Richard FeynmanRichard Feynman won a Nobel Prize for his work on quantum electrodynamics (QED) but he also developed several simple yet insightful explanations of the fundamental properties of quantum mechanics. In his famous Lectures on Physics, some of the lectures repeated in the 1967 Messenger Lectures at Cornell and published as The Character of Physical Law, Feynman famously said that "nobody understands quantum mechanics" and that the two-slit experiment contains the "one mystery" of quantum mechanics.
I will take just this one experiment, which has been designed to contain all of the mystery of quantum mechanics, to put you up against the paradoxes and mysteries and peculiarities of nature one hundred per cent. Any other situation in quantum mechanics, it turns out, can always be explained by saying, 'You remember the case of the experiment with the two holes? It's the same thing'. I am going to tell you about the experiment with the two holes. It does contain the general mystery; I am avoiding nothing; I am baring nature in her most elegant and difficult form.In some of the more accessible material from Lectures on Physics re-published as Six Easy Pieces, Feynman argued that the most important scientific knowledge - from physics to biology - is the simple fact that all things are made of atoms.
If, in some cataclysm, all of scientific knowledge were to be destroyed, and only one sentence passed on to the next generations of creatures, what statement would contain the most information in the fewest words? I believe it is the atomic hypothesis (or the atomic fact, or whatever you wish to call it) that all things are made of atoms—little particles that move around in perpetual motion, attracting each other when they are a little distance apart, but repelling upon being squeezed into one another. In that one sentence, you will see, there is an enormous amount of information about the world, if just a little imagination and thinking are applied...Feynman is quite right that everything is made up of discrete particles. We might rewrite his advice to the future this way:
The universe consists of discrete, discontinuous, and in some sense "digital," particles. There is no "classical" world, only a quantum world. The "classical" world emerges from the quantum world when a large enough number of particles get together. The continuous space (and time) in which we locate the particles is but a mathematical construct that allows us to describe the world.There are no continuous "fields" in which particles of matter (electrons, atoms, etc.) are thought to be singularities. The continuous, causal "forces" like gravity that we postulate are useful fictions. They are only statistical averages over other types of particles (photons, bosons, gravitons) that look continuous when very many such particles are present. At the microscopic level, quantum events are discontinuous and acausal. The analytic integral and differential equations that we assume deterministically govern the motions of material particles are idealizations only accurate for very large bodies.
The Path Integral Formulation of Quantum MechanicsIn 1948 Feynman developed his "sum over paths" approach to quantum mechanics. It was built on a 1933 article by P. A. M. Dirac to formulate quantum mechanics using a Lagrangian function rather than the standard Hamiltonian, and to use a variational method to solve for the least action. The idea of a single path for a quantum system (for example, the path of an electron or photon in the two-slit experiment) is replaced with a sum over an infinity of quantum-mechanically possible paths to compute a probability amplitude. It corresponds to the wave picture of spherical waves going in all directions that was critically questioned by Albert Einstein in his 1905 and 1909 papers on the light-quantum hypothesis and wave particle duality.
The Messenger Lectures at CornellLecture 1 - The Law of Gravitation (video only) Lecture 2 - The Relation of Mathematics to Physics (video only) Lecture 3 - The Great Conservation Principles (video only) Lecture 4 - Symmetry in Physical Law (video only) Lecture 5 - The Distinction of Past and Future (text and video) (video only) Lecture 6 - Probability and Uncertainty (text and video) (video only) Lecture 7 - Seeking New Laws (video only) In his sixth Messenger lecture, Feynman imagined a scenario like that Arthur Holly Compton used as a model for free will based on quantum uncertainty.
...we could cook up — we'd better not, but we could — a scheme by which we set up a photo cell, and one electron to go through, and if we see it behind hole No. 1 we set off the atomic bomb and start World War III, whereas if we see it behind hole No. 2 we make peace feelers and delay the war a little longer.Isn't it strange and somewhat diabolical the kind of examples some physicists come up with? Feynman on the Two-slit Experiment
Probability and Uncertainty - the Quantum Mechanical View of Nature Feynman on Irreversibility
The Distinction of Past and Future "No One Understands Quantum Mechanics" The Deep Mystery of Quantum Mechanics - Fundamentally Probabilistic