# A layman reflection

1. Jan 3, 2007

### lemma28

We have 1 particle.
Classical Mechanics says: If I know the initial state I can predict with absolute certainty what position (etc.) it will have the next second.
Quantum Mechanics says: I don't know the position the particle will hit. But If I know the state I can tell you the possible future positions and their probabilities.

Seems that QM has much less predicting power.

But...
We have 100, 1000, 10000 particles, all in the same state.
CM says: well... if they're all really identical and all really in the same state (seems strange to think of it...), all particles will hit the very same spot after a second's interval. And I know were this spot is.
QM says: if I know the identical states of all the identical particles I can tell you with higher and higher precision (as far as there are more particles adding on) the distribution we'll see. Since the particles are all identical, it's not really important to tell which one went where. The distribution is what counts. So I'd proudly say that I now have the same degree of information as you (classical observer) have.

(And all experiments with microscopic particles support the QM "distribution" view)

This simple picture suggest how the QM approach gets just the same level of information as CM with large ensembles of particles. It also suggest that physic reality is not to be looked after in the single particles, that have little meaning as far as prediction power is concerned, but rather in the whole of them, bringing naturally on the idea of something holistic or the concept of a field.....

What do you think?
It was just a stray musing, since I'm finally catching on with QM priciples and looking forward to confront with QTF.

2. Jan 3, 2007

### chroot

Staff Emeritus
There's no deeper predictive power to be found in either theory. Classical mechanics tells you everything there is to know about the motion of a particle, while quantum mechanics tells you everything there is to know about the distribution of experimental outcomes. In the limit of a large ensemble, the two theories become indistinguishable.

- Warren

3. Jan 3, 2007

### cesiumfrog

Note that classical mechanics would frequently predict something qualitatively different to QM, and that in such cases CM is proven wrong.

I think its very unfair to claim QM has the lesser predictive power.

4. Jan 3, 2007

### marlon

This is a rather poor formulation, IMO. The difference between QM and classical physics is "a bit" deeper than what you say above. Classical mechanics will tell you everything about the motion of a particle when we do not look at that particle at atomic scaled distances. You should have made the distinction based upon physical regimes of validity ! What happens to a single particle in the doubble slit exp (self interference, the intensity profile) cannot be explained using classical mechanics. The reason is very simple : "classical" particles have "a trajectory" which is not allowed by the HUP. Classical physics does NOT explain the Hydrogen atom or the H+ ion.

marlon

Last edited: Jan 3, 2007
5. Jan 7, 2007

### Kaspah_2k

cool

i used to try and study how quantum mechanics affected general relativity and what i learned is it doesnt matter..so long as we understand that we are here then we shouldnt need to know why.

too many people say 'what if' and 'why' but when they die their view of those subjects go with them or they may get passed on for someone else to fathom.

i dont see the need to work it out anymore because i have realised that it will never be fully explained.

:surprised

6. Jan 7, 2007

### ZapperZ

Staff Emeritus
We don't know that.

As scientists, by definition, we study things that we don't understand. We do not get employed to study things that are known. That is why we continue to expand the boundary of our knowledge. There are many things we barely know of just a year ago that we know more of now. It is called "progress".

If we go by your philosophy, then you're saying we should just give up. While I still reserve my judgement on the issue of GR-QM unification, I can see the worth of pursuing it. Even if the pursuit is difficult and did not reach its goal, just knowing what is wrong and what doesn't work is valuable in itself. Furthermore, in science, and physics in particular, there are many aspect of the study of one area of physics that affects other areas of physics. It is THAT interconnected. This is why one simply can't throw away one part of physics without considering how that will unravel other parts that where it has worked.

Zz.