what is the  main concept of quantum theory?

Quantum theory based on?

Experiment, observation, and a mathematical framework to describe them.

There are three main concepts:

1. Granularity: physical observables can only take on discrete values

2. Indeterminacy: the outcome of measurements is stochastic in nature (unless of course the system has been prepared in a definite state)

3. Relationality: It is not meaningful to attribute properties to quantum systems, until such time when they are actually measured (lack of counterfactual definiteness)

On ‎13‎/‎08‎/‎2018 at 12:07 PM, storm9 said:

what is the  main concept of quantum theory?

I would take wave particle duality as main concept.https://en.wikipedia.org/wiki/Wave–particle_duality

28 minutes ago, Itoero said:

I would take wave particle duality as main concept.https://en.wikipedia.org/wiki/Wave–particle_duality

That is a consequence of the underlying concept of describing things in terms of wavefunctions. 

15 hours ago, Strange said:

That is a consequence of the underlying concept of describing things in terms of wavefunctions. 

Also, wave particle duality is in a sense the same as quantum uncertainty. Wave-particle duality is the idea that a quantum object can behave like a wave, but that the wave behaviour disappears if you try to locate the object. (like in the double slit exp) The quantum uncertainty principle is the idea that it's impossible to know certain pairs of things about a quantum particle at once...like the exact position of a particle and its wave-behavior.https://arxiv.org/pdf/1403.4687.pdf