[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.

[hamdani yusuf (OP)]

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.

Afaik,  there is no guarantee that the reflected wave would behave exactly the same way as the icoming wave,  in terms of speed,  frequency,  and wavelength, especially when the reflectors move relative to the medium or light source.

[McQueen]

  Alan Calverd- That's an unjustified assumption!

The explanation [afaik] is as follows. Substitute the beam of light with a boy bouncing a ball. Suppose that the ball is constituted such that when dropped it returns to the boys hand.  If the boy is on a train running at 60 kmh  and he drops the ball from a height of 1m . Then according to t² = 2/0.5a it should take 0.63 seconds for the ball to return to the boys hand. During this time the train would have travelled approx. 10.5 m. Total distance travelled by ball equals 1² +  5.25²  = 6.25² =  sqrt 28.625 = 5.34 m x (2) = 10.68 m. So the ball travels a distance of 10.68 m approx. To the boy in the train it appears as if the ball has bounced and come back up in a straight line. If there were no windows the boy would not be able to tell if he were moving or not and he would assume that the ball is bouncing up and down in a straight line.  This is made possible because both the boy and the ball acquire the same velocity as the train.

For the observer on the platform it is quite obvious that the ball is travelling in a diagonal zig-zag pattern from the boys hand to the floor and back again to his hand. 

The situation with a beam of light travelling in the carriage is different because the speed of light remains constant.  Assuming that the distance between the mirror on the floor and the mirror on the roof of the railway carriage is 3m, then the total distance for one click of the clock is 6m. The time taken for one click of the clock is 0.00000002 s approx. During this time the train would have travelled 0.0000003 m.   Now reverse the situation, have the light clock situated on the platform with the train moving past. One tick of the clock would still take 0.00000002 s and the train would still have travelled 0.0000003 m . The speed of the light clock would be independent of the speed of the train.

No time dilation and no length contraction needed to explain what is happening here. Yet, this is one the favourite thought experiments quoted by famous physicists such as Brian Cox and Brian Greene to justify special relativity. As calculators become better and more accessible special relativity is slowly but surely unravelling.

This might be interesting, and relevant for the first 45 seconds: https://www.youtube.com/watch?v=kpuGjzdHqgI

[alancalverd]

So the ball travels a distance of 10.68 m approx.

Or not, depending on whether you are measuring  the distance travelled within the train (2m), or over the ground (10.5 m) or through the earth's atmosphere (10.68 m). But those who believe in an aether think differently about light, and would claim that you have arbitrarily defined 20 nanoseconds as the time it takes the clock to tick

[McQueen]

Alan Calverd: Or not, depending on whether you are measuring  the distance travelled within the train (2m), or over the ground (10.5 m) or through the earth's atmosphere (10.68 m). But those who believe in an aether think differently about light, and would claim that you have arbitrarily defined 20 nanoseconds as the time it takes the clock to tick.

The answer I had given is supposed to take both factors into consideration, since 10.68 m is the path travelled by the ball in a zig-zag diagonal pattern , from the boys hand to the moving floor of the train and back to the moving boys hand. Leaving that aside for the moment; there are far more critical problems to do with the special relativity.
   The crucial problem with relativity is that most people cannot come to terms with the fact that in special relativity lengths do actually contract and time does dilate for different observers. Look at the twin paradox, where one twin goes off into space travelling near the speed of light and the other twin stays at home. If the twins are living on a planet and twin A travels on a spaceship to a star 10 light years away at a speed of 0.8 c while twin B remains at home then the journey to the star and back again will take 20/0.8 = 25 years as observed from earth. According to Einstein both of these twins will age differently and it is possible to calculate the age difference between the twins by using the time dilation equation:



v is the speed of the space ship;
c is the speed of light; and
γ is called the gamma or Lorentz factor.
Δt' is the time as measured by the traveling observer;
Δt is the time as measured by a stationary observer;
Using the time dilation equation it is possible to see that :

t’ =    = 56.8 years

Therefore, according to special relativity the twin who remains behind on earth while his twin travels to a planet 10 light years away and back, will be 31.8 years older than the twin who went into space. Thus, if both twins were twenty years old when they parted, the twin on earth will be 20 + 56.8 years =  76.8 years old while  the twin who went into space will be only  20 +  25 = 45 years old.

Imagine not one set of twins but a million or more people out in space travelling at near light speed, each will measure a different distance and a different time. Therefore, what results is an n! (n factorial) number of possible speeds and times depending on the number of travellers. This yields impossibly huge numbers such as 1000! (one thousand factorial) for 1000 travellers all travelling at varying near light speeds  which is an absolutely unacceptable result.  Like it or not it is an impossible situation.  It is much more probable that the speed of light is constant because it is travelling through a medium even though such a medium has not as yet been identified than to suppose that length contracts and time dilates in order to keep the speed of light constant.

It might be profitable to continue arguing with these conclusions, or not.

[Bored chemist]

Imagine not one set of twins but a million or more people out in space travelling at near light speed, each will measure a different distance and a different time. Therefore, what results is an n! (n factorial) number of possible speeds and times depending on the number of travellers. This yields impossibly huge numbers such as 1000! (one thousand factorial) for 1000 travellers all travelling at varying near light speeds  which is an absolutely unacceptable result. 

Why is that unacceptable?
Imagine you consider groups of 3 people and the motion of the centroid of the triangle they form.
How many velocities are there?

Did you know that the results of statistical thermodynamics are derived using  quantities of the order of Avogadro's number factorial?
3X 10^6 !
is really big.
It's still the same number.
Why is one set of velocities acceptable, but the other isn't?

[McQueen]

Bored Chemist: Why is one set of velocities acceptable, but the other isn't?

It isn’t acceptable because people are involved.  It is actual space and time which is being chopped into these impossible numbers. This is illustrated by the differences in Age in the twin paradox.  Would it be acceptable to you if you could not synchronise your life with events happening around you. Even if you state that it would be acceptable, let me tell you that from a practical point of view, it isn’t. In fact such a situation precludes the existence of sentient life.

[alancalverd]

Would it be acceptable to you if you could not synchronise your life with events happening around you.

If people are travelling at relativistic velocities, they are not "around" each other.

I don't have a problem with the gravitational shift of GPS satellite clocks - it's a simple computation built into the firmware. Aiming a gun at  moving target is a bit of an art, but you can learn to synchronise the arrival of the shot with the predicted position of a bird. Though squirrels are more difficult because they run in random spurts. 

[Bored chemist]

It is actual space and time which is being chopped into these impossible numbers.

No it isn't.
Space isn't getting "chopped"; what would be in the gap between the two bits os separated space?

Would it be acceptable to you if you could not synchronise your life with events happening around you.

Yes.
I accept that, if I want to synchronise a clock in the the GPS system, I have to set it to run at the wrong rate while it is here on Earth.

Even if you state that it would be acceptable, let me tell you that from a practical point of view, it isn’t.

We are doing it; we always have. Up until fairly recently, we didn't even know we needed to.