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.