I thought I would break up a document I wrote a few year ago (after reading some books) into some discussions. Be advised that I am not a expert in this field. I hope to learn from these. Here goes:
Light can be shined through a small hole in an opaque plate, then continuing on, through two other small holes in another plate. After the two holes can be placed a photographic plate or some kind of viewing screen. What happens is that the light from the two holes interferes with each other to form a pattern of light and dark bands on the screen. This, they say, is proof positive that light travels as a wave. It is compared to ripples in a pond when pebbles are dropped in. The ripples add up in places causing the light bands, and cancel out in other places causing the dark bands. This is not too mysterious because there is plenty of light here, enough to go through both holes at once. Physicists can now send a single photon of light through the experiment at a time (it requires great skill and sophisticated apparatus).
Now imagine that the screen is a photographic plate which records the arrival of each photon as a small white dot. You see exactly what you would expect – a single photon leaves the light source, and makes a single white dot on the plate. But as hundreds, then thousands, and then millions of photons pass through, a pattern develops – the same interference pattern as before! Now we have two mysteries – how did each photon go through both holes at once and interfere with themselves, and how did they know where to place themselves in the overall pattern?
Is this weird behavior just a property of photons which have no mass and travel at the speed of light? No, electrons also behave this way, and they have mass, and travel much more slowly. Their speed depends on the forces acting upon them. As Ralph Baierlein has put it in his book Newton to Einstein, "Light travels as a wave but departs and arrives as a particle." The same could be said about electrons. The electron experiment was carried out in 1987 by a Japanese team from Hitachi Research Labs and Gakushuin University in Tokyo.
Not only photons and electrons have the wave-particle duality. Atoms do too. The atom experiment is described in Hans von Baeyer's book Taming the Atom. An atom of carbon has 22 million times as much mass as an electron. Researchers at the National Institute of Standards and Technology in Boulder Colorado have used two atoms (particles) in place of the holes, bouncing light off of them to produce the interference patterns.
How do physicists explain this behavior? The standard interpretation (since about 1930) is known as the Copenhagen interpretation because it was largely developed by the Danish physicist Niels Bohr, who worked in Copenhagen. The German physicists Werner Heisenberg and Max Born made major contributions. The key concept is called the 'collapse of the wave function.' They said that it was the act of observing the wave that made it collapse to become a particle. Another concept is that the wave going through the holes is actually a 'wave of probability', not a material wave at all. On this picture an electron (or atom) that is not being observed literally does not exist in the form of a particle at all. There is only the probability that it exists in one place, and another probability that it exists at another place. The Heisenberg uncertainty principle states that 'it is impossible to measure the position and the momentum of a quantum object at the same time.' The interpretation says that these properties do not even exist unless they are observed.
Does this explanation leave you cold? It does me! I will tell you of a book with a better explanation in Part 5.
Quantum Mysteries Part 1: Wave-Particle Duality
Re: Quantum Mysteries Part 1: Wave-Particle Duality
