7 ms·

What do these waves look like as they pass through us? Acoustic-like compression and expansion of particles as molecules temporarily reorient toward a “down” th

by derefr 3y ago

What do these waves look like as they pass through us? Acoustic-like compression and expansion of particles as molecules temporarily reorient toward a “down” that is ever-so-slightly off from the Center of mass of the Earth? Also, I assume that these waves are very gentle sinusoids? Could the opposite — a high-amplitude gravitational square wave — be possible? What would it do to the things it passes through?

readams 3y ago
It's not so much that you see a slightly different "down," but that space itself is changing such that the distance between e.g. your head and feet is (very) slightly altered.
- jessriedel 3y ago
  The two descriptions are equivalent. By the equivalence principle, the wave looks locally like neighboring bodies see different directions of down (and also slightly different strengths of the force of gravity in that direction).
- truculent 3y ago
  Dumb, but earnest question: if space itself changes, how can the distance change? What is the distance a measure of, if not space itself? What's the yardstick, speed of light?
  - ISL 3y ago
    The yardstick is indeed the light-travel time. Gravitational waves really do change the time it takes for light to travel between two points. We use light travel-times to measure distances, thus we say that the distance between the points has changed. If it feels counterintuitive for spacetime to be changing, that's good. It is outside our human experience and perception. The strongest gravitational waves ever observed by scientists passed through everyone who was alive in 2015. None of those people noticed before the instruments registered a detection.
    - truculent 3y ago
      Thank you. If the yardstick is light-speed, is there any meaningful distinction between saying that space itself changed and that there are local perturbations to the speed of light? The thing I struggle with is that we normally think of things occupying space. If space itself gets distorted, then the size of those things should change, too. Or is that mental model a useful but ultimately incorrect way to think about the world?
    - deleted 3y ago
      [deleted]
    - jballanc 3y ago
      >None of those people noticed before the instruments registered a detection. IIRC, the reason no one noticed is that even the strongest gravitational waves are only going to "stretch" space by something less than the diameter of a hydrogen atom.
      - MaxikCZ 3y ago
        Correct me if I am wrong, the gravitational waves we measure have such small effect, that if we blew proton to the size of the Earth, it would get squished by distance smaller than a human hair. Edit: from wikipedia LIGO page: "(interferometers) are capable of detecting a change of less than one ten-thousandth the charge diameter of a proton" I dont know why I remember the human hair analogy, perhaps I am confusing it with something else?
  - wlesieutre 3y ago
    Yes, and light is actually how the LIGO and similar detectors can measure such small changes in distance. Light emitted from a laser is split into two beams traveling different paths, and then merged back together. When the light merges back together, if the two paths traveled took exactly the same distance (or an even multiple of the wavelength at least), then the beams add together constructively and you put back together the light from the laser. But if one path becomes longer or shorter the other, the light is out of phase with itself (peaks of the waves no longer line up with each other) and you can detect the interference between them. LIGO can detect a change in distance of less than one ten-thousandth the charge diameter of a proton. https://en.wikipedia.org/wiki/LIGO#/media/File:Gravitational_wave_observatory_principle.svg https://en.wikipedia.org/wiki/LIGO#/media/File:Gravitational...
    - truculent 3y ago
      Incredible. Thank you
photonerd 3y ago
I think “look” is a misnomer here. It wouldn’t really look like anything, as it’s fundamentally all of space-time that is being stretched and rippled. There’s nothing to really see as such.
ortusdux 3y ago
I recommend looking into LIGO and other similar experiments. They use laser interferometry to accurately measure the distance between two points, to an extreme degree. From LIGO's website: Gravitational waves cause space itself to stretch in one direction and simultaneously compress in a perpendicular direction. In LIGO, this causes one arm of the interferometer to get longer while the other gets shorter, then vice versa, back and forth as long as the wave is passing. The technical term for this motion is "Differential Arm" motion, or differential displacement, since the arms are simultaneously changing lengths in opposing ways. As described above, as the lengths of the arms change, so too does the distance traveled by each laser beam. A beam in a shorter arm will return to the beam splitter before a beam in a longer arm--as the wave passes, each arm oscillates between being the shorter arm and the longer arm. When they arrive back at the beamsplitter (where they re-merge), the light waves no longer meet up nicely; they are out of phase. Instead, they shift in and out of alignment for as long as the wave is passing. https://www.ligo.caltech.edu/page/what-is-interferometer https://www.ligo.caltech.edu/page/what-is-interferometer https://en.wikipedia.org/wiki/LIGO https://en.wikipedia.org/wiki/LIGO
- samstave 3y ago
  >*Gravitational waves cause space itself to stretch* Please ELI5 specifically and empiracally what "Space Itself" actually is. Would it be possible to build a 'galactic clock & Compass' - a "clock" to the regular pulses of a pulsar and the galactic direction the pulsar is in relation to the terrestrial compass (magnetic) on earth...? What is the pulsar with the most reliable timings?
  - pantulis 3y ago
    Space itself means the metric of spacetime: how you measure distances. Locally, if there was no gravity, you could think of space as a plain reticule where distances would be calculated through eucledian geometry: that means cartesian distances, parallel lines would be parallel and the angles of a triangle would sum 180, and the shortest path between two points is the straight line. General Relativity successfully establishes that the presence of mass distorts this, so it defines a mathematical object (the Einstein tensor) that reacts to the distribution of mass and energy and precisely describes the changes to the metric. For example it can model how the mass of the sun distorts the space so that light from distant stars appear to follow a curved path because very close to the sun a curved path is now the shortest path. The Einstein tensor defines how distances --and time-- are measured and it's the best mathematical model that we have about what "space itself" is. Future theoretical advances could take us forward and demonstrate that space itself emerges from other more fundamental elements, but this needs bridging quantum mechanics and gravity. We don't really know what space is made of, but scientists have precisely modelled how it reacts to mass (and energy) with utmost precision. NB: At cosmic scales the exercise becomes more difficult, as there is an expansion of the metric of spacetime that is not due to the presence of mass, in fact it is caused by the _absence_ of mass as it seems to be due the energy of empty space: the phenomenon called Dark Energy. Hope this helps!
jessriedel 3y ago
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?
berniedurfee 3y ago
I was wondering if these waves could affect our brains, even subtly. Maybe cause a reaction in one part of one cell that tips the balance between a neuron firing or not.
- mr_toad 3y ago
  The amplitude of the waves we have detected are less than the width of a proton.
  - berniedurfee 3y ago
    Wavelength, right?
mr_toad 3y ago
The gravitational waves we detect are transverse waves. Longitudinal gravity waves have been proposed, discarded, and proposed again, I’m not sure what their current theoretical status is.

by derefr 3y ago

readams 3y ago

It's not so much that you see a slightly different "down," but that space itself is changing such that the distance between e.g. your head and feet is (very) slightly altered.

jessriedel 3y ago
The two descriptions are equivalent. By the equivalence principle, the wave looks locally like neighboring bodies see different directions of down (and also slightly different strengths of the force of gravity in that direction).
truculent 3y ago
Dumb, but earnest question: if space itself changes, how can the distance change? What is the distance a measure of, if not space itself? What's the yardstick, speed of light?
- ISL 3y ago
  The yardstick is indeed the light-travel time. Gravitational waves really do change the time it takes for light to travel between two points. We use light travel-times to measure distances, thus we say that the distance between the points has changed. If it feels counterintuitive for spacetime to be changing, that's good. It is outside our human experience and perception. The strongest gravitational waves ever observed by scientists passed through everyone who was alive in 2015. None of those people noticed before the instruments registered a detection.
  - truculent 3y ago
    Thank you. If the yardstick is light-speed, is there any meaningful distinction between saying that space itself changed and that there are local perturbations to the speed of light? The thing I struggle with is that we normally think of things occupying space. If space itself gets distorted, then the size of those things should change, too. Or is that mental model a useful but ultimately incorrect way to think about the world?
  - deleted 3y ago
    [deleted]
  - jballanc 3y ago
    >None of those people noticed before the instruments registered a detection. IIRC, the reason no one noticed is that even the strongest gravitational waves are only going to "stretch" space by something less than the diameter of a hydrogen atom.
    - MaxikCZ 3y ago
      Correct me if I am wrong, the gravitational waves we measure have such small effect, that if we blew proton to the size of the Earth, it would get squished by distance smaller than a human hair. Edit: from wikipedia LIGO page: "(interferometers) are capable of detecting a change of less than one ten-thousandth the charge diameter of a proton" I dont know why I remember the human hair analogy, perhaps I am confusing it with something else?
- wlesieutre 3y ago
  Yes, and light is actually how the LIGO and similar detectors can measure such small changes in distance. Light emitted from a laser is split into two beams traveling different paths, and then merged back together. When the light merges back together, if the two paths traveled took exactly the same distance (or an even multiple of the wavelength at least), then the beams add together constructively and you put back together the light from the laser. But if one path becomes longer or shorter the other, the light is out of phase with itself (peaks of the waves no longer line up with each other) and you can detect the interference between them. LIGO can detect a change in distance of less than one ten-thousandth the charge diameter of a proton. https://en.wikipedia.org/wiki/LIGO#/media/File:Gravitational_wave_observatory_principle.svg https://en.wikipedia.org/wiki/LIGO#/media/File:Gravitational...
  - truculent 3y ago
    Incredible. Thank you

photonerd 3y ago

I think “look” is a misnomer here. It wouldn’t really look like anything, as it’s fundamentally all of space-time that is being stretched and rippled. There’s nothing to really see as such.

ortusdux 3y ago

I recommend looking into LIGO and other similar experiments. They use laser interferometry to accurately measure the distance between two points, to an extreme degree. From LIGO's website: Gravitational waves cause space itself to stretch in one direction and simultaneously compress in a perpendicular direction. In LIGO, this causes one arm of the interferometer to get longer while the other gets shorter, then vice versa, back and forth as long as the wave is passing. The technical term for this motion is "Differential Arm" motion, or differential displacement, since the arms are simultaneously changing lengths in opposing ways. As described above, as the lengths of the arms change, so too does the distance traveled by each laser beam. A beam in a shorter arm will return to the beam splitter before a beam in a longer arm--as the wave passes, each arm oscillates between being the shorter arm and the longer arm. When they arrive back at the beamsplitter (where they re-merge), the light waves no longer meet up nicely; they are out of phase. Instead, they shift in and out of alignment for as long as the wave is passing. https://www.ligo.caltech.edu/page/what-is-interferometer https://www.ligo.caltech.edu/page/what-is-interferometer https://en.wikipedia.org/wiki/LIGO https://en.wikipedia.org/wiki/LIGO

samstave 3y ago
>*Gravitational waves cause space itself to stretch* Please ELI5 specifically and empiracally what "Space Itself" actually is. Would it be possible to build a 'galactic clock & Compass' - a "clock" to the regular pulses of a pulsar and the galactic direction the pulsar is in relation to the terrestrial compass (magnetic) on earth...? What is the pulsar with the most reliable timings?
- pantulis 3y ago
  Space itself means the metric of spacetime: how you measure distances. Locally, if there was no gravity, you could think of space as a plain reticule where distances would be calculated through eucledian geometry: that means cartesian distances, parallel lines would be parallel and the angles of a triangle would sum 180, and the shortest path between two points is the straight line. General Relativity successfully establishes that the presence of mass distorts this, so it defines a mathematical object (the Einstein tensor) that reacts to the distribution of mass and energy and precisely describes the changes to the metric. For example it can model how the mass of the sun distorts the space so that light from distant stars appear to follow a curved path because very close to the sun a curved path is now the shortest path. The Einstein tensor defines how distances --and time-- are measured and it's the best mathematical model that we have about what "space itself" is. Future theoretical advances could take us forward and demonstrate that space itself emerges from other more fundamental elements, but this needs bridging quantum mechanics and gravity. We don't really know what space is made of, but scientists have precisely modelled how it reacts to mass (and energy) with utmost precision. NB: At cosmic scales the exercise becomes more difficult, as there is an expansion of the metric of spacetime that is not due to the presence of mass, in fact it is caused by the _absence_ of mass as it seems to be due the energy of empty space: the phenomenon called Dark Energy. Hope this helps!

jessriedel 3y ago

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?

berniedurfee 3y ago

I was wondering if these waves could affect our brains, even subtly. Maybe cause a reaction in one part of one cell that tips the balance between a neuron firing or not.

mr_toad 3y ago
The amplitude of the waves we have detected are less than the width of a proton.
- berniedurfee 3y ago
  Wavelength, right?

mr_toad 3y ago

The gravitational waves we detect are transverse waves. Longitudinal gravity waves have been proposed, discarded, and proposed again, I’m not sure what their current theoretical status is.