# Tag Info

8

I think the key conceptual hurdle is that the vacuum state is not nothing. Quantum field theory describes matter as excitations in quantum fields. These quantum fields are very strange things, and I don't know of any easy way to explain to a non-physicist what a quantum field is. The key thing is that the quantum fields fill all of spacetime. So a vacuum is ...

6

A graviton is a theorised particle (specifically a boson). It is said to be its own antiparticle. It has a special role in general relativity (being that the science is all about gravitation and the warping of space/time). In relation to your mention of string theory, it is said to be a closed string with a very low-frequency vibration. Closed strings are ...

5

In quantum field theories, interactions are exerted by the exchange of a force carrier particle. For the electromagnetic force, this is the photon, for the strong force, the gluon and for the weak force, the W and Z bosons. All these force carrying particles have been observed. Now it is imagined, that if we are able to find a quantized version of gravity, ...

3

As John Rennie already put it, Hawking radiation is an semi-classical effect derived by treating the spacetime classically, including the source terms if any, and quantizing fields in the curved background. In addition general relativity has problems with distributional sources, so it is not clear how to treat elementary particles in this case. So one cannot ...

3

A quantum theory of gravity does make definite predictions. One such an example, which is the same for any theory of quantum gravity that reproduce GR at low energy, is the famous correction to the newton $1/r$ potential:  V(r)=\frac{M_{star}}{M_{Planck}r}\left(1-\frac{M_{star}}{M_{Planck}^2 r}-\frac{127}{30\pi^2}\frac{1}{M_{Planck}^2 r^2}+\ldots\right). ...

2

I would say that there is not too much experimental evidence for a quantum theory of gravity yet, the reasons why such a theory is desirable are mainly of conceptual/theoretical nature. I will give a (likely to be incomplete) list of motivations for studying quantum gravity. Unification of all four fundamental interactions: The Standard Model of particle ...

2

This is really a comment, but it got too long for the comment box. The problem is that the Hawking calculation is semi-classical. That is, it assumes the spacetime curvature is given by the (classical) Einstein equation. Once the radius of the event horizon decreases into the quantum regime the approximations Hawking used are no longer valid. You would need ...

2

The planck length is not necessarily an absolute limit to how small thing can be sub divided. The planck length is theoretical and it is empirically defined by dimensional analysis. At this length scale our knowledge of physics makes no sense. The planck length $\ell_P$ is defined as: $\ell_\text{P} =\sqrt\frac{\hbar G}{c^3} \approx 1.616\;199 (97) \times ... 1 Let's start by setting the scene. We've got a hyperdense (understatement) singularity containing everything at$t = 0\$. This is the beginning of time. Right now, we have no reason to assume that anything existed before then. Asking what happened before the Big-Bang ( depending on which model you use ) is not something that one can ask since we assume nothing ...

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