Inside the Centre: The Life of J. Robert Oppenheimer (48 page)

In four pages, mostly filled with the imposing equations of relativistic gravitational theory, Oppenheimer and Snyder provided a way of understanding the collapse of a neutron star into a black hole, the implications of which are still being explored today. Pick up a popular book on black holes now and the chances are that what you will see is a description extending over several pages, even several chapters, of the physical realities that correspond to the equations of Oppenheimer and Snyder. Almost certainly, the book will also attempt to convey the nature of black holes using the device adopted by Oppenheimer and Snyder of imagining two observers.

And yet, during Oppenheimer’s lifetime, this remarkable paper – and the ones preceding it written with Serber and Volkoff – were greeted with silence from both astronomers and physicists. This silence ended with the discovery in 1967 of ‘pulsars’, which, it was realised, are rotating neutron stars; the following year it was discovered that what had been known for a long time as the Crab Nebula was in fact the remnant of the 1064 supernova and that in the middle of it was a neutron star. Since then, neutron stars have even been photographed. As for black holes, though they have not been (and could not be) photographed, there is now abundant evidence that they exist and they are the subject of intensive theorising and observational work.

One of the leading figures in the study of black holes, John Archibald
Wheeler, was also one of the first people to revive interest in Oppenheimer’s work on the subject, and is credited with having introduced the term ‘black hole’. In the 1960s, shortly before Oppenheimer’s death, Wheeler tried to talk to him about his work on gravitational collapse, but Oppenheimer was not interested. Had he lived just a few years longer, Oppenheimer would have seen the empirical evidence which confirmed that the theory developed by him and his students in the late 1930s was not just a piece of mathematics, but was a description of physical reality.

One reason for the initial lack of interest in these great papers of Oppenheimer and his students has to do with the timing of two very different events. Oppenheimer’s paper with Volkoff was written in the very month that it was announced that scientists in Germany had discovered nuclear fission; his paper with Snyder, meanwhile, was published on the very day that the Second World War began. For the time being, the question of what happened inside a massive stellar core was of far less interest, and far less import, than the questions of what might be made to happen inside a uranium nucleus and what might become of Europe.

^fn37
Chevalier was evidently a little hazy on the exact years, as these do not match the dates he gave in his letter to Oppenheimer.

^fn38
It has worried some people that Griffiths mentions three members of the unit, while Chevalier mentions seven. However, Griffiths does not say it only had three members. He says rather: ‘Of the several hundred members of the faculty at Berkeley three were members of the communist group.’ As neither Addis nor Radin was at Berkeley during the period in question and the other two were not university people at all, this is perfectly consistent with Chevalier’s description of the group having seven members.

^fn39
‘Atomic electrons’ are those outside the nucleus, as opposed to those that are emitted from the nucleus in beta decay.

^fn40
The figures here are perhaps confusing. The Chandrasekhar limit of 1.4 solar masses given previously is a calculation of how much mass a white dwarf can have without collapsing into a neutron star. The Oppenheimer–Volkoff limit of 0.7 solar masses is a calculation of how much mass a neutron star can have and still be stable – that is, without collapsing further. What happens to a neutron star that continues to collapse is an unanswered question, which is why Oppenheimer says the question of what happens to large stars (those more massive than 1.5 suns) still remains unsolved. The full story about the gravitational collapse of large stars, he is indicating, has yet to be told.

THE RESPONSE OF
scientists to the news of nuclear fission in the New Year of 1939 was in itself a remarkable chain reaction, with Oppenheimer and his colleagues on the West Coast of America somewhat at the end of the chain.

It began with two chemists in Berlin, the eminent Otto Hahn and his young assistant Fritz Strassmann. They had been bombarding uranium with fairly slow, low-energy neutrons, trying to repeat the experiments conducted in Paris by Irène Curie and her assistant, which had produced some puzzling results. On 19 December 1938, Hahn wrote to his friend and former colleague Lise Meitner, who, because she was Jewish, had recently fled Germany and was now in Sweden. Meitner was a very able physicist to whom Hahn had often appealed in the past to explain his results. Now he asked her to explain something that had utterly perplexed him and Strassmann: when their slow neutrons hit uranium, the result seemed to be the emission of
barium
.

To understand why this was so puzzling, one has to take a step back and survey what had been achieved up to that point in the way of changing one element into another. Rutherford, back in 1919, had been the first modern alchemist, changing nitrogen into oxygen by bombarding it with alpha particles. What, exactly, was happening in this process was made clear by the photographs Blackett took in 1924: nitrogen, with atomic mass 14, was absorbing the alpha particle (mass 4), producing oxygen (mass 17) and emitting a proton (mass 1), or, in symbols: N
¹⁴
+α
⁴
→ O
¹⁷
+ p
¹
. Then, in 1932, Cockcroft and Walton had split a lithium atom by bombarding it with protons, and again there was no mystery about what was happening: lithium (mass 7) was absorbing a proton (mass 1) and then splitting into two helium nuclei, each with a mass of 4: Li
⁷
+ p
¹
→ α
⁴
+ α
⁴
.

At the heart of these processes is not only some fairly basic arithmetic
(14 + 4 = 17 + 1 and 7 + 1 = 4 + 4), but also some fairly basic chipping away at atomic nuclei, with nothing more dramatic than the absorption and emission here and there of an alpha particle and/or a proton. But it is impossible to understand how barium could be emitted from uranium by such means. Uranium is a very heavy element. In fact, it is the heaviest naturally occurring element. It has ninety-two protons and, in its most common and stable form, 146 neutrons, giving it an atomic mass of 238. Barium has fifty-six protons and, in its most common and stable form, eighty-two neutrons, giving it a mass of 138. You cannot, therefore, get barium from uranium by either adding or subtracting a proton or an alpha particle; you need to lose about 100 nucleons (protons and/or neutrons)! Whatever that is, it is not ‘chipping’.

Hahn and Strassmann had already strained credulity by suggesting earlier that what they had witnessed was the emission of an isotope of radium (atomic number 88, atomic mass 223–8), but it was just about conceivable how this might happen by, as they said, ‘the emission of two successive alpha particles’ (together with a couple of neutrons or protons). But no amount of juggling with the figures could explain how barium could be emitted from uranium on the assumption that transmutation was due to the emission or absorption of protons, neutrons or alpha particles. Something else was going on, something not previously encountered.

In Sweden, Meitner was joined by her nephew Otto Frisch, a young physicist who had lately been working in Copenhagen with Niels Bohr. On Christmas Eve 1938, Frisch and Meitner discussed the results obtained by Hahn and Strassmann. ‘But it’s impossible,’ Frisch remembers them thinking. ‘You couldn’t chip a hundred particles off a nucleus in one blow.’

Following a suggestion by George Gamow, Bohr had recently put forward the idea that an atomic nucleus is more like a liquid drop than a billiard ball; not a hard, stable object, but something continually moving, wobbling, with the forces acting not only on it but in it, pulling it in different directions. Among those forces in an atomic nucleus is the electrostatic repulsion that protons exert on one another. Seen like this, the heavier the nucleus is, the
less
stable it should be, because it will have more protons, all trying to pull away from the others. That is, in fact, why no elements heavier than uranium exist in nature; as soon as they are created, they pull themselves apart.

This fact was not well understood in 1938. Up to then, scientists thought that by bombarding uranium with neutrons they would create heavier, ‘transuranic’ elements. They thought that the uranium would absorb a neutron, which would then, through beta decay, transform into a proton, thus creating a new, heavier element. Thinking about the results Hahn and
Strassmann had obtained in terms of Bohr’s image of the nucleus as a drop of water, Frisch and Meitner realised that the opposite had happened: instead of the uranium absorbing a neutron, the neutron had hit a wobbling nucleus (which they pictured like a balloon full of water, pinched at the middle), making it wobble a bit more until it split in half. Frisch and Meitner also realised that this splitting – to which Frisch gave the name ‘fission’ – would release enormous amounts of energy, namely the binding energy holding the nucleons of the uranium nuclei together. They were able to be fairly precise about how much energy would be released, since they knew that the separated pieces of the split uranium nucleus – one of barium, the other (therefore) of krypton
^fn41
– would have a slightly smaller combined mass than that of the original nucleus, and were able to calculate what that difference would be. The answer is: a mass equal to one-fifth of a proton. Then, using the famous formula E = mc
²
, they could convert that mass into energy and thus work out that the amount of energy released by the fission of uranium is 200 million electron volts, which, not coincidently, is exactly the amount of energy Frisch and Meitner had calculated would be needed to pull the protons apart.

All this was understood by Frisch and Meitner on Christmas Eve 1938. For about a week they were the sole possessors of this (potentially, at least, literally) earth-shattering knowledge. Then, on 1 January 1939, Meitner wrote to Hahn, telling him that she and Frisch ‘consider it
perhaps
possible energetically after all that such a heavy nucleus bursts’. Two days later Frisch was back in Copenhagen, where he told Bohr the news. ‘I hadn’t spoken for half a minute,’ Frisch remembers, ‘when he struck his head with his fist and said, “Oh, what idiots we have been that we haven’t seen that before.”’

By 6 January, Frisch and Meitner, working together on the phone, had drafted a paper on fission that they intended to send to
Nature
. Bohr was leaving for the United States the next day, and before he left Frisch told him about the paper and handed him two pages of it, which was all he had been able to type out in the time available. He also told Bohr about an experiment he proposed to conduct in Copenhagen to confirm Hahn and Strassmann’s result. Bohr promised not to mention fission in America, until he had heard from Frisch that his paper had been received by
Nature
. Frisch decided not to send the theoretical paper he had written with his aunt until he had conducted his experiments. These were done quickly and did indeed confirm the remarkable fact that uranium can be split apart by slow neutrons, thus releasing enormous amounts of nuclear energy. Frisch dashed off a paper reporting on his experiments and on 16 January sent both papers to
Nature
.

On the same day, Bohr, accompanied by his colleague Léon Rosenfeld, arrived in New York. On the way over the two of them had read the pages that Frisch had given Bohr, and had spent almost the entire time discussing fission. Bohr, however, forgot to mention to Rosenfeld his promise to Frisch not to discuss it with the Americans. When they arrived in New York they were met by Enrico Fermi, who had, after receiving the 1938 Nobel Prize, fled Italy and was now working at Columbia University. With Fermi was the Princeton physicist John Archibald Wheeler. While Bohr went off with Fermi to spend the day and night as his guest in New York, Wheeler accompanied Rosenfeld to Princeton. Thus it was that, on the train from New York to Princeton, Wheeler became the first person in America to hear that nuclear fission had been achieved.

It so happened that that day, a Monday, was when the physics department at Princeton held its Journal Club, where they discussed new results in physics. Naturally, therefore, Wheeler asked Rosenfeld to give a short report of fission to the assembled faculty members and graduate students, and, of course, the news caused quite a stir. Isidor Rabi and Willis Lamb, who were both then working at Columbia University, happened to be at Princeton that week, so they returned to New York bearing the news to, among others, Fermi (to whom Bohr, of course, had said nothing). Fermi at once devised an experiment similar to Frisch’s to confirm the result, and meanwhile Bohr continued on his journey across the United States, now (having written a letter to
Nature
giving appropriate credit to Frisch and Meitner) feeling free to discuss fission with anyone who wanted to discuss it, which was almost everyone he met.

His next stop was Washington, to attend the annual Theoretical Physics Conference, co-sponsored by the Carnegie Institution and George Washington University. There, in front of fifty-one of America’s best physicists, including Harold Urey, George Gamow, Edward Teller, Hans Bethe and George Uhlenbeck (who was now back at the University of Michigan, having just returned from Holland), Bohr announced the news. Immediately two experimentalists at the Carnegie Institution returned to their labs to set up an experiment.

By now, before Frisch and Meitner’s paper had even appeared in print, it seemed as if every physicist on the East Coast knew about fission, and at least two laboratories had conducted experiments that confirmed the results. The news had still not hit the West Coast (as one physicist remarked: ‘We didn’t make long-distance calls in those days’), but this was about to change. Attending the Washington conference was a science writer from the
Washington Evening Star
, whose report on the sensational discovery was published on 28 January. The next day, the
San Francisco Chronicle
picked up the story.