Nuclear physicists have been studying heavy ion collisions for many years, where the aim is to collide heavy nuclei, such as lead, at nearly the speed of light. The center of these collisions produces a droplet of a novel state of matter called quark-gluon plasma. When we think of matter, most people consider three main states: solid, liquid, and gas. When matter (usually a gas) reaches extremely high temperatures, the electrons and nuclei making up the atom disassociate, producing another state of matter known as plasma.
The nuclei in that plasma, and all the matter we typically think of, are made up of protons and neutrons. Through the efforts of nuclear and particle physicists in the 20th century, it was discovered that the composition of matter goes one level deeper.1 Protons and neutrons are themselves made up of fundamental particles called quarks and gluons. Quarks and gluons interact via the strong nuclear force, unlike photons and electrons that interact via the electromagnetic force. One oddity of the strong nuclear force is that quarks need to be closely confined together, which is why particles like protons and neutrons that make up the nucleus of an atom are so stable. The potential (function describing energy) of quarks is so great and prefers confinement so strongly that if we were somehow able to tear two quarks apart (e.g., a pair that make up a π meson), it would require us to use so much energy that it would be energetically favorable to produce a quark-antiquark pair that each bind to the original quarks in the system, creating two stable particles.2 There are no bubble chambers, nor tracks, nor pictures of single quarks and gluons. Period.
Breaking the Confinement Rule Now let’s build a crazy machine to break that rule—sort of. Going back to states of matter, there is a lesser-known state of matter, a plasma actually, that takes the idea of disassociation in a plasma much, much further.
What if you take that plasma and keep heating and compressing it? Let’s ignore the electrons for now and focus on the nuclei. How could we create the most energy-dense, high-temperature, high-pressure system in the known universe? You take a really beefy nucleus like gold or lead3—just the nucleus, so it’s a dense, positively charged ball—and accelerate it as close to the speed of light as you can possibly manage. You do this with a bunch of nuclei at the same time, speeding them up through several highly synchronized superconducting magnets organized in a ring deep underground in Long Island, or, if you prefer, France and Switzerland. You now have what we call a beam of nuclei, or ions. Now repeat, but in the opposite direction around the ring.
That all might sound crazy, but that was the easy part. Now we want to cross these unimaginably small beams (only a few nuclei wide) dead on, exactly into each other, so that nuclei actually hit each other. That’s like shooting a bow and arrow at a regular target on Earth from the sun—drunk, blindfolded, and spun around a dozen times for good measure. A tremendous amount of work goes into this, but long story short, engineers and physicists solved this problem so well that our detectors have trouble keeping up with the frequency of the resulting explosions.
What’s amazing is we started with one of the densest objects we know of, nuclei, and actually made them even denser; as we accelerate the nuclei close to the speed of light, from our perspective they pancake, or ‘Lorentz contract,’ before they ever collide. Once nuclei collide, the energy and density are so high that the constituent protons and neutrons melt into the quark-gluon plasma.
The Mysterious Strong Nuclear Force The reason physicists are so interested in quark-gluon plasma is because the strong nuclear force is particularly mysterious. The confinement mentioned previously is only one side of the coin with the strong force. At larger distances, the strong force is unbelievably strong (100 times stronger than electromagnetic forces and 1,000,000 times stronger than the weak nuclear force), and it does not decrease at larger distances. Think about that for a second. Almost anything you can think of falls off the further away from it you are. Two magnets far apart are barely felt. The further you are from a mass, like a planet, you feel weightless. The strong force is instead like a focused beam of attraction that just doesn’t quit. Bizarre.
But then at smaller distances, when quarks and gluons are very close together, the strong nuclear force taps out almost completely. The strongest force in the Standard Model is extinguished, and the quarks and gluons inside stable particles are swimming around freely. Bizarre.
Anyway, the math to describe this doesn’t exist. We just can’t really describe these two phenomena cohesively. We’ve harnessed relativity and launched satellites in stable orbit around the Earth so you can yell at your Uber driver when he makes a wrong turn to pick you up. But we really can’t describe what’s going on with the basic building blocks of matter.
Quark-gluon plasma is unique because, for a fraction of a second, we create a primordial soup with such high energy and pressure that all the quarks and gluons are free. Then, somehow, it transitions into a spray of stable particles where all the quarks and gluons are confined. What better way to learn about the behavior that eludes rigorous description than to study a state of matter completely dictated by it? It’s a fluid, a nearly perfect fluid, with the lowest possible viscosity and entropy.4
Why Should You Care? Okay, it’s interesting for a few fringe scientists that play with gold and lead or whatever.5 But why should anyone else care? Well, you’re still reading this, and the sunk-cost fallacy dictates that you have to care about this now, else the past X minutes you spent reading this are wasted. Checkmate.
But let’s think about what we’ve been able to do with our current understanding of the strong nuclear force, limited as it may be. It’s actually not a very long list:
- Apocalypse-inducing bombs capable of instantly vaporizing entire cities and poison the planet forever.6
- One of the safest, densest, most stable, easily the cleanest sources of energy ever discovered that could usher humanity into a post-energy-scarcity future.7
There’s a huge amount to be learned from quark-gluon plasma (QGP) produced in heavy-ion collisions that can constrain models of the strong nuclear force. Is QGP produced in smaller collisions, between a proton and a lead ion, or even proton-proton collisions?8 How do we eke out important signals from the enormous background of random particles produced in these crazy collisions?9 How exactly does the soup of quarks and gluons combine into the final state particles we see in our detectors?10 And why is it that so many disparate final-state particles after the QGP has cooled continue to flow like a fluid, when there’s nowhere near enough time for them to reach any kind of equilibrium?11
Footnotes
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Why do physicist, people who have dedicated their lives and skills to understand the natural world instead of generating wealth, use such a luxurious material like gold? Because it’s shiny. Also it’s a very large, stable, and spherical nucleus. About 1 millionth of a gram of gold is used. ↩
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Sometimes, I like to imagine a grown adult smashing two gold or silver action figures together and swearing to others they’ve been doing science all day. ↩
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Can I link the Oppenheimer movie? ↩
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Fission is safer than wind, cleaner than solar. The problem of radioactive waste should be taken seriously, but is often blown completeley out of proportion. Wind and solar have made really fantastic progress, however. ↩
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Heavy quark production is easier to measure and understand in heavy ion collisions, and so it’s used as a probing stick for better understanding hadronization in general. ↩
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Hydrodynamics almost always assumes some kind of local thermodynamic equilibrium. This usually takes longer than the seconds QGP exists for. A relatively new and fascinating approach to model this is called far-from-equilibrium hydrodynamics. ↩