Question about gravity

[MattusMaximus]

http://nobelprize.org/nobel_prizes/physics/laureates/1993/illpres/discovery.html

I can't find anything on that page about gravity waves. Where are they mentioned?

"Gravity waves" is synonymous with "gravitational radiation". Here you go...

<snip>
Measuring gravitational radiation

Since the two neutron stars in PSR1913+16 are moving so fast and close together they should, according to General Relativity, emit large amounts of gravitational radiation. This makes them lose energy: Their orbits will therefore shrink and their orbiting period will shorten.

Indirect evidence: The binary pulsar has been observed continuously since its discovery, and the orbiting period has in fact decreased. Agreement with the prediction of General Relativity is better than 1/2%. This is considered to prove that gravitational radiation really exists. This in turn is currently one of our strongest supports for the validity of the General Theory of Relativity.
<snip>

As was pointed out earlier, this is indirect evidence. The LIGO and similar experiments are, I believe, designed to look for more concrete evidence.

Cheers - Mattus

 

[robinson]

Funny, I thought Einstein showed gravity was not radiation, but curving of space time. If mass is being lost through gravity, then there is a force involved, which leads us almost back to Newton.

Interesting ....

 

[Ziggurat]

Funny, I thought Einstein showed gravity was not radiation, but curving of space time.

The electric field of a static charge isn't waves either, and yet there are electromagnetic waves.

If you thought there was a contradiction, it's because you never understood it.

 

[sol invictus]

If you thought there was a contradiction, it's because you never understood it.

Nothing new there.

Gravitational radiation is a curving of spacetime - just a time-dependent one.

 

[Tumbleweed]

That's incorrect.




OK, that explains a lot. Out of curiosity, who was the professor?.

I can't remember his name, but he was a Black man with long hair

 

[The Man]

Funny, I thought Einstein showed gravity was not radiation, but curving of space time. If mass is being lost through gravity, then there is a force involved, which leads us almost back to Newton.

Interesting ....

Actually he showed it was both, a curvature of space time and ripples or waves in that curvature. Mass being lost by gravitational waves is one of the proposed results of the black holes have “no hair” theorem

http://en.wikipedia.org/wiki/No_hair_theorem


In the “hoop conjecture” of black hole formation, the entire mass of the body must be encompassed within a specific radial limit before the catastrophic gravitational collapse ensues.

http://en.wikipedia.org/wiki/Hoop_conjecture

In some conjectures asymmetric or aspheric attributes of mass just outside that radial limit might be radiated out as gravitational waves, but I can’t find a specific reference at this time.

For more information on current efforts to detect (and the generation of) gravitational waves from catastrophic gravitational collapse.

http://relativity.livingreviews.org/Articles/lrr-2003-2/

One of the fundamental precepts of general relativity is that what we generally perceive as a gravitational force is simply an imaginary or fictitious force, like the Corioliss force or centrifugal force, an apparent force that results due to a non-inertial reference frame. A body moving through curved space time does not experience a force but simply moves through the shortest proper time path in that curved space time.

http://en.wikipedia.org/wiki/Fictitious_force


http://en.wikipedia.org/wiki/Proper_time

 

[MattusMaximus]

The electric field of a static charge isn't waves either, and yet there are electromagnetic waves.

If you thought there was a contradiction, it's because you never understood it.

A good analogy. You can also think of it like this, using the "rubber sheet" anaolgy of spacetime...

Imagine that empty spacetime is represented by a flat, stretched rubber sheet. Now place a large amount of mass into that region of spacetime, such as a heavy bowling ball, and the sheet forms a divot or well where the mass is located. This divot or well is the warping of spacetime often referred to as "gravity".

Now, if the mass (our heavy bowling ball) were to suddenly disappear, then the rubber sheet would snap back up towards its original flat state. However, it wouldn't just snap flat and be done with it, it would oscillate up and down (like the surface of a vibrating drum that has just been struck) for a time causing ripples to wiggle along the rubber sheet. These ripples are "gravity waves" or "gravitational radiation".

I hope this picture helps with your view on what's going on.

And just an fyi for the un-informed on this particular point...

Electromagnetic Waves = Electromagnetic Radiation = Light

So the terms "waves" and "radiation" are also synonymous when discussing light - to physicists it is all the same thing. And yes, believe it or not, light is an electrical & magnetic phenomenon.

Cheers - Mattus

 

[sol invictus]

Now, if the mass (our heavy bowling ball) were to suddenly disappear, then the rubber sheet would snap back up towards its original flat state. However, it wouldn't just snap flat and be done with it, it would oscillate up and down (like the surface of a vibrating drum that has just been struck) for a time causing ripples to wiggle along the rubber sheet. These ripples are "gravity waves" or "gravitational radiation".

It's a good analogy, but as was pointed out earlier it's a bit of a misleading scenario. It violates the laws of physics for mass to disappear - mass a gauge charge, like electric charge, and its disappearance would violate Gauss' law.

To see why that's a problem, imagine a single electric charge at rest at the origin. In principle you can measure that charge by measuring the flux of the electric field through a spherical surface 1 light year away from the charge. Now if the charge were to suddenly disappear, by causality the electric field one light year away couldn't change until a year had past. But then Gauss' law would be wrong, since it would tell you the total charge enclosed is non-zero. Exactly the same logic works for mass and the gravitational field (including in GR - you measure the ADM mass with an integral over a large sphere).

So to modify your scenario a little, just imaging a smaller ball comes flying along and smacks into the big one, making it rock back and forth. That will send out ripples along the sheet, which are gravitational waves.

 

[Jimbo07]

It's a good analogy, but as was pointed out earlier it's a bit of a misleading scenario. It violates the laws of physics for mass to disappear - mass a gauge charge, like electric charge, and its disappearance would violate Gauss' law.

So to modify your scenario a little, just imaging a smaller ball comes flying along and smacks into the big one, making it rock back and forth. That will send out ripples along the sheet, which are gravitational waves.

Mass won't disappear, but say you have some Supernova-like Space Kablooey. Mass will definitely be ejected from the local region...

 

[sol invictus]

Mass won't disappear, but say you have some Supernova-like Space Kablooey. Mass will definitely be ejected from the local region...

Sure - there's no problem with that. You could imagine a bomb at the center of the sun which suddenly explodes, sending its pieces flying in all directions. The gravitational field a distance r away cannot change at all until light - and gravity waves - have had a time t=r/c to arrive there.

So the point that gravitational influences propagate at speed c is perfectly valid. The complaint was just that having mass/energy suddenly vanish violates the equations of the theory, so it's not the best example.

 

[Jimbo07]

Sure - there's no problem with that. You could imagine a bomb at the center of the sun which suddenly explodes, sending its pieces flying in all directions. The gravitational field a distance r away cannot change at all until light - and gravity waves - have had a time t=r/c to arrive there.

Which, not having studied cosmology per se is what I would have anticipated.

So the point that gravitational influences propagate at speed c is perfectly valid.

Again, agreed. I was just concerned that there was some formula buried in the heart of post-GR cosmology of the form:

((some Tensor) x (next Tuesday)) + purple = ?

The complaint was just that having mass/energy suddenly vanish violates the equations of the theory, so it's not the best example.

Agreed. It's like bothering to keep the trivial solution, or solutions which blow up, when it is clearly not the physical case...

 

[robinson]

That brings up an interesting question.

According to currently accepted theory, does gravity behave like light, in regards to relativity?

In simple words, does gravity propagate at the speed of light, no matter what the relative motion of a body? Does it behave like EM? Does it Doppler shift? Can it?

If it is like light, how is energy conserved when a moving body effects another body?
Light (EM) Doppler shifts up and down to obey the speed of light, conserving energy by changing frequency. Right? What does gravity do?

If Gravity is like light, how does motion change it?

Two bodies moving relative to each other, at very high velocities, what happens to gravity? Or is that a valid question?

And does anybody really know?

 

[sol invictus]

According to currently accepted theory, does gravity behave like light, in regards to relativity?

Yes. That's the what we've been talking about.

In simple words, does gravity propagate at the speed of light, no matter what the relative motion of a body? Does it behave like EM? Does it Doppler shift? Can it?

Yes, kind of, yes, and yes.

If it is like light, how is energy conserved when a moving body effects another body?
Light (EM) Doppler shifts up and down to obey the speed of light, conserving energy by changing frequency. Right? What does gravity do?

Gravity waves will Doppler shift if the source is in motion relative to the receiver.

If Gravity is like light, how does motion change it?

In a way similar to the way the electric field of a charge changes when you move the charge.

You should distinguish between the static gravitational fields of masses, which are analogous to electric and magnetic fields of static charges and currents, and gravity waves, which are analogous to EM radiation (such as visible light or radio waves).

Two bodies moving relative to each other, at very high velocities, what happens to gravity? Or is that a valid question?

If they pass near each other they will form a black hole. The faster they move, the larger the hole will be. Gravity acts on all forms of energy, including kinetic.

And does anybody really know?

Yes.

 

[robinson]

So there is a spectrum of gravity waves? Some are low energy, some are high? Like EM waves? How do we know? (this is really interesting)

Since it is thought galaxies are moving at high velocities to each other, well, pretty much everything is moving relative to each other, gravity comes in different frequencies?

Gravity is Doppler shifted?

How cool is that? So no matter how fast two bodies are moving, relative to each other, gravity is always at the speed of light? Doppler shifted gravity. How is that measured? Or is this theory?

Has an experiment been done?

 

[Ziggurat]

According to currently accepted theory, does gravity behave like light, in regards to relativity?

You keep making the same mistake of not distinguishing between the field and excitations of the field. Why is the difference so hard to understand? Do you not get the difference between an electric field and light?

Gravity is a field. The analogous field is the electric field. The field itself is not the same thing as waves in the field. If you have a stationary electric charge, it creates a static field, with no waves involved. If you have a stationary mass, it creates a gravitational field, with no waves involved.

Now, if you wiggle the source of the field (be it a charge or a mass), the field should wiggle too. Those wiggles do not propagate outwards from the source at infinite velocity, they travel at c. When a charge wiggles, it produces electromagnetic waves (light being electromagnetic waves within a certain frequency range). When you wiggle a mass, it produces gravitational waves. But the waves themselves are just variations of the fields - you can still have the fields themselves without any waves.

In simple words, does gravity propagate at the speed of light, no matter what the relative motion of a body?

It actually doesn't make a lot of sense to talk about the propagation of gravity or of the electric field. What propagates is any changes in the field. And changes in both electric and gravitational fields do indeed propagate at c, which is always constant regardless of your reference frame.

Does it Doppler shift? Can it?

Gravitational waves can doppler shift. Gravity doesn't doppler shift, and neither does the electric field, because Doppler shift only means a change in frequency, and the fields themselves need not have any frequency. If you change reference frames, you do need to change the gravitational field, but that also happens with electric fields too: the electric field of a moving charge gets squished along the direction of motion, and gravity will do something similar.

If Gravity is like light,

It isn't. I've said this before.

Two bodies moving relative to each other, at very high velocities, what happens to gravity? Or is that a valid question?

It's a valid question. The math is really ugly, but it can be calculated. But this problem doesn't really involve gravitational waves.

And does anybody really know?

Yes. And their ideas are being tested, too. In Newtonian physics, a rotating spherical mass has a field identical to a non-rotating spherical mass, but not in GR. Gravity Probe B was put in orbit to detect the gravitational effects of the earth's rotation, and the results so far confirm GR, which means that their predictions for how motion should affect gravity are correct.

 

[Ziggurat]

So there is a spectrum of gravity waves?

Yes.

Some are low energy, some are high? Like EM waves?

Not necessarily. Classically, the energy of a wave is a function of both its amplitude and its frequency, which means that any frequency of wave can have whatever energy you want, provided you pick the right amplitude. Light happens to be quantized, with the energy scale of quantization being proportional to the frequency, but that's a quantum mechanics effect. We only figured that out because it's got tangible, measurable effects (ie, the photoelectric effect). But quantum mechanics and GR are hard to mesh together. The truth is, we don't know whether or not gravitational waves should be quantized like light is. A lot of physicists think so, and I tend to suspect so as well, but the quantization energies might be so small that detecting such quantization directly might be impossible.

Gravity is Doppler shifted?

No. Gravity is lorenz-transformed (roughly speaking). Gravitational waves are doppler shifted. But if you don't have waves, there's nothing to doppler shift.

Has an experiment been done?

Indirectly, yes. Gravity Probe B.

If you look at just electricty and relativity, and how electric fields transform for different reference frames, then you find out that lorentz transformations require magnetism. And once you get magnetism, ectromagnetic radiation falls right out of the theory. You can't construct a theory of relativistic electric fields without both magnetism and electromagnetic radiation.

Something analogous happens with gravity. If you look at gravity in GR and ask how it transforms between different reference frames, you get a prediction for how rotation should change a gravitational field. Mathematically, the reasons it should change are exactly the same reasons that gravitational waves should exist. So demonstrating that gravity is affected by the rotation of the source is essentially the same thing, from the point of view of the theory, as demonstrating that there are gravitational waves. But experimentally, the former is easier to do than the latter. We've done the former. We haven't done the latter yet successfully, but that's to be expected because it requires greater sensitivity.

 

[robinson]

I think I got it. Gravity is like magnetism. Changes travel at the speed of light. So a moving object would have a gravity field moving as fast as it does. Not particles radiating like light, but more like a magnetic field.

So changes are limited by C, but gravity is like an EM field, it always extends to infinity. Unlike a magnetic field, we can't turn it on and off, so the issue of prorogation isn't really an issue, everything already has an infinite gravity field around it.

Is that correct?

 

[Ziggurat]

I think I got it. Gravity is like magnetism.

Not quite. Frame-dragging (what GPB measured) is to gravity what magnetism is to electricity.

Changes travel at the speed of light.

Yes.

So a moving object would have a gravity field moving as fast as it does.

Yes.

Not particles radiating like light, but more like a magnetic field.

More like the combined electromagnetic field (which in relativity is a single field, not two distinct fields) of a moving charge.

So changes are limited by C, but gravity is like an EM field, it always extends to infinity.

Yes.

Unlike a magnetic field, we can't turn it on and off, so the issue of prorogation isn't really an issue, everything already has an infinite gravity field around it.

Unlike an electric field, there's only positive mass, so we can't cancel it. In addition, since gravity is so weak compared to the other fields, large gravitational fields only come from large masses. It's hard to wiggle a large mass quickly and with a large amplitude, so gravitational waves tend to be very weak and low-frequency, making them very hard to detect. So how do you get a really big mass to wiggle back and forth really fast? Get two black holes to orbit each other.

 

[The Man]

I think I got it. Gravity is like magnetism. Changes travel at the speed of light. So a moving object would have a gravity field moving as fast as it does. Not particles radiating like light, but more like a magnetic field.

So changes are limited by C, but gravity is like an EM field, it always extends to infinity. Unlike a magnetic field, we can't turn it on and off, so the issue of prorogation isn't really an issue, everything already has an infinite gravity field around it.

Is that correct?

Please remember that EM means Electromagnetism, the two can not be separated in a time dependent (or changing in time) fashion. You turn off electrical and magnetic fields whenever you flip a light switch (although the magnetic considerations are of little concern for the end user). No one has, as yet, found a way to turn off a gravitational field.

You’re getting there, although the gravitational force mediating boson is proposed to be the graviton just like the force mediating particle for EM is the photon. The photon we know has no rest mass and the graviton is also theorized to have no rest mass, which is why those fields extend to infinity, as opposed to the other force mediating particles, the gluon and the intermediate vector boson that have rest mass, limiting the range of the strong and weak nuclear forces respectively.

For accelerating bodies interacting with (or compressing) their own EM or gravitational fields check out synchrotron radiation (for EM field) and frame dragging (for gravitational field).

http://en.wikipedia.org/wiki/Frame-dragging

http://en.wikipedia.org/wiki/Synchrotron_radiation

 

[robinson]

It's hard to wiggle a large mass quickly and with a large amplitude, so gravitational waves tend to be very weak and low-frequency, making them very hard to detect. So how do you get a really big mass to wiggle back and forth really fast? Get two black holes to orbit each other.

Thanks. A Super Nova would certainly produce profound gravitational waves, correct?