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pattern because your eyes are not very sensitive light detectors, but you can still record it with photographic film or a digital camera. Once recorded, the pattern is the same. Turn the intensity down a factor of 3000 to a trillion photons a second, and the pattern is unchanged. In the description in which half of the photons go into each leg of the apparatus, half a trillion photons per second go into each leg of the interferometer. Turn down the intensity to a billion photons per second, and the pattern is the same. Further reduce the intensity to a million photons per second, and there is still no change. Here is where the fallacy in the description becomes obvious. Turn down the intensity until there is only one photon per second entering the apparatus. Again, the pattern is unchanged. At one photon per second, it will take a long time to record enough signal to see the interference pattern, but if you wait long enough the pattern is the same.

When only one photon per second is entering the interferometer, there is only one photon at a time in the apparatus. A photon will take on the order of one hundred millionth of a second (10 -8 s) to traverse the interferometer. With one photon per second, there is virtually no chance that there is more than one photon at a time in the instrument, yet the interference pattern, once recorded, is the same. But the modified classical description of the interference effect in terms of photons said that half of the photons go into leg 1 and half of the photons go into leg 2. The photons in leg 1 interfere with the photons in leg 2 to produce the interference pattern. If there is only one photon in the apparatus at a time, there is no other photon for it to interfere with. The “half of the photons go into each leg of the apparatus model” predicts that the interference pattern should vanish at low enough intensity. The interference pattern does not disappear at low intensity. The model is wrong!

A NEW DESCRIPTION OF PHOTONS IN THE INTERFEROMETER

Here is where the complete change in thinking is required that will bring us back to Schrödinger’s Cats. How is it possible to have an interference pattern when only one photon enters the interferometer at a time? Our understanding of this problem and the nature of quantum mechanics in general is based on the conceptual interpretation of the mathematical formalism that is strongly associated with the work of Max Born (1882-1970). Born won the Nobel Prize in Physics in 1954 “for his fundamental research in quantum mechanics, especially for his statistical interpretation of the wavefunction.” This interpretation is frequently referred to as the Copenhagen interpretation.

The correct description of the interferometer experiment is that each photon goes into both legs of the interferometer. This is the big leap. A single photon encounters the 50% beam splitter. That means there is a 50% chance that the photon will be reflected and go into leg 1 of the interferometer (see Figure 5.1) and a 50% chance it will go into leg 2. Classically one would say the photon must go one way or the other, that is, it either goes into leg 1 or it goes into leg 2. This is not correct. When the photon encounters the beam splitter, its state is changed. If a photon is in fact moving in leg 1, call this state of motion “translation state 1,” abbreviated T1. If a photon is moving in leg 2, call this state of motion “translation state 2,” abbreviated T2. After a photon interacts with the beam splitter, it is not in T1 or T2. The state of the system after the beam splitter is referred to as a superposition state. It is an equal mixture of T1 and T2. In some sense, the photon is simultaneously in both T1 and T2. This sounds really strange. The single photon is in two regions of space simultaneously. It is in a superposition translation state T = T1 + T2. It is in a state that is an equal mixture of T1 and T2.

The photon is in the superposition translation state T = T1 + T2 because this is what is known about it. It has a 50% chance of being in leg 1 (T1) and a 50%