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?
4 1H --> 4He + energyThe 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.
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:
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.
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.
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
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.
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.
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:
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.
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: