When you heat things up, you can expect familiar effects. Heat ice and it melts. Heat water and it turns into steam. These processes occur at different temperatures for different materials, but the pattern repeats itself: solid becomes liquid and then gas. At sufficiently high temperatures, however, the known pattern breaks. At super high temperatures, a different type of liquid forms.
This surprising result is because solid, liquid, and gas are not the only states of matter known to modern science. When you heat a gas – say steam – to very high temperatures, strange things happen. Above a certain temperature, the steam becomes so hot that the water molecules no longer stick together. What were once water molecules with two hydrogen atoms and one oxygen atom (the well-known H2O) becomes unfamiliar. The molecules break down into individual hydrogen and oxygen atoms. And if you raise the temperature even more, eventually the atom can no longer hold its electrons, and you have naked nuclei marinated in a bath of energetic electrons. This is called plasma.
While water turns into vapor at 100°C (212°F), it doesn’t become plasma until it reaches about 10,000°C (18,000°F) — or at least twice the temperature of the Sun’s surface. However, using a large particle accelerator called the Relativistic Heavy Ion Collider (or RHIC), scientists are able to collide beams of bare gold cores (that is, gold atoms from which all electrons have been stripped) with one another. Using this technique, researchers can generate temperatures an astounding 4 trillion degrees Celsius, or about 250,000 times hotter than the center of the Sun.
At this temperature, not only are the atomic nuclei broken down into individual protons and neutrons, but the protons and neutrons literally melt, allowing the building blocks of protons and neutrons to mix freely. This form of matter is referred to as “quark-gluon plasma,” named after the components of protons and neutrons.
Such hot temperatures are not normally found 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 this heat existed generally in the universe was barely a millionth of a second after it began (Dec-6 S). Quite literally, these accelerators can recreate tiny versions of the Big Bang.
Generation of quark-gluon plasmas
The bizarre thing about quark-gluon plasmas is not that they exist, but how they behave. Our intuition, developed from our experience with human-scale temperatures, is that the hotter something gets, the more like a gas it should behave. So it’s perfectly reasonable to expect a quark-gluon plasma to be some sort of “super gas” or something; But that’s not correct.
In 2005, researchers using the RHIC accelerator found out that a quark-gluon plasma is not a gas but a “superfluid”, i.e. a liquid without viscosity. Viscosity is a measure of how difficult a liquid is to stir. Honey, for example, has a high viscosity.
In contrast, quark-gluon plasmas have no viscosity. Once stirred, they keep moving forever. This was a tremendously 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 in New York. While the accelerator began operations in 2000, it has been modernized and is scheduled to resume operations this spring with higher collision energies and more collisions per second. In addition to improvements to the accelerator itself, the two experiments used to record the data generated by these collisions have been significantly upgraded to cope with the more demanding operating conditions.
The RHIC accelerator has also caused other atomic nuclei to collide in order to better understand the conditions under which quark-gluon plasmas can form and how they behave.
RHIC isn’t the only collider in the world that can smash atomic nuclei together. The Large Hadron Collider (or LHC), located at the CERN lab in Europe, has similar capabilities and operates at an even higher energy than RHIC. About a month a year, the LHC collides nuclei of lead atoms. The LHC has been in operation since 2011 and quark-gluon plasmas have also been observed there.
While the LHC is capable of generating even higher temperatures than the RHIC (roughly double), the two facilities complement each other. The RHIC facility generates temperatures near the junction in quark-gluon plasmas, while the LHC studies the plasma farther from the junction. Together, the two facilities can study the properties of quark-gluon plasma better than either could individually.
With the improved operational capabilities of the RHIC accelerator and lead collision data expected at the LHC in the fall, 2023 is an exciting time for the study of quark-gluon plasmas.
