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Particles and waves: The central mystery of quantum mechanics - Chad Orzel


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One of the most amazing facts in physics is that everything in the universe, from light to electrons to atoms, behaves like both a particle and a wave at the same time. But how did physicists arrive at this mind-boggling conclusion? Chad Orzel recounts the string of scientists who built on each other’s discoveries to arrive at this ‘central mystery’ of quantum mechanics.

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While the story in the video starts around 1900, the question of whether light is a particle or a wave is much older, dating back to the 1600’s. Isaac Newton put forth a particle model of light as a stream of “corpuscles,” while the Dutch physicist Christiaan Huygens preferred a model of light as a wave. The question was thought to have been settled in 1800 by Thomas Young, who provided a definitive demonstration of wave behavior when light encounters a pair of small slits, an experiment.
You can see this dramatized in this Veritasium video.

You can also simulate the behavior of waves using a large number of Java applets collected here. Give it a try!

Young’s experiments and subsequent work by Jean-Augustin Fresnel convinced everyone that light was a wave, and that consensus held for about a hundred years. James Clerk Maxwell explained what was waving when he developed the theory of electromagnetism in 1865, and Heinrich Hertz experimentally demonstrated electromagnetic waves in 1888.

The wave model of light started to have problems, though, when Max Planck was trying to explain the spectrum of light emitted by a hot object. This “black-body radiation” is nicely described by a simple mathematical function, and depends only on the temperature of the object. You can play with the spectrum for various temperatures using this applet.

The next major challenge for the wave model was the photoelectric effect, in which light falling on a metal surface ejects electrons. There’s a simulation of the photoelectric effect here.

The wave model makes a bunch of predictions, listed in this blog post. Experimentally. Einstein’s particle model of light is much more successful, explaining all of the observations, but it’s a radical leap, and was an unpopular idea. One of many scientists who tried to disprove the theory was Robert Millikan, who ended up confirming it completely in a great display of scientific integrity. Photons remained controversial, though, and the first truly definitive proof of the particle nature of light wasn’t until 1977.

On the matter side, Rutherford’s alpha-particle scattering experiments were a total shock to everyone in physics. The prevailing model at the time, the “plum pudding” model of J.J. Thompson (so called because it had electrons embedded in an amorphous mass of positive charge like raisins in a plum pudding) predicted that alpha particles should hardly be deflected by a thin gold foil. Rutherford himself said that “It was quite the most incredible event that has ever happened to me in my life. It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you.” You can play with a simulation of the Rutherford and Thompson models at this link! Feel like a scientist making a discovery! See what Rutherford experienced. The American Institute of Physics has an online exhibit about Rutherford’s life and physics.

Bohr’s model of hydrogen was the first model that applied quantum physics to atoms, and explains the spectrum of light absorbed and emitted by hydrogen and hydrogen-like ions. This was refined by de Broglie and then Erwin Schrodinger and Werner Heisenberg developed more complete quantum theories in 1927. You can see what various models look like in terms of their interaction with light using this applet.

The wave nature of electrons was directly observed in 1927 by George Paget Thompson in the UK, the other by Clinton Davisson and Lester Germer in the US. Both saw interference patterns from electrons interacting with matter, just like those Young used to show light is a wave. Since then, numerous experiments have confirmed the wave nature of electrons, including the double-slit experiment with single electrons mentioned in the video, which you can see images and video of from Hitachi.

The group of Markus Arndt in Austria did essentially the same experiment with organic molecules in place of the electrons. They see the same result: individual molecules are detected like particles, but all the molecules together build up a wave-like interference pattern. This shows clearly that the dual nature is a general feature of everything in the universe, not just a quirk of electrons and light. Absolutely everything behaves like a particle and a wave at the same time, and once you have that idea, you can see it, if you’re clever enough with your own experiment.

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Meet The Creators

  • Educator Chad Orzel
  • Director Joana Bartolomeu
  • Narrator Addison Anderson

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