Friday, June 5, 2009

Feynman and what comes next...

As you may be aware of, I am a Feynman aficionado:
  1. My scientific motto is a (not so famous) quote of Feynman: "All we do is draw little arrows on a piece of paper - that's all!"
  2. My Twitter visual profile is dedicated to him.
  3. I love viewing videos of his lectures or his interviews.
  4. I regularly go across his written works.
  5. I've spotted errata in his Physics Lectures, volume III (Quantum mechanics), most of them typo, but some substantial errors.

Curiously, I discovered him relatively late. When I followed quantum mechanics courses at university (somewhere between 1985 and 1989), my courses didn't refer to his lectures. I consider that as a missing. It was only after I took time to dig deeper into the quantum foundations (after 1996) that I came across his works. Reading his works was so enlightening for my comprehension of the fundamental laws of nature, that there are pieces that I could read tens of times and each time I would learn new things. Or better said: approach known things from a new point of view.

Feynman is deservedly one of the most quoted people (at the Selected Pages section of Wikiquote, he figures along with people like Aristotle, Buddha, Confucius, Einstein, Jesus or Shakespeare...). His words are inspiring and often explain physical truths in plain language, comprehensible to the layman. As for all quotes, there is a caveat: they must not be seen as an absolute truth. Or as Feynman stated it himself: Learn from science that you must doubt the experts.

Very early I was skeptic about one of Feynman's most famous quotes: nobody understands quantum mechanics. This is often requoted in a more or less transformed way (for example Dawkins' version: "If you think you understand quantum mechanics, you don't understand quantum mechanics"). Does this quote have a general and definitive value of truth? Or was it just that Feynman didn't know of anyone who could explain quantum mechanics in an understandable common sense way?

Chapter 1 of Feynman's quantum lectures gives some insight in the reasons of his belief that nobody understands quantum mechanics: "We choose to examine a phenomenon which is impossible, absolutely impossible, to explain in any classical way, and which has in it the heart of quantum mechanics." He goes on to describe the double-slit experiment (with electrons), showing that it is impossible to think of waves alone or of bullets alone (such explanations have been taken over by popular media like that given by "Granddaddy of all Quantum Weirdness"). And Feynman concludes with "No one has found any machinery behind the law. No one can explain any more than we have just explained. No one will give you any deeper representation of the situation." These are terrible sentences when repeated to hundreds of thousands, maybe millions of physics students since 1965. They mark a halt for any further investigation of the subject.

Fortunately, there are inventive unconventional physicists. There were already deeper theoretical representations given by physicists like De Broglie or David Bohm, showing how a particle may be directed by a guiding wave and thus yield all the experimental results of the double slit experiment, but this had never been put to proof during their lifetime with an experimental model.

Today, I think I can safely say that the quote "nobody understands quantum mechanics" is experimentally outdated. Couder and Fort, two French physicists, experimented with bouncing droplets on a liquid subtract and discovered that they exhibited quantum behaviour, without looking for any quantum analogy:
  • droplet travelling in its wave,
  • diffraction and interference patterns of travelling droplets similar to photon and electron diffraction patterns,
  • attraction and repulsion of droplets embedded in their waves,
  • symmetric and anti-symmetric orbital motion of droplets.
Visuals presented by Couder are breathtaking. Even if you don't understand french, I highly recommend watching bouncing droplets orbiting around each other (for example at 25:35 of his 2006 presentation).

An upcoming paper of Couder's group in Physical Review Letters even suggests a quantum tunneling analogy with ordinary droplets: "Unpredictable tunneling of a classical wave-particle association", by A. Eddi, E. Fort, F. Moisy, and Y. Couder.

So today, Feynman's defeatist words about nobody understanding quantum mechanics are outdated. Please, experimental physicists, go ahead, be inventive and focus on experiments where ordinary macroscopic individual particles simulate quantum behaviour, polarization, bosonic and fermionic behaviour, inward bound forces, entanglement, quantum erasure, coupling of ordinary particles to their pilot-wave fields (gravitation, electromagnetism). Because all these quantum phenomena may be rationally understood with the help of experimental models. It's just a matter of inventivity. And we "will find someday that, after all, it isn't as horrible as it looks." ~ Feynman's Epilogue to his Lectures on Physics.


  1. In case anyone would like to see the "substantial errors" Mr. Dijksman found in FLP Vol III (mentioned above), here is a copy of his error report with the response it received from The Feynman Lectures website:

  2. Thank you for the link to the report. In fact, the great majority of the errors of the 1965 edition were already notified (mostly in the last chapters of volume III), so I didn't send them in my report. I particularly like the layout of The Feynman Lectures on Physics. Its broad margin is extremely handy. Reading the quantum lectures, I always had a pencil at hand in order to note thoughts, questions, doubts, that I could recheck later on. The layout gave the incentive to read the stuff many times. I've never seen a better layout for any other physics textbook.

    Kind regards,

  3. Hey Arjen, I like what you're doing with this site! I studied physics, but haven't gone through all of Feynman's lectures. Read a few books he wrote- he was an interesting character.

  4. Hi Neal. I didn't go through Feynmans classical physics and electromagnetism lectures. That's still on my to-do-list. You have an interesting site with a broad variety of physics subjects. Do you teach physics?

  5. You bring up DeBroglie and Bohm, but neglect to mention that no hidden variable / pilot wave version of quantum mechanics has been formulated which violates the Bell Inequalities the way the standard formulation of QM does.

  6. Hi Quantum Shinobi. The Bell test incompatibility of Bohmian models is a good point, so much the more because it is Bell's (a first class second generation quantum physicist) birth anniversary today. The point I wanted to address in this post was that double-slit and related interference experiments could be explained ordinarily with Broglian and Bohmian models, or as Bell stated it elegantly: De Broglie showed in detail how the motion of a particle, passing through just one of two holes in screen, could be influenced by waves propagating through both holes. And so influenced that the particle does not go where the waves cancel out, but is attracted to where they cooperate. This idea seems to me so natural and simple, to resolve the wave-particle dilemma in such a clear and ordinary way, that it is a great mystery to me that it was so generally ignored.
    Entanglement and Bell inequality violations is a different (yet related) subject. My opinion is that the assumption of single-valued reality flaws the current Bell tests interpretations (Bohmian and conventional). An assumption of single-valued reality is OK if the fundamental quantum entity is point-like but the assumption breaks down if the particle has some spatial extension. I began to discuss this in §3 of my paper on observation on macroscopic arrows. I'll have to reactivate it in order to handle your question in a blogpost;-)