Gravity: What Is It?



What is gravity? No-one bloody knows!


I love the fact that the answer to this has eluded the most intelligent and most active physics brains in the world.

We all know that an apple falls. I think people have always understood that if you let go of an will fall.

Genius, eh?
....which still does not explain it. We all know that it is there. We all know that it makes things attract other things. You and I could have deduced that a force called....."squeege" pulled things towards the ground.

What is Gravity?
1. Physics a. The natural force of attraction exerted by a celestial body, such as Earth, upon objects at or near its surface, tending to draw them toward the center of the body. b. The natural force of attraction between any two massive bodies, which is directly proportional to the product of their masses and inversely proportional to the square of the distance between them. c. Gravitation

apparently a lot of it is still theory, hence why it's still studied....

use the Force, Luke.....
and then there's always this malarky...
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and this...
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i'm sure i saw some TV show once were people realised gravity didn't really exist and they all started floating around...something about the whole theory of gravity being a government conspiracy to stop people form flying...daft... maybe i was dreaming....or it could've been The Simpsons... :)
Nope, you're right. It was a short film shown at the cinema before the main feature early this year. 'Low Price Gravity' or something like that.
Phill James said:
b. The natural force of attraction between any two massive bodies, which is directly proportional to the product of their masses and inversely proportional to the square of the distance between them.
Hmmm... ;)

The above only really applies to point masses. (Or if one mass is a sphere and the other a point mass, and the point mass is exterior to the surface of the sphere.)

In addition GR kinda knocks this one as well, as the source of the gravitational field includes a pressure term as well as a purely mass term. (This is why the gravitational collapse leading to black hole formation occurs. At a certain point, there is a runaway event where the pressure needed to withstand a particular gravitational force leads to an even greater gravitational force and no stable solution exists for a finite pressure.)

And as for Quantum Gravity... ;)
gravity is a dark ages concept created to make the average joe think that he must keep his feet on the ground. The church elders could not keep feeding the congregation the concepts they were trying to promote (i.e. ranks of angels, angelic powers being outside the ken of normal humans etc) if any-old-who could fly.

Thus we have the controlling church/governmental/educational system concept of "keeping your feet on the ground" even today.
Movement explains it

Here is a weird ramble...

If you take an hard-boiled egg and spin it in a shallow plate of water the water "rides" up the side of the egg's shell and, when the water reaches a certain hight, it spins off in a spinkler effect.

Bear with me...

If you move a iron bar in a coil of wire you generate a current.

Gravity could be the function of "movement" through a process that we have yet to understand. Nothing is "not moving" in the universe. A combination of forces, centrifigal, magnetic, etc. etc could all be used to explain gravity.

Einstine(sp?) tried to explain it in his Unified Field theory but from what I remember he did not complete it.

What would happen if all movement in the Universe came to a stop? No spining planets. No orbiting bodies. No galaxies spining and moving through the Universe. No atomic "movement".

Heat is generated through movement. (Atomic exchange of energy)

Magnitisim is generated through movement. (Spining iron in the Earth's core)

Light is generated by movement. (Energy transfer through photons I believe)

And many other examples probably show this fact.

Gravity is probably "movement potental while in a moving state in relation to another movement potental"

Almost like potental and kinetic energy are defined.

Just a thought :)

P.S. Like the egg example above, what would happen if the Earth suddenly stopped spining, stopped orbiting the sun and stopped moving with the sun through the galaxy and the rest of the universe? Would we sit on the planet and freeze to death or would we fly off the surface because there is no movement to generate Gravity?(provided we survived the initial catastrophe of inerta pushing us at miles a second into our house walls etc.)
Re: Movement explains it

Originally posted by Gerry Delmo

I've heard the argument before, and it is an interesting idea, though if the implication is that if the Earth stops spinning it would suddenly have no gravity (for example) there should be a way of testing the theory without actually stopping the earth rotating...
I was always taught that the reason every star, planet, asteroid was moving was simply that even the slightest tendency to movement would continue indefinitely with no friction to stop it.
Spin it up!

Of course, the corollary of the "gravity is caused by spinning" theory, would be that no spin means no gravitational attraction. Now whilst it would be very difficult (and I suspect that no funding body would support this as a research proposal ;) ) to stop the earth spinning, it is a relatively simple task to take a mass on the earth and stop it "spinning" relative to "the fixed stars" (following Mach.)

This should then become weightless. If it doesn't then the spinning gravity theory has problems.

Anybody want to give it a go and report back? :)

This is actually a great example of the way that science (usually) works, going from a theory to a prediction to an experiment/observation. :)
Excuse me for my ropey science (HAPPY NEW YEAR TO EVERYONE!!!:) ).

There was an old experiment we did in school that makes you think of gravity on a more 'personal';) level...

Try sitting on a swivel chair, and push yourself off. THere's one of two things you can do...

1) Stick your legs out, you'll notice how quickly you slow down.
2) Pull your legs in.. Egad! You'll keep on spinning!

I dunno if it's just one of my crazy ideas, but, maybe gravity has got more to do with the amount of weight revolving around a particular mass, as opposed to the idea of how much force a body in itself can exert on a mass revolving or sticking to itself. Maybe we could ask the moon to stick it's feet out, just to see.

In all seriousness, the theories of gravity, although mathematically objective to a point, leave a lot to be desired. Where is the consideration of mass Vs mass where the radius of orbit exceeds the optimal amount of swing to produce an effective and and constant inertia?

Thinking on a Bee-bop tip, maybe gravity has got a bit more to do with swing, or is, perchance, more understandable as such.

I hope this makes sense in some sort of twisted fashion; I just got back from a party near Sherwood where I has the pleasure of speaking my native tongue (Welsh) in England for a change! HUZZAH!

May your God love you all :)
If metal discs whose structures are weak are spun at the right speed, they can stretch and distort due to the (non-existant..) centrafugal force constantly pushing their molecules outward and away from the center. Why is planetary spinning different from this? Why does the spin not have the same power to throw us to the equatorial ring and then out into space? Gravity does it!

We are on a spinning ball, and instead of being pulled outwards, we are being pulled down. No matter where we stand, we are pulled straight down.

I know why certain people get defensive about these questions. It seems to be similar to those attitudes found in religious people whose theology is crumbling. This suggestion is not new, though.

I just think it makes it more fascinating and awe-inspiring.

Its not a threat!:)
Now where do gravitons (and anti-gravitons) fit into the scheme of things?
Gravitons are analogous to photons in the theory that gravity, like light, can be a wave or a particle i.e. when two objects are attracted by gravity they exchange gravitons - make sense?
A bit of a google, and after running away from some scarily complex looking calculations found this interesting site:

A nice theory, am a bit worried about all these little particles passing through me at super-light speed whilst spontaneously changing from energy to mass tho...
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This last one is the same theory practically as Phill James came up with...
to wit: gravity doesn't pull, it pushes (aka gravity doesn't suck, it blows.)

The medieval thinkers came up with the same explanation-
squillions of tiny angels all pushing against objects-
of course there are fewer wherever another object gets in the way, so the push is weaker.
This results in a net attraction.

Also the opposite of levity, as I understand it.
I'll admit I'm no physics major, so I apologize if this turns out to be an ignorant question.

Now, I'm under the impression that energy is finite. That is, in order for something to gain energy, something must give (and in turn) lose an equal amount of energy. Now, anything placed in a gravity field will immediately start accelerating towards the object. Now, I do realize that the two objects (say, a satellite and a planet) will impart momentum onto eachother (slingshot). However, the two objects do not impart gravity onto eachother.

My question is, is it that gravity simply is an infinite source of energy in regards to accelerating an object, or am I just missing something entirely here?
The extent of gravitational force depends on mass - THe greater the mass the greater the relative "pull" of gravity. which varies. it isn't infinite unless the mass of an object is infinite .Planets have finite mass,
Oh, that's not what I meant. What I mean is, for eternity you could keep throwing stuff at a planet, at it would keep attracting them. At no point would it run out of 'gravity'.

Assume you have a ball in a frictionless room, rolling west. I continually plink it with small balls from the west. Each ball it is hit with imparts energy and will eventually slow and stop the large ball. Every small ball I throw loses energy when it hits the large ball, with that energy slowing down the large ball.

With gravity, however, it doesn't change. You could drop an infinite number of objects on a stationary planet (removing them afterwards to mass and gravitation pull constant), and they will forever be attracted to it if you drop it again. They will accelerate towards the planet. Energy is obviously being applied to them. However, the planet doesn't lose 'gravity' energy if the only force it is applying is gravity (or does it? I know it will lose a tiny amount of momentum and the object will gain a tiny amount assuming their slingshotting, but I've never heard of such a thing with reference to gravity).
With gravity, however, it doesn't change. You could drop an infinite number of objects on a stationary planet (removing them afterwards to mass and gravitation pull constant),

of course, removing the objects would use up as much energy as was produced by dropping them onto the planet in the first place...
if you don't remove the objects, you will eventually cause the planet to become a black hole, and you can then directly harvest the energy of the acculated gravity by means of the Hawking Radiation...

but I think you are asking where does this energy come from?

Difficult one-
I'll let you know.
Gravity sets up a field of Potential Energy: the higher something is above the earth, say, the more PE it has. If it is allowed to fall freely under gravity, it accelerates: some of its potential energy is converted to Kinetic Energy. The sum of its PE and KE remains constant.

If you return it to its original position, you have to do work against gravity: this work increases the PE of the body again. Thus launching a satellite takes a lot of energy, which is provided by the chemical energy of the rocket fuel.

If a space-craft tries to return from orbit it has both KE (from its orbital velocity) and PE from its height. To land safely it has to get rid of this energy, which it does by using the air-braking effect of the atmosphere. The energy is transformed into thermal energy, causing tremendous heating of the air (and danger to the space craft, as we saw in the Columbia disaster).

Heat energy can be transformed into other forms of energy - engines of various types generally produce motion from the heat of burning fuel. (However, this process always results in a certain wastage, but I don't want to get into the Second law of Thermodynamics here!)

But for motion under gravity, eg for a body in an elliptical orbit, there is no energy loss, and some KE is transformed to PE, and vice-versa, orbit after orbit after orbit.
IIRC gravity is a result of a mass bending space/time so the said energy is space/time itself or the mass.

the picture i remember is as if space/time is like a elastic with graph paper on it. if u place a heavy object on it,and look from above the squares seem to be closer together as the weight presses on it

this is gravity.

the closer the squares ,heavier the object, the greater the gravity but ive never been able to understand this in 3d as space/time needent be in 2d should it ?


but isnt it that high voltages seem to interfere with gravity im at a total loss
unless the flow of electricity somehow either changes the mass or space/time
aint it more like a vandergraph grenerator with space/time as the supplyer of the energy and gravity being the path which it flows?

who knows.......................................
I do realize that removing an obejct will use up energy/give back energy.

What I'm thinking is more like this:

You take Earth. Remove it's atmosphere so that there is zero friction. Stick it in a vaccuum. Strap on some giant rockets to the Earth, and send it accelerating at 9.8 m/s. Stick something right above the surface. The Earth will continually accelerate until the engines run out fo fuel. Earth's energy comes from the fuel. The object, however, who'se energy comes form Earth's gravity, will never stop accelerating, and once the Earth stops accelerating it will slam into the ground.

Basically, I had jsut enver thoguht about it before, but it seems that gravity somehow creates it's own infinite source of energy (or rather, IS an infinite source of energy). I had just never heard the concept addressed before.
As far as I can make out, the energy in gravity comes fromthe creation of the universe.
The idea is that the universe was created from nothing, so if you add all the mass, gravity, electromagnetic enrgy and quintessence together you get zero.

Quintessence, or dark energy , is what ever it is that makes the universe expand. It is big, and seems to be increasing in power- it acts to expand the universe against the pull of all the gravity of all the galaxies in the universe, and also all the light (which has a weak gravity of it's own)

First imagined by Einstein as the cosmological constant, this tendency for everything to fly apart at long range is the reason that the universe is not a single point; it is the opposite of gravity and repels objects at long range rather than attracting them close up.
As the expansive force works to pull things apart, then gravity acts to attempt to reverse the process and pull them together. You are extracting energy from the entire universe when you gain energy from gravitational effects; this total is enormous, but finite-

if it were possible to gather all the matter together in the universe it would collapse, all he figures would balance once again and you would end up with nothing... no gravity, no dark energy, just nothing, ready to start again.

In some recent theories other dimensions, or branes, get involved, and the balance is spread over two (?or more?) branes, which probably means you could never cancel the universe out in this way. But carry on- the universe is a free lunch, if you know how to use it.

*edit -and of course, Tin Finger is right- spacetime curvature is the mechanism which allows these effects to happen and propagate at a distance.