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Gravitational waves are waves of the metric tensor. Einstein's field equations provide the "restoring force." Importantly, gravitational waves are not waves of

Gravitational waves are waves of the metric tensor. Einstein's field equations provide the "restoring force." 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. > E.g., if the moon suddenly lurched towards the Earth, we'd perceive an increase in gravity between the two. But that would be a semi-permanent change in the strength of that attraction between the two objects; not what I'd think of as a wave-like fluctuation. Indeed, gravitational waves do not work this way. Unfortunately it is hard to explain gravitational waves without significant math. In fairness, even Einstein himself waffled over whether gravitational waves would be a real effect predicted by the theory. Then it took nearly a century to detect them experimentally, and there were plenty of doubters along the way. The results one gets from intuition are generally incorrect in important ways. Here's a derivation of a wave equation from Einstein's field equations: https://en.wikipedia.org/wiki/Linearized_gravity https://en.wikipedia.org/wiki/Linearized_gravity A more helpful introduction for a layperson might be the paper titled "Gravitational Waves on the back of an envelope": https://aapt.scitation.org/doi/10.1119/1.13627 https://aapt.scitation.org/doi/10.1119/1.13627 But, infuriatingly, that paper does not seem to be open-access. Here's someone's scanned copy: https://www.ru.ac.za/media/rhodesuniversity/content/mathematics/images/Schutz1984-AJP000412-1.pdf https://www.ru.ac.za/media/rhodesuniversity/content/mathemat...

fsflover 5y ago
> But, infuriatingly, that paper does not seem to be open-access. Try sci-hub.
sildur 5y ago
> They stretch and compress space along axes perpendicular to the direction of propagation. Is it possible to compress space?
- drran 5y ago
  (Not a native speaker). Space is a mathematical abstraction. You can do anything with, because it's just math. If you want to bend, curve, distort, rip space, or add more dimensions — go for it. Physical medium should be compressible, like any other medium. Higgs «field» (let's call it Higgium — Higgs+Vacuum) is presented everywhere, because Higgs boson gives mass to every particle in the Visible Universe and beyond, so it should conduct gravitational waves. As demonstrated by LIGO/VIRGO, gravitational waves causes distortions in light travel. These distortions can be explained as changes of conductivity in Higgium, which can be caused by changes in density, so yes, it can be waves of compression.
  - evanb 5y ago
    I appreciate what you’re trying to get at, but it’s not right. Gravitational waves exist in GR even in theories with no matter / Higgs. The Higgs is a scalar field; gravitational waves are spin-2.
    - drran 5y ago
      Thank you for pointing it out. Spin-0 particles are round particles, like o (like ball), or something that cannot rotate at all. Spin-2 particles are symmetrical particles, like 8 (like H2 molecule). For spin-0, it can be changes in density only. For spin-2, it can be changes in orientation, in rotation speed, in rotation orientation, Dzhanibekov effect, wobbling, and some other if it has more complex shape, e.g. two spirals connected.
  - deleted 5y ago
    [deleted]
  - cobaltoxide 5y ago
    I believe this answer is nonsense.
  - deleted 5y ago
    [deleted]
- cobaltoxide 5y ago
  > Is it possible to compress space? The geodesic distance between inertial test masses increases and decreases. Colloquially we say that space has stretched and compressed. Geodesic distance can be measured by the time it takes light to travel between the points. Inertial test masses can be simulated by hanging them from fine wires (LIGO) or by setting them up in an orbital constellation (LISA).
DFHippie 5y ago
> 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]
shmerl 5y ago
Sci Hub has it apparently: https://sci-hubtw.hkvisa.net/10.1119/1.13627 https://sci-hubtw.hkvisa.net/10.1119/1.13627

fsflover 5y ago

> But, infuriatingly, that paper does not seem to be open-access. Try sci-hub.

sildur 5y ago

> They stretch and compress space along axes perpendicular to the direction of propagation. Is it possible to compress space?

drran 5y ago
(Not a native speaker). Space is a mathematical abstraction. You can do anything with, because it's just math. If you want to bend, curve, distort, rip space, or add more dimensions — go for it. Physical medium should be compressible, like any other medium. Higgs «field» (let's call it Higgium — Higgs+Vacuum) is presented everywhere, because Higgs boson gives mass to every particle in the Visible Universe and beyond, so it should conduct gravitational waves. As demonstrated by LIGO/VIRGO, gravitational waves causes distortions in light travel. These distortions can be explained as changes of conductivity in Higgium, which can be caused by changes in density, so yes, it can be waves of compression.
- evanb 5y ago
  I appreciate what you’re trying to get at, but it’s not right. Gravitational waves exist in GR even in theories with no matter / Higgs. The Higgs is a scalar field; gravitational waves are spin-2.
  - drran 5y ago
    Thank you for pointing it out. Spin-0 particles are round particles, like o (like ball), or something that cannot rotate at all. Spin-2 particles are symmetrical particles, like 8 (like H2 molecule). For spin-0, it can be changes in density only. For spin-2, it can be changes in orientation, in rotation speed, in rotation orientation, Dzhanibekov effect, wobbling, and some other if it has more complex shape, e.g. two spirals connected.
- deleted 5y ago
  [deleted]
- cobaltoxide 5y ago
  I believe this answer is nonsense.
- deleted 5y ago
  [deleted]
cobaltoxide 5y ago
> Is it possible to compress space? The geodesic distance between inertial test masses increases and decreases. Colloquially we say that space has stretched and compressed. Geodesic distance can be measured by the time it takes light to travel between the points. Inertial test masses can be simulated by hanging them from fine wires (LIGO) or by setting them up in an orbital constellation (LISA).

DFHippie 5y ago

> 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]

shmerl 5y ago

Sci Hub has it apparently: https://sci-hubtw.hkvisa.net/10.1119/1.13627 https://sci-hubtw.hkvisa.net/10.1119/1.13627