Steve3007 wrote: ↑September 5th, 2018, 2:57 am If anyone's interested, here's a Relativistic description of the Sagnac effect that I'm currently in the process of reading:I didn't find it very well explained. The effect seems intuitive (using classic explanation, not relativistic), but it took me a long time to glean the setup and thus to conclude that. The clocks are moving with the spinning disk, and they are at the same place as each other, so there is no synchronization issues.
http://www.physicsinsights.org/sagnac_1.html
I don't have any judgments on it yet because I haven't finished reading it.
The setup (clocks attached to disk in "Final case") is not shown until about 70% of the way through the article. Relativity doesn't significantly come into play, so the discussion in "peculiar case" is irrelevant (where they're syncing two linearly separated clocks in some frame).
If the signal is moving with the spinning disk, it must travel not just around the disk but the additional length to the clock that has moved away from the starting point. If signal is sent backwards, it meets the moving clock coming the other way. This effect far out-strips the relativistic effect of the clock running slow because it is moving in the frame of the disk. Similarly if I view a clock coming to me rapidly (linearly), the clock appears slowed due to dilation, but it speeds up due to Doppler effect, which is far more significant. The approaching clock appears to run fast, not slower. Receding clocks appear much slower because the two effects combine instead of cancel each other.
Some naive comments in the article: In fig 5, it says the two clocks are adjacent, and so "necessarily synced to each other when viewed from the lab frame". In fact, being adjacent at all times, they are necessarily synced to each other in any frame. They could be (and probably are) the same clock.
Another heading near the bottom announces: "Note that You Can't Synchronize the Clocks in a Rotating Frame"
Well you can if the clocks are adjacent, so this is irrelevant. There is no movement you can do to a pair of clocks to get them out of sync if they're always together.
Third paragraph from the top, the author seems to mistakenly take the wrong approach: "Throughout this page, I'm going to treat the effect using straight-line motion rather than circular motion as much as possible, which greatly simplifies the math."
In fact the math is unnecessary. I've described the effect by showing that the distance traveled by the signal is different one way than the other. I didn't need to express a formula to notice that.
Steve3007 wrote: ↑September 5th, 2018, 4:54 am In the interests of balance, here's another article about the Sagnac effect that appears, at a brief glance, to put the opposite point of view to the article I cited in my previous post:Where are you finding these amateur articles? This one is worse than the first one.
http://www.conspiracyoflight.com/Sagnac ... ndRel.html
I got as far as the first picture showing the two 'observers' and the red (with spin)/blue(against) arrows.
The text around that states: "If we now rotate the disk clockwise at [linear] velocity v, we find that the blue beam arrives back at the detector before the red beam – in fact, the difference in the velocity of light turns out to be 2*v, because the blue beam travels at C+v and the red beam at C-v"
where "C" means 'c' (light speed) except they should have mentioned that it is actually what the first article labels 'k' which is light speed through whatever medium they're sending it. The author apparently assumes that the refractive index is 1 and c can be used.
Anyway, the signal does not travel at c+v. Light moves at constant velocity c in any frame. You can't send a signal to Mars quicker by signalling down a fast-moving fibop cable or by firing a flashlight from a canon. This author doesn't know his science. If the red signal travels at c+v and blue at c-v, why does the blue signal reach the target first?
In fact, v does play a role if the refraction index is high. If the index is high, the light moves far slower than c and the v comes into play, speeding one signal and slowing the other, but only by a fraction of v. In fact, if you have say Glass with a refraction index of 3 and you move that glass at 1/3c in some frame, a light signal traveling through it against that movement will be stationary in that frame. If it is on our disk, it will have to wait for the detector to make one full lap. The signal going with the spin would have to go around 3 times to lap the detector going around twice. All that is without relativistic consideration, and there would be some adjustments at that speed.
The discussion goes on into the observer A on the disk: "On the other hand, the observer at A who is rotating with the disk has observed that the source and detector are stationary from his perspective, as are the paths, so he can only conclude (assuming no foreknowledge that he is rotating) that the light has actually travelled at two different velocities around the disk, being C+v and C-v. Who is right?"
This is nonsense. Observer A doesn't need to look out of the window to detect that he is spinning and which way. He knows damn well that the source/detector is moving, in any frame. He should expect the Sagnac effect.