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

[jbriggs444]

Past, present, and future events are not being defined as real. To paraphrase, the question was "what does it mean to say past, present, and future events are "real"? The answer I got to this at the time, and I see how it makes sense, is that we don't need to define what it means to be "real", its sufficient that whatever criteria apply to events in the present moment, equally apply to past and future events. They are as real, or as unreal as each other. What the actual meaning of the term "real" is, can be left to the philosophers.

If you do want to avoid constraining "real", that includes not specifying whether it does or does not apply equally to events in the "past" and "future", whatever those terms might mean.

 

[PAllen]

Ah, OK. This is certainly new to me, thank you!

Does this then mean that relativity permits a global "now", although observers will disagree about what the content of that global "now" is?

Not quite. I am proposing that you may believe there exists a unique global now, but you will never be able to find out what it is except that your local present is part of it. Such a belief in something you can't verify isn't really different from belief in the block universe. Both beliefs are compatible with relativity, neither can be verified.

 

[PAllen]

I think a Newtonian Block Universe would have certain experimental consequences, if I remember correctly. I'd have to try and refresh my memory, but I think that might be a subject for a different thread.

It could not possibly have any experimental consequences. Don't know what you might be thinking of.

 

[Nugatory]

Unfortuately, I wouldn't be sure how to go about rigorously defining the term "now".

Here it is then: The events that are happening now are the ones whose time coordinate is the same as the time coordinate of the event “I just said ‘now’”. That can be extended into the more general notion of “at the same time”: all events that have the same time coordinate are said to have happened at the same time. An important special case is when an event has the same time coordinate as the event “my wristwatch reads X”; I will say that the event happened at time X.

These terms ("now", "happened when", ...) tell us more about how we assign time coordinates to events than the relationships between events.

 

[Dale]

Yes, but Lorentz aether theory utilises a different conceptualisation of time.

Of course. If you compare presentism and eternalism you will clearly have a different conceptualization of time.

The point is that both conceptualizations are compatible with Newtonian physics and both conceptualizations are compatible with relativistic physics. Neither conceptualization is forced on us by the physics.

By “the physics” I mean the experimental evidence and the mathematical framework used to make experimental predictions. It seems like you may be adding some philosophy to the physics in what you understand relativity to be. So you are already assuming a specific conceptualization of time which is not part of “the physics”.

The key point, however, is that Lorentz aether theory does not incorporate the relativity of simultnaeity in truth, because it retains the absolute simultaneity of Newtonian mechanics.

So LET shares all of the mathematical framework and all of the experimental predictions of the block universe. The physics of both is the physics of SR. They differ in their conceptualization of time. This shows that the conceptualization of time is not part of the physics

 

[Lynch101]

That doesn't help because it doesn't tell me how to test whether it applies equally to past, present, and future events. Basically this looks to me like a way of dodging a question one doesn't have a good answer to.

You keep talking as if this is a viable alternative. It's not. As I've already explained, at a minimum, the events in the past light cone of your present event are fixed and certain, which would seem to mean they, at least, must "exist".

I also note that you keep switching terminology. First it was "real", now it's "exist", and in another post you used "form part of the structure of the Universe". None of those are really scientific terms; I've already explained why "real" isn't, and the explanation for "exist" or "form part of the structure of the Universe" would be similar. I suggested "fixed and certain" as a better alternative; see further comments below.

I've grouped these comments together because I think they all speak to the same point. I acknowledge that I keep changing terminology. That is an attempt to find a more suitable way of describing what the Block Universe says. That is what I am trying to do first, simply establish what the BU says.

===============================================
I'm returning to the top of this reply just to mention that I have been replying to your proposal of the term "fixed and certain" below. It certainly a preferable term to that of "real" but I think it needs some further clarification before it can be used in this context. Perhaps, more accurately, I need further clarification before I will be able to start using it in this context.
===============================================

What does the fox BU say?
You are obviously well familiar with the Block Universe, given your writings on it; and hopefully I'm not confusing you with another poster, but I think you've said that the Block Universe is a possible interpretation of relativity. But , you argue that it is not an absolute necessity, as often tends to be presented. Am I correct in that?

To start, we're just trying to establish what the familiar picture of the Block Universe says - aside from the claim that it is a necessary conclusion. Prior to that, we just need to be on the same page about what the BU says about past and future events. You might be able to state that more rigorously than I can but the BU definitely says something about past, present, and future events. It says something about past and future events that makes it different to a universe that is based only on a global "now".

Can we say past and future events have the same status?/ontology?/constitution?/[insert the most precise possible term here] as present events.

Ontology
Instead can we talk about the ontology of events that constitute a world line. The BU says that all points on a world line are ontologically equal. No single event is preferred over another. The observations we make in our scientific experiments might correspond to single events on our world line but that doesn't single it out as special.

To give a real world example, again, of our 10/30/50th birthdays. If now is our 30th birthday then we consider our 10th birthday to be in the past and our 50th birthday to be in the future. A universe with a global present comprises only the event of our 30th birthday, only a single event on our world line. The Block Universe, however comprises all of those events 10/30/50th birthdays. None of the events is singled out over the others.Dodging the question
I don't want to get bogged down on this particular point because I know how endless these philosophical rabbit holes can be, so I am keen to avoid such a debate. I think it's more important to just get on the same page as to what the BU says about past, present, and future events. I don't want to dodge the point above, however.

The way it is used here, the term "real" isn't a set of criteria which something must fulfil, it is instead simply a label that we can try to apply in our discussion. If we apply the label "real" to present events, say our 30th birthday, then the Block Universe says that label applies equally to past events, our 10th birthday, as well as future events, such as our 50th birthday. We don't need to know the true nature of reality to use the term in this way, instead, if we say that our 30th birthday is real, then we must say the same thing about our 10th and 50th birthdays.

We could use a different label, but "real" has the benefits of our preconceptions of what it means. It is a double edged sword however, for the reason you raise above, but any other term would probably cause more confusion.

No, it doesn't. Newtonian physics is perfectly consistent with viewing the past as fixed and certain. It is also consistent, as has already been pointed out, with a "block universe" view in which the future is fixed and certain as well as the present and past (this works because Newtonian physics is deterministic). So it seems like you were given an incorrect view of Newtonian physics as well as relativity.

Thank you Peter, yes. My apologies, when I refer to the Newtonian picture I am referring to the conception of it that is best juxtaposed with the Block Universe to offer a contrast. From here on, if I say the Newtonian picture, I am referring to the conception that is based on a global "now" not on a Newtonian Block Universe.

That what happens at them is fixed and will not change. Or, to put it another way, if we consider all possible 4-d spacetime models that could be realized, given what you know at your present event, all of them will have the same set of events (things that happen) in the past light cone of your present event. But not all of them will have the same set of events (things that happen) outside of the past light cone of your present event.

As you have mentioned above, the term "fixed and certain" can be applied equally to a Newtonian universe, either with a single global "now" or a Newtonian block universe, to a relativistic block universe, or in another way that isn't equivalent to the BU.

These represent very different pictures of the Universe, so the idea of "fixed and certain" would need some additional clarification. You mentioned above that the BU considers future events to be "fixed and certain", so, according to your application, we only consider events in the past light cone to be fixed and certain, it sounds very much like the conceptualisation usually referred to as "the growing block universe".

You say

My proposal is consistent with this view, as far as I can see, but my proposal has the advantage, from a scientific point of view, of not making claims that are not scientific.

As you have said, your proposal is consistent with all the different views, but all of the different views present very different pictures of the Universe, so I think it is necessary to clarify what we are referring to when we use it.

Is it intended to be more of a catch all phrase?

You say

As I've pointed out, block universe proponents ignore this obvious fact because they look at models that they have constructed, in which they declare by fiat what events happen everywhere in the 4-d spacetime of their model. But the real world doesn't work like that. You can't dictate by fiat what happens outside your past light cone. You can try to predict what will happen outside your past light cone, but those predictions can never be perfect, because you don't have sufficient data in your past light cone to determine for certain what will happen at any event outside your past light cone.

Note that this is true even if the fundamental physical laws are deterministic; even in a deterministic system, in order to have sufficient data to fix all events everywhere in the spacetime, you need to have initial data on an entire spacelike 3-surface. But no past light cone contains such data, and nobody ever has or ever will have such data. I discuss this in my Insights article (and IIRC there was more discussion of it in the comment thread on it).

Absolutely agree on this point. We cannot define what happens outside our past light cone.

I don't think this affects our conclusion though. We can talk strictly about the events that constitute our own world line and reason from there.
You mentioned

The fix for this is simple: don't try to learn actual science from pop science sources. Brian Greene in particular is a frequent offender--if I had a dollar for every PF thread where we've had to correct someone's misconceptions based on one of his pop science books or shows or videos, I'd be retired now.

If I could go back and choose again (and if free will really exists) I would choose to study physics. Unfortunately, my past is fixed and certain so that isn't an option.
I am grateful for pop-science though because it has allowed me to at least engage with physics, where otherwise I would not have. My understanding of physics is certainly better because of pop-science material, because without it, it would be non-existent. Even if it means the arduous task of unlearning some things, but I feel like it's at least given me a foundation from which to start.You say

This argument is simply wrong. It's not even as good as the argument I refuted in my Insights article, which at least made some pretense of using concepts specifically from relativity. This argument could just as well be made for Newtonian physics, and is just as wrong when applied to that. The obvious flaw is the smuggling in of "past and future events" as though they had to go together, when they obviously don't.

I'm not sure what you mean by smuggling in "past and future" events (with emphasis on and). Are you implying that relativity necessitates a block universe which comprises past and present events, but not necessarily future events?

Or are you saying that neither past nor future events can be "smuggled in"?I'm not actually sure how I'm smuggling anything in. I'm simply juxtaposing two alternatives and saying if not this, then that.

 

[Lynch101]

If you do want to avoid constraining "real", that includes not specifying whether it does or does not apply equally to events in the "past" and "future", whatever those terms might mean.

If now is your 30th birthday, then your 10th birthday would be a "past event", while your 50th birthday would be a "future event".

 

[Lynch101]

Not quite. I am proposing that you may believe there exists a unique global now, but you will never be able to find out what it is except that your local present is part of it. Such a belief in something you can't verify isn't really different from belief in the block universe. Both beliefs are compatible with relativity, neither can be verified.

That's true, but we can specify that either a global now exists, or it doesn't. If a global now does exist, then it would necessitate absolute simultaneity. Relativity tells us that this is not the case, doesn't it?

 

[Lynch101]

Here it is then: The events that are happening now are the ones whose time coordinate is the same as the time coordinate of the event “I just said ‘now’”. That can be extended into the more general notion of “at the same time”: all events that have the same time coordinate are said to have happened at the same time. An important special case is when an event has the same time coordinate as the event “my wristwatch reads X”; I will say that the event happened at time X.

These terms ("now", "happened when", ...) tell us more about how we assign time coordinates to events than the relationships between events.

Thanks Nugatory. That's similar to how Einstein defines simultaneity in his paper on relativity, isn't it?

I was thinking more along the lines of trying to define it without reference to clocks or simultaneity, but it might not be possible to do that rigorously.

 

[Lynch101]

Of course. If you compare presentism and eternalism you will clearly have a different conceptualization of time.

The point is that both conceptualizations are compatible with Newtonian physics and both conceptualizations are compatible with relativistic physics. Neither conceptualization is forced on us by the physics.

By “the physics” I mean the experimental evidence and the mathematical framework used to make experimental predictions. It seems like you may be adding some philosophy to the physics in what you understand relativity to be. So you are already assuming a specific conceptualization of time which is not part of “the physics”.

So LET shares all of the mathematical framework and all of the experimental predictions of the block universe. The physics of both is the physics of SR. They differ in their conceptualization of time. This shows that the conceptualization of time is not part of the physics

Presentism would necessitate absolute simultaneity though, which the relativity of simultaneity tells us isn't correct. So presentism can't be compatible with Einstein's relativity.

 

[Dale]

From here on, if I say the Newtonian picture, I am referring to the conception that is based on a global "now" not on a Newtonian Block Universe.

I would prefer if you not do that.

The correct term is “presentism”. If you use the term “Newtonian” to refer to presentism then the terminology makes the whole discussion more cumbersome. We know that relativistic picture of physics is different from the Newtonian picture of physics in many ways. So if you label “presentism” as Newtonian then it becomes difficult to disentangle the presentism philosophy from the Newtonian physics, the mathematical framework and experimental predictions. The philosophy and the physics are separate things.

Please use the correct philosophical term “presentism”.

If a global now does exist, then it would necessitate absolute simultaneity. Relativity tells us that this is not the case, doesn't it?

“The physics” (experimental predictions and mathematical framework) of relativity is perfectly compatible with an undetectable global now, just as it is compatible with an undetectable aether frame.

 

[PAllen]

That's true, but we can specify that either a global now exists, or it doesn't. If a global now does exist, then it would necessitate absolute simultaneity. Relativity tells us that this is not the case, doesn't it?

No, it would not. The relativity of simultaneity is what would make a global past / future surface undetectable. It doesn't prove its nonexistence.

 

[PeterDonis]

That is what I am trying to do first, simply establish what the BU says.

Then, instead of trying to make up your own terminology, why don't you read what the proponents of the BU interpretation actually say? For example, you could read the very statements I quoted in my Insights article, or the ones made in the references I gave there.

I have been considering your proposal of the term "fixed and certain". It certainly a preferable term to that of "real" but I think it needs some further clarification before it can be used in this context. Perhaps, more accurately, I need further clarification before I will be able to start using it in this context.

If you will read my Insights article, you will see that I chose that term because Roger Penrose, in the quote from The Emperor's New Mind that I gave, uses "certain" and "fixed" in his own description of the argument that I refute in the article.

I really wonder whether you have actually read my Insights article with proper attention, since you keep bringing up issues that I explicitly addressed there, precisely in order to try and forestall the kind of rehashing of them that we seem to be doing here.

I think you've said that the Block Universe is a possible interpretation of relativity. But , you argue that it is not an absolute necessity, as often tends to be presented. Am I correct in that?

Yes.

The way it is used here, the term "real" isn't a set of criteria which something must fulfil, it is instead simply a label that we can try to apply in our discussion.

And a useless one, as far as science is concerned, since it has no experimental consequences. Science is not concerned with applying labels to things that have no experimental consequences. Science is concerned with things we can actually test by experiment.

Of course people like to label things in ways that science doesn't address; that's fine. But it's not science. And to the extent the BU interpretation involves such labels, it's not science either. It's philosophy, or metaphysics, or whatever you want to call it other than "science", but it isn't science.

We could use a different label, but "real" has the benefits of our preconceptions of what it means.

What you see as a benefit, I see as a hindrance. Preconceptions are not useful if they lead you to make claims that are not valid, such as BU proponents claiming that BU is required by relativity.

any other term would probably cause more confusion.

"Fixed and certain", as I noted above, was the term used by Penrose, and it seems pretty clear what he means by it. It certainly works a lot better in his presentation of the argument than "real" would have.

These represent very different pictures of the Universe, so the idea of "fixed and certain" would need some additional clarification.

These represent very different pictures of the part of the Universe we don't know about yet, i.e., the part of the Universe that we haven't yet observed because no information from that part has reached us, because of the finite speed of light. But all of these very different pictures of the Universe share the belief that the things we already know about won't change. What you ate for breakfast yesterday isn't going to change depending on what you observe tomorrow. Where you were born, where your parents went to school, etc., aren't going to change depending on whether or not it rains tomorrow. And so on. All of these very different views of the Universe agree on simple mundane things like that.

That common property that all of these very different pictures of the Universe agree that the things we already know about have, is what I mean by "fixed and certain". The question is whether any other parts of the Universe, besides the part we already know about, have this property. The BU view is the view that all of the events everywhere in 4-d spacetime have this property. The argument I refute in the Insights article claims that that view is required by relativity. It's not.

according to your application, we only consider events in the past light cone to be fixed and certain, it sounds very much like the conceptualisation usually referred to as "the growing block universe".

I've already said that my proposed view is consistent with the "growing block universe" view. But your calling it the "growing block universe" view does not mean it's the same as the block universe view I discuss in my Insights article. It's not. The term "growing block universe" is thus a very unfortunate term to describe that view.

We can talk strictly about the events that constitute our own world line

Not all of them, since only the events on your worldline that are either your present event, or to the past of your present event, are in your past light cone. Events on your worldline to the future of your present event, such as you eating breakfast tomorrow, are not.

I'm not sure what you mean by smuggling in "past and future" events (with emphasis on and).

You are talking as if past events and future events must have the same "ontological status" (or whatever term you want to use). That's obviously false. There is nothing at all inconsistent about saying that past events are fixed and certain but future events are not. So there is nothing that requires past and future events to always be treated on the same footing.

But the argument you were quoting assumes that that is required: it claims that the only alternative to a block universe view, i.e., to having both past and future be "as real as" (or whatever term you want to use) the present, is to have only the present be "real" (or whatever term you want to use). If it's not required to treat the past and future on exactly the same footing, then the obvious other alternative is to treat the present and the past as "real" (or whatever term you want to use), but not the future. (And with the proper relativistic definition of "present" and "past", i.e., the "present" is your present event, and the "past" is its past light cone, that is exactly why my proposed alternative view does.)

The fact that this obvious alternative is not even mentioned is why I say the argument you were quoting here is not even as good as the one I refuted in my Insights article.

Are you implying that relativity necessitates a block universe which comprises past and present events, but not necessarily future events?

Not quite, no. First, "present" is only one event--your present event. Second, "past" is just your past light cone, which not everyone is clear about. Third, "future" is not all of the rest of spacetime; it's just your future light cone. There is also the region I called "elsewhere" in my Insights article (another useful term from Roger Penrose, who uses it in The Emperor's New Mind and, IIRC, other books as well), all of the events that are spacelike separated from your present event.

I'm simply juxtaposing two alternatives and saying if not this, then that.

Which is invalid reasoning if there are any other alternatives that you have not considered. Which there are in this case. See above. This logical fallacy is so common that it has a name: "False dichotomy".

 

[Nugatory]

Thanks Nugatory. That's similar to how Einstein defines simultaneity in his paper on relativity, isn't it?

It is the definition that he used, but it’s not his definition - it‘s the only definition that has ever been used (although usually not so explicitly) by anyone. Einstein’s contribution was to show the limitations of this generally accepted definition of “now”.

 

[Lynch101]

I would prefer if you not do that.

The correct term is “presentism”. If you use the term “Newtonian” to refer to presentism then the terminology makes the whole discussion more cumbersome. We know that relativistic picture of physics is different from the Newtonian picture of physics in many ways. So if you label “presentism” as Newtonian then it becomes difficult to disentangle the presentism philosophy from the Newtonian physics, the mathematical framework and experimental predictions. The philosophy and the physics are separate things.

Please use the correct philosophical term “presentism”.

Thank you Dale. I know that philosophy is off topic here, so I wanted to ground it in the context of scientific theory. Apologies if this caused confusion.

“The physics” (experimental predictions and mathematical framework) of relativity is perfectly compatible with an undetectable global now, just as it is compatible with an undetectable aether frame.

Maybe I'm interpreting this incorrectly then, but it would seem to suggest that relativity is compatible with absolute simultaneity, because presentism would necessitate absolute simultaneity. Is it compatible with absolute [undetectable] simultaneity?

 

[Lynch101]

No, it would not. The relativity of simultaneity is what would make a global past / future surface undetectable. It doesn't prove its nonexistence.

Ah I see. I was told that the relativity of simultaneity ruled out absolute simultaneity.

Or is it compatible on the basis of how simultaneity is defined, as opposed to on the basis of an underlying metaphysics?

 

[Dale]

it would seem to suggest that relativity is compatible with absolute simultaneity, because presentism would necessitate absolute simultaneity. Is it compatible with absolute [undetectable] simultaneity?

Yes. It is compatible with LET and from there presentism is easy to see.

I know that philosophy is off topic here, so I wanted to ground it in the context of scientific theory. Apologies if this caused confusion.

This is inherently a purely philosophical discussion. Using incorrect terminology doesn’t suddenly ground the discussion in science. “Block universe” is a standard synonym for “eternalism”. But “Newtonian view” is not a standard synonym for “presentism”, and I object to its use.

 

[PAllen]

Ah I see. I was told that the relativity of simultaneity ruled out absolute simultaneity.

Or is it compatible on the basis of how simultaneity is defined, as opposed to on the basis of an underlying metaphysics?

Not sure why we keep talking in circles. Try this: if Newtonian physics were true, there would be a procedure that could actually be performed to synchronize separated clocks such that any pair of clocks synchronized in one frame would be synchronized when compared to relatively moving clocks synchronized using the same procedure by someone comoving with those clocks. This is what absolute simultaneity means.

In relativity, this is false - it cannot be done.

None of this determines what is the boundary between what is fixed and certain versus not is. A block universe interpretation of either Newtonian physics or relativity simply posits that all that ever will be is as fixed as all that ever was. In neither theory is it possible to test this belief. Similarly either theory is compatible with the idea that there is a unique boundary defining a global now that separates what is fixed and certain from what is not. In neither theory is it possible to experimentally address this belief. The only difference is that in Newtonian physics it would be natural to identify this boundary with the absolutely simultaneous times as described above. However, you would still have no way to prove this over a block universe belief.

 

[Lynch101]

I just want to start this post by saying that I think I see the point you were trying to make, that we can't assume that the universe comprises events which are in our future light cone, we can't say that those events are fixed and certain. There are still one or two things I think I'm not be clear on. I'll reply below, but I just wanted to put this in at the start.

I may have confused the issue by using the term Block Universe because I have also been saying if past and/or future events, so I haven't necessarily been making an assumption about future events, although my use of the term Block Universe has clouded the issue.

Then, instead of trying to make up your own terminology, why don't you read what the proponents of the BU interpretation actually say? For example, you could read the very statements I quoted in my Insights article, or the ones made in the references I gave there.

If you will read my Insights article, you will see that I chose that term because Roger Penrose, in the quote from The Emperor's New Mind that I gave, uses "certain" and "fixed" in his own description of the argument that I refute in the article.

I really wonder whether you have actually read my Insights article with proper attention, since you keep bringing up issues that I explicitly addressed there, precisely in order to try and forestall the kind of rehashing of them that we seem to be doing here.

"Fixed and certain", as I noted above, was the term used by Penrose, and it seems pretty clear what he means by it. It certainly works a lot better in his presentation of the argument than "real" would have.

I wasn't familiar with the terminology you used in the article so it didn't stick in my mind. I understood what was meant as I read it, but the terminology I have become accustomed to is speaking about past and future as "real", or about the ontological nature of events on a world line. This seemed to be a major stumbling block, so I was trying to throw terms out there to see if I could illustrate the point I was trying to make, so that we would be on the same page. I was unfamiliar with your use of "fixed and certain", so I wasn't confident of using them in such a way as to convey my own understanding.

These represent very different pictures of the part of the Universe we don't know about yet, i.e., the part of the Universe that we haven't yet observed because no information from that part has reached us, because of the finite speed of light. But all of these very different pictures of the Universe share the belief that the things we already know about won't change. What you ate for breakfast yesterday isn't going to change depending on what you observe tomorrow. Where you were born, where your parents went to school, etc., aren't going to change depending on whether or not it rains tomorrow. And so on. All of these very different views of the Universe agree on simple mundane things like that.

That common property that all of these very different pictures of the Universe agree that the things we already know about have, is what I mean by "fixed and certain". The question is whether any other parts of the Universe, besides the part we already know about, have this property. The BU view is the view that all of the events everywhere in 4-d spacetime have this property. The argument I refute in the Insights article claims that that view is required by relativity. It's not.

If I understand correctly, your argument refutes the idea that events in our future light cone are fixed and certain, but allows for differing views about events in our past light cone.

It seems to be compatible with a presentist universe, but also with what is often colloquially referred to as the growing block, as you mention below.

I've already said that my proposed view is consistent with the "growing block universe" view. But your calling it the "growing block universe" view does not mean it's the same as the block universe view I discuss in my Insights article. It's not. The term "growing block universe" is thus a very unfortunate term to describe that view.

I understand, yes, the growing block is a different idea form the fully formed block.

This would mean that the "fixed and certain" definition applies to either a presentist or "growing block" type universe, am I correct in that?

Not all of them, since only the events on your worldline that are either your present event, or to the past of your present event, are in your past light cone. Events on your worldline to the future of your present event, such as you eating breakfast tomorrow, are not.

I understand your point now, I believe. This would mean that the Universe comprises either our present event, or our past and present events, but not necessarily our future events.

I was under the impression that when we add in a relatively moving observer, and their calculations about when events must have happened, based on when light from an event reaches them, leads us to the idea that the universe must also comprise events in our future light cone.

As in the example of the relatively moving observers on the train and platform, and the lightning strikes.

You are talking as if past events and future events must have the same "ontological status" (or whatever term you want to use). That's obviously false. There is nothing at all inconsistent about saying that past events are fixed and certain but future events are not. So there is nothing that requires past and future events to always be treated on the same footing.

I see how your criterion about events in our past light cone necessitates this. But my understanding was that, when we have two relatively moving observers, one of them either observes or calculates that an event which is part of their present (the flash of light to the front of the train) is in the future light cone of the other observer. Similarly, the observer on the platform can calculate, after the fact, that an event in their future light cone must have already happened when the other observer passed them.

Even if the observer on the platform can't say, in the present, that the event in their future light cone is fixed and certain, their calculations lead them to the conclusion that the future event must have been fixed and certain.

If I have understood the explanation correctly.

But the argument you were quoting assumes that that is required: it claims that the only alternative to a block universe view, i.e., to having both past and future be "as real as" (or whatever term you want to use) the present, is to have only the present be "real" (or whatever term you want to use). If it's not required to treat the past and future on exactly the same footing, then the obvious other alternative is to treat the present and the past as "real" (or whatever term you want to use), but not the future. (And with the proper relativistic definition of "present" and "past", i.e., the "present" is your present event, and the "past" is its past light cone, that is exactly why my proposed alternative view does.)

The fact that this obvious alternative is not even mentioned is why I say the argument you were quoting here is not even as good as the one I refuted in my Insights article.

I see the assumption I was making now, but, if my understanding of what was explained to me is correct, it means that the picture of the Universe that relativity allows us to calculate, isn't just dependent on what is in our own past light cone. We can calculate what a relatively moving observer would observe and when events happen in the relatively moving frame, cross reference with the location of the observer in our frame, and then make further deductions about events that would have been in our future light cone.

This is all calculated after the fact, but it allows us to build a more complete picture...I think.

Not quite, no. First, "present" is only one event--your present event. Second, "past" is just your past light cone, which not everyone is clear about. Third, "future" is not all of the rest of spacetime; it's just your future light cone. There is also the region I called "elsewhere" in my Insights article (another useful term from Roger Penrose, who uses it in The Emperor's New Mind and, IIRC, other books as well), all of the events that are spacelike separated from your present event.

OK, maybe I haven't fully understood your point then. Am I right in saying that we can't consider events in our future light cone as fixed and certain, at the very least?

If relativity doesn't necessitate that the Universe comprises

Is the above description talking about one single observer and what they can say is fixed and certain at any given moment?

My understanding is that, after the fact, observers could apply relativity theory and calculate a picture of the Universe that tells them that when a relatively moving observer passed them, an event they considered to be in their future light cone had already happened in the frame of the relatively moving observer. So, although they cannot say in advance that events in their future light cone are fixed and certain, they can calculate afterwards that such events must have been fixed and certain.

I might be misinterpreting that, but that is how I have understood it. It might be easier to explain by way of a thought experiment.

Which is invalid reasoning if there are any other alternatives that you have not considered. Which there are in this case. See above. This logical fallacy is so common that it has a name: "False dichotomy".

I have been talking about past and/or future events, so I haven't really been making an assumption about future events, although I can see how the Block Universe is the idea that past and future events "exist".

 

[PeterDonis]

If I understand correctly, your argument refutes the idea that events in our future light cone are fixed and certain, but allows for differing views about events in our past light cone.

No. I have said repeatedly that events in our past light cone are fixed and certain on all views (although I did not previously mention presentism, which is a possible exception--but see below). Certainly events in our past light cone are fixed and certain on the alternative view I proposed in the article. The question is whether any events other than our present event and its past light cone are fixed and certain.

In any case, my argument did not refute the claim that events in our future light cone are fixed and certain. It only refuted a particular argument that purports to claim that relativity requires events in our future light cone to be fixed and certain. I have explained this several times now.

It seems to be compatible with a presentist universe

It depends on what you mean by "presentism", and discussions of that view are too contaminated with vague terms like "real" for me to know what they are really saying. Also, the term "present" is ambiguous; on the alternative view I gave in the article, it means "your present event", but on many "presentist" views, it seems to mean "some spacelike 3-surface that contains your present event".

This would mean that the "fixed and certain" definition applies to either a presentist or "growing block" type universe, am I correct in that?

I am using the term "fixed and certain" as a general term that can be used by any viewpoint; what each viewpoint does is specify exactly which events are fixed and certain.

This would mean that the Universe comprises either our present event, or our past and present events, but not necessarily our future events.

This describes two possible viewpoints, yes.

I was under the impression that when we add in a relatively moving observer, and their calculations about when events must have happened, based on when light from an event reaches them, leads us to the idea that the universe must also comprise events in our future light cone.

Sorry to shout, but THIS IS EXACTLY THE ARGUMENT I REFUTED IN MY ARTICLE. Have you really read it? Really? Are you sure you don't need to go back and read it again?

Am I right in saying that we can't consider events in our future light cone as fixed and certain, at the very least?

No. You have repeatedly failed to understand what I actually said in the article.

Here is what I said in the article: Relativity does not require all events in 4-d spacetime to be fixed and certain.

Here are some things I explicitly did not say in the article:

I did not say that events in our future light cone are definitely not fixed and certain. (I only said relativity does not require that they are.)

I did not say that only our present event is fixed and certain. Nor did I say that it isn't.

I did not say that only some spacelike 3-surface containing our present event is fixed and certain. Nor did I say that it isn't. (I did propose an alternative that would say it isn't, except for our present event itself, but that's a different thing.)

I did not even say that relativity requires events in our past light cone to be fixed and certain. (I only said that this view is an obvious alternative that (a) makes sense, and (b) was not even proposed or considered by all the many people who have argued about the block universe.)

I hope this helps.

My understanding is that, after the fact, observers could apply relativity theory and calculate a picture of the Universe that tells them that when a relatively moving observer passed them, an event they considered to be in their future light cone had already happened in the frame of the relatively moving observer.

No, that's not what they can calculate after the fact. No calculation can tell you "when" some distant event happened according to some observer. That is something you assign by choosing coordinates. It has no physical meaning whatsoever.

What observers can calculate after the fact are the causal relationships between events: which events are in which other events' past or future light cones, which events are spacelike separated from which other events. Those things are invariants and have physical meaning. But in order to do those calculations, all of the events the calculations apply to must already be in the past light cone of the person calculating them. Which means nobody ever has to treat any events other than those that are in their past light cone as fixed and certain in order to do calculations. Which I pointed out in my article.

 

[Nugatory]

My understanding is that, after the fact, observers could apply relativity theory and calculate a picture of the Universe that tells them that when a relatively moving observer passed them, an event they considered to be in their future light cone had already happened in the frame of the relatively moving observer.

That cannot happen.

It can happen that an event in the past light cone of A ("A considers it to have happened") is in neither the past nor the future light cone of B ("Anything B says about whether it has happened or not depends on B's completely arbitrary choice of convention for assigning time coordinates to events outside the the light cones") or vice versa. And note that even that cannot happen when they're passing one another; when they're both at the same point in space they have teh same past and future lightcones.

 

[Dale]

It depends on what you mean by "presentism", and discussions of that view are too contaminated with vague terms like "real" for me to know what they are really saying.

I encouraged him to use the term “presentism” rather than “Newtonian”. As you say, the whole concept of presentism is focused on identifying what is “real”. Since that is a philosophical term of art it makes it clear that this discussion is a philosophical rather than a scientific discussion. I don’t think we should discourage the use of the correct terminology here.

I like “fixed and certain” also, but I am not sure how it relates to the philosophical concept of “real”. I suspect that a presentist would accept the future and past as “fixed and certain” under a deterministic theory even though he would not accept them as “real”.

 

[PeterDonis]

I like “fixed and certain” also, but I am not sure how it relates to the philosophical concept of “real”.

I'm not either. By suggesting that term, I was attempting to focus discussion on something that seemed less prone to vagueness and imprecision than "real". I'm not sure whether that attempt has actually worked.

 

[Lynch101]

No. I have said repeatedly that events in our past light cone are fixed and certain on all views (although I did not previously mention presentism, which is a possible exception--but see below). Certainly events in our past light cone are fixed and certain on the alternative view I proposed in the article. The question is whether any events other than our present event and its past light cone are fixed and certain.

In any case, my argument did not refute the claim that events in our future light cone are fixed and certain. It only refuted a particular argument that purports to claim that relativity requires events in our future light cone to be fixed and certain. I have explained this several times now.
...
Here is what I said in the article: Relativity does not require all events in 4-d spacetime to be fixed and certain.

Here are some things I explicitly did not say in the article:

I did not say that events in our future light cone are definitely not fixed and certain. (I only said relativity does not require that they are.)

I did not say that only our present event is fixed and certain. Nor did I say that it isn't.

I did not say that only some spacelike 3-surface containing our present event is fixed and certain. Nor did I say that it isn't. (I did propose an alternative that would say it isn't, except for our present event itself, but that's a different thing.)

I did not even say that relativity requires events in our past light cone to be fixed and certain. (I only said that this view is an obvious alternative that (a) makes sense, and (b) was not even proposed or considered by all the many people who have argued about the block universe.)

I hope this helps.

Thank you Peter, I appreciate your patience in all of this. I understand that it must get frustrating to have fielded such questions as these previously, gone to the trouble of writing an article that addresses these questions, and then have someone come along and raise the same old questions again.

I have read your article, but I didn't fully comprehend it, as is evident. The purpose of this thread is, in essence, an exercise in trying to understand it. I'm starting from a position where I have been taught that the BU = relativity, so I am only really familiar with arguments in favour of the BU. As with moving from a Newtonian picture to a relativistic one, I found that putting forward my own understanding and then having the errors and assumptions pointed out, helped me to understand, in a conceptual [but limited] way, what relativity says - apparently with a few added assumptions.

I think we were talking past each other a little up until this point. In the OP I tried to avoid the assumption that past and future events are necessitated by saying past and/or future. I think the problem is that I was using this interchangeably with the Block Universe which says that past and future events are fixed and certain.

Am I correct in saying that relativity allows for the following possibilities:

• [Whole] Block Universe
• Past Block Universe - colloquially the "growing block"
• Future Block - we might call this a "shrinking block".
• Presentist
• An alternative based on events in the past light cone are fixed and certain.

I'm not actually familiar with any advocates for a "shrinking block", and I'm not sure it is genuinely a possibility, but I've included it here just for [possible] completeness. Is there another alternative, as per the 5th option there? Is this what you suggest in the article? I'm not sure I fully understand it, if so.

If the above possibilities are correct, and if they are taken together with your statement in the other thread:

This also means there is no global concept of "now" in relativity.

Would this mean that we can rule out presentism?

If we rule out presentism, then wouldn't we be left with a universe which comprises past and/or future events, so either:
past, present, and future
past and present
present and future
Some alternative which includes some past events, but not all?

The confusion being that I was conflating the 2nd and 3rd options with "the Block Universe", which is the first option.Hopefully, I've got that much correct. I still don't fully understand why the full block isn't necessitated. I think I can see the destination of your argument but I'm not yet, fully sure how to get there. Some of the further points below are key to that I think.

It depends on what you mean by "presentism", and discussions of that view are too contaminated with vague terms like "real" for me to know what they are really saying. Also, the term "present" is ambiguous; on the alternative view I gave in the article, it means "your present event", but on many "presentist" views, it seems to mean "some spacelike 3-surface that contains your present event".

I appreciate the difficulty with such terms. If I were better versed I might be able to define it in more rigorous terms, instead I am left to try and describe around it, in the hope that I communicate what I mean.

I think of presentism in the context of Newtonian physics, the global/Universal "now" which would necessitate absolute simultaneity. Sometimes its helpful to think of this juxtaposed with the Block Universe, the growing block, and the shrinking block.

I'm not entirely sure how to define it rigorously.

I am using the term "fixed and certain" as a general term that can be used by any viewpoint; what each viewpoint does is specify exactly which events are fixed and certain.

I think I get you. I think I struggle a little with defining "fixed and certain" as being those events that are in the past light cone of an observer and then applying this to the future light cone of the observer, as the block universe does. But maybe, we can just think that all events in a block universe are in the past light cone of "the end of the universe"?

If the term "fixed and certain" can be applied to all of the different models, and each of the models describes a different picture of the Universe, do we need another term for describing what those different models say.

"Fixed and certain" seems to tell us what we can definitively say about events in the past light cone, but it doesn't distinguish between a presentist interpretation or the BU interpretation, or btween the BU interpretation and a growing block interpretation. Each of these says something somewhat different about past and future events.

I have started using the term comprise so as to avoid the word "real".

Sorry to shout, but THIS IS EXACTLY THE ARGUMENT I REFUTED IN MY ARTICLE. Have you really read it? Really? Are you sure you don't need to go back and read it again?

No. You have repeatedly failed to understand what I actually said in the article.

No worries, I get that this must be incredibly frustrating. That genuinely isn't my intention. I have gone back and read the article again and I think I understood a bit more this time, but I'm still not certain I fully grasp it. As I mention above, I think I can see the destination, I just haven't progressed through the route to get there. This thread is intended to be the means of getting there.

No, that's not what they can calculate after the fact. No calculation can tell you "when" some distant event happened according to some observer. That is something you assign by choosing coordinates. It has no physical meaning whatsoever.

What observers can calculate after the fact are the causal relationships between events: which events are in which other events' past or future light cones, which events are spacelike separated from which other events. Those things are invariants and have physical meaning. But in order to do those calculations, all of the events the calculations apply to must already be in the past light cone of the person calculating them. Which means nobody ever has to treat any events other than those that are in their past light cone as fixed and certain in order to do calculations. Which I pointed out in my article.

This is the part where the gap in my understanding lies. I will try to outline how I have understood the explanation and maybe you can identify where I am going wrong. Again, I can see the destination, but I was lead to believe something else.

I find it helpful to talk in terms of thought experiments because it helps to make things a little less abstract.

=============================================================================================
Returning to this point after having written the below. It is written in a very "matter of fact" tone, but that is just because that is how it flowed. I figured it would be easier to come back and point this out, rather than try to go through the whole thing and try to edit the language to make it less so. As I say, this is just how I have understood the arguments that were put to me, so I am very much open to correction.
==============================================================================================If we think about Albert Einstein and Hendrick Lorentz moving relative to each other. Albert is on the platform and Hendrick is on the train. Both know the theory of relativity inside out. Albert is located at point M, which is midway between two light sources L(eft) & R(ight), which are at rest relative to him. As Hendrick whizzes along the track, in the direction of L to R, he passes Albert at point M. A few moments later, Albert observes two simultaneous flashes of light coming from each light source. That is, photons from both L and R arrive to his location simultaneously. Hendrick, on the train, observes the light from R first and the light from L second.

Afterwards, both retire to their respective studies to perform their calculations and see if they can arrive at any additional information. Both can perform Lorentz transformations to build a picture of things from the relatively moving reference frame.

Based on the information from his own reference frame, Albert calculates that the light emission events from L and R happened prior to the photons arriving to his location. All of those events are now in his past light cone, but he can calculate that those light emission events must have occurred earlier, in order to arrive to his location when they did. That is, the emission events must have been fixed and certain prior to the photons arriving to his location and being in his past light cone. At the time, he could not know this, but afterwards he can calculate that it must have been so. The reason being, photons can only be emitted from events which have happened or are fixed and certain. Events which are not fixed and certain, cannot emit photons.

If "fixed and certain" doesn't work in this context, then I will need to find another term to describe what it is that I am saying.

This doesn't tell us anything particularly interesting. This would have been the case in the Newtonian picture also. It is when Albert performs the Lorentz transformation that we acquire additional information. One additional point that Albert notes about the emission events form L and R, is that the events must have coincided with the moment when Hendrick was at point M. This is important, because it gives us a non-clock related reference point.

Albert's Lorentz Transformation
Albert performs a Lorentz transformation to get the picture according to Hendrick's reference frame. As he expected, he notes that the light from the two light sources did not arrive to Hendrick simultaneously, instead, light from R arrived first and light from L arrived second. Again, nothing surprising here. Where we get the additional information, is when Albert calculates when the emission events must have happened according to Hendrick.

According to Hendricks frame of reference, the emission events were not simultaneous, not just because the light didn't reach him simultaneously but due to the relativity of simultaneity. According to Hendrick's frame the emission event from R happened before the event from L. The additional information that Albert gets from this is the location of Hendrick when these events happened.

According to Albert's frame, the emission events happened at the precise moment that Hendrick was located at point M. However, according to Hendrick's frame, the emission event from R must have happened prior to his arrival at M. The same is true for Henry as it is for Albert, photons can only be emitted from events that have happened or events which are fixed and certain. Events which are not fixed and certain, or have not happened can not emit photons.

This means, Albert must agree that prior to Hendrick's arrival at point, the emission event from R must have been fixed and certain, otherwise Hendrick wouldn't have observed the photon where he did (in his own reference frame) and his theory of relativity would be inaccurate.

This is what leads to the conclusion that the universe comprises future events because the emission event from R was in Albert's future in the moments prior to Hendrick's arrival at point M, but it must have been fixed and certain in order to emit a photon.

 

[Lynch101]

Yes. It is compatible with LET and from there presentism is easy to see.

Ah, I see. I am familiar with the notion of LET incorporating presentism but I had come to understand the relativity of simultaneity as being antithetical to absolute simultaneity i.e. that one precluded the other.

This is inherently a purely philosophical discussion. Using incorrect terminology doesn’t suddenly ground the discussion in science. “Block universe” is a standard synonym for “eternalism”. But “Newtonian view” is not a standard synonym for “presentism”, and I object to its use.

Newtonian physics wouldn't be synonymous with presentism, but presentism would represent a fairly standard interpretation of Newtonian physics, I would have thought. Although Newtonian physics is compatible with the concept of a block unvierse, I've never heard anyone advocate for such an interpretation.

 

[Lynch101]

That cannot happen.

It can happen that an event in the past light cone of A ("A considers it to have happened") is in neither the past nor the future light cone of B ("Anything B says about whether it has happened or not depends on B's completely arbitrary choice of convention for assigning time coordinates to events outside the the light cones") or vice versa. And note that even that cannot happen when they're passing one another; when they're both at the same point in space they have teh same past and future lightcones.

I hope you don't take umbridge with me linking you to my reply to Peter, just to save typing the whole thing again.

 

[PeterDonis]

Am I correct in saying that relativity allows for the following possibilities

Yes, all of those possibilities are allowed by relativity (though, as you note, there don't seem to be any advocates for the "shrinking block" interpretation, which is not surprising).

Is there another alternative, as per the 5th option there?

The 5th option is certainly allowed by relativity, otherwise I wouldn't have included it in my article.

Logically speaking, relativity allows you to claim that any set of events you like in 4-d spacetime is fixed and certain, or not. Relativity itself simply does not make any claims either way. Relativity combined with what seems to me like obvious common sense leads to the belief that, at least, all events in our past light cone should be fixed and certain; relativity is what tells us the "past light cone" part (since relativity is what tells us that light propagates with a finite speed) and common sense is what tells us the "fixed and certain" part (because, as I've noted, what happened at past events doesn't change based on what might happen in the future). But if you come across a sufficiently perverse person who insists that some other set of events is fixed and certain and the events in the past light cone aren't, there's no way you can refute him in a logical sense just based on relativity alone.

Would this mean that we can rule out presentism?

No. See above.

Note that when I say there is no global concept of "now" in relativity, that is only talking about the math and the physics of relativity. It does not in any way prevent someone from adding on some other concept of "now" in a philosophical or metaphysical interpretation, that is not contained in the math and the physics of relativity, as long as it is not inconsistent with the math or the physics. Adding on a presentist concept of "now" is not inconsistent with the math or the physics of relativity; it's just not contained in the math or the physics and doesn't change any of the math or the physics.

I had just come to associate the relativity of simultaneity as precluding the possibility of absolute simultaneity

It doesn't, because the term "simultaneity" is referring to two different things in the two terms (which means that the choice of the word "simultaneity" in at least one of them is a bad choice of words).

In "relativity of simultaneity", the word "simultaneity" refers to the notion of simultaneity given by a particular choice of reference frame.

In "absolute simultaneity", the word "simultaneity" refers to some other notion of "simultaneity" that has nothing to do with any choice of reference frame (and also nothing to do with the math or physics of relativity, as above) but comes from someone's chosen philosophical or metaphysical interpretation.

There is no logical connection at all between these two notions.

 

[Nugatory]

I hope you don't take umbridge with me linking you to my reply to Peter, just to save typing the whole thing again.

I'm not going to take umbrage, but I am unclear on how your reply to Peter is relevant to the correctness or incorrectness of your statement about possible relationships between events.

 

[Lynch101]

Yes, all of those possibilities are allowed by relativity (though, as you note, there don't seem to be any advocates for the "shrinking block" interpretation, which is not surprising).

Ah, glad I was able to get that far anyway, thank you!

The 5th option is certainly allowed by relativity, otherwise I wouldn't have included it in my article.

Logically speaking, relativity allows you to claim that any set of events you like in 4-d spacetime is fixed and certain, or not. Relativity itself simply does not make any claims either way. Relativity combined with what seems to me like obvious common sense leads to the belief that, at least, all events in our past light cone should be fixed and certain; relativity is what tells us the "past light cone" part (since relativity is what tells us that light propagates with a finite speed) and common sense is what tells us the "fixed and certain" part (because, as I've noted, what happened at past events doesn't change based on what might happen in the future). But if you come across a sufficiently perverse person who insists that some other set of events is fixed and certain and the events in the past light cone aren't, there's no way you can refute him in a logical sense just based on relativity alone.

I'm not entirely clear on what that 5th option is because I'm not sure in what sense you are using the term "fixed and certain".

In a presentist intepretation it says that events in the past light cone are fixed and certain in the sense that they are over and cannot be changed. A presentist universe comprises only present events. While the other interpretations would be universes which comprise [at least] present and past events.

Note that when I say there is no global concept of "now" in relativity, that is only talking about the math and the physics of relativity. It does not in any way prevent someone from adding on some other concept of "now" in a philosophical or metaphysical interpretation, that is not contained in the math and the physics of relativity, as long as it is not inconsistent with the math or the physics. Adding on a presentist concept of "now" is not inconsistent with the math or the physics of relativity; it's just not contained in the math or the physics and doesn't change any of the math or the physics.

Ah, I see. There is no preferred "now" in the mathematics. I think this is part of the justification that people offer for the Block Universe isn't it? There is nothing which singles out the events on an objects world line, so all events must have equal status. There is nothing to single out events in our past light cone.

It doesn't, because the term "simultaneity" is referring to two different things in the two terms (which means that the choice of the word "simultaneity" in at least one of them is a bad choice of words).

In "relativity of simultaneity", the word "simultaneity" refers to the notion of simultaneity given by a particular choice of reference frame.

In "absolute simultaneity", the word "simultaneity" refers to some other notion of "simultaneity" that has nothing to do with any choice of reference frame (and also nothing to do with the math or physics of relativity, as above) but comes from someone's chosen philosophical or metaphysical interpretation.

There is no logical connection at all between these two notions.

I see. I was thinking that it must be a matter of definition, or the lack thereof.

 

[Lynch101]

I'm not going to take umbrage, but I am unclear on how your reply to Peter is relevant to the correctness or incorrectness of your statement about possible relationships between events.

I was trying to outline the rationale that was given me as to why relativity necessitates a block universe. I was hoping that the errors in reasoning or mistaken assumptions might be explained to me because I cannot identify them myself.

Peter mentioned, in a proceeding post, the point that there is nothing in the mathematics of relativity which picks out the present moment - I presume this is neither globally nor locally. This is another argument I have heard in favour of the Block Universe, that no events are singled out over any other events in the mathematics, so all events must share the same status.

If a universe is said to comprise present events, then it must also be said to comprise past and future events, on the basis that nothing in the mathematics favours present events over the others.