E=mc2

by David Bodanis

  It was all empty air, and a camera down there would show the arriving meteor, once it pierced the outer film, falling through empty space. Only at the very bottom, down on the sea floor, was there some powerful device, extremely compact, that could grab an incoming meteor, and send it hurtling up through the atmosphere, and back into outer space. The equivalent inside an atom is the atom's nucleus, lost far in the center. Only up near the outer surface of an atom are there the flurries of electrons that are involved in ordinary reactions, such as burning a piece of wood in a fire. But they're far from the central nucleus, which is shimmering deep below, within all the empty space.

  If atoms were like little ball bearings, then Rutherford had found that these ball bearings were almost entirely hollow. There was just a tiny speck right at the center, called the nucleus. It was a disconcerting finding— the atoms we're composed of are mostly just empty space!—but by itself that still wouldn't have let anyone see how E=mc2 could apply. The "solid" electrons up on the outer surfaces of the atom weren't going to pop out of their material existence and turn into exploding clouds of energy.

  It was pretty clear that the nucleus was where scientists would have to turn next. There was a lot of electricity in the atom, and while half of it was spread diffusely, in the far-flung orbits of those electrons, the other half of it was crammed into the ultradense nucleus at the center. There was no known way to keep so much electricity concentrated in that small a volume. Yet something down there, in that nucleus, was able to squeeze down all that electricity, and hold it in a tight grasp, and keep it from squirmingly escaping. That must be where the storehouse—the hidden energy—that Einstein's equation hinted at could reside. There were positively charged particles—what we call the proton—in there, but no one could make out any greater detail.

  An assistant of Rutherford's, James Chadwick, finally got an important better view, in 1932, when he detected yet another item locked inside the nucleus. This was the neutron, which got its name because although it roughly resembled the proton in size, it was electrically entirely neutral. It had taken Chadwick more than fifteen years to identify it. At one point students had put on a play about his quest for this particle that had so few properties it might as well, they teased, be called the "Fewtron." But if you've spent years putting up with Rutherford's booming impatience, you can handle students having their fun. Although Chadwick was a quiet man, he was pretty determined about what he would do.

  Chadwick had originally been a slum boy from the Manchester streets, and his professional career had almost been destroyed just as it was about to begin. As a new postdoc under Rutherford, Chadwick had gone to Berlin, to study in the labs of the returned Hans Geiger. When World War I began, he meekly followed the advice of the local Thomas Cook's office that there was no reason to hasten to leave. As a result, he ended up spending four years as a POW, in the converted stables of a cold and windy Potsdam racecourse. He tried doing as much research as he could there, and even managed to get radioactive supplies. An enterprising firm, the Berlin Auer company, had extra thorium, and was marketing it to the German public in toothpaste as a way to make your teeth glow white. Chadwick simply ordered this miracle tooth whitener from the guards, then used it for his experiments. But he had such poor equipment that his tests never came to much. He was falling behind, and when he got back to England after the war ended in November 1918, barely managed to get back on track. Never again would he meekly follow anyone's advice.

  In theory Chadwick's 1932 discovery of the neutron should have led immediately to further discoveries. A number of radioactive substances release neutrons, and those could be aimed like a submicroscopic machine gun at waiting atoms. Because neutrons were neutral, they wouldn't be bothered by the negatively charged electrons at the surface of the target atoms. When they reached the nucleus at the center, they shouldn't be bothered by any charges down there either. They'd be able to slip right in. Maybe you could use them as probes to see what was happening in there.

  To Chadwick's disappointment, though, he could never get that to happen. The harder he blasted neutrons in at an atom, the less success he had in getting any of them to slip into the nucleus at the center. Only in 1934 did yet another researcher find a way around that problem, and manage to get neutrons to enter easily inside a target nucleus, to better see that nucleus's structure. And he wasn't working in an even more sophisticated research lab, but in the last place one might have expected.

  The city of Rome, where Enrico Fermi lived, had memories of grandeur, but in the long decades leading up to the 1930s, it had steadily been left further and further behind the rest of Europe. The lab that the government gave Fermi, who was respected as one of Europe's leading physicists, was on an out-of-the-way street, in a quiet gardened park. There were tiled ceilings, and cool marble shelves, and a goldfish pond under the shady almond trees out back. For someone wanting to make a break from the mainstream European consensus, it was ideal.

  What Fermi found in this gentle seclusion was that other research teams had been wrong to focus on blasting neutrons at higher and higher power to get them to enter the tiny nucleus inside an atom. Spraying fast neutrons directly at the great empty spaces inside a target atom meant that most of the neutrons simply raced right through. Only if the neutrons were slowed, so that they almost dawdled in their flight toward a distant nucleus, would they have a good chance of slipping inside. Slowed neutrons acted like sticky bullets. The reason they stuck so well to nuclei, one might visualize it, was that they became "spread out" in their relatively slow flight. Even if their main body missed the nucleus, the spread-out portions were still likely to connect.

  On the afternoon when Fermi realized slowed neutrons could do this, his assistants lugged up buckets of water from the goldfish pond out back. They sprayed fast neutrons from their usual radioactive source into the water. The water molecules were of a size that made the incoming fast neutrons rebound back and forth till they slowed down. When the neutrons finally emerged, they were traveling slowly enough to slip regularly into any target nuclei ahead of them.

  With Fermi's trick, scientists now had a probe that could get into the nucleus. But even that didn't make things entirely clear. For what was happening when the slowed neutrons entered? The full power that Einstein's equation spoke about still wasn't coming out. At most you got slightly changed forms of ordinary nuclei, which leaked out only a gentle sort of energy. It was useful for tracers that could be swallowed and then tracked to see what was going on inside the body. One of the first researchers to use similar tracers, George de Hevesy, employed it, his very first time, to prove that the "fresh" hash his Manchester boardinghouse landlady was serving was not quite as fresh as promised, but rather was coming back, slopped onto a fresh plate, steadily every day. But the slight energy leakages from elements that could be safely swallowed were not what the massive c2 in the equation promised.

  Somehow there had to be a further explanation; some further level of detail that physicists hadn't yet grasped. Atoms weren't solid massy spheres, but rather were almost entirely empty space—like an emptied ocean basin—with just the barest speck of a nucleus down at the center. That was what Rutherford had seen. The nucleus wasn't a simple solid speck either. It contained protons that crackled with positive electric charge, and pebblelike neutrons were packed in along with them. That was clear by 1932. The neutrons could go in and out of that nucleus pretty easily, once you did the unexpected twist of slowing them down when you sent them forward. That was what Fermi saw in 1934. But that's where matters stuck for several years.

  9 Quiet in the Midday Snow

  The solution to what was happening inside the nucleus— and so an unveiling of matter's deeper mechanisms, which would finally allow the energy promised by E=mc2 to emerge—only came in 1938. It was provided by a solitary Austrian woman, sixty years old, stuck on the edge of Europe, in Stockholm; who didn't even speak Swedish.

  "I have here . . ." she wrote, "no position th
at would entitle me to anything. Try to imagine what it would be like if. . . you had a room at an institute that wasn't your own, without any help, without any rights. . . ."

  It was a dispiriting change, for just a few months earlier, Lise Meitner had been one of Germany's leading scientists—" our Madame Curie," as Einstein put it. She'd first arrived in Berlin in 1907, an impossibly shy student from Austria. But she'd tried to open up, and quickly became friends with one exceptionally good-looking young man at her university named Otto Hahn. He had an easygoing confidence, a self-teasing Frankfurt accent, and seemed to feel it a personal obligation to put this quiet newcomer at ease.

  They were soon sharing a lab in the basement of the chemistry department. They were almost exactly the same age, in their late twenties. He persuaded her to hum two-part harmony songs from Brahms with him, despite her off-key voice. When their shared work was going especially well, she wrote, "[Hahn] would whistle large sections of the Beethoven violin concerto, sometimes purposely changing the rhythm of the last movement just so he could laugh at my protests. . . ." The Physics Institute was nearby, and other young researchers there "often visited us and would occasionally climb in through the window of the carpentry shop, instead of taking the usual way." After working hours, Meitner remained solitary, living in a succession of single rooms, and sitting in the cheapest student seats at concerts she went to by herself. It was only at the lab that she found community.

  Otto Hahn and Lise Meitner

  CHURCHILL COLLEGE, CAMBRIDE

  She was a much better analyst and theoretician than Hahn, but he was bright enough—and sensible enough—to realize this would only be to his good; he had a history of finding excellent mentors. The first joint discoveries of Meitner and Hahn led to their getting a large lab in the new Kaiser Wilhelm Institutes, on what was then the western outskirts of Berlin. There were rural windmills still within sight; a forest a little farther to the west. They were becoming known as an important and trustworthy research team; they contributed to building up a core of indispensable knowledge about how atoms worked; their findings were soon as necessary to consider as those of Rutherford in England.

  Through it all, she and Hahn kept their surface formality, carefully avoiding the informal "Du" form of address. In all her letters he was "Dear Herr Hahn." But there can be a special bond this way; a carefully unstated awareness that such dignified formality is blocking the pair off from any deeper links.

  In 1912, after four years of working together, with Meitner now age thirty-four, Hahn married a younger art student. Meitner told everyone that it didn't matter. But although she'd never officially dated Hahn, she never dated anyone else in the years after that. There was another young colleague Meitner had been friendly with, James Franck, and she stayed in touch with him for over half a century, even when he got married, and then later when he was forced out of Germany to distant America. "I've fallen in love with you," Franck teased when they were both in their eighties. "Spat! (Late!)" Lise laughed.

  In World War I, Meitner volunteered in hospitals, including some hellish ones near the eastern battlefields, while Hahn was on assignment with the army. The moral dilemmas of his work with poison gas seemed to bother neither of them. She sent letters regularly: lab gossip, and accounts of swimming trips with Hahn's wife, and occasionally the gentlest description of her hospital work. She also had a little time for research: "Dear Herr Hahn! . . . Take a deep breath before you begin reading. . . . I wanted to finish some of the measurements so that I could . . . tell you a variety of delightful things."

  Meitner had filled in one of the last gaps left in the periodic table listing all the elements. The work was her own, but she put both their names on it, and insisted to the Physikalische Zeitschrift editor that Hahn's name go first. During their wartime separations she tried not to push him for replies, but sometimes she slipped: "Dear Herr Hahn! . . . Be well, and write, at least about radioactivity. I remember a time very long ago when you would once in a while send a line even without radioactivity."

  A little after the war they switched to different labs. By the mid-1920s Meitner headed the theoretical physics division within the Kaiser Wilhelm Institute for Chemistry. She was still shy on the outside, but had become confident in her intellectual work, regularly sitting in the front row with Einstein or the great Max Planck at the most respected theoretical seminars. Hahn was aware he couldn't follow such explorations, and cautiously stuck to more straightforward chemistry. But when Fermi's 1934 advances showed how the neutron might offer an ideal probing tool into the nucleus, Meitner shifted once again, to studies of the nucleus's properties. This meant she could hire Hahn, for chemists were always needed to study the new substances that were being formed.

  In 1934 they started working together again, also taking on a recent doctoral student as their assistant, Fritz Strassmann. Hitler had come to power in 1933, but although Meitner was Jewish, and so immediately fired from the University of Berlin, she still was an Austrian citizen. The Kaiser Wilhelm Institutes had its own source of funding, and happily continued paying her as a full staff member.

  But in 1938, Germany took over Austria, and Meitner became a German citizen by default. The institute might still be able to keep her on, but it would depend a lot on what her colleagues said. An organic chemist named Kurt Hess had long had a small office at the institute. He was a minor researcher, full of envy, and he was one of the first at the institute to become an active Nazi. "The Jewess endangers our institute," he began to whisper, to anyone who would listen. Meitner heard this from one of her ex-students, who had remained loyal. She talked it over with Hahn. Hahn went straight to Heinrich Horlein, the treasurer of the organization that funded the Kaiser Wilhelm Institute for Chemistry.

  And Hahn asked Horlein to get rid of Meitner.

  To say that people have been charming, as Hahn had been all his life, is simply to say that they've developed a reflex to do what will put the individuals around them at ease. It says nothing about their having a moral compass deeper than that. Hahn may have been slightly troubled by what he was doing to his old colleague: "Lise was very unhappy now that I had left her in the lurch." But most other German physicists did what the new government wanted them to, and many of Hahn's past students, pro-Nazi, were in positions of power as well. They—more than she—were the people he was increasingly working with now, the ones he needed to please.

  He helped her a little bit with the details of leaving, but it's unclear how much Meitner understood in the shock. From her diary: "Hahn says I should not come to the Institute anymore. He has, in essence, thrown me out."

  By the time she'd settled in Stockholm, in August 1938, Meitner didn't mention to anyone else what Hahn had done. Instead, almost by reflex, she just remained involved from a distance with the work she had been leading. With Strassmann and Hahn's help, she'd been guiding the streams of slowed neutrons into uranium, the heaviest of all naturally occurring elements. Since neutrons slipped into and then stuck within the nuclei they hit, everyone expected that the result would be some new substance, even heavier in weight than the uranium they started with. But try as she and the researchers in Berlin might, they couldn't clearly identify whatever new substances they were creating.

  Hahn, as ever, seemed the slowest to grasp what was happening. Meitner met him, in neutral Copenhagen in November, and after he admitted he didn't have a clue, she sent him back with clear instructions for more experiments. He just had to use the top-quality neutron sources and counters and amplifiers she'd assembled, and which were still in place in their lab, right where she'd left them. The mail was so quick between Stockholm and Berlin that she could even talk him through the steps. "Meitner's opinion and judgment carried so much weight with us in Berlin," Strassmann recounted later, "that we immediately undertook the necessary . . . experiments." However much she was wounded, at least she could continue with the work that had been her focus for years.

  Meitner suggested they keep an eye o
ut for variants of radium that might be produced in the long bombardment process that had started with uranium. (Radium is a metal with a nucleus almost as massive as that of uranium. Both are so overstuffed with neutrons that they regularly end up spraying out radiation.) At this stage it was just a hunch, based on similarities between the two metals, and the fact that they were so often found together in mines.

  But it meant that the broader effects of E=mc2 were, finally, about to appear.

  Monday evening in the lab

  Dear Lise!

  . . . There is something about the "radium isotopes" that is so remarkable that for now we are telling only you. . . . Perhaps you can suggest some fantastic explanation. . . . If there is anything you could propose that you could publish, then it would still in a way be work by the three of us!

  Otto Hahn

  They had been using ordinary barium as something of an adhesive in the lab, to gather the fragments of neutron-loaded radium. Once the barium had done that job, it was collected with acids and then rinsed away. The problem now, though, was that Hahn could not get it to separate. Some of the barium that was left always seemed to have tiny bits of something radioactive stuck to it.

  He and Strassmann were at a loss. "Meitner was the intellectual leader of our team," Strassmann explained. But now she wasn't here. Hahn wrote her again, two days later: "You see, you will do a good deed if you can find a way out of this." They could do no more. The strange result—why couldn't they get the radiation away from the simple barium?—would be up to her to try to work out.

  It was nearly Christmas by this time, and a couple who knew that Meitner was alone in Stockholm invited her to stay at a hotel in their vacation village of Kungälv, on the west coast of Sweden. A nephew of hers whom she'd always liked, Robert Frisch, was in Copenhagen, and on Meitner's suggestion, the couple invited him too.