Catching a Nuclear Curveball

The following article is reprinted from Elizabeth Wade's "Fear of Physics" blog in the Columbia Spectator. Wade wrote about Columbia nuclear physicist William Zajc's involvement with the Relativistic Heavy Ion Collider. Article source.

Nuclear physics is like rocket science—so much a part of our cultural lexicon that it is hard to believe it actually exists. From presidential mispronunciations to debates about who should be allowed to have reactors and why, the technological and military possibilities of harnessing the energy of the atom’s nucleus are everywhere. With the potential benefits and dangers of nuclear energy always under discussion, it is easy to forget that we still do not completely understand what goes on in atomic nuclei in the first place.

Atomic nuclei were first discovered in 1909 by Ernest Rutherford, who fired positively charged alpha particles at a thin sheet of gold foil and observed a small percentage of them bouncing straight back at him. He concluded they must have been colliding with a tiny, positively charged part of the atom, now known as the nucleus.

Scientists soon figured out that the nucleus was made of protons and neutrons and called the force that held them together the strong force. The name is somewhat of an understatement, for, as Columbia nuclear physicist William Zajc explains, “Its relative strength compared to chemical forces is the difference between ordinary fire and nuclear explosions.” It is the process of splitting atomic nuclei, or fission, that leads to mushroom clouds and nuclear reactors alike.

But protons and neutrons are not the end of story. By the early 1970s, physicists realized that protons, neutrons, and the rest of the baryons, or the particles that make up normal matter, are made up of tiny particles called quarks. By 1995, they had observed all six kinds of quarks in particle accelerators. But unlike other kinds of particles, quarks have never been seen alone. They always come in pairs or triplets, held together by what turns out to be a deeper and stronger manifestation of strong force.

In the early 1970s, Zajc says, “We realized that the attraction between the protons and neutrons that make atomic nuclei was only a tiny leftover bit of the real strong force. The real strong force holds quarks together inside protons and neutrons.” We now understand that the true strong force governs the interactions between quarks and gluons, which do exactly what their name suggests—glue quarks together inside a particle.

For many years, Zajc and other physicists believed that quarks and gluons would be freed from their particle prisons at high enough energies, like those that existed in the microseconds after the Big Bang. They set out to see if they could recreate such a situation at the Relativistic Heavy Ion Collider (RHIC) at Long Island’s Brookhaven National Laboratory. RHIC collides gold nuclei and creates a substance known as quark-gluon plasma, which dominated the universe for about 10 of its first microseconds. As the ultra-hot and extremely dense substance expanded and cooled after the Big Bang, quarks and gluons were bound into protons and neutrons, never to escape. Physicists at RHIC were hoping to free them once again.

Collisions at RHIC produce 15 times the energy needed to liberate quarks and gluons. But since so many have been freed at once, the subatomic particles still find themselves confined to a tiny space. As Zajc explains it, once the prison break is complete, the yard is so crowded that the prisoners cannot go anywhere. This result was surprising because scientists expected the quark-gluon plasma to behave, well, like plasma, or ionized gas. Particles move freely in a gas, which is the reason smoke from your burned popcorn can travel from the microwave to the smoke detector. Even though the quarks and gluons had been freed in the collisions, they still could not move freely.

As the scientists at RHIC further examined the curious behavior of quark-gluon plasma, they discovered that it behaved more like a liquid than a gas. Moreover, they realized that it was a nearly perfect fluid, or one that flows with no viscosity. It is the liquid equivalent of the frictionless plane you spend most of first-semester physics imagining. This discovery, Zajc says, “was a forceful reminder that nature has surprises for you even when you understand the way that nature ought to work.”

Zajc and other physicists are planning to continue exploring the nature of quark-gluon plasma with upgrades at RHIC and at the Large Hadron Collider, the powerful accelerator set to open in Switzerland this year. It is still possible that at the higher energies the LHC will produce, the quark-gluon plasma will behave like a gas for a fraction of its tiny lifetime. Having received such a great surprise at RHIC, Zajc says, “The one thing everyone agrees on is that we’ll need experiment to decide.”

With the opening of the LHC, there has been a lot of discussion about whether it will lead to the end of physics—we might discover everything and have nothing left to investigate, or we might discover nothing and never be able to raise the funds for another experiment. But RHIC’s work reminds us that no matter how well we think we understand the universe, nature can still throw us a curveball. “We as a community are really grateful for having this chance to do this physics,” Zajc says. “We’ve been lucky enough that we’ve been able to build these experiments and accelerators and even luckier to have come upon a surprise.”

Elizabeth Wade is a Barnard senior majoring in comparative literature. Fear of Physics runs alternate weeks.

Tags: RHIC

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