Quantum physics has turned out to be extremely accurate in accounting for the properties of matter, and in that sense it is "correct." However, the conceptual foundations of the quantum theory are still the subject of discussion and research. The phenomena are so different from what we are used to in the macroscopic world and in "common sense" that it makes us wonder that a deeper layer of reality may be reflected in quantum physics. One of the most influential thinkers of the philosophical aspects of quantum mechanics has been Niels Bohr.

A free particle which moves with a constant, exactly known velocity was a basic entity for the old physics. But then Heisenberg's Uncertainty Principle tells that we do not know anything about its position; it is anywhere and nowhere in the universe! A classical particle simply cannot live in the quantum realm. Similarly, the familiar concept of an orbit becomes dubious.

Consider an electron which has left point A and is later observed in point B (Fig. 17.6). Laplace, the advocate of Newtonian mechanics, would calculate an orbit between the two points. He could tell you exactly where in the orbit the electron has been at every instant of the journey, and what the speed of the electron was. The Uncertainty Principle prevents this kind of continuous description of the journey. The electron has been observed in points A and B, but we really do not know where it has been at intermediate times. The best we can do is to make probability calculations of any electron orbit between the two points.

If the electron has no definite orbit, how does it know where it is going? We may say that the electron tries all possible routes all at once. Every route is represented by an electron wave. When the waves from all routes are combined, at most points the waves cancel each other. Only in some points the waves interfere constructively and a high probability of finding the electron remains; point B is just this kind of a point. So what was the real route from A to B then? All routes, or no route at all, as you like it. The concept of an orbit has lost its significance. When we discuss

Fig. 17.6 The travel of a particle from point A to point B. To find the shortest path the particle tests all possible tracks. The waves related to the particle destructively interfere with each other everywhere except on the straight (dashed) line connecting A and B. In quantum theory, the particle may be found with greater probability, but not necessarily, on this line

Fig. 17.6 The travel of a particle from point A to point B. To find the shortest path the particle tests all possible tracks. The waves related to the particle destructively interfere with each other everywhere except on the straight (dashed) line connecting A and B. In quantum theory, the particle may be found with greater probability, but not necessarily, on this line heavier bodies, then we approach the classical orbit. Then the interference pattern from all orbits produces a high probability narrow line connecting points A and B. In everyday phenomena we may safely apply Laplace's concepts.

What happens to Laplace's clockwork universe which progresses in a fully predictable way once set in motion? The Uncertainty Principle destroys the clockwork even before you can set it in motion. Laplace's assumption, "if the positions and speeds of all bodies were known at an initial moment of time," cannot be realized since there is fuzziness both in the position and the speed of the body; and even if one of them were measured momentarily, the other would remain undetermined. The accidental materialization of a particle even beyond an impenetrable "wall," as in tunneling, makes the prediction of the future impossible.

This may seem difficult to accept, and for many "old guard" physicists it was impossible. Even though they used the mathematical methods of quantum physics, these physicists could not accept their philosophical consequences. This is somewhat similar to the initial period after Copernicus' time when his methods of calculation were widely used while his Sun-centered system was not accepted.

Perhaps the foremost doubter of the interpretations of quantum mechanics was Albert Einstein who said: "God does not play dice." To disprove the philosophical foundations of quantum physics, he developed thought experiments on how one would get around the Uncertainty Principle. For most such arguments, Bohr and other developers of quantum philosophy had a ready answer. However, there was one experiment which had to be carried out before we knew who was right and who wrong. This experiment was proposed by Einstein together with his colleagues Boris Podolsky and Nathan Rosen.

The idea of Einstein, Podolsky, and Rosen was substantially as follows (presented somewhat differently by them): Let two particles collide and then separate from each other. Because of the collision, both the positions and the speeds of the two particles become interdependent. If we measure the speed of particle 1, then the speed of particle 2 is easily calculable without a measurement. On the other hand, by measuring at some other (later) time the position of particle 1, the position of particle 2 is determined through calculations. This would indicate that particle 2 has a well-defined speed and well-defined position at every moment of time since the collision. This is an apparent conflict with the Uncertainty Principle. Einstein, Podolsky, and Rosen used this example to claim that the system of quantum mechanics is incomplete. However, Niels Bohr argued that even though the position of particle 1 can be measured, the simultaneous measurement of its speed was not possible due to the inherent disturbance of the measuring process on the speed of the particle. Neither could one then calculate the speed of particle 2 with certainty; the Uncertainty Principle would apply also to particle 2.

In 1964 the Irish physicist John Bell (1928-1990) transformed the described thought experiment to a form where it could be tested in reality. In 1982 Alain Aspect carried out the experiment in Paris. It showed that Einstein and his associates had been wrong. You cannot fool particle 2: it "knows" about the measurement carried out on particle 1, even when there is no chance of transmitting information between them even with the speed of light. The two particles are really part of the same system.

Thus it has been shown that the Uncertainty Principle is a fundamental principle of Nature, and you cannot get around it. It is also exciting that one may apply it to situations which would be hard to understand without this principle. An example is the vacuum.

What is a vacuum? Take away all matter, radiation, and force fields from space; what is left over could be called a vacuum. Boring? On the contrary, vacuum is full of happenings. Heisenberg tells you that the energy involved in some "happening" is the more uncertain, the shorter time the phenomenon lasts. Even though the average energy in vacuum may be zero, over short intervals of time the Uncertainty Principle

Fig. 17.7 Particle-antiparticle pairs are born and annihilated even in the vacuum of space

Fig. 17.7 Particle-antiparticle pairs are born and annihilated even in the vacuum of space

allows particles to be born out of nothing and to disappear into nothing. Particles like that are said to live on "Heisenberg loan."

In this way a vacuum is automatically filled with particles. Although every particle lives only a minuscule length of time, new ones are constantly born to replace the ones which have disappeared. The more permanent ordinary particles swim through this "sea" of particles (Fig. 17.7). Later we will find out that a vacuum can have even stranger properties that rule the evolution of the whole universe.

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