Gravity - a non-physical entity

[vivekfw66]

How does QM come to a conclusion that gravity is quantizable?

 

[Drakkith]

Gravity is one of the fundamental forces of nature and is believed to be the result of spacetime curvature. Generally only objects made out of matter are considered to be "physical entities".

See here for more info on gravity: http://en.wikipedia.org/wiki/Gravity

 

[vivekfw66]

every wave is caused by disturbance caused in spacetime. but if spacetime curvature is resulting in gravity, then what is it that spacetime is disturbing?

 

[Drakkith]

every wave is caused by disturbance caused in spacetime.

I don't know what you mean by this.

but if spacetime curvature is resulting in gravity, then what is it that spacetime is disturbing?

That question is better asked in the Special and General Relativity subforum.

 

[vivekfw66]

My question is how does Quantum theory come to a conclusion that gravity comes in quanta just like photons?

 

[Drakkith]

My question is how does Quantum theory come to a conclusion that gravity comes in quanta just like photons?

The other 3 fundamental forces are described using exchanges of force carrier particles, and it is believed that gravity is no different. Note that we have not found any evidence of a graviton yet, so we aren't sure whether they exist or not. Currently they only exist in certain theories.

http://en.wikipedia.org/wiki/Graviton

Edit: Please don't change your posts after someone has quoted you, it leads to confusion.

 

[K^2]

From perspective of modern theory, all forces arise due to local gauge symmetries. There are gauge fields corresponding to every gauge symmetry, and these fields can be quantized. Electric field is one of the gauge fields of the U(1)xSU(2) symmetry group. Second quantization of electric field yields photon as the quantum of electromagnetic interaction.

Similarly, one can start with Poicare symmetry group and derive gravitational interaction. (This is backwards from the way GR was originally written, but all the physics is exactly the same.) As a result, you will get gauge field, which you can think of as the gravitational field. Albeit, this will be very different from gravitational field you would see in classical physics. Nonetheless, it is a physical field, and second quantization should yield a particle.

Unfortunately, gravity written in such a way is a non-renormalizeable theory, which means mathematical methods used to second-quantize the other three fundamental forces do not apply. People are still looking for a good way to get around that obstacle.

 

[bhobba]

How does QM come to a conclusion that gravity is quantizable?

Because it is:
http://arxiv.org/pdf/1209.3511v1.pdf
'The usual marriage of general relativity and quantum mechanics is ï¬ne at ordinary energies, but we now seek to uncover the modiï¬cations that must be present in more extreme conditions. This is the modern view of the problem of quantum gravity,and it represents progress over the outdated view of the past.'

Unfortunately some textbooks and popularizations are out of date and misrepresent the problem.

Thanks
Bill

 

[bhobba]

Unfortunately, gravity written in such a way is a non-renormalizeable theory, which means mathematical methods used to second-quantize the other three fundamental forces do not apply. People are still looking for a good way to get around that obstacle.

That's true.

But up to about the Plank scale an EFT is indeed possible.

Of course that's where the real 'action' takes place as far as gravity is concerned, but as a matter of principle it's not it can't be quantisized - its the problem of going beyond the Plank scale.

Thanks
Bill

 

[vivekfw66]

That's true.

But up to about the Plank scale an EFT is indeed possible.

Of course that's where the real 'action' takes place as far as gravity is concerned, but as a matter of principle it's not it can't be quantisized - its the problem of going beyond the Plank scale.

Thanks
Bill

Can you give me a little more mathematical insight into this as to exactly how the mathematical methods breakdown at Planck's scale?

 

[DevilsAvocado]

How does QM come to a conclusion that gravity is quantizable?

Because the other three fundamental forces (electromagnetic, strong and weak nuclear) is mediated through messenger particles (bosons) of underlying fields, that interact with matter particles, which can be described in relativistic quantum field theory (QFT), that is compatible to special relativity (SR).

Therefore, the logical assumption is that; the fourth fundamental force (gravity) is also mediated through a messenger particle (the graviton, still hypothetical), to achieve quantum gravity (QG).

every wave is caused by disturbance caused in spacetime. but if spacetime curvature is resulting in gravity, then what is it that spacetime is disturbing?

Spacetime is not disturbing anything, spacetime is 'deformed' by matter/mass.

The difference is that Newtonian mechanics and special relativity (and QM) has a fixed spacetime background, while in general relativity (GR) the spacetime geometry is dynamic.

Brian Greene explains the difference:

https://www.youtube.com/watch?v=0rocNtnD-yI


According to GR, matter always tries to move in straight trajectories, and if spacetime is curved the straightest path is the one through that curvature.

Another problem is gravity is not renormalizable (canceling of infinities) and when quantizing gravity we get infinitely many independent parameters, which at low energies is okay, but at very high energies (where QM effects take over), the infinitely many unknown parameters would begin to matter, and no predictions could made at all.

Another interesting fact is that gravity is an extremely weak force, compared to the others. The electromagnetic force is 10³⁶ times stronger, and that’s why the force of a tiny magnet can overcome the gravity of the entire Earth!

 

[ZapperZ]

My question is how does Quantum theory come to a conclusion that gravity comes in quanta just like photons?

I think there is a more fundamental misunderstanding here that should be addressed.

First of all, quantum theory does NOT come to such a conclusion ... YET. We do not have a well-defined, accepted, and verified theory of quantum gravity.

However, what you should try to understand is the idea behind an aspect of quantum theory, which is Quantum Field Theory (QFT). This is the idea where the interactions (or what we normally call "forces") that we observe is the result of an excitation in a quantum field that mediates this interaction. For example, in an electromagnetic interaction, photons mediates the interaction, resulting on what we know as the electromagnetic forces.

QFT says that if there is an interaction, then there's a possibility that such an interaction can be described using the QFT picture. We already have seen how this works in electromagnetic interaction, strong interaction, and the weak interaction. This accounts for 3 of the 4 fundamental forces that we know of in our world.

So naturally, there's an expectation that the 4th one, which is gravity, can also be described using such quantum field. This is why people are working on the theoretical aspect of trying to incorporate gravity within the quantum field picture, resulting in theoretical development of String theory, loop quantum gravity, etc... etc. However, this has turned out to be more difficult than originally expected. As of now, there is no one accepted quantum theory of gravity, and there have been no experimental verification of any of them. So officially, there is NO such theory.

However, this doesn't make gravity a "non-physical entity", as implied by the title of your thread. You need to be more careful in the future of making such characterization before you understand something.

Zz.

 

[bhobba]

Can you give me a little more mathematical insight into this as to exactly how the mathematical methods breakdown at Planck's scale?

Its to do with how EFT gets around the fact gravity is not renormalizeable. It replaces it with a theory that is valid up to about the plank scale but beyond that isnt, however it has the advantage finite quantities can be extracted and not blow up to infinity.

Renormalizeable theories, and gravity isn't one, have the great advantage you do not have to resort to the EFT trick to extract finite answers. But it's known you often can't push them too far - for example at high enough energies QED is replaced by the electroweak theory, and it is thought as you get to the plank scale the electroweak will break down as well. While finite answers can always be extracted from renormalizable theories physicists are pretty sure the standard model is not valid once you get to the plank scale.

So basically both the standard model (it is thought anyway - no one really knows for sure - but I am not a real expert in the area) and gravity break down there - but for different reasons - gravity because it's replaced by an EFT (to allow finite answers to be extracted) not valid beyond that, and the Standard model because it is suspected, like the electroweak takes over from QED and high enough energies, even though finite answers can be extracted for all energies.

This is the EFT approach to QFT:
http://en.wikipedia.org/wiki/Effective_field_theory

Thanks
Bill

 

[vivekfw66]

I think there is a more fundamental misunderstanding here that should be addressed.

However, what you should try to understand is the idea behind an aspect of quantum theory, which is Quantum Field Theory (QFT). This is the idea where the interactions (or what we normally call "forces") that we observe is the result of an excitation in a quantum field that mediates this interaction. For example, in an electromagnetic interaction, photons mediates the interaction, resulting on what we know as the electromagnetic forces.


Zz.

Is it motion that causes excitation?

 

[ChrisVer]

Can you give me a little more mathematical insight into this as to exactly how the mathematical methods breakdown at Planck's scale?

Because when you make the QFT of the other 3 interactions, you neglect the gravity effects. You can't do that after reaching the Planck's scale. I disagree with "beyond" the Planck's scale idea, since then we are talking about singularities.
For example when you are walking in the rain, you can "neglect" the effects of drops on your speed. But if the drops became equal in size to your body, you could not neglect them and expect to have a nice model.
So gravity becomes important, and you also need it in your game to make calculations

 

[ZapperZ]

Is it motion that causes excitation?

I have no idea what you are asking.

Maybe I shouldn't use the term "excitation", because you are obviously getting an inaccurate picture in your head.

But please verify that your original question has been answered sufficiently to your understanding before proceeding to trying to understand such thing as a QFT "excitation". Close one topic first before proceeding to the next.

Zz.

 

[ChrisVer]

you need a better insight in QFT. In QFT notation your fields don't move, but they are covering the whole space. Operators that create or annihilate states (excitation modes of the vacuum) are propagating in spacetime as waves, creating the particles you can measure and which interact with the field.

So for example, a picture for that -at least it works for me- is seeing an electron moving from point A to B with momentum k, there getting "destroyed" and recreated with momentum k', by acting with the according annihilation and creation operator on that point. So in fact there the particle interacted with something, changing its state