1. May 17, 2013

### durant

Can somebody explain the implications of the well known Andromeda paradox in a metaphysical sense? First, does it state that all events are determined? If one observer already knows what happens on the Andromeda galaxy, could it be possible that that event still happened in an indeterministic way (by chance)?

And also, what does the paradox say about the proper time of events? Does the Andromeda paradox imply that the future already exists, or that there only exists a 'latest state' of the object (or event) which will eventually occur sooner or later to every observer?

2. May 17, 2013

### Staff: Mentor

No. It just tells you that it is pointless to talk about what happens today (literally, not yesterday and not tomorrow) in other galaxies.
You cannot, as information cannot spread quicker than light in special relativity - and special relativity was used to create the setup you are considering.
There is no unique, global time-order, but locally everything happens ordered.

3. May 17, 2013

### durant

So if one observer theoretically founds out 'before' another what's happening in the Andromeda galaxy, it doesn't imply that the event was causally determined by the previous event?

So the proper time of some object/event remains invariant, and there exists a latest state of the object (in this case the Andromeda galaxy). If not it seems that somebody travelling by a great speed can see the future of an entity.

4. May 17, 2013

### Naty1

Then the 'one observer' is closer to the event than 'another'.....so light signal information arrives sooner....

5. May 17, 2013

### Staff: Mentor

The paradox says nothing about the "proper time of events" because there's no such thing. Proper time is defined as the distance along a path between two events, not something that you can attach to a single event (nor a pair of events, unless you're also going to specify a path between them).

I find that the most effective way of analyzing the Andromeda paradox is to be as rigorous as possible about exactly what it means to say that the fleet has or has not "already" taken off.... Avoid using any term that cannot in principle be backed up by observation by an observer on the spot... And I think you'll find that the paradox is not in the physics, but rather in the careless way that we use natural language to informally describe the paradox.

6. May 17, 2013

### Staff: Mentor

There is no such thing as "the proper time of some object/event". The invariant thing that we call "proper time" is the amount of time that elapses between two events as measured by a single clock that is present at both events. If you try to read any more than that into proper time, you WILL confuse yourself.

7. May 17, 2013

### durant

Thanks for the explanation, Nugatory.

8. May 17, 2013

### pervect

Staff Emeritus
We try to avoid metaphysics here and stick to physics. It's part of the PF policy.

No.

I think you might be misusing the term "proper time" here. As far as the metaphysics goes, the ONLY thing the Andromeda "paradox" says is that simultaneity is relative, that the notion of "now" is observer dependent and not universal.

9. May 18, 2013

### durant

But why does this effect occur? In order to see what's the 'latest state' of the object, we must be co-located with it. Depeding on the motion, in our reference frame 'the present' of the object may be 'the past' of it in some other reference frame. This doesn't make any sense to me. Would this imply that if we would travel really fast in the direction of the object we would see its future completely or something like that?

10. May 18, 2013

### Staff: Mentor

No!
You cannot see what happens now in the Andromeda galaxy. You can only see what happened far in the past, and all observers on earth will see the same thing at the same time, and all agree that this happened in the past. They will give it different timestamps ("happened 4 million years ago", "happened 4 million years and 1 day ago", ...), but that does not matter.

11. May 18, 2013

### durant

So in all frames of reference the Andromeda galaxy has the same state? It's just a matter of the distance between the observers and the Galaxy in judging the temporal parts of the galaxy?

By the first sentence I mean the following, for instance if the Andromeda galaxy explodes (I know this is very hard but just for an example :P), the event will be a part of the present of each frame of reference? Or not?

12. May 18, 2013

### Staff: Mentor

What do you mean with "the same state"? We all observe the same things here on earth.
I don't understand that question.
If the Andromeda galaxy explodes "tomorrow" for me and exploded "yesterday" for you, it does not matter - assuming we both stay on earth long enough, we will see it at the same time. We will just disagree about the time that has passed since then.

As you can see, arbitrary faster-than-light journeys automatically allow time travelling - if I can travel to Andromeda in a few minutes (as seem in my current frame), I can arrive before the explosion happens. I can travel a few minutes (as seen in your current frame) back, and arrive 2 days efore I started.

13. May 18, 2013

### durant

What if any of us was co-located (hypotethically) with the explosion, what would be the present in that situation? I understand that the distance from the object has an impact on the fact that we alway see its 'past', but I don't get why the motion changes what is present in the reference frame of any observer. And I don't mean the appearance of those entities, but I refer to the plane of simultaneity of each reference frame. Shouldn't it be the case that in every reference frame we should have the same present state of the object, just different order between the object and other events.

14. May 18, 2013

### Naty1

durant:

yes.

good, because that statement is incorrect. See my earlier post.

It doesn't. It seems that idea is what is causing your confusion. Everyone measures the same local speed of light regardless of their relative velocity. Two observers on earth, one 'stationary' one moving at the same location observe an events from Andromeda at the same time.

An analogy: If you and a another moving observer are adjacent at some moment, you will observe the flash of distant lightning at the same moment....and hear the resulting thunder together a few seconds later.

15. May 18, 2013

### durant

Thanks for the reply Naty. So in the particular case of the Andromeda paradox, the sequence of events on the Andromeda galaxy will be present in every distant reference frame, regardless of its motion. It's just the case that some observers will some events from the past of the Galaxy before another observer does (because of their relative distance). So it's something like the case that if we were really close to the sun we would see its specific past state before another observer on the Earth will, right?

16. May 18, 2013

### Staff: Mentor

This effect occurs because the speed of light is finite.

Yes. Note that you have no information about the present condition of a distant object in any frame. You only have information about its past. Furthermore, what you label as "the present" is a mere labeling convention with no more significance beyond the convention itself.

No. No velocity will ever let you see any part of the future of any object, regardless of distance.

17. May 18, 2013

### pervect

Staff Emeritus

http://www.bartleby.com/173/9.html

That has the detailed argument for why simultaneity must be relative.

18. May 18, 2013

### durant

I've understood relative simultaneity, I think, at least the part where the temporal order of the events in the universe isn't fixed. But the Andromeda paradox confused me because it is derived from the relativity of simultaneity, and I can't find the link between those two concepts.

19. May 18, 2013

### durant

So when moving away from the object (in this case the Andromeda galaxy) the different past segments of it will become present in our plane of simultaneity, but when moving towards to it all observers will have one 'future' state of it, regardless of their speed and distance towards the galaxy. So observers with different motions and distances away from the Andromeda galaxy will have different slices of the past of the Andromeda, but all observers who move towards the galaxy will have its latest state as the present in their frame of reference.

Please correct me if I'm wrong, this is what I've concluded regarding your last sentence. Thank you in advance.

20. May 18, 2013

### Staff: Mentor

The Andromeda paradox is just a visualization of relative simultaneity, I cannot understand how you can understand one and be confused by the other.