Alien astronomer?

[NickMilner]

ponder this . if you waved ,then somehow teleported yourself 1 light year away. would you see yourself waving at yourself in a years time ?

Correct, which is precisely the reason why lightspeed protects us. It's impossible for you to get to that point a light year away before the light that shows you waving.

Maybe in future we'll be able to teleport, but our signal would still be constrained by the speed of light.

[Stephen]

Correct, which is precisely the reason why lightspeed protects us. It's impossible for you to get to that point a light year away before the light that shows you waving.

Maybe in future we'll be able to teleport, but our signal would still be constrained by the speed of light.

Unless you used quantum transportation, doesn't quantum theory allow for instantaneous transfer of data?

[acey]

doesn't quantum theory allow for instantaneous transfer of data?

Nope. There are non-local effects in things like the EPR experiment but it's not transfer of data, which would violate special relativity.

[NickMilner]

Unless you used quantum transportation, doesn't quantum theory allow for instantaneous transfer of data?

Ah, but the entangled photon itself still needs to travel from A to B, and it can only do so at the speed of light. /edit - see below for why quantum teleportation doesn't constitute superluminal information transfer...

IIRC, GR doesn't prohibit events moving around in time or even swapping over, but those events cannot have a causal relationship. The information that determines A causing B always travels at the speed of light, so B must always follow A.

[Jonobiwan]

I was talking to someone about this exact thing about 2 weeks ago!!

It's actually mind blowing when you sit down and think about it.

My head hurts!

[NickMilner]

/removed

[jamespels]

As a mental exercise whilst cycling, I worked out you'd need a telescope with a primary mirror ~1 light year in diameter to get enough resolution to "see" a 2m high person in M81. So, for an f/5 telescope, you'd be looking 12 millions and 5 years into the past by the time the light got to the eyepiece

If we're using a "standard" newt measuring 5ly by 1ly, presumably we could be a bit crafty and observe a planet in M81 only 11,999,995 ly away, using the 5 years as the light travels back to us from the mirror to make up the full 12 million. That should save us the all-important couple of inches from the diameter of the primary... Of course, we'd only be able to watch aliens on stars on the closer half of M81...

Alternatively, doing a quick google for M81 suggests that Stargate Universe fans could pop through a stargate to a ship called Destiny which happens to be making its way to M81 at the moment. Or something.

[geoff_k]

12 million years ... might get a clear night by then.

24 million years - probably get two clear nights and you will be busy on both of them.

Geoff