Gravitons

Chucky

Could somebody explain to me why Gravitons were first postulated? Are they part of Quantum Theory only?


Personally, I do not see the need for Gravitons to exist. We have never detected them and even Einstein (In his General Theory of Relativity) stated that gravity was not a form of energy. Instead, he stated that gravity existed as a consequence of the fabric of the universe itself (It being curved).


If Gravitons were to exist then they would have to be particles that can travel at infinite speeds which just makes no sense. For instance, if the black hole at the center of our galaxy were to just vanish the gravitational effects on our solar system would be almost instant. In that scenario, if gravity was in fact due to particles called Gravitons then they would have to be travelling at infinite speeds - That cannot be...


...right?

Son Goku

Chucky wrote:

Could somebody explain to me why Gravitons were first postulated?

One moment.....

Chucky wrote:

Are they part of Quantum Theory only?

Yes. Or rather a specific part of it.
Quantum Theory can be broken down like this:
Quantum Mechanics:
Fundamental particles are actually a collection of numbers attached to a probabilistic field. Different Quantities such as Angular Momentum and Spin are numbers which effect the probability field.
However the Forces are still classical just like they were in Newtonian theory, i.e. ElectroMagnetism is still just the same as it was in Maxwell's Equations.

Quantum Field Theory:
The Unification of Quantum Mechanics and Special Relativity.
Fundamental Particles and the Forces are both probability fields, although much more complicated ones than in Quantum Mechanics.

There are four Quantum Field Theories:

QED
Describes the electromagnetic force. It consists of what is called the Dirac field being attached to the QED field.
The smallest lump of the Dirac Field is the electron, the smallest lump of the QED field is the photon.
Figured out and written down in two different ways by two different Americans, with some help from a Japanese Professor.
The two different ways were shown to be the same by a British Professor.

QCD
Describes the Strong Force. Consists of the Quark Field and Gluon Field. Currently the most difficult established field of physics.
Figured out slowly by over twenty people from 1958 to 1969.

QFD
Short lived theory of the Weak force. Although still used in the rare situations when the Weak force does something independant of the other forces.

Electro-Weak
A better version of QED, which shows that the QED field is just one half of a bigger field. The other half is the field which manifests itself as the Weak Force.

When we put QCD and Electro-Weak together we get the Standard Model.

The only force the Standard model is missing is gravity.
In the 80s some proposed that gravity is actually a probability Field just like the other Forces. The graviton would be the smallest lump of this field.
If they'd kept the naming convention it would have been called QGD, but it was instead called SuperGravity.

Personally, I do not see the need for Gravitons to exist. We have never detected them and even Einstein (In his General Theory of Relativity) stated that gravity was not a form of energy. Instead, he stated that gravity existed as a consequence of the fabric of the universe itself (It being curved).

SuperGravity would basically say that Einstein was wrong and that spacetime being curved is just what the graviton field looks like at the classical level.
Just like the QED field looks like electricity and magnetism at our level.

If Gravitons were to exist then they would have to be particles that can travel at infinite speeds which just makes no sense. For instance, if the black hole at the center of our galaxy were to just vanish the gravitational effects on our solar system would be almost instant. In that scenario, if gravity was in fact due to particles called Gravitons then they would have to be travelling at infinite speeds - That cannot be...

Gravity only travels at the speed of light and so would the gravitons.

Chucky

Thank you for the reply. By the way (And considering you are a moderator), can you change the title of this thread to 'Gravitons'? I appear to have accidentily named the thread after myself!


I never knew that Gravity travels at the speed of light - Where did you source this from? This would infer that Gravity is a wave-form, right? Presumably the wave would then spread out from the source just as light does and become progressively weaker?


What if Gravity-waves were actually light-waves themselves - Has it ever been tested to see that the hotter an object is in proportion to its mass, the more gravitational pull it has? This would infer that the Sun, being the hottest body in our solar system has the most gravitational pull. Then, the gravitational pull of each of the planets is due to how large and hot they are.


Kevster

mike65

Moved the physics

Mike.

Civilian_Target

Son Goku wrote:

QCD
Describes the Strong Force. Consists of the Quark Field and Gluon Field. Currently the most difficult established field of physics.

Dunno about that - I understand stong interactions pretty clearly myself, and coloured gluons, but I really don't understand the W particle interchanges in electroweak theory at all!

Like any wave or particle, gravity can't propogate any faster than c, the speed of light - although since nothing can propagate faster than c anyway, it's a moot point. And yes, while gravity is quantised, same as everything else, it does become weaker with distance. That's why your feet stay on the earth and you don't float away to the sun!

As for heat-related increases in gravity - no, that's not a possibilty because heat is just an electromagnetic effect that has no bearing on gravity. What does increase gravitation though is having more fermions (mass particles) in the same space, which can occur in dense plasmas.

The standard model actually predicts 13 field quanta, or bosons: Photons for EM, Gravitons for gravity, 8 gluons for strong, 2 weak quanta (W & Z) and the Higgs for spin, which hopefully will be found when the new CERN collider gets up and running.

Son Goku

Civilian_Target wrote:

Dunno about that - I understand stong interactions pretty clearly myself, and coloured gluons, but I really don't understand the W particle interchanges in electroweak theory at all!

It would take me about an hour or so to calculate the amplitude for W particle interchange (obviously with no loops), but a single quark-quark amplitude I'd honestly have to say I wouldn't be able to do at all.
Although it is easier to get the physics of high-energy QCD over Electroweak, I should have said it is the most challenging mathematically.

QCD is nonperturbative* at low-energies which is where most of the difficulty comes from, because technically quarks don't exist at that energy scale. It also has more open questions as well.

What is it about W exchange that you don't understand/like?
(Just wondering because I've difficulty with Z all the time, what the hell is that particle for?)

*For anybody reading, just so that I'm not spouting jargon, nonperturbative means we can't break the field up into particles.
At high-energies we can look at the what the QCD field is doing by breaking it up into gluons and quarks and saying that the quarks are absorbing gluons.
At low energies the field stops acting in a particle way, so we can't pick out a part of the field and say "this is a quark".
All we can say is that the field is doing stuff.

Son Goku

Chucky wrote:

I never knew that Gravity travels at the speed of light - Where did you source this from? This would infer that Gravity is a wave-form, right? Presumably the wave would then spread out from the source just as light does and become progressively weaker?

Yeah, pretty much. The experiment which tested it was done by two guys in the seventies, I forget their names.

What if Gravity-waves were actually light-waves themselves - Has it ever been tested to see that the hotter an object is in proportion to its mass, the more gravitational pull it has?

In generally relativity it isn't just mass that causes Gravity it's:
Mass, Stress, Heat, Pressure, Density, Potential Energy and (weirdly enough) Gravity.

This would infer that the Sun, being the hottest body in our solar system has the most gravitational pull. Then, the gravitational pull of each of the planets is due to how large and hot they are.

Exactly if the Sun were colder, but still had the same mass it would have less gravity.

Civilian_Target

Never done Z interactions - since I'm doing experimental Physics, not theoretical
The gluon "tube of force" maths is a bit messy but it's not that bad, as opposed to QM perturbation theory which just doesn't make sense to me!

Son Goku

Civilian_Target wrote:

Never done Z interactions - since I'm doing experimental Physics, not theoretical

It makes no sense. It just does nothing, particles exchange it and nothing happens. The only thing it seems to do is effect the probability of other measurements.

The gluon "tube of force" maths is a bit messy but it's not that bad, as opposed to QM perturbation theory which just doesn't make sense to me!

Never did quarks using the Quantum Mechanics "tube of force".
Perturbation Theory is definitely the hardest piece of Quantum Mechanics, although in Experimental Physics I'd imagine you'd do it to a greater depth than I would have.
You should see Quantum Field Theory. (Don't know if you have or not) In it you basically do a parturbation where the lowest term is Quantum Mechanics itself.

It's like:
(What QED says) = (What QM says) + (Correction to what QM says) + (better correction) + (better correction) +..............................

Gurgle

Son Goku wrote:

In the 80s some proposed that gravity is actually a probability Field just like the other Forces. The graviton would be the smallest lump of this field.

So is a graviton a particle, or the smallest possible change to a wave?
Has it got a fundamental frequency or spectrum?

Son Goku

Gurgle wrote:

So is a graviton a particle, or the smallest possible change to a wave?

More so the latter, I'll explain in a second.

Gurgle wrote:

Has it got a fundamental frequency or spectrum?

Yes.
In Quantum Field Theory every particle is a normal mode of a certain field.
A normal mode is when sections of something, the field in this case, all start oscillating with a certain frequency.

If you look at the first line, the black dots are where the field is strongest.
You can see pairs of black dots moving away and toward eachother over and over again.
A single particle is one of these black dot pairs.
The time it takes the dots to move away and come back would be the frequency of the particle.

An electron would be a normal mode of the Dirac Field. The graviton, if it exists, would be a normal mode of the SuperGravity field.

base2

Did you get the Callan Prize this year Son Goku?

Son Goku

base2 wrote:

Did you get the Callan Prize this year Son Goku?

I'm not eligable for it. I just finished up third year and I'm going into single honours Theoretical Physics next year.

Did you go to Maynooth?

Gobán Saor

Son Goku wrote:

In generally relativity it isn't just mass that causes Gravity it's:
Mass, Stress, Heat, Pressure, Density, Potential Energy and (weirdly enough) Gravity.


Exactly if the Sun were colder, but still had the same mass it would have less gravity.

Whoa! Is this part of standard physics or is it speculative and not universally accepted? I only ask cos I have a keen interest (non-professionally) in science and have read widely in the area but I've never heard of this. From my limited knowledge, gravitational force varies with G, M1, M2 and inversely with R squared. Where does temperature come in? (And pressure, stress, etc, etc?) I'm aware of relativistic variations in length, mass etc, as per special relativity but the idea of gravity varying with heat is a new one to me. Presumably the co-efficient of variation is tiny? Is it observable? HAS it been observed? Apologies if this is a trivial question but I just follow the thread in here from the Astronomy forum and it intrigues me - sad git that I am:D

Son Goku

Is this part of standard physics or is it speculative and not universally accepted?

Standard, but you usually don't hear about it because it takes quite a bit of explaining.

From my limited knowledge, gravitational force varies with G, M1, M2 and inversely with R squared. Where does temperature come in? (And pressure, stress, etc, etc?)

In General Relativity, they don't cause gravity in Newtonian physics.

Is it observable? HAS it been observed?

Yes and yes. Although it's quite hard to see.

At a basic level, Gravity is caused by what is called the Stress-Energy of matter. The Stress Energy of a piece matter is created by sixteen different things.
Three of these are different types of heat and so heat causes gravity.
One of the sixteen things is Mass, but usuallly it is the largest of the sixteen, because things are usually much heavier than they are hot.

If you're happy with that stop reading.






Okay here's the real explanation.
It might be a bit heavy going at times, but hopefully it'll be decent enough.

In Einstein's full Theory of Gravity, General Relativity, gravity is the bending of spacetime.
The following equation is used in General Relativity:

The terms to the right of the equals sign describe the shape of spacetime (in other words gravity) and the term to the left describes matter.
To make it easier, let's write it as:

So we have G for gravity and T for matter.

So let's take the term that describes matter.
This term is called the Stress-Energy Tensor and we use a capital T with any two letters beside it to represent it:


Now is a block of sixteen numbers each describing a different property of matter.

Energy density is basically the mass.
Energy flux is basically heat and viscosity is basically Stress.

Now remember from above that is gravity. This is also a block of sixteen numbers.
I won't go into the meaning of these numbers, but each number in is 8Pi times the corresponding number in .

So if the piece of matter is very warm then the energy flux numbers will be quite high and so the corresponding gravitational numbers will also be quite high.

Therefore Heat causes Gravity.

However in most situations the Energy Density(Mass) number is the largest and so that is what causes most of the Gravity in the Universe.

Gurgle

Three of these are different types of heat and so heat causes gravity.

I'll simplfy this right down.
Heat = Energy.

So heat causes gravity.

suggesting that gravity is a force not a shape?

EDIT: This post says edited by Son Goku because I accidently hit the edit button instead of the quote button.

Son Goku

Gurgle wrote:

Heat = Energy

There is a reason I didn't say that at the beggining.
Although heat is just a kind of energy at the Newtonian level, it causes more gravity than just the fact that it's a form of energy would allow.
The fact that it is energy is taken care of by the energy density term in the Stress-Energy Tensor.
The Energy density term measures energy contributions from every thing, heat, mass, e.t.c.

Now heat doesn't have much energy density, but it has huge energy flux. Which is what allows it to cause more gravity than we'd expect. Most of the warpage of spacetime due to heat doesn't come from the fact that heat is energy, but from the fact that heat has energy flux.

In large bodies, such as Red Super-Giants, heat becomes very important gravitationally.
Almost all of its gravitational strength comes from its energy flux and the fact that it is a form of energy can actually be ignored.

Gurgle wrote:

suggesting that gravity is a force not a shape?

Gravity is caused by every thing listed in the Stress-Energy Tensor, but it itself is geometric.
Gravity is a shape, not a force.

Gobán Saor

Thank you indeed, Sun Goku and Gurgle. That clears it up. Not saying I understand it, mind you, but at least I get the general idea.......I think My maths only extends to 1st year college level so I find it hard to grasp this stuff unless I can relate it to some physical reality.

If I could trouble you further, what would the the rough magnitude of the General Relativity components of gravity compared to a strictly Newtonian analysis? Say for the Sun-Earth gravitational attraction?

Son Goku

Gob&#225 wrote: »

If I could trouble you further, what would the the rough magnitude of the General Relativity components of gravity compared to a strictly Newtonian analysis? Say for the Sun-Earth gravitational attraction?

For the Sun-Earth system Newtonian analysis on its own is pretty much spot on.
General Relativity only corrects it by a tiny amount.

Let's say General Relativity's result is 100% correct then this is how correct Newtonian Gravity is for a few situations:

Sun-Earth : 99.999%
Sun-Mercury : 92%
Red-Super Giant : 88%
Neutron Star : 56%
Black Hole: 12%

Unless the field is very strong the Newtonian Component is fine enough on its own.

Gobán Saor

Thanks, thats quite interesting. I had no idea that the difference was so large in some of those scenarios.