## How can scientists measure the speed of light?

 Quote by physicsguy13 I know the textbooks say light travels 186,282.4 mps, but if time slows for a very fast object, how do scientists really know the speed of a photon?
There's certain rules for talking about space and time in relativity.

Never say something is slow or fast or stationary. Only say it is slow or fast or stationary RELATIVE to something else. Never talk about a velocity, only talk about a velocity RELATIVE to something else. If you see two particles traveling side by side at 99% of the speed of light RELATIVE to you, then they see you moving at 99% of the speed of light RELATIVE to them (in the opposite direction) and they see each other as not moving RELATIVE to each other.

Time does not slow for a fast object. If there is a fast object RELATIVE to you, then RELATIVE to you, its clock ticks more slowly than yours. But, to someone on that object, you are moving fast RELATIVE to them, and RELATIVE to them your clock ticks more slowly than their clock. That's what time dilation means.

So time doesn't slow for a fast object. In relativity, that statement makes no sense. A clock on a fast object RELATIVE to you ticks more slowly RELATIVE to your clock. When you measure how fast it is going, you use your clock, not the objects clock, so there is no problem with time dialation, you don't care about how fast or slow the object's clock is ticking RELATIVE to yours.

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 Quote by phinds I think you are still getting the concept wrong. I say again, time dilation is an artifact of remote observation. You in your own reference frame cannot measure any time dilation on you because there IS none, and you cannot help but measure time dilation in a different reference frame that is moving at relativistic speeds relative to you. You can COMPUTE the "local" time scale of a different reference frame that you see as time dilated, if you know its velocity relative to you.
Can you expand on that? Not sure it what sense you mean it is an "artifact of remote observation".

It sounds like you are saying it's an illusion of sorts, or has no physical meanings / consequence.

 Quote by nitsuj Can you expand on that? Not sure it what sense you mean it is an "artifact of remote observation". It sounds like you are saying it's an illusion of sorts, or has no physical meanings / consequence.
"artifact" is an unfortunate choice of words. What is meant is that time dilation needs two clocks in order to be defined. Your clock and the clock of some system moving with respect to you (the "remote" system). Time dilation means that you see the other clock is ticking slower than your clock. A better statement would be it is the "result of remote observation"

I think "artifact" was used to imply that the slowness of the other clock is not absolute, its only slow relative to your clock. To a person riding along with the other clock, your clock is ticking slower than theirs.

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 Quote by nitsuj Can you expand on that? Not sure it what sense you mean it is an "artifact of remote observation". It sounds like you are saying it's an illusion of sorts, or has no physical meanings / consequence.
I agree w/ Rap that "artifact" was a somewhat poor choice of word, mainly because the use of that word in the sense that I used it is not common nor is it well understood.

What I meant was not at all that it is an illusion, just that it is due to remote observation and it only happens with remote observation. As I stated earlier, you yourself, right now, are moving at .99999c from some frame of reference and from that frame of reference you are heavily time dilated. YOU, on the other hand, do not feel or measure any such thing in your local frame of reference.

I am not saying that you are or are not time dilated, I am saying that it depends on the observer. Things get weird in relativity. You would learn something from Googling "relativity of simultaneity"