# Teaching about light before quantum mechanics

1. Nov 18, 2013

### Avichal

I've been learning about the properties of light in school without quantum mechanical aspect of it. Now that I'm in college I'm looking to learn quantum mechanics and know about light in a different perspective.

Now this troubles me:-
I've been taught about light in the following way - Some properties were explained using wave equations and some assuming it behaves like a particle. Of course in reality nothing is true. Only the schrodinger equation perhaps best explains the properties of light. So why were the wrong things taught before. Now I have to unlearn everything right?

2. Nov 18, 2013

### ZapperZ

Staff Emeritus
You weren't taught the "wrong" things. The wave behavior works perfectly well in many circumstances. Try telling people who design RF accelerators that they can't use the wave picture of light, and they'll laugh hysterically at you. When it works, it works!

But we now know when such a picture breaks down. We know where the QM picture now will supersede the wave description. After all, you are not complaining that we're teaching the wrong thing when we still teach you Newton's laws, are you?

Zz.

3. Nov 18, 2013

### SteamKing

Staff Emeritus
Not really. Scientists studied light for many years before the development of QM and gained quite a bit of insight about its properties, even if they could not fully explain them. After all, Maxwell developed his theory of electromagnetism and he knew not one whit of QM or relativity.

Our minds develop gradually from birth. Sure, it would be nice to jump into QM in elementary school, but a child's mind is not capable of grasping the complexity of QM from such a young age. It takes quite a bit of intellectual development to grasp basic scientific concepts, and there is no use in waiting to teach certain things because one may not be sophisticated enough to grasp all of the math underlying QM.

For the most part, about 95% or more of the people can function just fine in life w/o knowing any QM. Newtonian physics is still quite workable for most things.

4. Nov 18, 2013

### gmax137

Two things:
First, as the biologists say, "Ontogeny Recapitulates Phylogeny" - - nobody ever really learned quantum mechanics without first learning classical mechanics and classical EM (maxwell's equations).

Second, learning physics *feels like* learning about how the world works, but what you're really learning is how the models of the world work. This feeling is (I think) due to the nice "word explanations" that accompany the mathematics. But don't be fooled: someday there will be theories (ie, models) that supersede today's QM and general relativity (just like they superseded classical physics). That doesn't mean we don't know what we're doing today, it just means we never stop learning more. It's a *good thing.*

5. Nov 18, 2013

Staff Emeritus
Not only were you taught "wrong" (i.e. oversimplified) things before, you're being taught them now. It's best to learn to not let that bother you.

6. Nov 18, 2013

### AlephZero

For the same reason that you were taught how to count apples in elementary school, without first having to learn http://en.wikipedia.org/wiki/Zermelo–Fraenkel_set_theory and then construct the integers from it.

The best way to learn most subjects is to start somewhere in the middle, not at the beginning of a formal exposition.

7. Nov 18, 2013

### Andy Resnick

Care to elaborate?

8. Nov 19, 2013

### Avichal

Well, I do not complain about Newton's laws because I already have a good intuition about mechanics and the laws work!
Sure the wave and particle visualization of light work well but it gives a wrong intuition to me that light sometimes behaves as light and sometimes as particle. I just do not understand this. But anyways, I have heard Quantum mechanics is even more wierd.

I meant that the wave and particle nature of light is not correct. Based on current knowledge, it is neither.

9. Nov 19, 2013

### Zarqon

Your own example of Newton's laws is a good one, because we now know that Newtonion gravity isn't a full description either! But is succeeded at least by Relativity theory, which is a more complete picture.

Does that mean that the Newontian picture was wrong?

No, it does not, it still works as well today as it did in Newtons time, and every experimental verification used back then, will still verify it today!

So it is not wrong, it is simply incomplete. Our new theories are more complete than the old ones, and the same is true for the picture of light. The old things you were taught was (mostly) not wrong, just incomplete. As was pointed out earlier in the thread, there are plenty of special situation where either a pure particle or a pure wave formulation works without problems, it's just that those formulations don't describe all possible situations, so they're incomplete.

10. Nov 19, 2013

### SteamKing

Staff Emeritus
Not to pile onto your discomfort (too much), QM may not be the last word in physical formulations, either.

QM covers certain things, relativity covers other things, and then there is gravity, which sort of stands alone, not covered by QM and only partly covered by relativity.

For a better understanding of these dilemmas, see:

http://en.wikipedia.org/wiki/Unified_field_theory

Even Einstein wrestled unsuccessfully with how to develop a single theory which would include relativity and QM.

11. Nov 20, 2013

### Avichal

Newton's Laws are a good approximation when speed is significantly less than speed of light.

I want to ask whether the wave behaviour is also an approximation of schrodinger's equation? In the case of newton's laws if we assume v << c then we get the newton's laws in a straight-forward way. So similarly can we get the particle behaviour and wave behaviour out of schrodinger's equations?