Is Quantum Gravity only related to Planck Scale?

[rogerl]

Is the need for Quantum Gravity only related to understand what goes on in the Planck Scale between quantum fields and curvature of spacetime where they collide. Or is it a more general solution to how quantum object is connected to spacetime (or quantum spacetime)? Let's take the example of a table. Does this table as object of quantum matter connection to spacetime occur in Planck scale or larger length dimension?

 

[fzero]

Quantum gravity is important in two basic situations. The first is when the object being studied itself causes a curvature in spacetime that is of order the (Planck mass)^2. Generally this happens for a black hole or big bang-type singularity, where there is actually a point in spacetime where the curvature is infinite.

The second case would be where our probe of a system is energetic enough that we cannot ignore the fact that the probe itself changes the background geometry. This is again related to the momentum of a single probe or, in the case of multiple probes, that the sum of energies approaches the Planck mass.

There are probably many philosophical or foundational questions that address properties of quantum gravity, but the fundamental issue that determines when QG is important is identifying the situations where classical GR is unreliable.

 

[marcus]

...
the fundamental issue that determines when QG is important is identifying the situations where classical GR is unreliable.

In other words Big Bang and Black Hole. I agree.

 

[rogerl]

Quantum gravity is important in two basic situations. The first is when the object being studied itself causes a curvature in spacetime that is of order the (Planck mass)^2. Generally this happens for a black hole or big bang-type singularity, where there is actually a point in spacetime where the curvature is infinite.

The second case would be where our probe of a system is energetic enough that we cannot ignore the fact that the probe itself changes the background geometry. This is again related to the momentum of a single probe or, in the case of multiple probes, that the sum of energies approaches the Planck mass.

There are probably many philosophical or foundational questions that address properties of quantum gravity, but the fundamental issue that determines when QG is important is identifying the situations where classical GR is unreliable.

Is the question of how matter is connected with spacetime independent of quantum gravity?? Or can we already answer now how matter made of quantum object is connected to spacetime? I mean, how does the interface work?

 

[fzero]

Is the question of how matter is connected with spacetime independent of quantum gravity?? Or can we already answer now how matter made of quantum object is connected to spacetime? I mean, how does the interface work?

Well classical GR already connects geometry to matter through the Einstein equation. This is an equation that relates the curvature of spacetime to something called the stress-energy tensor. The stress-energy tensor is an object that characterizes the energy of matter: kinetic, mass, pressure of a gas, etc. So the Einstein equation tells us how matter deforms geometry. However, at points where the curvature becomes very big, Einstein's equation is a singular differential equation. We can't trust it or the singular solutions that we obtain from it at those singularities.

We also know how to do quantum mechanics in a curved background geometry, at least in the approximation that we can neglect the change in geometry that the quantum objects cause.

So away from very special situations (at least compared with the usual phenomena we experience on Earth) classical GR and ordinary relativistic quantum physics are very good descriptions of nature. However, it is still very likely that if we understood quantum gravity, we would gain a large deal of additional insight into the interplay between matter and geometry. It would be similar to comparing general relativity with Newton's law of gravitation. One can derive Newton's law from GR as an approximation when gravitational fields are weak, but GR provides a much more detailed picture of the underlying physics.

 

[jal]

First ... we still don't know what happens inside the perfect liquid and what is in it.

Second ... if there are more than 3 space dimensions then Quantum Gravity will not related to Planck Scale.

jal

 

[rogerl]

First ... we still don't know what happens inside the perfect liquid and what is in it.

Second ... if there are more than 3 space dimensions then Quantum Gravity will not related to Planck Scale.

jal

Why. There is high chance there is more than 3 space dimensions. How come quantum gravity relating to the Planck scale is dependent only on the condition that there is 3 space dimensions?

 

[jal]

For a start, look at
http://en.wikipedia.org/wiki/Randall–Sundrum_model
Randall–Sundrum models
then follow the other links
Kaluza-Klein theory
Large extra dimension
DGP model