How can scientists measure the speed of light?

• physicsguy13

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?

Answers and Replies

Time dilation does not affect the speed of light. From all inertial frames of reference, aka those frames that are not accelerating, light will always be measured as traveling at c.

Please explain this to me Drakkith. I'm only 12 years old.

Please explain this to me Drakkith. I'm only 12 years old.

Is it because time dillation only affects objects with mass, and photons do not?

I think I will come back to physics forum when I am older. For now I will only look at posts, not post myself or reply. Thank you for clearing up my question, though.

Is it because time dillation only affects objects with mass, and photons do not?

No, you misunderstand time dilation. Time dilation is an artifact of remote measurement.

YOU, for example, right now, this very minute as you read this, are traveling at .9999c from some frame of reference, and observation from that frame of reference shows you to be seriously time dilated. Do you feel any different, knowing that? You shouldn't, since you are NOT time dilated from your own frame of reference.

No, you misunderstand time dilation. Time dilation is an artifact of remote measurement.

YOU, for example, right now, this very minute as you read this, are traveling at .9999c from some frame of reference, and observation from that frame of reference shows you to be seriously time dilated. Do you feel any different, knowing that? You shouldn't, since you are NOT time dilated from your own frame of reference.

I require more clarification. If I am not actually moving at 99% of c, is it because the frame of reference is moving at 99% of c? If so, would the frame of reference would see me as moving slower than c?

I require more clarification. If I am not actually moving at 99% of c, is it because the frame of reference is moving at 99% of c? If so, would the frame of reference would see me as moving slower than c?

You ARE moving at 99% c. Right now. This is because motion is NOT absolute. What I mean is that your velocity cannot be measured with respect to anything concrete in the universe because there is nothing concrete in the universe to measure from.

For example, we typically measure the velocity of objects on Earth with respect to the Earth's surface. If you are moving at 60 mph then you are only moving at 60 mph relative to the ground. Another vehicle passing you by on the highway would measure your velocity relative to ITSELF at a much lower 5 mph or so.

Say you measure the velocity of a cosmic ray. These are very high energy particles traveling at extremely high speeds over 99% c. To that particle YOU are the one traveling at 99% c. Neither of you can measure your velocity relative to anything that could be considered an absolute frame of reference. ALL objects are moving relative to something else at all times.

One of the postulates of special relativity is the constancy of the speed of light. No matter how fast you and your measurement apparatus are moving, light reaching you moves at speed c. It is counterintuitive, perhaps, which is one reason that relativity caused such a stir. Since light is always traveling at c, you can measure its speed accurately.

I require more clarification. If I am not actually moving at 99% of c, is it because the frame of reference is moving at 99% of c? If so, would the frame of reference would see me as moving slower than c?

Back up for a moment, and imagine that you have a telescope and you're using it to watch some guy on Mars. The Earth is moving around the sun at about 30 km/sec and Mars is moving around the Sun at about 24 km/sec. So you naturally say that you're at rest and Mars guy is moving at 6 km/sec. But from Mars guy's point of view, it's exactly the other way around - he's at rest and you're moving at 6 km/sec relative to him. Meanwhile, some other guy in the Andromeda galaxy (which is moving towards us at about 300 km/sec) is happily insisting that we're both wrong, it's plain to any fool that he's at rest and our entire solar system is rushing towards him at 300 km/sec.

So any time you say "moving at speed X", you always have to append the words "relative to whatever" to get a meaningful statement. When someone says "that car is moving at 60 mph" they almost mean "That car is moving at 60 mph relative to the ground it's standing on", even if they don't say it.

And that's why Drakkith can say you ARE moving at .99c right now - relative to someone, somewhere in the universe, you are. Of course, as far you're concerned he's the one who's moving at .99c.

If light is not affected by time dilation, are there any "light clocks" to measure time exactly without the effects of time dilation? Or am I still getting the concept wrong?

Also, I changed my mind. I will only post questions.

welcome to pf!

hi physicsguy13! welcome to pf!
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?

we measure the speed of an object as x/t,

where x is our measurement of its position, and t is our measurement of its time

(and t is measured on our clock, which of course is stationary relative to us)

if that object has a clock (or is a clock), then the time on that clock will be different … we usually use the letter t' instead of t, to show the difference

but t' is not used to calculate the speed
If light is not affected by time dilation, are there any "light clocks" to measure time exactly without the effects of time dilation? Or am I still getting the concept wrong?

a stationary clock will always measure time without time dilation

If light is not affected by time dilation, are there any "light clocks" to measure time exactly without the effects of time dilation? Or am I still getting the concept wrong?

You can use a light clock to measure time and two observers will STILL measure different times if they are moving relative to each other. This is a complicated concept that is not easy to explain, so don't feel bad if it doesn't make any sense. I suggest picking up a book about Relativity from a bookstore or from an online site and reading it.

If light is not affected by time dilation, are there any "light clocks" to measure time [strike]exactly[/strike] without the effects of time dilation?

Yes. Just use a clock that is at rest relative to you, so there is no time dilation, no length contraction, none of these funny relativistic effects.
You'll note that I've struck out the word "exactly" above though, because I'm not sure what you mean by it, and suspect that you are wandering onto some very shaky ground. Let's say that two explosions happen somewhere in space. You on earth, the Mars guy moving at 6km/sec relative to you, and the Andromeda guy moving at 300 km/sec relative to you will measure different times between the two explosions. All three measurements are "exact" and equally right; there is absolutely no basis for saying that one of those measurements is "right" and the other two are somehow distorted by time dilation.

Also, I changed my mind. I will only post questions.

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.

For example, see

http://www.phinds.com/time%20dilation/ [Broken]

Even though the guy on the planet sees the guy in the spaceship as time dilated, he can CALCULATE what the guy on the spaceship is actually seeing in his own reference frame. Of course that ends up just showing that everyone, in their own frame of refence measures time one second per second.

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Thank you Tiny-Tim! I believe I understand it now!

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.

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.

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.

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"