Constancy of light experiments

roosh

Just wondering, what are the specific experiments which demonstrate that the speed of light is constant for an observer, regardless of their motion relative to the source?

dlouth15

The classic one is the Michelson–Morley experiment.

Morbert

roosh wrote:

Just wondering, what are the specific experiments which demonstrate that the speed of light is constant for an observer, regardless of their motion relative to the source?

dlouth15 wrote: »

The classic one is the Michelson–Morley experiment.

dlouth15

Roosh, I see you have already asked about it here:

http://www.boards.ie/vbulletin/showthread.php?t=2056409403

and also here:

http://www.boards.ie/vbulletin/showthread.php?t=2056634338

So you already know about it.

roosh

dlouth15 wrote: »

Roosh, I see you have already asked about it here:

http://www.boards.ie/vbulletin/showthread.php?t=2056409403

and also here:

http://www.boards.ie/vbulletin/showthread.php?t=2056634338

So you already know about it.

what I had in mind was experiments other than interferometry experiments; when I started the thread I was thinking more of experiments where an observer is moving relative to a source of light.

Lbeard

roosh wrote: »

what I had in mind was experiments other than interferometry experiments; when I started the thread I was thinking more of experiments where an observer is moving relative to a source of light.

In astronomy the observance of the red shift of stars moving away from us.

For electromagnet radiation, the frequency multiplied by the wavelength always equals the speed of light. It has never been shown not to.

roosh

Lbeard wrote: »

In astronomy the observance of the red shift of stars moving away from us.

For electromagnet radiation, the frequency multiplied by the wavelength always equals the speed of light. It has never been shown not to.

Would the wavelength and frequency not be correlated in some way, such that their multiplication will always give the same result?

I would imagine a longer wavelength would result in a lesser frequency, and vice versa, meaning the change in one would offset the change in the other; but would a change in wavelength and frequency not represent a change in speed?

Lbeard

roosh wrote: »

Would the wavelength and frequency not be correlated in some way, such that their multiplication will always give the same result?

They're inversely proportional. If the one changes the other changes too, so the result is always the same speed.

Faraday did experiments on light and magnetism. He deflected a polarised light beam in a magnetic field. This gave a clue there was strong connection between light and electro-magnetism. Maxwell's later work, more on electric fields, that led to the discovery of radio waves. Maxwell's equations yield the speed of light as a constant. This is without doing any speed measurements. It comes out from doing measurements on magnets.

The speed of light is always constant. It has never been found to vary.

I would imagine a longer wavelength would result in a lesser frequency, and vice versa, meaning the change in one would offset the change in the other; but would a change in wavelength and frequency not represent a change in speed?

The proportionality always remains such that the wavelength by the frequency always produces the same constant value; the speed of light.

If someone throws a ball at you, and the ball is initially traveling at 30 miles an hour. If you run away at 10 miles an hour, the ball when it reaches you will be traveling at 20 miles an hour relative to you. This does not happen with light. No matter how fast you run, it will always be the same speed when it reaches you. The colour of the light will change, but not the speed of the light.

The speed always stays the same, but the energy changes. Simply put, the colour of light can change but not its' speed. That's why stars that are traveling fast away from us are red, when if they weren't moving they would be white - if they were coming towards us the would be blue. If light traveled like a thrown ball, it would always be white when it reached us, no matter what speed the stars were traveling at. We know this does not happen, and it's something you can literally see with the naked eye.


Not only has the constancy of the speed of light been proved to death, everything from electricity generation to radio transmissions would not work the way they do if the speed of light varied.

citrus burst

roosh wrote: »

Would the wavelength and frequency not be correlated in some way, such that their multiplication will always give the same result?

I would imagine a longer wavelength would result in a lesser frequency, and vice versa, meaning the change in one would offset the change in the other; but would a change in wavelength and frequency not represent a change in speed?

The speed that a wave propagates is given by [latex] v = \lambda\nu[/latex]

where [latex]\lambda[/latex] is the wavelength of the wave (m)
and [latex]\nu[/latex] is the frequency (s^-1).

In most applied sciences, wavelength, frequency and several other things (eg energy, wavenumber) are used interchangeably, usually at the discretion of the person, and or by convention, to mean the same thing while talking about light. This would not be possible if not for the speed of light being constant.

When measuring the spectrum of distant stars, it was found that further away stars have longer wavelengths then what would be expected, due to their relative motion away from the earth. If their frequency remained constant, then v would have to alter.

However it was found that energy of the photons emitted by these starts, also decreased, in accordance with;

[latex]E=\frac{hc}{\lambda}[/latex]

Since the energy of the photons is less then what would be expected, in order for the above equation to be consistent with the below equation, c must be a constant and the frequency of the wave is what changes.

[latex]E = h \nu[/latex]

If this were not the case we would find two different answers for the energy of the photon, depending on what we measured.

This would seem to suggest that the speed of light is constant, despite the relative motion of the source.