# Can QM be derived from Newton's Laws of motion

1. Mar 19, 2004

### Terry Giblin

Here is an answer I have just given to a question about the journey of light, which applying its logic you can derive QM and GR.

Newton's first law of motion state

"Every object in a state of uniform motion tends to remain in that state of motion unless an external force is applied to it."

Provided there is nothing to apply an external force to a photon then the above law will not be broken, therefore no additional force is required. It simply follow's the shortist path, where every its journey takes it.

But he has his revenge, with his third law,

"For every action there is an equal and opposite reaction. " - Hence the electron............

Using Newton's own laws of motion, proves the existance of electrons by the properties of photon's...........

Now that's an original thought - if I've ever had one.............

2. Mar 19, 2004

### lavalamp

I'm not sure I followed you around this bend. Do you care to be a little more specific for us less enlightened folk please.

3. Mar 20, 2004

### jcsd

The answer is unequivocally "no", you cannot derive quantum physics wholly from classical physics, hence the division.

4. Mar 22, 2004

### arivero

Now, you can get inspiration for quantum mechanics by reading Newton's principia.

5. Mar 22, 2004

### TeV

?
You mean treating the light in a corpuscular manner,or something else?
Personally,I've never got around to read Pricipia.
(My great sin,I know..)

6. Mar 22, 2004

### Chi Meson

Original thought, yes. But it turns out that Newtonian Physics can be derived as a special case of QM, where we have many many particles all together at relatively low energy levels. In the "macro" world quantum effects are too small to notice. In no way does a deep analysis of Newtonian mechanics lead us to quantum machanics; in fact, it was the application of newtonian mechanics that led Neils Bohr et al to realize that something was amiss, and a new set of rules was necessary to explain what was going on.

7. Mar 22, 2004

Staff Emeritus
I don't think Newton's corpuscular light theory is in his Principia; it's in his other book, Opticks.

8. Mar 22, 2004

### arivero

Yep but the real issue is in the proof of area law for central potentials. No the proposition, but the proof itself. It is already noticeable it is called to be the core of (Feynman) space-time approach to quantum mechanics.

9. Mar 22, 2004

### arivero

Bad grammar here in this statement. Perhaps it should be: But it turns out that Newtonian Physics can be derived as a special case of QM where we have many many particles all together at relatively high energy levels respect to the fundamental one.

10. Mar 22, 2004

### Chi Meson

OK. My mistake was trying to oversimplify again: I was considering thermal and kinetic energy levels.

But I think my grammar wasn't so bad for a forum post. Certainly warn't not worser than yorn.

11. Mar 22, 2004

### jcsd

For the key reason why you can't derive quantum mechanics from classical physics, you have to look at how the Schroedinger equation was derived. Basically it was derived using an inductive rather than deductive method and it is significantly different from a classical wave equation.

12. Mar 22, 2004

### jackle

For the less enlightened, what does this phrase mean please?

13. Mar 22, 2004

### jcsd

14. Mar 22, 2004

### jackle

Brilliant, very entertaining, thanks!

15. Mar 22, 2004

### jcsd

Id just add: don't get the impression that I mean classical physics doesn't involve induction. Taking the example of special relativity, Einstein inductively (of course Messrs Michelson, Morley et al deserve more than a little credit for this) derived the postulate that light travels the same speed in all inertial reference frames and then he deductively derived special relativity from previous classical theories. Special relativity is a classical theory.

Schroedinger inductively derived wave-particle duality (though the lionshare of the credit must go to Prince De Broglie for this), but he used induction again to derive his famous equaton (by first considering a classical wave equation then inductively making changes to it).

16. Mar 23, 2004

### arivero

It could be.

Hmm but one should compare quantum mechanics to classical point mechanics, not to classical waves. In origin, classical mechanics was very inductive too.

PS: yep my grammar is completely awful. You guys should hear my Spanish.

17. Mar 23, 2004

### TeV

Pardon my ignorance,but what is "the core of Feynman spacetime approach to QM"?

18. Mar 23, 2004

### arivero

I'd say, path discretization and consideration of the process of limit.

Feynman approach has two process of limit: "small time step", which drives one back to Schroedinger equation, and "small Planck constant", which lets one to go back to Newton equations (Euler Lagrange if you prefer).

19. Mar 23, 2004

### jcsd

Disagree, wave mechanics was the first real quantum theory and it comes straight from considering the form of classical wave equations.

Yes classical physics involves alot of induction, but if quantum mechanics was derived as a classical theory Schroedinger would of had his starting point of matter waves and the HUP and then he would of been able to deductively derive his equation from these (and classical physics), but didn't and he couldn't do this.

Simlairly you can't derive relativistic quantum mechanics from non-relativistic quantum mechanics and you apparently can't derive quantum gravity from quantum field theory, etc.

20. Mar 23, 2004

### Terry Giblin

When is the speed of light not!

Gentlemen,

Has anyone heard the joke,

"When is the speed of light, not the speed of light?"

"When it's an electron!!!"

In the quantum tunnelling effect, does the electron or photon pass through the potential barrier, based on probability alone - in a real world or chaos world?

Unless the 'infinitesimal volume element' is satified by the restrictions of the experiment, as predicted by the underlying harmonic equations.

In the YDSE does an electron not transform into an intereference pattern.

In the quantum tunnelling effect, does and electron or photon entering and existing a quantum barrier an electron emerging on the other side when tested for.

"The path of an electron is only created when tested for..."