# Spatially separated events are time relative but .

1. Mar 18, 2015

### Pleonasm

In relativity of simultaneity, two spatially seperated events are time relative as to the order in which they might occur to the observer (time is relative). However, if the two events are causally connected, the order is preserved in all frames of reference: http://en.wikipedia.org/wiki/Relativity_of_simultaneity

But what logically motivates this dinstinction in the first place? How can one make sense of this empirically proven universe, if you at the same time take the view of causality being a fact since the beginning of universe, entailing that all events are by necessity causally connected to each other.

Why then are spatially seperated events not subject to specific ordering in a cause-effect universe?

Last edited: Mar 18, 2015
2. Mar 18, 2015

### Orodruin

Staff Emeritus
No, this is not true. An event is a location and a time, not only a location. At any time, you are causally disconnected from what goes on in the Sun at the same time (in some frame), but things that happen in the Sun will become causally connected to your future at some point (in roughly 8 minutes time in the Earth rest frame).

3. Mar 18, 2015

### Pleonasm

How will I make sense of time delay in special relativity, if I don't believe it's an objectively existing property of the universe? Can perceived time passage be defined as different points in space, if one adheres to Minkowski/Einsteins 4-dimensional block-universe theory of time? Just curious.

Also: are events within earth causally disconnected to each other in a similiar way in the following: "a car crash in London and another in New York, which appear to happen at the same time to an observer on the earth, will appear to have occurred at slightly different times to an observer on an airplane flying between London and New York".

It's the example above that I found logically disturbing. But I guess it shouldn't be.

4. Mar 18, 2015

### Ibix

I think you are misunderstanding the term "causally disconnected" in this context. Two space-like separated (not merely spatially separated) events, A and B, are causally disconnected if A could not cause B and B could not cause A. The term is agnostic about how events A and B came about. For example, imagine a lamp half way between two mirrors. The lamp emits a pulse of light (call this event L) which spreads out and eventually arrives at the mirrors (events A and B). Event L is causally connected to both A and B (what happened at L caused what happened at A and B, after all). However events A and B are causally disconnected - what happens at A does not and cannot affect what happens at B, and vice versa. In this example, the point is that the reflection of light at the mirrors does not care how there comes to be a pulse of light travelling to it. It will reflect the light just the same whether it is in the experiment I described or one where there are two lamps, or five, or whatever.

The distinction between causally connected and disconnected events, like everything in relativity, follows from Einstein's postulate that the speed of light is independent of source and observer. This means that light always passes you at 3×108ms-1 whatever you are doing. That means that, in a universe governed by relativity, you can never accelerate past light speed - if you could, there would be a moment when you were stationary with respect to light, in which case it couldn't be passing you at 3×108ms-1. That, in turn, means that any two things that are far enough apart that light can't cross the gap between them in the time between them can't possibly affect each other. That means it doesn't really matter what order the events are in.

The point about the causal structure of the universe is that any point in the block universe (warning: that's just a model, not necessarily the reality!) can divide the universe into three regions: its future light cone (the 3+1 dimensional volume that can see the event), its past light cone (the 3+1 dimensional volume that can be seen from the event) and the rest of space-time (Penrose apparently just calls this "elsewhere"). It turns out that all observers will agree on what is inside the light cones and what is outside. So they will all agree that events in the future light cone happened after the "main" event, so to speak, and that events in the past light cone happened before it. They will also agree that the interval between the events (Δs2=(cΔt)2-(Δx2+Δy2+Δz2) is the same - this is analogous to the distance between two points in Euclidean geometry.

Everyone agrees that there is time. They just don't agree on exactly which direction it is, in the block universe. That is, fundamentally, no more mysterious than you and I disagreeing which way is left and which way is right because we aren't facing the same way.

Yes, this applies everywhere. The car crashes will not be simultaneous to all observers. But it doesn't matter, because if light from one crash can't have reached the other crash, then neither crash could have been caused or avoided by rubbernecking the other.

It's not logically disturbing. It just goes against your intuitive (Newtonian, with absolute time) feeling of how the world works. It takes a while for the Einsteinian model to settle in and get properly separated from your everyday intuitive model. You can go a long way by always asking "relative to what" when someone says "velocity", and "according to who?" when someone says "same place" or "same time".

5. Mar 18, 2015

### Pleonasm

That's exactly right. If I didn't know better and all I had to go on was philosophy/logic, like Newton did, I would have argued for the the Newtonian view of time. Now as to the block theory of time, Vesselin Petkov (PhD in both philosophy and physics) argues that the experimental confirmations of special relativity are not possible in a three dimensional world. http://philsci-archive.pitt.edu/2408/1/Petkov-BlockUniverse.pdf

6. Mar 19, 2015

### Staff: Mentor

He's arguing against a straw man. Nobody claims that SR describes a three-dimensional world. But the fact that the world is four-dimensional does not prove that the "block universe" viewpoint is true.

7. Mar 19, 2015

### Ibix

Newton based his theory on experimental evidence, not philosophy. The only problem was that the data was not precise enough to show relativistic effects. We only noticed them when we began to understand electromagnetism.

To add to Peter's earlier comment, I think the block universe is difficult to reconcile with the more complex geometries in general relativity. There was also a paper linked to recently here where someone was arguing that the block universe and quantum mechanics are irreconcilable. In short, take anything pertaining to the interpretation of the maths with a grain of salt - interpretations are helpful models, but which (if any) is correct (and whether "correct" has any meaning in this context) is very much
an open question.

8. Mar 31, 2015

### Pleonasm

Really? What alternative do you suggest in a 4 dimensional world, if not the block universe? Growing block universe? There are no alternatives that I know of.

I always imagined the universe as a ball, that is a confined source of energy, matter and so forth. Somehow I crave for ordering of events, just as I would expect from a dvd, comprimising events. I could play the dvd backwards, but the events would still be in order, only played in a different direction. That's not however what special relativity entails, but something even more radical than that.

How could our universe be preserved when certain events are of no important ordering? How does it all stick together as hole? Our view of what the universe is mechanically, must be revised.

9. Mar 31, 2015

### Staff: Mentor

This is just an argument from ignorance: you don't know of any alternatives, therefore there aren't any. It's not a valid argument.

Also, you are assuming that a choice has to be made among some defined set of alternatives to pick the "right" answer to this question. Why must that be the case? (See further comments below.)

I don't suggest any alternative. I merely note that the fact that the world is four-dimensional does not prove that the block universe viewpoint is true. It could be that we are simply not smart enough (yet) to figure out what the correct alternative is. It could be that the question of "does all of spacetime exist?" is not even well posed; it seems to us like it's well posed, but we may be mistaken. (Consider how you would respond to a sixteenth-century alchemist transported to the present day via time machine, who, upon finding out about the modern atomic theory, asks you whether phlogiston is made of atoms. When you start hemming and hawing, not quite sure how to explain to him that his entire conceptual scheme is simply not even wrong, he says, "well, if phlogiston isn't made of atoms, what other alternative is there?")

10. Apr 1, 2015

### Pleonasm

You don't seem to understand the implications, based on conceptual confusion. If we can conclude (which we have) that we don't live in 3 dimensional world, and with it abandon the common sense view of time - only the present moment is real, then by definition, the opposite of presentism is what remains, which ever form it might take. The opposite of only present moment is real, is that other events exists simultaneously. Special relativity does not merely support such conclusions, but requires it.

11. Apr 1, 2015

### Orodruin

Staff Emeritus
I am sorry but none of that makes sense and is utterly dependent on your very personal view of what is "real". Physics is about describing how the world around us behaves, not making claims about what "reality" is.

12. Apr 1, 2015

### Staff: Mentor

Agreed.

Who said we had to do that? Our "common sense view of time" is based on our experience of the passage of time along our own worldlines. It is not based on any concept of simultaneity that extends to far-distant portions of the universe. That is an artifact of Newtonian physics; it's not something we directly experience and it is not part of our "common sense view". The common sense view survives just fine in SR, as the concept of proper time: events along a particular timelike worldline always have an invariant time ordering, which is just what we experience.

I have made no such claim. You are attacking a straw man.

You should not have included the word "simultaneously" here, as it is not correct. SR is perfectly compatible with the view that events in our past light cone are "real", but events outside our past light cone are not, at least not in the same sense. This means there are plenty of events other than the present moment that are unproblematically "real". But none of those events are simultaneous with our present moment, since they are all in our past light cone.

13. Apr 1, 2015

### Pleonasm

This is a semantical objection. If you protest against my usage of the word "real", it doesn't make it less true. What matters is the content.

It appears that there are (from our perspective) future/comming events in spacetime which already take place in a 4-dimensional world. Events take place, that is points in spacetime, which we don't think have yet materialized (because we haven't experienced them yet). That is the essence of what 4 dimensional world entails, and what the block universe is.

14. Apr 1, 2015

### Pleonasm

That is what a 3 dimensional world is! Have you actually read Veselins paper?

15. Apr 1, 2015

### Ibix

Here's an alternative to the block universe. The point of Special Relativity is not that there is no absolute rest frame, just that it is not detectable. So it is possible that there genuinely is a global "now" and things only "really exist" on (what the block universe would describe as) one particular plane of simultaneity.

To illustrate this, imagine a computer simulation of the rod-and-barn problem. I record the ends of the barn as being at +L/2 and -L/2, and the ends of the rod as being at $vt+L/\gamma$ and $vt-L/\gamma$. Then I define L and set up a loop that runs from t=-T to t=T. That's very much a sketch - a true simulation would need to be at the particle level and incorporate some actual physical rules to permit interaction, but hopefully you get the picture.

As long as I stipulate that the arithmetic is of high enough precision that we haven't noticed the rounding errors, that isn't distinguishable from the block universe. It is purely 3d; history is encoded in the motions of particles and things like the relativity of simultaneity fall out of my choice of interaction laws.

I'm not really proposing that the universe is actually a computer simulation. It's another interpretation of the physics, however, and it's possibly a useful one if you are into making relativistic simulations.

16. Apr 1, 2015

### Pleonasm

Aha, I see what you are getting at it. An alternative view is that there are several events taking place all during one global "now"? It would still entail determinism though, since some event B that you believe is in the future, is already in someone elses past. The event must come about no matter what. You can still argue that the particular events creation was uncertain, but it if it wasn't uncertain to happen in your future, why would it have been uncertain for any other observer? I find this line of reasoning utterly uncompelling.

17. Apr 1, 2015

### Pleonasm

Why think theres' a global now at all, given all of this?

18. Apr 1, 2015

### Staff: Mentor

I am not claiming there is a 3-dimensional world. Nor am I claiming that only one particular spacelike hypersurface (the one labeled "now" in some chosen coordinate chart) is "real". You and the paper are both attacking claims that neither I nor SR make.

Who said there had to be one? An obvious interpretation of what SR is telling us about relativity of simultaneity is that there is no such thing as a global "now".

More precisely: before SR, everyone assumed, intuitively, that there are three classes of events: the past, "now", and the future. "Now" is just all the events that take place at the same time as "here and now", the event I am currently experiencing. The "past" is all the events that happened before "now", and the future is all events that will happen after "now".

What SR shows us is that there are actually four classes of events: the past, "here and now", the future, and what I'll call "elsewhere"--all the events that are spacelike separated from "here and now", the event I am currently experiencing. The past is the past light cone of "here and now", and the future is the future light cone of "here and now". Therefore, there is no such thing as a global "now"; events are simply not classified the way our pre-relativistic intuition would classify them.

No: these events are in "elsewhere", and when you say they "already take place", you are assuming your conclusion. Unless you already believe in the "block universe", there is no justification for claiming that events in "elsewhere" must have "already taken place".

19. Apr 1, 2015

### Staff: Mentor

20. Apr 1, 2015

### Orodruin

Staff Emeritus
I will just pile on that Peter is not the only one calling this "elsewhere". I know at least Wolfgang Rindler does it in his introductory SR text.

It should also be mentioned that GR crushes the notion of a global "now" to an even higher degree.