• Choose a category
• All
• Classic-Fiction

By Root 682 0

electrons, and the rest are not particles of the classical kind, visualized as pointlike or as solid spheres. Instead, what we call a particle corresponds to a wave function: an extended object, spread-out and diffuse due to its wavelike character. It does not have a distinct surface as a ball does; outside a central region it slowly fades away. Just as a water wave approaches lazily without allowing one to say when exactly it has passed a swimmer, the wave functions of quantum theory do not have sharply defined boundaries. Even though their tails are usually of tiny height, they can extend to regions far away from the center, the place best seen being analogous to the “position” of a classical particle. This is the origin of many phenomena that at first sight may appear impossible but can in fact be observed.

What may also seem counterintuitive is the interrelation of different objects described by wave functions. While classical particles can be put at separate places like billiard balls, and hit at different speeds to watch their motion and collisions, a single wave function already occupies all of space. Even though it may have nearly vanishing height far from a center, it does not have to be exactly zero. When a second wave function is positioned anywhere in space, its background differs from what it would be in the absence of the first wave. This is the case even if the respective central regions are far apart, which would allow us to ignore all kinds of attractive or repulsive forces. Just the presence of the first wave function, anywhere in the universe, has a certain influence on another wave function.

It is truly bizarre that completely different states of a single object can be superposed and coexist. Erwin Schrödinger, one of the prime movers in the early days of quantum mechanics, has described this pointedly in a thought experiment now called “Schrödinger’s Cat.” A cat is locked away in a cell enclosed by nontransparent walls, also containing a potentially deadly mechanism: a radioactive substance whose radiation, via a detector, triggers a hammer to break a flask containing poisonous gas. A substance of weak radioactivity has been chosen, which will only rarely trigger the detector and thus the murderous device.

If and when radiation will trigger the detector is, like the whole atomic world, governed by quantum mechanics. Macroscopic objects such as the cat must, in the end, obey the same laws of quantum mechanics, but this does not often make itself noticeable on the large scales we are used to in everyday life. That is why classical physics was able to describe everything so well until precise measurements revealed the true finesses of the quantum world. But here, via the hammer and the poison, the quantum moods of the radioactive substance are transferred directly to the macroscopic world of the cat, now helplessly delivered to the laws of quantum mechanics. There is no law for the exact time of the substance’s decay; there is a law only for the decay probability. According to quantum mechanics, this means that the atoms of the substance are not in one fixed state, either already decayed or not yet so, but in both of them! The atoms are in a superposition of both possibilities, and the poor cat, too, is hanging on in a zombic combination of life and death. When and only when a measurement is performed, for instance by peeking in to check on the fate of the cat, is a definitive state realized where the cat may or may not succumb. In the language of quantum mechanics, the wave function then collapses from its superposition into a state with a fixed measurement outcome.

The end result, however, is undetermined. Once the wave function has collapsed, to be sure, a second check would not lead to a different state; if the cat is found dead, it cannot be revived simply by looking away. After the measurement, the result is definitive, but before the first measurement is done, there can be only likelihood statements about the result. Performing the experiment with a hundred cats, all in the hands of their own