Creating temperatures higher than the core of the Sun to unlock the secrets of superfluidity

When you heat things up, you can expect familiar effects. Heat up the ice and it will melt. Heat the water and it will turn into steam. These processes occur at different temperatures for different materials, but the pattern is the same: a solid becomes a liquid and then a gas. However, at sufficiently high temperatures, the usual pattern is violated. At ultrahigh temperatures, another type of liquid is formed.

This surprising result is due to the fact that solid, liquid and gaseous states are not the only states of matter known to modern science. If you heat a gas—for example, steam—to very high temperatures, strange things will happen. At a certain temperature, the steam becomes so hot that the water molecules are no longer held together. What were once water molecules with two hydrogens and one oxygen (the familiar H₂O) becomes unfamiliar. Molecules break down into individual hydrogen and oxygen atoms. And if you raise the temperature even higher, eventually the atom can no longer hold its electrons, and you’re left with bare atomic nuclei marinated in a bath of energetic electrons. This is called plasma.

While water turns into steam at 100ºC (212ºF), it does not turn into plasma until about 10,000ºC (18,000ºF) – or at least twice as hot as on the surface of the Sun. However, with the help of a large particle accelerator called the Relativistic Heavy Ion Collider (or RHIC), scientists can push together beams of bare gold nuclei (i.e., gold atoms that have had all their electrons removed). Using this technique, researchers can get a staggering temperature of about 4 trillion degrees Celsius, about 250,000 times hotter than at the center of the sun.

At this temperature, atomic nuclei not only disintegrate into individual protons and neutrons, the protons and neutrons literally melt, allowing the building blocks of protons and neutrons to mix freely. This form of matter is called “quark-gluon plasma”, named after the constituent protons and neutrons.

Such high temperatures do not normally occur in nature. After all, 4 trillion degrees is at least 10 times hotter than the center of a supernova, which is the explosion of a star so powerful it can be seen billions of light years away. The last time such a high temperature usually existed in the universe was a millionth of a second after it began (10^-6 With). In a very real sense, these accelerators can recreate tiny versions of the Big Bang.

Generation of quark-gluon plasma

The strangest thing about quark-gluon plasma is not that it exists, but how it behaves. Our intuition, which we have developed from our experience with more human temperatures, is that the hotter something gets, the more it should behave like a gas. Thus, it is perfectly reasonable to expect a quark-gluon plasma to be some sort of “supergas” or something like that; but it is not.

In 2005, researchers using the RHIC accelerator discovered that quark-gluon plasma is not a gas, but “superfluidity”, that is, a liquid without viscosity. Viscosity is a measure of how hard a liquid is to mix. Honey, for example, has a high viscosity.

In contrast, a quark-gluon plasma has no viscosity. Once mixed, they keep moving forever. This was an extremely unexpected result and caused great excitement in the scientific community. It also changed our understanding of what the very first moments of the universe were like.

The RHIC facility is located at Brookhaven National Laboratory, a US Department of Energy science laboratory operated by Brookhaven Science Associates. It is located on Long Island, New York. Although the accelerator began operation in 2000, it has been upgraded and is expected to resume operation this spring with higher collision energy and more collisions per second. In addition to improvements to the accelerator itself, the two experiments that were used to record data from these collisions have been significantly improved to accommodate more difficult operating conditions.

The RHIC has also collided with other atomic nuclei to better understand the conditions under which quark-gluon plasmas can be generated and how they behave.

RHIC is not the only collider in the world capable of colliding atomic nuclei. The Large Hadron Collider (or LHC), located at the CERN laboratory in Europe, has similar capabilities and runs at even higher energy than the RHIC. About a month a year, the LHC collides with the nuclei of lead atoms. The LHC has been operating since 2011 and quark-gluon plasma has also been observed there.

Although the LHC is capable of generating even higher temperatures than the RHIC (by about a factor of two), the two units complement each other. The RHIC facility generates temperatures near the transition to the quark-gluon plasma, while the LHC probes the plasma further away from the transition. Together, these two objects can better explore the properties of the quark-gluon plasma better than either of them separately.

With the improved operational capabilities of the RHIC accelerator and expected lead collision data at the LHC, autumn 2023 will be an exciting time to study quark-gluon plasmas.