# Can the speed of light be measured only in a local inertial frame of reference?

Whether the speed of light can only be measured at the observer's place, in his local inertial frame of reference, that is, where the measurement is made. It is about the fact that the observer (that is, his measuring devices) are in his local reference frame, but he is measuring the speed of light in some distant reference frame.

If this is possible, will there be a difference in the speed of light between a local and such a remote measurement of the speed of light, and in what examples?

• The speed of light always connects two different locations. It is not possible to even talk about it without referencing two distinct points in space. I don't know what you mean by "different flow of time". Time is always that which the clocks show. The only difference in time is being observed when we look at a second clock in a different reference system. Commented Apr 5, 2023 at 8:18
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– Dale
Commented Apr 5, 2023 at 11:00
• Only one clock is needed to measure the speed of light, but we also need a ruler, and that means the speed of light is not a local quantity. It requires at least two locations and the entire connecting line between them. So while clocks are, in some sense, entirely local, rulers and derived quantities are not. Of course an observer can measure the speed of light in another reference frame. All it takes is a bunch of mirrors and a ruler in that other frame. Whatever you do, though, you always need that ruler. Commented Apr 6, 2023 at 16:31
• Physics with observers is, in general, not a good idea. It distracts from the physics. Negligible dimensions are also not "zero". They are simply physics speak for "tangent space". Unless there is no physics to first order (c is of first order in space and time), we can't go to second or higher order. Commented Apr 6, 2023 at 22:17
• Relativity is all about different conditions. I don't know why that should be a problem? That is the core idea of it... how local physics "translates". The speed of light is the most important property that translates in a trivial fashion. Another thing are scalars in general. Whatever can be counted in one reference frame, will lead to the same count in another. Commented Apr 6, 2023 at 22:20