Gravitons vs. Einstein?

[DivineNathicana]

As I understand it, if one uses the concept of gravitons, doesn't one have to abandon the concept of a curved space-time? I have heard that theories such as superstrings use flat Minkowski spacetime, but I am not sure how they can account for the experimentally proven effects of a curved spac-time. Can gravitons bend the path of light? That wouldn't really be a geodesic anymore, would it...

 

Do theories containing gravitons completely circumvent Einstein?

 

Thanks to all,

 

- Alisa

[Tormod]

Hi Alisa, cool avatar.

 

I am sure Bo or someone else will respond. Personally I don't understand why gravitons defy a curved space-time. I assume you mean that curved space-time IS gravity, so gravitons would ruin the purpose of the curvature.

 

Gravitons are supposed to be bosons which mediates the extremely weak gravitation between particles. Curved space-time is caused by massive objects. There is a huge difference here (someone correct me if I'm wrong).

[Bo]

Hi, and welcome to these fora!

The best way of thinking of gravitons vs curved space time is to think of an analogy with photons and the electromagnetic field.

I'll try to explain this...

 

We can always look at a general space time as flat space time+perturbations. The nice thing is now, that we can calculate a constraint on these perturbations (i'll call them h) in vacuum. It shows that h behves just like a free wave that travels at the speed of light (a so called gravitational wave). So what does that mean? well lets take flat space, without any mass in it, so no perturbations or whatever (we disregard quantum effects here, but that doesn't matter). Now at a certain point we make a small perturbation, e.g. by inserting a small mass in our flat space. It is known that a mass curves space, and the gravitational waves are exactly that proces. The perturbation 'spreads out', with the gravitational waves. There is an exact analogy with this and the electromagnetic field. For the EM field you can also calculate the effect of a disturbence in the EM field, and you get the result that that disturbence travels around, like a wave, with the speed of light. (the disturbence is better known as the photon of course ).

The particle/wave of the gravitational field is called the graviton, and So we see that it is equivalent to "the propagation of the manifestation of a disturbence in spacetime"

 

I hope this makes it all a bit clear;

 

Bo

[lindagarrette]

Bo, great explanation. Makes sense to me! Linda

[Freethinker]

However, unlike photons, we do not yet have a way to "detect" gravitrons.

[Tim_Lou]

is it possible for gravitons to interact with other particles?

what forces does it experience out of the 4 forces?

 

(man, Bo is the physics database right here!)

[DivineNathicana]

Hmm... In response to Tormod - I don't think that gravitons are only for small-mass objects. As I understand it, they are an alternate model of gravity. They are exchanged between two objects, forming the gravitational "pull". If we have gravitons, why do we need curvature?

 

I asked this same question of a physics teacher in my school, and he said that it can be logically deduced that we don't.

 

Thank you, Bo, for your detailed response. However, I am now torn between what to believe and would like to know why we still need a curved spacetime if we have gravitons. Also, why have gravitons anyway? Why isn't curved space-time enough? For purposes of supersymmetry or something?

 

Sorry to annoy you guys, thanks as always,

 

- Alisa

 

P.S. Thank you, Tormod, for appreciating the sexiness which is my avatar. = ) The Beatles rule.

[Tormod]

I wonder if what you're looking for is something called loop quantum gravity.

 

Nobody understands where gravity comes from. The gravitons are hypothetical, but there are already projects being planned to measure gravity waves (if they exist) - like the European Space Agency's LISA project.

 

Gravity is an extremely weak force. Gravitons may be the carriers of this force. That might seem to be an inconsistency in current cosmology - especially when we talk about curved space-time. But I still do not see why they rule out curvature. Curvature is caused by objects curving space-time around them, gravitons would be the particles that give objects this ability.

 

I don't understand your teacher's response and it would be interesting to hear the logic behind it.

 

Here is an article from Wikipedia:

http://www.yourencyclopedia.net/Quantum_loop_gravity

 

And no, you're not annyoing us. We like a challenge.

 

I'd also say there is no need to be torn between "what to believe". There are so many things we don't understand, yet we don't have to go for one or the other. They are all theories and that is the wonder of science. However, questioning the theories, testing them, making deductions - that is very, very important - and that is what you are doing.

[GAHD]

We don't need curved space time if we use gravitons. But then again, if that's true we don't need English if we have Latin.

 

They are two different Models, which can both be used to describe the same effects. It's really just two different ways of wrapping your brain around how things work.

 

When using curved space as a model, you think of space like a weight stretching a fabric which attracts anything on the curve of the stretch towards it.

 

With gravitons, you really have to think of it like a spring that attaches the different masses together.

 

Bo depicts gravitons as wave-particles causing a field effect similar to electromagnetism, which does have merit but the two work on different scales and have some crucial differences. EM has a similar fall-off, but is really material (and charge) specific on how strongly it can interact. Gravity is universally applicable to anything massive, and makes no differentiation to charge.

 

They all accomplish the same goals in the end.

 

 

Side note:

'Brane theory depicts gravitons a 4D particles interacting with our 3D universe, the reason gravity is 'weak' is that those particles bleed out into the 4th dimension and loose their effect on the 3 dimensions we exist in quickly because of that.

 

That help?

[Tormod]

GAHD's point is a better explanation of my own point: gravitons and curved space-time explain the same thing (although I was of the impression that they explain the small-scall and large-scale events, respectively).

 

However, gravitons still being hypothetical, curved space-time is the preferred model. As soon as someone can prove there are gravitons, theories will have to change.

[Bo]

Tim wrote:

is it possible for gravitons to interact with other particles?

what forces does it experience out of the 4 forces?

The Graviton IS the force.... But as Gahd pointed out, this is a dangerous area between general relativity and quantum field theory.

General relativity tells us that spacetime perturbations can travel around in gravitational waves, just like classical EM tells us that an EM perturbation travels around like a wave. Also, without quantum effects, it can be shown that a perturbation in space time causes a force on any mass (gravity) and perturbations in EMfield causes a force on any charge (EM-force)

 

Quantum field theory tells us that, when 2 particles interact, a 'stream of virtual particles' is sent between the 2. These virtual partiles can be created out of 'nothing' because of the Heisenberg uncertainty. And these virtual particles tehnicly 'carry' the force and change the motion of the 2 particles. Now in QFT one can quite easy show that the force carrier of the electromagnetic force has exactly the properties of the photon. with 1 difference, since we're dealing with quantum effects, the photon isn't just a wave, but the wave is quantised into parts of definitive energy. these quanta are called photons.

Unfortunatly we can't write down a Quantum theory of gravity, so we cant do the calculations as exact as for the EM force. but, since the complete analogy on the classical level, we can of course 'define' a carrier on the quantum level. And General relativity tells us all the properties of this carrier (the graviton), (remarkebly even some pure Quantum properties like spin!)

 

So to come back to your questions: Yes the graviton interacts with other particles, and in particular all particles with a nonzero mass, and no, the graviton doesn't experience any of the other 3 forces (e.g. it has no electric charge, so no EM force. Similar arguments hold for the strong and weak force)

 

DivineNathicana wrote:

would like to know why we still need a curved spacetime if we have gravitons. Also, why have gravitons anyway? Why isn't curved space-time enough? For purposes of supersymmetry or something?

i hope it is now more or less clear that gravitons and a curved space time are technicly the same, but on a quantum level we need the forces to be carried by some particles, but on the macroscopic scale, these particles cant exist, so we need something else, this is the curved space time. a nice analogy to your question is probably "why do we need a sea if we have waterdrops?"

Supersymmetry is something else; in fact Supersymmetry requires us to have more then 1 graviton (the extra gravitons are called gravitino's)

 

Tormod wrote:

wonder if what you're looking for is something called loop quantum gravity.

Loop quantum gravity is an effort to try to write down a quantum theory of gravity; it solves some problems, but gives many new

 

Bo

[Tormod]

Talking about supersymmetry - isn't the graviton also it's own anti-particle?

[Bo]

Talking about supersymmetry - isn't the graviton also it's own anti-particle?

 

yes, this comes from the fact that the graviton doesn't experience any of the forces, just like e.g. the photon. (but unlike the vector bosons and gluons; the carriers of the weak resp. strong force)

 

Bo

[Freethinker]

Would Dark Matter be anti-gravitrons? Or is it a monopole?

[Bo]

no, the graviton is just the carrier of the force; once again in comparison: the EM force can bothe attract and repulse, but the carrier is always the photon; the difference is in the charge.

Since we only know of 'positive' mass, for normal matter, gravity is attractive. But in principle gravity can be repulsive; Dark energy is the term for that special stuff that emits a repulsive gravity. (see previous posts on scalar fields, negative pressure or large extra dimensions)

Bo

[lindagarrette]

Originally posted by: Freethinker

However, unlike photons, we do not yet have a way to "detect" gravitrons.

 

 

Are you sure we can detect photons? Or just their effect.

[Tim_Lou]

"doesn't experience any of the other 3 forces"

 

so, is it affected by other gravitons?

gravitons interact with gravitons...its all messed up.

 

so, would it be just like the gluon, which carries the color charge and the gluon itself carries color charge?

 

i heard that some people are trying to bombard protons with each other and see if it creates gravitons......

well, if gravitons are not affected by the 3 forces, we will never be able to detect such particles.