aramis720

We're going in circles here. The speed of light is invariant with respect to what? I understand, of course, that c is invariant, but various parties have offered here that it is the invariant speed of light that gives rise to detectable effects from the interferometer. I'm asking how this is the case. If you assert that the interferometer can detect grav waves b/c of a difference in phases of the two light signals sent up and back in each arm, there must be a change in wavelength or length of the arms or both. Most here have agreed that there is no change in either, so what will happen? No difference in phase overlaps of the two light signals. "Time" in this context is inferred from change in phase fringes, but there can't be any change in phase fringes based on the definitions offered in this context.

"Change" implies detectable change. So if the arms are waving to exactly the same degree as the space they occupy (as they must under the definitions of these terms in this context), then it's entirely undetectable. So the lengths of the arms are not changing. Which brings us back to my initial question: what is being measured? So how in this case is the speed of light being changed by grav waves, and in such a way that any change in speed would be detectable by an interferometer?

Why would the invariance of c in this case lead to any length change? Again, the grav wave is waving space itself. The speed of light would be as affected by this wave as anything else in that space, as we know from various experiments finding the bending of light from gravitational masses like in 1919.

But the length of the arms isn't changing. That's been my point all along. There's no contradiction. The wave forms reported in the experiment are modeled and then data is matched to the modeled wave forms. Since both detectors got the same signals something was detected, yes, but if my objections here are correct that detection event wasn't from grav waves. We certainly shouldn't put the cart before the horse and argue that the premises must be correct b/c they found what they were looking for. If the premises aren't correct the experiment can't work, period. We then should look to what was detected in the apparent false positive. Again, I'm almost certainly wrong about my objections here, since I'm not even a physicist. But I do follow this stuff pretty closely and I still haven't seen any responses to my concerns that satisfactorily explain how the alleged detection event is taking place.

The point of my OP was that the premises underlying this experiment seem questionable. False positives happen pretty regularly in science.

I could have stated more clearly my summary of your previous statements, I'll agree on that, but my point remains. You had agreed that length and wavelength change to the same degree exactly as the wave, and thus are undetectable, that was my point. You now clarify that because the speed of light remains constant that the interferometer will register a difference. But think again about what is going on with the measurement apparatus. Light does not exist in some other realm than the space we exist in. So if space itself is being distorted so is any light occupying that space. You write:"Because if light takes a different amount of time to travel one arm than the other, it will have a different phase when it gets back." But why would it take a different amount of time to travel one arm? We know, of course, that light is affected by gravity in the same way as mass -- this was the basis for the famous 1919 Eddington experiment looking at the curving of light around the sun during a full eclipse. My point has been that there can't be any detectable length contraction b/c space itself is contracting, so anything occupying that space is contracted precisely the same amount -- and is thus undetectable. There is no contraction of either arm. Isn't that a pretty clear point?

You just stated in the previous post that you did agree that neither the arm nor the light wave are being contracted. Here's your quote: "you are correct: the length of the arm and "ruler" (wavelength of light) both change on the same way." Then you stated that even though the arm and the wavelength of light weren't changing, the speed of light must nevertheless change with respect to the interferometer b/c of the special relativity postulate that the speed of light is always constant. I then asked you why you made this conclusion b/c that's not how interferometers work. They work based on comparing phase overlaps. If the arm lengths or wavelengths change the phases won't overlap exactly. So there's no literal time measurement. Could you explain your reasoning further here? I'm always willing to consider that I may be mistaken (in fact, it's almost certain that I am mistaken here b/c of the vast weight of authority against me), yet no one here has answered my questions satisfactorily and there seems to be an ongoing confusion about my question. For example, your last two lines show that you don't understand my point about grav waves distorting space and anything in it. The point of anything in that space that is being waved also waving is that it makes such waves in principle undetectable b/c there is no physical difference that can be detected -- if indeed grav waves are defined as waves of space itself. So what I'm suggesting is the conceptual structure by which physicists are suggesting grav wave detection may in fact be erroneous. How? The only measurement is phase overlaps or lack thereof. There's no time measurement at issue here.

It seems that I'm not communicating well my basic point. Here it is: if a grav wave is defined, as it is, as a wave of space itself, anything occupying that space will wave to EXACTLY the same degree as the grav wave. So what can be used to detect such a wave when anything used to detect the wave must by necessity occupy the same space that is waving? Is this clear now? It doesn't matter what direction the wave is coming from. Any wave direction will wave space itself and anything occupying that space in exactly the same way.

Why is there a change in length? Again, a grav wave is defined as being a wave of space, so anything occuping that space will wave in exactly the same way as the grav wave. So why is there a change in length of the arm?

But it's not true that the time for the propagation of light is changing and thus detectable. What is being detected is a phase difference in the light signals in both arms. And if the phase is off this is interpreted as a change in the length of one or both arms. That's what is meant by a change in timing. So the change in timing is inferred from the phase difference, and that is due entirely to a change in the length of the arms OR a change in the wavelength. Since we both agree that there is no change in the arms or the wavelength from the grav waves we are back staring at the essential problem I raised in the OP.

I'm mainly questioning the basis for the experiment. False positives occur all the time in science and if the physical basis for this experiment is faulty then we're looking at false positives. Keep in mind also that the ability to tell where the apparent signal is coming from is extremely low granularity at this time b/c there are only two detectors. A third is coming online soon that will allow triangulation. So it's possible that we're seeing some kind of signal and a rough ex post explanation of where it's likely to come from, based on a faulty premise about how to detect grav waves. Now all of this is extremely unlikely, b/c I'm not even a Ph.D in physics, but this is why we have forums to discuss basic questions...

This isn't correct b/c the light wave and the interferometer arm are contracting to exactly the same degree and at exactly the same time as the grav wave -- again because the wave is a wave in space itself, so anything contained in that space will of course be moving precisely with the wave. So what exactly is the mechanism you're suggesting? Sounds like we agree that the arm does not contract in a detectable way, and nor does the wavelength -- both b/c the wave is a wave of space itself, and thus anything in that space is also waving to exactly the same degree as the grav. wave. If those two facts are true how is anything being detected.

Yes, I understand how interferometers work but no one here has explained why either of the arms of the interferometer is supposed to contract due to the grav wave. Again, the wave is defined as a wave of space itself, so anything (the arm or what have you) in that space will wave to exactly the same degree as the space itself that it occupies, thus the wave will be undetectable.

Let's zero in on the interferometer arm that is allegedly contracting. The arm is in the x direction of space, let's say. The grav wave comes in at the same x direction, waving up and down. So any physical object, whether it's an interferometer arm or a light wave, that occupies that space will wave to exactly the same degree as space itself. It won't matter how long or short the arm is because any distortion in that arm will be undetectable b/c it's distorting in exactly the same amount as space itself. Is that clear?

But what I'm suggesting is that as these terms are defined in the theory itself there is no contraction or expansion of these arms that is detectable b/c it's space itself that is contracting and expanding, so anything in that space will contract and expand in exactly the same degree as space. Changes in length of what? Space itself, right? So, if we agree that space itself is contacting, how can anything occupying that same space measure that contraction?

But again there is no detectable change in any dimension because the waves are literally waves of space, so anything occupying that space (whether it's falling masses or interferometer arms or a space gerbil) will be distorted by EXACTLY the same degree as space itself. So how is the oscillating squeeze of the ball you describe and as depicted on the wikipedia page taking place when there is no detectable change? Thanks for this additional link but I don't find it helpful b/c Shawhan is simply re-stating the conclusion as though it's an explanation. So the idea is this: the interferometer arm is stretched by the gravitational waves and therefore light takes a bit longer to travel that arm and therefore the phases of the light in the two arms doesn't match anymore, by a tiny amount (this is how all interferometers work). BUT, again, if the space in which the arm is located is stretching, the arm itself doesn't stretch in any detectable way. And therefore there is no additional distance for light to travel. It's not as though light continues to travel in a separate dimension at the "unstretched" speed. Light is occupying the same spacetime all physical things occupy. So we're back to the basic question of how LIGO is supposed to work.

I guess I'm still not seeing how the interferometer arm is detecting anything at all b/c if a gravitational wave is defined (as it is) as a ripple in spacetime itself, then anything in that ripple of spacetime will be distorted exactly the same degree to which spacetime is distorted. So how can an interferometer arm(s) occupying that spacetime detect the wave in any way?

Thanks, but it doesn't address the point I'm asking about: if space itself is waving/warping, how can any interferometer, no matter how long, detect it? It seems to me pretty clear that it can't, in principle, because any attempt to measure it will use tools that are distorted by exactly the same amount as space itself is warped. No? Thanks, but I don't see how measuring the speed of light gets around the problem I'm referring to. Any speed of light refers to distance traveled divided by time, but if that distance is distorted by gravitational waves any measuring instrument will also be distorted by exactly the same amount, making such detection in principle impossible, or so it seems to me. Or am I missing something?

I have a basic question that I've never actually seen answered in discussions of LIGO and gravitational waves: if these waves are warping space itself (actually spacetime), then all matter occuping that space will be warped to exactly the same degree that space is warped, making such warps in principle undetectable. So if an interferometer like LIGO, with two perpendicular arms, is set up to measure such waves, what is it actually measuring? Any distortion of the arms in the direction of the waves will not be detected because that arm(s) will be distorted to exactly the same degree that space itself is distorted. Help?