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This might be a naive question, but I'd like to clear it out. Do gravitational waves travel at the speed of light? I know the theory says nothing can travel f
by saganus 9y ago
This might be a naive question, but I'd like to clear it out.
Do gravitational waves travel at the speed of light?
I know the theory says nothing can travel faster than light. I also know that photons can be seen as quanta or as waves. So my guess is that gravitational waves travel at most, at the speed of light.
But do they? or do they travel slower? faster? Is there a doppler effect for GWs?
I ask because I would think ripples in the space-time fabric itself might be a bit different than light waves or other more studied phenomena.
Can anyone point me in the right direction?
- sleavey 9y agoThe most popular theories say that yes, they do travel at the speed of light. Only a coincident detection of a gravitational wave and an electromagnetic counterpart would confirm this (such as from a binary neutron star coalescence; the binary black holes seen so far by LIGO didn't emit any visible EM as far as we know).
- tzs 9y agoWould even a binary neutron star coalescence give us the information we'd need to answer this? Interstellar space is not actually empty. It contains a low density plasma, and so the speed of light in it is slightly slower than the speed of light in a vacuum. I have no idea if that plasma would also slow down gravity waves, but even if it does it wouldn't necessarily be the same amount [1]. Maybe from the differences in arrival time of light at different frequencies we could figure out enough about the intervening plasma to figure out when the light would have arrived if the space had been empty, and check if that is when the gravity waves arrived? [1] Edit: I have done some Googling. Gravity waves would not be slowed down by the interstellar plasma. The slowdown of light through a medium depends on the existence of positive and negative charges in the medium that can form dipoles in response to the passing electromagnetic wave. For gravity all the "charges" are positive so there is no dipole formation.
- yorwba 9y ago> For gravity all the "charges" are positive so there is no dipole formation. I'm no physicist, but couldn't lack of matter in some region otherwise filled with particles play the part of "virtual" negative matter?
- imaginenore 9y agoWe could build a second detector, sync their clocks, separate them, and measure the speed directly.
- greglindahl 9y agoLIGO is 2 pairs of detectors.
- diegoperini 9y agoCheck out LIGO episode of PBS SpaceTime youtube channel. Some very informative explanitions about gravity waves are given there.
- Sir_Cmpwn 9y ago+1 for PBS SpaceTime, easily one of the best channels on YouTube. Gravitational wave episodes: https://www.youtube.com/watch?v=1Tstyqz2g7o https://www.youtube.com/watch?v=1Tstyqz2g7o https://www.youtube.com/watch?v=gw-i_VKd6Wo https://www.youtube.com/watch?v=gw-i_VKd6Wo https://www.youtube.com/watch?v=eJ2RNBAFLj0 https://www.youtube.com/watch?v=eJ2RNBAFLj0
- erikpukinskis 9y agoI really wanted to like PBS SpaceTime. So many interesting subjects and I love physics videos on YouTube. But there's something about Matt O'Dowd that makes them unwatchable for me. Something about the pacing, I just can't stay focused. Another great channel is looking Glass Universe - https://youtu.be/r0plv_nIzsQ https://youtu.be/r0plv_nIzsQ And Udiprod doesn't have a habit of making physics vids, but this is my all time favorite physics videos on YouTube which explains quantum waves better than anything else I've seen - https://youtu.be/p7bzE1E5PMY https://youtu.be/p7bzE1E5PMY
- bramen 9y agoThe speed of light is sometimes referred to as the speed of causality, and it seems like it's more of a fundamental speed limit on the propagation of events or information through space.
- deepnotderp 9y ago+1 to this comment, it would remove a lot of mysticism to have called it "the speed of causality" instead. https://www.sciencealert.com/watch-why-the-speed-of-light-is-not-about-light https://www.sciencealert.com/watch-why-the-speed-of-light-is...
- reubenswartz 9y agoIIRC, everything moves through spacetime at c. Things with mass like people, planets, etc, move through the time portion as well as the space portion. As you go faster through space, you travel less through time, though at non- relativistic speeds you don't notice (GPS satellites do have to account for this). Electromagnetic waves have no mass, they don't travel in time, so the entire portion of their travel takes place in space, so we say they travel at the "speed of light."
- Swizec 9y ago> Electromagnetic waves have no mass, they don't travel in time, so the entire portion of their travel takes place in space, so we say they travel at the "speed of light." This part comfuses me. If they don't travel in time, how do they have a speed? Light is a type of electromagnetic wave right? And it takes many years to travel to us from a nearby star. If we can measure or calculate the time it takes for light from some place to reach us, does that not imply traveling through time?
- pducks32 9y agoYea so the universe says nothing can move faster than a massless particle in a vacuum. The photon is the most famous—hence the speed of light—but any massless this can move at it. So GWs do theoretically move at the speed of light, though checking to see if they do requires a lot of analysis and is tricky to figure out. But yes theoretically they do move at the speed of causality (or the speed that things happen in the universe unless someone slows them down) I’m a physicist who’s now listened to a ton of LIGO’s founders talk about it.
- neom 9y agoThese PBS Spacetime episodes should help: The Speed of Light is NOT About Light - https://www.youtube.com/watch?v=msVuCEs8Ydo https://www.youtube.com/watch?v=msVuCEs8Ydo Is Quantum Tunneling Faster than Light? - https://www.youtube.com/watch?v=-IfmgyXs7z8 https://www.youtube.com/watch?v=-IfmgyXs7z8 The Quantum Experiment that Broke Reality - https://www.youtube.com/watch?v=RlXdsyctD50 https://www.youtube.com/watch?v=RlXdsyctD50 Pilot Wave Theory and Quantum Realism - https://www.youtube.com/watch?v=RlXdsyctD50 https://www.youtube.com/watch?v=RlXdsyctD50 The Future of Gravitational Waves - https://www.youtube.com/watch?v=eJ2RNBAFLj0 https://www.youtube.com/watch?v=eJ2RNBAFLj0
- programbreeding 9y agoThank you for this. You linked to the youtube videos but I watch the PBS app on my Roku all the time and it's horrible at recommending what I should watch. "Watched 'City in the Sky' [a 3-part series about airports and planes]? You'll love Downtown Abbey or The Great British Baking Show!" I'm always looking for some actual good/educational shows on there, and there's so many great hidden gems, but they're all... hidden.
- iaw 9y agoSetup your youtube account with liking/subscribing to the content you're interested in (even if it's from PBS) and you'll get some okay recommendations.
- saganus 9y agoAwesome resources. Thanks a lot! I'll have to check them out and study up a bunch of stuff it seems.
- westurner 9y agoThanks! "Neutron star": https://en.wikipedia.org/wiki/Neutron_star https://en.wikipedia.org/wiki/Neutron_star
- 4ad 9y agoAll massless particles like photons and gravitons travel at the same speed, the speed of light. Yes, gravitational waves travel at the speed of light, and yes, of course they are affected by doppler shift.
- SidiousL 9y agoYour intuition is correct and the question of the speed of gravitational waves is pretty complicated. The usual treatment of gravitational waves is done by linearization of some non-linear equations. This is a very good approximation for the propagation of gravitational waves since they disturb the space-time only very slightly (for example, the mirrors in the interferometry experiment at LIGO get displaced by a tenth of a nucleus of a hydrogen atom, during the passage of a gravitational wave). In this linear approximation the gravitational waves are governed by the same wave equation as for electromagnetism (only the spin part is different since the spins of the gravitons and photons are different). Since in the linear approximation we recover the Lorentz symmetry, then the linearized wave equation has to be Lorentz invariant. Then, one can apply results from the representation theory of the Lorentz or Poincare groups; there are two major types of representations: massless and massive. They differ in striking ways when it comes to spin and when it comes to propagation, for example massless particles travel at the speed of light. If you want to have a massive graviton then you need to get the mass somehow from your theory. Einstein's theory predicts a massless graviton, which by the argument above has to travel at the speed of light. We still don't know experimentally if the graviton is massless (but last time I looked at the Particle Data Book there was an upper bound on the mass which was very small). Now, coming back to why your question is complicated. Remember that in Einstein's theory space-time itself is dynamical. Now suppose you follow the propagation of a gravitational wave. Since this takes some time, we need to take into account the fact that the shape of the space-time itself has changed in the meantime. In such dynamical situations it becomes complicated to even define what the speed of propagation between two points is. One way this becomes important is in cosmological situations. For example, the expansion of the universe stretches the distances and this allows us for example to see further that the distance you obtain multiplying the speed of light by the age of the universe. This being said, you can define the speed locally by studying propagation for very short distances and times and this will be a constant. However, you need to remember that the global situation is more complicated.
- saganus 9y agoThis is what I was thinking. I.e. that it would be very complicated. I wasn't even sure if it made sense to ask such a question. I have so much to read now...
- pedro_hab 9y agoI have the same question, based on the Alcubierre Drive math, (Wrap Drive) which is based on that space can travel at any speed. But I also know that the Alcubierre Drive have some weird implications, such as the inner of the ship is causally disconnected from the space wrap, basically you can't control it from the inside. Anyways, I'm curious if the gravitational waves are "space waves", in that case it wouldn't have limit, as space can move at any speed.
- yk 9y agoYes, though one has to go through quite a bit of trouble to actually show that. The big problem is, that speed of light is defined with reference to a background geometry and gravitational waves are a distortion of this background geometry. (And this has of course not be shown yet, however this observation, if the rumors are true, would be evidence for that.)
- saganus 9y agoThanks to everyone for their explanations! It took me a while to read them all and not sure I do understand them all. It's amazing how complicated can physics stuff get. I'll be sure to check all the linked resources. Thanks again!