn’t simple, or is wrong. The only argument, and IMO, comes only from “BU only” proponents, is that either BU or correct or “wild false caricature of any alternative”.
[As to the difficulties with reality – start debating this and you get into pure philosophy conundrums – how do we know we are not the dream of an intelligent machine?]
“They both share the same problem which is the “3D world” that Peter mentions. Different observers in relative motion, even in close proximity, would be assigning distant time-like separated events as having this physical attribute of “now”.
”
O.k., until now I can follow, although it is unclear how do you define “close proximity”, and by how much time these events would be separated depending on he observers’ relative velocity.
”
If “now” is to have any sort of non-local special physical manifestation and we can show that it applies to time-like separated events then it equally applies to ALL time-like separated events.”
Why do you insist on the non-locality?
Are you familiar with dynamic programming? For example, look at how the Floyd–Warshall algorithm fills up its result matrix. There is absolute no need for absolute simultaneity in order to allow the universe to grow successively. The universe can just grow anywhere as long as the past light cone of the growing event already exists. At least in this growing case I cannot see any issues at all.
“Why do you say that? How about the alternative presented in the article: reality is a meaningless term. However, the division between what is certain and what is not is the past light cone, and you just have to accept that this is different at different events. There is no global notion of what is certain, but what is certain is much more than local.
“Either everything exists or nothing does” simply rejects, on grounds of your philosophy, numerous other alternative.”
Perhaps I don’t understand what is being claimed if we say that reality is a meaningless term. Does non-local reality have a physical representation or not? What does existence mean? Does the past have a physicality which is different from the future? What about the local “now”?
Kind of seems like we need more definitive statements. Simply accepting a Block Universe is an extremely definitive statement, btw, and in my opinion attempts to refute it are rooted in fear of Fatalism. Even the name FATALISM is scary, like adherents to it are busy killing off Hope, Free Will and The Human Spirit.
“Perhaps I was confusing Peter’s criticism of a specific argument with having a counter argument for another position, but I can’t seem to actually find his stance on the subject. If A, B and C are the only possibilities considered, and Relativity is problematic for B and C, then I don’t see where else there is to go. Perhaps Peter is claiming
D) Due to Relativity we can make no non-local claims about Reality.
To me, that’s the only other alternative, either everything exists or nothing does. ;)”
Why do you say that? How about the alternative presented in the article: reality is a meaningless term. However, the division between what is certain and what is not is the past light cone, and you just have to accept that this is different at different events. There is no global notion of what is certain, but what is certain is much more than local.
“Either everything exists or nothing does” simply rejects, on grounds of your philosophy, numerous other alternative.
“Did he actually agree that relativity is problematic for B and C?
Anyway, I am still struggeling to understand why relativity should be problematic to either B or C. Maybe you can explain this to me.”
They both share the same problem which is the “3D world” that Peter mentions. Different observers in relative motion, even in close proximity, would be assigning distant time-like separated events as having this physical attribute of “now”. If “now” is to have any sort of non-local special physical manifestation and we can show that it applies to time-like separated events then it equally applies to ALL time-like separated events.
“If A, B and C are the only possibilities considered, and Relativity is problematic for B and C, then I don’t see where else there is to go.”
Did he actually agree that relativity is problematic for B and C?
Anyway, I am still struggeling to understand why relativity should be problematic to either B or C. Maybe you can explain this to me.
“But as far as I understand, Peter did not aim to refute any of these possibilities. What he refutes is a specific argument in favor of A and against B and C.”
Perhaps I was confusing Peter’s criticism of a specific argument with having a counter argument for another position, but I can’t seem to actually find his stance on the subject. If A, B and C are the only possibilities considered, and Relativity is problematic for B and C, then I don’t see where else there is to go. Perhaps Peter is claiming
D) Due to Relativity we can make no non-local claims about Reality.
To me, that’s the only other alternative, either everything exists or nothing does. ;)
”
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.
“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 [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?