This is pulled out of a post in the entanglement=attraction thread. The questions are pretty tangential to that discussion, and since they didn't seem to catch any attention there I figured I'd give 'em their own thread and add some flesh to 'em...

If gravitons travel and propagate as waves, that implies that gravity has a wavelength and amplitude.

Presumably, it is the wave's amplitude that decreases over distance by varying inversely with the square of distance between two particles.

So what is the effect of altering the wavelength? If two bodies are moving away from one another then redshift is going to lengthen the wavelength and if they are approaching one another then there will be blue shift shortening the wavelength. But what does that mean in terms of gravity?

Thought experiment: Let's say two extremely massive objects (let's go with neutron stars) get slingshot gravity boosts around two massive black holes and are accelerated directly toward one other, each traveling at relativistic speeds. This gives their gravity waves a massive blue shift.

Shortly before the collision, they will bath each other in extremely shortwave, high amplitude gravity waves. So how will that effect the collision? Will they feel a sudden pull towards each other stronger than it would if the objects where in the same relative places but not approaching so fast? Sounds logical to me. It means that the actual collision should produce more energy than would be expected if gravity was simply the warping of space and didn't involve waves.

And if that is so, then do objects moving towards us exert a slightly stronger gravitational pull at each measured position than they would if measured at the same positions without the relative motion towards us? That sounds logical too, and leads to objects moving away from us exerting less gravitational pull than expected. That might be an alternative to dark matter right there.

Have any tests ever been done on a concept like this? If so, is that already taken into account in current models of the cosmos?

w

Everything you have written about is purely speculative.

We have never seen a graviton.
Never measured a gravity wave.
Know nothing about relativistic effects.

And, IIRC, some of the theories that are out there give different answers to your questions.

Hi All,

In this regard have a look at this paper:

http://arxiv.org/abs/astro-ph/0302294

It is about measuring the speed of gravity. It has a rather drab title:

"The Measurement of the Light Deflection from Jupiter: Experimental Results"

that belies its significance.

Dr. R.

Okay, I don't think I mind admitting that 99.9% of that paper was a bit over my head.

However, what I got out of it, I think, is that gravity appears to travel at the same speed as light - at least insofar as the accuracy of the experiment allowed such measurement.

Did I get that right? Did I miss anything earth shattering?

w

You got it right.

Here's a simple thought experiment that will prove that gravity travels at the speed of light, or remarkably close to it.

Picture our solar system.

Picture a globe constructed of photographs of each of the planets in our solar system using the light that has come from them.

Picture a map constructed from the gravitational influences of each of these influences on the planet earth.

If gravity lagged behind light ... the oceans would not reach high-tide in the manner related by where the moon is in the sky. Neither would the gravitational influence of the sun on the earth and moon point back to the sun but rather to where the sun had been (past tense).

If gravity moved at a speed faster than light the opposite would be true. We would be pulled toward a location where visual observation indicated the sun had yet to move.

Hi Wayne,

Yep - you got it. This was something that troubled me for decades. No one had gotten around to checking the speed of gravity. For that matter it was hard to think about how to even do this.

Mostly, the technical stuff in the paper says "we were darn careful about how we did things."

Dr. R.

Hello Everyone

This is my first post and question.

Since the speed of gravity and the speed of light are the same, the universe 13.7 billion years old and according to the link below the universe 156 billion light years across

http://www.space.com/scienceastronomy/mystery_monday_040524.html

Does that mean that we are only affected by the gravity of the universe that is see within 13.7 billion light years?

Gary

Hi Gary. Welcome!

Yes. All electromagnetic waves and gravity (whatever it is, gravitons or whatever) have had only 13.7 billion yrs to reach us. That not only limits the observable universe to a radius 13.7 billion lt.yrs., but also means that gravity can reach us only if it originated within that radius.

It's fascinating and bizarre that we can't see the cosmos as it actually is 'now', and are not affected in any way by the vast majority of it. For all practical purposes, it seems that nothing exists beyond the observable sphere, even though theory states quite firmly that it does.

Does this mean that if we sent out a probe to the edge of the observable universe that it would register another portion of the universe, and eventually fade from our view as well? Would the probe's point of view shift so that we would eventually become unseen to it?

Rose

"Does this mean that if we sent out a probe to the edge of the observable universe that it would register another portion of the universe..."

Yes, but to make things simpler, let's suppose that the probe is already there at this very moment, at a point on the edge of the observable universe. Its view would still be confined to a sphere of radius 13.7 billion lt. yrs., but it would, indeed, be a diferent sphere. Our galaxy would not be observable, because the spacetime seen from that distance would still be that which existed shortly after the Big Bang, before our galaxy formed. Nothing could be seen beyond that location. Looking in the opposite direction, it would see a portion of the universe not observable from Earth, but which would still be limited to a view of the past extending back to the time of the Big Bang.

I know didn't I answer your question exactly, i.e., "if we sent out a probe". That's because I don't know enough maths and physics to take account of the rate of the expansion of space, and relativistic effects depending upon the velocity of the probe etc., so I hope the easy way out is OK.

"Would the probe's point of view shift so that we would eventually become unseen to it?

Yes, assuming that current theories are correct, and that space continues to expand. The probe would eventually vanish from our view, because the expansion of space would eventually result a mutual rate of recession that exceeded the speed of light.

Thanks, redewenur. Now I have more grist for my science fiction mill. I have to go now, I feel a new story coming along.

Amaranth