B What experiments confirmed the constant velocity of light?

[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...