Alice in Quantumland: An Allegory of Quantum Physics
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Faster and faster they went, guided around and around by magnetic fields. After some time, Alice began to notice that their speed was no longer increasing much, though she could still feel an acceleration. She asked one of the protons about this and was told that they were now going almost as fast as photons did and nothing could go much faster than that, but that their kinetic energy was still rising. This seemed odd to Alice and she was about to argue, when there was a sudden wrench and she felt herself flung out of the ring together with the protons.
Through the air she rushed at what now seemed an incredible velocity. Looking ahead, she was terrified to see a wall directly in front of her and to realize that she and the protons were headed straight toward it! She tensed for the collision as the wall rushed closer, but to her amazement the wall stopped her no more than would a fog or a dream.
She looked around her and saw that, although the wall had had little effect on her, the reverse was far from true. She passed an atom some way off and it burst asunder, the electrons spilling out and the nucleus cast free to drift on its own. All around her she trailed a deadly train of virtual photons. These tore at the atoms she passed, which seemed as gossamer, ripped apart by the distant effect of her passage. She came close to a nucleus and it too was shattered, the protons and neutrons scattered in every direction. In dismay she recollected the Cosmic Ray-der whom she had seen from Castle Rutherford and who had so effortlessly destroyed a nuclear castle. Now she was horrified to realize that she had become as he, leaving a wide swath of destruction among the atoms and nuclei which she passed!
She saw a neutron straight ahead for just a moment before she plowed into it. Briefly she glimpsed its three quarks, who were thrown into a panic by her passage. They were not cast individually out of the neutron, because they were too firmly bound to one another, but their chains stretched and broke, stretched and broke, with the creation of a host of quark-antiquark pairs. Where previously the neutron had been standing was now a great jet of mesons carried forward by the wake of Alice's own enormous momentum.
Alice hid her eyes to blot out the image of the chaos about her, lest she should see some even more violent catastrophe. She had a brief sensation of falling and felt a slight bump.
Alice quickly opened her eyes, to find that she had fallen off the couch in her own front room and was lying on the floor. She got up quickly and looked around. The sun was shining cheerfully in through the window and the rain had cleared away. She turned to look at the television, which was still operating. The screen showed a group of rather serious folk sitting around a studio, arranged carefully on either side of a commentator, who informed Alice that they were about to have a studio discussion on the future of scientific planning in the country.
"Boring," said Alice. She switched off the television firmly and went outside into the sunshine.
Notes
1. There have been many attempts to set up an experiment which would contradict the more extreme predictions of quantum theory, but so far quantum mechanics has always been vindicated.
An example is the Aspect experiment to investigate a form of the Einstein-Podolsky-Rosen (EPR) paradox. There are various forms of this paradox, which involves measurements of particle spin, the strange quantized rotation possessed by elementary particles such as electrons and also photons. The paradox treats the case of a system which has no spin but which emits two particles that do have spin and which travel directly away from one another. The restrictions of quantum theory tell us that a measure of the spin of either particle can give only one of two values: spin-up or spin-down. If the original system has no spin, then the spins of the two particles must compensate; that is, if one is spin-up, the other must be spin-down, so that the sum of the two gives a total spin of zero. If no measurement is made of the particle spins, then quantum mechanics says that they will be in a superposition of spin-up and spin-down states. When a measurement is made on the spin of one, then at that point its spin will be definite, either up or down. But at the same time, the spin of the other particle becomes definite also, as the two must be opposite. This would be true no matter how far apart the particles have moved since they separated. This is in essence the EPR paradox.
2. It would seem reasonable to explain the EPR paradox by saying that in some way the spins were predetermined from the start: that, in some way, the particles knew which would be spin-up and which spin-down when they set out. In that case it would not matter how far they had traveled as they would bring the information with them. The limits of the information which it would be possible for the particles to fix in advance are considered in Bell's theorem, which treats what happens if the spin measurements are not made along one predetermined direction, but at a selection of different angles for the two particles. The calculation is rather subtle, but the outcome is that, in some cases, quantum mechanics predicts a greater correlation between the measurements on the two particles than could be arranged by any advance information which could be sent with the particles without prior knowledge of the directions along which the spins would be measured. Alain Aspect in Paris has measured this effect and found, as usual, that quantum mechanics appears to be correct. It seems to involve some sort of information which travels more quickly than the speed of light.
The Aspect result does not directly contradict the normal understanding of Einstein's special theory of relativity. This says that no information, no message, may travel more quickly than the speed of light. The effect considered in the EPR paradox cannot be used to send messages. If you could decide whether you would measure the particle spin as up or down, then the opposite spin of the other particle would convey information in a sort of Morse code, but you cannot do this. You have no control whatsoever of the result of a measurement on a superposition of quantum states; the result is completely random and no signal can be forced onto it.
Richard Feynman, QED: The Strange Theory of Light and Matter, Penguin, New York
Table of Contents
Into Quantumland
The Heisenberg Bank
The Mechanic's Institute
The Copenhagen School
The Fermi-Bose Academy
Virtual Reality
Atoms in the Void
Castle Rutherford
The Particle MASSquerade
The Experimental Physics Phun Phair
See end-of-chapter note 1
See end-of-chapter note 2
See end-of-chapter note 3
p. 76
See end-of-chapter note 4
See end-of-chapter note 5
See end-of-chapter note 6
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