11 ms·

I still don't understand what the carrier of gravitational waves is. How are the waves propagated? Space is not an elastic fabric or water where water molecule

• deleted 3y ago
  [deleted]
• dotnet00 3y ago
  Space is like a fabric, it's a literal compression and stretching of space as the wave propagates through it. The distances physically get shorter and longer as the wave passes by. It doesn't necessarily mean that spacetime cannot be fundamental, as there's no rule that says that space itself must emerge from some sort of underlying medium (eg the way fabric is made of threads).
  • beders 3y ago
    What is stretching/compressing it? Spacetime is warped by mass but no mass is transmitted. So what exactly is being carried along?
    • pseudosudoer 3y ago
      You're essentially asking the same question after it's been answered. Space-time is the carrier that acts like a fabric, and some large cosmological mass is distorting that fabric. The distortion of the fabric of space-time ripples throughout the entire universe at the speed of light, and spreads it's effect proportional to the inverse square related to distance.
    • Sharlin 3y ago
      The surface of water is warped by a rock thrown into a pond, but no rock is transmitted by the waves. The electromagnetic field is disturbed by accelerating charges but no charges are transmitted by the EM waves. In any case, mass is not the only thing that warps spacetime. The components of the stress–energy tensor [1] include energy (which includes mass) density, energy flux, momentum density, and momentum flux, the latter of which is composed of pressure and shear stress components. [1] https://en.wikipedia.org/wiki/Stress%E2%80%93energy_tensor https://en.wikipedia.org/wiki/Stress%E2%80%93energy_tensor
    • dotnet00 3y ago
      Are you perhaps mis-applying how we say that photons carry the electromagnetic force? If so, for now we believe that gravity is simply the result of the warping of spacetime, with no known carrying particle. That's kind of the hurdle between uniting gravity with the quantum world. If say, it turns out that there is such a thing as a graviton - which carries gravity, then gravitational waves would be a bunch of gravitons.
• turndown 3y ago
  >Space is not an elastic fabric Space is an elastic fabric, if Einstein's explanation of a spacetime dimension is correct. This would mean that gravity is the warping and stretching of this dimension. If it is not then the other solution is in quantum field theory, which would state that gravity is a field with a messenger particle, the graviton, in which case we are just detecting a group of gravitons as they pass by.
  • beders 3y ago
    The confusing thing here is that space time warps in the presence of mass. But no mass is transmitted in a gravity wave. So what is the fabric made of?
    • ianburrell 3y ago
      Space is the fabric. Moving electric charges produce electromagnetic waves (light). No charge is transmitted with light, but it can cause charges to move. Space is also the fabric for electromagnetic waves.
    • ajuc 3y ago
      When you move mass around - the spacetime curvature responds with light-speed delay. I thought gravitational waves are just that - delayed change of curvature of space-time in response to the mass distribution changing in one place?
    • vl 3y ago
      We use words to describe abstractions which try to model reality as we observe it. In case of "space time warps in the presence of mass" it's important to understand that this is just a model. Another way to explain it that there is no space-time per se, only masses (with special case of photon et al) and masses interact. What we mean by space-time is that if there was a tiny mass in this given point, it would experience given force. Or, again, modeled differently, travel along given path. But if there is nothing to interact with, there is no "space-time" per se in this point, after all, it is an abstraction to describe interactions. So, to sum up, masses interact, space-time is an abstraction to conveniently describe how they interact. Gravitational wave is two massive masses rotating and shaking third small mass as a result of distance changes.
• consumer451 3y ago
  Along similar lines, I wonder if gravitational waves can be focused.
  • pfdietz 3y ago
    They should be focusable by objects with mass, just as light is. A difference is that gravitational waves could be focused by the Sun's core, and so would come to a focus closer to the Sun than light would be, which would have to be focused by the entire Sun (as any light passing through the Sun itself is blocked.)
  • raattgift 3y ago
    Yes, foreground massive objects can magnify gravitational waves emitted by a background source (the foreground objects can also induce a beating pattern on the background waves, or split the background waves into multiple wavefronts). The "foreground" emitter in a massive cluster of thousands of galaxies can be deep inside the cluster, and those are probably the gravitationally-lensed gravitational-wave sources we will identify first [more below at [1]]. A good starting point for a non-specialist is probably <https://astrobites.org/2021/05/25/gw-lensing/ https://astrobites.org/2021/05/25/gw-lensing/>. (You could compare a specialist paper <https://www.aanda.org/articles/aa/full_html/2020/11/aa38730-20/aa38730-20.html https://www.aanda.org/articles/aa/full_html/2020/11/aa38730-...> which describes the lensingGW software package https://gpagano.gitlab.io/lensinggw/ https://gpagano.gitlab.io/lensinggw/ ). - -- [1] When a candidate gravitational wave (GW) detection is found at LIGO et al., an electromagnetic (EM) detection may follow. Neutron stars collide explosively, generating a huge burst of very high-frequency (gamma ray) EM. Black holes don't explode but their accretion structures can interact very strongly producing high-frequency (X-ray) EM. These bright high-frequency EM flashes decay into a glow that can last for many days after a merger. Consequently they are looked for, as part of the overall project of multi-messenger astronomy (where EM and GW are two "messengers"; neutrinos and so forth make it "multi-"). Close to the source, peak EM tends to lag a bit behind the merger, mostly because of how the EM is produced at some distance outside merged black holes. Additionally, EM scatters/refracts/etc with dust and gas that one finds deep inside galaxies, which are more transparent to GW than to EM. Consequently in general GWs will be detected before EMs from the same source. Candidate GW detections provoke searches across the electromagnetic spectrum, since EM may arrive hours or even days later. GW from an inspiral and merger in general have much much longer wavelengths than the X-rays from the gas around the merging objects. Black holes merge roughly when their horizons touch, and horizon size is proportional to mass. GW wavelength is proportional to the orbital diameter, which must be larger than the sum of horizon radiuses, so supermassive black hole binary (SMBHB) mergers have much longer-wavelength GW than stellar-mass black hole binary mergers. The relatively short wavelengths of electromagnetic radiation will follow the laws of geometrical optics and in particular will experience Shapiro delay. GW wavelengths, being much longer, will not. This explains a further delay on EM from SMBHB mergers that are deep within a massive galaxy or cluster of galaxies. Foreground lenses (e.g. massive galaxies that the source does not live within) will further increase the delay suffered by EM produced by the merger that sourced the GW. Moreover, since foreground lenses can split the wavefronts of the GW and EM, there may be ~four detections, at different times, of the same merger in GW and EM. Alternatively, the foreground lens can magnify the source merger, but with the delays described above and geometric optics vs not, the focal point for GW and for EM will differ. All of this is active research, close to cutting edge, so sadly is mostly to be found via a literature search on Einstein lensing of gravitational waves. AFAICT most "pop sci" attempts to summarize some of the academic literature is simply awful. Hopefully I'm merely just not great above. :-/
• eis 3y ago
  I don't think they are a hint that spacetime is not fundamental. But I do think spacetime has to be some kind of real physical reality. The modifications of spacetime that we see as effects of gravity are relative changes to our immediate surroundings or reference frame. Similarly how you can't tell who is actually stationary and who is moving when two objects are in freefall and all you can note is the relative speed between the two, it would be equally valid to say the objects inside spacetime are getting distorted relative to spacetime.
• pfdietz 3y ago
  Gravitational waves follow because space time and gravity are described by equations that admit waves as solutions. Thinking about what space time is "made of" is contrary to what mathematical physics is about. This was a realization with electromagnetism: early attempts tried to interpret the EM field as something mechanical, but it was eventually realized that math making predictions was what one should focus on, and not try to create some sort of more comfortable interpretation in terms of materials or gears or something.
  • beders 3y ago
    Not looking for a comfortable interpretation. The confusing thing here is that spacetime warps in the presence of mass. But no mass is transmitted in a gravity wave. So how do the waves travel?
    • snarkconjecture 3y ago
      There are many ways for spacetime to warp, which can be put into two categories. The simpler kind, Ricci curvature, is the only kind of curvature in <4 dimensions and is produced by mass-energy, momentum, pressure, and shear stress, according to general relativity. The other kind, Weyl curvature, only exists in 4 or more spacetime dimensions and can exist in a vacuum. Gravitational waves are Weyl-curvature distortions of spacetime that propagate in a vacuum according to general relativity. (Also, gravitational waves do carry a little bit of energy, so they cause a small amount of Ricci curvature, but this is a secondary effect.)
      • raattgift 3y ago
        I'm not sure why you want to draw attention to non-Lorentzian spacetimes in this context. > ... that propagate ... "propagate". That requires a decomposition of spacetime into space+time, and of course the decomposition of the Riemann curvature tensor, the setting of a background value for the Weyl tensor, and the use of perturbation theory. But if you're going down that path, why not use the metric tensor? g_munu = eta_munu + h_munu + h.o.t. is standard in post-Newtonian expansion approximations, and in particular https://en.wikipedia.org/wiki/Linearized_gravity https://en.wikipedia.org/wiki/Linearized_gravity (which doesn't track the higher-order terms). The Weyl curvature tensor C_abcd is useful in understanding that in a spherical region of space (not spacetime, so really we're in the land of extracting 3-Cotton-York C_ab) where a GW is incident suffers not from a volume deficit but from an ellipsoidal stretch-squash. But conceptual understanding of and calculation are... well, not really on speaking terms. Even theorists who take the full covariant theory seriously will decompose further, into e.g. an electrical and magnetic part, and add further structure to match the worldlines to the Raychaudhuri equation in shear and vorticity. > they cause a small amount of Ricci curvature ??? If nothing else, I think you'd need to choose between explaining this or explaining why "Ricci curvature is produced by [matter but] Weyl curvature ... can exist in a vacuum" (or choose neither). The sticky bead apparatus is a breaking of the T_munu = 0 vacuum condition.
• jrmg 3y ago
  I think you’re looking for a ‘ether’? Many people have asked similar questions about all sorts of physical phenomena over the years. https://en.wikipedia.org/wiki/Aether_theories https://en.wikipedia.org/wiki/Aether_theories
• nwallin 3y ago
  So forget about waves for a second. Forget about spacetime. Forget about general relativity. The force of gravity is correlated to the mass of the object divided by the square of the distance to the object. The higher the mass, the lower the curvature. The larger the distance, the lower the curvature. The force of gravity doesn't change instantaneously. The speed is the speed of light. If you move the Sun around using Sufficiently Advanced Technology, (SAT) the orbit of the Earth won't change until 8 minutes later, when the gravity has had time to propagate from the Sun to the Earth. So what does it feel like if you're on the Earth and somebody is using SAT to wiggle the Sun around? The force felt from the Sun will go up and down. The math that you would use to quantify this changing force is the same math you'd use to characterize sound waves, or water waves, or electromagnetic waves. So we call this changing tidal acceleration 'gravitational waves'. That's how gravitational waves were described in the 19th and early 20th century, before general relativity. Not long after Einstein published his paper on General Relativity, he adapted the same process via which gravitational waves propagate in Newtonian F = m1 m2 G / r^2 gravity to General Relativity/curvature of spacetime gravity. The math is nasty, and it wasn't really ironed out until decades later, but the theoretical predictions of what gravitational waves will look like in any given detector was worked out well before we actually went and detected them. > Space is not an elastic fabric It's a 4D/(3,1) Lorentzian manifold. If you want a better analogy than the stretched elastic fabric analogy, unfortunately the next step is to grab a textbook and slog through the math.
• beders 3y ago
  Thanks a lot for all the answers! Many of them are outside of my math skills but I appreciate the different ways this was explained. Thanks!