swansont Posted March 28, 2016 Share Posted March 28, 2016 I mentioned in another thread that we had a colloquium from one of the LIGO collaborators. (He was one of the mirror coating people, not one ofthe GR theorists). Afterwards, my colleagues and I were chatting, and oneasked an excellent question:If the gravitational waves stretched space between the mirrors, doesn'tthat also stretch the light wave as well? One of the explanations aboutdetecting GWs was that you couldn't just look at some object compared to ameter stick (conceptually, and magnified many times, obviously), becausethe meter stick would stretch just as the object did. You would never seeanything. So if the light stretches as the distance between the testmasses change, why does the phase of the light change? IOW, what is the error in this thinking? Link to comment Share on other sites More sharing options...
Robittybob1 Posted March 28, 2016 Share Posted March 28, 2016 it is a good question. One point I can think of is that the photons in the laser light will have the same energy in the two arms, the stretching of spacetime will not change their energy content so their frequency should stay the same, so do they get out of phase because the distance the laser has to travel in one arm changes compared to the other? Link to comment Share on other sites More sharing options...
Strange Posted March 28, 2016 Share Posted March 28, 2016 I found some really good sources when this came up before (on another forum, perhaps). The only one I have been able to track down again is this one: https://www.aapt.org/doorway/tgrutalks/Saulson/SaulsonTalk-Teaching%20gravitational%20waves.pdf Aha. And this one: http://stuver.blogspot.co.uk/2012/09/q-if-light-is-stretchedcompressed-by-gw.html 1 Link to comment Share on other sites More sharing options...
StringJunky Posted March 28, 2016 Share Posted March 28, 2016 I found some really good sources when this came up before (on another forum, perhaps). The only one I have been able to track down again is this one: https://www.aapt.org/doorway/tgrutalks/Saulson/SaulsonTalk-Teaching%20gravitational%20waves.pdf Aha. And this one: http://stuver.blogspot.co.uk/2012/09/q-if-light-is-stretchedcompressed-by-gw.html I made a very rough guess it was something to do with the arrival time to each section of the interferometer, which should be different because the SOG is finite. It is almost natural to initially assume, as I did, that the whole equipment would be 'wallowing' in the wobble at the same time, leading to no detection. Link to comment Share on other sites More sharing options...
tracey Posted July 17, 2016 Share Posted July 17, 2016 the obvious error in this thinking is that light can not be measured, which it surely can. If light is being projected between two spaces it will simply become more, therefore changes in ways than can be detected. if you were two move the two mirriors wider apart than it would take longer for the light phase to change. this is a simple project that doesn't need an in depth reasoning. Now if you are solely working on this portion of the project to aid in something greater lets say..... bonding light to certain compounds to cement a physical representation of an image this would be much more interesting. actually this experiment was somewhat already presented in 1891 by nikola telsa which im sure could be related -1 Link to comment Share on other sites More sharing options...
John Cuthber Posted July 17, 2016 Share Posted July 17, 2016 the obvious error in this thinking is that light can not be measured, which it surely can. If light is being projected between two spaces it will simply become more, therefore changes in ways than can be detected. if you were two move the two mirriors wider apart than it would take longer for the light phase to change. this is a simple project that doesn't need an in depth reasoning. Now if you are solely working on this portion of the project to aid in something greater lets say..... bonding light to certain compounds to cement a physical representation of an image this would be much more interesting. actually this experiment was somewhat already presented in 1891 by nikola telsa which im sure could be related Word salad. Link to comment Share on other sites More sharing options...
Enthalpy Posted July 17, 2016 Share Posted July 17, 2016 I may have read somewhere - or I read it wrongly, or I misinterpretated - that the interferometer is sensitive to a tidal effect rather than directly to the contraction and expansion. Possibly something like: a portion of the light travels together with the gravitational wave and gets influenced for long, while the interference base experiences the change for a shorter time. Ahum. Take with due mistrust. Link to comment Share on other sites More sharing options...
StringJunky Posted July 17, 2016 Share Posted July 17, 2016 Word salad. With garnish Link to comment Share on other sites More sharing options...
Markus Hanke Posted July 18, 2016 Share Posted July 18, 2016 IOW, what is the error in this thinking? How exactly you interpret the effect of the wave depends on how you set up your coordinate system, but in the transverse traceless gauge the key to this is that the light traces out a geodesic in spacetime, whereas the constituent particles making up the arms of the LIGO setup do not, since they are part of a rigid structure. Furthermore, there are two perpendicular arms, and since a gravitational wave is quadrupole radiation, a passing wave will induce a phase difference since the two arms are effected differently. One way to think of it is a time-varying Shapiro delay - while the coordinate distance the light has to travel remains constant, the metric ( =proper ) distance does not, so in essence you are getting a fluctuation in the coordinate speed of light along one arm of the setup, but not along the other ( due to the polarisation of the wave ). This is detectable. 1 Link to comment Share on other sites More sharing options...
Daecon Posted July 18, 2016 Share Posted July 18, 2016 If it stretches the space between the mirrors and also stretches the light beam, would being unable to tell the difference imply that stretching a light beam is indistinguishable from making the light travel faster than c to reach the other side at the same time? That just seems... off. Link to comment Share on other sites More sharing options...
swansont Posted July 18, 2016 Author Share Posted July 18, 2016 How exactly you interpret the effect of the wave depends on how you set up your coordinate system, but in the transverse traceless gauge the key to this is that the light traces out a geodesic in spacetime, whereas the constituent particles making up the arms of the LIGO setup do not, since they are part of a rigid structure. Furthermore, there are two perpendicular arms, and since a gravitational wave is quadrupole radiation, a passing wave will induce a phase difference since the two arms are effected differently. One way to think of it is a time-varying Shapiro delay - while the coordinate distance the light has to travel remains constant, the metric ( =proper ) distance does not, so in essence you are getting a fluctuation in the coordinate speed of light along one arm of the setup, but not along the other ( due to the polarisation of the wave ). This is detectable. Thanks. We eventually were able to hash out the right answer, though I don't recall exactly how we worded it. Link to comment Share on other sites More sharing options...
Professional Strawman Posted July 19, 2016 Share Posted July 19, 2016 IOW, what is the error in this thinking? Yea, them interferometers! -2 Link to comment Share on other sites More sharing options...
Professional Strawman Posted July 24, 2016 Share Posted July 24, 2016 Thanks. We eventually were able to hash out the right answer, though I don't recall exactly how we worded it. And I started to think, you were one of those physicists who didn't have a clue. -3 Link to comment Share on other sites More sharing options...
swansont Posted July 24, 2016 Author Share Posted July 24, 2016 And I started to think, you were one of those physicists who didn't have a clue. The device has yet to be built that could measure how little I care about what you think. 4 Link to comment Share on other sites More sharing options...
Professional Strawman Posted July 24, 2016 Share Posted July 24, 2016 That's another colloquium, altogether I guess. Link to comment Share on other sites More sharing options...
Professional Strawman Posted July 25, 2016 Share Posted July 25, 2016 The device has yet to be built that could measure how little I care about what you think. I am not holding my breath. Still, interferometers have been around for a very very long time. And if there are physicists clueless about the workings of this device, it's honestly, laughable. If LEGO were about the size of my car, I wouldn't find that funny. -1 Link to comment Share on other sites More sharing options...
swansont Posted July 25, 2016 Author Share Posted July 25, 2016 I am not holding my breath. Still, interferometers have been around for a very very long time. And if there are physicists clueless about the workings of this device, it's honestly, laughable. If LEGO were about the size of my car, I wouldn't find that funny. It's OK that you didn't understand my question. 2 Link to comment Share on other sites More sharing options...
Professional Strawman Posted July 26, 2016 Share Posted July 26, 2016 (edited) LOL. Them, interferometers! Who knew physicists didn't understand, them. Now I know! Edited July 26, 2016 by Professional Strawman -4 Link to comment Share on other sites More sharing options...
Professional Strawman Posted July 27, 2016 Share Posted July 27, 2016 (edited) It's OK that you didn't understand my question. You asked a crank question. An Oscar winning crank question. Your question is equivalent to asking why does a solar panel need sun? Or why does a swimming pool need water? See the error in that thinking? If the cranks over at LEGO have their way, they'd replace, LASERS with a couple of Kenyan runners. One runs north, one runs east. When they return one of them will let you know if a gravity wave hit him. Heh! In my opinion, you and your colleagues ought to have a week's pay deducted from your salaries for engaging in such crankery. Honestly, your crank question belongs in trash, if not in speculation. Kind regards, Prof. Strawman Edited July 27, 2016 by Professional Strawman -2 Link to comment Share on other sites More sharing options...
swansont Posted July 27, 2016 Author Share Posted July 27, 2016 You asked a crank question. An Oscar winning crank question. Your question is equivalent to asking why does a solar panel need sun? Or why does a swimming pool need water? See the error in that thinking? If the cranks over at LEGO have their way, they'd replace, LASERS with a couple of Kenyan runners. One runs north, one runs east. When they return one of them will let you know if a gravity wave hit him. Heh! In my opinion, you and your colleagues ought to have a week's pay deducted from your salaries for engaging in such crankery. Honestly, your crank question belongs in trash, if not in speculation. Kind regards, Prof. Strawman I was having the conversation with my supervisor, so I'll have to pass this recommendation on to his supervisor. I'm sure he'll give your opinion all the weight it deserves. Link to comment Share on other sites More sharing options...
Strange Posted July 27, 2016 Share Posted July 27, 2016 That has to be a joke, right? It is too funny to be real. Link to comment Share on other sites More sharing options...
Daecon Posted July 28, 2016 Share Posted July 28, 2016 That has to be a joke, right? It is too funny to be real. I'm sure a comment about Poe's Law would be appropriate here... Link to comment Share on other sites More sharing options...
imatfaal Posted July 28, 2016 Share Posted July 28, 2016 ! Moderator Note Professional Strawman - stop trolling. You are clearly here to provoke a disagreement and to insult the membership - and not for a substantive discussion of the physics. If you feel the level of interaction is beneath you then the answer is to look elsewhere not to attempt to belittle the level of discourse here. Do not respond to this moderation within the thread. This post may be reported if there are concerns with this moderation note. And everyone else DFtT Link to comment Share on other sites More sharing options...
imatfaal Posted July 30, 2016 Share Posted July 30, 2016 ! Moderator Note Prof. Strawman Which bit of 'stop trolling' did you not understand? I have hidden your post. Do not respond to this moderation. Link to comment Share on other sites More sharing options...
Professional Strawman Posted July 31, 2016 Share Posted July 31, 2016 I've seen better fake forums. Link to comment Share on other sites More sharing options...
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