# Classical explanation for Fizeau & Sagnac

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Fizeau experiment and Sagnac effect are explained completely and correctly only using special relativity (Lorentz transformations), although the speeds involved are far smaller than c (the speed of light in vacuum).

Don't you think that we should have a "classical" explanation for them?

Do you know any?

As I wrote here, Sagnac effect is explained in a classical way only when light is travelling through vacuum or air. When the refractive index is greater than 1, SR is used.

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Fizeau experiment and Sagnac effect are explained completely and correctly only using special relativity (Lorentz transformations), although the speeds involved are far smaller than c (the speed of light in vacuum).

Don't you think that we should have a "classical" explanation for them?

Why? If there was a classical explanation for relativistic effects, we wouldn't need relativity. Relativity need not be invoked only for things traveling close to c — that's simply where the effects cannot be ignored under any circumstances. The ability to measure relativistic effects is also a function of the sensitivity of the measuring device. You could measure kinematic time dilation with a car going 100 km/hr if the clock was good enough — we're right on the edge of doing it with standard atomic clocks. And we already know it works on a plane, going just a few times faster than that.

Interferometry is often a fairly sensitive measuring technique.

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Why? If there was a classical explanation for relativistic effects, we wouldn't need relativity. ...

It is important, because, as I said, the speeds are not relativistic, so these are not relativistic effects. A good explanation/theory of how light travels in transparent materials can and should cover this. I have such a theory and, as I said, this has almost nothing to do with relativity.

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It is important, because, as I said, the speeds are not relativistic, so these are not relativistic effects. A good explanation/theory of how light travels in transparent materials can and should cover this. I have such a theory and, as I said, this has almost nothing to do with relativity.

And my response is that "relativistic" depends on how well you can measure. It's not a naive "going near the speed of light". With a good enough clock, for example, 100 kph is relativistic. Moving up or down a meter in elevation is relativistic. Interferometers are generally sensitive devices. Therefore, they may be able to measure relativistic effects at relatively low speeds.

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And my response is that "relativistic" depends on how well you can measure. It's not a naive "going near the speed of light". With a good enough clock, for example, 100 kph is relativistic. Moving up or down a meter in elevation is relativistic. Interferometers are generally sensitive devices. Therefore, they may be able to measure relativistic effects at relatively low speeds.

Ok, but this doesn't mean that we have to use GR, and only GR, to describe the movement of a falling ball. We can predict well enough with Newton's laws, when the speeds are less than 100kph. You agree?

And about the fact that "interferometers are generally sensitive devices", you should keep in mind that Sagnac effect in air/vacuum (rotating plate with mirrors) is very well explained in a classical manner, so there is not a problem of sensitivity ...

By the way, in Fizeau experiment, the result given by SR is not exactly the measured one. It tends to it (see the site), but it is just an aproximation ... The refractive index changes with wavelenght, so, to be really accurate, we should consider that. We have redshift/blueshift there ...

So, considering the above, we should have a classical explanation for Fizeau & Sagnac. Do you know one? Anyone?

Edited by DanMP
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Ok, but this doesn't mean that we have to use GR, and only GR, to describe the movement of a falling ball. We can predict well enough with Newton's laws, when the speeds are less than 100kph. You agree?

No. It depends on how well you can make the measurement. It may well be that the deviation is so small that such a measurement is not practical or possible. The difference between relativity and classical physics is small but not non-existant. So it all depends on what you are measuring. One cannot simply assume that there is a classical explanation for any phenomenon that is peculiar to relativity. It might exist, to a high degree of accuracy, but it might not.

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... You could measure kinematic time dilation with a car going 100 km/hr if the clock was good enough — we're right on the edge of doing it with standard atomic clocks. ...

So, "we're right on the edge" of measuring "kinematic time dilation with a car going 100 km/hr" ... That means that, for such speeds, we are not yet able to even detect the relativistic effects, but you still consider that we should use relativity and only relativity to describe that motion?!?

Edited by DanMP
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So, "we're right on the edge" of measuring "kinematic time dilation with a car going 100 km/hr" ... That means that, for such speeds, we are not yet able to even detect the relativistic effects, but you still consider that we should use relativity and only relativity to describe that motion?!?

No, that's not what I was saying. In fact, I already told you that it was not what I was saying. This is nothing but a straw man.

I said with standard atomic clocks (namely a 5071A). Use a better one, or an ensemble, and you would be able to measure it. Or go a bit faster, like on a plane. There would be no classical explanation for the discrepancy between a moving and stationary clock. The point being that even for travel nowhere close to c, you have effects that can't be explained classically. As I said before, "relativistic" is not the naive, pop-sci, boiled-down convention that speeds must be close to c. That's the story you get at the kids table.

Thus, it is not reasonable to assume that a relativistic effect must have a classical explanation, just because the speeds are well below c. It depends on the details, and requires analysis on a case-by-case basis. You have not done this.

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No, that's not what I was saying....

The point being that even for travel nowhere close to c, you have effects that can't be explained classically....

Yes, if you can measure such effects, with very accurate instruments, than the only way to explain them is by using relativity, but as I wrote above, the measurements made in Fizeau and Sagnac are not that accurate. Note that SR explains Fizeau quite loosely, ignoring the changes in refractive index and the fact that the laboratory frame is not quite inertial. Also, the classical result for Sagnac effect in air is as accurate as the SR one, beeing the same. So where exactly is the tiny difference that you call a relativistic effect?

...It depends on the details, and requires analysis on a case-by-case basis. You have not done this.

I did. You want to see it?

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Yes, if you can measure such effects, with very accurate instruments, than the only way to explain them is by using relativity, but as I wrote above, the measurements made in Fizeau and Sagnac are not that accurate. Note that SR explains Fizeau quite loosely, ignoring the changes in refractive index and the fact that the laboratory frame is not quite inertial. Also, the classical result for Sagnac effect in air is as accurate as the SR one, beeing the same. So where exactly is the tiny difference that you call a relativistic effect?

I didn't say there was one. I said you can't assume v<<c automatically means it's not relativistic.

I did. You want to see it?

That would be nice.

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I didn't say there was one. I said you can't assume v<<c automatically means it's not relativistic.

Relativistic effect is only the difference between the classical result and the relativistic one. And for low speeds, this difference is very small, usually very hard to detect. This is the "trend", as we seen in the car going 100 km/hr example and Sagnac in air example.

So, in the same way we still use classical physics for cars, we can and should have a complete classical explanation for Sagnac and Fizeau. Do you agree?

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Do you actually know the solutions to Sagnac for example?

you have a circle radius r - with angular velocity w you can calculate Delta t as follows

That seems to me as a very non-relativistic calc.

And BTW can you drop using the term classical as an opposite to relativistic - classical physics tends to be used as that which does not include Quantum Mechanics not that which does not include Einstein's Relativity

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This is the Sagnac in air/vacuum example I mentioned at least 3 times above.

How about the fiber optic conveyor depicted in Fig.5 of the wiki page?

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Relativistic effect is only the difference between the classical result and the relativistic one. And for low speeds, this difference is very small, usually very hard to detect. This is the "trend", as we seen in the car going 100 km/hr example and Sagnac in air example.

So, in the same way we still use classical physics for cars, we can and should have a complete classical explanation for Sagnac and Fizeau. Do you agree?

Nobody agrees?

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• 2 weeks later...

Nobody agrees?

swansont, you wrote some things to consider. Between them there is:

6. When you have been shown to be wrong, acknowledge it.

You don't have to observe this rule?

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swansont, you wrote some things to consider. Between them there is:

6. When you have been shown to be wrong, acknowledge it.

You don't have to observe this rule?

Where was I shown to be wrong?

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Where was I shown to be wrong?

imatfaal posted here a non-relativistic explanation for Sagnac effect. It is incorrect (he lost an r; typical for imatfaal) and the same thing I mentioned above it 3 times, but the point is that it shows that a non-relativistic solution is possible. You insisted that Sagnac effect is relativistic. I said:

Relativistic effect is only the difference between the classical result and the relativistic one. And for low speeds, this difference is very small, usually very hard to detect.

You never agreed to that, so please agree or explain how imatfaal solution is entirely relativistic.

Edited by DanMP
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imatfaal posted here a non-relativistic explanation for Sagnac effect. It is incorrect (he lost an r; typical for imatfaal) and the same thing I mentioned above it 3 times, but the point is that it shows that a non-relativistic solution is possible. You insisted that Sagnac effect is relativistic. I said:

Where did I insist on that?

What I did say was

I didn't say there was one. I said you can't assume v<<c automatically means it's not relativistic.

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Where did I insist on that?

What I did say was

You never agreed with

Relativistic effect is only the difference between the classical result and the relativistic one. And for low speeds, this difference is very small, usually very hard to detect.

so it appears that you consider Fizeau & Sagnac entirely relativistic. If you do, please explain how imatfaal solution is entirely relativistic. If you don't, please agree with my statement above, statement that implies that we should have a (complete, better) non-relativistic explanation for Fizeau & Sagnac.

This is post #19. How many more do you need to agree with that or anything from me?

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You never agreed with

so it appears that you consider Fizeau & Sagnac entirely relativistic.

It would be a mistake to equate a non-response to a specific topic I was never discussing as agreement. (The fallacy of argument from silence). My sole discussion point has been your erroneous claim that v<<c means that there must be a nonrelativistic explanation for a result. I provided examples of this not being true.

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imatfaal posted here a non-relativistic explanation for Sagnac effect. It is incorrect (he lost an r; typical for imatfaal) and the same thing I mentioned above it 3 times, but the point is that it shows that a non-relativistic solution is possible. You insisted that Sagnac effect is relativistic. I said:

A cheap shot from someone who has posted little to no maths - I will leave you to your ignorance from now on. Sorry to have disappointed you.

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My sole discussion point has been your erroneous claim that v<<c means that there must be a nonrelativistic explanation for a result. I provided examples of this not being true.

Where? If you know examples, other than Fizeau exp. and Sagnac effect in materials (n>1), where, with v<<c and no GR effects, we have only a relativistic explanation for a result, please provide them.

A cheap shot from someone who has posted little to no maths - I will leave you to your ignorance from now on. Sorry to have disappointed you.

Sorry for the "shot". Your mistakes were indeed small, but I was bothered by the fact that you didn't acknowledged the second one, the one where you considered I made a humongous mistake, when in fact what I claimed was correct: all inertial frames/observers agree with the differences between H-K exp. clocks.

My Fizeau/Sagnac solution includes maths and agrees with the relativistic ones. Still, I'm pretty sure that you will be not satisfied or interested ... You prefer to remain ignorant about how light really travels trough transparent materials (and many more).

Edited by DanMP
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Where? If you know examples, other than Fizeau exp. and Sagnac effect in materials (n>1), where, with v<<c and no GR effects, we have only a relativistic explanation for a result, please provide them.

Clock measurements. Earlier in this thread.

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Clock measurements. Earlier in this thread.

with v<<c and no GR effects

For the car going 100km/h we do have a pretty good non-relativistic model/result.

We need to get to page 3 until you admit it?

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For the car going 100km/h we do have a pretty good non-relativistic model/result.

We need to get to page 3 until you admit it?

The nonrelativistic calculation does not predict the result to arbitrary accuracy, and the difference can be measured for v<<c. v/c in this case is 10^-7. So we could be going several orders of magnitude faster and still be in the v<<c regime. 100x faster would mean ~10,000x increase in the effect.

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