A. Douglas Stone is the Carl A. Morse Professor of Applied Physics and Physics at Yale University. His book, Einstein and the Quantum: The Quest of the Valiant Swabian, reveals for the first time the full significance of Albert Einstein’s contributions to quantum theory. Einstein famously rejected quantum mechanics, observing that God does not play dice. But, in fact, he thought more about the nature of atoms, molecules, and the emission and absorption of light–the core of what we now know as quantum theory–than he did about relativity.
In a recent interview, A. Douglas Stone talked about Einstein’s contributions to the scientific community, quantum theory, and his new book, Einstein and the Quantum: The Quest of the Valiant Swabian.
Why does quantum theory matter?
At the beginning of the 20th century science was facing a fundamental roadblock: scientists did not understand the laws governing the atoms and molecules of which all materials are made, but which are unobservable due to their size.
At that time there was a real question whether the human mind was capable of understanding this microscopic realm, outside of all our direct experience of the world. The development and success of quantum theory was a turning point for modern civilization, enabling most of the scientific advances and revolutionary technologies of the century that followed.
What are some of the ways that quantum theory has changed our lives?
There is a common misconception that quantum mechanics is mainly about very weird phenomena, remote from everyday life, such as Schrodinger’s cat, exotic sub-atomic particles, black holes, or the Big Bang. Actually it is a precise quantitative tool to understand the materials, chemical reactions and devices we employ in modern industries, such as semiconductors, solar cells, and lasers. An early success of the quantum theory was to help predict how to extract ammonia from the air, which could then be used as fertilizer for the green revolution that revolutionized 20th century agriculture. And of course our ability to develop both nuclear weapons and nuclear power was completely dependent upon quantum theory.
Why is Einstein’s role in quantum theory important and interesting?
It is important because a careful examination of the historical record shows that Einstein was responsible for more of the fundamental new concepts of the theory than any other single scientist. This is arguably his greatest scientific legacy, despite his fame for Relativity Theory. He himself said, “I have thought a hundred times more about the quantum problems than I have about Relativity Theory”. It is interesting because he ultimately refused to accept quantum theory as the ultimate truth about Nature, because it violated his core philosophical principles.
So you are saying that Einstein is famous for the wrong theory?
In a certain sense, yes. All physicists agree that the theory of relativity, particularly general relativity, is a work of staggering individual genius. But in terms of impact on human society and history, quantum mechanics is simply much more important. In fact, relativity theory is incorporated into important parts of modern quantum mechanics, but in many contexts it is irrelevant.
In what ways was Einstein central to the development of the theory?
I estimate that his contributions to quantum theory would have been worthy of four Nobel Prizes if different scientists had done them, compared to the one that he received. I go through each of these contributions in its historical and biographical context in the book.
Can you give a few examples?
Quantum theory gets its name because it says that certain physical quantities, including the energies of electrons bound to atomic nuclei are quantized, meaning that only certain energies are allowed, whereas in macroscopic physics energy is a continuously varying quantity. Typically the German physicist, Max Planck, is credited with the insight that energy must be quantized at the molecular scale, but the detailed history shows Einstein role in this conceptual breakthrough was greater.
Another key thing in quantum theory is that fundamental particles, while they move in space, sometimes behave as if they were spread out, like a wave in water, but in other contexts they appear as particles, i.e. very localized point-like objects. Einstein introduced this “wave-particle duality” first, in 1905 (his “miracle year”), when he proposed that light, long thought to be an electromagnetic wave, also could behave like a particle, now known as the photon.
Yet another, very unusual concept in quantum theory is that fundamental particles, such as photons, are “indistinguishable” in a technical sense. When many photons are bunched together it makes no sense to ask which is which. This changes their physical properties in a very important way, and this insight is often attributed to the Indian physicist, S. N. Bose (hence the term “boson”). In my view Einstein played a larger role in this advance than did Bose, although he always very generously gave Bose a great deal of credit.
The stories of these and other findings are fully told in the book and they illustrate new aspects of Einstein’s genius, unknown to the public and even to many working scientists.
What did Einstein object to about quantum theory?
Initially he reacted strongly against the intrinsic randomness and uncertainty of quantum mechanics, saying “God does not play dice”. But after that his main objection was that quantum theory seems to break down the distinction between the subjective world of human experience and the objective description of physical reality that he considered the goal of physics, and his central mission in life. Many physicists struggle with this issue even today.
Why is Einstein’s role in quantum theory underappreciated?
Einstein ultimately rejected the theory and moved on to other areas of research, so he never emphasized the extent of his contributions. His own autobiographical notes, written in his seventies, understate his role to an almost laughable degree. Second, Einstein’s version of quantum theory, wave mechanics, did not create a school of followers, whereas Niels Bohr, Werner Heisenberg and others reached the same point be a different route. Their school fostered the primary research thrust in atomic and nuclear physics, gradually causing the memory of Einstein’s role to fade. Finally, the history of Einstein’s involvement with quantum theory was long (1905-1925) and complex, and few people really understand it all; I try to remedy that in this book.
Did Einstein do anything important in quantum physics after the basic theory was known?
No and yes. He did not work in the main stream of elementary particle physics which developed shortly after the basic theory was discovered in the late nineteen twenties, since he refused to employ the standard mathematical machinery of quantum theory which everyone else used. However, in the early 1930’s he identified a conceptual feature of quantum theory missed by all the other pioneers, which became known by the term “entanglement”. This concept, ironically, is critical to the most revolutionary area of modern quantum physics, quantum information theory and quantum computing.
What does the subtitle of the book refer to? Who is the “Valiant Swabian”?
The Valiant Swabian was a fictional crusader knight, the hero of a poem by Ludwig Uhland, a poet from Swabia where Einstein was born. In his twenties, Einstein used to refer to himself jokingly by this name, particularly with his first wife, Mileva Maric. It was a similar to someone today calling himself “Indiana Jones” for fun. The young Einstein was a charismatic and memorable personality, with great joie de vivre, as this nickname indicates. He was known for his sense of humor, his rebelliousness, and for his attractiveness to women, in contrast to the benevolent, grandfatherly, star-gazer we associate with iconic pictures of the white-maned sage of later years.
How did you research this book? What materials did you have access to?
There is a very extensive trove of letters and private papers that survive in Einstein’s estate, all of which have been translated and published for the period 1886 to 1922. From reading all of these I got a good sense of his personality. And all of his important scientific papers in the relevant time period are available in English now, so I was able to go back and see exactly how he arrived at his revolutionary ideas about quantum theory, which I then did my best to interpret in layman’s terms. In addition I relied on several excellent biographies by Folsing, Isaacson and Pais, and historical articles by many leading historians of science, such as T.S. Kuhn and Martin Klein.
What do you hope readers take away from reading Einstein and the Quantum?
First, new insight into Einstein’s genius, and a sense of the personality of the young Einstein, before his fame. Second, appreciation of the historic significance of the successful attempt to understand the atom through quantum theory, a turning point in human civilization. Third, an understanding of how science advances as a creative, human process, with both brilliant insights and embarrassing blunders, affected by psychological and philosophical influences.