Naked Science Forum

Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: MarianaM on 24/09/2019 17:45:49

Title: Are gravitational waves faster than light?
Post by: MarianaM on 24/09/2019 17:45:49
Roy is asking...

Reading between the lines of what Joris said on the 3rd of September podcast, is it now accepted that gravity and gravitational waves travel FASTER than the speed of light? Or is he suggesting that there is some kind of gravitational mechanism where the speed of the photons produced by the event are retarded (held back, impeded?) by the accompanying gravitational incident that allows the gravitational wave to reach us ahead of the visible spectacle?

Can you help clear up this light wrinkle?
Title: Re: Are gravitational waves faster than light?
Post by: Halc on 24/09/2019 18:19:46
I know not of the podcast in question, but an event like a supernova has the light being delayed by the shock wave needing to travel to the surface of something like a star going supernova.  The light is also not subsequently traveling in a vacuum.  Gravity waves on the other hand are unimpeded by any of this and get here first.  So do neutrinos, which don't even travel at light speed, but fast enough that light (given the initial delay) never manages to catch up.
Title: Re: Are gravitational waves faster than light?
Post by: alancalverd on 25/09/2019 06:48:11
So do neutrinos, which don't even travel at light speed, but fast enough that light (given the initial delay) never manages to catch up.
Never? Surely if A travels faster than B, it will eventually overtake? Or am I missing something?
Title: Re: Are gravitational waves faster than light?
Post by: Hayseed on 25/09/2019 07:23:22
I'll bet the first thing out of a super nova explosion is light(or EM).  Just like a nuc.  Probably a lot of x and g ray.  You might delay light a little, but not hard x and g ray.
Title: Re: Are gravitational waves faster than light?
Post by: Halc on 25/09/2019 09:23:00
So do neutrinos, which don't even travel at light speed, but fast enough that light (given the initial delay) never manages to catch up.
Never? Surely if A travels faster than B, it will eventually overtake? Or am I missing something?
Not before the two of them are detected here.  Not sure how far away a supernova needs to be to have the light win the race.  Neutrinos are so fast that they were thought to not have rest mass, similar to light, but it has since been demonstrated otherwise.
Title: Re: Are gravitational waves faster than light?
Post by: evan_au on 25/09/2019 11:31:06
Quote from: OP
on the  3rd of September podcast, is it now accepted that gravity and gravitational waves travel FASTER than the speed of light?
on 3rd September 2019 there was a segment on the possible collission of a black hole and neutron star:
https://www.thenakedscientists.com/podcasts/naked-scientists-podcast/stripping-down-stis

I think that the the speed discussion of gravitational waves centers around the neutron star collision detected almost simultaneously (1.7s apart) by LIGO (gravitational waves) and by the Fermi gamma-ray satellite (electromagnetic waves, aka "light").
- Crucially for this discussion, the gravitational waves were earlier than the light.
- But this does not mean that gravitational waves are faster than light!
- The gravitational waves were detected for 100 seconds prior to the colission, as the two neutron stars spiralled closer together.
- The gravitational waves stopped when the two bodies collided
- However, the gamma rays can only start after the two bodies collided, so the gravitational waves have a "head start"
- The gamma rays would originate from the "splatter" of neutron-star stuff that is sprayed into space from the impact.
- suddenly freed from the gravitational field that was holding it together, this would be like the nuclear decay of an immensely heavy nucleus, which would emit all sorts of nuclear radiation, including gamma rays.
- The brightness of the source is affected by the size of the source.
- At the instant of collision, the source is only about 20km across
- But the velocity of the collision would have sprayed radioactive matter into space at a significant fraction of the speed of light, increasing the size of the source significantly in the subsequent few seconds
- So the peak output of gamma rays would have been slightly after the collision.
- The best we can say is that gamma rays travel at almost exactly the same speed as light (within 1.7 seconds out of a travel time of 130 million years)
   
See: https://en.wikipedia.org/wiki/GW170817#Scientific_importance

As for neutrinos, we know that they travel slower than light, but by a very small margin, < 10-9
- This is such a small difference that scientists have not yet managed to measure it on Earth, although the LHC participated in an experiment to try and measure it.
- We know the energy spectrum of neutrinos emitted by the Sun
- If we knew the rest-mass of the neutrino, we would be able to calculate the speed of neutrinos required to produce this energy according to special relativity
- Unfortunately, there are only some very loose bounds on the mass of the neutrino
- Neutrinos escape from an exploding star well before the shock wave makes it to the surface, so it is hard to compare the speeds.
- If the neutrinos had some visible-light impact on (say) a gas shell surrounding the star, we might have a common "starting gun" for a cosmic race. But neutrinos don't have much impact on normal matter.
See: https://en.wikipedia.org/wiki/Measurements_of_neutrino_speed
Title: Re: Are gravitational waves faster than light?
Post by: evan_au on 25/09/2019 21:50:41
Quote from: evan_au
the "splatter" of neutron-star stuff that is sprayed into space ... which would emit all sorts of nuclear radiation, including gamma rays
PS: This would also include neutrinos, released as the neutrons decayed into protons & electrons.

So the next time a neutron star merger occurs in our galaxy, there might be enough neutrinos to detect on Earth, and we might be able to compare the arrival time of gravitational waves, neutrinos, and gamma rays.

By the way, when we refer to c, we mean "the speed of light in a vacuum".
- But even intergalactic space contains a wisp of matter, so not even light travels at c.
- This is most obvious in the case of the rather mysterious Fast Radio Bursts (FRBs)
- These arise from a very short event, lasting perhaps 2ms: A very clear "starting gun"
- But the radio signal is delayed by the intergalactic medium, so the high frequencies arrive first, and lower frequencies arrive later
- This is because lower frequencies are delayed more as they travel through the ionised intergalactic medium.
- This mechanism would not delay gamma rays very much, as they have very high frequency
- This mechanism would not delay neutrinos at all, as they don't interact with electric fields

See: https://en.wikipedia.org/wiki/Fast_radio_burst#Features