[trevorjohnson32]

Maybe the question should be why gravity waves and light waves move the same speed at all. A gravity wave doesn't have weight like energy does and would assumably travel faster then light. I think saying gravity waves travel light speed is like saying light waves travel as fast as sound waves. They are three different things.

[Bogie_smiles]

Maybe the question should be why gravity waves and light waves move the same speed at all. A gravity wave doesn't have weight like energy does and would assumably travel faster then light. I think saying gravity waves travel light speed is like saying light waves travel as fast as sound waves. They are three different things.

There are some observations which might be clues to why they both travel at the speed of light. Light has a wave nature, and their frequencies and wavelengths define their energy. The nature of gravitational waves shows they also traverse vast distances of space in the form of waves. Light waves all travel at the speed of light, and yet they carry a vast range of different amounts of energy as shown by the wide scope of the electromagnetic spectrum. Gravity waves are only detected at the highest end of their energy range, but it is not hard to conclude that there is also a wide range of energies in the gravitational wave spectrum as well.

Light waves and gravitational waves both carry energy through space, and each have a vast range of energies that are carried by their various frequencies of waves. So maybe it is a characteristic of space itself that governs the speed of energy carrying waves through it? Could the factors like permittivity and permeability in space be associated with the same space traversed by both light and gravitational waves, and so would seemly affect all energy waves the same way, thus invoking a particular seed, the same speed for both light waves and gravitational waves?.

[Bill S]

  A "classical" way of looking at this broadband effect is traceable back to Maxwell. Space has a certain permittivity and permeability, which affect it's "stiffness", and how fast a wave propagates through this medium.

If space were “empty”, presumably there would be no “stiffness” involved.  Does this mean that permittivity and permeability are the result of matter in space, or the activity of virtual particles “coming and going”?

Is the "broadband effect" due to the fact that, unlike the atomic orbitals which are discrete, the phonon spectrum can be broad and continuous over a large frequency range?

[evan_au]

If space were “empty”, presumably there would be no “stiffness” involved.

If a region of space in our universe had no matter (or even dark matter, for that matter...), and was far from any gravitational well, light would still travel at "c" (as seen by a local, immaterial observer). This is far faster than the speed of vibrations through hard substances like steel or diamond.

This means that empty space is very "stiff".

This is one consideration that eventually killed the "aether" theory - space is known to be very stiff, and yet the Earth circles around the Sun, apparently with no impediment.

The presence of atomic matter in space can only make it less stiff - light travels more slowly through matter.

Why does the light arrive after the gravity waves from colliding neutron stars?

Matter in the universe curves spacetime (according to Einstein). So light traveling near a mass must travel farther into and out of the gravitational well, so it takes longer than light which travels the same distance, but does not pass near a mass.

The burst of gravitational waves and gamma rays must escape from the gravitational well of:
- the colliding neutron stars (which together may have been enough to form a black hole at the end of the collision)
- the galaxy in which the collision happened
- in and out of various gas clouds in between
- past the edge of several other galaxies
- and finally into our galaxy
- and this gravitational well is even deeper because of the mysterious Dark Matter that seems to surround galaxies

This curvature of spacetime affects both light and gravitational waves, delaying both.

However, light is an electromagnetic wave, and is additionally delayed due to the presence of:
- Matter which is displaced by electric fields, eg electrons(-) & protons(+) = interstellar gas
- Matter which is displaced by magnetic fields, eg nuclei with an odd number of spins, magnetic materials, etc
- I assume that light would not suffer an additional delay from the presence of Dark Matter on the path; current theories suggest that Dark Matter particles do not respond to electric or magnetic fields.

[Victoriab]

Why does the light arrive after the gravity waves from colliding neutron stars?

[I'd say because the waves are caused by the MOVEMENT of the neutron stars, while the radiation is given off due to the COLLISION itself. The acceleration also increases each time the neutron stars lap; coming closer together.
* Gravitational ripples - kinetic energy
*Light- Light and heat energy.

[Bill S]

Hi Victoriab; welcome.

The acceleration also increases each time the neutron stars lap;

Is it the acceleration, the velocity or the speed that increases?  I can tie myself up in knots on that subject very easily.

[Victoriab]

Hi Victoriab; welcome.

The acceleration also increases each time the neutron stars lap;

Is it the acceleration, the velocity or the speed that increases?  I can tie myself up in knots on that subject very easily.
[the ACCELERATION, as it is gaining momentum.]

[evan_au]

Is it the acceleration, the velocity or the speed that increases?

All 3 increase as 2 neutron stars spiral together.

As the neutron stars spiral closer together:
- Their mass stays constant, in their own frames of reference
- As Newton's gravity predicts, as you move two masses closer together, the force between them increases
- As Netwon's F=ma predicts, if the force increases, the acceleration increases.
- To keep two objects in orbit at a reduced distance, the orbital velocity must increase (which can be deduced from Kepler's laws)
- The orbital speed is the same as orbital velocity (just ignoring the direction)

All 3 increase as 2 neutron stars spiral together - until they actually touch, in which case the velocity of their centers of mass drops abruptly. An abrupt change in velocity represents an extreme acceleration (or deceleration).

[Bill S]

Thanks, Evan; even I can get my head round that. 

Would the negative acceleration, on impact, cause the kinetic energy to convert to heat, which, in turn, would be responsible for the emission of EM radiation?

[yor_on]

" A big difference between the light and gravitational waves in this scenario is that gravitational waves have been emitted constantly over the course of the neutron star pair's orbit, whereas the light was only emitted during the collision itself. As the pair spiral closer to each other, the gravitational waves grow stronger and stronger. I imagine the reason that we detected the waves before we detected the light was because the waves grew strong enough to detect 2 seconds before the actual collision took place. " By Kryptid

:)

Have to cite that

Gravity waves are presumed to travel at 'c'.

[PmbPhy]

A big difference between the light and gravitational waves in this scenario is that gravitational waves have been emitted constantly over the course of the neutron star pair's orbit, whereas the light was only emitted during the collision itself.

Where did you get that notion from? Neutron stars are always emitting light at various wavelengths. If this wasn't the case we'd never have detected them. We know their there due to fact that they emit light. By light I mean electromagnetic wave. This is especially true for pulsars, which is in the class of neutron stars. The light is to dim to be seen with the naked eye. They're observed using radio telescopes.

Evan was right. Light travels at the exact same speed as gravitational waves.

[evan_au]

Neutron stars are always emitting light at various wavelengths.

This is true; the surface temperature of a neutron star approaches 1,000,000K, so it radiates well at X-Ray wavelengths. However, its surface area is very small, so the total power output is pretty low - not detectable at a range of 130 million light-years.

However, when you gouge great gobs of neutronium out of a neutron star and spray it into space, then you have a kilonova, and it is comparable in brightness to the whole galaxy in which it resides (for a short time). The short burst of gamma rays from 130 million LY away  outshone all gamma-ray sources in our galaxy - for under 1 second.

See: https://en.wikipedia.org/wiki/Kilonova

Would the negative acceleration, on impact, cause the kinetic energy to convert to heat, which, in turn, would be responsible for the emission of EM radiation?

Yes, there would be some pretty phenomenal temperatures generated during the neutron star impact - unless their combined mass was enough to form a black hole, in which case two hot neutron stars would be replaced by an event horizon at practically absolute zero.

But a tiny source at phenomenal temperature does not emit as much radiation as a huge debris field at a phenomenal temperature.

One description suggested that as the neutron stars crushed inwards, some of the colliding matter would be squirted out the axis like toothpaste - at a good fraction of the speed of light. It was this debris field consisting of Einsteinium and other rapidly decaying heavy elements that produced the X-ray, visible and infra-red radiation that was detectable for weeks afterwards.

The next holy grail for astronomy is to do a simultaneous detection of gravity waves, gamma rays and neutrinos. We know that neutrinos travel very slightly slower than the speed of light. Particle physicists would really love to know how much slower, as that would give them another clue about the mass of the neutrino - a number that has proven rather difficult to pin down. But this triple detection would take a much closer neutron star merger to produce a detectable burst of neutrinos.

[jeffreyH]

One a side note, does a black hole have an electric field?

[evan_au]

does a black hole have an electric field?

The "No Hair Theorem" suggests that there are only 3 externally-visible characteristics of a black hole: mass, angular momentum and electric charge.

Due to the strength of the electric field, any volume of normal matter has a pretty equal number of electrons and protons. A black hole formed out of neutral matter will itself be electrically neutral. And if it did gain an electric charge, it would attract the opposite charge more strongly (and repel the like charges), so it would neutralise itself pretty quickly.
See: https://en.wikipedia.org/wiki/No-hair_theorem

If and when we resolve the quantum paradoxes around the firewall hypothesis, we may find that information is not totally lost inside a black hole.
See: https://en.wikipedia.org/wiki/Black_hole_information_paradox

[jeffreyH]

So all wave functions may collapse at the event horizon. I am currently looking at the Heisenberg matrix mechanics picture of quantum mechanics which is equivalent to Schrödinger's wave mechanics picture. I will bear this in mind as I proceed.

[Bill S]

The next holy grail for astronomy is to do a simultaneous detection of gravity waves, gamma rays and neutrinos……. But this triple detection would take a much closer neutron star merger to produce a detectable burst of neutrinos.

Should one assume that neutrinos were emitted in the observed event, but that it was too far away for our instruments to detect them?

[jeffreyH]

Neutrinos are hard to detect. The detectors are built underground to mask out interference.
https://en.m.wikipedia.org/wiki/Neutrino_detector

[Bill S]

I was thinking more about factors relevant to emission.  E.g. What conditions have to be present so that we can be (reasonably) sure that neutrinos will be emitted?

It would seem reasonable to assume that the further the neutrinos had to travel, the more dispersed they would become, which would have to be a limiting factor in their detection.

[evan_au]

Should one assume that neutrinos were emitted in the observed event, but that it was too far away for our instruments to detect them?

Yes.
The ghostly neutrino is hard to detect. We can measure them from:
- The Sun,  since the 1960s
- Nuclear reactors
- Atomic bombs
- Particle accelerators
- But the only time neutrinos have been detected from an "Astronomical" event was a supernova in 1987: SN1987A

A burst of 25 neutrinos were detected in 13 seconds. This was from a supernova in the Large Magellenic Cloud, a dwarf galaxy just 168,000 light years away. In this kind of supernova, a plasma of negative electrons and positive nuclei are crushed into neutrons, forming a neutron star: electron + proton = neutron + (anti-)neutrino. Something like half the mass of the Sun is converted from protons to neutrons.

In a neutron star merger, perhaps 1-10% of the mass of the Sun is scraped off the colliding neutron stars, and ejected into space. Freed from the bonds of incredible surface gravity, this "neutronium" will quickly (< seconds) decay into electrons and atomic nuclei. neutron = proton + electron + neutrino.

As for detectability, neutrino emissions follow an inverse-square law; the recent neutron star merger was over 1,000 times more distant than SN1987A, so the neutrino flux would be reduced by more than a factor of 1,000,000. The number of neutrinos emitted from the neutron star collision would have been perhaps 10 to 100 times lower.

Detecting a neutron star merger via neutrinos will probably have to wait for an event in our own galaxy - something that is estimated to happen about once in every 80,000 years.
See: https://en.wikipedia.org/wiki/SN_1987A#Neutrino_emissions