# I need your help teaching physics

1. Nov 24, 2007

### ΔxΔp≥ћ/2

For my physics classes I have to create presentations on a subject in physics. I was feeling ambitious and chose QM for my physics 3 class. I must do a 5 minute presentation (am shooting for 15 minutes).

Now, I know I will have to surprise them with how reality is perceived in quantum mechanics, I will do this mostly by showing experiments and physical phenomena: Davisson-Germer, Double-slit, maybe Aspect, blackbody radiation, nucleosynthesis, Hawking radiation, half-life, electron in a box, the Compton effect and I will demonstrate the photo-electric effect and maybe demonstrate the emission spectrum of a gas.

I plan on dividing the presentation in to sections : complementarity, the uncertainty principle and maybe the exclusion principle (I don't think time will allow).

I also have to create some kind of model. I will be demonstrating the photoelectric effect, but I would like to have another one too. Any ideas on that one? Is it possible to do something like the Young experiment and show the interference of light in the classroom?

That’s the presentation in a nutshell.

I know that you guys have explained quantum mechanics many times and some of you even teach quantum mechanics, so I'm hoping that you can help me in making the presentation as logical and easy to understand in the allotted time.

Remember: I want to cover the most quantum mechanics possible in a short period, without missing any major parts of the theory and without losing the class.

2. Nov 24, 2007

### blechman

One really nice demo of QM is the following: take a polarizer (sun glasses) and shine light through it. Now take another polarizer and rotate it. You will see the light get dimmer until it is black (when the two polarizers are perpendicular to each other). So far, nothing but classical E&M.

Now insert a third polarizer in between them, and POOF! It is no longer black! By inserting the middle polarizer you have collapsed the wavefunction of the photons a different way and this allows light through the final polarizer. Very simple, yet very profound.

Treating QM in 15 minutes, that's quite an ambitious project! May I suggest that you try to focus on only a couple of the many beautiful experiments you mentioned. I know you want to do it all, but there's just no time, and you'd be better off doing one or two of them REALLY well rather than doing all of them terribly. That's my advice.

Good Luck! Let us know how it goes!

Let me also emphasize (as you seem to be doing) that you should STAY AWAY from the equations, and stick to demos - this is coming from a particle theorist! If you start writing equations down, you'll just lose your audience. But with a good demo, they'll be yours for life!

Last edited: Nov 24, 2007
3. Nov 25, 2007

### ΔxΔp≥ћ/2

Thanks for the help. I think I will do that experiment.

As for the presentation, I would probably be better off just doing complementarity and the uncertainty principle then (in that order).

As for all the experiments, I wasn't planning on doing most of them in depth. For example: it is theoretically impossible to prove that the sun shines unless we allow for tunnelling. (Hopefully with a better wording than that though).

I would have really liked to do like Feynman and Brian Greene and start with the double slit experiment, but it is so good at explaining QM (the important principles are all present) that I fear that it would be impossible to explain them one by one. Should I maybe finish with it to go through all the concepts again?

As for the math, as a general rule, I will have no time to go through the proofs (particularly that some of the math does not even have a proof). However I am considering the possibility of including ΔxΔp≥ћ/2. I know that it is a special case, but how special is it and is it easy to prove? We are also learning matrixes in math class, should I talk about this representation at all?

I would like to get into the history of QM too (it is a passion of mine), but only in passing.

There are some other things that I was planning on staying away from due to complexity or disagreement among scientists, but you guys might have a different opinion. Schrondinger's cat (multiple states) and the interpretations: Feynman(all possibilities cancel each other, could be a mess), Copenhagen(the moon is not there when you are not looking?!?), Bohm, Multiple Worlds(messiest) etc.

I have already read a few books on QM, are there any that I should absolutely read before I present? Any of them with nice pictures I can show?

As always, I really appreciate your help.

4. Nov 25, 2007

### blechman

the double-slit experiment is where it all begins! It should be very easy to construct for light (Young's experiment, pre-QM), and then you can explain that the exact same thing happens when you shine a neutron beam through the slits! So much for neutrons being "particles"! Unfortunately, it's a bit harder to actually do that experiment.

NO MATH! - you can "prove" the uncertainty principle with words: let's say you want to measure the position of a particle very precisely. How do you do it, physically? You have to shine light on it to "see" it and measure it. More precisely, you scatter photons off of it and watch the diffraction patterns that result. HOWEVER, by the principles of QM, some of the momentum of the light will be passed onto the particle you want to measure, screwing up your measurement of its momentum.

This "proof" should be enough for high-school students (and teachers!) The mathematical proof of the HUP is not difficult, but it involves some rather high-level mathematics. You can prove it explicitly by considering various specific examples (famous QM1 problem: calculate Dx*Dp for the physical problem at hand and show that it is always >= hbar/2) - but you don't want to go this route in a 15 minute presentation - no one will understand you.

BTW: the HUP is always true - it's quite general and mathematically precise (see the QM1 problem that I mentioned earlier - you can explicitly calculate Dx and Dp and show that the HUP holds).

And don't talk about "matrix mechanics" and "path integrals" - this is *much* too advanced for your audience. Skip that entirely. There's no physics there - just mathematical formalisms. If you *must* talk about something, stick to wave mechanics - it highlights the wave-particle duality that is hidden in the other formalisms. But if you want my advice, I wouldn't even mention it. I suppose you could say something about "wave-function collapse" and all that (see below).

It's such a fascinating subject, but I worry that you just won't have the time. Remember the rule I said above: better to do fewer things REALLY well than to do too many things crapily!

I *hate* Schrodinger's cat! I find that it always leads laypeople to make mistakes. Did you know that Schrodinger proposed his cat example because he thought QM was garbage, and he wanted to emphasize that it made ridiculous predictions? His luck, it became the standard example of how "cool" QM can be.

As I said above: stay away from Feynman's path integrals. This is a mathematical technicality, not "deep physics".

You can talk about wavefunction collapse for real (is the electron spin-up or spin-down?) and mention that there are various ways to "interpret" this (Copenhagen, many worlds, etc). But I would be careful here: there are many pitfalls, and you don't want to sound like you don't know what you're doing if this is for a class!

The first book on QM I ever read was Nick Herbert's "Quantum Reality" (to be honest, it was so long ago, I don't remember if I thought it was a good book or not!). Gibbons's pop-science book, "In Search of Schrodinger's Cat" is also very good.

I hope this does help. Let us know how it goes.

Last edited: Nov 25, 2007
5. Nov 25, 2007

### ΔxΔp≥ћ/2

I am in the process of finishing "In Search of Schrodinger's Cat" for the second time.

I am concerned that the students (and my uber-classical teacher) will think that I am wrong or will get the wrong picture by clinging on to a classical reality. How do I show that the uncertainty principle is not just from the awkwardness of experiment?

After further thought, the experiment with the Polaroid lenses bothers me too. I fear that all the class will get out of it is that we change the photon’s spin, instead of what they should. If I did choose to do it, could I just bust a bunch of regular sunglasses and run a laser pointer trough?

I am also scared that they favour either particles or waves (probably the former). I know how to explain the double-slit experiment in terms of particles, but how do you explain blackbody radiation and the photoelectric effect in terms of waves?

I have done a lot of reading and think I have a good grasp of the theory, I just want to get these things right.

Thank you so much blechman for taking the time to read and respond to my posts, your help is greatly appreciated.

6. Nov 25, 2007

### blechman

That's a good book!

But it isn't: it is that awkwardness of experiment that makes the HUP happen. The new feature in QM is that the awkwardness of the experiment is built in to the theory! That is: there is NO WAY to get around it, even in principle!

There is also a "classical" HUP that follows from wave mechanics (classical waves, this time) - if a wave is damped (which every physical wave is), there is a relationship:

$$\omega\Gamma\ge 1$$

where $\omega$ is the frequency and $\Gamma$ is the inverse lifetime. This follows from the equations for waves, and it is sometimes called the "Classical Uncertainty Principle". So the HUP also comes out of wave mechanics.

I say this to you, not suggesting that you should talk about it in your presentation, but just so you can see personally where these things come from (Heisenberg didn't "discover" the HUP, he just related it to "particles").

No to your first point: there is no way to explain this phenomena without QM and the collapse of the wavefunction. If you think of these as "classical" spins, you would still get zero transmission through the second (perpendicular) polarizer. Remember that a polarizer doesn't "change" the spins; rather, it blocks all the photons that have the "wrong" spin. So if it were classical, once the first polarizer (horrizontal, say) blocks the "vertical" photons, then they're gone, so there should be no transmission through the vertical polarizer whether the third polarizer is there or not. But with QM, the situation is different, since the polarizer at a 45 degree angle will re-introduce vertical photons due to QM, and that would never happen in a classical system.

As to how to do this experiment: see if you can get a polarizer from your teacher - they're *very* cheap. If not, you can pick it up at a science supply store or a local hardware store probably. If that fails, then (polarized) sunglass lenses will work.

Don't use a laser - that's coherent light, and may not have all the same effects you want. Just use a flashlight, or an incandescent bulb.

I'll get back to you on this last one. I"m off to dinner!

7. Nov 25, 2007

### nrqed

It's sad that a student would have to convince a teacher about something established for almost 100 years!!
I think you should really focus on a single concept and make it clear rather than pile up a bunch of stuff covered very quickly.

I think that the double slit experiment conveys all the weirdness of the particle-wave duality in a simple setup and can be explained with no maths. I would think that would be a good example to focus on. The wave-particle duality can be made explicit.

8. Nov 25, 2007

### blechman

Once again, I would like to explicitly agree with what nrqed said. I would consider either the polarizer experiment or the double-slit experiment and give a great presentation on that. I think it would really be a hit.

You're not going to teach anyone QM in a 15 minute presentation!

9. Nov 25, 2007

### ΔxΔp≥ћ/2

I'll get back to you on this one.

So... ...we use the wave function to establish probabilities of finding particles in a certain spot. Observation (interaction with particles real or virtual?) causes the wave function to collapse, we see a particle.

I believe you, but how can we say that the photons are not just deflected (thinking of a classical spin) by the 45degree polarizer and re-deflected by the perpendicular one instead of reintroducing photons?

I think he is waiting for them (physicists) to agree...

By the way, my presentation is in January, so you will hear plenty from me between now and then. I am also doing a project on special relativity.
Thanks, you guys are great.

10. Nov 25, 2007

### blechman

That's a little confusing to me. I would say, rather, that we make a "measurement" and this collapses the wavefunction, so that all future measurements yield the same result. What, precisely, this "measurement" thing is, is actually a deeply troubling and still-unsolved problem of QM.

I'm not sure I understand you. Think of a polarizer as a (vertical, say) metal grate with a string going through it. Wiggle the string in the vertical direction (making vertically polarized waves) and they go through the grate, no trouble. Send horizontal waves through, they're blocked. Send a wave polarized at an angle: you get a (smaller) vertical wave coming out! So polarizers don't "deflect" anything, they BLOCK!

Have fun!

11. Nov 26, 2007

### ΔxΔp≥ћ/2

That example helps a lot.

If I physically did the string experiment, it would wiggle horizontally though right (seeing as the grate doesn't block the string in the same way)?

I know you guys are really insisting on ONE concept, but do you really think it would be hard to explain both wave-particle duality and the uncertainty principle in 15 minutes? Seems to me like I have done more stuff in less time before (a good special relativity presentation in 10 minutes).

Thanks again.

12. Nov 26, 2007

### blechman

If the slits of the grate a vertical, then wiggling the string vertically would send the wave through, no blocking. If you wiggle the string horizontally, no wave would pass through the grate.

Hey, HUP, if you really want to do two things, none of us will stop you! But just keep in mind that "less, but better" is always preferable to "more, but worse"! The other point: this is not a public lecture, this is a class, and you don't want to look bad in front of your teacher and his red pen! So my advice (and nrqed, I think, would agree with me) is to consolidate, choose one (alright, maybe two!) things that you can explain *very* well with no math, and stick strictly to that. Remember that it takes time to run experiments/demos, even when it all goes right! You don't want to be rushed.

13. Nov 26, 2007

### ΔxΔp≥ћ/2

... ...after reading these words HUP runs and jumps out a window...

...but comes back to finish his post.

If I had a vertical polarizer followed by a second one 45degrees to it, wouldn't all the light coming out of this experiment be polarized at 45degrees from the vertical? If not, shouldn't no light get trough a system of two polarizers if they are not both parallel? I hope you see where I am going with this.

14. Nov 26, 2007

### blechman

OH, I think I see what you're saying now. Yes, you're right. You can rotate the wave polarization in this way (and, in fact, that's how you do it!).

But you can also interpret light as a particle (photon), and it has one of two polarizations (that is is, a number +1 or -1). These quantities never "talk" to each other (they are "orthogonal states"). Now send it through the first polarizer, so only the +1 photons come out. Now there are no more -1 photons left, so you should never see them again! And yet I can regenerate them as if by magic with the middle polarizer!

Mad at me? Maybe we should just say: To Hell with the particle description! Except I can do the exact same experiment with neutrons (I stay away from electrons since they're charged and that complicates things, but you can do electrons also if you want!). Now, SURELY these are particles , and yet they will reproduce the same funny result of this experiment.

15. Nov 26, 2007

### nrqed

I have seen numerous students giving their first technical presentations 9typically it was a talk to present their undergraduate thesis) and the most common mistake they make is to try to cram too much stuff in a one hour presentation and to get too technical (they presnet too many equations too quickly). This is recipe for disaster.

You have to keep things simple and clear. Especially given that you are presenting ideas that are very counterintuitive, so hard to grasp the first time one hears them! Focus on one concept illustrated with one experiment. And then just make sure that you present the setup very clearly (don't rush! Take the time to make things very clear, even things that may seem obvious to you) and explain the implications of the experiment in details, emphasizing the weird part. Practice several times and present the talk to people you know before you do it in class (preferably people who know nothing about physics and who will tell you if there are some stuff you say that are not clear to you). You will know it's a godo talk if someone with no physics background at all "gets it" and comes out of the presentation feeling amazed at the wirdness of the quantum world.

16. Nov 26, 2007

### nrqed

It's because of possible confusion about what a polarizer actually does (one must be already familiar with them in the contect of classical optics before really understanding the implications for the particle aspect of light) that I personally prefer the double slit experiment when I want to introduce the wave-particle duality to neophytes.

First, the interference pattern through a double slit is easy to viusalize with classical waves (one can even show an interference pattern produced with actual water waves or one can shine a laser beam through a double slit and see the pattern right away). The fun begins when discussing shooting electrons through a double slit and seeing an interference pattern appear. This is truly amazing. What is the wave here? what is "waving"? Then the next step is to explain that the wave involved here is a probability wave (and one must explain why this is the correct interpretation).

Then the fun begins when one tries to "catch" the electron going through one hole .

I could easily fill 15 minutes with just that!!

One trick: try to build the "suspense" by asking leading questions, wondering aloud about some issues, emphasizing what is weird, etc.

Last edited: Nov 26, 2007
17. Nov 26, 2007

### ΔxΔp≥ћ/2

...ok, I think I'll scrap the polarizer. I still don't fully understand the concept and I am very familliar with the double-slit experiment.

I will go through the double-slit experiment in depth and use other "real world" examples to reinforce my point. I still want to demonstrate the photo-electric effect when I talk about the wave-particle duality though (it's pretty darn cool). Or should I consider dropping that too?

18. Nov 26, 2007

### nrqed

To explain the photo-electric effect clearly (all the implications about changing the potential, changing the intensity of the light, the frequency, etc) would require more than 15 minutes in itself and there is a lot of background to get through before getting to the point. Of course you can do what you want but I would suggest to drop it.

19. Nov 26, 2007

### ΔxΔp≥ћ/2

Don't you think that I need an example of light as particles or is particles as waves the best part?

20. Nov 26, 2007

### blechman

Sure, get rid of my suggestion. Thanks for the support, nrqed!

Just kidding. If you are at all confused about it and you feel like you understand double slit better, then you're quite right to focus on that instead.

Again, if you had more time, then it would be nice to do this. But you said you're interested in science history, so consider this: when Newton wrote his (other Magnum Opus) "Optiks", he thought that light was a particle! It wasn't until much later, when Young did his experiment, that people decided that Newton was wrong and that light is actually a wave. So from a historical point of view, the idea of light being a particle was much less of a shock than matter particles being a wave!

Maybe that convinces you to focus on the double-slit; maybe it doesn't.