Thread: About the measurements of the spped of light.

#1- Correct me if I am wrong: All meaurements of the speed of llight are done by measuring a back and forth travel of the light. We send a beam of light from point A to point B (a mirror) and count the time it took the light to return to the source(A). Are there any other methods that i am not aware off?

#2- A photon "bouncing" off a mirror will lose some of its energy to the mirror. Correct?

#1+#2: 1+2=.....3? About #1 - If there are no other methods of measuring the speed of light other then the "back and forth" experiment, then we are meausring an average speed of the A to B and B to A travles. Aren't we? We can not single out a measurment only for A to B travel or B to A, due to synchronisation diffuclties. Correct?


So taking to consideration the above #2 that the photon has lost some of its energy..... maybe the back and the forth are no the same speed?

Funny question, I know.

THANKS FOR YOUR WISE ANSWERS!

Here's a good article on how the speed of light is and was measured.
How is the speed of light measured?

If you send light at a mirror, most of it will be reflected back at you, unlike if you shine it at a wall where more of it will be absorbed.

Light always moves at speed C. When you send light at something, the way that it "loses" energy is by absorbing photons and converting that energy into kinetic energy.

Originally Posted by chaimc

#1- Correct me if I am wrong: All meaurements of the speed of llight are done by measuring a back and forth travel of the light. We send a beam of light from point A to point B (a mirror) and count the time it took the light to return to the source(A). Are there any other methods that i am not aware off?

#2- A photon "bouncing" off a mirror will lose some of its energy to the mirror. Correct?

Correct, so far.

#1+#2: 1+2=.....3? About #1 - If there are no other methods of measuring the speed of light other then the "back and forth" experiment, then we are meausring an average speed of the A to B and B to A travles. Aren't we? We can not single out a measurment only for A to B travel or B to A, due to synchronisation diffuclties. Correct?

Correct.

So taking to consideration the above #2 that the photon has lost some of its energy..... maybe the back and the forth are no the same speed?

This part is incorrect. For photons , the energy E and the momentum p are tied by the relationship where c is the (two-way)speed of light. When energy E decreases, momentum p decreases as well, c stays the SAME.

Originally Posted by chaimc

#1- Correct me if I am wrong: All meaurements of the speed of llight are done by measuring a back and forth travel of the light. We send a beam of light from point A to point B (a mirror) and count the time it took the light to return to the source(A).

One of the earliest measurements of the speed of light was done by Fizeau using a rotating wheel. See - Fizeau, Armand-Hippolyte-Louis: Fizeau

Originally Posted by Physicist

One of the earliest measurements of the speed of light was done by Fizeau using a rotating wheel. See - Fizeau, Armand-Hippolyte-Louis: Fizeau

This is not what the OP, asked.

Originally Posted by chaimc

#1- Correct me if I am wrong: All meaurements of the speed of llight are done by measuring a back and forth travel of the light. We send a beam of light from point A to point B (a mirror) and count the time it took the light to return to the source(A). Are there any other methods that i am not aware off?

The earliest methods used the eclipses of Jupiter's moon (predicted time versus observed time) and stellar aberration.

#2- A photon "bouncing" off a mirror will lose some of its energy to the mirror. Correct?

Yes, but very little.

So taking to consideration the above #2 that the photon has lost some of its energy..... maybe the back and the forth are no the same speed?

The speed of the photon does not depend on its energy? it's speed is always c.

You are all, naturally, answering the same answer about the "rigidity" of the speed of light. That is well established and known. ...even to me hahah.
But isn't it really a convenient postulation?

In the conventional measurements of the speed of light, in the the forth and back method, can anyone measure the" forth" alone and the "back" alone? NO! So it is an average speed measurment!

I can calculate the speed of a soccer ball hitting me in the head if I know its mass and my mass etc according to Newton.
But, as light does not have a mass there is no way I can measure(!!!) or calulate the speed of a light coming from space. I can presume, I can postulate but not more than that. Am I right?

Thanks as usual for your answers.

Originally Posted by chaimc

But isn't it really a convenient postulation?

Which postulation ? That C is constant ? There is no postulation of that when you are measuring it.

In the conventional measurements of the speed of light, in the the forth and back method, can anyone measure the" forth" alone and the "back" alone? NO! So it is an average speed measurment!

YES ! Why do you say NO ! You can split experiment into micro sub experiment. It's useless and complicate the setup, but why not ?

All measure on all sorts of thing are average. When to try to get more and more precision, you start to encounter randomness and start a new physics for those scales (QM).

I can calculate the speed of a soccer ball hitting me in the head if I know its mass and my mass etc according to Newton.

Yes you can calculate the speed of a soccer ball, or use GR to have more accurate numbers.
But I've been hit enough by them to know that don't match experiment
Seriously, if its rotating, the speed and steadiness of the win, the humidity, how the ball is made, are all important factor. All chaotic and unpredictable, at normal scale.
All calculus is approximation of situation. Light is in no way different.
All setup spoke about speed of light "in vacuum" for example. You don't put you mirrors in water inside a centrifuge either (the so called inertial FoR)

But, as light does not have a mass there is no way I can measure(!!!) or calculate the speed of a light coming from space

Why having no mass is important ? Light is there, and then there. Just measure that. It will also work very well for light from space, there are Gillionz of light source out there (but apparently not enough

Originally Posted by chaimc

In the conventional measurements of the speed of light, in the the forth and back method, can anyone measure the" forth" alone and the "back" alone? NO! So it is an average speed measurment!

I don't think so. See One-way speed of light - Wikipedia, the free encyclopedia

Originally Posted by Boing3000

Which postulation ? That C is constant ? There is no postulation of that when you are measuring it.


YES ! Why do you say NO ! You can split experiment into micro sub experiment. It's useless and complicate the setup, but why not ?

There is the synchroniztion problem.

All measure on all sorts of thing are average. When to try to get more and more precision, you start to encounter randomness and start a new physics for those scales (QM).


Yes you can calculate the speed of a soccer ball, or use GR to have more accurate numbers.

You read wrong - I said you can with a ball...

But I've been hit enough by them to know that don't match experiment
Seriously, if its rotating, the speed and steadiness of the win, the humidity, how the ball is made, are all important factor. All chaotic and unpredictable, at normal scale.
All calculus is approximation of situation. Light is in no way different.
All setup spoke about speed of light "in vacuum" for example. You don't put you mirrors in water inside a centrifuge either (the so called inertial FoR)


Why having no mass is important ? Light is there, and then there. Just measure that. It will also work very well for light from space, there are Gillionz of light source out there (but apparently not enough

i think you are wrong.... If light comes from an unknown distance i can not meaure it speed.


PHSICIST has put for us a link to WIKI on the excat issues. I didnt know of what is written there about the "one way" methods and i will read them later. I suggest you do too...

Thanks.

Originally Posted by chaimc

i think you are wrong.... If light comes from an unknown distance i can not meaure it speed.

You should always read the link before commenting on it. It confirms what I said.
One way setup are just more complicated (especially synchronizing the clocks). I've still no clue about why on earth you think is would NOT be possible...

It is also possible to do two way measurement with light from distant stars (If you prefer two ways). You just cannot choose were to put the mirrors.

It's just ordinary light...

You read wrong - I said you can with a ball...

That's what I read, and what I wrote : "Yes you can calculate the speed of a soccer ball, or use GR to have more accurate numbers."

The problem of experiments are not about the theory computational power. It is about experimental precision/setup/conditions.

Originally Posted by Boing3000

You should always read the link before commenting on it. It confirms what I said.
One way setup are just more complicated (especially synchronizing the clocks). I've still no clue about why on earth you think is would NOT be possible...

Synchronization of clock is a major problem! I am still struggling with the wiki page PHSICIST has suggested but what I gathered so far is that the one way measurments are in a way (which I did not comprehend really) a two way experiment. Read the above wiki page and see for youself - it is a major issue.

It is also possible to do two way measurement with light from distant stars (If you prefer two ways). You just cannot choose were to put the mirrors.

It's just ordinary light...




That's what I read, and what I wrote : "Yes you can calculate the speed of a soccer ball, or use GR to have more accurate numbers."

The problem of experiments are not about the theory computational power. It is about experimental precision/setup/conditions.

OK
How can you measure a two-way experimrnt with a thousand light year distance galaxy...?

Thx !

Originally Posted by chaimc

OK
How can you measure a two-way experimrnt with a thousand light year distance galaxy...?

Thx !

You send a light pulse and you wait 2000 years until it comes back to you.

Originally Posted by chaimc

How can you measure a two-way experiment with a thousand light year distance galaxy...?

You put the clock and the measuring apparatus on a upper satellite the mirror on lower satelite, and there you go...
The problem is not the distance. Every long distance measures are not in vacuum. Space is not empty. Only labs are. Light bend and twist at those scales.

You did not ask to measure round trip of light from distance stars, but the speed of light coming from those stars.

It would be like asking the speed of my holidays, by knowing I started and arrived at the same place, and having done that much distance. Kind of useless.

BOING : You make it sound so simple.... In the example you gave with the two synchronised satelites, I can tell when the signal arrived. As it is a thousand light years away, how can I tell when was the signal sent?

You wrote:"You did not ask to measure round trip of light from distance stars, but the speed of light coming from those stars."
Well, exactly that....

And I am asking in the most naive and ignorant fashion: Is it not all just a convension? I read about the two way experiments, about the Jupiter moon.... But what about light that is coming from extreme distances? Can you REALLY measure its speed?

Thanks

Originally Posted by chaimc

BOING : You make it sound so simple.... In the example you gave with the two synchronised satelites, I can tell when the signal arrived. As it is a thousand light years away, how can I tell when was the signal sent?

You wrote:"You did not ask to measure round trip of light from distance stars, but the speed of light coming from those stars."
Well, exactly that....

And I am asking in the most naive and ignorant fashion: Is it not all just a convension? I read about the two way experiments, about the Jupiter moon.... But what about light that is coming from extreme distances? Can you REALLY measure its speed?

Thanks

OK,

Here is the deal.

1. You are right, we cannot measure one-way light speed (OWLS). This is due to the fact that OWLS is tied to the clock synchronization method, so, if we change the clock synchronization method we get certain situations where OWLS is anisotropic (direction dependent)

2. We can measure two way light speed (TWLS) and we have done so for centuries.

3. As of 2002 w have figured a way of constraining OWLS anisotropy via some very clever tests. So, what we can do is measure TWLS, use the fact that there is no anisotropy and assign the TWLS measured value to OWLS. And this is what we have been doing since 2002.

Originally Posted by chaimc

BOING : You make it sound so simple

I am trying to, and fail apparently

Originally Posted by chaimc

In the example you gave with the two synchronised satellites, I can tell when the signal arrived. As it is a thousand light years away, how can I tell when was the signal sent?

You can tell when he arrives and bounce back, to have the "somewhat local" speed of that star's light. You can do classic interferometry if you don't like clocks. It sound easy for sure, but doing those setup for real is another peace of cake altogether.

Originally Posted by chaimc

but the speed of light coming from those stars." Well, exactly that....

I am not talking of the propagation speed (holiday example speed) just the local "arriving" speed.
The round-trip propagation speed is not interesting. The photons way have traveled entirely different path. That's a moot question except to measure hypothetical dark matter.

Originally Posted by chaimc

Is it not all just a convension? I read about the two way experiments, about the Jupiter moon.... But what about light that is coming from extreme distances? Can you REALLY measure its speed?

I have done my best to explain to you there is absolutely no difference between measuring light coming from a lazer, the moon, or alpha centaury.
When astrophysicist "date" light, they do account for lots fuzzy errors (space curvature, shift and so forth). They have big margin of errors, and they live with it. That's not in anyway relevant to your question.

I hope to have help you a little. I am not to going to post any time soon. So good luck.

OK but before you vanish can you enlight me with your wisdom over this (Please...)?

http://www.thephysicsforum.com/gener...-key-hole.html

Thx again