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• Where did quantum theory come from?

• It started, not as a crazy idea, but with a light bulb. In the early 1890s, the German

• Bureau of Standards asked Max Planck how to make light bulbs more efficient so that they

• would give out the maximum light for the least electrical power.

• The first task Planck faced was to predict how much light a hot filament gives off. He

• knew that light consists of electromagnetic waves, with different colors of light carried

• by different frequency waves. The problem was to ensure that as much light as possible

• was given off by visible waves rather than ultraviolet or infrared.

• He tried to work out how much light of each color a hot object emits, but his predictions

• based on electromagnetic theory kept disagreeing with experiments.

• Instead, in what he later called anact of despair,” he threw the existing theory

• out the window and worked backwards from experimental measurements. The data pointed him to a new

• rule of physics: light waves carry energy only in packets, with high frequency light

• consisting of large packets of energy and low frequency light consisting of small packets

• of energy.

• The idea that light comes in packets, or "quanta", may sound crazy, and it was at the time, but

• Einstein soon related it to a much more familiar problem: sharing.

• If you want to make a kid happy... give them a cookie! But if there are two kids, and you

• only have one cookie, you'll only be able to cheer them up half as much. And if there

• are four, or eight, or sixteen hundred thousand, you're not going to make them very happy at

• all if they have to share one cookie between them.

• In fact, if you have a room with infinitely many kids but not infinitely many cookies,

• if you share the cookies evenly each kid will only get an infinitesimally small crumb, and

• none of them will be cheered up. And they'll still eat all your cookies.

• The difference between light waves and kids is that you can't actually have infinitely

• many kids in a room. But because light waves come in all sizes, you can have arbitrarily

• small light waves, so you can fit infinitely many into a room. And then the light waves

• In fact, all these infinitesimal waves together would have an infinite capacity to absorb

• energy, and they'd suck all the heat out of anything you put into the roominstantly

• freezing the tea in your cup, or the sun, or even a supernova.

• Luckily, the universe doesn't work that waybecause, as Planck guessed, the tiny, high

• frequency waves can only carry away energy in huge packets. They're like fussy kids who'll

• only accept exactly thirty-seven cookies, or a hundred and sixty-two thousand cookies,

• no more and no less. Because they're so picky, the fussy high frequency waves lose out and

• most of the energy is carried away in lower-frequency packets that are willing to take an equal

• share. This common, average energy that the packets carry, is in fact what we mean by

• "temperature."

• So a higher temperature just means higher average energy, and thus by Planck's rule,

• a higher frequency of light emitted. That's why as an object gets hotter it glows first

• infrared, then red, yellow, white; hotter and hotter towards blue, violet, ultraviolet

• and so on.

• Specifically, Planck's quantum theory of fussy light tells us that light bulb filaments should

• be heated to a temperature of about 3200 Kelvin to ensure that most of the energy is emitted

• as visible waves - much hotter, and we'd start tanning from the ultraviolet light.

• Actually, quantum physics has been staring us in the face since long before lightbulbs

• and tanning beds: human beings have been making fires for millennia, with the color of the

• flames spelling out "quantum" all along.

Where did quantum theory come from?

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The Origin of Quantum Mechanics (feat. Neil Turok)

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Matt posted on 2013/09/19
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