Nov 13, 2018, 10:43am, Chad Orzel
As noted in my bio that appears at the end of these posts, I have a new book on the ways in which quantum phenomena manifest in the course of ordinary morning activities (it’s already out in the UK and the US edition will be out in early December). The title of the book is Breakfast with Einstein, which might seem an odd choice for a book about quantum mechanics, given Einstein’s famous disdain for the theory late in his life. Most popular treatments of Einstein emphasize his decades-long argument with Niels Bohr about the philosophical foundations of the theory, and it’s pretty much obligatory to quote (or at least paraphrase) his comment from a letter to Max Born about the statistical nature of the theory:
“Quantum mechanics is very impressive. But an inner voice tells me that it is not yet the real thing. The theory produces a good deal but hardly brings us closer to the secret of the Old One. I am at all events convinced that He does not play dice.”
So, is this just a blatant cash grab, sticking Einstein’s name in the title on the theory that it will help sell books? Not really– in fact, it’s entirely appropriate to have Einstein’s name in there, because he played a pivotal role in launching quantum mechanics. And even his parting shot as he got out of the quantum game for good has advanced the field– the was wrong about the fundamental physics, but in a way that created enormous new possibilities for physicists and engineers.
Author copies of the US edition of BREAKFAST WITH EINSTEIN.CHAD ORZEL
Einstein’s best-known contribution to quantum theory was one of his “miracle year” papers in 1905, in which he adapted Max Planck’s original quantum hypothesis to suggest a particle nature for light. While many people tout relativity as a revolutionary change in physics, in fact the photoelectric effect paper was Einstein’s truly radical contribution.
The photoelectric effect paper was not an immediate hit– Max Planck, whose relationship between energy and frequency was key to the theory, wrote in a letter of reference that:
“That [Einstein] may sometimes have missed the target in his speculations, as for example, in his hypothesis of light quanta, cannot really be held too much against him, for it is not possible to introduce fundamentally new ideas, even in the most exact sciences, without occasionally taking a risk.”
Even its experimental confirmation, in work by Robert Millikan in 1916, was not hugely enthusiastic. Millikan’s papers are a brilliant example of the passive-aggressive style in academic writing– he opens with the statement that Einstein’s model “cannot in my judgment be looked upon at present as resting upon any sort of a satisfactory theoretical foundation,” before grudgingly admitting that ” it actually represents very accurately the behavior… of all the substances with which I have worked.” In another paper, he remarks that for all its empirical success, “the semi-corpuscular theory by which Einstein arrived at his equation seems at present to be wholly untenable.”
There’s a nice bit of irony, then, that the citations for both Einstein’s Nobel Prize in 1921 and Millikan’s in 1923 specifically cite the photoelectric effect. (It’s the only specific theory mentioned in Einstein’s Nobel citation (“for his services to Theoretical Physics, and especially for his discovery of the law of the photoelectric effect”), because of petty wrangling over relativity in the Swedish academy.) It’s a fine topic for a prize, though, because the particle nature of light is central to a lot of modern technologies. If you use a digital camera, for example, that’s taking advantage of the particle nature of light exciting electrons between bands in a semiconductor, in much the same way that a photon ejects an electron from the material in the photoelectric effect. And if you send your digital pictures out into the world via social media, that also relies on the quantum nature of light, through Einstein’s other great contribution to the field.
Portrait of Einstein by Alice Reischer, based on a sketch made during a 1939 lecture. Hangs in the Department of Physics and Astronomy at Union College, a gift of Carl George. (Photo by Chad Orzel)CHAD ORZEL
A lot of textbook treatments of early quantum physics mention the photoelectric effect from 1905, and then Einstein leaves the story until the late 1920’s when he starting having philosophical arguments with Bohr. In fact, though, arguably his most important to quantum physics came in 1917, when he published a monumental work on the interaction between light and atoms.
This came in the context of the “old quantum theory,” when Bohr had put forth his quantum model of atoms with discrete energy states that absorb or emit light only when moving between states. Einstein combined this idea with his earlier hypothesis, and set out to see how much he could deduce about the properties of these “light quanta” (the word “photon” was still a decade or more from being coined). To do this, he necessarily introduced an element of randomness.
Quantum physics books with dice.CREDIT: CHAD ORZEL
That probably seems like a weird thing to do, given his well-known distaste for a craps-shooting God, but in fact it was a perfectly sensible trick, using randomness to cover for ignorance. Einstein noted that while they didn’t have a theory that would describe when a particular atom would absorb or emit, they knew perfectly well that these are processes that occur in nature, and for a large collection of atoms, that means you can treat them probabilistically. That lets you gloss over the details of the deterministic quantum theory that he assumed must be waiting to be found, and just assign probabilities to everything. This is a technique physicists use all the time, even in situations that aren’t the least bit quantum– a responsible treatment of classical projectile motion, for example, will give a range of probabilities of landing at various positions to cover for our ignorance of microscopic instantaneous interactions that we can’t hope to measure.
Einstein identified three processes that can occur in a system of Bohr-type atoms with two energy states interacting with light of the appropriate frequency, two of which were well-known: the process of spontaneous emission, in which an atom in a high-energy state drops down to a low-energy state, emitting some light, and the process of absorption, in which an atom exposed to light absorbs some of the light and moves to a high-energy state. He also introduced a new process, spontaneous emission, where a high-energy atom exposed to light will be stimulated to emit more light of the same frequency, and drop back to a low-energy state. This hadn’t previously been considered, but logically must exist, he argued using a mechanical analogy.
Given those three processes, Einstein assigned probabilities to them (these days, the probability of spontaneous emission is given the symbol “A” and the probabilities of absorption or stimulated emission get represented by “B” with some subscripts), and then looked at what could be deduced about these probabilities and the properties of light. He found a wealth of stuff that could be experimentally tested, including a simple relationship between the A and B coefficients, and a formula for A in terms of the frequency of light and some other well-known properties. These “Einstein coefficients” remain a useful way to describe the interaction between light and atoms. He also showed that the interaction must transfer energy and momentum between atoms and light, and that this pictures gave yet another way to explain the Planck formula for the black-body spectrum of light emitted by hot objects. It’s a masterful bit of work, and was important in convincing physicists to accept the quantum nature of light.
This paper also planted the seeds of what would become one of the most essential modern technologies. Einstein’s new process of stimulated emission is at the heart of the laser, a word that started as an acronym for “Light Amplification by Stimulated Emission of Radiation.” A laser makes a bright and narrow beam of light by exploiting stimulated emission: a collection of high-energy atoms are placed between two mirrors, and any photons they emit bounce back and forth between the mirrors, stimulating the emission of more and more photons, each new photon having exactly the same frequency and heading in the same direction as the one that stimulated it. Lasers are everywhere these days– from pet toys to supermarket scanners to industrial cutters to powering the fiber-optic telecommunications systems that carry the Internet and allow you to read these words. None of that would be possible without the quantum nature of light, as first described by Einstein.
So, while his eventual disenchantment with quantum mechanics has been exhaustively documented, Einstein in fact played a pivotal role in launching the theory. Even his departure from the field was enormously influential, as the 1935 “EPR” paper he wrote with younger colleagues Boris Podolsky and Nathan Rosen provided one of the first clear examples of what’s now known as “entanglement,” and provided the foundation for revolutionary work in both the philosophical foundations of quantum physics and the technology of quantum information. I won’t dwell on entanglement here, as it’s been the source of numerous prior blog posts (start here and follow links…), but it’s fascinating how even when Einstein was wrong about quantum physics, it turned out to be brilliantly influential.
And that, ultimately, is why it makes sense to have Einstein’s name in the title of my forthcoming book on quantum physics.
I’m an Associate Professor in the Department of Physics and Astronomy at Union College, and I write books about science for non-scientists. I have a BA in physics from Williams College and a Ph.D. in Chemical Physics from the University of Maryland, College Park (studying la…MORE
Chad Orzel is a physics professor, pop-science author, and blogger. His next book, Breakfast with Einstein: The Exotic Physics of Everyday Objects, will be released in December 2018.
Collected at: www.forbes.com