# What qualifies as a quantum theory and why are we seeking a quantum theory of gravity? [duplicate]

When can a theory be called a quantum theory? Does it have to do with the existence of certain quantities which take discrete values (they increase in quanta)? Or does it have to do with the existence of non-commuting operators?

If yes, then classical mechanics qualifies as a quantum theory. To get standing waves on a string, one can have DISCRETE wavelengths and finite rotations about x and y-axes are NON-COMMUTING.

Also, why are we seeking a quantum theory of gravity?

## marked as duplicate by John Rennie gravity StackExchange.ready(function() { if (StackExchange.options.isMobile) return; $('.dupe-hammer-message-hover:not(.hover-bound)').each(function() { var$hover = $(this).addClass('hover-bound'),$msg = $hover.siblings('.dupe-hammer-message');$hover.hover( function() { $hover.showInfoMessage('', { messageElement:$msg.clone().show(), transient: false, position: { my: 'bottom left', at: 'top center', offsetTop: -7 }, dismissable: false, relativeToBody: true }); }, function() { StackExchange.helpers.removeMessages(); } ); }); }); Mar 28 at 6:57

• This question is hard to understand. The second paragraph begins "If yes,...", but the first paragraph doesn't not ask a yes/no question. Please clarify. – DanielSank Mar 28 at 5:54
• @DanielSank The first paragraph does have two yes/no type questions. They are about non commuting observables and quantities taking discrete values. – Sashwat Tanay Mar 29 at 1:01

Here you will find a list of the postulates of quantum mechanics.

To have a quantum mechanical theory one needs a wave equation whose solutions are consistent with the postulates of quantum mechanics.

It is the wavefunction postulate that separates quantum mechanical models from classical physics models.

The solution of a wave equation $$Ψ(x,t)$$ , in space and time is interpreted differently in quantum mechanics, as the probability of finding a particle at a particular time and space.

It is not the energy that is "waving" in a quantum mechanical model, but the probability of finding a particle at a specific space time point is calculated by use of the wavefunction. Same equations and wave functions in classical and quantum mechanics, but different quantities are represented by the solutions. In quantum probabilities, in classical energy densities .

See my answer here for an experimental illustration.

Also, why are we seeking a quantum theory of gravity

For aesthetic reasons, the hypothesis that a mathematical unified theory of all four fundamental forces must exist, and since the three , electromagnetic, weak and strong are quantum theories, gravitation should follow suit. There are various unifying theories for the first three, and the hypothesis is that gravity should also be quantized.

An additional reason is that quantum theories avoid singularities, due to the probabilistic nature. For example the 1/r of the coulomb potential is no problem in quantum mechanics: the electron cannot fall on the proton in the hydrogen atom.

Already in the Big Bang cosmological model effective quantization is used for the beginning of the BB instead of the classical singularity.