Exploring the Implications of Block Time in General Relativity

[Gerinski]

It seems to me that block time is an unavoidable consequence of GR. The fact that events that still lie in our future must have already been observed by other (hypotetical) observers seems to leave no room for escaping the fact that (at least some of) the events in our future "have already happened".

To me (a layman) that seems to leave just 2 possibilities: either the future is already totally fixed or we live in Everett's-Deutsch many-worlds multiverse.

Or are there any other alternative interpretations? I know about quantum undeterminacy, but I still don't really get how does it get along with the seemingly unavoidable reality of block time derived from GR

 

[turbo]

It's my understanding that the block time concept arose not out of the consequences GR, but out of Special Relativity, to try to standardize the treatment of space-time across all possible reference frames. Block time might make some calculations and conceptualizations more "do-able", but there are some real difficulties with it.

For instance, if space-time is immutable and our futures are "fixed", as some propose, then the concept of free will has to be revised pretty drastically. This one is purely philosophical, since the deterministic people will say "of course you will act this way in this situation due to your ethical (or moral) values, and even though you feel that you have made a choice, you have not". Hindus have grappled with variants of determinism for centuries, claiming that one's position in life is determined by past actions, but that rebirth in a better station can be attained through improved behavior in each rebirth. Interestingly, more serious difficulties arise on the quantum scale, where probabilities of the positions or states of sub-atomic particles can collapse into "reality". Predetermination raises H-E-double hockey-sticks with this.

 

[Nenad]

I think this thread belongs on the philosophy forums.

 

[Gerinski]

Thanks, I'm aware that this discussion may easily drift into philosophy, but I'd like to know what mainstream objective science has to say about it too.

The fact that 2 events A and B may be perceived by one observer as A happening before B and for another observer as B happening before A is a scientifically accepted relativistic effect.
It's therefore scientifically sound that events that still lie in our future may have already objectively happened, as we are not supposed to have any privileged status versus other hypotetical observers. Past present and future are subjective labels (or time stamps) attached to objective events, and the sequence of the time stamps is not necessarily unique.

Until here it seems to me this is hard science and not yet into philosophy.
Quantum underterminism is also mainstream science.

Multiverse is a scientific theory finding a way out from the apparently contradictory solutions arising from our 2 most respected scientific theories to the scientifically legitimate question wether the future is determined or not.

My question is if there are other scientific way outs to this contradiction.
And anyway I agree with you that posting in phylosophy of science will broaden the perspective of the question !

 

[Gerinski]

Sorry guys, since there was such little response to this post, I'm wondering that my question was probably completely wrong or silly. That's no surprise since I'm just a humble amateur layman, but I'd like to know what was wrong anyway, i.e:

1. Several relativistic effects seem to suggest that past / present / future are subjective labels (or time stamps) that observers attach to objective events. The sequence of the time-stamps is not necessarily unique.
Is there something wrong in these statements?

2. i.e: Observer S sees event A happening before event B, but due to relativistic effects observer S' sees event B happening before event A (I believe this is an accepted possible relativistic effect, provided A and B are not causally related).
if so:
What does science say about the events which have happened according to observer S' but not yet to observer S? (event B).
If we are observer S and we have observed event A but not yet B, does science accept that some event still lying in our future (event B) is probably already determined, since observer S' saw it happening before the event A which we have already observed?

Or is there a flaw in this reasoning? (i.e. maybe as the events can not be causally related, event B does not need to be the same event for observers S and S' ?)

Thanks, and sorry if we laymen are a pain in the ass for these forums ! but I believe it's a good sign we get interested in science anyway :-)

 

[selfAdjoint]

Let's see if I can restate your argument so I understand it. Relativivity of simultanaity meant that if you have two events alpha and beta, then of three observers one can see alpha coourring at the same time as beta, another sees alpha before beta, and the third can see beta before alpha, and all these observations are each as good as any other. Therefore you say the ontological status of beta for the second observer, being in the future of alpha is the same as the ontological status of beta for the third observer, being in the past of alpha. So the ontological status of all the events in spacetime is the same, i.e.they exist.

I would point out that all the observers were seeing the events by light, so both events were in their past light cones. So it is not surprising that both events exist, since they are past events for all three observers. And indeed spacetime comes with a "causal structure" consisting of all the families of observer light cones. The observers cannot see events in their future lightcones, and so those events may be said not to exist, at least for those observers.

 

[Gerinski]

Thanks SelfAdjoint !
Yes I think you got my meaning, but I still think I'm getting something wrong.

As you say, "all observers see the events by light", and "the observers cannot see events in their future lightcones, and so those events may be said not to exist, at least for those observers"

Yet this seems to me a consequence of the finite speed of light, not convincing enough to state that the event "does not exist" for those observers.

If I use the analogy of the middle ages when the fastest that men could travel was by horse, an attack to the kingdom in south of France could not be known by the king in Paris until 2 or 3 days later, the time a messenger took riding a horse to bring the news.
Yet this does not mean the attack did not happen objectively, it just means that different observers received the information of a certain event at different times.
Even if the king couldn't yet know about it, an event in his future (i.e learning about how many people were killed in the attack in the south) was already determined.
Doesn't this principle apply to observers / events separated by cosmic distances?

I mean, if event B (the attack) has been observed by observer S' (the messenger), just because we are oberver S (the king) and we have not yet got the information about it, can we say that event B (the attack) did not happen ?

TX again !

 

[Gerinski]

I'm not sure it's applicable, but another case I think of pointing that "the future already exists":

I have read that an hypotetical observer falling into a black hole would see the universe behind him aging at a super-fast rate, in between the event horizon and the center singularity, if he looked back he would see the universe aging in "super-fast-forward-motion" "until the end of time".

Surely we can say that due to time dilation, such observer will be kept hovering around the event horizon and it will take the whole eternity until he will get to the center (from our point of view).

But honestly I find it a bit hard to imagine that even under such a huge gravitational force the observer will actually take "the whole eternity" to be pulled down to the center.
I feel more comfortable imagining that this effect is just our perception of what really happens (the poor observer falls fast into the center, but because the light from those events is held by the black hole's gravitation, we will only receive it after a very long time into our future), therefore we will see him hovering around, not falling into the center until the end of our days ... yet that's only due to our perception, because of the time light from the actual events will take to get to us.

Such observer will be able to witness the future of the universe "much before" we will witness the same events.
Conclusion once again, this seems to suggest that (unless multiverse), our future "is already there", certain observers under certain conditions can witness it, but we just can't.

 

[Garth]

We have to distinguish between the consideration of a space-time diagram, such as is used to illustrate SR, and our experience, or observation, of the real universe. In a space-time diagram the cosmos has to be a "block universe", in which past, present, and future "exist" all-at-once because time is one of the axes of the diagram. The "block universe" is an artefact of the diagram.
The diagram is a mapping of the real world - but how does it relate to our experience?

We have to be careful with our language because if we say "the future already exists" we could be simply confusing ourselves semantically, by confusing our tenses; 'the future' should be predicated by the future tense whereas 'exists' is in the present tense. The question is, "Is this just an ungrammatical sentence, or is there a real meaning in it?"
(I am sorry if this sounds like philosophy rather than science but scientists also have to be able to communicate in a logical manner!)

When we turn to SR the question is "What do we mean by 'exists' when applied to distant stars/galaxies etc.?" Often in a basic astronomy lecture a novice will ask that if we are seeing the Andromeda galaxy as it was 2.2 million years ago, because of the time light has taken to reach us, does that mean that it might not exist now?"

What do we mean by 'now'? We only observe the galaxy as its world-line passes through our past light cone. To arrive at concept of 'now' for Andromeda we have then to project that observation of the galaxy 2.2 mega-years ago 2.2 mega-years into its future. We can do so likewise for all observations, and thence arrive at a conceptual 'now' membrane emanating out from us into all of space ‘at our moment in time’. This membrane consists of all events that we consider to be simultaneous with us.

It is this ‘now membrane’, which conceptually divides past from future, which is frame dependent, different observers will conclude that different events are ‘in front of’ or ‘behind’ their individual ‘now membranes’. Simultaneity is frame dependent. However nobody can see their own simultaneous ‘now’ membrane, we can only see along our past light cones. Because of the Lorentz transformations between moving observers nobody can see into the future of another co-existing observer’s light cone.
So as far as our experience, or ability to observe the universe, is concerned the future does not already ‘exist’ in the sense that we cannot observe it, either for ourselves or for any other co-existing observer. That is unless we simply wait for the future to become ‘now’!
This is because although we can conceive of a space-time diagram from the ‘outside’ with its ‘block universe’, we are not ‘outside’ it. In so far as it is mapping our world ‘looking in’, we are actually inside it ‘looking out’, we are at a particular location in space and at a particular moment in time.

However it is possible to conceive of a being/observer who is outside the space-time diagram, as we mentally project ourselves to be when we develop our conceptual model of it. To that observer the universe would be a block of past-present-and-future co-existing simultaneously. So whether the universe is conceived as a ‘block’ or not is observer dependent!

Furthermore, you might want to call such an observer ‘God’ but then we would be straying from physics through philosophy to theology and this post would definitely be moved!

 

[Gerinski]

Thanks Garth, I'm aware of the risk that the essence of the question may be blurred by my using incorrect semantics. Your comments were very clarifying!
But I have to admit that I'm left with the feeling of you giving me a great lesson yet not getting an answer to my wondering ...

If I understand correctly, you mean that block time is not considered to have any physical significance, but it's just an artifact, like a graph to help understanding relativity?

I will try to be concise about the issue which I do not understand. No need to involve the future, to make it easier I will put the question into the past.

An event A (let's say for example, we launch a rocket Apollo X to outer space) happens in Earth's year 1950 (I think it was 1969, but for simplicity of figures :-).

Another event B (let's say the light from a supernova in the constellation of Virgo arrives to the Earth and we emit a radio signal recording the event) happens in Earth year 1990.
(I'm not sure if the possibility exists that such 2 events may not be causally related, but if not, just replace my examples by 2 events which are not causally related)

Now, I think according to relativity, some hypotetical observer could receive the radio signal announcing the supernova (event B) before seeing the Apollo rocket leaving the Earth (event A). Of course the time stamps 1950 and 1990 are not valid for him, he would label B at an earlier date than A according to his clock / calendar.

Now the question:
when we were on Earth's year 1970 we were certain that we had launched Apollo X in 1950, yet we hadn't seen any supernova in Virgo at all. If someone would tell us that 20 years later we will emit a radio signal to record the sight of a supernova, we would probably say: "hey man, you can not know the future, we can't know what we're going to be doing after 20 years !"

On the other hand, even if we could not communicate with our hypotetical 2nd observer, we can reasonably believe that at some time in his calendar, he had received a radio signal from Earth announcing a supernova in Virgo, yet he had not seen any Apollo X rocket being launched from earth.
But as soon as he saw the Apollo X launch, he had knowledge of both events.
From his frame, the Earth radio announcing a supernova sight was as real as the Apollo X being launched.

Now, do we have any privileged condition versus the 2nd observer? obviously not.
Then, when we were back in 1970, were we right if we questioned or denied that in 1990 we will witness a supernova in Virgo?
Of couse we didn't know then, we just knew we did launch Apollo X rocket in 1950.
But must not we admit that some hypotetical observer, having seen the launch of the Apollo X, was also certain that we eventually must see a supernova and emit a radio signal? (as from his frame, he saw that happening before the Apollo launch)

 

[gonzo]

I'm pretty sure that if two observers will see two separate events occurring in different orders, they will be unablet to communicate with each other (because they would have to be able to communicate faster than the speed of light) before both events happen, which renders the problem meaningless.

 

[Garth]

gonzo Yes, I concur– To give an example: If the Earth, A, sends out two radio/light signals the order 1, 2 in 1950 and 1990 respectively then another observer, B, moving relative to the Earth would have to receive them in the same order 1,2 although not 40 years apart.

The order of reception could be reversed if the observer was traveling away greater than the speed of light but this is normally thought to be impossible.

However if two supernovae were observed at particular times by A and A calculated when the stars actually exploded taking the light time travel into account to give the order 1, 2 then their respective order calculated by a relatively moving observer B could be 2, 1.

 

[Gerinski]

I'm pretty sure that if two observers will see two separate events occurring in different orders, they will be unablet to communicate with each other, which renders the problem meaningless.

Hmmm, the impossibility of communication between both observers, is really an argument enough to dismiss a question as meaningless?
the physical limitation that something can not be seen (it lies beyond our light-cone), is really a reason enough to consider that for all what matters, "it does not exist" or "it didn't happen"?

Sorry I may be getting into muddy waters again due to my semantics :-(

I think you mean that for events that lie beyond our visibility horizon, it's useless to wonder if they "happened" or not, since they can never be related to our time coordinates, we can never say if they happened or not at any particular moment of our time.

Hmmmm, sorry can't help it feeling like we are saying that trees are not there when we don't look at them :-(

 

[Garth]

The question is, "Does the future for one observer already exist for another?"
If you want your future to come into existence now all you have to do is wait. The problem is that of the future of a distant and moving observer. Can we see their future now, or vice versa? As in the post #9 above we can construct our 'now membrane' to define what we think is their future; but we will not see it happen until they have! We cannot communicate what is in their future until it has happened to them so causality is preserved. We are not saying their future will never exist because we cannot see it, just that it will have to wait!

 

[Gerinski]

Thanks again, getting clearer step by step :-)

I now think my whole confusion came from misunderstanding the famous sentence that "due to relativistic effects, 2 events A and B may happen in reverse sequence according to different observers"

I was taking it as saying that while for observer S, A happens before B, "for observer S', B happens before A".

Yet if we could use a "cosmic universal time", even for oberver S' A happens before B too (or the other way around for both observers, that doesn't really matter). The "universal time sequence" is one and only one. It's just that he sees B before seeing A, but that is just because of the physical limitation of the finite speed of light.

I hope I got it right this time :-)

 

[Gerinski]

But just let me say that the problem about block time didn't go away ...

The idea that spacetime may be "cut in slices" at different angles by different observers, still leaves the feeling that past, present and future are subjective labels ...
Knowing what my slice looks like, if I put myself in the shoes of the other observer, I can't help thinking that when he is (or was, or will be) seeing some event I see now, he can also see together with it some other event that I have not yet seen, due to the different angle of his slice.

pfffff, I admit I don't see the way scientists avoid this problem :-(

 

[Garth]

Remember Archimedes saying "Give me but one firm spot on which to stand and I will move the Earth"?

Block time does not go away if an observer can "find one firm spot on which to stand", outside of space and time, to observe the space-time diagram of the entire history of the universe. Block time is an artefact of that diagram and that observer's perspective.
However all we have done is use our imaginations to conceive of the space-time diagram itself and ourselves standing on that "firm spot". We have found it a very useful tool in Relativity theory.

But the question is,"Is there an observer who can really do that?" We can't, as we are very much 3+1 observers within space and time.

If you are an Idealist you believe such a concept as block time and the curvature of space-time to actually 'exist' in a Platonic way.

If you are an Instrumentalist you simply see them as conceptual models that are good at getting the right answers, they are conceptual instruments, but they do not really 'exist' reality is something even deeper.

Take your choice!

 

[iq2150]

Hello ... I'm having the same thoughts

Gerinsky, you are asking exactly the question I've been trawling the web for an answer to, I think!

I've been having a discussion with a friend at work about this, which started about the implications of SR for free will. Having progressed the debate a little, what troubles me now is the following;

Does the lack of a unique "now" in SR (which I accept) necessarily lead to a single four-space in which the human perception of the flow of time is an artefact of the nature of conciousness?

- with the key word being "necessarily".

I am not troubled particularly if the answer is "yes", but like you I want to understand if this is a necessary (i.e. the only) conclusion, or if there is a formalism that expresses what I intuitively feel the solution to be (see later).

I am aware that the possibility of 'multiverses' allows individual observers an open, undetermined future, but this seems 'overkill' to me. Similarly, it seems to me that SR leads only to 'at least' 4 dimensions, and there is a possibility of "meta-time" that allows each conciousness an individual 4-space (but perhaps I am geting confused here).

My friend argues that the lack of a unique now, and the lack of a preferred frame of reference, leads inevitably to the conclusion that events that we judge to be in our future are "already" (care !) determined. I struggle with the route to this conclusion and it is this that I wish to explore.

My intuitive conclusion, since (I assert - please criticise) that saying that my future is undetermined leads to no paradoxes in SR (for instance no observer can disagree on the order of events on my worldline), is that it is permissible to view all observers as having an open future, with a "real" (care again!) flow of time in which events, "eventually" 'freeze together' into a past on which all observers can agree.

What I want to do, rather than open this question out, is to boil it down to the essential logical steps so that I can explore each one.

All the best

Alan

 

[Gerinski]

Welcome to the club Alan!

As you can see from the thread, I don't have any personal bets on the subject, that's precisely what I'm looking for !
The way I see it leads to some paradoxical consequences, but it seems scientists don't actually see any paradoxes at all about it, so as you, I just want to understand how physicists got rid of the problem.

Garth's and others' comments have been great so far, but I have to admit they have not really made me finally see the light ...

My last post mentioning about the different angles of the spacetime views (slices) according to different observers, I think that summarizes in short my problem...

 

[chronon]

Does the lack of a unique "now" in SR (which I accept) necessarily lead to a single four-space in which the human perception of the flow of time is an artefact of the nature of conciousness?
- with the key word being "necessarily".

I would say that the answer is definitely "no".

Every event has a definite past,present and future in SR, its just that these don't fill the whole of space-time. There are events with spacelike separations which cannot be assigned to these categories without choosing a reference frame. But I don't see that as being a problem.

My intuitive conclusion, since (I assert - please criticise) that saying that my future is undetermined leads to no paradoxes in SR (for instance no observer can disagree on the order of events on my worldline), is that it is permissible to view all observers as having an open future, with a "real" (care again!) flow of time in which events, "eventually" 'freeze together' into a past on which all observers can agree.

I would agree with that, clearly a universe where "freewill" events occur is not logically excluded by SR, even if some people don't believe that our universe is actually like that.

 

[kjones000]

The universe does not preclude paradox. The universe only precludes internal inconsistency of its own rules. Just because relativity tells you that observers A and B see events 1 and 2 in opposite orders doesn't mean that you have to invent something extra beyond relativity to explain it (it doesn't even make it necessarily reasonable to do so). Sure, there is almost certainly something else going on below the surface that we have yet to discover. Perhaps your resolution of this paradox will point to the answer. Perhaps a totally different resolution of this paradox with point to the answer. Perhaps the paradox simply does not need resolving.

 

[Garth]

Does the lack of a unique "now" in SR (which I accept) necessarily lead to a single four-space in which the human perception of the flow of time is an artefact of the nature of conciousness?

- with the key word being "necessarily".

We are conscious of the flow of time so it is an artefact of our consciousness!

Or is our consciousness an artefact of the flow of time?

 

[Gerinski]

I will try once more to express more clearly ...
As indeed there is no universal now, I will try to avoid words such as "already", "simultaneous" etc

If we are humble enough to assume that we don't have a privileged status versus other potential observers in the universe, how do so many people (even scientists) defend that our future is open (undetermined)?

Do we agree that in our future we will SURELY receive light (or any influence) which is on its way to us? (the famous hypotesis that a supernova blast or other cosmic disaster might be traveling towards us right now)

If we assume that the possibility exists, then I think the issue is subject to an infinite regress (because at the event point that the blast will hit the earth, the same could be said of light which is also on its way towards meeting that world line a bit further ahead).
And of course no need for supernovas or disasters, SURELY SOME light and matter is traveling towards us right now (our "now" :-) ), but the disaster case just makes it more clear that the outcome of the future meeting of both world lines is not so clearly "open" !.

Is this reasoning wrong? apart from multiverse theory, how do other theories avoid the problem?
Sure it feels better to live in an undetermined universe, I just want to understand how they invalidate the above arguments.

 

[chronon]

Do we agree that in our future we will SURELY receive light (or any influence) which is on its way to us? (the famous hypotesis that a supernova blast or other cosmic disaster might be traveling towards us right now)

No. If I choose to look down a telescope at a particular time my eyes could receive photons which had been traveling from a supernova. If I choose to do something else instead then my eyes wouldn't receive those photons.