# What gravity waves?

amt
What gravity waves?

Considering Gravity is the warping of space, then how is gravitational waves possible? Aren't we all sliding down the slopes of space due to the massive warping of space by ojects? isn't space pushing us down?

Then why is gravitational waves a topic?

Thanks.

Antiphon
If you shake the object, the ripples will move away as waves.

It's an interesting aspect of GR so it's a topic.

Homework Helper
Gold Member
Doesn't gravitational waves exist in Newtonian mechanics also? Shake a mass harmonically and watch the field at some point. It will oscillate. How are relativistic waves any different that that other then we can detect them through length contraction and time dilatation?

Staff Emeritus
Newtonian gravity doesn't have any gravity waves because it doesn't have any gravitational equivalent to magnetism.

Magnetism is important, because acording to Maxwell's equations, a changing electric field gives rise to a magnetic field, and a changing magnetic field gives rise to an electric field. A self-sustining oscillation of electirc and magnetic fields which satisfies Maxwell's equations is responsible for light, radio waves, and all electromagnetic radiation.

GR does have a gravitational equivalent to magnetism (gravitomagnetism, aka frame-dragging). Because weak-field GR obeys equations which are very similar to Maxwell's equations, the theory has self supporting gravitational "waves".

Antiphon
Also, Newtonian gravity does not have a finite speed of propagation.

amt
So, the only way I can comprehend gravitational waves is by thinking that they are continuous (oscillation as explained by you all). So is it correct to think of it as continuous ripples accross space? though the frequency and speed of this ripple is still unknown?

rbj
quasar987 said:
Doesn't gravitational waves exist in Newtonian mechanics also? Shake a mass harmonically and watch the field at some point. It will oscillate. How are relativistic waves any different...?

the difference is that the Newtonian gravity waves would be traveling at infinite speed. the GR waves travel at the same speed that EM waves do in a vacuum.

by the way, for weak gravitation the equations look exactly like Maxwell's Equations, except that charge density is replaced by mass density, and $$\frac{1}{4 \pi \epsilon_0}$$ is replaced by $$G$$. there is some 1/2 factor with the B field having to do with something "spin 2" that i don't completely understand, but i think that sort of comes out in the wash.

we really should not think of $$c$$ as merely the "speed of light" but as the speed of propagation of all things or actions "instantaneous".

r b-j

Staff Emeritus
amt said:
So, the only way I can comprehend gravitational waves is by thinking that they are continuous (oscillation as explained by you all). So is it correct to think of it as continuous ripples accross space? though the frequency and speed of this ripple is still unknown?

That's the best way of understanding them, the way that General Relativity describes them. General relativity is a classical theory, so it doesn't include quantum effects. Thus, the picture GR draws of gravity waves is the classical picture that - that of waves in spacs - not the quantum picture of particles following all possible paths.

Gold Member
Dearly Missed
Do these waves get stronger when they encounter a gravity well ?

Antiphon
In a way- the wavelength should shorten as they fall in and lengthen as
they leave the well. Shorter wavelengths have higher energy but not
higher amplitude.

Gold Member
Dearly Missed
This is a fascinating subject, i can not understand what is actually oscillating
though, i have read words like the, "metric", or the "vacuum", it confuses me.

Creator
Wolram said:
Do these waves get stronger when they encounter a gravity well ?

Significant question.

Antiphon said:
In a way- the wavelength should shorten as they fall in and lengthen as
they leave the well. Shorter wavelengths have higher energy but not
higher amplitude.

Would you mind stating what empirical or theoretical evidence you have for believing that? Not that I have evidence to the contrary; just want to see how you arrived at that conclusion.

Creator

Last edited:
Gold Member
Dearly Missed
This is the point where things get,"sticky", and only for the brave.

Gold Member
At the last that I heard anything about it, there are still experiments under way to detect and measure gravity waves. They involve immensely heavy metal cylinders hung like pendulems, or lying on their sides, but I can't recall the set-up.

kaos
Danger said:
At the last that I heard anything about it, there are still experiments under way to detect and measure gravity waves. They involve immensely heavy metal cylinders hung like pendulems, or lying on their sides, but I can't recall the set-up.

I think that LIGO is supposed to detect gravitational waves.

amt
The Laser Interferometer Space Antenna is supposed to be launched in 2013. This will be used to measure Gravity waves that is still resonating from the Big Bang (Article from DISCOVER magazine- August 2005 edition).

Gold Member
kaos said:
I think that LIGO is supposed to detect gravitational waves.
I'm not familiar with that acronym, but it might very well be the one that I'm thinking of. If memory serves, it's based in Australia... possibly in an opal mine. I'm afraid that it's been several years since I saw anything about it.

amt said:
The Laser Interferometer Space Antenna is supposed to be launched in 2013. This will be used to measure Gravity waves that is still resonating from the Big Bang (Article from DISCOVER magazine- August 2005 edition).
Very cool. That's a bit far off for a fellow of my age, but I shall attempt to retain my grip upon the mortal coil until such time as results come back.

Staff Emeritus
Gold Member
Danger said:
I'm not familiar with that acronym, but it might very well be the one that I'm thinking of. If memory serves, it's based in Australia... possibly in an opal mine. I'm afraid that it's been several years since I saw anything about it.

LIGO is based in the U.S. It is basically an interferometer (a la Michelson-Morley, just 4km length, L-shaped). There will be two of them to reject local noise.

Another possible way to ponder on gravity waves is to watch the ripples on the surface of a pond or river, preferably with lots of ducks or boats moving around and just around sunset to get the most shimmering and glistening effect.

Gold Member
Dearly Missed
jammieg said:
Another possible way to ponder on gravity waves is to watch the ripples on the surface of a pond or river, preferably with lots of ducks or boats moving around and just around sunset to get the most shimmering and glistening effect.

But what, "medium", is rippling, if space itself can ripple it must have some
mechanical properties.

Gold Member
ahrkron said:
LIGO is based in the U.S. It is basically an interferometer (a la Michelson-Morley, just 4km length, L-shaped). There will be two of them to reject local noise.
Thanks for the clarification. I assume that you mean a laser interferometer. So what's the thing that I'm thinking of that involves (tungsten?) cylinders?

jammieg said:
Another possible way to ponder on gravity waves is to watch the ripples on the surface of a pond or river, preferably with lots of ducks or boats moving around and just around sunset to get the most shimmering and glistening effect.
Well... that to me would ideally involve a 12-gauge so as to supply me with roast duck for a couple of months. (Oh, come on... they don't call me Danger because of my humanistic nature. If it's breathing, it's food.)

wolram said:
But what, "medium", is rippling, if space itself can ripple it must have some
mechanical properties.
Negatory, good buddy. That's sort of like saying that EM can't propogate as a wave because nothing is 'waving'. It's one of those situations where classical thinking doesn't work, sort of like thinking of the 'gravity well' around a mass. What's indented to make the 'well'?

I agree with Wolram if something is something or doing something or detectable or something it must have mass and things with mass have properties, or maybe we just don't have the means of detecting this kind of mass or maybe we don't understand mass because mass is gravity dependent or mass is gravity oh never mind.

Gold Member
jammieg said:
detecting this kind of mass or maybe we don't understand mass because mass is gravity dependent or mass is gravity oh never mind.
:rofl: It would appear that you buy the same brand of beer that I do.

Gold Member
Dearly Missed
by Danger
Negatory, good buddy. That's sort of like saying that EM can't propogate as a wave because nothing is 'waving'. It's one of those situations where classical thinking doesn't work, sort of like thinking of the 'gravity well' around a mass. What's indented to make the 'well'?

As i understand a," gravity wave", will cause a ridged rod to stretch as it
passes, when it has passed the rod will return to its original dimensions.
Im not sure but i guess there could be many such rods.

Gravity has done mechanical work by stretching the rod/s but how many can it strech before its energy is spent ? this seems different
from warped space time or a gravity well which are, "static", in space, and
quite a step from the rubber sheet analogy.

Last edited:
Gold Member
wolram said:
As i understand a," gravity wave", will cause a ridged rod to stretch as it
passes, when it has passed the rod will return to its original dimensions... ...but how many can it strech before its energy is spent ?
I'm lost on that, dude. I would have thought that since gravity propogates at 'c' in vacuum, it would drop to the speed of sound while traveling through a metallic medium. That might indeed be what the 'massive cylinder' experiment that I was thinking of is based upon. The idea of gravity losing its energy when passing through something never crossed my mind. (But neither did the idea of it going through something.) You just had to go and bring that up right when I have to try to sleep... :grumpy:

Gold Member
Dearly Missed
I guess if a gravity wave encounters enough mass the work it does on that
mass uses up the energy that created it, but that is to simple I am sure.

amt
Darn it!

So let me get this-
A body with Mass warps space. This warp is the result of the ripple effect when the body first appeared in space at that particular location. A good example is- placing a boat in a pond. The boat displaces water and sends out ripples.

Gold Member
amt said:
A good example is- placing a boat in a pond. The boat displaces water and sends out ripples.
That would be an analogy, not an example, and unfortunately not accurate. One thing to keep in mind is that water ripples are essentially a 2-dimensional phenomenon (amplitude and wavelength; width is usually ignored). Gravity waves are 4-dimensional at least. This is keeping in mind that they not only radiate spherically from the originating mass, but also affect relative time. Also, no mass just 'appears'. Anything of significant gravitational influence has accrued over who-knows-how-many years or millennia or aeons.
I would appreciate it if an expert in the subject can clarify something for me, though. Most waves (of whatever sort) are produced by something moving. How would that apply to gravity waves? If, for instance, a mass were at rest relative to the universe in general, it would still have a gravitational field. Would it also produce 'waves', or would some other mechanism have to take over? This has been bugging me for years.

Gold Member
Dearly Missed
Danger said:
That would be an analogy, not an example, and unfortunately not accurate. One thing to keep in mind is that water ripples are essentially a 2-dimensional phenomenon (amplitude and wavelength; width is usually ignored). Gravity waves are 4-dimensional at least. This is keeping in mind that they not only radiate spherically from the originating mass, but also affect relative time. Also, no mass just 'appears'. Anything of significant gravitational influence has accrued over who-knows-how-many years or millennia or aeons.
I would appreciate it if an expert in the subject can clarify something for me, though. Most waves (of whatever sort) are produced by something moving. How would that apply to gravity waves? If, for instance, a mass were at rest relative to the universe in general, it would still have a gravitational field. Would it also produce 'waves', or would some other mechanism have to take over? This has been bugging me for years.

Have a look here Danger, The gist is that a static body does not, "radiate",
gravity waves, a body has to be accelerated.
http://en.wikipedia.org/wiki/Gravitational_wave

Gold Member
wolram said:
Have a look here Danger
Cowabunga! There's one ****load of blue text there! I don't have time to even start reading that now, but I have a day off coming up. Thanks for the link, man.

Antiphon
Creator said:
Antiphon said:
In a way- the wavelength should shorten as they fall in and lengthen as
they leave the [gravity] well. Shorter wavelengths have higher energy but not
higher amplitude.

Would you mind stating what empirical or theoretical evidence you have for believing that? Not that I have evidence to the contrary; just want to see how you arrived at that conclusion.

When the gravity wave (which carries energy and momentum) is climbing
out of the gravity well, it has to do work againt the well's field. This work
equals the energy that it picked up on the way down into the well.

While in the well, the extra energy shows up as a higher frequency which
is also a shorter wavelength since the speed is always measured as c.

Last edited: