Melting Subatomic Ice Cubes
Look out solid, liquid and gas: There’s a new form of matter in town. Actually, this “new” matter isn’t new at all—it is one of the most ancient forms of matter in the universe. Last seen more than 13 billion years ago, just millionths of a second after the Big Bang, this exotic stuff is making a comeback thanks to particle accelerators like the Relativistic Heavy Ion Collider (RHIC) on Long Island and the Large Hadron Collider (LHC) in Europe, where physicists can generate temperatures of more than a trillion degrees centigrade. These enormous temperatures allow scientists to push back the clock of the cosmos and witness matter in the extreme energy environment that existed within microseconds of the Big Bang.
At such high temperatures, the protons and neutrons inside atomic nuclei literally melt, releasing the quarks and gluons inside them and creating a form of matter called a quark-gluon plasma. You can think of the quarks as the “matter” particles and gluons as the particles of force that hold the protons and neutrons together. A reasonable mental image of a proton or neutron would be like a few flecks of Styrofoam (quarks) inside a lottery ball machine. The wind in the lottery machine is analogous to the force field, while the air molecules represent the gluons.
Under ordinary conditions, quarks and gluons are forever locked inside protons and neutrons. They’re like the water held frozen into ice cubes in a glass. But just as ice can be melted into water when energy is added to the system (by pouring hot tea over them, for instance) allowing the molecules from one ice cube to mix with molecules from other cubes, so too it is possible to melt protons and neutrons and have the quarks and gluons scamper around willy-nilly.
To melt protons and neutrons, you need to heat them up to about a trillion degrees. The only way to generate this kind of temperature is to smash together atomic nuclei at high velocities in huge particle accelerators. That’s what physicists are now doing at the LHC and the RHIC, accelerators that take atoms (lead and gold, as well as some others), strip off all of the electrons, and then slam the bare atomic nuclei together. The most violent of these collisions can heat up the nuclear matter enough to free the quarks and gluons to wander as they will. Though experimental calibration issues add some uncertainty to the mix, the current temperature record seems to belong to the ALICE experiment at the LHC, which measured an astounding 5.5 trillion degrees centigrade.
While the first lab-made quark-gluon plasma was created in 2000, physicists are only now beginning to understand how this form of matter behaves. At the LHC and RHIC, they are mapping out in more detail the temperatures and pressures at which ordinary matter transforms into quark-gluon plasma. They are also tracing the boundaries between quark-gluon plasmas and even more exotic forms of matter like the stuff of neutrons stars, which is thought to be so dense that, at the center of the stars, quarks get “smooshed” together into an exotic kind of solid.
In fact, physicists believe that there are many different phases of matter involving quarks. While I’ve focused on two states of matter, atomic nuclei and quark-gluon plasma, this just scratches the surface of the possible.
By studying quark-gluon plasma, physicists are able to explore a period in the history of the universe that has thus far eluded us—a period in which protons and neutrons, the basic ingredients of ordinary matter, were coalescing for the first time. Thanks to accelerators like the LHC and the RHIC, we are finally beginning to probe this pivotal chapter in the story of cosmos.
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