Acceleration towards c without a reference frame and changes

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Discussion Overview

The discussion revolves around the concept of acceleration in a vacuum without a defined reference frame, specifically examining two hypothetical scenarios involving spaceships accelerating to relativistic speeds. Participants explore the implications of relative motion, energy expenditure during acceleration phases, and the nature of reference frames in the context of special relativity.

Discussion Character

  • Exploratory
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • Some participants question the validity of discussing motion without a defined reference frame, arguing that speed is inherently relative and requires a frame of reference to be meaningful.
  • One participant suggests that the spaceship's pilot could consider their own frame of reference as valid, but raises issues about how instruments like speedometers would function without external references.
  • Another participant posits that acceleration does not lead to an absolute state of motion but rather a change in inertial reference frames.
  • There is a discussion about whether energy expenditures for acceleration phases would differ, with some suggesting they would be the same while others indicate that mass loss during acceleration could complicate the scenario.
  • Philosophical analogies, such as the "tree in the forest" thought experiment, are used to frame the discussion about the existence of reference frames in a void.
  • Clarifications are made regarding the conditions under which one spaceship catches up to another, with emphasis on the need for precise definitions of motion and acceleration in the scenarios presented.

Areas of Agreement / Disagreement

Participants express disagreement regarding the necessity and definition of reference frames, with some asserting that a reference frame is essential for discussing motion, while others explore the implications of a pilot's perspective. The discussion remains unresolved on the specifics of energy expenditure during acceleration phases and the effects of relativistic mass.

Contextual Notes

Participants highlight limitations in the original problem statement, particularly regarding the clarity of the scenarios and the assumptions about reference frames. The discussion also reflects varying interpretations of relativistic effects and the philosophical implications of motion without external references.

  • #31
Buckethead said:
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.
 
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  • #32
Buckethead said:
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.
 
  • #33
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.
 
  • #34
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:
PeterDonis said:
"speed" can be defined relative to a particular inertial frame (the one in which the rocket started out at rest) in that spacetime geometry.
 
  • #35
Buckethead said:
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.
 
  • #36
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.
 
  • #37
Chris Miller said:
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).
 
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