: Let’s say I’m alone in the universe. Am I moving at all ?

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According to (my understanding of) Einstein, the slower I go through space, the faster I go through time. So If I’m alone, with no point of reference, am I travelling through time at maximum speed, or am I travelling through space at the speed of light ?

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In your own frame, you are always at rest. Time dilation applies only to things that you observe that are moving with respect to you.

All motion is relative. There is no absolute reference frame. Your local experience of time is always constant and never changes. Other things around you that are moving quickly relative to you will experience time at a different rate than you.

If you’re alone in a universe with no other point of reference, then the words “motion” or “speed” would have no meaning. Even the “speed of light” would be meaningless because if there was light in this universe, then there would necessarily be other objects (photons) in the universe that exist in a different reference frame than you.

If you are all alone and there is literally nothing left in the universe then the very concept of motion , movement becomes useless.

>the slower I go through space, the faster I go through time

This is not correct either. You can’t move “faster through time”. Faster with respect to what? You’re always moving through time at the rate of 1 second per 1 second.

This probably come from the misunderstanding of velocity 4-vector. Velocity 4-vector is always normalized. When you move through spacetime, the only thing that describe how you move is the worldline. If you think of spacetime as a piece of paper, the worldline is the trace of a curve you draw on a paper using a pen. But *how* you draw that curve does not matter, the way you move your pen doesn’t matter, your pen could move fast or slow and you still get the same trace. For mathematical simplicity, it’s common to normalize the speed of this pen so that it always move at constant speed. And that’s what 4-velocity does. But that does not mean you are physically moving through spacetime like that. The “pen” is just a mathematical tool, an abstraction to describe the worldline.

This question is not well defined. Since all motion is relative, you can’t really be “moving” without a reference point.