Generally speaking, waves are alternations of some state or condition, the pattern propagating outward from a source.
Water waves: regions of heaped-up water vs. regions of air (ie, troughs).
Waves on a string: a bend in one direction vs. a bend in the other.
Electromagnetic waves: alternations in the strength and direction of the magnetic component and the electrostatic component.
Sound waves: higher-density regions of air vs. lower densities.
So what about gravity waves? Gravity that pulls vs. gravity that pushes? Naw. High and low DENSITY of gravitation? What does that even MEAN? But what else?
(By the way–can one theoretically produce waves of the strong and weak nuclear forces, too? If not, why not?)
But I think you were close with your “high and low density” of gravitation idea.
A visualization of gravity I’ve heard a few times is the rubber sheet method. Visualize a rubber sheet that distorts whenever you put anything in it. If you put a bowling ball in the middle, it will deform a lot, and have a sort of pit leading toward the bowling ball. Anything else on the sheet will move toward the bowling ball.
Now imagine that for some reason, the pit leading from the bowling ball has ripples in it. These ripples would represent gravity waves. If you dropped a ballbearing onto the sheet, it would roll faster toward the bowling ball (more attracted to it) at some places than others.
Of course, in space, things would work a bit differently. The object would not slow down upon encountering an area with weaker gravity (the “trough” of the wave…) unless the gravity were somehow pushing the object away.
Gravity waves are quadropole radiation. What that means in this context is that at one point in the wave cycle, the wave is compressing vertically and stretching horizontonally. At the opposite point in the cycle, the wave is stretching vertically and compressing horizontally. So, if you were to look toward a gravitational wave source, you’d feel your head alternating between being stretched and compressed, both up & down, and left & right.
Well, since gravity is always attractive, what I imagine you’d get at the trough would be a reduced gravitational attraction, not a negative attraction.
For example, say you have a handy black hole. This would be very useful for studying gravity waves because it’s very small with a measurable gravity. Now, get a way to move it around. Now, jiggle it back and forth about a foot.
The difference in attraction between when it’s close to you and when it’s far correspond to the peaks and troughs of the waves. It’s always towards the black hole, but with a different magnitude.
Pleonast:
Never come across that before, is there a cite? I’m having a hard time imagining a gravitational effect in any direction other than radially toward the source. If course, it could simply be a lack in my imagination.
RE: “A visualization of gravity I’ve heard a few times is the rubber sheet method. Visualize a rubber sheet that distorts whenever you put anything in it. If you put a bowling ball in the middle, it will deform a lot, and have a sort of pit leading toward the bowling ball. Anything else on the sheet will move toward the bowling ball.”
I’ve heard this too and the description bothers me ; doesn’t it presume that there is a gravitational force/attraction/whatever under the rubber sheet that is making things roll towards the bowling ball?
This ‘description’ smacks of circular reasoning - it is using gravity to describe gravity.
This is the best way to describe the general relativistic view of gravity in terms of common intuition–we can all imagine a bowling ball on a rubber sheet. We have a hard time visualizing a distortion in Riemannian 4-space. However, your concern is valid. The basic idea is that space itself is warped by gravitating objects in such a way that other gravitating objects want to be closer together. Why do they want to get closer? Who knows? In the 2D rubber sheet example, “gravity” is the why. In reality? Nobody knows.
I would posture that it doesn’t even matter “why”; or at least that we’ll never answer that question to our satisfaction. Suffice to say this: In classical physics, we think of momentum as being conserved (so objects do not accelerate unless a force is applied). The reason momentum is conserved, however, is that space is homogeneous–so a particle might think “that place is exactly the same as this place–thus I have no reason to want to be in either spot more than the other.” Thus, it doesn’t move, or doesn’t stop moving if it was moving. In the GR framework, however, space is not really homogeneous, due to the curvature induced by gravitating objects–thus particles might “prefer” one region of space to another, and so desire to move there.
Pleonast’s description of what you would feel in a gravity wave is accurate (although whether or not you would feel it is a different story–it is space-time itself which is being distorted, so it’s difficult to measure it with objects that exist in space-time (i.e., your head)). My source for this information is various informal discussions with with researchers on LIGO (Laser Interferometric Gravitational-Wave Observatory), and lectures on the subject.
So basically, gravity waves are transverse waves in spacetime–the distortion in space-time is felt in a direction perpendicular to the direction of propagation. They can be called quadrupole radiation because the distortion is felt first in one orthogonal direction, then the other orthogonal direction (i.e., orthogonal to the direction of propagation).
Doh! So, to answer the OP, basically what is happening that space itself is being alternately stretched and squeezed, in directions transverse to the direction of propagation.
Not exactly true. Even though it’s space itself which is being distorted, things like electromagnetic interactions can still be used to measure the changes in “length”. In the gravitational wave detectors currently being built, they use lasers shined over long distances, but you can also, in principle, use a single object held together by electromagnetic forces, like a big block of aluminum, or even your head.
It is true that you wouldn’t feel anything, but that’s just because gravitational waves from any sort of plausible source are extremely weak: You might, for example, get a change in the separation between two points of one part in 10[sup]20[/sup]. Your head, for instance, would change size by about a millionth of the size of the nucleus of a single atom. Needless to say, this is very difficult to test. If you were within a few radii of a pair of merging black holes, say, you probably would feel something, but it’s hard to say what you would feel: The structure of the waves in such a region is very difficult to model satisfactorially.
Sure they can, though all the places where wave concepts crop up explicitly in discussions about them are fairly esoteric at best. Talk of things like s-waves and p-waves, etc., is the sort of stuff that particle physicists take for granted in talking about anything.
In volume 2 of the Lectures on Physics, Feynman uses the analogue of trying to measure distances on a flat hot-plate with a metal ruler. Depending on where you are, the temperature can be different and hence your ruler can have “different” lengths in different places. You’ll quickly get confused about trying to confirm Euclidean geometry in such circumstances, even though you’re on a flat surface.
Now I await my man on a white horse who will paint for me a nicer, neater (kinder, gentler?) picture of these moving regions of lateral compression and stretching (and what they have to do with the force of gravity, which is just a “pull” sort of thing–isn’t it?).
Maybe G-waves are waves associated with gravity…but not “made of” gravity??
As a quick aside on the rubber sheet model of gravity: I think that the rubber sheet model got started by someone to show the difference in curvature inside and outside of massive bodies. To wit, the rubber sheet under the bowling ball has positive curvature (sphere-like) which is the case inside of a massive body. Away from the bowling ball the rubber sheet has negative curvature (saddle-like) which is the case in the vicinity of a massive body.
I sometimes think that the whole “other balls roll towards the big ball” thing is just another aspect…
The frequency of gravitational waves is determined by the frequency of whatever’s creating them. A typical source, for instance, is two objects orbiting each other, in which case the waves produced will have a frequency of twice the orbital frequency.
Well, I can at least try to paint a picture of what they would look like. Picture a chain, which fits tightly inside a flexible hose. Because of the chain, the hose will bulge out, sideways in some places, and vertically in other places. Now pull the chain through the hose, so that those bulges move. That’s about what gravitational waves are shaped like. I don’t pretend, by the way, that this model has anything to do with how the waves are actually produced: It’s strictly just a visualization aid.
And gravity isn’t exactly just a “pull” sort of thing. It’s a “distortion of space” sort of thing. A static mass will cause a distortion which acts like a pull, but the result of a gravitational wave isn’t really like a pull or a push: It would not cause you to move towards the source or away, but sideways.
And as to my question about the strong and weak nuclear forces and their waves…?
CHRONOS: Are you saying that unilinear acceleration does not produce G-waves? Something with the mass of Jupiter, say, falling in a straight line from outside the solar system toward the exact center of the sun?