
Ask HN: Why do gravity waves travel at the speed of light? - AnimalMuppet
ELI5 (OK, ELI20, but not a general relativist).<p>The speed of light is the speed of propagation of electromagnetic radiation.  It is fixed by two constants:  the vacuum (electrical) permittivity, and the vacuum (magnetic) permeability.<p>Why should the speed of propagation of gravity waves have anything to do with either of those?  They aren&#x27;t electromagnetic at all.<p>Or does something more fundamental force both electromagnetic and gravity waves to travel at c, and that more fundamental something thereby imposes a relationship between the vacuum permittivity and the vacuum permeability?  If so, <i>how</i>?
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gus_massa
c is a constant of the universe, it's not related to light :)

There are some important constants of the universe, one of them is c. Let's
call it "c" without a friendly name in English.

You can (theoretically) measure c in the laboratory without in pure mechanical
experiments in experiments that are not about light. The easiest experiment
that I can imagine now is the change of the half life of the muons
[https://en.wikipedia.org/wiki/Time_dilation_of_moving_partic...](https://en.wikipedia.org/wiki/Time_dilation_of_moving_particles#Experiments)
but I'm not sure to classify it as "pure mechanical experiment".

c is the only speed that doesn't change when you go from a inertial reference
frame to another inertial reference frame (i.e. if you have a train moving at
a constant velocity and you measure something inside the train or measure it
though the window from the platform.)

For some technical reasons the speed of all massless particles must be c.
(Because they must travel at the same velocity in every reference frame,
because all the reference frames are equivalent.)

And all the waves that can travel in empty space for a long distance must be
massless. So the photons and gravitons travel at c. (The theory of gravitons
is far for complete, but just ignore this detail.)

For historical reasons, the first time that someone measure c was while trying
to measure the speed of light. So they called it "the speed of light". Much
later Physicist realized that c was an important constant of the universe and
not only the speed of light, but the name stuck and is too late to change it.

So ...

* gravitational waves travel at c

* light waves travel at c. Actually it's better to define mu0=1/(c^2.epsilon0) or something like that. In special relativity electric and magnetic fields are very related, (it's similar to the relation between space and time, but not exactly the same relation). So it's "better" to think that they have only one constant and c, than thinking that they have two independent different constants. Physicist still use two constants, for historical reasons and because sometimes it makes the calculation easier.

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db48x
First a nitpick: gravity waves are those things you see on the surface of the
ocean. You're asking about gravitational waves, which are a disturbance in the
curvature of spacetime.

As for the speed, nobody knows! In fact for the last 100 years it was only
conjectured that they travel at the same speed as light. It was only last year
that a really solid measurement was taken by simultaneously observing a
neutron star inspiral in both visible light and gravitational wave
observatories.

Presumably when you make your Grand Unified Theory you'll have to explain
this.

