Why do Gravity waves move at c?

Originally posted by sonhouse
I think it safe to say that, which means by definition, the universe is bigger than what we can see with telescopes

No, actually, it is not safe to say that. If the universe is finite then it could be smaller than the region we can see. Confusing, but true.

which we think goes out 13.7 billion light years. I guess that means we are the center of the universe from out perspective so we would be describing a sphere some 28 billion light years across in our 3 dimension world. I think🙂
Do not forget that telescopes are time machines and given that we have a pretty good understanding of the expansion of the universe we can work out where those objects 13.7 billion light years away when we last saw them have now got to, which is a lot further away.
Wikipedia puts it at a diameter of about 93 billion light years:
https://en.wikipedia.org/wiki/Observable_universe#Misconceptions_on_its_size

As far as I know there are no indications whatsoever as to the universes actual size other than a lower bound.

A rather exciting rumour almost on subject...

http://www.zmescience.com/science/physics/gravity-waves-rumor-13012016/

Originally posted by avalanchethecat
A rather exciting rumour almost on subject...

http://www.zmescience.com/science/physics/gravity-waves-rumor-13012016/

It will be several months before an announcement will be made one way or another. There is a lot of analysis to do which has been ongoing. It is not as simple as seeing a blip on a screen. The signals are buried in the mud of noise and it is the mathematical analysis of the noise that will or will not suss out a genuine gravitational wave signal from the mud of the noise band.

I dealt with that when I worked on Apollo at Goddard. A training mission I had there had us students first find Mars, then using the student dish on the roof, MAYBE 20 foot across, pretty small for such things, anyway, the exercise of the day was to lock on to the signal coming from a then active Mars orbiter, which had all of 5 watts transmitted signal and it was about 100 million miles away. So finding Mars, step one, getting the co-ordinates to feed the antenna dish motors, moving it to those co-ordinates and hitting the acquire button and lo and behold, I was successful. Of course NOTHING like the difficulty of LIGO detections but I was proud of myself for doing that job successfully. You could put the signal on an oscilloscope to view it. Which was an exercise in futility, there was no signal to view, it was pure noise, the real signal was buried some 80 DB below the noise level but the modulation technique was so powerful it was able to see the clock ticks as a minute bump in the noise level and so was able to lock on to the target orbiting Mars. An exciting day for me for sure.

I was thinking, here is a transmitter with the power of a frigging CB set, 5 watts and we are locking on to that signal with a student dish and 100 million miles away.

As a ham, that was the furthest signal I ever worked!

Originally posted by sonhouse
I was thinking, gravity waves are ripples in spacetime. At the BB the universe expanded about 22 orders of magnitude greater than c (speed of light).

So, if space itself can exceed c than why are gravity waves which are ripples in spacetime, why is the propagation of gravity waves limited to c?

Waves are propagated through a medium of some sort, in the case of waves in the sea it is water, for sound we have air, for light the electromagnetic field, and for gravity waves the medium is space-time itself. To first order - the speed at which the material moves is independent of the speed the waves move at - what is moving when a wave passes through a spring (say) is the material backwards and forwards, if you choose some point on the spring and paint a dot there and send a wave through the spring then the peak speed the dot moves at will depend on the amplitude of the wave and the frequency. In the ideal case, so no losses and an infinitely long spring, the speed of the wave depends on the mass per unit length and the spring constant and is independent of the amplitude. So the speed gravity waves travel at is, to first order, independent of the rate at which the metric is changing at some point in the trajectory of the wave.

During the inflationary phase believed to have occurred in the early universe, any gravity waves would be dragged along with the inflating universe and so the distance between the wave and its source will have been increasing faster than if the wave was propagating at the speed of light in a flat background, in other words faster than light. However, any given inertial observer will measure the speed of a passing gravity wave to be the speed of light. Of course, in the context of cosmological inflation there is a problem with the words "any given inertial observer will measure ..." as all other fields are effectively zero in that phase because no signals can get between particles for force to be transmitted and so no observation of anything is possible.

Originally posted by DeepThought
Waves are propagated through a medium of some sort, in the case of waves in the sea it is water, for sound we have air, for light the electromagnetic field, and for gravity waves the medium is space-time itself. To first order - the speed at which the material moves is independent of the speed the waves move at - what is moving when a wave passes ...[text shortened]... get between particles for force to be transmitted and so no observation of anything is possible.

I wonder if there is yet any explanation for the shape of the gravitational wave. If I understand it, the wave is a squeezing of space say in the Y direction, followed by a pull in the X direction, where the motion of the wave is in the Z direction, then followed by a squeeze in the Y followed by a pull in X and so forth.

Does that sound correct?

There must be some deeper fundamental aspect of space/time that prevents even gravity waves from propagating faster than c.

To say nothing of why EM radiation is also at c.

Originally posted by sonhouse
I wonder if there is yet any explanation for the shape of the gravitational wave. If I understand it, the wave is a squeezing of space say in the Y direction, followed by a pull in the X direction, where the motion of the wave is in the Z direction, then followed by a squeeze in the Y followed by a pull in X and so forth.

Does that sound correct?

The ...[text shortened]... ravity waves from propagating faster than c.

To say nothing of why EM radiation is also at c.

That sounds about right. There's some graphics on the Wikipedia page. Gravitational waves are solutions of Einstein's Field equations, which explains why they are as they are.

https://en.wikipedia.org/wiki/Gravitational_wave

One point, a gravity wave is a wave on the surface of a fluid like the sea - a gravitational wave is a solution of the Einstein Field Equations.