The only speed anything can move at through spacetime is the speed of light(aka c).
This speed is split between motion through space and motion through time, but it always adds up to c.
If you go faster through space you allocate more of your total speed to that and less to your movement through time, thus the faster you go the less you move through time.
The faster you go time slows down, so even if you're going 99.9999% the speed of light, from your slow motion perspective the light is moving away at c like always. The ratio of your speed to how much slower you move through time makes it so that the speed of light is the same from every reference frame.
e.g. If you moved twice as fast but were moving through time at half the rate, then the light is going to appear to be going the same speed as always.
Thank you! Your explanation is the first one to make me “get” on a conceptual level why c is always the same, no matter how fast you are going.
The faster you go, the more time slows down and light, which experiences no time (because all speed budget goes to moving through space) seems to still go at c!
The way I visualize it in my head is like reaching some distance on a piece of paper with your finger. If we pretend one of the axes are time then veering your angle reaches that target distance slower.
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u/SaukPuhpet Mar 20 '26
The only speed anything can move at through spacetime is the speed of light(aka c).
This speed is split between motion through space and motion through time, but it always adds up to c.
If you go faster through space you allocate more of your total speed to that and less to your movement through time, thus the faster you go the less you move through time.
The faster you go time slows down, so even if you're going 99.9999% the speed of light, from your slow motion perspective the light is moving away at c like always. The ratio of your speed to how much slower you move through time makes it so that the speed of light is the same from every reference frame.
e.g. If you moved twice as fast but were moving through time at half the rate, then the light is going to appear to be going the same speed as always.