what is gravity?

[Rev. Green]

Einstein thought gravity was a virtual force, like centrifugal force or Coriolis force, caused by curved space-time. What is the proof that it is a real force, like the electromagnetic force, but transferred by gravitons instead of by photons?

[Clanad]

If you receive a definitive, supportable definition please let the powers that be in science know. They'd be delighted at the revelation (no pun intended). Fact is the Newtonian theroies don't stand up very well when exposed to Einsteinian General Relativity theories. However, Mr. Einstein's somehat plausible explanation falls apart when exposed to the winds of quantum physics. It's mathematically incompatible with the quantum laws that govern subatomic particles. So... scinetists everywhere are searching for a clue, of which they have very few right now. The favored theory at the moment involves the vagaries of the string theory. But, I'll leave it to you to enlighten yourself (think Macbeth, Act One, Scene One)...

[jake-the-peg]

There's no proof that gravity is mediated by gravitons, but the other 3 known forces, strong weak and electromagnetic are mediated by so called "carrier bosons" and so that's a rather strong hint that gravity is as well.

Since Maxwell first unified the Electric and magnetic forces and then the Electromagnetic and Weak forces were shown to be part of the same force at high energies the obvious direction is to work on the principle that all the forces are intimately related and therefore operate in the same way.

This means lots of people should be out hunting for gravitons, but they are too weak to be detectable individually so the effort is to infer properties about them from detecting gravitational waves.

The issue that Clanad refers to is about what happens when you try to make predictions about gravity's behaviour at high energies. Basically put the equations go infinite.

This happens in the predictions about the electro-weak as well but there a nifty little trick called re-normalisation that Richard Feynman and others used to make the infinities go away - unfortunately we can't do that with gravity.

The place to keep an eye on is CERN in a few months the new collider (the LHC) will start up. This should be able to find the Higgs Boson believed to give matter it's mass.

If this is found we're on the right track

[Rev. Green]

Thanks Jake-the-peg, but he strength of the other three forces is independent of mass: gravity is exactly proportional to mass, just like the virtual forces. Infinities are not a problem: with finite energy mass cannot be confined so closely.

[matt1e]

I always believed that gravity was to do with mass ie larger bodies always have a stronger gravitational pull on smaller bodies
earth huge - us tiny in comparison so we stick to it

[jake-the-peg]

I think Reverend you are looking at gravity in a Newtonian context.

Gravity is indeed simply proportional to the product of the masses when things are nice and mid range.

Of course as you doubtlessly know this simple view doesn't explain effects like the precession of the orbit of Mercury, gravitational lensing and time dilation on a gravitational field - we need to invoke Einstein for this and the maths gets more complex.

The infinities arise when you try to explain the action of gravity by the exchange of gravitons. Because the graviton has to be a spin 2 partical the renormalisation trick doesn't work and you can't use it to get good answers out the way you can in Quantum Electro Dynamics.

If you're interested in this stuff I can strongly recommend this book:
http://www.worldscibooks.com/physics/5088.html

Veltman won a nobel prize for the quantum behaviour of the electroweak force and the book starts off with the predictable " what is an atom stuff" and goes through the standard model, virtual particles the practical issues of accelerators ending up with all this stuff on perturbation theory, Feynmann diagrams and the Higgs.

It's technical but rarely mathematical so if you follow through it you can get a good grasp of what's going on without needing a degree in maths.

If you have a mathematical background you may be interested in
http://itunes.stanford.edu/

Stanford university have put on Itunes an enire course of lectures on quantum mechanics by Leonard Susskind the so called "father of string theory" It assumes some basic familiarity with concepts like complex numbers, vectors and matrices but is easily accessible if you've a good grasp of A level maths.

09:11 Thu 07th Feb 2008