Author
Max Planck
1 reading card from 1 book · 1931.
1 card
Scrisoare către Robert W. Wood (1931), despre ipoteza cuantelor din 1900 · 1931
The energy of light does not flow down a ramp; it climbs in steps, and the blue step is taller than the red one.
Put an iron rod in a fire. At first you see nothing, though you feel the heat. Then it turns dark red, then orange, then yellow-white. It never goes through blue first. The physics of 1890 could not explain this. The calculations of the day said a hot body should radiate more and more energy towards blue and ultraviolet colours, without limit. Every stove would have burned us with ultraviolet. Later the problem got a dramatic name: the ultraviolet catastrophe. In 1900 Planck found a formula that matched the measurements perfectly. To justify it, he had to make a strange assumption: energy can be given or taken only in whole packets, not in any amount. And the size of the packet depends on the colour. A packet of blue light is more ‘expensive’ than one of red light, and an ultraviolet one is more expensive still. An analogy, with its limits: think of a vending machine that takes only whole coins, where each product has a different price. The body's heat is like a pocket of small change. The cheap products, red and infrared, are bought often. The expensive product, blue, is bought very rarely, because many coins have to come together at once. The analogy stops there: in nature there is no machine, only the rule that energy comes in steps. Planck himself did not believe at first that the packets were real. He called the whole thing ‘an act of desperation’. But his steps opened the road to all of quantum physics, and in 1918 he received the Nobel Prize for them.
“It was a purely formal assumption and I did not really think much about it, only that I had to bring about a positive result under all circumstances, whatever the cost. [...] Briefly summarized, I can call the whole deed an act of desperation.”