I Acceleration towards c without a reference frame and changes

[PeterDonis]

In a purely empty universe there is no previous speed

Yes, there is, because you have a spacetime geometry and "speed" can be defined relative to a particular inertial frame (the one in which the rocket started out at rest) in that spacetime geometry. See my previous post.

 

[PeterDonis]

With regard to your first sentence, think of it as a polygraph.

You are getting very close to a warning for personal theory. You are not describing how motion in flat Minkowski spacetime works.

 

[Buckethead]

OK - I concede. I am in no position to argue if Minkowski spacetime remains intact or not in a universe that is void of everything except for one spaceship. It is very difficult for me to step away from the notion of Mach's principle.

 

[infector]

Thank you all for your input, very interesting discussion emerged here.
To all my (relatively little) knowledge on the topic I agree with @PeterDonis that:

"speed" can be defined relative to a particular inertial frame (the one in which the rocket started out at rest) in that spacetime geometry.

 

[PeterDonis]

I am in no position to argue if Minkowski spacetime remains intact or not in a universe that is void of everything except for one spaceship.

The question is not so much whether Minkowski spacetime "remains intact" -- Minkowski spacetime is a highly idealized model, everyone recognizes that. The question is, do we have any other model that could possibly describe "a universe that is void of everything except for one spaceship". The answer to that question is no, we don't. And in the absence of any other model, Minkowski spacetime is the best we can do, and anything else is just speculation and is off topic here.

 

[Chris Miller]

A universe devoid of everything but one spaceship would = that spaceship. Hard to make any sort of physical predictions about such a hypothetical, small and different "universe." In this universe, possibly expanded to some future point where nothing is observable (as most of it already isn't) you'd still have the CMB to measure your relative speed and direction against.

 

[PeterDonis]

A universe devoid of everything but one spaceship would = that spaceship

Not if you are using SR/GR as your theory. In SR/GR, if we assume the mass of the spaceship is negligible, the universe would be Minkowski spacetime. If we assume the mass of the spaceship is non-negligible, the universe would be some curved spacetime (if we idealize the spaceship as spherically symmetric, it would be a vacuum region described by the Schwarzschild geometry surrounding a non-vacuum region occupied by the spaceship).