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  • One of the most amazing facts in physics is this:

  • everything in the universe, from light to electrons to atoms,

  • behaves like both a particle and a wave at the same time.

  • All of the other weird stuff you might have heard about quantum physics,

  • Schrodinger's Cat, God playing dice, spooky action at a distance,

  • all of it follows directly from the fact

  • that everything has both particle and wave nature.

  • This might sound crazy.

  • If you look around, you'll see waves in water and particles of rock,

  • and they're nothing alike.

  • So why would you think to combine them?

  • Physicists didn't just decide to mash these things together out of no where.

  • Rather, they were led to the dual nature of the universe

  • through a process of small steps,

  • fitting together lots of bits of evidence, like pieces in a puzzle.

  • The first person to seriously suggest the dual nature of light

  • was Albert Einstein in 1905,

  • but he was picking up an earlier idea from Max Planck.

  • Planck explained the colors of light emitted by hot objects,

  • like the filament in a light bulb,

  • but to do it, he needed a desperate trick:

  • he said the object was made up of oscillators

  • that could only emit light in discrete chunks,

  • units of energy that depend on the frequency of the light.

  • Planck was never really happy with this, but Einstein picked it up and ran with it.

  • He applied Planck's idea to light itself, saying that light,

  • which everybody knew was a wave, is really a stream of photons,

  • each with a discrete amount of energy.

  • Einstein himself called this the only truly revolutionary thing he did,

  • but it explains the way light shining on a metal surface knocks loose electrons.

  • Even people who hated the idea had to agree that it works brilliantly.

  • The next puzzle piece came from Ernest Rutherford in England.

  • In 1909, Ernest Marsden and Hans Geiger, working for Rutherford,

  • shot alpha particles at gold atoms

  • and were stunned to find that some bounced straight backwards.

  • This showed that most of the mass of the atom is concentrated in a tiny nucleus.

  • The cartoon atom you learn in grade school,

  • with electrons orbiting like a miniature solar system,

  • that's Rutherford's.

  • There's one little problem with Rutherford's atom: it can't work.

  • Classical physics tells us that an electron

  • whipping around in a circle emits light,

  • and we use this all the time to generate radio waves and X-rays.

  • Rutherford's atoms should spray X-rays in all directions for a brief instant

  • before the electron spirals in to crash into the nucleus.

  • But Niels Bohr, a Danish theoretical physicist working with Rutherford,

  • pointed out that atoms obviously exist,

  • so maybe the rules of physics needed to change.

  • Bohr proposed that an electron in certain special orbits

  • doesn't emit any light at all.

  • Atoms absorb and emit light only when electrons change orbits,

  • and the frequency of the light depends on the energy difference

  • in just the way Planck and Einstein introduced.

  • Bohr's atom fixes Rutherford's problem

  • and explains why atoms emit only very specific colors of light.

  • Each element has its own special orbits,

  • and thus its own unique set of frequencies.

  • The Bohr model has one tiny problem:

  • there's no reason for those orbits to be special.

  • But Louis de Broglie, a French PhD student,

  • brought everything full circle.

  • He pointed out that if light, which everyone knew is a wave,

  • behaves like a particle,

  • maybe the electron, which everyone knew is a particle,

  • behaves like a wave.

  • And if electrons are waves,

  • it's easy to explain Bohr's rule for picking out the special orbits.

  • Once you have the idea that electrons behave like waves,

  • you can go look for it.

  • And within a few years, scientists in the US and UK

  • had observed wave behavior from electrons.

  • These days we have a wonderfully clear demonstration of this:

  • shooting single electrons at a barrier with slits cut in it.

  • Each electron is detected at a specific place at a specific time,

  • like a particle.

  • But when you repeat the experiment many times,

  • all the individual electrons trace out a pattern of stripes,

  • characteristic of wave behavior.

  • The idea that particles behave like waves, and vice versa,

  • is one of the strangest and most powerful in physics.

  • Richard Feynman famously said

  • that this illustrates the central mystery of quantum mechanics.

  • Everything else follows from this,

  • like pieces of a puzzle falling into place.

One of the most amazing facts in physics is this:

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TED-ED】粒子と波動。量子力学の中心的な謎 - チャド・オーゼル (【TED-Ed】Particles and waves: The central mystery of quantum mechanics - Chad Orzel)

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    稲葉白兎 に公開 2021 年 01 月 14 日
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