The premise of relativity is that the speed of light remains constant, such that in an accelerated frame, both time and distance are dilated equally. In the frame traveling at the speed of light, the clock stops and the ruler shrinks to zero, as no measurements are possible, since that would require additional motion and thus exceed the speed of light.

Presumably then, the frame where they are the least dilated would be closest to the equilibrium of the vacuum.

If I may add, the problem with spacetime as the physical basis for the math of Special Relativity is that it treats time as an actual dimension, measured as units of duration.

We are mobile organisms, necessitating this sequential process of perception, in order to navigate, so our experience of time is as the point of the present, moving past to future, but the reality is that change turns future to past. Tomorrow becomes yesterday, because the earth turns. Duration is the present, as the events rise and fall.

There is no literal dimension of time, because the past is consumed by the present, to inform and drive it, aka, causality and conservation of energy. Cause becomes effect.

Time is asymmetric, because it is a measure of action and action is inertial. The earth only turns one direction. Enropy is a second order effect and not what is measured as time.

So time is an effect, similar to temperature, pressure, color and sound. Time is frequency, events are amplitude.

Ideal gas laws correlate volume with temperature and pressure, but we don't refer to them as dimensions of space, because they are only foundational to our emotions, bodily functions and environment, just not the sequence of perception.

Consider that epicycles were brilliant geometry, as description, but the crystalline spheres were lousy physics, as explanation.

Spacetime is a lousy explanation.