On November 24, 1803, Thomas Young presented his historic double-slit experiment with light to the Royal Society of London. He said, “The experiments I am about to relate ... may be repeated with great ease, whenever the sun shines.”1 After that, the double-slit experiment has been carried out more precisely with photons, electrons, atoms, and even molecules. In September 2002, the double-slit experiment with single electrons was voted as “the most beautiful experiment in physics” by readers of Physics World.Nowadays many textbooks on quantum mechanics use the double-slit experiment to illustrate the wave- particle duality of light and matter. The importance of the experiment lies in that it demonstrates the central mystery of quantum mechanics in a vivid and elegant way. After we have disclosed the mystery, we will be able to understand quantum mechanics.
The predictions of quantum mechanics agree with the result of the double-slit experiment to astonishing precision. What is the mystery then? The mystery lies in the understanding of the startling and counterintuitive result. What does it imply for the nature of reality? How on earth does a single electron pass through two slits at the same time? Exactly what is an electron? The mystery is not that classical mechanics fails to explain the experiment. This is already according to expectation, as it has been replaced by quantum mechanics. The mystery is that quantum mechanics cannot explain the experiment either. It seems that there exists no imaginable physical picture that can explain exactly what happens in the double-slit experiment of single electrons.
Is Nature really not understandable? Or it is just that we have not found the way to understand her? According to Niels Bohr, one of the founding fathers of quantum mechanics, the task of physics is not to find out how nature is, and the above questions are all pointless. In Bohr’s times, when a student curiously asked how a single electron passes through the two slits, the standard response from a professor would be “shut up and calculate”.
Fortunately, Bohr’s “Copenhagen oracle” has been broken, and the door to quantum reality has already been opened since 1950s. This gradually gave birth to a new research field called quantum foundations. But unfortunately, we have so many possible pictures of quantum reality nowadays, and correspondingly so many explanations of the double-slit experiment. From pilot wave and Brownian motion to many worlds and dynamical collapse, every interpretation claims that it gives the right answer, but none of them has particular experimental support. This may be the main reason why most textbooks never refer to these interpretations of quantum mechanics. Which one is right then? Or none of them is right? Obviously there is only one real picture of quantum reality. Can we find it now?
This lecture, as the first one of Everyone Can Understand Quantum Physics Lectures Series, will show us the way out of the maze of quantum interpretations along a logical way, and in the end the real picture of quantum reality will naturally appear. To our great surprise, quantum mechanics already tells us how a single electron passes through two slits.
Shan Gao
Sydney
January 2011
1 It is worth noting that Young’s original “double slit” experiment was not done with a double slit. In his simpler experiment, light were actually split with what Young described as “a slip of card, about one thirtieth of an inch in breadth (thickness)”, and passed on its both sides.
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