Is the Sagnac Effect Misleading in Understanding Length Contraction?

[PeterDonis]

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Edit: thread reopened.

 

[PeterDonis]

The Sagnac effect is a phase shift observed between two beams of light traversing in opposite directions the same closed path around a rotating object.

This description, as it stands, is ambiguous. Take the example of a ring laser gyro. By "closed path around a rotating object", do you mean that light beams travel in opposite directions around the ring, which is itself rotating? (In this case, no other objects of significance would be present.) Or do you mean that light beams travel in opposite directions around the ring, which is not itself rotating, but which has a rotating massive object at its center?

 

[rude man]

You're not claiming that the experiment can be described using general relativity. You're claiming that the experiment can only be described using general relativity, which is claiming that the effect is not present in flat spacetime. That is a much stronger claim which you have not justified.

Thread closed for moderation.

Edit: thread reopened.

Oh, thank you.

From the French Wikipedie: http://www.ecoresults.info/search.p...000000;GFNT:0000FF;GIMP:0000FF;FORID:11&hl=en

"… On the contrary, since this emitter-receptor is in rotation, its reference is no longer inertial and therefore special relativity does not permit the direct determination of the observed shift."

And:

In employing general relativity one finds a shift ….

I worked in Honeywell's world-leading FOG program for over 20 years and can promise you I read that fact in English also, just don't have access to those papers any more.

So it would seem that the French wikipedie is somewhat better informed than the English wikipedia.

 

[rude man]

This description, as it stands, is ambiguous. Take the example of a ring laser gyro. By "closed path around a rotating object", do you mean that light beams travel in opposite directions around the ring, which is itself rotating? (In this case, no other objects of significance would be present.) Or do you mean that light beams travel in opposite directions around the ring, which is not itself rotating, but which has a rotating massive object at its center?

If you need more info on the FOG there are lots of papers available on the Web. And I assume you know that the ring laser gyro operates on the Sagnac principle also.

 

[PeterDonis]

I assume you know that the ring laser gyro operates on the Sagnac principle also.

Yes, of course. Nobody is questioning that the Sagnac effect exists or that ring laser gyros use it. What I am questioning is your claim that GR is required to explain it. The sources you give only show that GR can be used to explain it--and even here your claim is not based on the actual content of the sources you have given, but simply on the fact that they use the words "general relativity" in various places. That is not a valid argument.

What you need to do is show, based on the actual content of these papers, that they use curved spacetime in their models, since the use of curved spacetime is the key content that distinguishes GR from SR. You have not done this. In fact, the quotes you do give from your sources indicate that they, like you, are confused on this point. For example:

"… On the contrary, since this emitter-receptor is in rotation, its reference is no longer inertial and therefore special relativity does not permit the direct determination of the observed shift."

This claim is simply false. Using a non-inertial frame in flat spacetime is still SR. A rotating frame in flat spacetime is still SR. And the Sagnac effect can be predicted using exactly these tools (though they are not needed to make the prediction--see below).

What's more, the apparent implication of the above quote that the Sagnac effect cannot be derived using an inertial frame in flat spacetime (which would unproblematically be "special relativity" even by their definition) is also false. Deriving the prediction of a phase shift using an inertial frame is simple; as I said in a previous post, even the Wikipedia page on the Sagnac effect does it. (It's true that the motion of the emitter-receptor in the ring laser is not inertial, but non-inertial motion of objects does not require a non-inertial frame to describe it.) This clearly shows that "special relativity", even by the strictest possible definition of that term, can explain the Sagnac effect, contrary to your claim that GR is required to do so.

 

[PeterDonis]

Thread closed again for moderation. rude man, if you want to discuss further, please PM me.