Imagine there exists only a single object (say a 1 metre sphere). There is nothing else in all directions.
- Is the object moving or at rest? Is it even possible to tell, given that there is no frame of reference?
Extending the idea, suppose the sphere has some form of (rocket-like) propulsion, constantly accelerating it an arbitrary but single direction. The sphere's velocity, relative to what it was when we first imagined it, approaches the speed of light.
The propulsion then stops for a while (while we think about it). At this point we - I assume - still can't tell the difference between it being at rest or moving.
- The propulsion resumes. What exactly prevents the above from repeating, and the sphere continuing to accelerate to a velocity any arbitrary number of times the speed of light, relative to its velocity when we first imagined it?
(To preempt answers such as it's all irrelevant because there's no frame of reference,
I forgot to tell you we then discover there is actually a second sphere, exceedingly far away, but in the direction of propulsion).