# What is Quantum Physics?

1. Aug 7, 2014

### samgrace

I have a basic understanding of QM, i.e the experiments and some results, photoelectric effect, Compton scattering, double slit experiment; heisenburg uncertaintly principle, de broglie hypothesis, blackbody, spectroscopy and the schrodinger equation, of which I can kind of pin together as an understanding of what it is.

What I'm asking is how do you explain what quantum physics is to someone who doesn't do physics?
I got asked by someone in town and all I could say what it's about the quantization of energy and the wave-particle duality of matter on a very small scale. This didn't satisfy the question.

What would you say?

Also is there any experiments/phenomenon I've missed?

Thanks

Last edited: Aug 7, 2014
2. Aug 7, 2014

### phinds

I think "wave-particle duality" is misleading. What I would say is that QM is about moving beyond classical physics in which there are waves and particles to a more realistic view of reality which shows us that photons and electrons, etc, are NOT waves OR particle OR "waves AND particles" but rather are quantum objects that behave differently than what classical physics tells us.

3. Aug 7, 2014

### samgrace

I agree, it'd explain why atoms of different size vary in characteristics and properties so drastically. I'm studying solid state and thermal/statistical physics this coming year and they adopt quantum principles, phonons, photon gases with canonical ensemble etc, it's going to be a huge learning curve. nervous times.

4. Aug 7, 2014

### atyy

At the lay level, I think wave-particle duality is basically fine. Technically Schrodinger's equation is a wave equation, and the wave propagates in a space called Hilbert space, which is an abstract space whose dimensions are determined by the number of particles.

A key feature of quantum mechanics is that although we measure particle positions in ordinary space, we need this abstract space to predict the results. A second feature is that a quantum particle does not have a classical trajectory, during which position and momentum are simultaneously well defined. A third feature is that all predictions are probabilistic. A fourth feature is that the ordinary reality we see is nonlocal, and there can be correlations between measurements that are widely separated in space, yet this nonlocality does not permit any faster-than-light communication.

A very important aspect of quantum mechanics is that quantum mechanical theories describe everything we have observed to date, and there are no observable deviations from the predictions of quantum mechanics.

Last edited: Aug 7, 2014
5. Aug 7, 2014

### Staff: Mentor

The following gives its conceptual core:
http://www.scottaaronson.com/democritus/lec9.html

I would say its what you inevitably get if you want continuous transformation between so called pure states in probability.

The argument goes like this.

Suppose we have a system in 2 states represented by the vectors [0,1] and [1,0]. These states are called pure. These can be randomly presented for observation and you get the vector [p1, p2] where p1 and p2 give the probabilities of observing the pure state. Such states are called mixed. Now consider the matrix A that say after 1 second transforms one pure state to another with rows [0, 1] and [1, 0]. But what happens when A is applied for half a second. Well that would be a matrix U^2 = A. You can work this out and low and behold U is complex. Apply it to a pure state and you get a complex vector. This is something new. Its not a mixed state - but you are forced to it if you want continuous transformations between pure states.

QM is basically the theory that makes sense out of pure states that are complex numbers. There is really only one reasonable way to do it - by the Born rule (you make the assumption of non contextuality - ie the probability is not basis dependant, plus a few other things no need to go into here) - as shown by Gleason's theorem.

A more detailed answer to your question can be found in the following papers where is is viewed from different perspectives:
http://arxiv.org/pdf/quantph/0101012.pdf
http://arxiv.org/abs/0911.0695
http://arxiv.org/abs/1204.0653

The view of the first paper is a detailed development of the continuous idea.
The second shows its the same as requiring entanglement
The third shows its what's required if we require something weird - complex numbers in classical equations - but that deeply is related to Feynman's sum over history view of QM.

Thanks
Bill

6. Aug 8, 2014

### ChrisVer

QM for people who have never seen classical mechanics or anything like that? Well then, it wouldn't be much to say that QM is magic....
QM is the mechanics we have built to study and describe the micro-world, since through time and by experiments we found out that the already known classical treatments would give reliable results.

7. Aug 8, 2014

### Fredrik

Staff Emeritus
I don't think that listing the things that QM can be applied to really explains what QM is. I think the best you can do is to explain briefly what classical mechanics is, and then describe some of the differences between QM and CM.

The essential feature of CM is that at every time t, a particle has a position x(t). The theory is built up around Newton's second law (F=ma), which is a differential equation of a kind that ensures that the function x (which tells us the position at all times) can be found if you know both the position and the velocity at one specific time.

QM on the other hand doesn't say that particles have positions (and therefore neither should we). The best we can do is to associate a wavefunction with each particle. The wavefunction is determined by the preparation procedure that the particle has been subjected to before the experiment. It's a mathematical "thing" that can be used to assign probabilities to possible results of experiments.

The problem with this explanation is that it can give them the impression that QM is just probability theory in the usual sense. But it's not. If it was, the Bell inequalities would hold, and they don't. It's a generalization of probability theory. Unfortunately this last comment can't be understood without advanced mathematics.

8. Aug 8, 2014

### phinds

I REALLY think that most of the answers here are technically correct and utterly useless for explaining things to a non-physicist. I still think that something like my response in post #2 is what's needed, then follow up with examples if pressed for details.

9. Aug 10, 2014

### Barry911

I think that the essential message is that all interpretations are premature.

10. Aug 10, 2014

### phinds

Probably, but anyway, interpretations of WHAT? Your sentence does nothing to explain to anyone what QM is which is the question that we are supposed to be answering.

11. Aug 10, 2014

### Jilang

And that all interpretations should be discouraged.

12. Aug 10, 2014

### Barry911

Hello phinds: My point is that any interpretation or explanation is inappropriate at this time. We end up
with inappropriate speculation. Consider the "Copenhagen Interpretation" A wholly observer dependent
reality. Bohr's stubbornness on the issue of "no objective reality" was based on von Neumanns's so
called proof, later proven incorrect by John Bell. Notions such as the Everett "Many worlds solution" for the
measurement problem. Note that it was not so much a solution as it was an "end run" around the problem.
An incredibly inelegant conjecture at that. So I'm simply saying that it is ok to accept the very elegant and powerful mathematical formalism and forgo interpretation until more is known.

thanks

Barry911

13. Aug 10, 2014

### vanhees71

Well, the only thing you need from a physics point of view is, how to apply the mathematical formalism of quantum mechanics when describing real-world phenomena, and for this you just look at what (theoretical and experimental!) physicists do when they use quantum theory do describe an observation: They use the minimal statistical interpretation. No more no less. No esoterics or quasi-religious believes on the meaning of the quantum-theoretical mathematical objects are needed. Bohr was a great physicist, but bringing in all this metaphysics into the debate didn't help much to understand quantum physics. Better read Pauli (who kept his strong tendency to esoterics strictly out of his scientific writings), Sommerfeld, Dirac, Feynman, and other physicists following the "no-nonsense approach". This helps you to concentrate on what quantum theory really is as an important (if not the most important) part of physics, and it's a marvelous theory describing a lot of phenomena and no phenomena are known contradicting it!

14. Aug 10, 2014

### ChrisVer

Just to understand what you're trying to say (or learn it if it's a used phrase on the topic)... what is the "no-nonsense approach" of the above mentioned physicists?

15. Aug 10, 2014

### phinds

And I ask again ... what does any of that have to do with the OP's question? You think talking to a rank beginner about interpretations, is going to be helpful?

16. Aug 10, 2014

### samgrace

I kinda follow, reading Griffiths 2nd edition QM, just finished first chapter. Questionably metaphysical.

17. Aug 10, 2014

### atyy

I think interpretation is one of the most important aspects of QM. The important points are that a Copenhagen-like interpretation (eg. Landau & Lifhistz or Weinberg) in which a Heisenberg cut is present has something called a "measurement problem". This problem has at least one solution. The full range of possible solutions is not yet known, and as long as quantum mechanics is not experimentally falsified, there is no way to distinguish between the solutions.

A great introduction to the "measurement problem" is found in http://www.tau.ac.il/~quantum/Vaidman/IQM/BellAM.pdf.

Of course, this doesn't mean one should spend hours on interpretation. But an introduction without mentioning interpretation is not physics, since physics is all about the interpretation of mathematics.

18. Aug 10, 2014

### RonL

The study of things so small they can't be seen with our eyes or interacted with in a physical way in the world we are in.

If one can remember back to the very point they knew nothing about quantum physics then that might not sound too simple or dismissive. How can "someone in town" be interested in what they have absolutely no knowledge ?

My trigger point was 2-1/2 years ago when looking for information about magnetic fields of a toroidal transformer. I may be slow of mind but now can read a post in the quantum physics threads and at least have a very shallow understanding of what is being discussed, getting to know most acronyms like (HUP, ERP< BT, FAPP) and many others has been a big challenge.

It seems to me that most people are asleep when you reach the electron level

19. Aug 10, 2014

### phinds

I'm not following you at all on that since photons and electrons, for example, are quantum objects and are interacted with physically quite regularly.

20. Aug 10, 2014

### ChrisVer

Like what? The electron-photon interactions can either be Compton scattering, Photoelectric effect or pair production... all these 3 processes are quantum mechanical.

21. Aug 10, 2014

### mishima

I would say something like this: an engineer designing a bridge can use normal physics to make predictions about his design. He can calculate around what load will cause the bridge to fail, for example. Engineers designing advanced, modern technology (for example anything containing semiconductors, like a cell phone) can likewise predict the behavior of their designs using quantum physics.

This is of course misleading since engineers designing cell phones don't actually use quantum mechanics as far as I know, but of course the results of physicists studying semiconductors has trickled down to the practical application level enabling the same type of predictions that makes normal physics so useful. Lasers might be another good example technology. Its all about predicting the future for physical systems.

22. Aug 10, 2014

### RonL

Guess I'm in a tight spot here:uhh: what I think I'm getting at....what point does a quantum photon and electron become visible to the eye ? (only in quantity ) same with atoms.
Could I as an individual do what is necessary to expose a quark ? I'm pretty sure not.

I was thinking of interaction more in a single person setting.

Not sure if this will help at all.

23. Aug 10, 2014

### phinds

OK, that makes more sense to me than your original statement, although I'd still contend that the interaction IS "physical", but yeah, it's not a macro-level thing for individual particles.

24. Aug 11, 2014

### Barry911

quantum objects require the "R" process (irreversible and information destroying). You can observe
the effect of single quantum interactions as in the two slit experiment as each point on the screen
represents a photon, electron whatever as a "particle" interaction.
Why is it that modern physicists can glibly talk about the wave function of the universe!?? while
dismissing environmental decoherence?

25. Aug 13, 2014

### Barry911

Shouldn't all correlation events be treated as QM processes?
I find it interesting that the original description of a correlation event was in fact classical i.e.- "a collision
of two electrons" The combined momenta is knowable so the "rebounding" particles are then assumed to
have equal momenta?? I would guess that know that the momentum and energy can be conserved by
two exiting particles with different momenta and position variables. The requisite symmetry of classical
collisions is no where in evidence in QM interactions.