Once Before Time - Martin Bojowald [33]
Here, too, the image of a lake may be helpful, but now of a lake in the rain. Large waves are still visible and can be tracked in their propagation for long times; but every single raindrop contributes a new little wave. In this sea of irregular perturbations, details of the entangled interrelations drown. Similarly, the world on a large scale looks classical because fluctuations and entanglement are easily destroyed. On the small scale, however—in microscopic physics or in precisely constructed experiments—perturbations can often be subdued long enough to let the unfamiliar world of quantum physics be revealed.
Thus Schrödinger’s cat is at last delivered from uncertainty: In any life-form, so many processes take place that quantum mechanical superpositions are always perturbed. Even if no measurement is performed on the apparatus or the cat, the superposition of decay and nondecay of the radioactive substance—the life and death of the cat—is rapidly transformed to a definite state. For some time, the decay probability is very small, owing to the weakness of the radioactivity, and the cat remains alive. Strictly speaking, the atoms in the substance are indeed in a superposition, but the contribution from a state of decay is very small. Perturbations of the system quickly and most likely turn the superposition into a definite nondecay state. But at some time, the probability of decay becomes large enough to make the superposition register as a definite decay, and the cat must suffer its fate.
BLACK BODY RADIATION:
THE SIMPLICITY OF WEARING BLACK
In addition to the emission and absorption spectra of atoms, the measurement of heat radiation in a closed dark box at a uniform temperature, the so-called black body, was an important subject of experiments and theoretical modeling in the early years of quantum theory. What we perceive as heat are tiny vibrations of atoms and molecules. These constituents, being electrically charged, radiate energy when vibrating: heat radiation. Radiation in a closed box can be controlled very well, and precise data were collected in the course of the nineteenth century.
Initially, these observations were consistent with classical ideas about radiation. In particular, experimenters had measured how the radiation energy is distributed over different frequencies. With increasing frequency, energy should grow, since atoms and molecules emitting radiation vibrate more strongly. If one has seen a certain amount of energy realized for one frequency, there must be more energy in every higher frequency. Indeed, this increase was measured—but it also led to a serious problem in understanding: According to classical calculations, the energy should grow without limit at increasingly high frequencies; at sufficiently large frequencies, the energy would exceed any conceivable upper bound. In such a way, the total energy, summed over all frequencies, becomes infinite. Here we encounter a problem in some ways resembling the infinite increase of energy at cosmological singularities or in the interiors of black holes. Black body radiation and its quantum theory are much better understood and much more scrutinized experimentally than quantum gravity; it is thus instructive to see how the problem of infinite energies is resolved here.
As discovered by Max Planck even before quantum mechanics was developed, the solution to the problem is the discrete nature of energy emission. The atoms and molecules of the black body walls have discrete emission spectra and cannot emit arbitrary energy packets. Taking this into account—something Planck did intuitively even without