Are we (not) limited by our own speed through time (1 sec / sec) to measure highest possible speeds(c)?

I fire a laser which hits a mirror and comes back and I precisely measure the time, and knowing the distance I calculate the speed.. but when I measured time, wasn't I limited by my own speed in time?

Here I'm imagining time in a 2-d graph with me just moving upwards (stationary) and light making a |>. Let's call bottom part where the light leaves A and top where it comes back B. I measured AB. Which is my own speed...


1 Answer 1


Since 1967, the second has had a very precise definition. As shown in Wikipedia, it is:

the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom

On the other hand, the meter is defined as the distance light travels in exactly one 299,792,458th of a second.

In other words, both the second and the meter are very precisely defined: the second by counting Cesium waves (which can be done with absolute precision), and the meter as the distance light travels while we're counting those waves. That leaves no room for any errors.

Even before these definitions came into effect, the lightspeed you measured was as good as your measurements of the time and the distance. For example, when Fizeau measured the time light travelled between distant mountains, he used a toothed wheel. If the wheel had turned an integer number of teeth, the reflected light could pass through; if it had (n+1/2) teeth, the light would be occluded. From the speed of the wheel and the number of teeth he could get a very precise idea of the time light took for the return trip.


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