/sci/ - Science & Math

>source of gravitational force
>gravitational force
>force

You do realize that there is no gravitational force, right? It is the relationship between the presence of mass and the resultant curvature of spacetime that we do not understand. I doubt it would be possible to have mass without the curving of spacetime, but it may well be possible to have the curving of spacetime without mass.

>mfw gravity has been explained as the curvature of spacetime for almost 100 years
>mfw i have no face

your picture made me sad OP i do not want to answer your post

waves are actually as predominant as particles - almost everything has a clock speed - why can't gravity simply be defined as a wave?

>>4686614

what if it doesn't have anything to do with the curvature of space - and please define, simply, the curvature of space. thanks

>>4686679
The curvature of spactime may refer to a number of things, but the typical one is the Riemann curvature tensor. It's a rank 4 (1,3) tensor built from second derivatives of the metric tensor.

Imagine space as a trampoline.
As you stand on the trampoline it indents.
Place a ball on the trampoline, and it will roll to the bottom of the said indention.
>now, instead of trampoline, you have space-time, in 3-dimensions (or n-dimensions, whatever)
>instead of you "standing on it" anything with mass is "standing" on it
>That force (which acted on the ball) is called gravity in space-time.
voila.

>>4686702
That analogy is awful.

>Mandatory xkcd reference.

Whatever you do, if you act in a professional setting, never imagine gravity as some trampoline. The picture is wrong and misleading. There is no simple picture for relativistic gravity. Just do the math.

Put simply: energy and momentum tell spacetime how to curve, and spacetime tells objects how to move.

If you want to get into detail, my answers will probably go over your head (not trying to be condescending, just honest).

Not one single poster in this thread has explained why gravitons are necessary.

Just sayin'.

>>4686738
you seem unaware that gravitons are hypothetical particles which do not really exist.

the metric of spacetime is "really" a coherent background of gravitons, that the field equations for gravitons look geometrical to leading order is a cool "coincidence". We know that GR cant be the final theory, since it ain't quantum mechanical, and the quantum corrections to the field equations will probably mean that we no longer can see gravity as classically curved spacetime (although ofcourse that view will still be valid as an excellent effective field theory).

>>4686738
They are necessary since they are what you get when you quantize gravity. We need to quantize gravity since otherwise our theory breaks down at sufficiently small length scales where both gravity and quantum mechanics becomes important.

>>4686738
They are necessary for quantization. In analogy with electromagnetism: the electromagnetic field in classical theory, when quantized, is built up from photons. Likewise with gravity - the metric field is built up from gravitons.

>>4686745
They are present in (so far) every attempt to quantize gravity.

>>4686613
>mfw Maxwell's equations described light as a wave for nearly 50 years
>mfw I have no face
There is empirical evidence suggesting gravity occupies quantum states. Relativity fails at the subatomic scales unless you use strings.

If you like curved space-time so much, keep in mind the gravitons are just fluctuations in the metric, they reproduce the same effect when scaled up. The possible states of the field above vacuum energy are just quantized.

>>4686595
>What happens if we could eliminate this particle from an object?
Do you mean remove the coupling from the graviton field? Gravitons are associated intrinsically with the structure of space-time. The graviton, being a spin-2 particle, couples (listens) to mass-energy, analogous to electromagnetic current coupling to the photon field. You could not prevent the field from coupling as it would violate some local space-time symmetry that we are not yet aware of, just like conservation of charge.

For a field theory in flat spacetime
<div class="math">S(h) = S(h=0)-\int \mathrm{d}^{4} x \left [ \frac{1}{2} h_{\mu v} T^{\mu v} + O(h^{2}) \right ]</div>
You can see <span class="math">h_{\mu v}(x)[/spoiler] (the graviton field) still couples to the stress-energy tensor <span class="math">T^{\mu v}(x)[/spoiler] just like in relativity.

Of course, this action fails when you try to calculate graviton-graviton scattering or any other amplitude.

Why does it fail? What is so hard about quantizing gravity?

>>4686885
GR isn't renormalizable.

>>4686885
It is a non-linear field theory, and the graviton couples to itself because of this. This is similar in Yang-Mills theories, but the action terminates and is renormalizable.

When you try to setup a field theory for the Einstein-Hilbert action, you get an infinite series (<span class="math">1+g E^{2} + (g E^{2})^{2} + \cdots[/spoiler]) in the graviton field with the divergence worse and worse (quadratic divengence actually from <span class="math">\sqrt{-g}[/spoiler] in the classical action) at higher and higher order, so it is nonrenormalizable.

So either something is wrong with relativitiy, or we could extend the 0 dimensional particles to 1 dimensional stringy excitations and add extra dimensions. It is still background dependent like this though, so although you have a graviton field, the stress-energy tensor still curves the conformal field theory that defines the moving strings which isn't true quantum gravity. You can use AdS/CFT at this point and get graviton scattering amplitudes with perturbation theory, or you can go the background independent route with String field theory and N=8 sugra which describes the string field excitations for all fundamental forces and particles in just one flat background.

>>4686730
Nope. Just nope.

>>4686840
OP is talking out his/her ASS!
There are no gravitons. Gravity is simply another word for the changed geodesic in quantum foam.
There are no gravitons needed for this.

OR we simply can quantize gravity because its a fundamental relation of spacetime geometry to mass/energy.

Nothing suggests that a massless particle with spin 2 exists, other than the fact that qft proports that the presence of one would be indistinguishable from gravity.

>>4686922
1/10, just because you got me to respond.

>>4686926
It's a reasonable assumption, given the other forces in the SM as analogies.

>>4686926
How would you make it compatible with Lorentz group transformations? The entire point of quantizing gravity is to unify physics so we can write things out simpler, as they were at the early age of the universe. If you have all of these weird cases and some things behaving in some sort of fashion not related to the way other things behave, it gets convoluted and does not agree with anything before the Inflationary epoch.

The graviton needs to be spin-2 to couple to the stress-energy tensor, <span class="math"> h_{\mu v} T^{\mu v} [/spoiler], you need a tensor field symmetric in both its indices for it to be attractive or the math won't work. I don't understand how you could write it any other way without scrapping relativity.

How would you make it compatible with Lorentz group transformations? The entire point of quantizing gravity is to unify physics so we can write things out simpler, as they were at the early age of the universe. If you have all of these weird cases and some things behaving in some sort of fashion not related to the way other things behave, it gets convoluted and does not agree with anything before the Inflationary epoch.

The graviton needs to be spin-2 to couple to the stress-energy tensor, <span class="math">h_{\mu v} T^{\mu v}[/spoiler] , you need a tensor field symmetric in both its indices for it to be attractive or the math won't work. I don't understand how you could write it any other way without scrapping relativity.

>>4686944

But the other forces are different from gravity.
I don't see how quantizing gravity can account for observed phenomena such as gravitational lensing, time dialtion, etc since it presupposes a fixed spacetime background.

I understand the assumption is warranted based on the success of qft though, it just doesn't seem to me that a graviton can account for all the nuances of GR

>>4686710
pulled down by gravity?
damn, xkcd is dumb

>>4686972
You are free to like a background-dependent gravity theory, there is nothing wrong with it. Many people like background independence though (with every force) as it describes states of the force field as being discrete, which is what we observe, and what comes out of the math. Maxwell had not a clue of a photon, he thought the electromagnetic field was continuous.

For example, with electrodynamics, in order for U(1) symmetry to work (and naturally to have conservation of charge and excitations which do not propagate FTL) you have to have a discrete photon field. When we excite the field, we see this. We cannot observe internal lines of Feynman diagrams, but under this basis it is easy to assert they behave the same way (with perturbation theory) even though they are off the mass shell.

The only reason the Riemannian manifold comes up is because it is a classical limit of quantum gravity. It is what gravity looks like on the large scale. If you zoom in, you would find that all of space, time, matter, energy, etc. are all a result of the same phenomena, discrete excitations in a fixed background. Or at least this is our current understanding. The excitations collectively add up and produce the curvature and effects we see such as gravitational lensing. Why does gravity need to be separate from this? It just makes the picture more complicated.