Tag Archives: particle physics

Women in Science at the End of the World

It is one thing to use science to better understand the world, another to fear the world itself is crumbling all around you. And yet the scientists who were pursuing research during World War II must have felt both these things keenly, as the Great Powers became embroiled in the second major war in a generation.

Against this backdrop, scientific advances were about to become very
important to the course of the war, and the public perception of science was about to be changed indelibly. Researchers in Europe and the United States were digging to the heart of nuclear fission, an understanding of how the nuclei at the heart of atoms could split, changing into other elements in a naturally-occurring process. Fission was also thought to release an unheard of amount of energy, which in wartime led to one obvious thought: was it possible to use fission to build a bomb?

The Project
After a report from the UK was shared with the US Army, which coordinated the results of a series of secret conferences to discuss the possibility of a fission bomb and how it might be designed, the Army Corps of Engineers launched what was called ‘the Manhattan Project’. Major General Leslie Groves was put in charge, and appointed as scientific director Robert Oppenheimer, an expert in neutron collisions at the University of California Berkeley (the only university with a particle accelerator powerful enough to make plutonium, which had been recently discovered in 1941). Oppenheimer’s first task was to find a suitable location to build a lab where a fission bomb could be designed, built, and eventually tested.

As a child, Oppenheimer suffered from tuberculosis and recovered at the Los Alamos Ranch School in the New Mexico mountains. Far from any major settlements, this location seemed ideal to Oppenheimer and he suggested it as the main site for the Manhattan Project. The land at Los Alamos was purchased by the US government in late 1942, with scientific work beginning there in 1943. Initially General Groves had imagined a military installation, with the scientists in uniforms and posted away from their families. But key scientists balked at uniforms and many wished to bring their families with them. The Los Alamos scientists, working in secret, are often considered a boys’ club, plus wives. Yet even in wartime, and facing prejudice their male counterparts did not, women made huge scientific contributions to the success of the Manhattan Project.

Lise Meitner

The physicist whose work set the scene for the development of the fission bomb, though she would not have wished it, was Austrian-born Lise Meitner. Working in pre-war Germany, she and long-time collaborator Otto Hahn developed the theory and the experimental understanding of nuclear fission. But Meitner was uprooted from Germany due to the Nuremberg laws and her Jewish heritage, and she was forced to flee to Stockholm to continue her work. Her German-based colleagues left her name off several key papers, fearing repercussions from the Nazi authorities. And so the Nobel Prize for Physics in 1944 was then awarded to Meitner’s main collaborator, Otto Hahn, for the discovery of nuclear fission. Although the Nobel committee later revealed that the complications of wartime and the difficulty of assessing interdisciplinary work had contributed to Meitner’s omission from the prize, the US Army had recognized her contribution and invited her to join the Manhattan Project. Her experimental and theoretical insight had clearly been critical, and she was no longer ensnared in Nazi Germany as Hahn was. But Meitner refused to join the Project, saying ‘I will
have nothing to do with a bomb!’ Her advancement of the theory of nuclear fission was, nevertheless, critical to the Project’s success.

Lilli Hornig

Among the women who did join the Project was Lilli Hornig, a Czech chemist who specialized in the newly discovered element plutonium. She had a master’s degree from Harvard when the Manhattan Project began and came to Los Alamos married to an explosives scientist, having been told that anyone
with a chemistry background would be welcomed on board.

Upon arrival, however, Horning was offered a typing job, and was only permitted to work on plutonium chemistry after saying she did not know how to type. Hornig was moved to the explosives group once lab management realised the intense radioactivity of plutonium might cause reproductive damage. ‘I tried delicately to point out that they might be more susceptible than I was; that didn’t
go over well,’ she said in an interview with Manhattan Project Voices (a public
archive of oral histories).

Hornig was eventually a witness to the ‘Trinity’ test of the first so-called ‘atomic
bomb’ in Alamogordo, New Mexico. The Trinity detonation used plutonium as its fissile material, standing on the shoulders of Hornig’s work, but after seeing the devastation it was capable of, she signed a letter along with 100 other scientists requesting that the bomb be demonstrated to the Japanese on an uninhabited island. The next two nuclear detonations occurred over the cities of Hiroshima and Nagasaki, killing over 100,000 people. After the war had ended, Hornig went back to graduate school, getting her PhD and becoming a chemistry professor at Brown University. She was a feminist and a passionate advocate for women in science, studying inequality in the sciences alongside her first love of chemistry.

Maria Goeppert Mayer

Maria Goeppert Mayer was a German physicist, whose doctoral thesis was super-
vised by Max Born, the father of quantum mechanics. Goeppert Mayer came to America when her husband took employment as a professor at Johns Hopkins, but nepotism laws at the time prevented the wife of a professor from being employed at the same institution. Initially she worked unpaid, collaborating with others and eventually studying the separation of different atomic isotopes. The couple then moved to Columbia University and there Goeppert Mayer began her work with the Manhattan Project, studying isotope separation of uranium compounds to be purified into fissile fuel.

While comparing different isotopes, she began to notice ‘magic numbers’ of nucleons which led to more stable atomic nuclei. In 1945, she went to Los Alamos to work directly with Edward Teller on the successor to the atomic bomb: the hydrogen bomb, which exploited the energy from the atomic fusion of elements.

After the war, Goeppert Mayer continued to develop her shell model of the atomic nucleus, which she likened to pairs of waltzers at a dance: each nucleon was a waltzer paired with another waltzer, and more waltzing couples could be fitted into the nucleus by having some go clockwise, some anticlockwise, paralleling nuclear spin. She finally achieved her first full-time paid position as a scientist in 1960 at the University of California San Diego, after receiving her PhD in 1930. When she received the Nobel Prize in Physics in 1963, for her shell model of the atomic nucleus, the local newspaper headline read ‘S. D. mother wins Nobel Prize’. Goeppert Mayer was the second female Nobel Laureate in physics, after Marie Curie.

Each of these women had critical scientific and technical contributions to the Manhattan project, but the politics of the time and secrecy surrounding the wartime effort shrouded the impact of their work. The contributions of male scientists to the Project are more widely touted, and although Hornig, Goeppert Mayer, and Meitner are long gone, the scientific establishment today still struggles to appropriately acknowledge the contributions of women.

Furthermore, that a Czech, a German, and an Austrian were so central to ending an international war, displaced from their countries of origin, shows the value of immigration and even of wartime refugees. Despite difficulties accessing academia and key resources, female scientists played a major role in the Manhattan Project, the building of the first atomic bomb, and the end of World War II.

Particles, Field Theory, and the Higgs Boson

The buzz around the discovery of the Higgs boson last week induced Erin to challenge me to explain what it is! Well, I’m not a particle physicist, but I do like talking about science, so here goes.

It’s my opinion that the easiest way to understand the Higgs boson is by starting from forces and fields. In day to day life, there are two broad sorts of forces that we are used to encountering. There are forces that come from expending energy to create mechanical action, like pushing a door or throwing a ball. But there are also forces that arise due to intrinsic fields, such as gravitational force on an apple falling or magnetic force on a compass. These fields provide a way to quantify the fact that at every point in space, there are gravitational and electromagnetic and other forces coming from other near or not so near objects. If a new object is introduced to a point in space, it feels forces due to those fields. One way to think about it is that the fields transmit forces between objects, like the gravitational field which transmits forces between the Earth and the Sun. Thus, there is no true vacuum, in part because there is a measurable  gravitational field. Quantum field theory takes things a step further and describes everything in terms of fields. Then, what we have been calling ‘particles’ are special mathematical solutions to the field equations, like oscillations of the underlying field.

But when thinking about fields providing force, a very good question to ask is: what’s the physical mechanism for that? When I push on a door, I generate movement by activating muscles, which turns one chemical into another, turning energy that was stored in chemical bonds into a mechanical form of energy. My hand transfers that energy to the door, via the interface between door and hand at the spot where I’m pushing, and then the door moves. So if there is really a magnetic field pushing a compass needle, how is energy transferred to the needle in order for it to move?

The current thinking in particle physics is that each of these fields has a corresponding particle that transfers the forces from that field. So for an electromagnetic interaction between two particles, there is actually a third particle type that is being exchanged, which is what conveys energy from the field. Usually these mediating particles are ‘virtual’, meaning they exist over very short distances but can have high energies (the requirement of short lifetime for high energy comes from the uncertainty principle between energy and time). In the Standard Model of particle physics, these mediators are called gauge bosons. For example, the electromagnetic field is mediated by the photon, which you may know as the quantum of light. There are special forces that are only noticeable on very short length scales, such as those in the nucleus of atoms. These are the nuclear strong and weak forces, mediated by gluons and W and Z bosons. An additional gauge boson for the force of gravity, called the graviton, has been theorized but not yet detected.

The Standard Model of particle interactions was intended as a framework for unifying the electromagnetic, strong, and weak forces, meaning that it had to account for the properties of gauge bosons. Accounting for the mass of these gauge bosons has been complex: the photon is massless, but the W and Z bosons have a significant mass. But in early formulations of the Standard Model, all particles were treated as massless, and it was a big issue to find a way to fit non-zero particle mass into the picture. The Higgs field is a means to that end, dreamt up by the theoretical physicist Peter Higgs in 1964. It is another field which interacts with particles and contributes to the forces that they experience, this time by giving them mass. The Higgs field is often described as a field which slows some particles down as if they were moving through treacle, but Higgs himself has mentioned his dislike of the idea that the particles experience drag or turbulence due to the Higgs field. The analogy with drag and turbulence implies that energy is being dissipated, but the Higgs field affects particles without reducing their energy. Higgs proposes thinking of it as similar to the refraction of light as it enters a medium like water. As the properties of light are changed by moving through water, so the properties of the particle are being changed by interaction with the Higgs field. How particles interact with the Higgs field determines their mass.

If the Higgs field is real, the corresponding gauge boson must exist. And, a new boson with many of the expected properties of the Higgs boson has now been found in two corroborating experiments at CERN, a particle physics laboratory in Geneva.

There are many different theoretical ways to add the Higgs field into the Standard Model. But most of them predict a fairly high energy for the Higgs boson, and so as accelerator energies rose progressively over the decades and the Higgs boson was not found, Higgs bosons of low mass were eliminated as possibilities. The Large Hadron Collider, which went online in 2008, was expected to have an energy range capable of either finding a Higgs boson that fit with one version of the Standard Model, or proving that the Standard Model contained a serious error. The newly discovered Higgs boson seems to fit the Standard Model, though more work is needed to figure out which version of the Standard Model was correct.

There are still lots of questions to be answered regarding the Standard Model, though. The Standard Model does not incorporate gravity or relativity. It doesn’t explain why the range of mass values for other fundamental particles is huge and kind of arbitrary, as opposed to the few fixed values for charge that fundamental particles can have. It also doesn’t explain why there is more matter than anti-matter (sometimes called CP-violation), or what dark matter or dark energy (both of which have been observed by astronomers) might be. So the discovery of the Higgs boson is definitely a triumph for the particle physics community, but there are still plenty of discoveries to be made!