# Questions on Gravity

what makes gravty work? like it takes energy to move things right? it seams like gravity has infenite of that stuff…so then whats up? my teacher told me it has something to do wit time or something…help?

Supposedly, there are a couple of particles that “carry” gravity–the Graviton and the Gravitino (?).

These, however, have yet to be observed, AFAIK.

One way for you to think about gravity is kinda strange, but here goes:

Consider all of space-time to be a huge rubber mat. When something is placed on the mat, it leaves an indention.

Small objects, like paperclips and cat whiskers, leave very tiny indentions. Large objects, like planets and stars, leave huge, deep indentions.

Now…consider something, a rocketship in this case, travelling through that rubber mat (remember, the rubber mat = space and time).

As the ship approaches one of the indentions left by an object, it begins to move closer to the object because the ship is “falling” into the hole left by the object.

If the object is a paperclip, then the hole isn’t very big, so the speed and direction of the ship isn’t affected much–in other words, there isn’t much of a gravitational pull.

If the object is a star or a planet, the hole is deep and wide, so the ship will have an easier time falling in, and a harder time getting out–thus, the gravitational pull is stronger.

If the object it a black hole, the indention in the mat is so deep that you can’t get out–in fact, the rubber mat itself can be ripped in that area.

One crucial thing to remember is that all of the indentions are as wide as the universe. They’re very shallow far away from the object that made the indention, but they get deeper and deeper as you move closer to the object.

Therefore, Jupiter has an incredible pull nearby, where it’s indention is deepest.

Jupiter’s pull is not as strong here on Earth, because we’re far away and the indention is not as deep.

On the other side of the universe, Jupiter’s pull is almost–but never quite–zero, because the indention is about as shallow as it can get.

Let’s see if this makes any sense in the morning…

Hope that helped some, but if I’ve confused you, just forget about it.

-David

It’s really very simple. The world just sucks.

JACK

HA! You may have a point, Jack! &nbsp

A couple of things that I want to add to the very basic outline above:

1. &nbsp There may very well be a limit to the distance of gravity’s effects, but I think that the jury’s still out on that one.

2. &nbsp The rocketship I mentioned is ALSO making an indention in the mat. Paperclips and planets alike are falling into the ship’s indention, as well–but the ship isn’t massive enough to move a planet enough to measure. &nbsp Probably couldn’t measure the ship’s gravitational effect on the paperclip either, for that matter…

3. &nbsp I said that size was what determined how deep the indention was–I should have said mass, instead.

-David

Soulfrost, I like your secription of the bent space model, mind if I use it?

twISTed, this makes a lot more sense once you learn the differences and relationships between force, energy and work. The kinetic energy of an object falling due to the force of gravity is just a trade for the potential energy it had from being in a position where it could fall in the first place.

And it gained the potential energy by moving it away from the planet because it takes energy to change the warpage of space?

I think that two bodies separated from each other constitute more warpage than the two of them together, therefore they the elasticity of space tends to move them together?

Here is what I don’t get about the trampoline / rubber mat / flexible stage analogy. It seems to me that it is still dependant on our (sometimes) friend gravity to work. A marble rolling along is held to the surface by gravity, no? OK, so make it two sheets of flexible material and squoosh a marble between the two. This effectively limits the marble’s movement to the topography of the mat. Great, I think I get by that one. But…

and

This analogy seems to do a fine example of describing why a planet has a greater gravitational effect than I do, but (to me) it does not answer the OP. The analogy still relies on gravity to cause the ship to ‘fall’ into the hole, and it relies on gravity to give the ship the harder time getting out. The analogy fails if you move the location of the rubber mats to deep space, where gravitational effects are negligible.

First of all, why would a bowling ball distort the mat? What force is it exerting on the mat to cause the distortion? Is that gravity? I also have a hard time separating bowling balls from the mat. If the mat represents time/space, why is matter (energy) separate from that? As you can see, I barely understand what I don’t understand about this. But maybe someone with greater insight can explain it. If so, maybe the explanation will also satisfy another problem I have understanding this analogy.

How does this analogy account for acceleration? A marble rolled along the surface in a zero gravity environment will (assuming there is no friction between it and the mat) will roll happily along at a constant speed. Doesn’t it loose a bit of energy when it is constrained to the surface of the mat to ‘fall’ into it? Besides that bit of energy, why would it change speed when it encounters an indentation in the surface fo the mat? What the hell is gravity ‘doing’ that makes it (appear) to fall faster? [speculation]Maybe if one superimposes a fixed grid over the surface and labels one axis Distance and one Time? Moving along a straight line at a constant speed, it would cross a fixed number of Distance lines for a certain amount of Time lines. As it gets closer to the bowling ball, the ratio changes, and as it moves (again, in our analogy in outer space it is moving to us at constant speed) between two Time lines it crosses a greater number of Distance lines. Crossing a greater distance in the same amount of time is tantamount to acceleration, no? [speculation] Simple simple simple, I know, but it almost makes sense to me. And besides, they persecuted Einstein and Galileo for… oops. Nevermind.

Anyway, I’ve read the rubber mat analogy before. It has been used by some pretty great minds. I can’t help but assume that the real flaw lies in my own misunderstanding of the analogy, so I hope someone can point me in the proper direction. Thanks for listening!

## Rhythmdvl

Once in a while you can get shown the light
in the strangest of places
if you look at it right…

Rhythmdvl, all analogies break down after a while but sometimes they can be the best way to introduce a new concept. The rubber graph paper works because we can visualize it. If someone finds a way to draw four dimensional space-time on on two dimensional paper we won’t need it anymore.

All models of the physical world are an attempt to describve what’s causing the things we observe in terms we can understand.

Using Newton’s model of gravity gravity is just a force that attracts every particle in the universe to every other particle. The force is proportional to the masses of the objects and inversly proportional to the square of the distance. The force just is and never gets used up.

The warped space model is extremely hard to visualize so we cheat buy using gravity in the model. It might even be misleading to use a grid but we do because we understand grids.

A physics teacher gave me a thought experiment that puts it into perspective at least for me. Imagine being iside a closed box that is in one of two places: sitting still on the surface of a planet that is not revolving or rotating or moving through “free” space being accelerated by a rocket motor that simulates the planet’s gravity. Is there any experiment you can easily do that will tell you which place you’re in?

Well, of the four forces, Gravity is by far the weakest. But it has one property that the others don’t–it’s range is (probably) unlimited.

Theoretically, this is because Gravitons and Gravitinos “carry” the force of Gravity, or the effect of Gravitation, to every other particle in the universe. (Just the thought of it boggles my mind!)

But as to how they carry the force and why it works, man, I really can’t tell you.

I’m not a particle physicist, but there are some on these boards, so hopefully they can help.

And I really hope that somebody points out a fatal flaw in my above definition of the whole Graviton business, because the concept opens a lot of cans of worms…

-David
Padeye…Thanks! And feel free to use it.

Ahh… using gravity to define gravity…

Actually, this analogy works OK… as long as you remember it’s just an analogy. Here are a few of the shortcomings of the analogy.
(1) The rubber sheet is two dimensional (three if you count time) and real space is three dimensonal (four if you count time).

(2) The rubber sheet has friction that affects moving objects where space does not.
The other thing is the question: Does the deformation of space cause gravity or does gravity cause the deformation of space or are both the symptoms of something else.

This is actually one of the strongest parts of the rubber sheet analogy. Realize that when the rubber sheet is deformed, it’s distortion is non liner - it’s logarithmic. If you look at the path a point mass would take towards the center of gravitation will be a curve, it looks just like an acceleration curve… until it reaches the mass, at which point the deceleration is rather abrupt.

Hello Soulfrost, you said, “Well, of the four forces, Gravity is by far the weakest. But it has one property that the others don’t–it’s range is (probably) unlimited.”
The Electromagnetic force has infinate range too, not just Gravity. Not to be a dick, I just wanted to point that out.

If at first you don’t succeed you’re about average.

Naw, you’re not being a dick–if I’m wrong, I wanna stop being wrong ASAP!

Thanks for pointing that out! I was unaware that E-M was infinite.

But hey–that’s just one more reason to believe that a GUT can be worked out, after all…

-David

Actually this “thought experiment” came from Einstein. He used it to explain why inertial mass and gravitational mass must be equal.

I can only think of one way to tell the difference without looking outside the box.

Virtually yours,

DrMatrix
These words are mine and they are true - Chief Meninock

One trick to making sense of the rubber sheet analogy is to remember that the sheet represents space itself. The masses involved are not held to the sheet by some ‘supergravity.’ They are held to the sheet merely because they exist in space and couldn’t leave it if they tried.

The important part is the curvature. Since the space is curved, the shortest path curves within it. Masses are pulled toward each other in an attempt to go straight in a curved universe. Gravitation is basically how curved space looks to things within it.

“If you prick me, do I not…leak?” --Lt. Commander Data

There are “tidal forces”. If the force is due to gravity, then it should be very slightly stronger at your feet than at your head, due to the fact that your feet are closer to the center of the earth than your head. This results in a very small force stretching you slightly. Also if you spread your arms, the force at your right hand will not be entirely parallel to the force at your left hand, since they are both being pulled towards a single point. This will result in a slight force squeezing you from the sides.

These forces are what causes the tides on the earth, hence “tidal forces”. Since the moon, IIRC, is held together by its own gravitation, there is a distance from the earth at which the tidal forces caused by the earth will be stronger than the moon’s own gravity, and will break the moon apart. I have been meaning for quite some time to figure this distance out, but it isn’t quite as easy as it sounds.

waterj2
Very good. I was thinking about using plumb bobs and noticing whether they converge or are parallel. Either method assumes very accurate measurements

Virtually yours,

DrMatrix
These words are mine and they are true - Chief Meninock

PlumbBob and Dr. Matrix:

Yes, in the situation you described you could detect the differnce between gravity and acceleration by tidal effects or by measuring the radial characteristic of the Earth’s gravitational field. There may be other ways.

However, it is important to note that the ability to measure the difference between acceleration and gravitation in that instance is a failure of the analogy, not a failure of teh equivalence principle. Just as many of the failures of the rubber sheet analogy are not failures of the underlying theory.

jrf

All of these answers are pretty good, but I don’t think they answer the original question – what makes gravity work? The answers given, to me, just seem to describe what gravity does.

Anyway, I don’t know why gravity is. I don’t know if anyone else does either. All I know, based on my liberal arts degree understanding of physics, is that gravity is basically a byproduct of matter. Matter attracts matter, from the hydrogen atom to a black hole. The force of attraction rises proportionately to the amount of matter involved. Therefore, my farts to not exert much attraction on their surroundings (actually, some repulsion), but the earth is a huge glob of dirt and rock and all the people and things on it, so its attraction is more than enough to keep me safely in my recliner in front of the TV. So why does matter attract matter? Heck if I know. The important thing is it does, but if anyone has an explanation, I’d like to know.

Here’s a fun question for the real novices, which everyone in my sophomore (college) physics class got wrong, including the teacher. We all know that, as you increase in altitude, the “pull” of gravity diminishes. Therefore, you would “weigh” more at the ocean than you would standing at the peak of Mt. Everest. The question is, where (on our home planet) is the pull of gravity strongest?

twISTed_LogiK:
Physics FAQ:What causes Gravity?
More detail is being sought after with
LIGO:
The Laser Interferometer Gravitational - Wave Observatory.
which is the brain child of Kip Thorn. I don’t think its fully functional yet so stay tuned.

trouts1-
That cite has a great point, about treating gravity like a condition rather than a force. It’s like the way they insist on the term “centripetal acceleration” rather than “centrifugal force” in physics. We feel the force (Luke) that opposes the unfeelable acceleration.

## It’s a bit OT, but I can’t resist: in reply to paintsvillecom’s question, my guess would be the South Pole. The poles are closest to Earth’s center of mass and the South Pole is solid land (denser than the North Poles ice-covered ocean).

Sure, I’m all for moderation – as long as it’s not excessive.