[vampster (OP)]

Hi,
My first post here! I was wondering about yesterdays publication about gravity waves and the discovery of the colliding neutron stars. Apparently the light from the collision of the neutron stars arrived 2 seconds after the gravity waves hit us.
Why is this? Both are traveling at light speed. What caused the difference?
I see many options but I am not clear on which one is the correct one:

• gravity waves follow a different path than the light
• light has to travel through intergalactic dust and gas clouds that cause it to go slower
• light could not escape the first 2 seconds of the cataclysmic event from 2 colliding neutron stars
• the gravity waves where strong enough to detect before the actual collision
• probably many more options I did not think about

Do gravity waves follow the exact same path as the light? I assume no, since the whole idea of detecting them is based on the longer path the light follows when a big enough gravity wave passes through us. Maybe it is wrong to understand this as a "path" to begin with, since the actual spacetime itself is what is "shaking"...

[Kryptid]

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.

[evan_au]

I imagine ...the waves grew strong enough to detect 2 seconds before the actual collision took place.

It was reported that these gravitational waves were detectable for about 100 seconds before the merger, which is far longer than the 100ms or so in the previous black hole mergers. This was aided by the much closer range of this event (130M Light Years, instead of 1 billion LY for the black holes).

The trace of frequency vs time really zooms asymptotically from around 50Hz to 500Hz in the 4 seconds before the merger.
It is 1.7s after the merger that the gamma-ray burst was detected by the Fermi satellite.

But it wouldn't be the first time that operators of radically different instruments found that their clocks weren't synchronised to the necessary accuracy!

The gravitational wave chirp is quite apparent in the following audio file - but they had to cheat with the gamma ray pulse!
Watch & listen to 10 second video: https://svs.gsfc.nasa.gov/vis/a010000/a012700/a012740/Fermi-LIGO_Graph_Sound.mp4

[syhprum]

I cannot believe that clocks weren't synchronised to the necessary accuracy these days it is very easy to synchronise clocks to a few milliseconds via standard time transmissions from boulder Colorado and other standard time sources. 

[chiralSPO]

but they had to cheat with the gamma ray pulse!

That is the gentlest sounding gamma ray burst I have ever heard!

[chiralSPO]

Hi,
My first post here! I was wondering about yesterdays publication about gravity waves and the discovery of the colliding neutron stars. Apparently the light from the collision of the neutron stars arrived 2 seconds after the gravity waves hit us.
Why is this? Both are traveling at light speed. What caused the difference?
I see many options but I am not clear on which one is the correct one:

• gravity waves follow a different path than the light
• light has to travel through intergalactic dust and gas clouds that cause it to go slower
• light could not escape the first 2 seconds of the cataclysmic event from 2 colliding neutron stars
• the gravity waves where strong enough to detect before the actual collision
• probably many more options I did not think about

Do gravity waves follow the exact same path as the light? I assume no, since the whole idea of detecting them is based on the longer path the light follows when a big enough gravity wave passes through us. Maybe it is wrong to understand this as a "path" to begin with, since the actual spacetime itself is what is "shaking"...

My vote is for:
• light has to travel through intergalactic dust and gas clouds that cause it to go slower

We know that there is some "stuff" between here and there (ions, atoms, small molecules, big molecules, dust etc. there may only be a few particles per cubic meter, but there are plenty of meters between the burst and here!) And we know light will interact with this type of stuff, slowing it down ever so slightly. As far as I understand it we don't expect gravity waves to interact with any of this "stuff." But I also imagine we are likely to learn a lot bout gravity waves over the next few decades, thanks to the LIGO and Virgo detectors (and whatever else springs up kn the coming years).

[Bogie_smiles]

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.

That is a good answer.

[jeffreyH]

There are some theoretical consequences if gravity propagated at light speed. Gravity is a source of gravity. Gravity slows gravity and stops it at the event horizon. Inside the horizon the field can only point towards the singularity.

Let me modify that a bit. ALL fields will point towards the singularity so that all the forces are focussed together. It may also be the case that gravitational acceleration approaches zero as the radial separation approaches the horizon's radius. This would leave only inertial motion at the horizon. What is this final velocity?

[evan_au]

That is the gentlest sounding gamma ray burst I have ever heard!

Why, how many gamma-ray bursts have you heard?

I found that clip on the NASA website for the Fermi gamma-ray satellite.
As far as they were concerned, the gravitational waves were merely the prelude to the real prize - the gamma ray burst...

[Bill S]

My vote is for: light has to travel through intergalactic dust and gas clouds that cause it to go slower

If it’s a democratic matter; my vote goes with Kryptid, for the major cause; but seeing it as the only factor involved would almost certainly be an oversimplification. Considering all the “stuff” it might have met on the way, I think the EM radiation made good time.

Even if the GWs and EM radiation left at the same time the coincidence of arrival times is close.  At a very rough estimate I think that would make their speeds the same to about one part in 10^12. 

Could be we are all wasting our time discussing this.  I understand that the US vice-president has denounced the report of the merger as a “conspiracy” by NASA to further the cause of gay marriage,  I wonder what, if anything, he will say when a black hole/neutron star merger is detected. 

[Bill S]

Gravity is a source of gravity.

There was some discussion of this at: https://www.thenakedscientists.com/forum/index.php?topic=71536.msg524674#msg524674

Have you thought it through yet, Jeffrey?

[jeffreyH]

You can think of it like an infinite series. There will be a limit to the sum of the series that is not necessarily infinity. So adding all the gravitational contributions will give a particular limit based upon the original mass. It would only become significant in the strong field.

[Bill S]

Could one also reason that:

M is the original mass/energy that created gravity.
G is the gravity (curvature) created by M.
G carries energy, but the total mass/energy of M + G cannot be greater than the original mass/energy of M, because the energy contained in G does not appear from nowhere.
Thus, a statement like "gravity creates gravity" says simply that the relevant equations are non-linear?

[jeffreyH]

Could one also reason that:

M is the original mass/energy that created gravity.
G is the gravity (curvature) created by M.
G carries energy, but the total mass/energy of M + G cannot be greater than the original mass/energy of M, because the energy contained in G does not appear from nowhere.
Thus, a statement like "gravity creates gravity" says simply that the relevant equations are non-linear?

I would agree with all the above. With the modification that there has to be a distinction between M, the rest mass and m the reduced gravitational mass so that m + G < M. I am not sure if that is a correct limit but it is a maximum.

[Bogie_smiles]

Could one also reason that:

M is the original mass/energy that created gravity.
G is the gravity (curvature) created by M.
G carries energy, but the total mass/energy of M + G cannot be greater than the original mass/energy of M, because the energy contained in G does not appear from nowhere.
Thus, a statement like "gravity creates gravity" says simply that the relevant equations are non-linear?

That raises the question, given two separate Masses, M1 and M2. When M1 emits gravity, giving us M1 - G1, and thus giving us space that contains G1 sub m, when that G1 Sub m reaches M2, does M2 absorb any of the G1 sub m from space?

[jeffreyH]

Could one also reason that:

M is the original mass/energy that created gravity.
G is the gravity (curvature) created by M.
G carries energy, but the total mass/energy of M + G cannot be greater than the original mass/energy of M, because the energy contained in G does not appear from nowhere.
Thus, a statement like "gravity creates gravity" says simply that the relevant equations are non-linear?

That raises the question, given two separate Masses, M1 and M2. When M1 emits gravity, giving us M1 - G1, and thus giving us space that contains G1 sub m, when that G1 Sub m reaches M2, does M2 absorb any of the G1 sub m from space?

If you are asking if gravitational fields cancel the yes they do. If not then I think you need to clarify what you mean.

[Bogie_smiles]

If you are asking if gravitational fields cancel the yes they do. If not then I think you need to clarify what you mean.

The way I asked it probably implies some bad science thinking on my part. I’m good with your answer.
 

[evan_au]

If you are asking if gravitational fields cancel the yes they do

Some examples of this are the Lagrange points; the gravitational fields of two large objects combine in such a way that a small object can float in a stable position, not really in orbit around either body.
See: https://en.wikipedia.org/wiki/Lagrangian_point

From another angle, yesterday I heard an interview with a LIGO physicist, whose PhD thesis was on collision of gravitational waves. He found that if two very strong gravitational waves collided, they could create black holes. However, the gravitational waves would have to be very strong. That implies two powerful collisions which are very close in time and space. That is incredibly unlikely.

All the gravitational waves we have detected so far have been extremely weak.
Listen at: https://www.sciencefriday.com/episodes/october-20-2017/

[Bill S]

Returning to the question of light being slowed by a medium: if this apparent slowing results from the absorption and re-emission of the photons by atoms in the medium; why would astronomers be able to see the emission spectra of distant stars?
 
Wouldn’t these spectra be converted into the emission spectra of the atoms in the lenses of their telescopes?

[evan_au]

Wouldn’t these spectra be converted into the emission spectra of the atoms

Light of the exact right frequency can be absorbed by an atom, kicking an electron into a higher orbital.
Later, the electron will drop back down, perhaps in a single step, emitting a photon of light in a random direction (or in multiple steps, emitting several lower-frequency photons in random directions).
This will result in this particular frequency being greatly reduced in the light of a far star, producing an absorption spectrum of the gas in the light path.

However, the mechanism being discussed which changes the speed of light in a gas has a broadband characteristic, which affects all frequencies, not just particular frequencies corresponding to electron energy levels.

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. However, a gas has a slightly different permittivity than a vacuum (it has slightly more capacitance), and so light travels slightly slower through the atmosphere than through a vacuum,  and almost immeasurably more slowly through intergalactic space than through a vacuum - that is, not measurable until the recent neutron star merger.

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