Is gravity incompatible with quantum theory?

Answer 1

General relativity (GR) is the theory of gravity, while quantum mechanics (QM) is the theory of subatomic particles. GR therefore applies to massive objects, while QM applies to extremely small objects.

Both do extremely well in their respective domain.

But what happens when an object is both small but subject to a strong gravitional field? There are two such situations known. One are black holes, the other is the Big Bang, close to its beginning. There we need both theories, but the mathematical results are either absurb or incompatible.

The reason for this lies in the way matter is represented in the two theories. GR describes space as being smooth. But in QM space is filled with constant fluctuations and activity. Worse, if one studies smaller and smaller time and length scales, the more violent and frequent the fluctuations are.

Answer 2

Combining General Relativity and quantum mechanics is an active field in theoretical physics called "quantum gravity". There is no final theory of quantum gravity, but a lot of work has been done and there is some success---for example, it is possible to predict that Black Holes explode, and how and why and when that happens.

I think the "deep" reason that quantum gravity is a problem is that in General Relativity, the shape of space itself is dynamic---to describe the shape of space requires a knowledge of gravity, in particular a knowledge of the location of all the mass and energy. Quantum mechanics, even relativistic quantum mechanics, starts out with a fixed background geometry, and the masses and energies move around in this fixed background. The two approaches are fundamentally different, from the very first page of the textbook.

Answer 3

Gravity is not compatible with quantum theory. Quantum theory applies only to the atomic and subatomic scales, and gravity doesn't fit the equation at these scales. Einstein's and Newton's equations work well for gravity, but not forces at the subatomic level.

Which is why a single unified theory is the Holy Grail of Physics.

Answer 4

Not with relativistic quantum theory. In fact, there are emerging quantum models wherein gravity is caused by the contraction of space around each and every atom. That is, wherever there is mass (the atoms), space itself is bent and the effect of that bending is called gravity. [See source.]

One of the fundamental problems with considering mass as the origin of gravity is that mass appears to have no energy that it shares with the outside world. That is, disregarding the energy potential from E = mc^2, mass just sits there like a lump of...mass. Where's the energy in that for causing gravity?

The quantum gravity boys think the energy comes from the atoms that make up that mass. Further, they feel that the protons, neutrons, etc. are in a constant struggle to fly apart and stay connected, and that constant struggle causes space around each atom to relativistically contract and relax, which sets up the gravity waves of space outside each atom.

Answer 5

Gravity is not compatible in the quantum world because gravity is the weakest of the forces, it is not strong enough to hold matter's constituents together. The subatomic realm is governed by the other forces ( Weak, Electromagnetic, Strong forces)

Answer 6

So far.