PeterDonis said:
It's the same argument that I refute in the Insights article. I don't see the point of refuting it again here.
A good example is the quote you gave from Friedel Weinter, which, now that I read what that person actually said, turns out to be the same argument I already refuted in the Insights article. If I had only your description to go on, I would not have known that and would not have been able to save time by stating that I've already refuted that argument once and don't see the point of doing it again.
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No, but what the argument actually is is best assessed by reading the argument as it is presented by the person who originally made it. That is not you. I don't want to read what you think someone else's argument says. I want to read what they say.
Thank you Peter, I appreciate your taking the time thus far. I understand that it must be frustrating to be going over the same ground again. The purpose of this thread is, essentially, to understand your article better.
My understanding of your argument is that you say BU proponents "help themselves" to the second premise of the argument, that all events are "equally real". That is similar to a point I had raised myself, when first learning about relativity, but the counter argument that was put to me is that proponents don't just "help themselves" to this part of the premise, 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. This is demonstrated by the quote from Theodore Sider.
That part of the premise is arrived at by taking the mathematics at face value and not adding any additional assumptions or intuitions. If that is true, as you seem to have confirmed yourself, then I don't think that the contention stands - that the BU helps itself to the second premise.
PeterDonis said:
That's correct: the causal relationship between events, i.e., whether they are timelike, null, or spacelike separated, is invariant and independent of any choice of coordinates.
This, however, is wrong. Events are points in spacetime. The causal relationship between E and any other point in spacetime is what it is; it can't change, because "where" in spacetime a particular point is can't change.
I might not have elucidated that point clearly enough. I'm not suggesting that the causal relationship between E and any other point in spacetime can change, its simply a statement about E's ability to have a causal influence on other, unspecified events. If it cannot have a causal influence on other events, then it cannot happen and will never be observed.
Of course, we can only work out after the fact when an event happened, and started having a causal influence, but that allows us to fill in the blanks in our pictures of previous "nows". Take the example of Albert standing on the platform, midway between two light sources, which happen simultaneously in his frame. He can only know that the events happened simultaneously after the light has reached him. However, once the light has reached him, he can calculate that those events must have happened at a moment prior to when the light reached him, given the finite speed of light. Here therefore knows that these events were having a causal influence on other events "elsewhere" as the light made its way towards him. He concludes that in his frame, the events happened at the precise moment when Hendrick was co-located with him (or when he passed him at the midway point). Studying is spacetime diagrams, he can see that, according to Hendrick's frame, the event/light was having a causal influence on other events prior to the moment when they crossed paths. It must have been having such a casual influence because otherwise, it would not have arrived to Hendrick when it did, or it wouldn't have arrived to either of them.In the example above, if we say that t1=t2=0 and we also say that t1=t2=0= O1's 30th birhtday and O2's 30th birhtday. At t=0 event E is having a causal influence on events "eleswhere" in the universe. Neither O1 nor O2 can know this at t=0, but after the fact, they can do their calculations and draw their spacetime diagrams and analyse the information.
Both can see that at t=0, event E was already having a causal influence on other events before O1 and O2 crossed paths. How can an event have a casual influence if it hasn't happened yet? How can it have happened prior to the moment when O1 and O2 cross paths, according to O1, yet not yet have happened at exactly that moment according to O2, where O2 would say that it was in her future? The co-ordinate reference frames might be arbitrary but the causal influence of an event is not dependent on the choice of co-ordinates.
If we think in terms of 30th birhtday's, at t=0 both turned 30. O1 says event E happened before their joint 30th birthdays. That is, O1 says that the event was already having a causal influence on other events in the Universe prior to their 30th birhtday encounter. O1 says that event E was having a causal influence on other events at a moment when O2 says the event hadn't happened yet. If O1's calcuations are correct, how can an event be influencing other events cuausally, if it hasn't yet happened? The Block Universe would appear to be the only reasonable solution to this, from the arguments I've heard.
PeterDonis said:
What would be true is to say that, while E cannot have a causal influence on an event spacelike separated from it, say some event A, it can have a causal influence on events in the future light cone of A, some of which might be on the worldline of some object whose worldline also passes through event A. I suspect this sort of thing is what you are actually thinking of.
Yes, but that is not all we can say, as outlined above.
PeterDonis said:
Yes. But we're not in a general context here. We're in a specific context in which a claim is not scientific unless there is a way to test it. There's no way to test what events "comprise" the Universe that I can see.
I didn't find this answer very satisfying when I first heard it myself, but I found it hard to dispute and ultimately convincing of the case for the Block Universe. Maybe you will be able to help me: the criteria which you apply for testing events - events in the past light cone - applies equally in a Block Universe. At every point along your world line the you that corresponds to that moment has observed all the events in the past light cone. We do not have the knowledge of future events because of our location in spacetime, but there is a future version of ourselves for whom those events are in the past light cone, and so your own criteria would be fulfilled.
We arrive at this conclusion, not by assuming it, but by taking the mathematics at face value and not adding any assumptions or intuitions.
PeterDonis said:
The block and growing block unvierses would say that events are fixed and certain because they are in our past light cone and the universe comprises them (in the general meaning of the word), or they persist in the overall structure of the universe - this is what gives it a block strucutre.
Contrast this with a presentist universe, which doesn't have such a block structure. In a presentist universe events are fixed and certain because they are over. However, the Universe does not comprise (in the general meaning of the word) past events. They do not persist in the structure of the Universe, hence why there is no block structure.
It might help to think of a presentist universe as one which is extended in only 3 spatial dimensions, but not in a temporal dimension (its more of a single temporal point), while a 4D block universe is extended in 3 spatial dimensions as well as extended temporally. In these different contexts "fixed and certain" has different connotations.
PeterDonis said:
Wrong. Our observations are of information reaching us at our present event, but that information comes from events in our past light cone, and only those events--it is impossible for us to get information at our present event that is outside the past light cone of that event, because information cannot travel faster than light.
Not only that, but you can in principle continue to get new information about the same past events. For example, you find out that someone else also took a photo at your 10th birthday party, and what is in their photo is consistent with what is in your photo. That is a fresh test of the fact that past events are fixed and certain.
Thank you, I should have been more precise.
We cannot test events which are in our past, which are over according to us. We cannot test our 10th birthday to see if it has changed. To do so would require time travel. The photo of our 10th birthday is a record that was taken at the time, but they are separate events in and of themselves i.e. the records are not the events in themselves. We can see this by virtue of the fact that the records persist in the present. We can test the photo to see if it has changed, but we cannot test the past event.
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. If we burn the photos, it has no bearing on the past events because they are separate. Now, that is not to say that the past does change, its simply that we cannot test it.
We can only carry out tests in the present on light which reaches us from events which happened in the past. This is part of the reason how, in the example given above, O1 and O2 can calculate when event E had to start having a causal influence in the Universe in order to arrive at O1 when it does. They conclude that, it must have started having this causal influence prior to t=0. However, at t=0, event E was in O2's future. So, O2 would have to agree - on the basis of knowing what relativity shows - that an event in their future was having a causal influence in the universe. O2 would have to conclude that, at t=0, the Universe comprises (in a general sense) events which O2 considered to be in the future. If it didn't, then it wouldn't have the causal influence on O1 that it ends up having.
This conclusion is supported by the fact that the mathematics doesn't privilege any particular events on an objects world line over any other events i.e. by taking the math at face value and not adding any additional assumptions or intuitions.
PeterDonis said:
How would you test this? The only way to test it is to observe such an influence, and the only way to do that is for all the events in question to be in your past light cone. So any event for which you have evidence of it being "real" in this sense must be in your past light cone, and hence is fixed and certain according to my proposed view.
Indeed, all of the evidence points to its validity. If an event which doesn't happen is demonstrated to have a causal influence, the the hypothesis would be invalidated.
The above doesn't contradict the Block Universe however, as outlined above. If no moment along an observers world line is more special than any of the others, as the mathematics when we don't append anything to them, then at every moment that observer observes what is in their past light cone. This is true for every moment along the world line, up until the observers death, and beyond (given that their corpse persists). Each moment can only observe the past light cone for that moment, but if no moment is singled out over any other, then all events are observed up until the death of the observer.
Again, this isn't assumed it's what the mathematics seems to imply.
No. See above.
PeterDonis said:
That's what you typed, and that's what I read. So I don't know what you think you have changed here.
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My response has not changed, since "fixed and present", as above, is what you typed before, and what I responded to.
My apologies, I did it twice!
I meant "fixed and certain"
PeterDonis said:
No, but what the argument actually is is best assessed by reading the argument as it is presented by the person who originally made it. That is not you. I don't want to read what you think someone else's argument says. I want to read what they say.
OK, so there are two arguments. One w
PeterDonis said:
The argument I refuted in my Insights article is the argument I am familiar with. So far, the only actual reference to an argument that you have given (the Friedel Weinter quote you gave) is making that same argument, as I noted above. I don't see the point of refuting the same argument over and over; the whole reason I wrote the Insights article was to not have to do that.
There are two references there, one from Friedel Weiner and the other form Theodore Sider in
this post (at the bottom). The argument from Sider is a refutation of your contention that Block Universe proponents "help themselves" to the second premise. His is an example of the point I was making, for which you asked a reference on; that the second premise is assumed. Sider points out the fact, which you reiterated, that the mathematics does not single out any event(s) over any others on the world line of an object. It is on this basis that BU proponents base the statement that all events on an objects world line are "equally real".
The argument from Weiner is separate, but related. I'm not sure your article actually refutes that point either because it seems to focus on the contention that the conclusion is assumed. You mention the Andromeda paradox, but only in the context of assuming the conclusion. The argument from Sider speaks to this point and, I believe, demonstrates that the premise isn't assumed.
The argument from Weiner would still require an answer, however, as to how an event which you say hasn't happened yet can have a causal influence in the Universe - not on you, but on other objects in the Universe - as it must do, in order for the observer moving relatively to you, to observe it when they do.
PeterDonis said:
This context is pretty useless without knowing what the different views he refers to actually say.
Not for the point that is being made. The point being made is that BU proponents don't assume the premise, it is based on the fact that the math doesn't privilege any events on an objects world line over any others. You asked for a reference for this particular point, to which Sider is that reference.
The context he provides says that other hybrid models do privilege specific events. If you are interested in those other models, I have referenced the name of the book there and the pages. But, that isn't necessary for the point that the math doesn't privilege any events over any others, this just offers a juxtaposition for added content. He also makes the explicit statement on the point, which preceded it and which emboldened. The latter part was just for additional context.
PeterDonis said:
However, I think there is a general point that can be made. It seems to me that this author is implicitly struggling with an idea that he doesn't state explicitly: the idea that what is "real" depends on which event in spacetime is your present event. That is what he seems to be objecting to when he talks about "privileged" points in spacetime. But of course which point is "privileged" in this sense is just whichever point is our present event--our "here and now". And this event changes as we experience our lives. So there is no point that is always privileged; there is just the obvious fact that which event in spacetime is our "here and now" is not always the same. Every point on our worldline gets its chance to be "privileged", and other points on other observers' worldlines get their chance to be "privileged" as well.
What would lead you to conclude that the author is implicitly struggling with that?
Indeed, you might be correct, but he still points out that the reason for not privileging any event over any other is because the mathematics doesn't stipulate this. Therefore, to designate any event as the "real" present, for any reason whatsoever, is to add an additional assumption that isn't implicit in the mathematics. As he says, there is an interpretation which doesn't require this assumption, and that is the Block Universe. So, I don't think the contention of your article stands.
In addition, none of the above contradicts the Block Universe interpretation.
PeterDonis said:
I discussed this point in the Insights article. Relativity does not require that there has to be any criterion for what is "real" that is the same for all events in spacetime. It is perfectly consistent with the view that what is "real" for us, here and now, might be different from what is "real" for us tomorrow, or what is "real" for observers somewhere in the Andromeda galaxy at an event spacelike separated from us here and now. This, of course, is an obvious implication (if we treat "real" as synonymous with "fixed and certain") of my proposed view that only events in the past light cone of our present event are fixed and certain.
It is precisely that relativity does not set out any criteria for what is "real" that all events should be considered equally "real". There is nothing in the mathematics to distinguish a "real" present or a "here and now" form any other event on the world line.
The Block Universe is perfectly consistent with the points you raise above, it just doesn't add anything to the math to designate something as "real for us" or "here and now" because the math doesn't set out the criteria for any such ideas. As you have said, the "fixed and certain" criteria you set out is perfectly consistent with the Block Universe.