The application of the scientific method. There was a problem, he formulated a hypothesis, tested it against an experiment, and then concluded that he was on to something. After chalking up a few more successes, which we won’t go into here, it appeared that his hypotheses were consistent with all relevant observations of the photoelectric effect.
Nevertheless, the very concept of the photon blatantly contradicted the wave theory of light, which by the early twentieth century had become the widely accepted paradigm. As a result, the initial reaction among Einstein’s peers was not exactly one of enthusiasm. Initially, most wrote it off as a misguided and adventurous (albeit clever) line of thinking. With time, however, more and more careful experiments were conducted, each of which could be explained in terms of photons. The balance eventually tilted in Einstein’s favor.
QUANTUM QUOTE
That he may have sometimes missed the target in his speculations as, for example, in his hypothesis of light quanta (photons), cannot really be held too much against him, for it is not possible to introduce fundamentally new ideas … without occasionally taking a risk.
—excerpt from the Prussian Academy of Sciences recommendation in favor of Albert Einstein’s membership
One of the most important problems to which Einstein applied his photon hypothesis just happens to be one we’ve already discussed: blackbody radiation. It was a natural step to take, given the obvious parallels between the hypotheses of Einstein and Planck. After attacking this problem, Einstein not only demonstrated that Planck’s results were consistent with the photon point of view; he also showed that Planck’s hypotheses were simply a special case of Einstein’s more fundamental concept.
Recall that Planck asserted that the oscillating charges in the blackbody were limited to distinct, equally spaced energy levels. He further surmised that they could emit radiation only by dropping from one energy level to the one below. Though these hypotheses led to the proper conclusions, Planck admitted that he could not explain why this was the case in the first place.
Einstein’s photon picture, however, offered an elegant physical interpretation: when the oscillators drop from one energy level down to the next, they emit … a single photon. From Einstein’s perspective, then, it is not the oscillating charges that are quantized. Instead, the quantized entity is none other than light itself!
QUANTUM LEAP
Albert Einstein published his paper on the photoelectric effect in the German journal Annalen der Physik. The editor who first reviewed it was none other than Max Planck. Planck was impressed by Einstein and became a strong advocate for his relativity theory. For years, however, he failed to accept Einstein’s quantum theory of light, despite the fact that it added further meaning to Planck’s own work.
At this point, you may be wondering why the first experimental evidence of the photon didn’t come about until the turn of the twentieth century. Much like our consideration of a little girl on a backyard swing, the answer is simply a matter of scale.
For example, consider a 60 watt incandescent light bulb, which produces 60 joules of light energy every second. While it emits a broad (blackbody) frequency spectrum, the emitted light is centered at approximately 200,000 GHz. This may seem big, but when multiplied by Planck’s exceptionally small constant h, we find that the energy of a single photon is a mere 1 × 10-19 joules. An awful, awful lot of photons are therefore required to produce an amount of energy detectable by humans.
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