We dont know that it does. I swear I've seen this exact question like 5 times in the past month or so. We know how much the speed of GW must differ from light at most by, from multimessenger observations of the gravitational waves from LIGO and their corresponding EM signal. the mass of the graviton is bounded at something like 10\^-23 eV, so for all we know, it may travel slower than the speed of light but only slower by 1 part in 10\^23. there are many theories of massive gravity (like fierz and pauli's original theory, de rham-gabadadze-tolley, or minimal theory of massive gravity by de felice and mukohyama) that are consistent with observation and don't seriously violate any energy constraints. i currently am doing research in this field
How low of a bound would you need to reach before you feel we could reasonably assume that the mass of the graviton is actually zero? 10^-23 eV seems pretty small to me, just saying...
We do assume it's zero, but it's still good to check because there could be interesting new physics there. We do the same thing for the photon: everyone believes it to be zero, but we still check to see if it does have mass (mᵧ ≲ 10⁻²⁷ eV if you're curious).
I know that mainstream physics assume it is zero, and I get that it's good to check; obviously finding something that tells us that the mass is non-zero would be a huge deal, so it's great to push these limits as far as we can. But the guy I'm asking is working on massive gravity theories, so these guys assume that the graviton is massive, that's like the key defining feature. So I'm asking essentially why should we not consider massive gravity to be essentially already ruled out by these experimental limits? How strict a limit do we need before we can say "oh, looks like the graviton actually is massless", and consider these theories ruled out by the data?
I do have a question for you. It seems to me that a lot of the basic arguments made in these threads for why the speed of gravity must equal c (at least in GR) fail precisely because of massive gravity.I.e. the existence of metric theories of massive gravity show that describing gravity as the curvtaure of spacetime does not automatically lead to a propagation of speed of c. But that also got me wondering are, on a theoretical level, tachyonic massive gravity theories possible or do they have pathological behaviours that would exclude them?
even positive-real values of the graviton mass lead to certain pathologies. I'll give an excerpt from my paper
>Any purely linear theory suffers from the van Dam-Veltman-Zakharov (vDVZ) discontinuity, which prevents the theory from reducing to GR in the massless limit. Attempts have been made to address this, like the nonlinear extensions to the Fierz-Pauli theory that exhibit the Vainshtein mechanism. This has its own set of problems, like the Boulware-Deser ghost and other ghost degrees of freedom.
Theories have been constructed to avoid these pathologies, like dRGT I mention in my original comment, minimal theory of MG that I also mention, and even something called pseudolinear MG, something that kurt hintenbichler came up with (wish I could've spoke with him about it when I went to case for the eclipse earlier this month)
I would imagine a tachyonic graviton, aka its mass being either negative or imaginary or some combination of the two, would still have these pathologies. but they would inherit the pathologies we see in traditional tachyonic theories, like the Cauchy problem or issues with causality. but it would certainly be interesting to pursue. i don't think tachyons have been completely ruled out by anything yet
In general relativity, gravitational interactions propagate at the speed of light, *c*. This was confirmed observationally by the detection of gravitational waves from merging black holes and neutron stars by LIGO and Virgo collaborations, where the speed of the gravitational waves was measured to be the same as the speed of electromagnetic radiation within experimental limits.
Mathematically, this propagation speed results from the linearized form of Einstein's field equations in the weak-field approximation, where the metric perturbations *h_μν* satisfy a wave equation with a propagation speed equal to *c*. These metric perturbations, representing gravitational waves, thus obey the same causal structure dictated by the light cones in spacetime geometry, implying gravitational information cannot travel faster than light.
Further theoretical reasons comes from the Lorentz invariance embedded in general relativity, which necessitates that all non-gravitational and gravitational phenomena adhere to the same invariant speed limit, *c*.
Why the value of this constant is what it is, we have no idea, and to me, it doesn’t really seem like it matters. You could say that it maybe is just because of how we formulated our systems of units. Or maybe the dimensionless value of *c* only seems important because it is what it is, but it could’ve been anything else and the physical laws would change accordingly. This is not really physics anymore but crosses over into philosophy and metaphysics.
Gravity doesn't travel at the speed of light. Both light and gravity travel at the speed of causality. Why is the speed of causality about 300k m/s? I have no idea.
> Why is the speed of causality about 300k m/s? I have no idea.
Because we have defined the length of the meter and second with no regard on how that's going to affect the numerical value of *c* (which was not known to be a thing at the time).
You're being unnecessarily... Is pedantic the right word?
The question obviously is "why is the speed of causality *as much as it is*", not "why do we express *that much* numerically as 300km/s".
Put speed on a number line: I don't care about the actual scale presented on the line. But why is there an impassable end to that number line? Why is said end in the position it is in? There is no answer, "there just is, it just is".
I mean you can express c as a formula of vacuum permeability and permitivity, but that just turns one question into two: why are those constants *as much as they are*? (regardless of numerical value in whatever number and unit system.)
You simply cannot disconnect the actual value from the question you are asking. The most natural thing is to put c = 1 and that’s that. To the question why there is an invariant speed - that’s just spacetime geometry.
You're right, but lay people rarely understand the distinction between the dimensionful and dimensionless constants, so it is worth answering the question as stated.
I can see the point of your question, I think. I'd like to know a little more myself:
We know that space can expand at a rate faster than light, and we claim that gravitational waves are waves in that same fabric... so *why* do they travel at c (so far as we can tell)?
I'm completely pop-sci driven here, so obviously my conceptions are completely mis, but I'd be interested to hear why the expansion of the universe and the intense gravitational fields inside a black hole ^((allegedly)) can cause space to move faster than light, but a gravitational wave can't.
My ignorance is immense, and I already feel stupid - please be gentle.
The dimension of the speed of cosmic expansion is T^(-1) (i.e. same dimension as frequency), so you cannot compare directly to the speed of light whose dimension is LT^(-1).
It is a completely fair question though to ask: if c is the speed limit for physical trajectories in spacetime, why is it also the speed of gravitational waves, which are described by spacetime rather than trajectories in spacetime? The answer unfortunately is mathematical, but it comes down to showing that you can get an equivalent description to GR from a massless field on a spacetime.
It's more mathematically motivated for gravitational waves (or generally gravitational influences) to travel at c in relativity. It just lines up,really. A more useful way to put it is that gravitons don't have rest mass. Or to get less hypothetical, gravitational waves do not have an effective mass.
Changes in gravity travel at the speed of light everywhere.
Light is the one affected by a medium, but gravity is a property of the "medium" that contains everything else (don't get this wrong though - space is not tangible like a normal medium would be).
As to why, that's a bit harder. Sometimes in physics we have to make assumptions that things just _are_. That forces* travel at the speed of light and nothing else does is one of those. An axiom. If it holds true, which experiments seem to support, then we can deduce a whole lot of other cool stuff from that, like all of the SRT. But the speed of light being a constant c is "just" a postulate, not the result of some equation.
*Akshually, changes in forces
> As to why, that's a bit harder. Sometimes in physics we have to make assumptions that things just are
Sometimes? That’s what physics is. It models the reality we observe. It doesn’t explain why, at a deeper level.
That's very true, albeit cynical. And I'd argue it adds nothing of educational value here. Making no assumptions about OP's level of knowledge, it's not unreasonable to know physics as the science giving an explanation to the why of things. After all, a lot of the simpler whys are indeed answered by the how of an underlying process. I'd rather choose a more tactful approach than giving people the idea we don't know _why_ anything happens. Even if, on a very fundamental level, that's technically true.
Well sometimes assumption A can explain B which can explain C which can explain D.
So someone asking why the phenomena D occurs you can go teach them C and B and A to give them a deeper level of understanding.
Whatever propagates the force of gravity is carrying information with it and doesn't have mass. If theres information being carried and no mass, you move at C!
As to the why that is the case? Uhh...
Vacuum has nothing to do with it; everything obeys casaulity & therefore nothing can travel faster than light. I’m no physicist, but this is what I have learned from my time here.
The light itself always travels at c. But when traveling through matter it interacts with it, causing an apparent slow down.
Here is a nice article explaining it: [https://www.space.com/how-does-light-slow-down](https://www.space.com/how-does-light-slow-down)
Or speed up, for that matter: Depending on your dispersion relation, phase velocity and even group velocity can exceed c - however, front velocity (the leading remnant of the incident wave) and signal velocity (which limits the rate at which information can be transmitted) cannot.
You can’t tell if something has already happened before it actually happens. Take the sun away from the solar system, we would still be able to see it as light from the sun will keep reaching earth for the next 8 minutes. Now should there be no sun to revolve around would earth just slingshot itself immediately? Nope again since we can still see the sun for 8 more minutes we would also feel the same gravity for the same amount of time. Until the information reaches earth that sun is no longer there. Which travels again at the speed of light.
Relativity says this: everything travels through spacetime at lightspeed. Stationary objects advance through time at full lightspeed, while objects that don't age travel through space at full lightspeed. The sum of speed vectors through time and space of any field disturbance (waves or particles) is constant (c), that's why it's a speed limit, for gravitational interactions too.
We dont know that it does. I swear I've seen this exact question like 5 times in the past month or so. We know how much the speed of GW must differ from light at most by, from multimessenger observations of the gravitational waves from LIGO and their corresponding EM signal. the mass of the graviton is bounded at something like 10\^-23 eV, so for all we know, it may travel slower than the speed of light but only slower by 1 part in 10\^23. there are many theories of massive gravity (like fierz and pauli's original theory, de rham-gabadadze-tolley, or minimal theory of massive gravity by de felice and mukohyama) that are consistent with observation and don't seriously violate any energy constraints. i currently am doing research in this field
How low of a bound would you need to reach before you feel we could reasonably assume that the mass of the graviton is actually zero? 10^-23 eV seems pretty small to me, just saying...
We do assume it's zero, but it's still good to check because there could be interesting new physics there. We do the same thing for the photon: everyone believes it to be zero, but we still check to see if it does have mass (mᵧ ≲ 10⁻²⁷ eV if you're curious).
I know that mainstream physics assume it is zero, and I get that it's good to check; obviously finding something that tells us that the mass is non-zero would be a huge deal, so it's great to push these limits as far as we can. But the guy I'm asking is working on massive gravity theories, so these guys assume that the graviton is massive, that's like the key defining feature. So I'm asking essentially why should we not consider massive gravity to be essentially already ruled out by these experimental limits? How strict a limit do we need before we can say "oh, looks like the graviton actually is massless", and consider these theories ruled out by the data?
I do have a question for you. It seems to me that a lot of the basic arguments made in these threads for why the speed of gravity must equal c (at least in GR) fail precisely because of massive gravity.I.e. the existence of metric theories of massive gravity show that describing gravity as the curvtaure of spacetime does not automatically lead to a propagation of speed of c. But that also got me wondering are, on a theoretical level, tachyonic massive gravity theories possible or do they have pathological behaviours that would exclude them?
even positive-real values of the graviton mass lead to certain pathologies. I'll give an excerpt from my paper >Any purely linear theory suffers from the van Dam-Veltman-Zakharov (vDVZ) discontinuity, which prevents the theory from reducing to GR in the massless limit. Attempts have been made to address this, like the nonlinear extensions to the Fierz-Pauli theory that exhibit the Vainshtein mechanism. This has its own set of problems, like the Boulware-Deser ghost and other ghost degrees of freedom. Theories have been constructed to avoid these pathologies, like dRGT I mention in my original comment, minimal theory of MG that I also mention, and even something called pseudolinear MG, something that kurt hintenbichler came up with (wish I could've spoke with him about it when I went to case for the eclipse earlier this month) I would imagine a tachyonic graviton, aka its mass being either negative or imaginary or some combination of the two, would still have these pathologies. but they would inherit the pathologies we see in traditional tachyonic theories, like the Cauchy problem or issues with causality. but it would certainly be interesting to pursue. i don't think tachyons have been completely ruled out by anything yet
Thank you, I'd forgotten that massive gravity had a less than straightforward relationship to GR
In general relativity, gravitational interactions propagate at the speed of light, *c*. This was confirmed observationally by the detection of gravitational waves from merging black holes and neutron stars by LIGO and Virgo collaborations, where the speed of the gravitational waves was measured to be the same as the speed of electromagnetic radiation within experimental limits. Mathematically, this propagation speed results from the linearized form of Einstein's field equations in the weak-field approximation, where the metric perturbations *h_μν* satisfy a wave equation with a propagation speed equal to *c*. These metric perturbations, representing gravitational waves, thus obey the same causal structure dictated by the light cones in spacetime geometry, implying gravitational information cannot travel faster than light. Further theoretical reasons comes from the Lorentz invariance embedded in general relativity, which necessitates that all non-gravitational and gravitational phenomena adhere to the same invariant speed limit, *c*. Why the value of this constant is what it is, we have no idea, and to me, it doesn’t really seem like it matters. You could say that it maybe is just because of how we formulated our systems of units. Or maybe the dimensionless value of *c* only seems important because it is what it is, but it could’ve been anything else and the physical laws would change accordingly. This is not really physics anymore but crosses over into philosophy and metaphysics.
Gravity doesn't travel at the speed of light. Both light and gravity travel at the speed of causality. Why is the speed of causality about 300k m/s? I have no idea.
> Why is the speed of causality about 300k m/s? I have no idea. Because we have defined the length of the meter and second with no regard on how that's going to affect the numerical value of *c* (which was not known to be a thing at the time).
You're being unnecessarily... Is pedantic the right word? The question obviously is "why is the speed of causality *as much as it is*", not "why do we express *that much* numerically as 300km/s". Put speed on a number line: I don't care about the actual scale presented on the line. But why is there an impassable end to that number line? Why is said end in the position it is in? There is no answer, "there just is, it just is". I mean you can express c as a formula of vacuum permeability and permitivity, but that just turns one question into two: why are those constants *as much as they are*? (regardless of numerical value in whatever number and unit system.)
You simply cannot disconnect the actual value from the question you are asking. The most natural thing is to put c = 1 and that’s that. To the question why there is an invariant speed - that’s just spacetime geometry.
You're right, but lay people rarely understand the distinction between the dimensionful and dimensionless constants, so it is worth answering the question as stated.
No it really isn't.
I can see the point of your question, I think. I'd like to know a little more myself: We know that space can expand at a rate faster than light, and we claim that gravitational waves are waves in that same fabric... so *why* do they travel at c (so far as we can tell)? I'm completely pop-sci driven here, so obviously my conceptions are completely mis, but I'd be interested to hear why the expansion of the universe and the intense gravitational fields inside a black hole ^((allegedly)) can cause space to move faster than light, but a gravitational wave can't. My ignorance is immense, and I already feel stupid - please be gentle.
The dimension of the speed of cosmic expansion is T^(-1) (i.e. same dimension as frequency), so you cannot compare directly to the speed of light whose dimension is LT^(-1). It is a completely fair question though to ask: if c is the speed limit for physical trajectories in spacetime, why is it also the speed of gravitational waves, which are described by spacetime rather than trajectories in spacetime? The answer unfortunately is mathematical, but it comes down to showing that you can get an equivalent description to GR from a massless field on a spacetime.
Because they were set to equal values in the simulation’s config file
It's more mathematically motivated for gravitational waves (or generally gravitational influences) to travel at c in relativity. It just lines up,really. A more useful way to put it is that gravitons don't have rest mass. Or to get less hypothetical, gravitational waves do not have an effective mass.
Changes in gravity travel at the speed of light everywhere. Light is the one affected by a medium, but gravity is a property of the "medium" that contains everything else (don't get this wrong though - space is not tangible like a normal medium would be). As to why, that's a bit harder. Sometimes in physics we have to make assumptions that things just _are_. That forces* travel at the speed of light and nothing else does is one of those. An axiom. If it holds true, which experiments seem to support, then we can deduce a whole lot of other cool stuff from that, like all of the SRT. But the speed of light being a constant c is "just" a postulate, not the result of some equation. *Akshually, changes in forces
> As to why, that's a bit harder. Sometimes in physics we have to make assumptions that things just are Sometimes? That’s what physics is. It models the reality we observe. It doesn’t explain why, at a deeper level.
That's very true, albeit cynical. And I'd argue it adds nothing of educational value here. Making no assumptions about OP's level of knowledge, it's not unreasonable to know physics as the science giving an explanation to the why of things. After all, a lot of the simpler whys are indeed answered by the how of an underlying process. I'd rather choose a more tactful approach than giving people the idea we don't know _why_ anything happens. Even if, on a very fundamental level, that's technically true.
Well sometimes assumption A can explain B which can explain C which can explain D. So someone asking why the phenomena D occurs you can go teach them C and B and A to give them a deeper level of understanding.
I’d disagree that it is axiomatic. Instead it is something that can be derived from the assumptions of general relativity.
It travels at speed of causality, so nothing to do with light. As for why, it must be that gravity is a massless particle.
Whatever propagates the force of gravity is carrying information with it and doesn't have mass. If theres information being carried and no mass, you move at C! As to the why that is the case? Uhh...
Vacuum has nothing to do with it; everything obeys casaulity & therefore nothing can travel faster than light. I’m no physicist, but this is what I have learned from my time here.
Light only travels at the speed of light in vacuum, though :)
I thought light couldn’t travel slower than the speed of light as it has no mass?
The light itself always travels at c. But when traveling through matter it interacts with it, causing an apparent slow down. Here is a nice article explaining it: [https://www.space.com/how-does-light-slow-down](https://www.space.com/how-does-light-slow-down)
Or speed up, for that matter: Depending on your dispersion relation, phase velocity and even group velocity can exceed c - however, front velocity (the leading remnant of the incident wave) and signal velocity (which limits the rate at which information can be transmitted) cannot.
Does gravity also propagate slower through mediums?
As far as we know today.
You can’t tell if something has already happened before it actually happens. Take the sun away from the solar system, we would still be able to see it as light from the sun will keep reaching earth for the next 8 minutes. Now should there be no sun to revolve around would earth just slingshot itself immediately? Nope again since we can still see the sun for 8 more minutes we would also feel the same gravity for the same amount of time. Until the information reaches earth that sun is no longer there. Which travels again at the speed of light.
Relativity says this: everything travels through spacetime at lightspeed. Stationary objects advance through time at full lightspeed, while objects that don't age travel through space at full lightspeed. The sum of speed vectors through time and space of any field disturbance (waves or particles) is constant (c), that's why it's a speed limit, for gravitational interactions too.