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What experimental verification exists to substantiate that light from a receding light source travels at c to a relatively stationary destination?


jamesfairclear
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My understanding is that Doppler redshifted light received at a relatively stationary destination from a receding light source is still deemed to be travelling at the same speed of c as light arriving from a relatively stationary source.

Has this been experimentally verified and if so how?

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"Relatively stationary"? Relative to what?  "Receding  light source"?  Receding from what?

I assume you are referring to an  observer who is, of course, stationary relative to himself, observing a light source receding relative to him.  The measurement of the speed of light, with light from whatever source is probably the most repeated experiment in physics.

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8 hours ago, Country Boy said:

"Relatively stationary"? Relative to what?  "Receding  light source"?  Receding from what?

I assume you are referring to an  observer who is, of course, stationary relative to himself, observing a light source receding relative to him.  The measurement of the speed of light, with light from whatever source is probably the most repeated experiment in physics.

Thank you for your response. Your assumptions are correct.

One could envisage an experiment whereby a light source is set in motion at a constant speed S and then illuminated at a distance D from a relatively stationary detector. A clock at the detector measures the time it takes the Doppler redshifted light to arrive from distance D in order to establish its speed. 

Are you aware of an experiment of this type that has explicitly measured the speed of Doppler redshifted light emitted from a receding light source?

 

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3 hours ago, jamesfairclear said:

One could envisage an experiment whereby a light source is set in motion at a constant speed S and then illuminated at a distance D from a relatively stationary detector. A clock at the detector measures the time it takes the Doppler redshifted light to arrive from distance D in order to establish its speed.

That doesn't work unless you know the time at the light source, and you can't measure that from the detector. You theoretically can't measure the one-way speed of light, but you don't have to measure it since it is by definition equal to c. You can measure the 2-way timing of light, and find the one-way speed because literally by definition, the time that it takes for the light to go 1 way is the same as the time it takes to go the other way.

You can however confirm that light from a distance D takes the same time regardless of the motion of the light source. For example, if you have 2 sources moving in opposite directions, and a signal from each of them when they're at the same location, you can verify that the 2 signals arrive at the same time. You don't have to know what time they're sent at, if all you care about is that they were sent at the same time, and you can make sure that happens by sending them from the same location.

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16 hours ago, jamesfairclear said:

My understanding is that Doppler redshifted light received at a relatively stationary destination from a receding light source is still deemed to be travelling at the same speed of c as light arriving from a relatively stationary source.

Has this been experimentally verified and if so how?

The invariance of c is a direct consequence of Lorentz invariance, so any experiment testing this symmetry would verify that finding. There is a large body of such experimental tests.

But you don’t even need any reference to relativity for this, because it also follows directly from Maxwell’s equations - the propagation velocity of light in vacuum depends only on vacuum permittivity and permeability, and nothing else. So it is naturally invariant between frames, since these are fundamental constants. And I think we can all agree that Maxwell is not under contention.

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If you want verification of Maxwell, all you have to do is test if your radio works when you are in a moving car, which is evidence we've had for some time now. The EM wave equation works, and relies on c being invariant.

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17 hours ago, md65536 said:

That doesn't work unless you know the time at the light source, and you can't measure that from the detector. You theoretically can't measure the one-way speed of light, but you don't have to measure it since it is by definition equal to c. You can measure the 2-way timing of light, and find the one-way speed because literally by definition, the time that it takes for the light to go 1 way is the same as the time it takes to go the other way.

You can however confirm that light from a distance D takes the same time regardless of the motion of the light source. For example, if you have 2 sources moving in opposite directions, and a signal from each of them when they're at the same location, you can verify that the 2 signals arrive at the same time. You don't have to know what time they're sent at, if all you care about is that they were sent at the same time, and you can make sure that happens by sending them from the same location.

It should work ok. The light source is set in motion at a constant speed S and then illuminated at a distance D from a relatively stationary detector. The illumination of the light source could either be triggered by coinciding with a relatively stationary device at distance D or via an onboard clock that calculates distance D. Thus we know exactly when the light source has travelled a distance D from the detector (t1) and the clock at the detector begins measuring time from that moment. 

Theoretically the one way speed of light is the same as the 2 way speed. However there has been a lot of debate about this over many years. I am specifically interested in measuring the speed of light from a receding light source. We know that the motion of the receding light source affects the wavelength of light received at a relatively stationary destination but I am looking for any experiments that have conclusively proven that it does not affect the speed of propagation.

6 hours ago, swansont said:

If you want verification of Maxwell, all you have to do is test if your radio works when you are in a moving car, which is evidence we've had for some time now. The EM wave equation works, and relies on c being invariant.

That is a interesting point. However I would have thought that the only potential issue would be extremely minimal doppler shift which probably wouldn't  be sufficient for there to be any audible artefacts. Any variation from c of the speed of the radio waves reaching the car aerial would be insignificantly small to make any audible difference. 

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18 minutes ago, jamesfairclear said:

That is a interesting point. However I would have thought that the only potential issue would be extremely minimal doppler shift which probably wouldn't  be sufficient for there to be any audible artefacts. Any variation from c of the speed of the radio waves reaching the car aerial would be insignificantly small to make any audible difference. 

I invite you to try solving Maxwell's equations to obtain a wave equation with c being variable.

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42 minutes ago, swansont said:

I invite you to try solving Maxwell's equations to obtain a wave equation with c being variable.

I don't doubt the mathematics. My interest is in finding experimental evidence to substantiate that light from a receding light source travels at c to a relatively stationary destination. Have you come across such an experiment?

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2 hours ago, jamesfairclear said:

I don't doubt the mathematics. My interest is in finding experimental evidence to substantiate that light from a receding light source travels at c to a relatively stationary destination. Have you come across such an experiment?

Define 'relatively stationary'. It doesn't matter from which position you take, in time or space, even moving with uniform velocity, you will measure c.

Edited by StringJunky
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3 hours ago, jamesfairclear said:

I don't doubt the mathematics. My interest is in finding experimental evidence to substantiate that light from a receding light source travels at c to a relatively stationary destination. Have you come across such an experiment?

Not a direct measurement, to my recollection. Since c can be measured and the invariance can be demonstrated, by the transitive property you know c stays the same when there’s relative motion. There’s little incentive to do the harder experiment when you can do easier ones.

You can do experiments that rely on it, and that confirms the behavior without doing a direct measurement. It’s not really an experiment, but the technology relies on it: GPS works. Would that be the case if c wasn’t invariant?

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12 hours ago, swansont said:

Not a direct measurement, to my recollection. Since c can be measured and the invariance can be demonstrated, by the transitive property you know c stays the same when there’s relative motion. There’s little incentive to do the harder experiment when you can do easier ones.

You can do experiments that rely on it, and that confirms the behavior without doing a direct measurement. It’s not really an experiment, but the technology relies on it: GPS works. Would that be the case if c wasn’t invariant?

 GPS supports the invariance of light speed in a given IFOR but not for a receding light source. I wouldn't agree that performing experiments in a single IFOR necessarily confirms the behaviour in multiple IFORs especially when we can already confirm that one property of light (the wavelength) behaves differently with a receding light source.

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5 minutes ago, jamesfairclear said:

 GPS supports the invariance of light speed in a given IFOR but not for a receding light source. I wouldn't agree that performing experiments in a single IFOR necessarily confirms the behaviour in multiple IFORs especially when we can already confirm that one property of light (the wavelength) behaves differently with a receding light source.

We have particle accelerators that accelerate particles away from the source at speeds where relativity should show up.

Perhaps those that know more about particle accelerators than I do could say if any measurements have been done on radiation from these accelerated particles ?

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14 hours ago, StringJunky said:

Define 'relatively stationary'. It doesn't matter from which position you take, in time or space, even moving with uniform velocity, you will measure c.

By relatively stationary I mean that the detector is for example on Earth at a location on the equator and the emitter is receding at a velocity v along the equator. It is of course a principal of SR that the speed of light is invariant but my interest is in finding experimental evidence of this invariance measured with a receding light source. 

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12 minutes ago, jamesfairclear said:

 GPS supports the invariance of light speed in a given IFOR but not for a receding light source.

GPS satellites don't recede when moving from overhead to horizon?

 

12 minutes ago, jamesfairclear said:

I wouldn't agree that performing experiments in a single IFOR necessarily confirms the behaviour in multiple IFORs especially when we can already confirm that one property of light (the wavelength) behaves differently with a receding light source.

Wavelength is not an invariant, so how does that matter? By "behaves differently" do you mean that there is a change, but in agreement with theory, or are you contending there is a deviation from this?

9 minutes ago, jamesfairclear said:

By relatively stationary I mean that the detector is for example on Earth at a location on the equator and the emitter is receding at a velocity v along the equator. It is of course a principal of SR that the speed of light is invariant but my interest is in finding experimental evidence of this invariance measured with a receding light source. 

Like a transmitter/receiver communicating with a target moving somewhere out in the solar system? Something that NASA does all the time?

 

There are also these experiments:

https://math.ucr.edu/home/baez/physics/Relativity/SR/experiments.html#moving-source_tests

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De Sitter's experiment

Consider a distant binary star system (two stars, A and B, orbiting each other) and assume that the orbits of the stars obey Kepler's laws (so they trace out ellipses). Assume Ritz's theory so that the speed of light depends on the velocity of the star. When star A is moving towards us (at speed vv) in its orbit, the light it emits in our direction will be moving at speed c+vc+v. When it is moving away from us (also at speed vv), the light it emits in our direction will be moving slower, with speed cvc−v. Therefore the motion will appear very nonuniform: the star will seem to speed up as it comes towards us and slow down as it moves away. This is not consistent with what we observe in practice, which is uniform Keplerian motion. This means that Ritz's theory cannot be an accurate description of the motion of light.

This is an incredibly simple explanation. There are many ways in which you could criticise this argument (maybe the motion we see is the result of Ritz's theory and highly eccentric elliptical orbits?) and he gives a more detailed argument with reference to specific binary systems in a follow-up paper. In this follow-up paper he phrases the conclusion slightly more conservatively: one can put an upper bound on the dependence of the speed of light on the velocity of its source using astronomical observations of binary systems according to the argument sketched above.

Of course, there are many other experiments which confirm the predictions of special relativity, but this one has the advantage that it only requires you to have a good telescope rather than some complicated configuration of interferometers. It's also the simplest imaginable experiment you could design to directly test the constancy of the speed of light: essentially racing lightbeams against one another!

https://www.homepages.ucl.ac.uk/~ucahjde/blog/lightspeed.html

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