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> Importantly, gravitational waves are not waves of Newtonian gravity. Gravitational waves do not "push and pull" along the direction of propagation. They stret

> Importantly, gravitational waves are not waves of Newtonian gravity. Gravitational waves do not "push and pull" along the direction of propagation. They stretch and compress space along axes perpendicular to the direction of propagation. This may be, likely is, a stupid question, but how do you measure this if space itself is stretched or compressed? Won't any yardstick lying in that space also be stretched or compressed? Or is it a matter of this distortion of space changing geodesics through it so light following a straight line will end up in a different place? It seems that LIGO uses interferometry to measure a change in distance between two points, though, and I don't understand how that can be measured if space is stretched, only if space is added. Perhaps that is the answer right there: when space is stretched dimensionless particles traveling through that space are not stretched, because they are dimensionless. Or perhaps better, objects in space stretch, but their momentum vectors don't stretch.

cobaltoxide 5y ago
Here is an introductory paper on that exact topic: "If light waves are stretched by gravitational waves, how can we use light as a ruler to detect gravitational waves?" https://universe.sonoma.edu/moodle/pluginfile.php/89/mod_resource/content/1/saulson.pdf https://universe.sonoma.edu/moodle/pluginfile.php/89/mod_res... The way I explain it: The light already inside the arm cavity is stretched. But newly entering light is not. We are basically using the light as clock to measure the length of the arm. When the arm is in the "stretched" state, light will take longer to make a roundtrip in the arm. It will accumulate slightly more phase, which is measured interferometrically. It is important that the storage time of the light in the arm is short compared to the period of the gravitational wave.
- pixel_fcker 5y ago
  On a similar note - if a huge gravity wave passed through the earth, say one that stretched the space I’m in by a factor of 10, would I feel anything?
  - deleted 5y ago
    [deleted]
cobaltoxide 5y ago
> when space is stretched dimensionless particles traveling through that space are not stretched, because they are dimensionless. Or perhaps better, objects in space stretch, but their momentum vectors don't stretch. This is an interesting idea, but I think it's not right. To be honest, even as an experimentalist who worked on LIGO, I'm not sure how to treat the effect of the g.w. on photons in the arm. Instead I think of the entire arm cavity vacuum as a giant phase modulator.
lightsighter 5y ago
The clever thing about interferometers is that they're actually measuring space and time in two different dimensions concurrently and then looking for changes in the interference pattern between light moving along each of the dimensions. Simplistically, imagine a gravitational wave propagating along one dimension of the interferometer (realistically gravitational waves will never be perfectly aligned with any direction of the interferometer). Space will be distorted in that dimension but not the other and we can notice the change in the resulting interference pattern. In practice, gravitational waves will come from all sorts of weird angles, but they will distort each of the two dimensions differently and allow us to figure what direction they were propagating by the interference pattern that's observed.
- cobaltoxide 5y ago
  > The clever thing about interferometers is that they're actually measuring space and time in two different dimensions concurrently I would say that we measure space in two different directions concurrently. > and then looking for changes in the interference pattern between light moving along each of the dimensions This is true, but I like to push back against the use of the phrase "interference pattern." We're not looking at a "pattern", which to me brings to mind a complicated interference pattern resolved spatially. We do not resolve the "interference pattern" spatially. We measure the amplitude (er, power) of the light coming out of the interferometer with a photodiode (a single pixel, if you will.) > In practice, gravitational waves will come from all sorts of weird angles, but they will distort each of the two dimensions differently This is true. The detector's sensitivity to waves coming from different directions is the "antenna pattern." > t they will distort each of the two dimensions differently and allow us to figure what direction they were propagating by the interference pattern that's observed. With a single detector, we cannot determine the direction in which a g.w. is propagating. With a single detector and a transient (short-lived) source, we cannot tell the difference between a loud source in a direction where the detector is not very sensitive versus a quieter source in a direction where we have good sensitivity. With a network of detectors and/or signals that persist for a long time (compared to the rotation of the earth, etc) we can resolve the source direction.
dimator 5y ago
You might enjoy this video which goes into that question: https://youtu.be/iphcyNWFD10 https://youtu.be/iphcyNWFD10

cobaltoxide 5y ago

Here is an introductory paper on that exact topic: "If light waves are stretched by gravitational waves, how can we use light as a ruler to detect gravitational waves?" https://universe.sonoma.edu/moodle/pluginfile.php/89/mod_resource/content/1/saulson.pdf https://universe.sonoma.edu/moodle/pluginfile.php/89/mod_res... The way I explain it: The light already inside the arm cavity is stretched. But newly entering light is not. We are basically using the light as clock to measure the length of the arm. When the arm is in the "stretched" state, light will take longer to make a roundtrip in the arm. It will accumulate slightly more phase, which is measured interferometrically. It is important that the storage time of the light in the arm is short compared to the period of the gravitational wave.

pixel_fcker 5y ago
On a similar note - if a huge gravity wave passed through the earth, say one that stretched the space I’m in by a factor of 10, would I feel anything?
- deleted 5y ago
  [deleted]

cobaltoxide 5y ago

> when space is stretched dimensionless particles traveling through that space are not stretched, because they are dimensionless. Or perhaps better, objects in space stretch, but their momentum vectors don't stretch. This is an interesting idea, but I think it's not right. To be honest, even as an experimentalist who worked on LIGO, I'm not sure how to treat the effect of the g.w. on photons in the arm. Instead I think of the entire arm cavity vacuum as a giant phase modulator.

lightsighter 5y ago

The clever thing about interferometers is that they're actually measuring space and time in two different dimensions concurrently and then looking for changes in the interference pattern between light moving along each of the dimensions. Simplistically, imagine a gravitational wave propagating along one dimension of the interferometer (realistically gravitational waves will never be perfectly aligned with any direction of the interferometer). Space will be distorted in that dimension but not the other and we can notice the change in the resulting interference pattern. In practice, gravitational waves will come from all sorts of weird angles, but they will distort each of the two dimensions differently and allow us to figure what direction they were propagating by the interference pattern that's observed.

cobaltoxide 5y ago
> The clever thing about interferometers is that they're actually measuring space and time in two different dimensions concurrently I would say that we measure space in two different directions concurrently. > and then looking for changes in the interference pattern between light moving along each of the dimensions This is true, but I like to push back against the use of the phrase "interference pattern." We're not looking at a "pattern", which to me brings to mind a complicated interference pattern resolved spatially. We do not resolve the "interference pattern" spatially. We measure the amplitude (er, power) of the light coming out of the interferometer with a photodiode (a single pixel, if you will.) > In practice, gravitational waves will come from all sorts of weird angles, but they will distort each of the two dimensions differently This is true. The detector's sensitivity to waves coming from different directions is the "antenna pattern." > t they will distort each of the two dimensions differently and allow us to figure what direction they were propagating by the interference pattern that's observed. With a single detector, we cannot determine the direction in which a g.w. is propagating. With a single detector and a transient (short-lived) source, we cannot tell the difference between a loud source in a direction where the detector is not very sensitive versus a quieter source in a direction where we have good sensitivity. With a network of detectors and/or signals that persist for a long time (compared to the rotation of the earth, etc) we can resolve the source direction.

dimator 5y ago

You might enjoy this video which goes into that question: https://youtu.be/iphcyNWFD10 https://youtu.be/iphcyNWFD10