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IAQ (Infrequently Asked Questions

Update: Aug 23, 2022

"An acquaintance with physics is a propadeutic to a life of curious investigation."
-- Nobody ever said this

Where did the name "quark" come from?
What is the source of energy of the sun?
Did Heisenberg sabatoge the German atom bomb effort during WW2?
Why did Julius Robert Oppenheimer lose his security clearance?
What famous physicist received the Nobel prize in part for an incorrect interpretation of important physics?
Can two photons interact?
Where can I read about the Manhattan Project?

Where did the name "quark" come from?

Murray Gell-Mann (CalTech) named the hypothetical constitutent of the proton in 1964 after a quotation "Three quarks for Muster Mark," from James Joyce's Ulysses. (You can tell that Gell-Mann is a literary type; not many scientists had the patience to read that book.) Gell-Mann's suggestion was that there were three of these things in each proton and neutron, and he was led to the hypothesis from symmetries of the existing heavy relatives of the proton. The mysterious "quarks" had the unusal property of having fractional charges (+2/3 and -1/3), and because no free particles had every been observed with fractional charge, even Gell-Mann didn't believe in their existence as fundamental objects.

What is the source of energy of the Sun?

This is not a dumb question. And it was the source of a big scientific controversy. By 1920, biologists understood that evolution was a slow process, and that the Earth was at least a billion years old. But before the 1930s, when nuclear reactions were first studied, leading to an understanding in the late 1930s of the basic nuclear reactions in the Sun, it was assumed that the Sun's energy came from chemical reactions. However, the energy liberated in a chemical reaction is roughly a million times less than the energy liberated by nuclear fusion of hydrogen to helium:
       4 1H --> 4He + energy
The fusion reaction liberates about 28 MeV (million electron volts) of energy for each He atom made. By contrast, a chemical reaction liberates a few electron volts (eV) of energy, and a photon of visible light is between 2 and 3 eV. Without nuclear energy, the Sun shouldn't have lasted more than a few million years, even including the gravitational energy liberated during its collapse. This was far too short a timespan to agree with the biological record on Earth.

Did Heisenberg sabotage the German atom bomb effort during WW2?

People have been trying to answer this question definitively since 1945, when Heisenberg was captured by the allies and held for six months under "house arrest" at Farm Hall in Britain. All their conversations were secretly recorded, transcribed, and translated into English. Robert Jungk wrote the first book that discussed the Farm Hall prisoners, Brighter than a Thousand Suns, in 1957. Jungk did not have access to the transcripts (he may not have even known about them), and he accepted the story that the Farm Hall prisoners concocted after their capture: namely, that they knew how to make the bomb but deliberately sabotaged the effort.

Some of the Farm Hall transcripts were finally released in the early '90s. Thomas Powers wrote Heisenberg's War in 1993, giving the history of the German atomic project, including an analysis of the released transcripts. Like Jungk, Powers concluded that it was likely that Heisenberg had willingly sabotaged the project, but he wasn't as decisive about this conclusion because the evidence was murky. Heisenberg was in a tough spot. He wanted people to believe three things. First, that he was a loyal German and not a traitor to his country. (Heisenberg continued to live and work in Germany after the war.) Second, that they had not attempted to build an atom bomb because the German military was too short-sighted and they didn't have the resources to do it. And third, that he was smart enough to know how to build a bomb, and specifically, how much uranium was required to build a 235U bomb. So Heisenberg was claiming that although he was the head of the German atomic project, because the project was not feasable he never had to face the morality of deciding whether or not to build an atomic weapon for the Third Reich. In this way, Heisenberg walked a tightrope, attempting to keep his honor, his scientific reputation, and his reputation for humanity all intact.

A strange visit by Heisenberg in 1941 to his mentor and former colleague Niels Bohr in Copenhagen bears directly on the question. It is the subject of a recent play, Copenhagen, by Michael Frayn. Was Heisenberg trying to get Bohr to help him prevent the Allies from making a bomb? Was he trying to get information from Bohr about a possible Allied bomb project? (The Manhattan Project started the following year.) Or was he indirectly trying to tell Bohr that he wouldn't pursue a German bomb? If the latter was Heisenberg's intent, he failed miserably. Bohr, understanding him to say that he was planning to build a bomb, threw him out.

More of the transcripts were released in 2000, and then on February 2, 2002, the Niels Bohr Archive released a "bombshell" -- 11 letters that Bohr wrote, but never sent, to Heisenberg. Bohr wrote them in response to statements that Heisenberg had made to Robert Jungk, who then printed them in his book, published in Danish in 1957 (and translated into English the next year). Here is an excerpt from the first of those letters:

Personally, I remember every word of our conversations, which took place on a background of extreme sorrow and tension for us here in Denmark. In particular, it made a strong impression both on Margrethe and me, and on everyone at the Institute that the two of you spoke to, that you and Weizsäcker expressed your definite conviction that Germany would win and that it was therefore quite foolish for us to maintain the hope of a different outcome of the war and to be reticent as regards all German offers of cooperation. I also remember quite clearly our conversation in my room at the Institute, where in vague terms you spoke in a manner that could only give me the firm impression that, under your leadership, everything was being done in Germany to develop atomic weapons and that you said that there was no need to talk about details since you were completely familiar with them and had spent the past two years working more or less exclusively on such preparations.

This throws a new light on Heisenberg's war efforts, and it explains why Bohr would have thrown out his longtime friend and star student, as well as Weizsäcker, who was Heisenberg's student, his best physicist, and who accompanied Heisenberg on his visit to Bohr. Based on this new evidence, it appears that Heisenberg would have built a German bomb if he had thought it was possible. However, Thomas Powers still believes that Heisenberg did not think it was possible for the German scientists to make this weapon, as he explains in a recent article in the New York Review of Books.

Gerald Holton expresses a very different view in the April 2002 APS News. In brief, Holton believes that Bohr's memory is accurate. Heisenberg, in 1941, was full of confidence in both the ability of Germany to win the war and in his abilities to help with weapons. This scared and appalled Bohr. By the end of the war, in 1945, Heisenberg was in captivity at Farm Hall, and he was busy revising the historical record. When Heisenberg wrote to Jungk in 1957, he gave Jungk the revisionist story, admitting that he may not have remembered everything correctly. This story was in such contradiction to Bohr's version that Bohr was impelled to write the letters. Why did Bohr never send them? Possibly out of respect for Heisenberg and a wish not to engage in a public debate that would surely damage Heisenberg's reputation.

In October 2003, John Cornwell published a thorough history of the German effort: Hitler's Scientists: Science, War and the Devil's Pact. Like Holton, Cornwell believes that Heisenberg misled the German effort, not as a willing saboteur, but because he did not understand the fundamental physics of uranium fission due to slow neutrons. (Specifically, the cross-section for slow neutron capture.) As a consequence, Heisenberg believed that they needed not 10 kg, but approximately 1000 kg of 235U to build a bomb, an amount that was far beyond their ability to manufacture.

The American Institute of Physics has a very fine historical web page on Werner Heisenberg.

Why did Julius Robert Oppenheimer lose his security clearance?

World War II was a time of transition for physics and for the relation between physics and society. The Manhattan Project was perhaps the first (and certainly the most ambitious) attempt at "Big Physics," and it was also a clear merging of a large development project using state-of-the-art physics with weapons and national defense. Oppenheimer was at the center of the effort, and, as the "Father of the A-bomb" he was often quoted as saying, "In some sort of crude sense, which no vulgarity, no humor, no overstatement can quite extinguish, the physicists have known sin, and this is a knowledge which they cannot lose."

Many books have been written about the Manhattan Project and the strange events leading up to revocation of Oppenheimer's security clearance in 1953 and the hearings on the matter the following year. Oppenheimer himself never explained why he got into the trouble, except during an interrogation where he said "I was an idiot!" in response to the question as to why he fabricated a story of attempted espionage. How could Oppenheimer -- the man who orchestrated the activites at Los Alamos and associated laboratories through to success, a person who was universally acknowledged as a great administrator and teacher -- have been brought to the disgrace and humiliation of exile from the access to secrets and corridors of power?

This much is fairly certain. In early 1942, Oppenheimer reported to General Groves (his boss, in charge of the Manhattan Project) that one or more "intermediaries," whom he refused to name, contacted three scientists, with the message that a communist named George Eltenton could get information they provided to the Soviet Union. This was at a time when it was still almost fashionable to be a communist; the Nazi-Soviet non-aggression pact had been abrogated by Hitler, who had then proceeded to attack the USSR, so the US was now an ally with the USSR in the war, and Oppenheimer himself was closely associated with communists (including his past and present wives, and his brother Frank). Oppenheimer had been approached with Eltenton's request by his friend, Haakon Chevalier, a French professor who was also a communist. Chevalier later claimed that he was simply informing Oppenheimer that their acquaintance, Eltenton, was trying to get secret information -- not that he was asking Oppenheimer to spy. Two military men, Pash and Borden, naturally wanted more information, but Oppenheimer refused to name the intermediary or the other people who were contacted. Pash then asked that Oppenheimer be removed from the Manhattan Project. Under pressure from Groves, Oppenheimer admitted that Chevalier was the intermediary, and Groves agreed to put the matter to rest for the duration of the war.

After the war, Pash and Borden wanted the case reopened. But it took another eight years before the opportunity arose to review the situation in depth. By 1954, the Cold War was in full swing, the University of California demanded loyalty oaths from all employees, and the Soviets were building their own nuclear arsenals. Oppenheimer had been on several influential committees from 1946 onward, recommending consistently against R&D on fusion (hydrogen) bombs, but in spite of this opposition, Ernest Lawrence and Edward Teller had succeeded in opening the Livermore lab, and H-bombs had been designed and tested by both Los Alamos and Livermore. Lewis Strauss of the AEC believed that Oppenheimer's opposition to R&D work on H-bombs was treasonous, and this, combined with the unresolved security issues from 1942, resulted in a hearing to consider revoking Oppenheimer's security clearance. After three weeks and tens of thousands of pages of testimony and documents, the committee voted 2 to 1 (with the one physicist on the committee in opposition) to revoke.

Edward Teller was a lightning rod in this farce. He testified that Oppenheimer was not a security risk, but he also expressed ambiguity about whether he should retain his top secret clearance. Teller explained at length the many occasions that Oppenheimer had impeded work on the H-bomb, giving the impression that this was the reason for his (Teller's) concern. However, in his Memoirs (2001), Teller says he was prepared to advocate that Oppenheimer retain his clearance, but he became confused because shortly before he testified, he was shown a recent transcript of an interrogation of Oppenheimer, where Oppenheimer admitted he lied in 1942, that he had unfairly implicated his friend Chevalier (who was fired from UC Berkeley), and gave no reason for this strange behavior. At this point, Oppenheimer appeared so complicated that Teller lost the ability to defend him. However, in his own testimony, Teller did not mention being shown the transcript, thus leaving others to infer that his non-confidence vote on Oppenheimer was over Oppenheimer's opposition to work on the H-bomb. (In fact, Oppenheimer was in plentiful company. Because the H-bomb was apparently unlimited in its destructiveness, university physicists were in widespread opposition to any work on it.)

The whole thing ended badly for both Oppenheimer, who thereafter appeared to be a broken man, and for Teller, who thereafter was shunned by much of the scientific community. Teller points out that Eisenhower should have made an executive decision to simply remove Oppenheimer from the committees, without going through the disaster of the security hearing, thus sparing everyone the embarrassment and shame. Hindsight is 20-20.

Oppenheimer was truly a complicated person. Why did he make this fabrication based on elements of truth? My guess is that in early 1942, when Los Alamos was in its first stages of formation, there was considerable pressure on Groves not to appoint Oppenheimer as scientific leader. Further, after Groves made the appointment, because of Oppenheimer's past activities and his close connections with known communists, there was pressure to replace him. Groves would have informed Oppenheimer of this, at least in general terms if not in details. Oppenheimer wanted badly to prove himself as a scientfic leader, so he didn't want to be fired. He also wanted to demonstrate that he was a loyal citizen, so he reported the conversation with Chevalier to Groves. He should have known that Groves would have to report this to intelligence officials. But Oppenheimer, for a reason that only he would know, embellished the story by having the intermediary (or intermediaries) contact three people, not just himself. This would make the situation much more serious, including Chevalier's part, which then couldn't be simply explained as informing Oppenheimer of Eltenton's intentions. With three contacts, the intermediary appears to be part of the plot; hence, Oppenheimer's reluctance to name him.

What famous physicist received the Nobel prize in part for an incorrect interpretation of important physics?

Enrico Fermi was awarded the Nobel prize in Physics in 1938 for his work in systematically bombarding elements with neutrons, starting in 1932 when Chadwick discovered the neutron. Being uncharged, neutrons can more easily enter a (positively charged) atomic nucleus. Further, they can enter at low speeds, down to thermal energies, which are a fraction of an electron volt. Fermi found that slow neutrons are generally more likely to be absorbed than fast ones, with the cross-section going approximately inversely with speed. Consequently, he used paraffin, which has many hydrogen atoms, to slow the neutrons, and increase the probability of absorption.

When neutrons are absorbed, an isotope of the element with an extra neutron is produced. This is often in an excited state, and typically decays in one of three ways: electromagnetically, with emission of a gamma ray; by the weak interaction with electron or positron emission, producing an element with one more or one less proton; or by the strong interaction, expelling a helium nucleus (alpha particle) and generating an element with two less protons.

The most interesting results occurred when uranium and thorium were bombarded with slow neutrons. Uranium disintegrates naturally by a path that throws off a large number of alpha particles, bringing the atomic weight down from 238 to (eventually) 192, which is a stable isotope of lead. Therefore, Fermi was looking for high-energy alpha particles in his geiger counter. He covered the uranium sample with a thin layer of a material (cadmium?) to absorb the slow alpha particles. But this layer also absorbed the various fission products of uranium, so Fermi never observed them. If he had not covered the uranium, even once, he would likely have discovered fission in 1933, so it is very fortunate that he was such a careful experimenter! Lise Meitner performed the experiment in 1935 without an absorbing layer around the uranium target, and found that dozens of radioactive substances were observed, when there should only have been one. Fermi's explanation, that these were all transuranic elements, was accepted, even though it didn't make much sense. Swiss physicist Paul Scherrer also barely missed discovering fission. He bombarded thorium with neutrons, recorded the fission fragments in his geiger counter, and decided that his detector was malfunctioning because this was not supposed to happen! The correct interpretation, that the uranium nucleus was breaking up into two pieces of approximately equal mass, was finally made in early 1939, by Hahn and Strassmann.

Fermi received his Nobel for his general work on investigating neutron capture, but he was specifically cited for production of transuranic elements, the first two of which he named ausenium (93) and hesperium (94). (These are now called neptunium and plutonium.) That Fermi made this mistake is one of the most fortunate events of 20th century physics.

Fermi was nicknamed "The Pope" because of his reputation for near infallibility. Therefore, it's amusing that another error that Fermi made was immortalized by the U.S. Post Office in 2001, when they issued a stamp in honor of the 100th anniversary of Fermi's birth. They used a photograph made in the late '40s that has Fermi standing at a blackboard next to some equations and figures he has drawn.

At the upper left, you can see part of the equation for the fine structure constant, a dimensionless measure of the strength of the electromagnetic interaction (the square of the electron charge divided by the product of Planck's constant and the speed of light to make it dimensionless). Fermi wrote it incorrectly!

Can two photons interact?

Let's consider only interactions in vacuum, because we know that within materials there are nonlinear optical effects associated with absorption and emission of photons at different frequencies.

Why would this be an interesting question to ask for photons in a vacuum? After all, Maxwell's classical theory of electromagnetism is linear. Electric fields from all sources simply add at every place in space and time. In the quantum version, these electric fields come from photons, so one would expect that photons do not interact. And in fact, in quantum electrodynamics, the fundamental interaction between photons and charged leptons is described as a vertex somewhere in spacetime with two leptons and a photon. This vertex can be pictured as a lepton "current" (a particle coming in to the vertex and then going out) plus a photon either going in or coming out. There is only one photon in the interaction, so it appears that two photons can never interact. This was Paul Dirac's belief, and this is what he wrote on page 9 of The Principles of Quantum Mechanics, a masterpiece of clarity and conciseness that has been a standard reference for 70 years:

Some time before the discovery of quantum mechanics people realized that the connexion between light waves and photons must be of a statistical character. What they did not clearly realize, however, was that the wave function gives information about the probability of one photon being in a particular place and not the probable number of photons in that place. The importance of the distinction can be made clear in the following way. Suppose we have a beam of light consisting of a large number of photons split up into two components of equal intensity. On the assumption that the intensity of a beam is connected with the probably number of photons in it, we should have half the total number of photons going into each component. If the two components are now made to interfere, we should require a photon in one component to be able to interfere with one in the other. Sometimes these two photons would have to annihilate one another and other times they would have to produce four photons. This would contradict the conservation of energy. The new theory, which connects the wave function with probabilities for one photon, gets over the difficulty by making each photon go partly into each of the two components. Each photon then interferes only with itself. Interference between two different photons never occurs.
A beautiful description of the probability interpretion of the wavefunction, and one that Feynman emphasized as the basic linear interference effect of quantum phenomena.

But the very last sentence is wrong! It is possible for two photons to interfere with each other. The simplest way this can happen is a fourth order (i.e., 4-vertex) Feynman diagram in quantum electrodynamics:

This can be understood as follows. As the photon moves through the vacuum, it can generate virtual pairs of electrons and positrons, which typically proceed to annihilate themselves. This looks like a little lepton "bubble" stuck in the middle of the photon propagator. (A photon propagator is just the amplitude for a photon to go from one place to another. It is drawn in a Feynman diagram by a wiggly line, whereas an electron propagator is drawn as a straight or slowly curving line.)

The lepton bubble shows an electron going around a loop. If we think of time increasing to the right, then one of the electrons is going backwards in time. In QED, an electron going backwards in time is equivalent to a positron going forwards in time. So the lepton loop can be interpreted as an electron-positron pair forming at the left vertex and annihilating at the right vertex, allowing the photon to continue on its journey.

Now suppose two photons that are close to each other both generate such a virtual pair, but the electron of one pair happens to annihilate with the positron of the other pair, and vice-versa. Can you see how you can form the fourth-order diagram above by combining two photon propagators with loops? The net effect is that the two photons have collided and exchanged energy and momentum. It's an unlikely event, but it can happen. So the answer is that two proximate photons can interact, but it is very unlikely that they will.

Where can I read about the Manhattan Project and its sequels?

Many books have been written on the history of the Manhattan Project. In addition to the fearsome goal of the Manhattan Project, the subject is of great interest because of the interactions of the scientists and engineers who were involved. These people were highly focused, had strong personalities, and behaved in ways that were almost always honorable but sometimes eccentric. The problems they faced were more difficult than those in any previous engineering project in history. They had to keep their work secret from the outside world, and they had to coordinate their efforts across the continent. To solve physics problems they invented elaborate techniques of calculation, and John von Neuman invented the digital computer with stored programs and sequential operations on data, as we know it today.

Some historians focus on two or three of the individuals, and others on some aspect of the project. I have mentioned above Edward Teller's Memoirs, Thomas Powers' Heisenberg's War, and John Cornwell's Hitler's Scientists: Science, War, and the Devil's Pact. However, for the history of the development of nuclear weapons, I would suggest the following:

There are some older books. For example, Jungk's Brighter than a Thousand Suns, 1958, is the first widely read history of the bomb project. It is historically inaccurate and not recommended.
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