[hamdani yusuf (OP)]

It is likely that astronomers have looked at the microwaves from distant stars (which are moving rapidly WRT us) using both frequency and wavelength measurements.
If the measurements didn't agree, they would have noticed.

Those experiments assumed that interstellar medium has negligible effects, which may or may not be true. To get more reliable answer,  we need a controlled environment.

[alancalverd]

Hamdani Yusuf,
would you be prepared to accept using a laser at 674 nm instead of 300 nm?

No problem,  I used arbitrarily round number just to make calculation easier.

Excellent. And if you are prepared to go to microwave frequencies you can get an easily-measured Doppler shift on a moving train.

[Bored chemist]

Doppler radar guns have been used by police for decades.
They have been tested in court.
If there was any chance that the outcome was different from what was expected, someone would have challenged it.

Those experiments assumed that interstellar medium has negligible effects, which may or may not be true. To get more reliable answer,  we need a controlled environment.

The interstellar medium is a much  better vacuum that we can achieve in our experiments.

[alancalverd]

It strikes me that this solves the "one way speed of light" question that turns up here from time to time.

Let A and B have identical apparatus for sending, receiving and analysing an electromagnetic signal. Each measures the frequency and wavelength of a pulse from the other, and calculates c.

[Bored chemist]

It strikes me that this solves the "one way speed of light" question that turns up here from time to time.

Let A and B have identical apparatus for sending, receiving and analysing an electromagnetic signal. Each measures the frequency and wavelength of a pulse from the other, and calculates c.

Due to budget cuts there's only enough money for 1 set of apparatus, and a mirror.
Does that affect the outcome?

[alancalverd]

According to several correspondents to this forum, it might, and the possibility confirms their belief in the aether, flat earth and fairies.

[Bored chemist]

I can imagine scenarios where it might- for example, if you put a black hole there so its event horizon is just behind the mirror.
But most of the discussions are carefully framed to avoid weird things like that.

[Halc]

It strikes me that this solves the "one way speed of light" question that turns up here from time to time.

Let A and B have identical apparatus for sending, receiving and analysing an electromagnetic signal. Each measures the frequency and wavelength of a pulse from the other, and calculates c.

It does calculate c, but it does not measure the one-way speed of a given light pulse.  Similarly, Roemer first measured c using a one way method, something I occasionally point out to the absolutists. What he did is entirely valid, but measuring c using a one-way method is very different than measuring the one-way speed of light.
Ditto for the measuring the frequency and wavelength thing, both measurements yielding incorrect results in an absolutist interpretation of the universe. The joke is that they claim this is a simplification, and yet they are forced to use Einstein's relativistic (not abolute) mathematics when computing anything. I've never seen the computations of any complex system done the 'simpler' absolutist way. Fairies indeed.

Due to budget cuts there's only enough money for 1 set of apparatus, and a mirror.

If there's a mirror involved, it isn't a one-way experiment. Roemer used no mirror, but he did use a pair of clocks separated by a very large distance. It was the recent invention of the clock that made this first measurement of c possible. At his time, the most accurate clock for long term (months) measurement was still a sundial.

I can imagine scenarios where it might- for example, if you put a black hole there so its event horizon is just behind the mirror.

Again a 2-way measurement which will yield exactly c if you use Alan's method.
And a black hole is not necessary. There are reflectors placed on the moon for such purposes, and a light pulse sent there and back again makes the round trip at faster than light. If they did the same on the moon with a reflector on Earth, the round trip would take place at less than light speed. This is a different method of measuring c than Alan's method which doesn't rely on mirrors or round trips.  But the method assumes relatively stationary equipment, making it invalid for computing one-way speed under a view that does not assume the postulates of relativity.

[Bored chemist]

And a black hole is not necessary.

It is necessary to make the point.
If there is one set of apparatus and a mirror (with a BH behind it) you get some sort of measurement.
If there are two sets of apparatus, but one of them is within the EH of a BH then there will be no measurement.

So, in those circumstances the budget cut version gives a different outcome.
And that's the point I was making.

[alancalverd]

Ditto for the measuring the frequency and wavelength thing, both measurements yielding incorrect results in an absolutist interpretation of the universe.

I don't recommend interpreting anything. For a travelling wave, v = fλ by definition of the terms. Nothing to to with relativity as my experiment works when A and B are not moving relative to one another. The theoretical and demonstrable constancy of c is in fact the basis, not the consequence, of relativity.

[Halc]

I don't recommend interpreting anything. For a travelling wave, v = fλ by definition of the terms.

Yes, but your measurement of both depend on some postulates made by the relativistic view, postulates not proven true.
Those postulates are assumptions, and yes, are the basis of relativity. But being assumptions, they're not necessarily true in a theory which doesn't assume them.

The theoretical and demonstrable constancy of c is in fact the basis, not the consequence, of relativity.

Agree. Nobody is suggesting that c is a different figure. But in the case under discussion, we're not measuring the constancy of c, but rather attempting to measure the one-way speed of light, something which cannot be measured.

[alancalverd]

Yes, but your measurement of both depend on some postulates made by the relativistic view, postulates not proven true.

Really? I'm using two identical stationary clocks and two diffraction gratings, none of whose properties depend on relativistic postulates, and since their relative velocity  is zero, even if you insisted on introducing relativity, you'd find that the measurements are exactly as predicted by classical nonrelativistic postulates because v_rel/c = 0 in all the relativistic equations.

Indeed the first test of relativity is that its predictions must degenerate to classical mechanics if v_rel = 0.

v_wave = f λ is an obvious definition that does not depend on the constancy of c or the value of v, however it is measured. λ is the distance between wave peaks and f is the number of peaks passing a point per second.

Now using my technique I measure the speed of light transmitted from A, as received at B. I can move B to any distance and find that c_B is invariant, so that must be the speed c_A→B - the one-way speed of light.

[Halc]

Yes, but your measurement of both depend on some postulates made by the relativistic view, postulates not proven true.

Really? I'm using two identical stationary clocks and two diffraction gratings, none of whose properties depend on relativistic postulates

You apparently used one of those postulates when you declared your clocks and gratings to be stationary, something that cannot be demonstrated. How can you measure the 1WSoL with a moving device?

While I'm on your case, what's the second clock for?  I thought this was a local test to determine c. Seems only one clock is necessary to do it that way. Are we changing the test now?

even if you insisted on introducing relativity

That's my point. I'm deliberately not invoking relativity.

you'd find that the measurements are exactly as predicted by classical nonrelativistic postulates.

No argument there, but neither of the views claims they can measure the 1WSoL nor can a device that measures the absolute time be produced.  For instance I notice that the absolutists do not posit an actual age of the universe corrected for all the relativistic dilations that act on an Earth clock.

Indeed the first test of relativity is that its predictions must degenerate to classical mechanics if v_rel = 0.

That doesn't change even if the 1WSoL is frame dependent. If you disagree, then you have the test nobody seems to be able to come up with.

Now using my technique I measure the speed of light transmitted from A, as received at B. I can move B to any distance and find that c_B is invariant, so that must be the speed c_A→B - the one-way speed of light.

That's pretty much what Roemer did.  Now why doesn't that constitute a valid measurement of the 1WSoL?  Assume A and B are on the same path moving at 0.8c. How would that change my results given an absolute interpretation?  Light would go from A to B in a 9th the time it takes it to go from B to A. How would you go about demonstrating that? If you can, you'd have a falsification test for one theory or the other.

[alancalverd]

You apparently used one of those postulates when you declared your clocks and gratings to be stationary, something that cannot be demonstrated.

Stationary relative to each other. Like joined together with a stick - or more likely an optical bench.

what's the second clock for?

So that the frequency I measure at B does not depend on the clock at A.

 

nor can a device that measures the absolute time be produced.

Irrelevant. All I need to do is count the number of waves that reach B in a second as measured at B with a clock like the one at A. Any second will do!

Light would go from A to B in a 9th the time it takes it to go from B to A.

I'm not measuring the transit time from A to B, but the speed of the travelling wave as it passes B

[McQueen]

One of the experiments that is often quoted a proof of special relativity is called the photon clock experiment. The photon clock consists of a coach in a train whose interior is equipped with mirrors on the floor and the roof. A beam of light shines from the mirror on the floor to the mirror on the roof and is reflected back. One whole reflection, from the floor to the roof and back again is taken as one tick of the clock. Alice is sitting in the coach with the photon clock.

To her the beam of light appears to go straight up and down.  To Bob, assume the coach has glass walls, standing on the platform and watching the train go by, the beam of light appears to travel a longer distance than it does for Alice sitting in the coach with the clock. How can one account for this discrepancy in measurement. Alice is measuring one length that the beam in the photon clock travels while Bob on the platform watching the photon clock perform the same up and down journey in the same identical time frame, measures a much longer length over which the beam travels. What is happening here? The answer is simple, light travels the same distance in both instances, but to Alice who is moving with the same momentum as the train the light appears to go straight up and down, while to Bob it appears to be moving a greater distance. In actual fact Alice would see the light travel up and down (in reality it travels a triangular route)and arrive a fraction behind her own position. If the roof of the carriage is 3m above the floor of the carriage then in 1 tick of the photon clock, light would travel 6m (3m up and 3m down), and it would take 6 ÷ 3 x 10⁸  or 2 x 10^-8  s for 1 tick of the photon clock. If the train is travelling at 60 kmh (16.666m/s) then in 2 x 10^-8 s the train would travel 3.33 x 10^-7 m in one second. So the difference in distance that either Alice or Bob would see would be too tiny to differentiate. But Bob and Alice would both measure the correct distance that the light travels. Since light does not acquire the velocity of the moving train. The light travels the same distance for both of them.   The above assumption, often quoted in explanation of this thought experiment, that the light would assume the velocity of the train, is false. For instance if you had a ball and you bounced it up and down in a coach in a moving train, then the ball would acquire the momentum of the train, in other words it would behave as if it were part of the train and moving with the same speed. This is a pure Galilean transformation. The same does not hold good for a wave like light or sound. The speed of light will always be independent of any vehicle it is travelling on.  It is important to note that in both instances, the boy bouncing a ball up and down in a train moving at constant speed and light bouncing between two mirrors, both observers, the stationary observer (Bob) and the moving observer (Alice) measure the correct distance. There is no question of time dilation or length contraction.

There are some strange facts associated with this problem. First of all special relativity admits that the speed of waves in a medium behaves exactly like light does, meaning that its speed remains constant irrespective of the frame of reference from which it is being viewed. The difference, special relativity claims arises when waves like sound are travelling with (inside a vehicle) or not. If the source of the sound is merely fixed to the outside of a car,  the sound from that siren will travel at a constant speed depending on the properties  of the medium it is travelling through. If, however, the source of the sound is inside the cabin of the vehicle, the sound will acquire the speed of the vehicle. Special relativity claims that this is the reason that light is different from a wave travelling in a medium. Even if the light were in the cabin of the vehicle its speed would remain constant unlike the speed of a wave which would vary because the medium it travels through take air, acquires the speed of the vehicle it is travelling in.  Surely a spurious excuse, when viewed from the point of Newton, Rutherford or any sane scientist. What about dark matter for instance, what if light travels through dark matter and dark matter is the medium that light travels through. Because of the low interaction of dark matter with matter, if light were travelling through it, the light would not acquire the speed of the vehicle it is travelling in.

[Halc]

I'm not measuring the transit time from A to B, but the speed of the travelling wave as it passes B

But you're not measuring that, or if you are, you're assuming that the one way speed of light is c in all directions, which would be begging your conclusion. To demonstrate the speed of the travelling wave as it passes B, you need to not start with the assumption that it is travelling at c.

That's why I picked the absolute interpretation which suggests that the absolute speed of light is  constant, and thus the relative speed of light is not, and thus the eastbound light measured by an observer moving (absolutely to the west) at 0.8c passes him at a vastly different relative speed (1.8c) than the westbound light (0.2c), despite its identical appearance from our observer. He measures the same wavelength and frequency for both, and thus does not measure the speed at which it is passing him by. The actual frequency and wavelengths of the two beams are significantly different, but our observer happens to be in the exact frame where their appearance is identical, which is no surprise since both identical emitters are moving with him.  In an absolute interpretation, the frequency of the light emitted by a moving laser is very dependent on the direction you point it, just like in regular SR where its frequency is frame dependent.

My point is that both theories acknowledge that there is no way for our observer to measure what you're suggesting above.

[alancalverd]

To demonstrate the speed of the travelling wave as it passes B, you need to not start with the assumption that it is travelling at c.

I haven't. I merely state that v = fλ, which is the definition of v for all travelling waves in any medium, then measure f and λ at B by independent means.

[alancalverd]

To her the beam of light appears to go straight up and down. 

That's an unjustified assumption!

[Bored chemist]

To demonstrate the speed of the travelling wave as it passes B, you need to not start with the assumption that it is travelling at c.

I haven't. I merely state that v = fλ, which is the definition of v for all travelling waves in any medium, then measure f and λ at B by independent means.

The weird thing about this is that it's not a 2 way measurement (in any way I can spot)
and it's not even a one way measurement- you aren't timing a flash of light over a distance.
So what is it?
You can easily imagine splitting the incoming light into two paths and measuring the wavelength of one beam, and the frequency of the other; so the two measurements are independent.
Practically speaking, getting a precise measurement is going to be tricky- you would need large equipment but that's just a human technology problem It will get easier as we learn how to do mm wave stuff better

[alancalverd]

So what is it?

It is a measurement of the speed of a wave travelling in one direction.

The joy of a continuous travelling wave is that you don't have to time a pulse over a distance because the speed at any point defines two independently measurable quantities: how many peaks pass that point in a second, and what angle the beam is deflected by if you place a grating at that point.

I didn't subscribe to your earlier etalon suggestion because that requires multiple reflections, so could be argued to be a two-way measurement. The angle at which a beam is diffracted from a simple transmission grating depends only on the wavelength of the incoming radiation and the periodicity of the grating.  Or you can use a zone plate and just measure the focal distance, which is an inverse function of wavelength.

As we both pointed out earlier, these measurements (particularly of f) are difficult at optical frequencies but very easy with microwaves.

The fun bit is that the measured value of c turns out to be exactly that calculated by Maxwell from independent electrostatic and electromagnetic measurements.