|
In 1804, a physician named Thomas Young did a memorable experiment. He aimed a beam of light at a screen with two very fine parallel slits just a tiny distance apart. On the other side of the screen he placed a second movable screen. When it was located very near the first screen, there were just two "bars" of light. But as this second screen was moved further away, a series of bright and dark parallel bars appeared. If either slit was covered, there was only one bright bar. With both slits open, the pattern of bright and dark bars returned.
Most of us have seen what happens if a stone is thrown into a still pond. Where the stone lands in the water, a series of circular waves move away towards the edges of the pond. Throw in another stone so that the wave patterns will meet and we see that where wave crest meets wave crest, they add together to make a bigger wave but if a crest meets a trough, the two cancel and the wave disappears. This phenomenon gets the name "interference."
The interference patterns that Young observed provided the evidence that, for the next 100 years, led physicists to accept that light was a wave. But then Einstein shattered this conclusion when he was able to interpret the so-called photoelectric effect (used with solar panels to get electricity from the sun) as evidence for a particle nature of light.
In 1927, Clinton Davisson at Bell Laboratories did a similar experiment to Young but used electrons. He fired an electron beam at a screen with two slits behind which he had columns of tiny Geiger tubes, each of which recorded a hit if an electron reached it. With one of the slits closed off, the electrons going through the single slit scattered sufficiently to cause every Geiger tube to, sooner or later, register a hit. But when he had both slits open, some columns of Geiger tubes registered no hits at all. This is just like the light and dark bands observed by Young in his experiment using a light beam.
Coffee break?
Davisson could do more. He could reduce the number of electrons being fired at the screen to about one per minute. Left for long enough, he obtained the same result as previously. With a single slit open, all Geiger tubes eventually fired. With both slits open, some columns of tubes never fired.
How did electrons passing through Davisson's screen at the rate of one per minute manage to make all the Geiger tubes fire if only one slit is open but to prevent whole columns of tubes from firing when both slits are open? Does a single electron go through both of the slits to create an interference pattern? Note too that it is the experimenter who makes a conscious decision to open or close a slit and that his conscious decision affects whether the electron performs purely like a particle or as a wave.
Unbelievable!
It gets worse. What would happen if we delayed our choice to open or close one of the slits until after an electron (or photon) had passed through but before it reached the detector set up?
Many of this "delayed choice" type of experiment has been performed, often using a split-beam approach. The eerie result is that individual electrons or photons appear to travel via both pathways but register either as a wave or a particle depending only on the decision of the observer--what he/she wants to observe, a wave or a particle.
How can such a crazy thing occur. And how can the mind of the observer causally determine whether a electron or a photon shall act as a wave or as a particle?
Einstein was one of those who could not cope with mental gymnastics required from the disciples of quantum physics. Yet he was the one who told us that distances get shorter, clocks run slower, weights gets heavier as objects go faster and faster. He also did away with the attractive force of gravity and told us we are stuck to the earth because space is curved! It seems that he was right on all counts but he still could not cope with the indeterminacy implied by quantum theory. Hence his famous exclamation that God does not play dice with us.
Einstein also realized that the determinate world he advocated left no room for free will. In answer to a question, he was forced to agree that his deterministic philosophy meant that criminals could not be held responsible for their actions. They do what they do because they cannot do otherwise.
Locality
Einstein was undoubtedly among the top half dozen of the most creative geniuses of all time. Another of his discoveries was that no physical object can exceed the speed of light. This speed limit also means that all influences between material objects happening in space-time must be local. That is they must travel through space one bit at a time with a finite velocity--which gives us the principle physicists call "locality."
|
|
