6 ms·

Yes, that's basically right. The gravitational way has a direction (say, z) in which its propagating. Within the plane perpendicular to that direction (x-y),

Yes, that's basically right. The gravitational way has a direction (say, z) in which its propagating. Within the plane perpendicular to that direction (x-y), a circular ring of particles will at at one moment experience squeezing in one direction (x) and stretching the perpendicular direction (y). As the wave passes through and you move from the peak of the wave to the trough, the directions reverse, so the first direction (x) stretches and the other direction (y) squeezes. By "stretching" and "squeezing" I mean instantaneous additional (positive and negative) acceleration on top of the (much, much larger) acceleration from the background gravitational field provided by the Earth. Here's a visualization: https://www.researchgate.net/publication/313828462/figure/fig1/AS:963462419189762@1606718749775/Gravitational-waves-are-transverse-quadrupole-waves-If-a-wave-passes-through-the-ring-of.gif https://www.researchgate.net/publication/313828462/figure/fi... Just as a child can swing their legs at the resonant frequency of a swing to pump up their sinusoidal amplitude, a very weak gravitational wave can pump up a ring oscillator if it's oscillating at the ring's resonant frequency. Exactly square gravitational waves are of course not possible, just as for electromagnetic waves. (They would have infinite energy at the corners.) But in principle you could get a close approximation. However, spacetime is incredibly stiff, and I think all the known real-world sources produce pretty smooth waves. I presume most violent events are mergers of existing black holes, and essentially always result from a smooth in-spiral rather, say, a sharp collision event. This is what the "chirp" signal looks like to the LIGO detector: https://www.youtube.com/watch?v=TWqhUANNFXw https://www.youtube.com/watch?v=TWqhUANNFXw The effects of a square wave would be roughly as you would expect: instead of smoothly pumping up an oscillator, it would give it a sharp kick, just as with electromagnetism.

derefr 3y ago
Acoustic waves propagate through what are essentially elastic deformations of the material they travel through. Can gravitational waves be thought of as propagating by elastically deforming spacetime? If this analogy holds, then can it be taken further? Acoustic waves dissipate their energy insofar as they trigger plastic deformation in a material. Could gravitational waves plastically deform... spacetime itself? Or would they just be deforming the material? Or is gravitational energy not dissipated into other forms of energy at all?
- jessriedel 3y ago
  Acoustic waves dissipate their energy into non-acoustic (molecular) degrees of freedom. In the absence of matter, I think there are no other degrees of freedom for gravitational waves to dissipate into, unless they are strong enough to form a black hole (which they can do under extreme conditions, I think). (This is waaaay outside my expertise though, so take my answer with a grain of salt. Everything I've said in this thread is basically based off the rudimentary understanding from taking a GR course in grad school. I've never done research on this topic.)
- cyberax 3y ago
  > Or is gravitational energy not dissipated into other forms of energy at all? Extremely weakly. The mechanism is similar to acoustic waves, but the coupling constant is so small, that the amount of dissipated power would be insignificantly small. In theory, you can use gravitational waves to extract energy. For example, you can wait for the "compression" part of the wave, and push a cart uphill during it. Then let it slide downhill when the compression peak passes. You'll be able to extract some useful energy, because the distance that you pushed the car uphill will be shorter than the "normal" distance. You can make more elaborate systems on this principle. E.g. a tuned resonator: two orbiting masses with a period selected to match the frequency of the gravitational waves.
  - treeman79 3y ago
    The earth going around the sun produces gravitational waves, and some of the energy is lost. If all of that energy could be harnessed; it would be sufficient to power a small toaster oven.
    - staunton 3y ago
      > If all of that energy could be harnessed; it would be sufficient to power a small toaster oven. Do you have a source? Sounds like an interesting calculation.
      - 3y ago
nativeit 3y ago
Thank you for that! As an EE and radio enthusiast, I feel like I have a fairly good grasp of how gravitational waves behave from that description. The way in which the mechanics of these different energies all sort of share characteristics is rather beautiful.
asdff 3y ago
Could you set up massive pendulums to convert this gravitational wave energy into mechanical energy? Would such a device diminish the strength of the wave downstream?
- thatcherc 3y ago
  Yes! In principle at least, and only miniscule/undetectable amounts of energy. The first gravitational wave detectors build were these "Weber bars" [1] - big block of aluminum that were supposed to resonant mechanically (like a bell) when a gravitation wave of the right frequency passed through them. Looks like these things haven't detected a real gravitational wave, but if a strong enough one at the right frequency came through, they might start ringing like (very quiet) bells! [1] - https://en.wikipedia.org/wiki/Weber_bar https://en.wikipedia.org/wiki/Weber_bar
xattt 3y ago
> Just as a child can swing their legs at the resonant frequency of a swing to pump up their sinusoidal amplitude, a very weak gravitational wave can pump up a ring oscillator if it's oscillating at the ring's resonant frequency. So is it possible that a passing gravitational wave could initiate some natural process that otherwise might have not happened?
- mirekrusin 3y ago
  It can at least trigger hn front page submission, this we know. You mean something like tectonic event trigger or something physics specific?
  - xattt 3y ago
    I was thinking more along the lines of a chemical or nuclear reaction (or some yet-to-be-conceptualized space-time reaction) at near an initiation point, but not quite there. I assume the gravity wave could push the reaction to initiate by warping a subatomic element (like an electron orbital) into an otherwise impossible configuration on a scale of picometers for a split second.
    - zeven7 3y ago
      These gravity waves aren't strong enough to pull us off Earth. Everything on Earth is already subjected to gravity, and it doesn't cause nuclear reactions. Gravity waves won't cause impossible configurations to become possible.
      - taeric 3y ago
        Could it impact orbits? Or tides?
        stouset 3y ago
        I think you are overestimating the amount of power in these gravitational waves by several dozen orders of magnitude. To even detect these, we need to observe multiple pulsars over long periods of time in order to find minute effects only visible at galactic scales due to the nanohertz frequency of these waves. In other words, spacetime is being stretched and compressed at subatomic scales on a sinusoidal wave with a period of a month or so. It’s a bit like asking if cosmic rays from the Pinwheel Galaxy are affecting cancer rates.
Solvency 3y ago
"However, spacetime is incredibly stiff, and I think all the known real-world sources produce pretty smooth waves." In English, why does stiffness correlate to smooth waves? What does stiff spacetime mean? I'd have thought a square wave would be "stiff" as it's quite the opposite of smooth.
- jessriedel 3y ago
  Imagine using a flick of your wrist to create a traveling transverse square wave in a string vs. in a long metal dowel. It's actually not so hard to create a smooth sinusoidal wave in a metal dowel. (It will be long wavelength, but it will be easily visible if you put your eye near one end and look down it.) But it would be impossible for you to make a square wave.
- derefr 3y ago
  I think what the GP was stating here were two independent reasons why we would find it hard to know what would happen if a gravitational square-ish wave interacted with spacetime: 1. spacetime is "stiff", and 2. there are no gravitational square-ish wave generators around to observe. The two things are kind of related, though. One "natural" way to create a square wave in nature, is to "interrupt" a material transmitting a sinusoid wave, at the peak of its transmittance. And one way to do that, is to break through the modulus of elasticity of the material transmitting the wave, such that it switches from the elastic-deformation domain (transmitting the wave) to the plastic-deformation domain (ripping apart.) Imagine a speaker cone tearing at the peak of a high-amplitude drum beat. The cone pushes "out" — and then doesn't push back "in", because instead the air behind it rips forward through it. The air created by the speaker cone wants to rush back "in", but now there's no longer a speaker cone acting as a waveguide for the inward flow, so the natural turbulence cancels out much of the "falling" energy of the wave, making it look much more like a square-wave drop. I believe that the GP is saying that, because spacetime has such a high effective "modulus of elasticity", we haven't yet observed any practical way to perturb so as to create the conditions that would generate a gravitational square wave.
- tremon 3y ago
  In order for a square wave to travel through a material, it would need to allow for infinitely high frequencies (see the animation on wikipedia[0] for a visual demonstation). The lower the maximum frequency possible, the more every wave will resemble a sine wave because it's the most basic shape: any non-smooth wave has higher-frequency components to give it its shape. Filter out those higher frequencies, and you're left with the basic smooth sine wave again. A stiff material tends to dampen high frequencies, simply because it cannot deform fast enough to follow the wave's shape. In a way, the medium acts as a low-pass filter; compare, fow example, how fast you can clap your hands in air vs in water: the stiffness of water slows down your movements so you cannot reach high clapping frequencies. [0] https://en.wikipedia.org/wiki/Square_wave?useskin=timeless#Characteristics_of_imperfect_square_waves https://en.wikipedia.org/wiki/Square_wave?useskin=timeless#C...

derefr 3y ago

Acoustic waves propagate through what are essentially elastic deformations of the material they travel through. Can gravitational waves be thought of as propagating by elastically deforming spacetime? If this analogy holds, then can it be taken further? Acoustic waves dissipate their energy insofar as they trigger plastic deformation in a material. Could gravitational waves plastically deform... spacetime itself? Or would they just be deforming the material? Or is gravitational energy not dissipated into other forms of energy at all?

jessriedel 3y ago
Acoustic waves dissipate their energy into non-acoustic (molecular) degrees of freedom. In the absence of matter, I think there are no other degrees of freedom for gravitational waves to dissipate into, unless they are strong enough to form a black hole (which they can do under extreme conditions, I think). (This is waaaay outside my expertise though, so take my answer with a grain of salt. Everything I've said in this thread is basically based off the rudimentary understanding from taking a GR course in grad school. I've never done research on this topic.)
cyberax 3y ago
> Or is gravitational energy not dissipated into other forms of energy at all? Extremely weakly. The mechanism is similar to acoustic waves, but the coupling constant is so small, that the amount of dissipated power would be insignificantly small. In theory, you can use gravitational waves to extract energy. For example, you can wait for the "compression" part of the wave, and push a cart uphill during it. Then let it slide downhill when the compression peak passes. You'll be able to extract some useful energy, because the distance that you pushed the car uphill will be shorter than the "normal" distance. You can make more elaborate systems on this principle. E.g. a tuned resonator: two orbiting masses with a period selected to match the frequency of the gravitational waves.
- treeman79 3y ago
  The earth going around the sun produces gravitational waves, and some of the energy is lost. If all of that energy could be harnessed; it would be sufficient to power a small toaster oven.
  - staunton 3y ago
    > If all of that energy could be harnessed; it would be sufficient to power a small toaster oven. Do you have a source? Sounds like an interesting calculation.
    - 3y ago

nativeit 3y ago

Thank you for that! As an EE and radio enthusiast, I feel like I have a fairly good grasp of how gravitational waves behave from that description. The way in which the mechanics of these different energies all sort of share characteristics is rather beautiful.

asdff 3y ago

Could you set up massive pendulums to convert this gravitational wave energy into mechanical energy? Would such a device diminish the strength of the wave downstream?

thatcherc 3y ago
Yes! In principle at least, and only miniscule/undetectable amounts of energy. The first gravitational wave detectors build were these "Weber bars" [1] - big block of aluminum that were supposed to resonant mechanically (like a bell) when a gravitation wave of the right frequency passed through them. Looks like these things haven't detected a real gravitational wave, but if a strong enough one at the right frequency came through, they might start ringing like (very quiet) bells! [1] - https://en.wikipedia.org/wiki/Weber_bar https://en.wikipedia.org/wiki/Weber_bar

xattt 3y ago

> Just as a child can swing their legs at the resonant frequency of a swing to pump up their sinusoidal amplitude, a very weak gravitational wave can pump up a ring oscillator if it's oscillating at the ring's resonant frequency. So is it possible that a passing gravitational wave could initiate some natural process that otherwise might have not happened?

mirekrusin 3y ago
It can at least trigger hn front page submission, this we know. You mean something like tectonic event trigger or something physics specific?
- xattt 3y ago
  I was thinking more along the lines of a chemical or nuclear reaction (or some yet-to-be-conceptualized space-time reaction) at near an initiation point, but not quite there. I assume the gravity wave could push the reaction to initiate by warping a subatomic element (like an electron orbital) into an otherwise impossible configuration on a scale of picometers for a split second.
  - zeven7 3y ago
    These gravity waves aren't strong enough to pull us off Earth. Everything on Earth is already subjected to gravity, and it doesn't cause nuclear reactions. Gravity waves won't cause impossible configurations to become possible.
    - taeric 3y ago
      Could it impact orbits? Or tides?
      - stouset 3y ago
        I think you are overestimating the amount of power in these gravitational waves by several dozen orders of magnitude. To even detect these, we need to observe multiple pulsars over long periods of time in order to find minute effects only visible at galactic scales due to the nanohertz frequency of these waves. In other words, spacetime is being stretched and compressed at subatomic scales on a sinusoidal wave with a period of a month or so. It’s a bit like asking if cosmic rays from the Pinwheel Galaxy are affecting cancer rates.

Solvency 3y ago

"However, spacetime is incredibly stiff, and I think all the known real-world sources produce pretty smooth waves." In English, why does stiffness correlate to smooth waves? What does stiff spacetime mean? I'd have thought a square wave would be "stiff" as it's quite the opposite of smooth.

jessriedel 3y ago
Imagine using a flick of your wrist to create a traveling transverse square wave in a string vs. in a long metal dowel. It's actually not so hard to create a smooth sinusoidal wave in a metal dowel. (It will be long wavelength, but it will be easily visible if you put your eye near one end and look down it.) But it would be impossible for you to make a square wave.
derefr 3y ago
I think what the GP was stating here were two independent reasons why we would find it hard to know what would happen if a gravitational square-ish wave interacted with spacetime: 1. spacetime is "stiff", and 2. there are no gravitational square-ish wave generators around to observe. The two things are kind of related, though. One "natural" way to create a square wave in nature, is to "interrupt" a material transmitting a sinusoid wave, at the peak of its transmittance. And one way to do that, is to break through the modulus of elasticity of the material transmitting the wave, such that it switches from the elastic-deformation domain (transmitting the wave) to the plastic-deformation domain (ripping apart.) Imagine a speaker cone tearing at the peak of a high-amplitude drum beat. The cone pushes "out" — and then doesn't push back "in", because instead the air behind it rips forward through it. The air created by the speaker cone wants to rush back "in", but now there's no longer a speaker cone acting as a waveguide for the inward flow, so the natural turbulence cancels out much of the "falling" energy of the wave, making it look much more like a square-wave drop. I believe that the GP is saying that, because spacetime has such a high effective "modulus of elasticity", we haven't yet observed any practical way to perturb so as to create the conditions that would generate a gravitational square wave.
tremon 3y ago
In order for a square wave to travel through a material, it would need to allow for infinitely high frequencies (see the animation on wikipedia[0] for a visual demonstation). The lower the maximum frequency possible, the more every wave will resemble a sine wave because it's the most basic shape: any non-smooth wave has higher-frequency components to give it its shape. Filter out those higher frequencies, and you're left with the basic smooth sine wave again. A stiff material tends to dampen high frequencies, simply because it cannot deform fast enough to follow the wave's shape. In a way, the medium acts as a low-pass filter; compare, fow example, how fast you can clap your hands in air vs in water: the stiffness of water slows down your movements so you cannot reach high clapping frequencies. [0] https://en.wikipedia.org/wiki/Square_wave?useskin=timeless#Characteristics_of_imperfect_square_waves https://en.wikipedia.org/wiki/Square_wave?useskin=timeless#C...