Naked Science Forum

Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: geordief on 03/01/2019 14:11:24

Title: Does acceleration (also) curve spacetime?
Post by: geordief on 03/01/2019 14:11:24
So ,if we are in a spacecraft  that was accelerating at any particular moment and an observer on it attempted to make sense of what he or she was seeing ,would the curved spacetime model  give a correct interpretation of the data he or she was processing?

Is the difference between acceleration-caused spacetime curvature and that caused by gravity only really apparent for phenomena such as tidal effects?
Title: Re: Does acceleration (also) curve spacetime?
Post by: Halc on 03/01/2019 15:23:56
So ,if we are in a spacecraft  that was accelerating at any particular moment and an observer on it attempted to make sense of what he or she was seeing ,would the curved spacetime model  give a correct interpretation of the data he or she was processing?

Is the difference between acceleration-caused spacetime curvature and that caused by gravity only really apparent for phenomena such as tidal effects?
Acceleration is equivalent to a uniform gravitational field, and while a sufficiently large object can get close to that over limited distance, such a field doesn't actually exist anywhere.  So there will be non-local differences over a large enough scale.  Light will still bend, but in a parabolic curve, not a hyperbolic one.  From an accelerating frame, there is still an event horizon beyond which light cannot reach the accelerating observer.
There are no Newtonian tidal effects since those are generated from nonuniformity of the field, but there are relativistic tidal effects due to higher acceleration behind the accelerating reference, and lower acceleration at points forward of the reference.
Title: Re: Does acceleration (also) curve spacetime?
Post by: geordief on 03/01/2019 15:40:02
If there was a light emitter at the rear of the accelerating spacecraft and the observer was at the front (in the direction of acceleration) what effect would that have on the  way that the observer perceived the light coming from the emitter?

Just red shifted?

The observer can never accelerate to such an extent that the light never reaches him can he?
Title: Re: Does acceleration (also) curve spacetime?
Post by: Halc on 03/01/2019 15:47:11
If there was a light emitter at the rear of the accelerating spacecraft and the observer was at the front (in the direction of acceleration) what effect would that have on the  way that the observer perceived the light coming from the emitter?

Just red shifted?

The observer can never accelerate to such an extent that the light never reaches him can he?
Red shifted, yes.  If the observer was accelerating enough and the emitter was far enough in the rear, the rear part of the ship would not be able to keep up even given any amount of acceleration.  If the rear can keep up, the light emitted from there will reach the front.
Title: Re: Does acceleration (also) curve spacetime?
Post by: geordief on 03/01/2019 15:52:13
Red shifted, yes.  If the observer was accelerating enough and the emitter was far enough in the rear, the rear part of the ship would not be able to keep up even given any amount of acceleration.  If the rear can keep up, the light emitted from there will reach the front.
Are you saying that if there was light coming from a distant enough star and this star was accelerating away by even only a small amount that that star would be forever invisible to an observer on the earth?
Title: Re: Does acceleration (also) curve spacetime?
Post by: Halc on 03/01/2019 17:47:32
Are you saying that if there was light coming from a distant enough star and this star was accelerating away by even only a small amount that that star would be forever invisible to an observer on the earth?
It wouldn't matter if the star was accelerating.  If Earth is accelerating, that puts a limit on how far it can see behind it.
Earth is accelerating, but not continuously in any particular direction, so none of this applies.
Title: Re: Does acceleration (also) curve spacetime?
Post by: Halc on 03/01/2019 17:52:18
Earth already has an event horizon in all directions due to the acceleration of spatial expansion.  That horizon would not exist were space to be expanding at a constant rate, or a slowing rate.  Were Earth to begin permanently accelerating at some rate in any one direction, it would change the location of the current event horizon.  1G of proper acceleration would put that horizon only a little more than a light-year away.

The event horizon is currently about 15 billion light years away, not far outside the Hubble sphere.  That means that light from stars that are current receding at a little more than light speed will still get here eventually, but light from stars much more distant than that will never get here in any amount of time, assuming the expansion rate continues its current acceleration.
Title: Re: Does acceleration (also) curve spacetime?
Post by: geordief on 03/01/2019 18:01:01
Earth already has an event horizon in all directions due to the acceleration of spatial expansion.  That horizon would not exist were space to be expanding at a constant rate, or a slowing rate.  Were Earth to begin permanently accelerating at some rate in any one direction, it would change the location of the current event horizon.  1G of acceleration would put that horizon only a little more than a light-year away.

The horizon is about 15 billion light years away, not far outside the Hubble sphere.  That means that light from stars that are current receding at a little more than light speed will still get here eventually, but light from stars much more distant than that will never get here in any amount of time, assuming the expansion rate continues its current acceleration.
Thanks,I feel a bit more  comfortable with these ideas now.

The accelerated expansion everyone talks about is  different from everything accelerating away from everything else is it?

Maybe it is not yet understood why it happens-just that it does happen....?

Title: Re: Does acceleration (also) curve spacetime?
Post by: Halc on 03/01/2019 18:15:52
The accelerated expansion everyone talks about is  different from everything accelerating away from everything else is it?
Sort of.   A galaxy X many billions of LY away will be moving away at speed S today, and S+1 tomorrow.  That is acceleration.  If expansion was constant, it would always be moving away from us at S on average.  So I'd say it is the same sort of acceleration that Newton and everybody else is talking about.