Simultaneity in Non-Observable Universe: Is it Possible?

[Chris Miller]

Is there any method/model for determining simultaneity of events farther apart than light can ever travel? I'm guessing not. Would it be wrong to say that the non-observable universe exists only in our past (or future)?

 

[Ibix]

You can adopt any simultaneity convention you like. Typically people use co-moving coordinates as "the" time, in which case one can define simultaneous as meaning two events which occur the same time (as determined by a co-moving observer) after the big bang singularity. Or that occurred when the temperature of the CMB (measured by co-moving observers at each event) was the same.

 

[stoomart]

Would it be wrong to say that the non-observable universe exists only in our past (or future)?

My understanding of light cones tells me events in the non-observable universe are not in our past or future, since their information cannot reach us.

 

[Chris Miller]

Thanks Ibix. Problem for me here is that I have no idea what my distant co-moving observer's measurements are, was under the impression that I had to be able to "see" her clock through some hypothetically super-powerful telescope and then subtract the light travel time to know, say, that my 2018: Jan 1: 00:00:00 is her 2035: Mar 6: 13:30:05. Or whatever.

I share your confusion here stoomart. Practically speaking, simultaneity beyond light's reach seems undefined.

 

[Ibix]

You don't know what's happened until you see it, no. And my understanding is that there are parts of the universe you'll never see. But you can talk hypothetically about what's happening now somewhere you can't see (or won't see until later). The only extra wrinkle relativity introduces is a requirement to define what you mean by "now" before you do.

Saying simultaneity is undefined where you can't see is a bit like saying the back of your head is undefined because you aren't sitting between a pair of mirrors. I suppose it's possible that the back of my head doesn't exist at the moment, but if so it's strange how it comes into existence the instant someone comes into the room behind me.

 

[Chris Miller]

You don't know what's happened until you see it, no. And my understanding is that there are parts of the universe you'll never see. But you can talk hypothetically about what's happening now somewhere you can't see (or won't see until later). The only extra wrinkle relativity introduces is a requirement to define what you mean by "now" before you do.

Saying simultaneity is undefined where you can't see is a bit like saying the back of your head is undefined because you aren't sitting between a pair of mirrors. I suppose it's possible that the back of my head doesn't exist at the moment, but if so it's strange how it comes into existence the instant someone comes into the room behind me.

I understand relativity doesn't apply to Hubble expansion where even though co-moving objects are separating at >c, their light clocks would each appear (if they could each be seen by the other) to run at the same speed. Also Minkowski space doesn't factor in expansion. So maybe this was the wrong forum for my question. I suppose your analogy is valid, although I have seen the back of my head and can examine it in ways other than visually.

 

[PeterDonis]

So maybe this was the wrong forum for my question.

General relativity covers all spacetime geometries, not just Minkowski spacetime. So this is the right forum.

Another way of phrasing the answer you are getting to the original question of this thread is that you can always adopt a simultaneity convention that includes events outside your observable universe. But you won't be able to connect that convention to any physical observation that you can actually make involving those events, the way you can, for example, connect the standard simultaneity convention for inertial frames in flat spacetime to the Einstein clock synchronization procedure.

 

[Ibix]

I understand relativity doesn't apply to Hubble expansion where even though co-moving objects are separating at >c, their light clocks would each appear (if they could each be seen by the other) to run at the same speed. Also Minkowski space doesn't factor in expansion. So maybe this was the wrong forum for my question.

In cosmology there's an identifiable "time zero", the Big Bang singularity. And there's an identifiable way to define "two people in the 'same' state of motion", by finding when the CMB is isotropic. Neither of these things can be done in special relativity because the singularity and the CMB don't exist in that case. So you can actually define simultaneity in this case without communication where you can't in special relativity.

You can't actually do anything with the simultaneity convention because you can't communicate. But "at the same time" can be defined perfectly well.

I suppose your analogy is valid, although I have seen the back of my head and can examine it in ways other than visually.

It's not my best analogy.