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No. 3085:
Einstein and the Quantum

by Andy Boyd

Today Einstein's bigger deal. The University of Houston presents this series about the machines that make our civilization run, and the people whose ingenuity created them.

From special relativity's time travel to the famous E = mc2, we're all aware of Albert Einstein's achievements. Yet, there's one achievement most people haven't heard of. Einstein himself called it "very revolutionary," and one historian calls it "the miraculous argument."

To appreciate the miracle we need to step back two centuries to the work of Sir Isaac Newton. Newton's most famous treatise was the Principia, which tackled gravity and motion. But he also wrote Opticks, a highly regarded treatise on the nature of light. In it, Newton describes his many scientific experiments with light. But Opticks contains one rather bold assertion that Newton really didn't have adequate evidence to support: that light was made of particles.

Illustration taken from Newton's original letter to the Royal Society. S represents sunlight. The light between the planes BC and DE are in colour. These colours are recombined to form sunlight on the pane GH. Photo Credit: Wikimedia Commons.

Newton was held with such high esteem that his particle theory of light became widely accepted for more than a century. But in 1803, standing before the Royal Society of London, Thomas Young performed an experiment showing light was a wave. It was a terrific experiment, leading to a careful reexamination of light's true nature. Over the following century evidence continued to mount in favor of light being a wave. By the time of Einstein things were pretty well settled. Newton was wrong. Light was a wave. Or was it?

Sine waves of several frequencies. Waves colored like the frequencies of the visible spectrum. Photo Credit: Wikimedia Commons.

One particular experiment was causing trouble for the wave theory of light. When light was shone on certain metal plates, electrons jumped off, but only when the light was above a threshold frequency. At low frequencies nothing happened. Wave theory, with its smooth transmission of energy, predicted all frequencies should cause electrons to jump, though some frequencies would take longer than others.

An example of the photoelectric effect. Photo Credit: Wikimedia Commons.

Scientists were at a loss. And this is where Einstein stepped in with the "miraculous argument," an argument that led him to conclude the unthinkable: light traveled in packets. Only the packets associated with high frequency light carried enough energy to dislodge the electrons. Einstein had brought Newton's idea full circle, suggesting once again that light was a particle.

A picture of a single particle. Photo Credit: E.A. Boyd.

So which is it? Both waves and particles carry energy, but apart from that they're entirely different beasts. The rhythmic motion you feel floating in the ocean isn't anything like the pain you feel when hit with a rock. We need to pick our poison.

Yet we don't. Scientists get around the enigma by allowing that light sometimes behaves like a wave, and sometimes like a particle. If that sounds like doubletalk, it is. But then again, what choice do we have given the experimental evidence?

Wave-particle duality is just one of many conundrums posed by quantum theory; conundrums that leave scientists' heads swimming. Rather than fret the philosophical implications, many turn to a well-known pragmatic interpretation that preserves their sanity: Just "shut up and calculate."

I'm Andy Boyd at the University of Houston, where we're interested in the way inventive minds work.

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J. Norton. 2007. "Einstein's Miraculous Argument of 1905: The Thermodynamic Grounding of Light Quanta." PhilSci Archive. See: Accessed September 6, 2016.

Opticks. From the Wikipedia website: Accessed September 6, 2016.

Photoelectric Effect. From the Wikipedia website: Accessed September 6, 2016.

Photoelectric Effect. From the website of the Encyclopedia Britannica: Accessed September 6, 2016. 

Thomas Young (Scientist). From the Wikipedia website: Accessed September 6, 2016.

Young's Interference Experiment. From the Wikipedia website: Accessed September 6, 2016.

This episode was first aired on September 15, 2016