Unlearning the Block Universe: How Relativity Challenges Our Perception of Time

[PeterDonis]

this part of the premise derives from the fact that the mathematics do not privilege any events on the world lines of objects, over any other events.

The statement you make here is irrelevant to the second premise described in my article, because it is about the wrong set of events. The statement you make here is about the set of events on some observer's worldline. But a "3D world", which is what the second premise referred to in my article is about, is a spacelike 3-surface, not a timelike worldline. So it is simply a non sequitur to go from your statement here to any statement about 3D worlds, including the second premise I talk about in my article.

If it cannot have a causal influence on other events, then it cannot happen and will never be observed.

This is irrelevant, since nowhere has anyone claimed that there is any event in 4D spacetime that cannot have a causal influence on any other events. Such a statement is obviously false for Minkowski spacetime, and indeed for any globally hyperbolic spacetime, which includes any spacetime considered to be physically relevant in General Relativity. So you appear to be attacking a straw man here.

that allows us to fill in the blanks in our pictures of previous "nows"

Only the events in our previous "nows" that have since entered our past light cone. So this is perfectly consistent with my proposed alternative viewpoint.

A comment: you are using way too many words to belabor simple points. You could have stopped your discussion of this particular point right at the sentence I quoted above, and it would have gotten the point across just as well. Everyone else in this discussion is more familiar than you are with the math of relativity, and none of us get paid for this. Brevity is good.

If O1's calcuations are correct, how can an event be influencing other events cuausally, if it hasn't yet happened?

For every pair of events A, B for which O1 (or any observer) knows that A causally influenced B, both A and B are in O1's past light cone. So this is perfectly consistent with my proposed alternative viewpoint.

the criteria which you apply for testing events - events in the past light cone - applies equally in a Block Universe

No, it doesn't, because my criterion allows the set of which events are fixed and certain to be different for different events in spacetime. The Block Universe does not.

Btw, the statement I just italicized above is a crucial one, which I made and emphasized in my Insights article, and which I have made and emphasized previously in this very discussion, to make the point that no philosopher who has discussed this issue, to my knowledge, has ever addressed it. It might be a good idea for you to stop and take a step back and think very carefully about that statement, before posting again. It would save a lot of time and avoid us having to go back over and over the same ground, since if your response to that statement is simply "Well, I don't think that is even possible", then there is no more to discuss since we simply disagree on a fundamental point.

Contrast this with a presentist universe

To be frank, I don't see the point of discussing presentism any further here. We are not here to elucidate all of the fine points of all of the philosophical interpretations of relativity that have been proposed. I would strongly suggest that we focus discussion on the two alternatives that I discussed in my article: the Block Universe, and my proposed alternative in which only events in our past light cone are fixed and certain. Otherwise we are likely to be here long enough to make the point moot because all of 4-d spacetime will be in our past light cone anyway.

The corroborating photo simply shows that, during our 10th birthday other observers observed the same thing, but it doesn't tell us about the state of past events.

It doesn't tell us whether past events "still exist", because that's not a testable question.

It does tell us what happened at past events. If you deny that, then you are denying that our present records can give reliable information about past events, in which case, once again, there is no more to discuss since we simply disagree on a fundamental point.

Furthermore, denying that past events are fixed and certain, which is what you are arguing for here, is pointless if you are trying to argue that relativity requires the Block Universe, since the Block Universe accepts that past events are fixed and certain. The point of disagreement between my proposed alternative viewpoint and the Block Universe, which is really the only substantive point of discussion here, has entirely to do with how to treat events outside of our past light cone. So harping on how to treat events inside of our past light cone, the one set of events that my proposed alternative and BU agree about, strikes me as a waste of time. (See also my remarks about presentism above.)

The above doesn't contradict the Block Universe

You. Are. Missing. The. Point.

What was the point of my article? Was it that the Block Universe was false? Go back and read my post about what I said and didn't say in my article. We really need to keep this discussion focused and not continue to get bogged down in irrelevancies. Either there is a valid argument that relativity requires the Block Universe interpretation, or there isn't. You claim there is, or at any rate that you can't see why relativity doesn't require the Block Universe. Arguing that such and such does not contradict the Block Universe says nothing whatever about your claim. So please don't waste any more time doing it.

The rest of your post is just more repetition of the same errors or irrelevancies that I have already pointed out, so I won't bother responding any further to it.

 

[PeterDonis]

how an event which you say hasn't happened yet can have a causal influence in the Universe

Say event A causally influences event B. When event B happens, event A must be in its past light cone (otherwise event A couldn't causally influence event B), so any observer who observes event B will also have event A in his past light cone when he observes event B. So all the evidence anyone can ever have about which events causally influence which other events comes from their past light cones.

Once again, the key point here is that "hasn't happened" is relative to which event is your present event. Relative to some events, event A hasn't happened yet. Relative to other events, it has happened. The latter category includes all events that A can possibly causal influence (all the events in A's future light cone), so there is never any chance of any event that hasn't happened causally influencing anything.

You and all of the people you quote simply ignore this possibility altogether. You talk as if "hasn't happened" must be an absolute, that the "happening" status of an event can never change. But that is precisely the Block Universe interpretation! In other words, you are simply assuming your conclusion and arguing in a circle.

 

[Ibix]

Forgive me if this is naive, but isn't this entire argument just "how literally should I take a spacetime diagram"? You can always draw one. If you take it as a literal representation of spacetime (or as close as you can get on a Euclidean plane) then you have the block universe/eternalism, or a post hoc view of an accumulating spacetime. If you regard it as a stack of "snapshots" of "the universe now" you have presentism.

No interpretation is implausible because you can't actually draw the diagram until the whole thing is in your past light cone and you have local access to your memories or records. Or at least, parts of it must be forecasts if you draw it earlier.

 

[vanhees71]

I don't know, but I've the impression one should introduce a subforum about "Foundations" (aka "philosophical gibberish") for the Relativity Forum too, as for the Quantum Forum. SCNR.

 

[martinbn]

I haven't read the whole thread, but from the first post I got the impression that you wanted to unlearn this in order to learn relativity better. The whole thread seems to aim at the exact opposite. Why don't you just forget about block universe, LET, presentism and all that is not physics and just study relativity.

 

[Lynch101]

A comment: you are using way too many words to belabor simple points. You could have stopped your discussion of this particular point right at the sentence I quoted above, and it would have gotten the point across just as well. Everyone else in this discussion is more familiar than you are with the math of relativity, and none of us get paid for this. Brevity is good.

Thank you, Peter. I have a tendency to write too much in an effort to express my thoughts more clearly. I know my use of language is not as precise as it should or could be, so I end up writing more and more in the hope that I clarify my thinking. I will err on the side of brevity in future.

In the interest of Brevity, I haven't responded to every point of discussion but I've restricted it to those you've highlighted as the key issues, and the ones I believe are key. If you would like me to reply to the other points, I can certainly do that.

All emphasis in any quotes below are my own.

The point of disagreement between my proposed alternative viewpoint and the Block Universe, which is really the only substantive point of discussion here, has entirely to do with how to treat events outside of our past light cone. So harping on how to treat events inside of our past light cone, the one set of events that my proposed alternative and BU agree about, strikes me as a waste of time. (See also my remarks about presentism above.)

Thank you! These clarifications help me to understand your article better.

It's more likely that the error is on my side, but is it possible that you have, ever so slightly, mischaracterised Penrose's argument and that for the Block Universe? In the article you say:

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.

I don't think this is borne out by the argument you cite, however. I actually think that Penrose tacitly applies your "fixed and certain" criterion.

As per the cited argument:

In fact neither of the people can [at the time of passing each other] 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.

This would suggest that it is not at the given event (passing each other) that each observer divides the universe into the "uncertain future" and the "certain past", rather it is when the event enters their past light cone. Armed with this information:

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.

It is only after the fact that, with the information that has arrived from the event, that they designate events below one observers surface of simultnaeity as being in the "certain past". Penrose could perhaps have talked about the event laying in the other observer's "certain future" because of course the event is later observed. But he uses "uncertain future" to illustrate that, according to the other observer, it hadn't happened at the moment they met.

So, it is after the fact that they calculate what the state of the Universe must have been at that time. According to one of them, the event had already happened, while according to the other, it had not. This means, that when they met each other (at t=0) , the event was already having a causal influence in the Universe. As Weinert puts it, "if an event is already determinate for one observer, it must be determinate for all observers."

The statement you make here is irrelevant to the second premise described in my article, because it is about the wrong set of events. The statement you make here is about the set of events on some observer's worldline. But a "3D world", which is what the second premise referred to in my article is about, is a spacelike 3-surface, not a timelike worldline. So it is simply a non sequitur to go from your statement here to any statement about 3D worlds, including the second premise I talk about in my article.

It must have been this paragraph that caused me to think of the Sider's argument.

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.

Am I correct in thinking that the paper doesn't refute Sider's particular point then?

If we think of Penrose's Andromeda paradox in terms of the events on the Andromeda world line, instersected by the simutlaneity surfaces of the observers on earth, then we can talk about two different events on the Andromeda world line at t=0. The mathematics doesn't indicate any preference for one event over the other. The BU ineterpretation takes the mathematics at face value and says that the Universe comprises (in a general senes) all the events on an objects world line.

This is irrelevant, since nowhere has anyone claimed that there is any event in 4D spacetime that cannot have a causal influence on any other events. Such a statement is obviously false for Minkowski spacetime, and indeed for any globally hyperbolic spacetime, which includes any spacetime considered to be physically relevant in General Relativity. So you appear to be attacking a straw man here.

Events which have not happened cannot have a causal influence on other events. These would be deemed "unreal" under the criteria. Events which have happened can have a causal influence on other events. These would be called "real events". Any event which can be said to be having a causal influence on other events is considered "real" under this criterion.

In the Andromeda parpadox, at t=0 the decision to launch the fleet is having a causal influence because according to one observer, the fleet has already left.

 

[Ibix]

In the Andromeda parpadox, at t=0 the decision to launch the fleet is having a causal influence because according to one observer, the fleet has already left.

No! The observer is guessing that the fleet was launched, based on 2.5 million year old light. The launch can have no causal effects on Earth for another 2.5 million years.

 

[Lynch101]

Say event A causally influences event B. When event B happens, event A must be in its past light cone (otherwise event A couldn't causally influence event B), so any observer who observes event B will also have event A in his past light cone when he observes event B. So all the evidence anyone can ever have about which events causally influence which other events comes from their past light cones.

This past light cone criterion is the one that Penrose utilises in the Andromeda paradox. The observers then "hark back" and reconstruct a picture of the world based on the gathered information.

Once again, the key point here is that "hasn't happened" is relative to which event is your present event. Relative to some events, event A hasn't happened yet. Relative to other events, it has happened. The latter category includes all events that A can possibly causal influence (all the events in A's future light cone), so there is never any chance of any event that hasn't happened causally influencing anything.

You and all of the people you quote simply ignore this possibility altogether. You talk as if "hasn't happened" must be an absolute, that the "happening" status of an event can never change. But that is precisely the Block Universe interpretation! In other words, you are simply assuming your conclusion and arguing in a circle.

The present event in question is shared by both O1 and O2. It is their meeting at t=0. Both observer's are in each others present. According to O1's calculations (at a later time, when the events are in the past light cone) the event E happened before that now, while according to O2's calculations it happened after that "now". Before they cross paths and after they cross paths is not a frame dependent event.[/QUOTE]

 

[Lynch101]

Forgive me if this is naive, but isn't this entire argument just "how literally should I take a spacetime diagram"? You can always draw one. If you take it as a literal representation of spacetime (or as close as you can get on a Euclidean plane) then you have the block universe/eternalism, or a post hoc view of an accumulating spacetime. If you regard it as a stack of "snapshots" of "the universe now" you have presentism.

No interpretation is implausible because you can't actually draw the diagram until the whole thing is in your past light cone and you have local access to your memories or records. Or at least, parts of it must be forecasts if you draw it earlier.

The question of how literally to take a spacetime diagram is one prong of the argument, with that argument being there is nothing in the mathematics which privileges certain events over others.

The other prong is how to make sense of the observations that would happen. The Andromeda paradox is a good illustration of that particular argument. If you are walking down the road and and meet your friend and there is an event which, according to your reference frame has already happened, but according to your friend's frame, it has not yet happened, then how do we make sense of that, without a block unvierse?

At the time, of course, neither of you know about this event, but later you each make your own observations and then calculate when the event happened, your calculations show that it happened before you met your friend, while her calculations tell her that it happened after the two of you met.

 

[Lynch101]

I don't know, but I've the impression one should introduce a subforum about "Foundations" (aka "philosophical gibberish") for the Relativity Forum too, as for the Quantum Forum. SCNR.

A philosophy of science forum would be great! My primary interest is to try to understand what implications of physical theories are, or what they say about the world. I know this is not everyone's cup of tea though.

 

[Ibix]

If you are walking down the road and and meet your friend and there is an event which, according to your reference frame has already happened, but according to your friend's frame, it has not yet happened, then how do we make sense of that, without a block unvierse?

You agree that you described the situation differently. There is no detectable effect of the difference in description. The end.

The only addition from using something like presentism is that at least one of you is using a frame that doesn't have thesame definition of past and future as the "real" frame. But there's no consequence to this in a relativistic universe, so who cares?

 

[Lynch101]

I haven't read the whole thread, but from the first post I got the impression that you wanted to unlearn this in order to learn relativity better. The whole thread seems to aim at the exact opposite. Why don't you just forget about block universe, LET, presentism and all that is not physics and just study relativity.

What I have learned about the BU, I have done through the lens of the BU because that is the version that seems to be most prominent in pop-science. That was really my first encounter and so I tried to learn more and more, because it was fascinating to me. Pop-science material was the most accessible to me at the time and so I started discussing it online. Maybe it was just happenstance that those I engaged with at that time were ardent "Blockers" but it was in that context I developed a conceptual understanding of relativity.

A lot of the arguments that are being put forward here are similar to arguments I made myself when I first encountered relativity - mine, of course, were not as rigorous. I was coming from a more Newtonian perspective so to speak, and so I was challenging things from the perspective of their being a global "now". Those I discussed with put forward the counter arguments, in favour of the Block Universe.

It is from this position now that I am trying to see how relativity doesn't necessitate the BU. In that sense I am putting forward my understanding of the arguments I have previously learned and hoping that the assumptions can be pointed out.

 

[Lynch101]

No! The observer is guessing that the fleet was launched, based on 2.5 million year old light. The launch can have no causal effects on Earth for another 2.5 million years.

This is the citation that Peter uses in his insight article.

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.

It is of course, entirely hypothetical, but there is no guessing involved. They make an observation at a later time and then "hark back" i.e. they reconstruct a picture of the world based on their calculations.

 

[Nugatory]

The Andromeda paradox is a good illustration of that particular argument. If you are walking down the road and and meet your friend and there is an event which, according to your reference frame has already happened, but according to your friend's frame, it has not yet happened, then how do we make sense of that, without a block unvierse?

To make sense of it without a block universe you recognize that in this context both you and your friend are using the word “happened” to mean something different than if you and your friend were discussing what “happened”, for example, last night when your neighbor’s house sadly caught fire.

 

[Lynch101]

You agree that you described the situation differently. There is no detectable effect of the difference in description. The end.

The detectable effect is what leads to them calculating when the event must have happened and when it must have started having a causal influence on other events.

The only addition from using something like presentism is that at least one of you is using a frame that doesn't have the same definition of past and future as the "real" frame. But there's no consequence to this in a relativistic universe, so who cares?

Presentism requires the additional ad hoc postulation of an undetectable reference frame doesn't it? As well as invoking some mysterious dynamics to explain phenomena such as length contraction.

Lorentz's aether theory would be an example of a presentist interpretation wouldn't it? Is it possible to generalize that to General Relativity?

 

[Lynch101]

To make sense of it without a block universe you recognize that in this context both you and your friend are using the word “happened” to mean something different than if you and your friend were discussing what “happened”, for example, last night when your neighbor’s house sadly caught fire.

Are they not using it in the same way? If both observes meet at their neighbours house as it catches fire, one says the event happened before the fire ignited, the other says it happened after the first ignited, they are both using it in the same way. The event started having a causal influence on other [distant] events prior to this moment, the other says it started having a material effect after that moment.

 

[Ibix]

It is of course, entirely hypothetical, but there is no guessing involved.

Not at that point, no. But that's 2.5 million years after the fleet left, when it has had a causal effect. 2.5 million years earlier, at around the time the fleet left according to one frame, it does not have a causal effect and the observer is just guessing that the fleet has left. So the fleet's departure (or failure to depart) does not have a causal effect at $t=0$, just at $t=2.5\mathrm{million}$, contrary to your statement in the post I quoted.

 

[Lynch101]

Not at that point, no. But that's 2.5 million years after the fleet left, when it has had a causal effect. 2.5 million years earlier, at around the time the fleet left according to one frame, it does not have a causal effect and the observer is just guessing that the fleet has left. So the fleet's departure (or failure to depart) does not have a causal effect at $t=0$, just at $t=2.5\mathrm{million}$, contrary to your statement in the post I quoted.

I haven't said it has a causal effect on the observers at t=0. I'm saying it had started having a causal effect in the Universe prior to that moment. This is calculated after the fact.

The observation is made 2.5m years later. After that observation, the observers "hark back" to their meeting and reconstruct a picture of the world as it must have been at that moment. One says the fleet had launched prior to their meeting, the other says it launched after.

If we think in terms of a prisoner being executed. One observer says that the prisoner was dead prior to the meeting, the other says he was alive during the meeting. This is calucalted years later, but this is the picture of the world they reconstruct.

 

[PeterDonis]

This would suggest that it is not at the given event (passing each other) that each observer divides the universe into the "uncertain future" and the "certain past"

You're missing the point. There are two issues involved.

First, the division into "uncertain future" and "certain past" is being made based on surfaces of simultaneity, which are frame-dependent, instead of light cones, which are invariant. The argument is based on objecting to the fact that what is "uncertain" vs. "certain" should not be frame-dependent (as a quote you give later says, it should be the same for all observers); but it is only the choice of criterion for division (surfaces of simultaneity) that makes the division frame-dependent. Doing the division based on the past light cone (what is in the past light cone is "certain", what is outside it is "uncertain") would make the division invariant. So the argument fails to consider a third alternative that would equally well avoid what it wants to avoid (having what is "uncertain" vs. "certain" be frame-dependent), and is a false dichotomy.

Second, the argument uses future data ("future" meaning in the future light cone from the standpoint of the two observers at the event on Earth at which whether or not the launching of the Andromedan fleet is in the "future" vs. the "past" based on surfaces of simultaneity) to figure out after the fact how the launching of the Andromedan fleet relates to the two surfaces of simultaneity. But, as you correctly point out, the launching of the fleet is certain (as in "in the past light cone") at the future event on Earth when all the data is available. The argument implicitly assumes that, if the launching of the fleet is certain at that future event on Earth, it must also have been certain at the previous event on Earth when the two observers pass each other. But that implicitly assumes that what is "certain" cannot change from event to event--which, as I have already pointed out, is assuming the conclusion and arguing in a circle.

Events which have not happened cannot have a causal influence on other events

I've already addressed this: any event A can only causally influence another event B if B is in A's future light cone, and if B is in A's future light cone, A is in B's past light cone and therefore has "happened" according to B. Further, A will also be in the past light cone of any event that has B in its past light cone, so all observers for which B has happened will also agree that A has happened.

Again, you are ignoring a third alternative that avoids what you want to avoid, so you are arguing from a false dichotomy. Your avoidance of the alternative amounts to implicitly assuming that what has "happened" cannot change from event to event, which, again, is assuming your conclusion and arguing in a circle.

Are you beginning to see a repetitive pattern here? I sure am.

This past light cone criterion is the one that Penrose utilises in the Andromeda paradox.

No, it isn't. See above.

I am putting forward my understanding of the arguments I have previously learned and hoping that the assumptions can be pointed out.

This has already been done, repeatedly. You are simply not addressing those responses, but continuing to repeat the same points, which then get the same responses again and again.

this is the picture of the world they reconstruct.

Only if they treat frame-dependent things as if they were "real", and assume that what is "real", or has "happened", or is "certain" cannot change from event to event. This point has been repeatedly made and you have failed to respond to it.

 

[PeterDonis]

Thread closed for moderator review.

 

[Dale]

After moderator review we will keep it closed. There seems to be no progress and no point in repeating the same comments.