# Absolute Position

1. Jul 21, 2009

### DavidSnider

"Absolute Position"

This is a bit of a contrived scenario, but I wonder how it might be solved..

Say you have two highly advanced civilizations that want to meet each other, but in order to do so the trip will take billions of years.

Now, in this time, solar systems collapse, orbits change, etc, etc..

Since you have no idea whether a relative point in space will still be valid in the future, how would you communicate the rendezvous point?

(Nevermind how they communicated with each other in the first place)

2. Jul 21, 2009

### maverick_starstrider

Re: "Absolute Position"

Well assuming you have a perfect star map you could simply say "let's meet on planet X" and just compute where planet X is going to be in a million years and take off. Of course planet X might not be there in a million years but if nothing had changed for it by the time your message was received and your alien buddies took off it wouldn't matter because you'd still meet at the same spot (where planet X was supposed to be). If "where planet X is supposed to be" changed during transmission (say it got hit by a swarm of meteor's that you didn't consider and it changed its trajectory) then you're screwed.

3. Jul 21, 2009

### maverick_starstrider

Re: "Absolute Position"

That's why I never make plans for a million years in the future. I never know what my schedule is going to be like.

4. Jul 21, 2009

### Staff: Mentor

Re: "Absolute Position"

Well you provided an inarguable answer to the question:
Well if you have no idea, then you have no idea.

That said, it is probably wrong to say you have no idea. The easy answer is that you simply track and predict the changes in order to keep your eye fixed on the location of the rendezvous. That's how it is done now with spacecraft sent to the outer solar system!

5. Jul 21, 2009

### DavidSnider

Re: "Absolute Position"

Tracking things in the outer solar system in a period of years is relatively deterministic I would think. I don't think you could track it on a larger scale...

So there is nothing absolute in the universe from which to make a reference point?

6. Jul 21, 2009

### maverick_starstrider

Re: "Absolute Position"

No, however space if very, very, very, empty so on the horizon of a couple million years the probability of a given solar system (where planet x is) passing close enough to another celestial entity (star, solar system, black hole, quasar, etc.) in that amount of time is slim. Therefore, even though it's millions of years, by the odds, it'll be a good approximation to treat the solar system where planet X is as a closed system. Of course you could also predict if this solar system has another object of interest in its path. All in all you could pick a rendezvous destination with an extremely high probability of being unchanged over the passage of a couple million years (given good star maps).

7. Jul 21, 2009

### DavidSnider

Re: "Absolute Position"

Since space is expanding, and from what I hear the expansion is accelerating.. doesn't this mean that at a certain distance it would be impossible for two entities to meet?

8. Jul 21, 2009

### maverick_starstrider

Re: "Absolute Position"

That sounds like a very good question to me but I haven't a clue. I don't do/know GR at all really.