I have just finished reading the biography of P.A.M Dirac by Graham Farmelo and it brought back to me the power and strangeness of Quantum Mechanics. Dirac was the first person who defined a logical basis for Quantum Mechanics in his classic book “Principles of Quantum Mechanics”. This book also inspired me as a young high energy physicist and cleared the fog. I was always unhappy with the historic descriptive books writting about quasi-non scientific concepts like “complementarity” and the like which seemed semi-mystical. Dirac was one of the physicists acknowledged as the greats of Quantum Mechanics, others among them being Heisenberg and Schroedinger. His greatest achievement was unifying relativity and quantum mechanics in his famous Dirac Equation of the electron and thereby predicting anti-matter, electron spin and founding  Quantum field theory. He was 27 at the time. Richard Feynman once said that anyone who claims they intuitively understands Quantum Mechanics is lying, however the mathematical theory is precise and exact. This underlying disconnect between theory and “common sense” is still even true today because atomic phenomena will allways seem counter intuitive on the macroscopic level.

One way of seeing the reality of Quantum mechanics is thinking about why solid objects like tables don’t just collapse. After all atoms are just electrons orbiting a few protons and neutrons in the nucleus which classically would be highly unstable. The electrons would quickly collapse into the nucleus. Dirac, Pauli and Fermi showed that “fermions” (spin 1/2) - matter particles can only occupy just one state (orbit in the atom) as a consequence of  quantum mechanics. No matter how you compress an atom on the macroscopic scale it remains solid for this reason. It also explains how covalent bonds arise in chemistry and “explains” how the periodic table forms as orbit shells of  atoms.

At the same time, I have been reading a book by Lewis E. Little who claims to “explain” quantum mechanics. His explanation to some of the strange effects in quantum mechanics seems to be that particles react and follow waves that travel  backwards to them in time to then determine their consequent behaviour . One problem with his argument is that he then spoils it by accusing all physicists as being stupid because they do not accept this idea. One “advantage” of his model is that it avoids particles behaving like waves unless we try to observer them as particles. The big mistake of quantum physicists (he claims) is that they have assumed that Schroedinger(Dirac’s) equation describe waves following a particle history whereas he claims that  the waves really travel from any detector to the source. He hates the idea of an observation changing the state as described by Heisenberg’s uncertainty principle so reverses it to propose the observation causes the effect earlier, almost inverse causality. However for me his model seems even more non intutitive because never once does he explain how a detector would emit waves to trigger the emmission of a particle it would later observe, nor does he propose what these waves physically are. His model would imply that every target was emitting waves across the universe to an infinite set of sources so that they could then follow these waves later back to the detctor. In other words all 14 billion eyes of humans emit waves to the sun so that we may later observer the photons consequently emitted in response.

Quantum mechanics describes how a physical state evolves with time and the mathematics was originally  set as a wave equation. The waves associated with this state ( e.g. a particle) must be interpreted mathematically. The evolution of the wave can (in principal) be calculated exactly as it interacts with a force field. The problem is that the wave function turns out to be  a complex number which can subsequently only be measured by observing the particle. Such an observation then “collapses” the wave to some real “measurement value”. The procedure to do this is well defined and Quantum theory calculates the probability of such a measurement giving a certain value. When dealing with say one million particles the theory predicts exactly the outcome - say 66.7 % go up and 33.3% go down. Most of  the philosophical  and reasoning difficulties arise when dealing with just one or two particles. In this case quantum theory cannot predict the outcome of a measurement with certainty. It can only predict the probability of what the result will be.

All the confusion arises from this “weird” effect because we are not used to a single event having two or more possible outcomes. When you hit a tennis ball against a wall you would be very surprised if 1 time in 20 it went through the wall - but this does happens with electrons. An extreme example is when two particles are created and move in different directions. These particles must conserve angular momentum so if they have spin 1/2 a spin measurement on one of them must yield either +1/2 or -1/2. Suppose these electrons travel to apart by 100 light years and then someone captures one of them and measures the spin. If they measure +1/2 then they know 100% for sure that the other electron 100 light years away is -1/2 and likewise if they measure (50% probablity) -1/2 then the opposite is true. Does this mean that information is flowing faster than the speed of light ?

In fact such “mysteries” already existed in classical physics - the best example I know is that of angular momentum. If I take a bucket of water and spin it then the water moves to the sides of the bucket because ift feels a centrifugal force. But think what this really means - Newton’s 2nd  law of motion states that a force is proportional to rate of change of momentum,  which is also just a mathematical formula. Momentum is a vector so a fixed rotational velocity is continuously changing momentum because the direction of motion is rotating at a constant angle per second. But what is this direction relevative to ? In an empty universe angular velcocity would have  no meaning,  so rotation can only be meaningful relative to fixed stars which are part of and  define space itself. So how then does the universe tell the bucket that it is rotating? How do these galaxies 1000’s of light years away effect a rotating bucket instantly  ?