B Classical physics emerging from quantum physics

[Andrew Wright]

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

Thanks for your thoughts.

 

[phinds]

What do you think and why?

 

[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.

 

[phinds]

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.

You might want to check out the Heisenberg Uncertainty Principle

 

[atyy]

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

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.

 

[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.

 

[atyy]

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

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.