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What is gravity?

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raid517    0

Well, what do you think it is?

GJ

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username    0

A theory that objects with mass have an attractive force on other objects with mass

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Evolution    16

Isn't it that objects with mass create a displacement in the time-space (or possibly just space, they could be seperate, and only appear together) fabric thus bringing objects closer together :/ ? After all light bends with gravity.

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raid517    0
A theory that objects with mass have an attractive force on other objects with mass.

Good start... Do you want to add more detail about what exactly you think that attractive force is 'made of'?

Surely it isn't just the theory that is holding planets together?

There is reason in my madness for starting this thread BTW. I'm not really asking. I'm just really interested in other people's opinions.

GJ

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sundayx    127

gravitons

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vincent    154

The force of attraction between bodies of mass. It's basically the sole force that holds the universe in place, well... alot of things in the universe in place. Without gravity there would be alot of chaos in the universe, gravity plays a role in supernova which in turn are responsible for the distribution of heavy elements which play an integral role on Earth itself.

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raid517    0

Erm.. just a hint, this thread isn't quite aimed at people like rigput - not yet anyway. ;)

GJ

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vincent    154
Erm.. just a hint, this thread isn't quite aimed at people like rigput - not yet anyway. ;)

GJ

586629873[/snapback]

:p, why bro?

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Lexxan    0

Noone really knows what it is.. They just have a name for it and a theory on it.

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raid517    0
why bro?[

Because you are too damned smart. :p

There is not much point asking you what you think gravity is because you are likely to have a pretty decent grasp of it already.

I am just interested in getting everyone's views.

Everybody's opinions are appreciated.

GJ

Edited by raid517

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davemania    1

String theory, open loop, that would at least explain why its so weak compare to the other attractive forces

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neowin_hipster    0

Gravity is something that even scientists are trying to explain. For example, we know of gluons, photons, bosons but the graviton's existence is still being debated. Gravity is the weakest of all forces by far and we're still looking for gravitational waves even.

In fact, string theory is the leading candidate for explaining many things about the mysterious gravity and how it all relates to everything else.

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SaLiVa    3

Funny nobody mentions Newtons Laws of Gravitation.

Every particle of matter in the Universe attracts ever other particle with a gravitational force that is directly proportional to the product of the masses of the particles and inversley proportional to the square of the distance between them.

Do what you may with that statement, but all it is saying is that every particle that has a mass will be attracted by a force due to the presence of another similar particle. It mainly depends how far they are and how much matter they both contain.

That's the basic A Level definition, there are more complex equations such as Potential and what not, that I will not point out in this post.

"Gravitation is not responsible for people falling in love." - Albert Einstein.

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headless_armadillo    0

g = G(Mi.Mii)/(r^2) (gravitational field strength - ms^-2)

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remix17    0
Funny nobody mentions Newtons Laws of Gravitation.

Every particle of matter in the Universe attracts ever other particle with a gravitational force that is directly proportional to the product of the masses of the particles and inversley proportional to the square of the distance between them.

Do what you may with that statement, but all it is saying is that every particle that has a mass will be attracted by a force due to the presence of another similar particle. It mainly depends how far they are and how much matter they both contain.

That's the basic A Level definition, there are more complex equations such as Potential and what not, that I will not point out in this post.

"Gravitation is not responsible for people falling in love." - Albert Einstein.

586630904[/snapback]

Everyone knows this. But this doesn't explain what gravity actually is.

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raid517    0
g = G(Mi.Mii)/(r^2) (gravitational field strength - ms^-2)

586631458[/snapback]

Well that is the text book answer. But in this instance I am trying to understand what people's views are on how it actually works, what the mechanism for it is - and just plum how eactly do we manage to keep our feet on the ground?

BTW that is a classical Netoninan interprtation of gravity, I am trying to come at this from a more Einsteine/relativity type angle. There is not much point either just quoting physics text books or maths you found on the web or whatever - as not a lot of people here will understand that. I am just keen to find out what people's basic perception of gravity is, how they think it works, what causes it an so on. In this sense those calculations aren't much use, because all they say is how gravity works, but they do not say at all what it is.

What is this force of attraction? Again, you would tend to think that it was 'made' of something - as really just saying that something is a 'force' isn't enough. The only force that is alleged to have no natural mechanism and consist of exactly nothing is supernatural - as in God. So if it is a force, what exactly does this force consist of? Does gravity emit particles? Is it a wave? Or maybe it really is something supernatural that keeps everything in place?

GJ

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j.nudd    0

Well, it may be a bit on the simple side, but if you believe that nothing can travel faster than the speed of light, how does gravity have an immediate effect on objects over imense distances? That is if you believe that nothing can travel beyond that barrier :)

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SaLiVa    3
Everyone knows this. But this doesn't explain what gravity actually is.

586631497[/snapback]

That question is quite useless then. You're better off asking "Why does Nuclear fission produce so much energy from separating such small particles?".

For that, it's caused by a mass defect, a reduction in mass. Mass is basically energy in itself.

This relates to Gravity a bit. Since mass is a form of energy this can make gravity as a form of force that brings these energy together at a bigger scale. Nuclear energy being at a molecular level, while gravity takes control of the product of nuclear and binding energies.

It's all dependent on how you view the world, and I'd rather much depend on Mathematical and Physical figures. They have proven to be accurate in predicting motion due to gravity and energy content of molecules. The Philosophical viewpoint relies on proving "What is real?" or "What is normal?".

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headless_armadillo    0

*Wants to learn General Relativity*

Ive heard of ideas where Gravity is analogous to a "well" in space-time - IE an object creates a dent in space-time, like a cherry in a pool of water. Objects are then drawn towards the object creating the dent (they even have their own smaller dent surrounding them) and in the case of planets, can cause orbiting around one of the isobars of gravity (equipotentials). I can's explain it very well :(

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insanekiwi    0

gravity is: what goes up must come down. period. :rofl:

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raid517    0
*Wants to learn General Relativity*

Ive heard of ideas where Gravity is analogous to a "well" in space-time - IE an object creates a dent in space-time, like a cherry in a pool of water. Objects are then drawn towards the object creating the dent (they even have their own smaller dent surrounding them) and in the case of planets, can cause orbiting around one of the isobars of gravity (equipotentials). I can's explain it very well :(

586651681[/snapback]

That is a very good answer.

In a sense (indeed in a very specific sense) gravity is a depression caused by a body of mass in the very fabric of spacetime. To envisage this, if you imagine a sheet of thin rubber to be analogous to the fabric of space time, clearly if you stretch this at each of its 4 corners and place a cricket ball, or a baseball (or any semi spherical object in the middle of it) this will very clearly create a depression in the fabric of the rubber sheet. So also is this true when considering the effects of mass on the fabric of spacetime. Gravity in this sense is almost exactly similar to this in that mass possesses the inherent capability to bend the very fabric of space.

You can even if you wish consider the 'shape' that of space time that such a depression causes. This shape is what we know as a parabola - essentially a geometric shape that starts out with a gentle slope at its edges, but who's gradient increases and becomes ever more steep the closer to the centre of the body of mass that we come. (Indeed this effect continues right to the very centre of the body of mass where the force of gravity is strongest and is concentrated to an area smaller than even the smallest pin prick).

All of these ideas lead up to Einstein's more generalised theory of relativity, a.k.a. the General Theory of Relativity.

This theory take the ideas of Special Relativity and combines them with the idea of gravity.

Basically, the idea is that mass is energy (E=mc2), and this mass, or energy. distorts spacetime, causing it to curve.

Thus, a really massive object will dramatically curve spacetime.

curved_space.gif

This isn't possibly the best shape - as the curve/parabola would probably be much wider than this - but it does I suppose serve to demonstrate the principal (so clearly from this you can also see that there is no need for gravity to act 'faster than light' as a previous poster suggested - since (although it's influence can reach over vast distances) its effects are entirely localised.

Here are a couple of nice videos of this effect in action. (Warning don't click unless you have broadband).

http://www.irtc.org/ftp/pub/anims/2002-01-15/gravity.mpg

http://blueox.uoregon.edu/~karen/astro123/images/precess.mpg

Now because we know from Newtons Universal laws of gravitation that any object travelling in space if given a sufficient push will continue travelling in straight line and at the same speed forever (unless another force acts on it - such as friction or another object being placed in its path) then clearly the tendency of any object orbiting another object is space would also be to travel on in a straight line forever. But because spacetime can be bent or warped by bodies of mass, a straight line in space isn't necessarily the same as a straight line on Earth.

Because it is the actual fabric of reality that is being bent (or of spacetime) then if we were to have a very long and somewhat flexible ruler, it is clear to see that if we took this ruler and tried to measure the Earth from space from one pole to another, that the ruler too would be bent, or curved by the effects of gravity.

Another good example of such a ruler to use would be a laser beam.

Laser beams always follow perfectly straight lines. But if you were to take a laser beam and shine it at a planet so that it it just skimmed past the upper edges of the atmosphere of this planet, you would see also how gravity was able to bend the light from the laser beam around it. (See video below).

http://blueox.uoregon.edu/~karen/astro123/...s/lightbend.mpg

This does not mean that the light beam is bent, because in actual fact it is spacetime and/or reality that is bent - so as far as the light beam is concerned it is simply continuing to travel in a perfectly straight line.

Therefore when we consider how a body of mass (such as a moon) orbits another body of mass (such as a planet) it is clear that although it they do indeed orbit each other, they do so in what to them essentially seems like perfectly straight lines. This is exactly in accord with Newton's work and in fact using this kind of reasoning we can use Newton's calculations to work out exactly just how much influence a specific body of mass will exert over another body of mass and - also given Einstein's insight to what degree that body of mass will distort the fabric of spacetime. Newton tells us what effects gravity has on bodies interacting with each other within the same local system - whereas Einstein actually says what gravity actually is. So it is really quite a remarkable insight!

It is not the whole story - and we do not have all the answers yet! But it is certainly at the very least a very large part of the picture.

GJ

Edited by raid517

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Oompa    1
*Wants to learn General Relativity*

Ive heard of ideas where Gravity is analogous to a "well" in space-time - IE an object creates a dent in space-time, like a cherry in a pool of water. Objects are then drawn towards the object creating the dent (they even have their own smaller dent surrounding them) and in the case of planets, can cause orbiting around one of the isobars of gravity (equipotentials). I can's explain it very well :(

586651681[/snapback]

Exactly what I think.

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raid517    0

And now that a little about the geometry of spacetime for normal bodies. Here is a nice little image of the somewhat more pronounced geometry of a black hole

geom.gif

This is what as known as the Schwarzschild geometry for a black hole.

The Schwarzschild geometry describes the spacetime geometry of empty space surrounding any spherical mass. Karl Schwarzschild derived this geometry at the close of 1915, within a few weeks of Albert Einstein publishing his fundamental paper on the Theory of General Relativity. The history of this discovery and much more is wonderfully recounted in Kip Thorne's book ``Black Holes & Time Warps: Einstein's Outrageous Legacy'' which is a book I would recommend to any curious reader.

Note as I said how all such geometries (even in a black hole) concentrate the curvature of spacetime (and therefore of gravity) towards the centre.

GJ

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