B What experiments confirmed the constant velocity of light?

[Grampa Dee]

What other experiments, besides the M&M experiments confirmed the constant velocity of light?

Thank you in advance for the response.

 

[Dale]

The best compilation of experiments is here: http://www.edu-observatory.org/physics-faq/Relativity/SR/experiments.html

You are interested in section 3.

 

[PeroK]

What other experiments, besides the M&M experiments confirmed the constant velocity of light?

Thank you in advance for the response.

The key property about the speed of light is its invariance, which means it has the same measured value in all inertial reference frames.

 

[Herman Trivilino]

Here's a link to a few:
http://www.edu-observatory.org/physics-faq/Relativity/SR/experiments.html#modern-laser

Remember that when you are studying the connection between the MM experiments and Einstein's development of special relativity, you are studying the history of relativity.

It's been well over 100 years!

Experiments are no longer needed to verify that level of validity of the theory. It's used every day in every part of the world by thousands of technicians, engineers, and scientists.

 

[Grampa Dee]

The best compilation of experiments is here: http://www.edu-observatory.org/physics-faq/Relativity/SR/experiments.html

You are interested in section 3.

thank you Dale

 

[Grampa Dee]

The key property about the speed of light is its invariance, which means it has the same measured value in all inertial reference frames.

Thank you for your response,PeroK:
Yes; but what proof exists of this postulate?

 

[Grampa Dee]

Here's a link to a few:
http://www.edu-observatory.org/physics-faq/Relativity/SR/experiments.html#modern-laser

Remember that when you are studying the connection between the MM experiments and Einstein's development of special relativity, you are studying the history of relativity.

It's been well over 100 years!

Experiments are no longer needed to verify that level of validity of the theory. It's used every day in every part of the world by thousands of technicians, engineers, and scientists.

Thank you for the link Mister T

 

[PeroK]

Thank you for your response,PeroK:
Yes; but what proof exists of this postulate?

You don't prove a postulate! Obviously, you need some experimental confirmation of it. Which @Dale has provided a link too.

Fundamentally, however, the point of SR is not about the speed of light. SR is a theory of spacetime and a theory of particle energy-momentum. The simplest and best confirmation of SR is perhaps what happens when you put a particle in a particle accelerator. Particles may be given energy that would classically represent a speed of hundreds or thousands of times the speed of light. That doesn't happen. Instead, they get closer and closer to the speed of light, in agreement with SR energy-momentum.

This is why it's pointless to suspect that modern physics is based on some semi-religious like pronouncements of A. Einstein that no one ever bothered to check! The reality is that all of the 20th Century physics has been based on theories that are either relativistic, quantum mechanical or both. And the whole body of 20th century experimental physics supports these theories.

The future of physics is extending these theories, where required.

 

[cianfa72]

Fundamentally, however, the point of SR is not about the speed of light. SR is a theory of spacetime and a theory of particle energy-momentum.

I believe the point is that in our universe the light propagation process occurs exactly with that constant invariant speed $c$. If light propagation didn't happen in that way, SR would continue to hold though.

 

[pervect]

Possibly relevant is the following youtube linked, an old instructional video based on a published experiment. What's notable about this experiment is not it's precision, but the simple matter in which it is presented.

 

[Nugatory]

What other experiments, besides the M&M experiments confirmed the constant velocity of light?

It has been confirmed so many times in so many different ways that we no longer think of the confirmations as "experiments", any more than you or I would think that we're performing an experiment confirming Newton's theory of gravity every time we pick up or drop an object. For example... GPS receivers assume that the speed of light is always $c$ and would report ridiculously wrong positions if this assumption were off by any measurable amount. So that's millions of experiments performed by millions of people every hour of every day.

 

[Grampa Dee]

The best compilation of experiments is here: http://www.edu-observatory.org/physics-faq/Relativity/SR/experiments.html

You are interested in section 3.

Thank you Dale and everyone else...I didn't expect so many responses.-Thanks for the link, Dale; I saved it as a favourite.
I looked at section 3 and it's mostly experiments performed in order to find the Earth's velocity through an ether...which is what the M&M experiment was all about in the first place. However, there were experiments done using a moving source of light, being what I was looking for.
Now, the problem that I have with these experiments is due to the idea of light having different velocities when passing through different mediums.. it's not due to the light's velocity as such, but it is due to the time needed for a molecule/atom to absorb and re-emit a new photon of light.
When light comes from a moving source such as the sun, it enters the experiment by being absorbed and re emitted and so making the new source, stationary, relative to the experiment.

This is mentioned in the document you gave me.

-------------------------------------------------------​

3.3 Tests of Light Speed from Moving Sources​

If the light emitted from a source moving with velocity v toward the observer has a speed c+kv in the observer's frame, then these experiments place a limit on k. Many but not all of these experiments are subject to criticism due to Optical Extinction.

Within "Optical extinction", it reads: Many measurements of the speed of light involve the passage of the light through some material medium. This can invalidate some conclusions of the measurement due to the extinction theorem of Ewald and Oseen.

----------------------------------------------------------------------------

What I was looking at, Dale, was an experiment that would deffinitely rule out the possibility of the particle theory of light being responsible for the nul results of M&M.

Thank you for your time.

 

[Grampa Dee]

You don't prove a postulate! Obviously, you need some experimental confirmation of it. Which @Dale has provided a link too.

Fundamentally, however, the point of SR is not about the speed of light. SR is a theory of spacetime and a theory of particle energy-momentum. The simplest and best confirmation of SR is perhaps what happens when you put a particle in a particle accelerator. Particles may be given energy that would classically represent a speed of hundreds or thousands of times the speed of light. That doesn't happen. Instead, they get closer and closer to the speed of light, in agreement with SR energy-momentum.

This is why it's pointless to suspect that modern physics is based on some semi-religious like pronouncements of A. Einstein that no one ever bothered to check! The reality is that all of the 20th Century physics has been based on theories that are either relativistic, quantum mechanical or both. And the whole body of 20th century experimental physics supports these theories.

The future of physics is extending these theories, where required.

Thank you PeroK: the experiment of the particle accelerator, as I would understand it (and believe me I know that I can very well be wrong, as I'm not a scientist), would have something to do with PE and KE, something like the Lagrangian, or something similar.
Just as the force applied on a piston is reduced (leaving the gas peddle at the same position) due to the increased velocity of the piston, it would seem to me that the acceleration force upon the electron,would also be reduced as the electron increases in speed.

L = PE -KE

 

[Grampa Dee]

Possibly relevant is the following youtube linked, an old instructional video based on a published experiment. What's notable about this experiment is not it's precision, but the simple matter in which it is presented.

Thank you perfect: very nice video... I like it very much...my concern here is what I wrote to PerocK.

Thank you PeroK: the experiment of the particle accelerator, as I would understand it (and believe me I know that I can very well be wrong, as I'm not a scientist), would have something to do with PE and KE, something like the Lagrangian, or something similar.
Just as the force applied on a piston is reduced (leaving the gas peddle at the same position) due to the increased velocity of the piston, it would seem to me that the acceleration force upon the electron,would also be reduced as the electron increases in speed.

L = PE -KE

Again, I'm not claiming this to be a problem as such, as I am not a scientist...it is just the way I understand things to mean...

 

[PeroK]

Thank you PeroK: the experiment of the particle accelerator, as I would understand it (and believe me I know that I can very well be wrong, as I'm not a scientist), would have something to do with PE and KE, something like the Lagrangian, or something similar.
Just as the force applied on a piston is reduced (leaving the gas peddle at the same position) due to the increased velocity of the piston, it would seem to me that the acceleration force upon the electron,would also be reduced as the electron increases in speed.

L = PE -KE

I think that's what's called clutching at straws!

 

[Grampa Dee]

It has been confirmed so many times in so many different ways that we no longer think of the confirmations as "experiments", any more than you or I would think that we're performing an experiment confirming Newton's theory of gravity every time we pick up or drop an object. For example... GPS receivers assume that the speed of light is always $c$ and would report ridiculously wrong positions if this assumption were off by any measurable amount. So that's millions of experiments performed by millions of people every hour of every day.

Thank you for your response, Nugatory:
I understand that there are many experiments performed agreeing with Relativity, but they all assume that the velocity of light is always "c" in the first place...the reason for applying "length contractions and time dilations" within the equations...What if, the GPS, instead, used the different frequencies (red shift/blue shift) for different light speeds?

 

[PeroK]

For example:

https://home.cern/science/accelerators/large-hadron-collider

 

[Grampa Dee]

For example:

https://home.cern/science/accelerators/large-hadron-collider

Hello PerocK:

Thank you for the link; however, for some reason, my computer has blocked it...I will try to get at it somehow.

 

[DaveE]

Now, the problem that I have with these experiments is due to the idea of light having different velocities when passing through different mediums.. it's not due to the light's velocity as such, but it is due to the time needed for a molecule/atom to absorb and re-emit a new photon of light.

Not really absorbed and re-emitted. Check out these videos.

 

[PeroK]

Hello PerocK:

Thank you for the link; however, for some reason, my computer has blocked it...I will try to get at it somehow.

Basically, the highest energy particles at CERN have energies of about $6.5TeV$. In classical terms that equates to a speed of about $3 \times 10^{10}m/s$, or about 100 times the speed of light(!)

Clearly, that is not what is happening. So, the classical formula for KE of a particle ($\frac 1 2 mv^2$) is not generally valid. Not to mention that particles of greater total mass emerge from these experiments using the relationship $E = mc^2$ etc.

This is point one: classical (Newtonian) physics cannot be valid in the realm of high-energy sub-atomic particles. Something has to change. We need some new ideas! (These new ideas came in 1905, by the way, so they are not so new anymore.)

It's not a question of whether classical Lagrangian mechanics and classical KE can explain the last 100 years of experimental physics. It can't. The question is whether SR can explain particle accelerator experiments. If SR wants to prove itself, then that's what it has to do: successfully predict the outcome of particle accelerator experiments. Which it does and which is why it is generally accepted.

Note that just because you make your predictions using the assumptions of SR doesn't mean the experiments have to fall in line. If you assume something that is false, then the physical experiments will expose the mistake. The experiments at CERN are not a self-fulfilling prophecy. The experiments are an independent test of the theory.

That's how physics work.

 

[Dale]

What I was looking at, Dale, was an experiment that would deffinitely rule out the possibility of the particle theory of light being responsible for the nul results of M&M.

So for example, the most stringent astronomical constraint is:

K. Brecher, “Is the Speed of Light Independent of the Velocity of the Source?”, Phys. Rev. Lett. 39 1051–1054, 1236(E) (1977).

Uses observations of binary pulsars to put a limit on the source-velocity dependence of the speed of light. k < 2×10−9. Optical Extinction is not a problem here, because the high-energy X-rays used have an extinction length considerably longer than the distance to the sources.

While the most stringent terrestrial constraint is:

Operation of FLASH, a free-electron laser, http://vuv-fel.desy.de/.

A free-electron laser generates highly collimated X-rays parallel to the relativistic electron beam that is their source. If the region that generates the X-rays is L meters long, and the speed of light emitted from the moving electrons is c+kv (here v is essentially c), then at the downstream end of that region the minimum pulse width is k(L/c)/(1+k), because light emitted at the beginning arrives before light emitted at the downstream end. For FLASH, L=30 meters, v=0.9999997 c (700 MeV), and the observed X-ray pulse width is as short as 25 fs. This puts an upper limit on k of 2.5×10−7. Optical extinction is not present, as the entire process occurs in very high vacuum.

This one is fun just because the experimenter was anti-relativity but his experiment proves the point you are asking about:

Beckmann and Mandics, “Test of the Constancy of the Velocity of Electromagnetic Radiation in High Vacuum”, Radio Science, 69D, no. 4, pg 623 (1965).

A direct experiment with coherent light reflected from a moving mirror was performed in vacuum better than 10−6 torr. Its result is consistent with the constant velocity of light. This experiment is notable because Beckmann was a perennial critic of SR. Optical Extinction is not a problem.

I understand that there are many experiments performed agreeing with Relativity, but they all assume that the velocity of light is always "c" in the first place.

This is simply factually untrue. When testing relativity or measuring the speed of light, you cannot and do not assume it is c. That would defeat the whole point of the experiment. This is a completely false and very uninformed complaint, particularly when it is made so broadly such that all experiments are implicated.

 

[Grampa Dee]

Not really absorbed and re-emitted. Check out these videos.

Very interesting videos, Dave...thanks.

I did know that the process of light refraction as such was much more complicated than simply an absorption and emission process; however, the process of light interfering with matter seems, in my opinion, enough to disqualify the moving source, such as the sun, as being the true source.

 

[Grampa Dee]

Basically, the highest energy particles at CERN have energies of about $6.5TeV$. In classical terms that equates to a speed of about $3 \times 10^{10}m/s$, or about 100 times the speed of light(!)

Clearly, that is not what is happening. So, the classical formula for KE of a particle ($\frac 1 2 mv^2$) is not generally valid. Not to mention that particles of greater total mass emerge from these experiments using the relationship $E = mc^2$ etc.

This is point one: classical (Newtonian) physics cannot be valid in the realm of high-energy sub-atomic particles. Something has to change. We need some new ideas! (These new ideas came in 1905, by the way, so they are not so new anymore.)

It's not a question of whether classical Lagrangian mechanics and classical KE can explain the last 100 years of experimental physics. It can't. The question is whether SR can explain particle accelerator experiments. If SR wants to prove itself, then that's what it has to do: successfully predict the outcome of particle accelerator experiments. Which it does and which is why it is generally accepted.

Note that just because you make your predictions using the assumptions of SR doesn't mean the experiments have to fall in line. If you assume something that is false, then the physical experiments will expose the mistake. The experiments at CERN are not a self-fulfilling prophecy. The experiments are an independent test of the theory.

That's how physics work.

Thank you for your response, PeroK;

There is much in what you've written and I'll digest what you wrote before commenting .
I will only add this; In my opinion, it doesn't make any difference how "much" energy one gives into a system, but only at what speed does the vehicle of energy interacts with the system. For example, if a small car or a big truck rolling at 25mph hits, let's say a box; while the power will be different for each case, the box will never go beyond 25mph in either case. It seems, therefore, that the electron could never speed beyond "c" relative to the source of electromagnetic waves.

 

[Grampa Dee]

So for example, the most stringent astronomical constraint is:While the most stringent terrestrial constraint is:This one is fun just because the experimenter was anti-relativity but his experiment proves the point you are asking about:This is simply factually untrue. When testing relativity or measuring the speed of light, you cannot and do not assume it is c. That would defeat the whole point of the experiment. This is a completely false and very uninformed complaint, particularly when it is made so broadly such that all experiments are implicated.

Thank you Dale: There is a lot in your post to digest, so, I'll reply to it later.

 

[PAllen]

Thank you for your response, PeroK;

There is much in what you've written and I'll digest what you wrote before commenting .
I will only add this; In my opinion, it doesn't make any difference how "much" energy one gives into a system, but only at what speed does the vehicle of energy interacts with the system. For example, if a small car or a big truck rolling at 25mph hits, let's say a box; while the power will be different for each case, the box will never go beyond 25mph in either case. It seems, therefore, that the electron could never speed beyond "c" relative to the source of electromagnetic waves.

What do you think about the scenario of someone with a large basket of baseballs, sitting on a cart on a low friction track. They start throwing them off the rear of the cart. Per Newtonian mechanics, there is no upper limit to the speed of the car relative to the track that can be achieved this way.

Independently, your statement is just nonsense per Newtonian mechanics. In Newtonian mechanics, if a ball being stopped by a wall is measured to have delivered a certain amount of energy to the wall, this uniquely determines its speed, given its mass. It does not matter how it got that kinetic energy.

It seems you either don’t understand either Newtonian mechanics or relativity, or you reject both.

 

[DaveE]

I will only add this; In my opinion, it doesn't make any difference how "much" energy one gives into a system, but only at what speed does the vehicle of energy interacts with the system. For example, if a small car or a big truck rolling at 25mph hits, let's say a box; while the power will be different for each case, the box will never go beyond 25mph in either case. It seems, therefore, that the electron could never speed beyond "c" relative to the source of electromagnetic waves.

Aside from the fact that even Newton would disagree with this, you are going to have rough going if you think relativistic electrons and particle accelerators can be compared to cars and trucks at 25mph.

Also, most of us think how much energy you put into a system is really, really important; much more so than velocity.

 

[PAllen]

Thank you for your response, PeroK;

There is much in what you've written and I'll digest what you wrote before commenting .
I will only add this; In my opinion, it doesn't make any difference how "much" energy one gives into a system, but only at what speed does the vehicle of energy interacts with the system. For example, if a small car or a big truck rolling at 25mph hits, let's say a box; while the power will be different for each case, the box will never go beyond 25mph in either case. It seems, therefore, that the electron could never speed beyond "c" relative to the source of electromagnetic waves.

Actually, just using your example, Newton says you are flat out wrong. Using specific numbers, if a 10 kg ball moving at 10 m/s hits a 1 kg ball, and the collision is elastic (no KE converted to heat), then the 1 kg ball will have velocity 18.182 (appx) after the collision (the 10 kg ball will have velocity about 8.182).

 

[PeroK]

Actually, just using your example, Newton says you are flat out wrong. Using specific numbers, if a 10 kg ball moving at 10 m/s hits a 1 kg ball, and the collision is elastic (no KE converted to heat), then the 1 kg ball will have velocity 18.182 (appx) after the collision (the 10 kg ball will have velocity about 8.182).

And, we see this phenomenon in sports all the time: tennis balls going faster than the racket; a golf ball going faster than the clubhead; and, baseballs very occasionally getting hit out of the park.

And, indeed, with there would be no explanation for a bullet from a gun moving at all, given that the gun itself has no initial speed!

 

[Nugatory]

What if, the GPS, instead, used the different frequencies (red shift/blue shift) for different light speeds?

The GPS system depends (with oversimplification appropriate for this B-level thread) on the receiver being able to calculate the exact distances between it and multiple GPS satellites. It does this by taking the time $T1$ at which it receives a signal from the satellite saying "I sent this signal at time $T0$", subtracting $T0$ from $T1$ to get the time it took the signal to cover the distance between it and the satellite, then multiplying by the speed of light (the good old distance-speed-time relationship that we all learned in elementary school) to get the distance to where the satellite was when the signal was emitted.

It should be obvious that this procedure would not give the correct distances.if the speed of light were not constant and independent of any red/blue shifts from the relative motion of the satellite and receiver.

 

[DaveE]

OK, so, a lot of us are pointing out that what you are thinking about this stuff is incorrect. Don't think it's meant as an insult. Trust me, we all get things wrong sometimes. Much better for you to get correct answers to your questions than people leading you on a path of misunderstanding. That's really what PF is for, pointing you in the right direction.

If you find these questions interesting, then you can study, at whatever pace you like, to learn more. I would start with @PAllen's reply:

Actually, just using your example, Newton says you are flat out wrong. Using specific numbers, if a 10 kg ball moving at 10 m/s hits a 1 kg ball, and the collision is elastic (no KE converted to heat), then the 1 kg ball will have velocity 18.182 (appx) after the collision (the 10 kg ball will have velocity about 8.182).

How can you use conservation of momentum and conservation of energy to arrive at the same conclusion as he did? What if they don't travel in a straight line so that velocity and momentum are vectors?

 

[Grampa Dee]

So for example, the most stringent astronomical constraint is:While the most stringent terrestrial constraint is:This one is fun just because the experimenter was anti-relativity but his experiment proves the point you are asking about:This is simply factually untrue. When testing relativity or measuring the speed of light, you cannot and do not assume it is c. That would defeat the whole point of the experiment. This is a completely false and very uninformed complaint, particularly when it is made so broadly such that all experiments are implicated.

Thank you Dale; this is pretty much what I was looking for...I will try to read on these experiments.

 

[Grampa Dee]

What do you think about the scenario of someone with a large basket of baseballs, sitting on a cart on a low friction track. They start throwing them off the rear of the cart. Per Newtonian mechanics, there is no upper limit to the speed of the car relative to the track that can be achieved this way.

Independently, your statement is just nonsense per Newtonian mechanics. In Newtonian mechanics, if a ball being stopped by a wall is measured to have delivered a certain amount of energy to the wall, this uniquely determines its speed, given its mass. It does not matter how it got that kinetic energy.

It seems you either don’t understand either Newtonian mechanics or relativity, or you reject both.

Yes, PAllen, I don't know what I was trying to convey . I was certainly wrong in my example as it destroys the conservation of momentum. I certainly am not a pro in understanding science; However, I do not reject anything, but do have many questions about the subject, which might seem to you a form of rejection.
Dale is pretty close to what I'm asking though...
The theory of relativity is based ( or seems to be) on the invariant speed of light. If this can be proven, than clearly all else must stand.

 

[Grampa Dee]

The GPS system depends (with oversimplification appropriate for this B-level thread) on the receiver being able to calculate the exact distances between it and multiple GPS satellites. It does this by taking the time $T1$ at which it receives a signal from the satellite saying "I sent this signal at time $T0$", subtracting $T0$ from $T1$ to get the time it took the signal to cover the distance between it and the satellite, then multiplying by the speed of light (the good old distance-speed-time relationship that we all learned in elementary school) to get the distance to where the satellite was when the signal was emitted.

It should be obvious that this procedure would not give the correct distances.if the speed of light were not constant and independent of any red/blue shifts from the relative motion of the satellite and receiver.

Thank you, Nugatory; I'll read up on this ... understand that my original question did not centre on this...I'm not an expert on these particular issues, being the reason why I wanted to start with the basics...being the invariant speed of light as such..

 

[Grampa Dee]

OK, so, a lot of us are pointing out that what you are thinking about this stuff is incorrect. Don't think it's meant as an insult. Trust me, we all get things wrong sometimes. Much better for you to get correct answers to your questions than people leading you on a path of misunderstanding. That's really what PF is for, pointing you in the right direction.

If you find these questions interesting, then you can study, at whatever pace you like, to learn more. I would start with @PAllen's reply:
How can you use conservation of momentum and conservation of energy to arrive at the same conclusion as he did? What if they don't travel in a straight line so that velocity and momentum are vectors?

Thank you Dave: I think that I am trying to respond to too many people at the same time, and making huge errors in doing so.There are many side views of Relativity here that are too developed for me at this moment.
I was not trying to reply in a way to correct them, it is just that all those particular experiments, GPS or particle accelerators do not seem, at this moment, to be direct proofs for me .

I thank you for giving me those videos and explanations...I am not taking this as an attack...I am simply interested in learning more about the subject as a hobby, no more.

 

[Grampa Dee]

Basically, the highest energy particles at CERN have energies of about $6.5TeV$. In classical terms that equates to a speed of about $3 \times 10^{10}m/s$, or about 100 times the speed of light(!)

Clearly, that is not what is happening. So, the classical formula for KE of a particle ($\frac 1 2 mv^2$) is not generally valid. Not to mention that particles of greater total mass emerge from these experiments using the relationship $E = mc^2$ etc.

This is point one: classical (Newtonian) physics cannot be valid in the realm of high-energy sub-atomic particles. Something has to change. We need some new ideas! (These new ideas came in 1905, by the way, so they are not so new anymore.)

It's not a question of whether classical Lagrangian mechanics and classical KE can explain the last 100 years of experimental physics. It can't. The question is whether SR can explain particle accelerator experiments. If SR wants to prove itself, then that's what it has to do: successfully predict the outcome of particle accelerator experiments. Which it does and which is why it is generally accepted.

Note that just because you make your predictions using the assumptions of SR doesn't mean the experiments have to fall in line. If you assume something that is false, then the physical experiments will expose the mistake. The experiments at CERN are not a self-fulfilling prophecy. The experiments are an independent test of the theory.

That's how physics work.

Hi PerocK:

I wanted to answer back this particular post...I don't know what I was thinking in my last response; it was soon wrong...
I will try to explain a little better; however I am not trying to claim what I'm about to write as fact, but simply something that bothers me.

Take a light coming towards you; it has energy. If you change your velocity away from the light, it will be red shifted, meaning that it has now less energy...now, again, I'm not claiming that this is what is happening to the electron relative to the magnetic field, I'm mearly asking the question.
Dale has given me enough material for me to search, so I probably won't be back for a little while.

Thank you for your help everyone.

 

[Nugatory]

it is just that all those particular experiments, GPS or particle accelerators do not seem, at this moment, to be direct proofs for me .

Of course they aren't - no experiment is ever a direct proof of any physical theory. Instead, experiments disprove theories, by demonstrating results that are inconsistent with the predictions of the disproven theory.

 

[Dale]

The theory of relativity is based ( or seems to be) on the invariant speed of light. If this can be proven, than clearly all else must stand.

It is proven, not in the deductive sense of a mathematical proof, but in the inductive “beyond a reasonable doubt” sense.

For that, you should not rely merely on the three experiments I highlighted addressing your specific interest. Instead you should consider the totality of the evidence. Any alternative to relativity must explain ALL of those experiments. Not merely in vague qualitative terms, but with specific quantitative predictions matching the experimental results for each experiment.

 

[jbriggs444]

The theory of relativity is based ( or seems to be) on the invariant speed of light. If this can be proven, than clearly all else must stand.

This is an attractive way to think about things. But it is (in my opinion) also a wrong way to go about deciding where to look for evidence in favor of relativity. Or for a proof of the validity of the theory.

Yes, the theory of relativity is based on the postulated invariance of the speed of light. However, the original paper is not based on any experimental proof that the speed of light is invariant.

In mathematics, a "postulate" is not a self-evident truth. It is a statement that is accepted without proof. [One could go into depth on that and talk about things like the parallel postulate, the continuum hypothesis or the axiom of choice, but that is material for another sub-forum]

Maxwell's equations had suggested that the speed of light should be invariant. You measure a couple of parameters, take the square root of the inverse of their product and out pops the velocity of an electromagnetic wave according to those equations. Which matches the speed of light.

It would be nice if this velocity were an invariant -- the same in every reference frame. Unfortunately, according to our standard way of translating from one reference frame to another, there was no such thing as an invariant velocity. According to our standard translation scheme (Galilean relativity), if you switch to a new frame moving at +v compared to the old, all of your old velocities get v subtracted from them as a vector. The new speed of light should be c+v, c-v or something in between depending on the angle of the light compared to the relative motion of your two frames.

But what if the speed of light actually were an invariant? Always equal to c regardless of frame. Take that as a postulate. See if it is possible to build a coherent system of physics that adheres to that postulate without reaching a contradiction.

It turns out to be possible to do so. The resulting theory gives up some properties of time that we had previously taken for granted. But it does not reach an outright contradiction. The resulting theory (Special Relativity) also makes some unexpected predictions.

When experiment conforms to those predictions and fails to conform to the Newtonian predictions, that is what makes us take special relativity seriously.

 

[cianfa72]

Maybe is not entirely on topic, anyway we are actually talking about the constancy/invariance of the two-way (round-trip) speed of the light from an experimental point of view.

BTW one-speed of light is actually a matter of convention. I found this old post from @PAllen

Let's say you build two atomic clocks right next to each other, get them exactly synchronized, and move them slowly 1 meter apart, with exactly symmetric acceleration profile. You assume this keeps them synchronized. And it does - exactly equivalently to as if you synched them using Einstein's clock sync based on assuming isotropy plus invariance of two way light speed. Maybe not emphasized enough is that any procedure that relies on any assumption of isotropy is informationally worthless for resolving one way light speed, because it builds in the answer given invariance of two way light speed. The assumption that slow separation with the same acceleration profile (in opposite directions) leaves the clocks synched is an isotropy assumption [it also builds in a homogeneity assumption].

Isotropy & invariance of two-way speed of light results in constancy/invariance of the one-way speed of light, I believe.

 

[berkeman]

Thread closed for Moderation...

 

[berkeman]

After a Mentor discussion, thread is re-opened...

 

[Grampa Dee]

So for example, the most stringent astronomical constraint is:While the most stringent terrestrial constraint is:This one is fun just because the experimenter was anti-relativity but his experiment proves the point you are asking about:This is simply factually untrue. When testing relativity or measuring the speed of light, you cannot and do not assume it is c. That would defeat the whole point of the experiment. This is a completely false and very uninformed complaint, particularly when it is made so broadly such that all experiments are implicated.

Beckmann and Mandics, “Test of the Constancy of the Velocity of Electromagnetic Radiation in High Vacuum”, Radio Science, 69D, no. 4, pg 623 (1965).

A direct experiment with coherent light reflected from a moving mirror was performed in vacuum better than 10−6 torr. Its result is consistent with the constant velocity of light. This experiment is notable because Beckmann was a perennial critic of SR. Optical Extinction is not a problem.

Perfect! This is exactly what I was looking for; thank you Dale...I downloaded a description of the experiment and will try to understand the process.

One might ask why do I not simply accept a theory being over a century old, having good observational evidence ? For me, it is because of the manner the theory came into being.
After Young's double slit experiment, light was proven to be a wave; so, now, a medium was needed which was called the "ether". Next, the Earth must travel also within this ether, hence the M&M experiment. But there was a nul result, meaning no velocity was detected.
Afterwards, it seemed things started to go a little weird, in my opinion. Fitzgerald, I believe, claimed that there must be a length contraction in the direction of the Earth's movement relative to the ether.
The contraction is in fact the Lorentz transformation. Next,Einstein claimed that there is no ether, but the Lorentz transformation still stands as the velocity of light is now postulated as being invariant. Besides, there was Ritz' hypothesis that claimed the velocity of light was source dependant.
Now all three theories mathematically agreed with the M&M experimental observation.
For the ether theory, the Lorentz transformation was used, Relativity didn't need the transformation at all since everything was stationary within the experiment, and the light's velocity for the Ritz's theory was also "c" since the source was stationary as well.
So, my question was what evidence do we have for the invariance velocity of light? While the DeSitter's observation supported the invariant velocity of light, we needed to be certain that the light observed had not past through some medium such as H or He gas clouds, for then, the (c+v)(c-v) effect would have been lost.
Therefore, for me personally, an experiment highly supporting the invariant velocity of light was needed...afterwards, everything else would then follow, for all the paradoxical elements in the relativistic observations, are due to the postulate in the first place...
I am satisfied with what you've given me, I thank you again...this thread is over for me, personally..I may however answer a couple of more posts because I want to try to correct the unforgivable error I wrote in one of my post. :)

 

[berkeman]

Aaand, thread is closed again for more Moderation...