# B Classical physics emerging from quantum physics

1. Jul 6, 2017

### Andrew Wright

Is classical physics more than just the quantum physics of a large number of particles and quanta?

2. Jul 6, 2017

### phinds

What do you think and why?

3. Jul 6, 2017

### Andrew Wright

Some people have told me there is this thing called "emergence", where nature behaves in unexpected ways when objects are put together. I feel that if all the parts were well enough understood, you could derive the behaviour of collection - right up to the behaviour of the whole universe.

4. Jul 6, 2017

### phinds

You might want to check out the Heisenberg Uncertainty Principle

5. Jul 6, 2017

### atyy

The standard Copenhagen interpretation of quantum mechanics assumes the existence of classical measurement apparatus.

The Bohmian interpretation of quantum mechanics assumes the existence of hidden variables in addition to the quantum wave function

The Many-Worlds interpretation tries to say that there are many parallel realities.

Last edited: Jul 8, 2017
6. Jul 7, 2017

### DeathbyGreen

I think you should look into the correspondence principle. To quote Wikipedia the correspondence principle is "the behavior of systems described by the theory of quantum mechanics (or by the old quantum theory) reproduces classical physics in the limit of large quantum numbers." So basically yes, classical physics is essentially an extension of a large number of quantum objects. When you only have a few objects, the small quantum effects become important, but when you have an enormous number of them, like in a macroscopic object, they essentially disappear. There is no deeper meaning. Think about how $\hbar$ is an important number in quantum mechanics, but is of the magnitude $10^{-27}$, so you can see why it would become negligible at large scales.

7. Jul 8, 2017

### atyy

Here is the answer, from Landau and Lifshitz's famous text, in the standard Copenhagen interpretation:
Thus quantum mechanics occupies a very unusual place among physical theories: it contains classical mechanics as a limiting case, yet at the same time it requires this limiting case for its own formulation.