# Why isn't classical physics probabilistic in nature?

1. Dec 17, 2013

### yugeci

Hello friends,

Why does classical physics not follow a probabilistic nature? And why is conventional predicting absent at microscopic levels? I have searched a little including sites like physics.exchange but only see responses that are "classical physics fails to predict this" or "it cannot explain this" or responses that are far beyond my syllabus. I know that for example you can't explain the photo-electric effect at a macroscopic level because the energy isn't related to the wave nature but rather the particulate nature. But what has that got to do with probability? This isn't really a homework question but I question I feel could come in my exam, and my understanding of quantum physics being probabilistic and classical physics being deterministic is quite poor. Still, I didn't think it was fit in the "Classical Physics" or "Quantum Physics" forums.

2. Dec 17, 2013

### Decimae

To understand this, you first need to understand what it means when something is not probabilistic.
Something is not probabilistic when given a closed(no interactions with anything else) system at a certain time, there is an unique description of the system for every point in the future.

Classical physics is not probabilistic because it can fully determine what happens in the future, there is no chance involved. The description of classical physics does not fully correspond with reality, but the mathematics of it are not probabilistic.

Quantum mechanics, on the other hand, is probabilistic, as chance is involved, because for certain states, we can only calculate chances for different outcomes, not the exact outcome.

3. Dec 18, 2013

### voko

It is not correct to say that classical physics is not probabilistic. It actually is. It is only non-probabilistic for a very particular class of situations. The term "statistical mechanics" was coined before the term "quantum mechanics", and the ideas and methods of statistical/probabilistic approach to classical physics were in circulation well before that time.

The perfectly deterministic laws of classical mechanics result in chaotic behavior where determinism is effectively lost even without having to introduce the quantum machinery. For example, we cannot make accurate predictions of the planetary motion in your very own Solar System over significant periods of time.