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Shelf life: Alan Lightman

What are the three best popular-science books?

Chaos by James Gleick. A beautifully written and fascinating book that introduced the idea of chaos theory for the first time to the public. Gleick is a master storyteller and writer.

The Making of the Atomic Bomb by Richard Rhodes. A meticulously researched, powerfully written, definitive account of the most significant, and horrifying, application of science in our era. Rhodes does more deep research on his subjects than anyone.

The First Three Minutes by Steven Weinberg. An elegant account of the beginning of the universe. Weinberg has a rare combination of literary skill, wide reading and indisputable authority in physics.

What science books are you currently reading?

Einstein in Berlin by Tom Levenson.

What else are you reading?

A non-science book that I am currently reading is the biography Pol Pot by Philip Short. I am currently doing some work in Cambodia, and this book provides a good understanding of the Cambodian mind and culture.

Which popular-science book have you never read, but feel you ought to have tackled?

All the books by [the astrophysicist] Martin Rees. He’s an extraordinary scientist and a fine writer. I’ve read his essays and articles, but never his books.

Ten of the best for BEC

Ultracold atomic gases can also serve as textbook examples of many basic physical phenomena. To create a Bose condensate or a degenerate Fermi gases it is necessary to cool the atoms until their de Broglie wavelength, which is inversely proportional to their temperature, becomes comparable with the average distance between them. However, if the atoms get too close together, they might form a solid, so the gas has to be dilute, which calls for even lower temperatures.

As of mid-2005 five alkali metals have been condensed – lithium-7, sodium-23, potassium-41, two isotopes of rubidium (⁸⁵Rb and ⁸⁷Rb) and caesium-133. For many years hydrogen was the front-runner in the race, but unexpected problems meant that it was not condensed until 1998. Helium, which succumbed in 2000, was also difficult to condense because it exists in a metastable state that contains some 20 eV of energy, which can be a problem if you are working at thermal energies of 10^-10 eV (see “Triple first for Bose condensates” Physics World May 2001 pp21-22).

The first condensates were produced with a combination of optical and magnetic cooling techniques (which themselves are also splendid textbook examples of basic physics), with all-optical traps arriving a few years later. The advantage of the all-optical approach is that magnetic fields can then be used to control the interactions between the atoms – making attractive interactions repulsive, for example – with the powerful Feshbach-resonance technique. Optical traps have allowed ytterbium and chromium to be condensed in recent years. Ytterbium is notable because it is the only atom with two valance electrons to be condensed so far, while chromium has a very large magnetic dipole moment. Both elements are important in industrial applications as well.

Ultracold gases are also textbook examples of quantum statistics: the fact that we can see a cloud of bosonic ⁷Li atoms get smaller as it is cooled, while a gas of fermionic 6Li atoms remains the same size, is a striking example of the exclusion principle in action. Of course, the most salient aspect of condensates and Fermi gases is that they are quantum gases – macroscopic objects that exhibit purely quantum phenomena such as superfluidity. The next frontier will be performing what were previously thought to be “thought experiments” in condensed-matter physics by trapping Fermi gases on optical lattices – “crystals of light” created by laser beams. These experiments will give physicists the chance to explore many-body phenomena such as superfluidity and superconductivity in systems that are much cleaner than the samples used in most condensed-matter experiments. A decade from now there will be even more to celebrate.

The revolution that has not stopped

At 10.54 a.m. on Monday 5 June 1995 a group of physicists at the JILA laboratory in Boulder, Colorado, created something that had never been seen before – a Bose-Einstein condensate made of atoms. Eric Cornell, Carl Wieman and colleagues had made a new state of matter by cooling a gas of rubidium-87 atoms to a temperature of just 170 nK. This was so cold that the de Broglie wavelength of the atoms was comparable with the distance between them, causing the atoms to condense into the same quantum ground state. The breakthrough in Boulder kick-started an intense period of research into the properties of ultracold atoms that continues to this day, and has implications for topics as diverse as superfluidity and the early universe.

Within months of the first observation, Bose-Einstein condensation had also been seen in lithium-7 by Randy Hulet’s group at Rice University in Texas and in sodium-23 by Wolfgang Ketterle and co-workers at the Massachusetts Institute of Technology (MIT). And by the time Cornell, Ketterle and Wieman shared the 2001 Nobel Prize for Physics, condensates had also been seen in hydrogen, helium and potassium-41. Moreover, a whole new frontier of research was opening up – the study of ultracold Fermi gases.

“What has happened in the past year with Fermi gases has only been topped by the discovery of the first condensates in 1995,” says Ketterle. “But this is just the beginning – research into Fermi gases is now connecting to a long list of intellectual challenges in condensed-matter physics, such as superfluidity, superconductivity, magnetism and so on.”

Fermions and bosons

All atoms, indeed all particles, fall into one of two fundamental classes depending on the value of their intrinsic angular momentum or “spin” in quantum units – atoms with spins of 1/2, 3/2 and so on are fermions, whereas those with integer spins are bosons. The difference becomes clear at very low temperatures: large numbers of bosons can collapse into the same quantum state to form a condensate, whereas the Pauli exclusion principle prevents fermions from doing the same. Rather, as a Fermi gas is cooled, more and more of the lowest energy levels fill up until all the levels below the Fermi energy are occupied. The first such “degenerate” Fermi gas was produced in potassium-40 by Deborah Jin and Brian DeMarco in Boulder in 1999.

Last year Jin and colleagues went a step further by producing the first Fermi condensate, again in potassium-40 (see “Fermionic first for condensates”). A Fermi condensate might seem like a contradiction in terms but it is not: two fermionic atoms can join forces to form a molecule or the atomic equivalent of a Cooper pair, just like electrons form pairs in the Bardeen-Cooper-Schrieffer (BCS) theory of superconductivity. The molecules and the pairs both behave as bosons because they both have integer spin.

Most of these experiments rely on a phenomenon known as a Feshbach resonance to control the interactions between the atoms. First demonstrated in condensates by Ketterle and co-workers in 1998, the Feshbach resonance has become an essential technique in many experiments on ultracold atomic gases, including the condensation of molecules. “In the early days of BEC the strength of the interactions between the atoms was fixed,” says Rudolf Grimm of the University of Innsbruck in Austria. “The discovery of Feshbach resonances changed this completely and led to a new world of experiments.”

Fermi gases have proved to be a very rich source of new results. Indeed, since quarks, electrons and all the fundamental matter particles in nature are fermions, degenerate Fermi gases have the potential to be even more interesting than their bosonic counterparts. However, this comes at a cost. “In general, fermions are more challenging to work with than bosons,” says Randy Hulet at Rice, “due to the difficulty in cooling them and measuring their temperature. These challenges make any experiment more difficult.”

One of the problems is that collisions between atoms play a central role in the cooling of atoms, but the exclusion principle prevents identical fermions getting too close to one another. One way to overcome this problem is to mix bosons and fermions in the same trap and rely on collisions between the two species – which are not forbidden by the exclusion principle – to do the cooling. This approach is known as sympathetic cooling.

However, the absence of collisions can be an advantage in some experiments, according to Massimo Inguscio of the University of Florence in Italy. When using beams of ultracold atoms to measure small forces in interferometer experiments, for instance, fermionic atoms give better results because collisions between bosons reduce the precision of the measurements.

Building bridges

Most researchers working on ultracold atoms believe that the next big goals in the field are to make connections with other areas of physics. The observation of superfluidity in a Fermi gas is widely seen as the next milestone, and it is common knowledge in the community that Ketterle and co-workers have just seen conclusive evidence for this in the form of quantized vortices in a rotating gas of lithium-6 atoms. This should confirm less direct evidence for superfluidity seen at a number of other labs.

Another challenge is to explore the cross-over between the BEC region in which molecules are condensed and the BCS regime in which pairs of atoms are condensed (see “Superfluidity in Fermi gases”). “A number of experiments are under way but we need some better theory of the phenomena to guide them,” says Keith Burnett, a theorist at Oxford University in the UK.

The race is also on to explore the properties of a Fermi gas trapped in an optical lattice – a 3D landscape in which the energy varies to form a perfect lattice of peaks and troughs. If the lasers producing the lattice are intense enough, atoms can be trapped at the peaks (or troughs). Such an experiment would basically reproduce what happens inside a crystal, with the atoms playing the role of the electrons and the lattice representing the ions. The advantage of the ultracold approach, however, is that it is possible to control the interactions between the atoms, which is impossible with the electrons in an ordinary solid (see “Quantum gases in optical lattices”).

In late 2001 Immanuel Bloch, then based in Munich, and co-workers managed to trap ultracold rubidium atoms in an optical lattice and observe phase transitions between the normal superfluid state of a Bose condensate and a state known as a Mott insulator. However, to date only two groups – Inguscio and co-workers in Florence and Tilman Esslinger’s group at ETH Zurich – have managed to repeat this feat with a Fermi gas.

“The next steps will be to understand the influence of atom-atom interactions in the lattice,” says Esslinger. “Various phases that have been predicted by theorists may then become available in the experiment. From the experimental point of view, the challenge is to reach low enough temperatures and to extract the information from the experimental observables.”

Thinking outside the box

While most researchers are looking to the parallels with condensed-matter systems, a small number are moving in a completely different direction. For instance, some groups are exploring the use of condensates to make quantum computers that can perform certain tasks much faster than is possible on a classical computer. However, there is a lot of competition from other approaches. “Condensates have lots of potential quantum bits,” says Chris Monroe of the University of Michigan, “but it remains very difficult to address and control individual neutral atoms at the same level as ions.”

Other groups are hoping to show that the electron has a small electric dipole moment, as predicted by certain extensions of the Standard Model of particle physics. These experiments currently involve making extremely precise measurements on atoms such as caesium and thallium, or on dipolar molecules such as ytterbium fluoride.

Another line of research involves experiments in which the Feshbach resonance is used to make the scattering length – which describes the interactions between the atoms – much longer than the average distance between them. Such gases are said to be strongly interacting. In 2002 John Thomas and co-workers at Duke University in North Carolina produced the first Fermi gas that was both strongly interacting and degenerate. However, when they released the gas from the trap – which was shaped like a cigar – they noticed something unusual: it expanded more rapidly in the narrow direction than in the long direction.

It then emerged that similar behaviour had been seen in experiments built to produce a quark-gluon plasma – a state containing free quarks and gluons that last existed just a fraction of a second after the Big Bang (see “Liquid universe hints at strings”).

“The basic connection between a quark-gluon plasma and our strongly interacting Fermi gas is that they both obey nearly perfect hydrodynamics, where perfect means zero damping and zero viscosity,” says Thomas, who has recently started working with Ed Shuryak, a nuclear theorist at Stony Brook University, on these topics. More generally, Thomas sees strongly interacting Fermi gases as a way of testing the theoretical methods that are used in other areas of physics where strong interactions dominate. Indeed, a recent paper on string theory referenced the Duke group’s 2002 experiment.

And this is not the only connection between ultracold gases and strings. Only last month a group of theorists at Utrecht University in the Netherlands proposed that superstrings could be made in the laboratory by trapping an ultracold cloud of fermionic atoms inside a vortex in a Bose condensate (see “A recipe for making strings in the lab”). Given the progress that has been made over the past 10 years, it should surprise nobody if someone actually manages to do the experiment.

Where are they now?

When Eric Cornell, Wolfgang Ketterle and Carl Wieman shared the Nobel prize in 2001, they became three of the best-known physicists in the world. Cornell and Ketterle remain very active in condensate research, although Cornell was diagnosed with necrotizing fasciitis last October and subsequently had to have his left arm and shoulder amputated. He returned to his lab part-time in April, with plans to search for the dipole moment of the electron. “The doctors tell me I am lucky to be alive,” he said, “and I am much inclined to agree with them.”

Famous five

Wieman, meanwhile, claims to have retired from thinking about big goals in physics. “Nowadays I am satisfied to work on physics problems that seem interesting to me but are not aimed at big goals,” he says. “However, I am thinking about big goals in science education and how to reach them. My main conclusion is that current science education is failing badly at achieving the needs of the 21st century – it needs to be effective and relevant to a large fraction of the population, and not just the small fraction going into science.”

Wieman, who was named US professor of the year last year for “his unwavering dedication to undergraduate teaching”, is convinced that science education requires a scientific approach. “When practices based on good data, disseminating and duplicating what works and so on are combined with effective uses of technology, it is possible to vastly improve science education without it requiring a great deal of additional cost or faculty time,” he says.

And what happened to the graduate students and postdocs who worked on the original BEC experiments at Boulder and MIT? Michael Anderson, the first author on the Boulder paper, joined a company called Meadowlark Optics, where he worked his way up to be vice-president in charge of R&D and manufacturing.

“I worked there until the autumn of 2001, when the bottom fell out of the telecoms market,” he recalls. “Developing reliable manufacturing processes and product improvements was quite challenging technically. However, I found learning about business and the ‘how to win friends and influence people’ side of the job especially challenging – business skills are something that a physics education does not prepare you for!” After considering a career as a cabinet maker, Anderson recently set up his own company, Vescent Photonics, to make sensors based on a new type of diode laser.

Anderson’s co-workers also followed careers in industry. Jason Ensher is a systems engineer for Ball Aerospace & Technologies, while Mike Matthews currently works for 3M in Texas. “It was a great experience to attend the Nobel-prize ceremony in 2001,” says Matthews, “but I don’t regret leaving the field of BEC specifically. However, I do miss the almost boundless curiosity and the energy that you find at a place like JILA.”

Two of the co-authors on the MIT paper – Ken Davies and Marc-Oliver Mewes – are management consultants, while a third, Michael Andrews, has worked for Lucent and start-ups. The other three have remained in academic research: Dallin Durfee and Dan Stamper-Kurn have faculty positions at Brigham Young University and the University of California at Berkeley, respectively; while Klaasjan van Druten is project leader at the University of Amsterdam.

New light on Hitler’s bomb

This year marks the 60th anniversary of the American nuclear attack on the Japanese cities of Hiroshima and Nagasaki. The atomic bombs that were dropped on Japan in August 1945 were the fruit of a herculean wartime effort by the American, British and émigré scientists involved in the Manhattan Project. They had to overcome great obstacles and were only able to test their first atomic bomb after Germany surrendered in May of that year. The main motivation for these scientists when the project began in 1941 was the possibility that they were engaged in a race with their German counterparts to harness nuclear fission for war.

Even Albert Einstein had been involved, signing a letter to President Roosevelt in 1939 urging that the US take nuclear weapons seriously. And in December 1943 the Danish physicist Niels Bohr visited Los Alamos – the home of the Manhattan Project – to offer both scientific and moral support. But when the war was over, it was clear that the Germans did not have atomic bombs like those used against Japan.

The German “uranium project” – which had been set up in 1939 to investigate nuclear reactors, isotope separation and nuclear explosives – amounted to no more than a few dozen scientists scattered across the country. Many of them did not even devote all of their time to nuclear-weapons research. The Manhattan Project, in contrast, employed thousands of scientists, engineers and technicians, and cost several billion dollars.

Not surprisingly, historians have concluded that Germany was not even close to building a working nuclear device. However, newly discovered historical material makes this story more complicated – and much more interesting.

Germany and the bomb: a turbulent tale

Our understanding of the German nuclear-weapons project during the Second World War has changed over time because important new sources of information keep turning up. For example, in 1992 the British government released transcripts of secretly recorded conversations between 10 German scientists who had been interned at Farm Hall near Cambridge in 1945. With the exception of Max van Laue, all the scientists – Erich Bagge, Kurt Diebner, Walther Gerlach, Otto Hahn, Paul Harteck, Werner Heisenberg, Horst Korsching, Carl Friedrich von Weizsäcker and Karl Wirtz – had been involved in the uranium project. What was most interesting was the surprise with which the scientists greeted the news that Hiroshima had been bombed. Ironically, at the end of the war German scientists had been convinced that they were ahead of the Allies in the race for nuclear energy and nuclear weapons.

Further intriguing material appeared in 2002 when the Niels Bohr Archives in Copenhagen released drafts of letters that had been written by Bohr in the late 1950s about a visit to occupied Denmark by Heisenberg and von Weizsäcker in September 1941. After the war, the two German physicists claimed that they had merely gone to Copenhagen to assist Bohr and enlist his help in their efforts to forestall all nuclear weapons. But in the letters, Bohr denied that their actions or motivations had been so noble. The intrigue surrounding the visit has been well dramatized in Michael Frayn’s play Copenhagen.

Figure 2

We now have an extra twist to the tale with new documents that were recently discovered in Russian archives, including papers from the Kaiser Wilhelm Institute of Physics in Berlin. There are four particularly notable items among this material: an official report written by von Weizsäcker after a visit to Copenhagen in March 1941; a draft patent application written by von Weizsäcker sometime in 1941; a revised patent application in November of that year; and the text of a popular lecture given by Heisenberg in June 1942.

One of us (RK) has used these documents – as well as many other sources – as the basis of a new book Hitlers Bombe. The book, which was published in March, prompted a heated debate about how close Germany was to acquiring nuclear weapons and how significant these weapons were (see Physics World April 2005 p7). Working with the journalist Heiko Petermann, RK discovered that a group of German scientists had carried out a hitherto-unknown nuclear-reactor experiment and tested some sort of a nuclear device in Thüringia, eastern Germany, in March 1945. According to eyewitness accounts given at the end of that month and two decades later, the test killed several hundred prisoners of war and concentration-camp inmates. Although it is not clear if the device (figure 1) worked as intended, it was designed to use nuclear fission and fusion reactions. It was, therefore, a nuclear weapon.

Following the publication of Hitlers Bombe, another document has turned up from a private archive. Written immediately after the end of the war in Europe, the undated document contains the only known German drawing of a nuclear weapon (figure 2).

What did German scientists know?

Over the years, several authors have concluded that Heisenberg and his colleagues did not understand how an atomic bomb would work. These authors include the physicist Samuel Goudsmit, who in 1947 published the results of a US Army investigation – entitled Alsos – into Germany’s bomb effort. The historian Paul Lawrence Rose came to the same conclusion in his 1998 book Heisenberg and the Nazi Atomic Bomb Project 1939-1945. These critics argue that the German scientists did not understand the physics of a nuclear-fission chain reaction, in which fast neutrons emitted by a uranium-235 or plutonium nucleus trigger further fission reactions. Both Goudsmit and Rose also say the Germans failed to realize that plutonium can be a nuclear explosive.

These criticisms of the Germans’ scientific incompetence are apparently reinforced by the Farm Hall conversations, which reveal that Heisenberg initially responded to the news of Hiroshima with a flawed calculation of critical mass, although within a few days he had improved it and provided a very good estimate. However, there was other evidence that, no matter how Heisenberg responded at Farm Hall, he and his colleagues understood that atomic bombs would use fast-neutron chain reactions and that both plutonium and uranium-235 were fissionable materials.

For example, in February 1942 the German army officials who were responsible for weapons development described the progress of the uranium project in a report entitled “Energy production from uranium”. This overview, which was discovered in the 1980s, drew upon all classified material from Hahn, Harteck, Heisenberg and the other scientists working on the project. The report concluded that pure uranium-235 – which forms just 0.7% of natural uranium, the rest being non-fissionable uranium-238 – would be a nuclear explosive a million times more powerful than conventional explosives. It also argued that a nuclear reactor, once operating, could be used to make plutonium, which would be an explosive of comparable force. The critical mass of such a weapon would be “around 10-100 kg”, which was comparable to the Allies’ estimate from 6 November 1941 of 2-100 kg that is recorded in the official history of the Manhattan Project – the so-called Smyth report.

Von Weizsäcker’s draft patent application of 1941, which is perhaps the most surprising find from the new Russian documents, makes it crystal clear that he did indeed understand both the properties and the military applications of plutonium. “The production of element 94 [i.e. plutonium] in practically useful amounts is best done with the ‘uranium machine’ [nuclear reactor],” the application states. “It is especially advantageous – and this is the main benefit of the invention – that the element 94 thereby produced can easily be separated from uranium chemically.”

Von Weizsäcker also makes it clear that plutonium could be used in a powerful bomb. “With regard to energy per unit weight this explosive would be around ten million times greater than any other [existing explosive] and comparable only to pure uranium 235,” he writes. Later in the patent application, he describes a “process for the explosive production of energy from the fission of element 94, whereby element 94…is brought together in such amounts in one place, for example a bomb, so that the overwhelming majority of neutrons produced by fission excite new fissions and do not leave the substance”.

This is nothing less than a patent claim on a plutonium bomb.

On 3 November 1941 the patent application was resubmitted with the same title: “Technical extraction of energy, production of neutrons, and manufacture of new elements by the fission of uranium or related heavier elements”. This submission differed in two significant ways. First, the patent was now filed on behalf of the entire Kaiser Wilhelm Institute, instead of just von Weizsäcker. Second, every mention of nuclear explosive or bomb had been removed.

The removal of any reference to weapons could reflect the change of fortunes in the Second World War: in November 1941 a quick German victory no longer appeared as certain as it had done earlier in the year. Another possible explanation is that von Weizsäcker and his colleagues had a change of heart – perhaps their initial enthusiasm for the military applications of nuclear fission had cooled. This would support Heisenberg’s and von Weizsäcker’s post-war claims that they had visited Bohr in September 1941 because they were ambivalent about working on nuclear weapons. Perhaps the most forceful exponent of this thesis is Thomas Powers in his 1993 book Heisenberg’s War.

But another of the new Russian documents – von Weizsäcker’s report on his visit to Copenhagen in spring 1941 – suggests that, at least at that time, he was enthusiastic about the uranium work. Indeed, we know that, after the war, scientists from Bohr’s institute accused Heisenberg and von Weizsäcker of acting as German spies when they came to Copenhagen. There may at least be some truth to this because in March 1941, when Germany had not yet invaded the Soviet Union and victory appeared likely, von Weizsäcker reported the following to the Army.

“The technical extraction of energy from uranium fission is not being worked on in Copenhagen. They know that in America Fermi has started research into these questions in particular; however, no more news has arrived since the beginning of the war. Obviously Professor Bohr does not know that we are working on these questions; of course, I encouraged him in this belief…The American journal Physical Review was complete in Copenhagen up to the January 15, 1941 issue. I have brought back photocopies of the most important papers. We arranged that the German Embassy will regularly photocopy [make photographs of] the issues for us.”

The spotlight turns to Diebner

RK’s book Hitlers Bombe draws upon what was already known about the German wartime work on nuclear reactors and isotope separation, and uses documents from Russian archives, oral history and industrial archaeology to open up a new chapter in the history of German nuclear weapons. For most of the war, there were two competing groups working on nuclear reactors: a team under the Army physicist Kurt Diebner in Gottow near Berlin; and scientists directed by Werner Heisenberg in Leipzig and Berlin.

Whereas the experiments under Heisenberg used alternating layers of uranium and moderator, Diebner’s team developed a superior 3D lattice of uranium cubes embedded in moderator. Heisenberg never gave Diebner and the scientists working under him the credit they were due, but the Nobel laureate did take up Diebner’s design for the last experiment carried out in Haigerloch in south-west Germany. RK now reveals that Diebner managed to carry out one last experiment in the last months of the war. The exact details of the experiment are unclear. After a series of measurements had been taken, Diebner wrote a short letter to Heisenberg on 10 November 1944 that informed him of the experiment and hinted that there had been problems with the reactor. Unfortunately, no more written sources have been found relating to this final reactor experiment in Gottow. Industrial archaeology done at the site during 2002 and 2003 suggests that this reactor sustained a chain reaction – if only for a short period of time – and may have ended in an accident.

In 1955 Diebner submitted a patent application for a new type of “two-stage” reactor that could breed plutonium. An internal section would use enriched uranium to achieve a self-sustaining chain reaction, while a much larger external section would surround the internal reactor and run at a subcritical level. Plutonium could then be removed from internal section. It appears likely that Diebner’s 1955 patent application drew upon his last wartime experiment.

More surprising, if not shocking, is another revelation in RK’s book: a group of scientists under Diebner built and tested a nuclear weapon with the strong support of both Walther Gerlach – an experimental nuclear physicist who by 1944 was in charge of the uranium project for the Reich Research Council. (Hahn, Heisenberg, von Weizsäcker and most of the better-known scientists in the uranium project apparently were not informed about this weapon.) This device was designed to use fission reactions, but it was not an “atomic” bomb like the weapons used against Nagasaki and Hiroshima (figures 1a and b). And although it was also designed to exploit fusion reactions, it was nothing like the “hydrogen” bombs tested by the US and the Soviet Union in the 1950s.

Instead, conventional high explosives were formed into a hollow shape, rather than a solid mass, to focus the energy and heat from the explosion to one point inside the shell (figure 1c). Small amounts of enriched uranium, as well as a source of neutrons, were combined with a deuterium-lithium mixture inside the shell. This weapon would have been more of a tactical than a strategic weapon, and could not have won the war for Hitler in any case. It is not clear how successful this design was and whether fission and fusion reactions were provoked. But what is important is the revelation that a small group of scientists working in the last desperate months of the war were trying to do this.

Blueprint for a bomb

Shortly after the end of the war in Europe, an unknown German or Austrian scientist wrote a report that describes work on nuclear weapons during the war. This report, which RK discovered after Hitlers Bombe was published, contains both accurate information and less accurate speculation about nuclear weapons, and may well include some information from the Manhattan Project – the word “plutonium” is used, for example. Unfortunately, the title page is not included and there is no other evidence of who composed it. However, this individual does not appear to have been a member of either the mainstream German uranium project or the group working under Diebner.

What the report does demonstrate is that the knowledge that uranium could be used to make powerful new weapons was fairly widespread in the German technical community during the war, and it contains the only known German diagram of a nuclear weapon (figure 2). This diagram is schematic and is far removed from a practical blueprint for an “atomic bomb”. The unknown author also mentions a critical mass of slightly more than 5 kg for a plutonium bomb. This estimate is fairly accurate, because the use of a tamper to reflect neutrons back into the plutonium would cut the critical mass by a factor of two. Moreover, this estimate is particularly significant because such detailed information was not included in the Smyth report.

The new report is also interesting because it makes clear that German scientists had worked intensively on theoretical questions concerned with the construction of a hydrogen bomb. Two additional sources con- firm this. The papers of Erich Schumann, director of the Army’s weapons-research department, include many documents and theoretical calculations of nuclear fusion. The Viennese physicist Hans Thirring also discussed this topic in his book The History of the Atomic Bomb, which was published in the summer of 1946.

Not the last word

Historians, scientists and others have debated for decades whether Heisenberg and von Weizsäcker wanted to build atomic bombs.Taken together, the new revelations change our picture of German nuclear weapons. None of this new information supports in any way either the interpretation of Heisenberg and his colleagues as resistance fighters (Powers) or as incompetents with Nazi sympathies (Rose).

However, these new documents and RK’s revelations do place Heisenberg and von Weizsäcker in a different context by making their ambivalence about nuclear weapons much clearer. Although they continued to work on nuclear reactors and isotope separation, and dangled the prospect of nuclear weapons in front of powerful men in the Nazi state, they did not try as hard as they could to create nuclear weapons for Hitler’s regime. Other scientists were doing that, notably Walther Gerlach,Kurt Diebner and the researchers working under him.

It would be rash indeed to believe that this is the last word on the matter. The German atomic bomb is like a zombie: just when we think we know what happened, how and why, it rises again from the dead.

Box 1: Heisenberg’s role

During the Second World War, Werner Heisenberg was one of the most influential scientists in Germany and its leading theoretical physicist. He had won a Nobel prize for his work on quantum mechanics and the uncertainty principle, had become one of the youngest full professors in Germany when he began teaching at the University of Leipzig, and in 1942 at the age of 40 was appointed director of the prestigious Kaiser Wilhelm Institute for Physics as well as professor at the University of Berlin.

However, in the early years of the Third Reich, Heisenberg had been attacked by his fellow Nobel laureate Johannes Stark in an SS publication for being a “white Jew” and “Jewish in spirit”. A subsequent investigation by the SS ended in 1939 with his public and political rehabilitation. The result was that, by 1942, Heisenberg enjoyed the support of influential figures in the Nazi regime, including the armaments minister Albert Speer, as well as the industrialist Albert Vögler, who was president of the Kaiser Wilhelm Society.

Pulled both ways

In February 1942 Heisenberg gave a popular lecture to an influential audience of politicians, bureaucrats, military officers and industrialists. At the time, the future of Germany’s uranium project was in doubt because the Army was only interested in weapons that could be delivered in time to influence the outcome of the war. As we know from a transcript of the talk, which was discovered by the historian David Irving in the 1960s, Heisenberg emphasized both the potential of nuclear weapons and how difficult it would be to make them. His conclusion was clear.

“1) Energy generation from uranium fission is undoubtedly possible, provided the enrichment of isotope uranium-235 is successful. Isolating uranium-235 would lead to an explosive of unimaginable potency. 2) Common uranium can also be exploited to generate energy when layered with heavy water. In a layered arrangement these materials can transfer their great energy reserves over a period of time to a heat-engine. It thus provides a means of storing very large amounts of energy that are technically measurable in relatively small quantities of substances. Once in operation, the machine can also lead to the production of an incredibly powerful explosive.”

However, by the summer of 1942, the uranium project had been transferred from the German Army to the civilian Reich Research Council and the German uranium-project scientists once again enjoyed secure institutional support. In June of that year Heisenberg gave a lecture at the Kaiser Wilhelm Society in Berlin before Speer and other military and industrial leaders of the Nazi state. The lecture has become famous because of the story that Heisenberg responded to a question about the size of an atomic bomb by saying that it would be about as big as a pineapple.

This anecdote was first reported in Irving’s 1968 book The Virus House, but a transcript of the talk had never been found. However, it has now been discovered in the new Russian documents. The text of the June lecture – entitled “The work on uranium problems” – differs significantly from the February talk. Heisenberg begins by mentioning the discovery of nuclear fission in 1939, noting that interest in this new development had been “exceptionally great”, especially in the US. “A few days after the discovery,” he notes, “American radio provided extensive reports and half a year later a large number of scientific papers had appeared on this subject.”

Heisenberg continues by describing Germany’s work on isotope separation and nuclear reactors since the start of the war, cautioning that “naturally a series of scientific and practical problems will have to be cleared up before the technical goals can be realized”. Mid-way through the talk, Heisenberg makes his only mention of nuclear weapons in a rather understated way. “Given the positive results achieved up until now,” he says, “it does not appear impossible that, once an uranium burner has been constructed, we will one day be able to follow the path revealed by von Weizsäcker to explosives that are more than a million times more effective that those currently available.”

But even if that did not happen, the nuclear reactor would have an “almost unlimited field of technical applications”. These include boats and even planes that could travel long distances on small amounts of fuel, as well as new radioactive substances that could be useful for many scientific and technical problems. Heisenberg concludes by saying that new discoveries of “the greatest significance for technology” will be made “in the next few years”.

Since the Germans knew that “many of the best laboratories” in America were working on this problem, they could hardly afford “not to follow these questions”, Heisenberg points out. Even if “most such developments take a long time”, they had to reckon with the possibility that – if the “war with America lasted for several years” – the “technical realization of atomic nuclear energies” might “play a decisive role in the war”.

Heisenberg was right about that, of course. But fortunately for him and his countrymen, the first atomic bombs fell on Hiroshima and Nagasaki instead of Frankfurt and Berlin.

Box 2: A timeline to the bomb

January 1933              Nazis come to power in Germany
December 1938          Otto Hahn, Lise Meitner and Fritz
                                   Strassmann discover nuclear fission in
                                   uranium
2 August 1939            Einstein warns President Roosevelt of
                                   dangers of an atomic bomb
1 September 1939      Germany invades Poland and launches
                                   ”uranium project”
3 September 1939      Britain and France declare war on Germany
1941                           Von Weizsäcker files a draft patent
                                   application that refers to a plutonium bomb
March 1941                 Von Weizsäcker visits Bohr in Copenhagen
June 1941                   Germany invades Soviet Union
September 1941         Von Weizsäcker visits Bohr again, this time
                                   with Heisenberg
6 December 1941       Manhattan Project begins in Los Alamos
7 December 1941       Japan attacks Pearl Harbour
8 December 1941       US enters Second World War
February/June 1942    Heisenberg gives popular lectures on
                                   nuclear weapons
December 1943          Bohr visits Los Alamos
March 1945                 Germany tests a nuclear device in
                                   Thüringia, eastern Germany
7 May 1945                 Germany surrenders
16 July 1945                Trinity test – world’s first atomic blast
6 August 1945             US bombs Hiroshima
9 August 1945             US bombs Nagasaki
14 August 1945           Japan surrenders

Shelf life: Graham Farmelo

What are the three best popular-science books?

The notion of a popular-science book is often something of a misnomer as very few such books are actually very popular. Ask anyone in the book trade. Bill Bryson’s A Short History of Everything is a shining exception, and, although it is marred by mistakes that could easily have been corrected, there is little doubt that it has hit the spot with an extremely wide audience. Part of Bryson’s reward has been a lot of patronizing comment from some other science writers. Anyway, I will be selfish and make my choice according to the criteria that many lapsed scientists use when judging “popular” books – which one taught me the most while remaining widely accessible.

First, I would be hard-pressed to name any science book that more compellingly sets out its story than Richard Dawkins’ The Blind Watchmaker. This was my introduction to evolutionary biology, and I will never forget the impact it had on me. I still remember where I was when I read Dawkins’ line about “raining DNA”.

My second choice is James Gleick’s Chaos – the book that introduced me to a subject I had scarcely heard of. Gleick is an excellent storyteller and masterly at interweaving human interest with gems of science.

Finally, I would pick Jared Diamond’s Guns, Germs and Steel, which is a tour de force of the interdisciplinary study of human history. The continuing sales of this challenging book show that there is a market for big scientific ideas, when they are marshalled by an articulate polymath. Physics has yet to find its Diamond.

What science books are you currently reading?

In preparation for my forthcoming biography of Paul Dirac, I am reading Nancy Thorndike Greenspan’s terrific new biography of Max Born entitled The End of the Certain World. Although its science is a little sketchy, her painting of Born’s social and political milieu is superb (see “Max Born and the peace movement”> Physics World April pp35-38). I am also impressed by The Fly in the Cathedral, the account by journalist Brian Cathcart of James Cockcroft and Ernest Walton’s atom-splitting experiment. He too has done a first-rate job of bringing the heyday of the Cavendish Laboratory thrillingly to life.