# How to explain the term quantum mechanics in 10 minutes?

1. Aug 1, 2015

### td21

How to explain the term quantum mechanics to a class in 10 minutes? I may need to do this in an oral exam where the professors are from English department. Whiteboard can be used.
My presentation should include one or more of the following:

· Examples (practical and/or personal)

· Analogies (e.g. "The structure of an atom is similar to that of the solar system.”)

· Comparisons and/or contrasts

· Word origin (meanings of prefix, suffix, root)

· Drawings or diagrams on board

Thank you very much.

2. Aug 1, 2015

### Dr. Courtney

Everything is a wave. Quantum mechanics is the theory that describes this mathematically and allows accurate predictions from considering which wave states exist due constructive interference.

I would bring a stringed instrument, a computer running Audacity, and display the sound waveforms and the Fourier transform on the screen.

I would use the time to make an analogy between the allowed waves on the string and the allowed waves for the 1-D particle in a box.

3. Aug 9, 2015

### micromass

Staff Emeritus
You should probably only bring up this analogy to show that this is a very inaccurate analogy.

Anyway, the crucial thing about quantum mechanics that students need to realize is how completely unintuitive and weird it is. You need to stress that it is not something you can understand intuitively or something you can visualize. Any attempt to give an intuitive explanation or visualization is in my opinion doing more harm than good.

If you want ideas, check the amazing Feynman lecture on quantum mechanics:
This is of course much too long, but it is essentially the only video of quantum mechanics for "laymen" that I really like. Unlike many other videos, it doesn't attempt to appeal to your intuition with false analogies, but instead he straight up tells you it's not possible to. So what he tells you is that quantum mechanics is a way to predict the outcome of experiments, but does not attempt to explain why the (weird) outcomes are the way they are. This is crucial.

4. Aug 9, 2015

### atyy

Feynman's presentation about what is unintuitive and weird about quantum mechanics is controversial. Perhaps it is even wrong. When talking about non-relativistic quantum mechanics, it is certainly wrong. At that time, perhaps Feynman did not know about Bohmian Mechanics. Although not everyone at that time had similar misconceptions, eg. the book by Messiah does not make the argument and explicitly says hidden variables have not been ruled out, Bohm himself made a wrong argument very similar to Feynman's.

5. Aug 9, 2015

### micromass

Staff Emeritus
At that time, Feynman indeed did not know about Bohmian mechanics. But the point is that Bohmian mechanics is a very fringe part of QM. It explains all the results, but it is essentially untestable. So while Feynman might go to far in saying that there are no hidden variables, it is still fair to say that the hidden variable theory offers no advantage over the usual theories, and that it cannot be tested. It is just not a useful theory in science.

6. Aug 9, 2015

### atyy

"Fringe" is a sociological term, so you may be correct.

However, there is the idea from many physicists that QM is incomplete, because of the measurement problem. One way of stating it is that QM has deterministic evolution by Schroedinger's equation and random evolution by wave function collapse. What determines which rule to apply? That comes from outside quantum mechanics, suggesting that QM is incomplete.

Bohmian Mechanics is the first solution of the measurement problem, if only for non-relativistic quantum mechanics. That alone makes it significant since it suggests the measurement problem can be solved.

Furthermore, Bohmian Mechanics does predict deviations from quantum mechanics. Bohmian Mechanics is analogous to statistical mechanics in that it requires a notion of equilibrium. Just as in statistical mechanics, we do not expect equilibrium to hold always, Bohmian Mechanics will deviate from quantum mechanics under non-equilibriunm conditions. So it is testable.

7. Aug 9, 2015

### atyy

I will add my own view here, since the thread is asking for a personal opinion. I don't think quantum mechanics is unintuitive. In fact, quantum mechanics in the Copenhagen interpretation says: use your common sense - you know intuitively when you see a definite outcome! Furthermore, in the Copenhagen interpretation, although we do not know whether the wave function is real, we do not deny that once we have a classical/quantum cut that we can treat the wave function as if it is real, for the purposes of making predictions. "Reality" is just a tool to predict the outcomes seen in reality. Measurement is an interaction, which in many cases disturbs the system - what is unintuitive about that? Measurements requiring different placements of the equipment cannot be made at the same time, since the different arrangements will interfere with each other - what is unintuitive about that? So I deny that quantum mechanics is unintuitive or weird.

8. Aug 10, 2015

### CWatters

Going back to the OP...

These are English Professors so if you go into too much detail in 10mins they will be lost.

I would start by explaining why QM is necessary eg what paradox does it help explain?

What Quantum theory actually is. The origin of the words "Quantum" and "Mechanics" and why they are appropriate names for the theory.

Perhaps mention some of the surprising things that QM leads to such as not being able to pin down the position of a quantum particle, only draw a graph of it's probable position and leads to things like Quantum tunnelling.

Perhaps explain why we don't see QM effects at large scales.

For a comparison/contrast perhaps explain how QM differs from Newtonian. Newtonian suggests everything is predictable (snooker ball analogy) vs QM (all about probability).

Something like that.

9. Aug 10, 2015

### Fewmet

On thing that usually comes up when I talk with high school students about quantum mechanics is this from Einstein:
"Common sense is the collection of prejudices acquired by age eighteen."

I like to imagine he said it in response to someone saying quantum mechanics violates "common sense". I suspect English professors would be open to the idea that what we take for granted is based on personal experience and that we have no reason to expect it to apply to circumstances where we have no experience.

I might also use the listeners' knowledge of atoms to develop the idea of the quantum: you cannot have, for example, any arbitrary mass of gold (or of a given isotope of gold, if you prefer to more technical). There must always be the mass of a whole number of atoms. Building on this, energy (and arguably distance and time) are quantized. Then move into consequences of the premise.

10. Aug 10, 2015

### atyy

My view is that there is nothing contradictory in thinking that quantum mechanics is common-sensical in the standard interpretation, as well as thinking that quantum mechanics is incomplete in the standard interpretation.

In fact, the reason for thinking quantum mechanics incomplete is because quantum mechanics appeals to common sense.

Thus I think Feynman's argument is doubly wrong. Quantum mechanics is neither unintuitive nor incompletable (or at least Feynman's argument for its incompletability was wrong).

11. Aug 10, 2015

### atyy

Ok, I admit one thing is unintuitive about QM - QM's formalism is manifestly nonlocal if we pretend the wave function is real, and it violates causality, yet it does not violate special relativity. Classically, special relativity is a theory of causality. This is violated by quantum mechanics, showing that special relativity in some sense does not require causality. In his later years, Feynman changed his mind to the now conventional view - it is not the double slit that shows the mysteries of quantum mechanics - rather it is a Bell test.

http://www.cs.berkeley.edu/~christos/classics/Feynman.pdf
Feynman, Simulating Physics with Computers

12. Aug 11, 2015

### Fewmet

I fully agree that quantum mechanics (at least my primarily qualitative grasp of it) follows logically from the reality that careful, subtle experiments implies. Few people, however, are aware of those experimental results and the huge majority of people very likely assume the world works according the way daily experience shows. (A panel of English professors are likely to fall into that group.) Daily experience tends to lead to the Aristotelian view for most, although those of us fortunate enough to have had at least a decent high school course in physics will probably take a Newtonian view. I intended to get that across in my post, but apparently failed.

The relevance here is that I see value in alerting listeners that they have no first hand experience with the quantum realm and so their filters for what seems reasonable and what seems impossible might not be reliable when considering quantum mechanics. It is point of rhetoric designed to encourage being open-minded rather than dismissive.

13. Aug 11, 2015

### atyy

Gosh, I'd made exactly the opposite assumption, that English professors only understand weird things!

Time present and time past
Are both perhaps present in time future
And time future contained in time past.

What??? Maybe that's talking about the unitary evolution of the wave function :P

What might have been is an abstraction
Remaining a perpetual possibility
Only in a world of speculation.