5 ms·

> If your explanation is that gravitational waves stretch and squash spacetime so light takes different amount of time to bounce from an emitter back to a measu

by knzhou 6y ago

> If your explanation is that gravitational waves stretch and squash spacetime so light takes different amount of time to bounce from an emitter back to a measurer, then you don't have even a slight understanding of how it works. Well, I'll bite. I'm a physicist and I understand LIGO. What's your alternative explanation?

stevebmark 6y ago
I have no idea how it works, I don't have an explanation, I just know the one spouted by people doesn't make sense
- knzhou 6y ago
  Well, if you're uncomfortable with LIGO, you're in good company. Once a whole decade went by in physics where people couldn't agree if it would work in principle or not. And it is true that a lot of common explanations are bad (e.g. "it's just a ruler" is not complete by itself because "why doesn't the light get stretched too?"). Nonetheless, today we have a variety of independent explanations. Maybe you'll find this paper helpful: https://aapt.scitation.org/doi/10.1119/1.18578 https://aapt.scitation.org/doi/10.1119/1.18578
  - rrss 6y ago
    Would you say it's accurate to say LIGO is a big interferometer measuring changes in lengths of the arms due to gravitational waves, with some extra analysis to figure why it still works even though the light waves get stretched? Or is OP correct that such an explanation means I "don't have even a slight understanding of how it works" and a correct explanation must "involve higher spatial dimensions (not time)?"
    - knzhou 6y ago
      I'd say that's a perfectly fine explanation. And in fact the light does get stretched. For sufficiently low frequencies, that's fine because there's enough time for new (unstretched) light to enter and exit the apparatus. At higher frequencies this causes the sensitivity of LIGO to fall. Part of the popular confusion around how LIGO works is the freedom in coordinates: there are different, perfectly good definitions of space and time you can use, and the explanation sounds different in each one. So people can get them mixed up. For example, my previous paragraph makes sense in "transverse traceless gauge", but not in others. I'm not sure what GP was referring to with "higher spatial dimensions".
  - stevebmark 6y ago
    Thank you for this link. I read it once and think I've gotten more of the picture and I'm going to re-read it to see if I can get more. What's baffling to me is everyone who has tried to explain the LIGO detector doesn't even realize this question exists. I've independently thought this question and when people start explaining LIGO to me, and I take the time to spell out the question, they realize they don't understand LIGO either. If light is emitted at a constant wavelength independent of the stretching of the universe, doesn't that imply light is traveling through a higher spatial dimension, otherwise the emitter itself would be stretched with the universe and we'd never be able to observe differences in the speed of light? If I understand this paper, once light is emitted, it's "stuck" to space and will stretch along with it. But if the emitter wavelength stays constant doesn't that imply it's waving through a higher dimension?
    - knzhou 6y ago
      > If light is emitted at a constant wavelength independent of the stretching of the universe, doesn't that imply light is traveling through a higher spatial dimension, otherwise the emitter itself would be stretched with the universe and we'd never be able to observe differences in the speed of light? If I understand this paper, once light is emitted, it's "stuck" to space and will stretch along with it. But if the emitter wavelength stays constant doesn't that imply it's waving through a higher dimension? I'm not totally sure what you mean by a higher dimension. The properties of the emitter (which is, e.g. a laser cavity) aren't affected by the gravitational wave because the emitter is a rigid body, which doesn't get stretched. (It's the same thing as described here: https://news.ycombinator.com/item?id=22990753 https://news.ycombinator.com/item?id=22990753 ) So it puts out light of a given frequency. By contrast, LIGO is not a rigid body, because the mirrors at the ends of the arms hang freely, hence allowing gravitational waves to change the distance between them. > What's baffling to me is everyone who has tried to explain the LIGO detector doesn't even realize this question exists. I've independently thought this question and when people start explaining LIGO to me, and I take the time to spell out the question, they realize they don't understand LIGO either. Yup, it generally is the case in physics that over 95% of people who claim they can explain any given thing don't actually understand it! But the professionals are aware. I even know a LIGO guy who goes to popular talks armed with a pile of copies of the paper I linked.

by knzhou 6y ago

stevebmark 6y ago

I have no idea how it works, I don't have an explanation, I just know the one spouted by people doesn't make sense

knzhou 6y ago
Well, if you're uncomfortable with LIGO, you're in good company. Once a whole decade went by in physics where people couldn't agree if it would work in principle or not. And it is true that a lot of common explanations are bad (e.g. "it's just a ruler" is not complete by itself because "why doesn't the light get stretched too?"). Nonetheless, today we have a variety of independent explanations. Maybe you'll find this paper helpful: https://aapt.scitation.org/doi/10.1119/1.18578 https://aapt.scitation.org/doi/10.1119/1.18578
- rrss 6y ago
  Would you say it's accurate to say LIGO is a big interferometer measuring changes in lengths of the arms due to gravitational waves, with some extra analysis to figure why it still works even though the light waves get stretched? Or is OP correct that such an explanation means I "don't have even a slight understanding of how it works" and a correct explanation must "involve higher spatial dimensions (not time)?"
  - knzhou 6y ago
    I'd say that's a perfectly fine explanation. And in fact the light does get stretched. For sufficiently low frequencies, that's fine because there's enough time for new (unstretched) light to enter and exit the apparatus. At higher frequencies this causes the sensitivity of LIGO to fall. Part of the popular confusion around how LIGO works is the freedom in coordinates: there are different, perfectly good definitions of space and time you can use, and the explanation sounds different in each one. So people can get them mixed up. For example, my previous paragraph makes sense in "transverse traceless gauge", but not in others. I'm not sure what GP was referring to with "higher spatial dimensions".
- stevebmark 6y ago
  Thank you for this link. I read it once and think I've gotten more of the picture and I'm going to re-read it to see if I can get more. What's baffling to me is everyone who has tried to explain the LIGO detector doesn't even realize this question exists. I've independently thought this question and when people start explaining LIGO to me, and I take the time to spell out the question, they realize they don't understand LIGO either. If light is emitted at a constant wavelength independent of the stretching of the universe, doesn't that imply light is traveling through a higher spatial dimension, otherwise the emitter itself would be stretched with the universe and we'd never be able to observe differences in the speed of light? If I understand this paper, once light is emitted, it's "stuck" to space and will stretch along with it. But if the emitter wavelength stays constant doesn't that imply it's waving through a higher dimension?
  - knzhou 6y ago
    > If light is emitted at a constant wavelength independent of the stretching of the universe, doesn't that imply light is traveling through a higher spatial dimension, otherwise the emitter itself would be stretched with the universe and we'd never be able to observe differences in the speed of light? If I understand this paper, once light is emitted, it's "stuck" to space and will stretch along with it. But if the emitter wavelength stays constant doesn't that imply it's waving through a higher dimension? I'm not totally sure what you mean by a higher dimension. The properties of the emitter (which is, e.g. a laser cavity) aren't affected by the gravitational wave because the emitter is a rigid body, which doesn't get stretched. (It's the same thing as described here: https://news.ycombinator.com/item?id=22990753 https://news.ycombinator.com/item?id=22990753 ) So it puts out light of a given frequency. By contrast, LIGO is not a rigid body, because the mirrors at the ends of the arms hang freely, hence allowing gravitational waves to change the distance between them. > What's baffling to me is everyone who has tried to explain the LIGO detector doesn't even realize this question exists. I've independently thought this question and when people start explaining LIGO to me, and I take the time to spell out the question, they realize they don't understand LIGO either. Yup, it generally is the case in physics that over 95% of people who claim they can explain any given thing don't actually understand it! But the professionals are aware. I even know a LIGO guy who goes to popular talks armed with a pile of copies of the paper I linked.