# Understanding classical stochastic systems

1. Aug 4, 2011

### spaghetti3451

This is an extract from my lecture notes:

"For classical stochastic systems, w(p,x,t)dpdx = prob. particle is in dpdx.
$w\geq0 \int dp \int dx w(p,x) = 1.$"

1. Can anyone please explain what a classical stochastic system is?
2. Why is there a question of probability in analysing such a system?

2. Aug 4, 2011

3. Aug 6, 2011

### Bill_K

failexam, Stochastic is just a fancy word for random. A stochastic system in physics is one that's governed by statistical mechanics.

The word 'classical' is in there because for a classical system x and p may be specified independently and take on any value, so you can speak of w(p,x) as a continuous function in phase space. For a quantum system only some values of x and p are allowed. For example for a harmonic oscillator, ½(p2 + x2) = nħ, and so the distribution in phase space is discontinuous.

4. Aug 8, 2011

### atyy

In classical systems, randomness arises because of lack of experimental control. For example, the experimentalist may not be able to prepare the system in exactly the same state every time. He may only be able to prepare the system in some states with higher probability than others. This is how probability enters classical systems.

In quantum systems, even if the experimentalist has perfect control, probability still enters the physics.

Also, for a classical system, w is always greater than or equal to zero. Also, for a quantum system w is not always greater than or equal to zero, but can be negative. This is because a classical particle can have definite position and momentum. However, a quantum particle with definite position cannot have definite momentum.