[Kris Kuitkowski]

"Your rod cannot be as rigid as you imagine it to be - if you push it from one end, there will be a delay before the other end starts moving, and the same applies if you pull it from the other end. If you try to avoid delays by accelerating the whole rod evenly at the same moment in time, you run into synchronisation issues. Whichever frame of reference you use to synchronise your moment of time along the rod will determine the number your experiment produces for the speed of light - all it does is give you a speed that relates to the frame you use for synchronisation, thereby rendering the experiment useless."

The rigidity of the rod should be enough to withstand the wind caused by the rod travelling through the air with the speed of ~10m/s. I think the Styrofoam would be sufficient, but of course we can find slightly better materials. Because the distance between the lasers A and B is EXACTLY (to the limit of our ability to mark it - not measure; just mark it) equal to the length of the rod, if the end of the rod at B will start allowing the light from laser at B to travel perpendicularly to B' we will know that the light from laser at A will start to be blocked by front end of the rod BECAUSE THE LENGHT OF THE ROD is exactly the same (disregard length contraction; at that speed it is really insignificant) as the distance d between the lasers.
The speed of the rod is not important either; the limited precision in positioning the lasers in regards to the length of the rod d would be compensated if we repeat the experiment from the other direction since 2 way speed of light has to be c.

[PmbPhy]

In SR there is a convention which says no mater what direction you are traveling speed of light is the same. However the one way speed of light can't be measured.

That's incorrect. First of all if that were true then it would violate the conservation of energy law. Second, all one has to do is pick up and read a good SR text like Ohanian's to see that statement to be in error. The one way speed of light has been measured.

If you don't wish to read an SR text then see the MIT Technology Review at
https://www.technologyreview.com/s/421603/the-one-way-speed-of-light-conundrum/

Specifically

One interesting question immediately arises: how do you measure the one way speed of light? It turns out there are various methods. One idea involves the emission and absorption of gamma rays by certain kinds of atoms in a solid. The process of absorption is very sensitive to the energy of the gamma rays. So if the speed of light (and therefore its energy) varies with direction, then the rate of absorption ought to change too.

In the 1960s and 70s, various physicists looked for a directional dependence by placing a gamma ray emitter at the edge of a rotating disc and an absorber at the centre. They then looked for any difference in the rate of absorption as the disc rotates but found none.

Neither have physicists using other techniques found any variation either. (The controversial Bulgarian Stefan Marinov claimed to have found evidence of a variation in the one way speed of light but his claims are not considered valid by most mainstream physicists.)

[alancalverd]

You can calculate the one-way speed of light from Maxwell's equations. Then, remarkably, however you measure  the propagation velocity of electromagnetic radiation in vacuo, it turns out to be exactly the calculated value, so either there is a selfcorrecting error in the calculation, or the two-way speed is the same as the one-way speed.

An everyday example is the average ground speed of an airplane flying a distance d with a tailwind w and returning against the same wind. The constant airspeed is a

Outbound time = d/(a + w). Return time = d/(a - w).

Total journey time = d((a - w) + (a +w)) / ((a + w) (a - w)) = 2da/(a² - w²) > 2 d/a if w > 0. In other words, any difference between the "coming" and "going" speeds will result in a measured speed different from the calculated value.

Since the observed value of c is exactly the calculated value, it therefore must be the case that the two-way speed is exactly the same as the one-way speed.

[David Cooper]

The rigidity of the rod should be enough to withstand the wind caused by the rod travelling through the air with the speed of ~10m/s.

Why do I have to explain this a third time? Your rod can never be rigid (and the air is irrelevant). When you move one end of the rod, there is a delay before the other end moves unless you try to move both ends of the rod simultaneously by accelerating them both individually, but if you do that you run straight into the issue of what is simultaneous, so you have to pick a frame of reference in which the two events are simultaneous, at which point you've already selected a speed of light for your experiment to "confirm".

...if the end of the rod at B will start allowing the light from laser at B to travel perpendicularly to B' we will know that the light from laser at A will start to be blocked by front end of the rod BECAUSE THE LENGHT OF THE ROD is exactly the same

Not so - you'd need the movement of the rod to be transmitted from one end to the other faster than the speed of light for such instantaneous coordination. A rod will actually communicate the movement at the speed of sound, so it's worse than using a light pulse. What you need to understand is that when you push or pull one end of your rod, the other end doesn't respond immediately, and the delay in the reaction of the other end is magnitudes worse than the delay from sending a light signal.

(disregard length contraction; at that speed it is really insignificant) as the distance d between the lasers.

Length contraction can indeed be ignored as the rod can move at very low speed, but that's not the issue - it's the delay between one end moving and the other end reacting to it that renders your experiment worthless.
 

The speed of the rod is not important either; the limited precision in positioning the lasers in regards to the length of the rod d would be compensated if we repeat the experiment from the other direction since 2 way speed of light has to be c.

As I've told you in two previous posts, you're up against a synchronisation issue. If you choose to synchronise events at the two ends of the rod by using the frame of reference in which the apparatus is stationary, you will measure the speed of light as c in both directions. If you choose a different frame of reference, you will measure a higher speed of light one way and a lower speed of light the other. If you do this with a hundred different frames of reference, you will get a hundred sets of different results and you'll be no closer to knowing which of them might be closest to being true.

[David Cooper]

The one way speed of light has been measured.

That's incorrect. There are experiments which are often presented as having measured the one-way speed of light, but none have actually done so.

One interesting question immediately arises: how do you measure the one way speed of light? It turns out there are various methods. One idea involves the emission and absorption of gamma rays by certain kinds of atoms in a solid. The process of absorption is very sensitive to the energy of the gamma rays. So if the speed of light (and therefore its energy) varies with direction, then the rate of absorption ought to change too.

In the 1960s and 70s, various physicists looked for a directional dependence by placing a gamma ray emitter at the edge of a rotating disc and an absorber at the centre. They then looked for any difference in the rate of absorption as the disc rotates but found none.

The experiment fails to work because of the Doppler shift. The idea with the experiment is that the frequency of the radiation has a fixed value and the detector can only detect it if it remains constant at the absorber end. If the centre of the turntable is stationary in space, the absorber will be moving at a constant speed relative to it as it goes round, but it has to be tuned precisely to that speed of movement if it is to detect the radiation, so if you slow down the turntable or speed it up just a little (or a lot), the detector won't detect anything.

The idea of the experiment is that if the centre of the turntable is moving through space, the speed of the absorber will continually change (while the speed of the emitter is constant), so the idea was that it should stop detecting the radiation most of the time. The movement of the emitter causes the frequency of the radiation to fall due to its "clock" being slowed by that movement - it runs slightly in slow motion. The frequency the absorber is tuned to will likewise fall due to its extra movement through space. However, if you work through the maths properly, you find that as the "clock" of the absorber speeds up and slows down, the frequency of the radiation hitting it rises and falls to match due to the Doppler effect, and the match remains perfect at all times. There is no possible way for the experiment to show up any difference in the speed of light in different directions no matter how much that speed of light across it varies.

[alancalverd]

Simple thought. How does a photon know whether it is coming or going? Obviously it can't know, so there can't be a difference between the propagation speeds.

[GoC]

Simple thought. How does a photon know whether it is coming or going? Obviously it can't know, so there can't be a difference between the propagation speeds.

Sometimes the simple thoughts are the best.

[Le Repteux]

Simple thought. How does a photon know whether it is coming or going? Obviously it can't know, so there can't be a difference between the propagation speeds.

Hi Alan!

Two bonded protons that form a hydrogen molecule may not need to know which one of them is moving to follow precisely the direction and the speed they were given in the beginning, providing they kind of keep in mind the motion parameters of the information that allows them to stay bonded, because due to the limited speed of any information, one of them necessarily has to move before the other in the process, which necessarily produces doppler effect on the energy that carries the information, what should produce shifted steps between them, a step like motion that should necessarily follow the original acceleration if the exchanged energy has to stay constant. Here how this motion should look like at first glance.


But if we give it a second thought, if we consider the problem of the one way speed of light for instance, a question arises: the left proton makes a full step before the information from that step reaches the right proton, and that right proton starts its step when the left one is close to it, so it doesn't have the time to make a full step before the information from that step starts to hit the left proton, and it should since I thought that those steps could represent what we call inertial motion. Of course the steps could be infinitely small for instance, but it seems to me that it would only postpone the issue, an issue that I did not yet succeed to imagine. Anybody can? Maybe Kris since he seems quite concerned by the limited speed of the information, or PmbPhy since he seems concerned by doppler effect?

Notice that this particular problem gives us a new tool to analyze the way light behaves, because if those steps really exist, light wouldn't just help us to measure motion, it would also participate to its production at the particles' level.

[David Cooper]

Simple thought. How does a photon know whether it is coming or going? Obviously it can't know, so there can't be a difference between the propagation speeds.

There is no difference in the propagation speeds - the speed is imposed by the fabric of space, but there is a difference in the propagation speeds relative to the object the light came from if that object is moving through space. The light doesn't need to know anything, but simply sets off at the speed it's allowed to move at.

Let me remind you of my thought experiment with a fibre-optic cable running east-west right round the planet. If we send a pulse of light through it in both directions, we'll get a pulse returning from the west and another from the east, but they won't arrive at the same time because one of them has had to travel further due to the rotation of the Earth. The pulse moving west goes a shorter distance through space than the pulse moving east. If we try to measure the speed of those pulses of light at different points on their journey round the cable, we'll find that it is always c, and Einsteinists will assert that it is moving at c in both directions relative to each point at which they measure it rather than faster in one direction than the other. However, if it is really moving at c in both directions at each of an infinite number of points around the ring, it would be impossible for the pulses of light to arrive at the finish at different times - they would have to arrive simultaneously. The fact that the pulses do not return simultaneously proves that the speed of light at some of those points along the way cannot be the same in both directions relative to those points. This invalidates SR, but what Einsteinists do is what they always do - they tolerate contradictions and stick rigidly to a disproven theory.

[Le Repteux]

Hi David!

Unless a new theory on motion gets more useful than SR, there is no reason for physicists to reject it. We don't reject our old ideas just because they contain contradictions, otherwise we would never get the ground to build new ideas. I also think SR is wrong, and I am also pointing at its contradictions to discuss it, but I know it is not sufficient to convince anybody that my own theory is promising. To convince people, I know I have to show the benefits.

[David Cooper]

Unless a new theory on motion gets more useful than SR, there is no reason for physicists to reject it.

Not so - if another theory does so much as produce the exact same numbers while making better sense (as with LET), they should switch to it instead of carrying on regardless while continuing to assert superiority of the irrational theory over the rational one. At the very least, they should drop all the irrational dogma and recognise that it cannot be the case that light moves at the same speed in opposite directions relative to all objects, but they persist in asserting that it does even though their position has been shown to be wrong on that score.

We don't reject our old ideas just because they contain contradictions, otherwise we would never get the ground to build new ideas.

Where theories produce contradictions, we should accept that they must be wrong instead of asserting that they are right, and then we should put more effort into finding out ways of correcting or replacing the faulty theory so that we have something that doesn't generate contradictions. The establishment's insistence in sticking with a disproven theory while rejecting one that does the same job while making better sense is actively holding back progress.

I also think SR is wrong, and I am also pointing at its contradictions to discuss it, but I know it is not sufficient to convince anybody that my own theory is promising. To convince people, I know I have to show the benefits.

The benefits are that you encourage more rational people to go into science instead of making it appeal to the irrational ones who are happy to tolerate contradictions and dig in to defend faulty models rather than working to correct them.

[Kris Kuitkowski]

"Why do I have to explain this a third time? Your rod can never be rigid (and the air is irrelevant). When you move one end of the rod, there is a delay before the other end moves unless you try to move both ends of the rod simultaneously by accelerating them both individually, but if you do that you run straight into the issue of what is simultaneous, so you have to pick a frame of reference in which the two events are simultaneous, at which point you've already selected a speed of light for your experiment to "confirm"

If the rod of length d=10m is moving with constant speed and its end is at point B, then its front must be at point A 10m forward. It has nothing to do with the speed of sound .

"Length contraction can indeed be ignored as the rod can move at very low speed, but that's not the issue - it's the delay between one end moving and the other end reacting to it that renders your experiment worthless."

If the rod has been moved out (let's say 10km away) and accelerated to the constant speed v and then let it glide (without any acceleration) past lasers at A and B, then if front end reaches point A, the rear end must be at point B; there is no delay between one end moving and the other reacting. They just both move with constant speed (or if you prefer, they are at rest and embankment is moving with constant speed -v)

[Le Repteux]

Not so - if another theory does so much as to produce the exact same numbers while making better sense (as with LET), they should switch to it instead of carrying on regardless while continuing to assert superiority of the irrational theory over the rational one. At the very least, they should drop all the irrational dogma and recognize that it cannot be the case that light moves at the same speed in opposite directions relative to all objects, but they persist in asserting that it does even though their position has been shown to be wrong on that score.

I agree that they should try to understand, and I noticed that some do, but can they change their mind? Take someone who believes in god for instance, can he change his mind just because it is more logical to think the contrary? Moreover, what would happen if everyone would change his mind every day just because another idea looks better? We have to resist to change otherwise things would not have the time to exist at all. That's probably why bodies resist to acceleration, and also why species stay the same while their mutations offer new possibilities to the natural selection. It's by hazard and coincidence that we change, otherwise we must stay the same.

Where theories produce contradictions, we should accept that they must be wrong instead of asserting that they are right, and then we should put more effort into finding out ways of correcting or replacing the faulty theory so that we have something that doesn't generate contradictions. The establishment's insistence in sticking with a disproved theory while rejecting one that does the same job while making better sense is actively holding back progress.

That would be true if developing new ideas did not depend on chance, but if it does, then it might be wrong, because trying to develop new ideas would not mean that one of them will work for sure, so that putting a lot of energy on that one could lead to disaster. The evolution of species doesn't work like that: it produces mutations everywhere at random, and it kind of hopes that one of them will do the job. We may have the feeling that we know how to control our evolution, but I think it's just an illusion due to the fact that changing is always uncertain, so that we kind of need that good feeling to help us jump off the cliff.   

The benefits are that you encourage more rational people to go into science instead of making it appeal to the irrational ones who are happy to tolerate contradictions and dig in to defend faulty models rather than working to correct them.

Unless somebody suddenly suffers a mutation in his ideas, I'm afraid nobody changes ideas only while discussing with others. Even if I am aware of the phenomenon, I can't avoid the feeling that, at first sight, others' ideas do not work as well as mine, and I realize that I can't study them as well as I would like because of that feeling. It's like trying to study the bible while being atheist: you almost need to be masochist. I can easily observe my own resistance and I am alone to think the way I do, so I can imagine how resistant I would be if I had the whole scientific community behind me. :0)

[David Cooper]

If the rod of length d=10m is moving with constant speed and its end is at point B, then its front must be at point A 10m forward. It has nothing to do with the speed of sound .

I thought the rod was stationary relative to the rest of the apparatus to begin with, but having looked back at what you originally said, I got that wrong because I was distracted by the word "rigid" and the bit about measuring it "at rest" - that set me off on the wrong track. So, I apologise for wasting your time with my previous comments on your thought experiment, and I'll attempt to deal with it correctly this time. I've looked at this kind of thought experiment more than once in the past and thought it was the same old thing coming round again in another form, but it's fully possible that on each occasion it involved a rod starting from stationary such that communication speed issues are involved in getting it moving. You've already got it moving though, and because the speed can be very low relative to the rest of the apparatus, length-contraction may not be be a factor (although it might be too soon to rule it out). Is it really possible that we've all missed a trick? I was sure I'd looked at this myself (a version with the rod already moving), but maybe I was diverted off into looking at a version with a stationary rod being accelerated and never got back to looking at the original idea. Whatever the case, your thought experiment is definitely worth looking at carefully, so lets test it to destruction. I'll quote it again as it's a long way up the page:-

Let’s have two light sources at points A and B separated by distance d and sending constantly (perpendicular to AB) signals to clocks at A’ and B’
Let’s have an opaque rigid rod of the length d (it can be measured against AB while at rest with AB, so accuracy can be high) traveling with constant speed v (non-relativistic) parallel (and very close to) the line AB from B towards A. Initially the light from B to B’ will be blocked and the light from A to A’ will be allowed to be transmitted. When front end of the rod will start cutting off the light from A to A’ the light from B will start to be transmitted to B’ . At this moment, we will have both clock at A’ and B’ synchronized.

We can improve the accuracy of the measurement sending the rod from A to B with the same speed v and measure one way speed of light from A to B. The average 2 way speed of light from A to B and from B to A has to be c.

What you're really doing is using your rod to start two clocks, the leading end of the rod starting one clock and the trailing end starting the other, and the clocks simply start as these ends run past them. You then send light from one clock to the other (or a radio signal) and note the time at which it is sent out on one clock, then the other clock records the time at which it is received. If the apparatus is stationary in space, the delay will be the same in both directions and will show the speed of light in both directions to be c, but if the apparatus is moving in the AB direction, the two timings will be different and different values for the speed of light will be generated for the two directions.

Has this really synchronised the clocks in an absolute way? A conventional method of synchronising the clocks might be to send a radio signal from half way between the two clocks and for the two clocks to start when they receive that signal, which means that if the apparatus is moving through space in the AB direction, clock A could start up long before clock B. If your system of synchronisation synchronises them differently, then you might have found something big. Let's add another rod going in the opposite direction. We can arrange things such that the leading end of the new rod passes clock A at the same time as the trailing end of your original rod passes clock A. If we do this, the trailing end of the new rod will pass clock B at the same time as the leading end of the original rod passes clock B. That locks things down very tightly. We can check the rods before doing the experiment, placing them with their ends at A and B to confirm that they are the right length. Then we take them away and set them moving towards the clocks from opposite directions.

Any compression or stretch from how we accelerate them will be lost as they rearrange themselves and settle down, and we can prove this by using two rods for each direction, one of each pair pushed up to speed while the one next to it is pulled. We now have two pairs of two rods with the two rods in each pair moving side by side, their front ends directly side by side and their trailing ends directly side by side too, thereby proving that any compression and stretch from the accelerations has been lost. Having dealt with this issue, lets simplify it back to two rods moving in opposite directions for the rest of the discussion.

Let's assume the clocks are not moving through space. The rods approach, and the ends pass clocks simultaneously, synchronising the clocks. We can also send a radio signal from half way between the clocks and have it arrive at both clocks at the same moment. We have perfect synchronisation with both synchronisation systems agreeing with each other.

Now let's assume the clocks are moving through space at high speed such as 0.867c. Our length AB is length-contracted to half its rest length, but so are the rods because they're moving at 0.867c plus or minus a tiny amount. We run our experiment again (well, it's your experiment, so if this works, it's your Nobel prize), and this time when we send out our radio signal from half way between the clocks it will reach clock A long before it gets to clock B. But what are the two rods doing? Let's look at the one moving in the AB direction first, then do the BA direction afterwards. For the two synchronisation methods to match up, the trailing end of this rod must reach clock A at the same time as the radio signal gets there. We know that the leading end of the other rod should be there at the same time too, so let's assume it's there. What's happening at clock B?

If there's any length-contraction issue with the rods, one will be infinitesimally shorter than the length AB and the other will be infinitesimally longer, so that means the leading end of our first rod might not have reached clock B yet. and although the leading end of our second rod has reached clock A, its trailing end may not have reached clock B. That leaves room for a delay, and given the slow movement of the rods relative to the clocks, it's just possible that our radio signal will still be able to reach clock B at the same time as those ends of the rods pass that clock, because even a tiny amount of length-contraction could add up to a significant difference. If it can, then the thought experiment will be defeated, and I have a nasty feeling that that's what's going to happen.

How can we check? Let's try moving the rods faster relative to the clocks so that we can get a decent value for the length-contractions acting on them and work out what the delays actually are. We can then repeat it with slower speed differences, but avoiding values which take us into places where calculators introduce significant rounding errors. We only need to do this with one rod from now on, so let's move the clocks through space at 0.867c and the rod at 0.968c. That's a high speed for the rod, but it's convenient because it gives us length-contractions to 1/2 for the distance between the clocks and 1/4 for the the rod. We now have a rod that's only half the length required to span the distance between the clocks. The radio signal is sent out from half way between A and B, then when it arrives at clock A, the trailing end of the rod passes clock A. At that moment, the leading end of the rod is now at the place where the radio signal was sent out from, so there is no way it can reach clock B at the same time as the radio signal which is already far ahead of it. Of course, we could use a rod that's twice as long so that the other end would be at clock B at that moment, but if we did that, we'd be assuming that our clocks are moving at 0.867c through space. If we assume that they are stationary instead, a double-length rod would need to be moving at 0.867c to be length-contracted to half the length AB, and the problem with that is that the relationship is wrong - to match up with this, the relative speed difference between clocks and rod would mean that if the clocks were moving at 0.867c, the rod would be moving through space at 0.99c rather than 0.968c.

[Edit: it wouldn't need to be double the length though as there's a delay before the radio signal reaches clock B, so we'd only need to lengthen the rod to get the end of the rod far enough ahead of the radio signal to reach clock B at the same time.]

My suspicion is that length-contraction is going to wreck the ability of the experiment to synchronise the clocks in the way it initially looked as if it could, but I'm going to post this now and take a break before getting back to it later (if someone else doesn't finish it off first).

[Edit 2: I'll get back to this tomorrow.]

[Kris Kuitkowski]

The experiment has been designed in such a way that it should be possible to carry it out in even modest lab. Most crucial part is positioning the lasers against the rod (the experiment could be modified with 2 narrow slots cut in the rod so both lasers could shine through only if they are perfectly aligned) and the clocks which should measure the time to at least 100ps precision.( As I have mentioned before, we could modify the experiment that only one clock would be required)  The length of the rod doesn't need to be measured extremely precise; accuracy to 1mm will do. Now let's do some calculations:
time for the light to cover 10m is d/c=10m/3x10^8=~33ns.
The length contraction of the rod of length d=10 for v=100m/s will be dx((v^2)/(c^2)=10mx((10^4)/9x10^16)=~(10^-12)m.
If we can position the lasers against the rod with 1nm accuracy, the length contraction will be 1000 time smaller then our uncertainity in positioning the lasers..
For 1nm accuracy in positioning the lasers there will be uncertainty in synchronizing the clocks:
delta t= 1nm/100m/s=~10^-11s, which still will allow us to measure one way speed of light with 0.5% accuracy.
I think it could be possible to carry out such experiment without spending exuberant amount of money on necessary equipment.

[PmbPhy]

That's incorrect. There are experiments which are often presented as having measured the one-way speed of light, but none have actually done so.

If you know me at all I then you know that I never accept mere claims like that. You'd have to demonstrate that all the published papers which demonstrate that its been measured area all wrong or state why you're making that claim.

[David Cooper]

That's incorrect. There are experiments which are often presented as having measured the one-way speed of light, but none have actually done so.

If you know me at all I then you know that I never accept mere claims like that. You'd have to demonstrate that all the published papers which demonstrate that its been measured area all wrong or state why you're making that claim.

The example you provided was one that was debunked long ago. I'm in contact with other people in the LET camp who've spent years going through all the experiments that supposedly measure the speed of light in a single direction who have found fatal flaws in every single experiment of that kind that they've been able to find. The best one they'd found involved a rotating rod with a fan-like device on each end (slits to let light through), but it failed to account for twist in the rod due to communication delays - when they tested me on it, I immediately realised that if you had two separate fans and coordinated them using radio communications, they'd move out of alignment due to delays and that the exact same delays would operate within the rod.

The reason you believe that experiments exist which have measured the one-way speed of light is that the people who put out these experiments rarely admit that they've got flaws in them once they've been debunked, so documentation is left in place asserting that they have succeeded in doing the job originally claimed of them. There's also a reason why you pointed to a particular example that's been debunked, because that's about as good as they get - if there was an experiment that actually did what they claim of it, it would be up there as the supreme example with a Nobel prize tied to it. The hard reality is that you have no experiment to point to that can measure the one-way speed of light because no such experiment has been found, and my thought experiment with the fibre-optic cable round the Earth should be a clue as to the fact that no such experiment has been found, because if such an experiment was known it would have demonstrated that light moves at different speeds in different directions across it.

[David Cooper]

Hi Kris,

I'm not sure your way of analysing it is right, so I'll continue to blunder along with mine to see what happens. I made a mistake in what I said before:-

The radio signal is sent out from half way between A and B, then when it arrives at clock A, the trailing end of the rod passes clock A. At that moment, the leading end of the rod is now at the place where the radio signal was sent out from, so there is no way it can reach clock B at the same time as the radio signal which is already far ahead of it.

I was forgetting that the apparatus has moved past the midpoint, so the leading edge of the rod is still ahead of the radio signal heading for clock B. That makes sense, because for the two synchronisation methods to clash so badly would have shown that the experiment could indeed measure the one-way speed of light. For the experiment to fail to do so, the synchronisation methods should have matching results no matter how fast the apparatus is moving through space.

The method I'm using to test this is as follows.

(1) Pick a speed for the clocks A and B to move through space.

(2) Pick a distance of separation between A and B and adjust it for the length-contraction required for the speed in (1)

(3) Put a radio transmitter at point M, half way between A and B. Work out how long light (or the radio ping) will take to get from M to A and from M to B. Remember that A and B are moving, so the signal will reach A first (if they're moving in the AB direction rather than BA).

(4) Calculate how close to B the radio signal has got to at the point in time when it has reached A.

(5) Pick a speed for the rod to move through space and work out the length-contraction on it.

(6) Calculate how close to B the leading end of the rod will be at the point in time when its trailing end has reached A.

(7) Compare how long it will take for the radio signal and the leading end of the rod to catch clock B (from the starting points calculated in steps 4 and 6). Remember that clock B is racing away from them, so it's a moving target.

If we do this for a range of speeds for clocks and rods, I suspect we'll see an interesting pattern that will show that they both reach clock B simultaneously in every case. I don't have time to fiddle around with a calculator to do all that, but I might write a program to do it instead. I'll do that when I've got a bit more time free, unless you (or someone else) get in first. I expect there are more elegant ways to check it that a mathematician would apply straight away, but this one should do the job.

[Kris Kuitkowski]

Hi David,
I think it would be possible to carry out the experiment. Would you be  (or would you know somebody ) interested in helping to set it up?

[alancalverd]

Let's try again.

A photon has no idea whether it is travelling towards or away from you. Until it reaches you, it doesn't even know if you exist. Therefore its speed in vacuo is independent of direction.

Now suppose that the vacuum is in fact filled with aether, which is in some way essential for the propagation of light,  and we are travelling through it in some direction. The measured two-way speed will depend on whether we are measuring "into wind" or "across wind", and the Michelson-Morley experiment (and its descendants) have clearly and consistently shown no difference.