TO LOOK deep into the fundamental structure of matter is to look billions of years back in time, to the moment when matter first blinked into being.
Recreating the conditions of that moment has long been an aim for physicists wanting to understand how the universe evolved from the cosmic fireball that existed a fraction of a second after the big bang. Now researchers at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory in Upton, New York, have, almost certainly, finally recreated the moments after creation.
By colliding nuclei together at enormous speeds, RHIC experimenters were able to break down the structure of nuclear matter. This resulted, most experts agree, in the formation of a long-sought-after plasma that is believed to be the primal stuff of the cosmos, the state of matter at the beginning of time.
Advertisement
It turns out, though, that the nature of matter is inextricably tied to the vacuum in which it resides. And the RHIC experiments have thrown up some surprises. They seem to show that the vacuum is a richer and more complicated place than was previously imagined. They suggest the boundary between something and nothing is more blurred than experts had predicted.
The stuff made at RHIC is a plasma consisting of quarks and gluons, the most basic building blocks of everything we see around us. Quarks combine in threes to form the protons and neutrons that comprise the nucleus of every atom. But while we can observe a single proton or neutron, we cannot observe a single quark. Quarks are perpetually confined to group living. In fact, the harder you try to pull quarks apart, the stronger the force between them becomes. This is part of the theory of quantum chromodynamics (QCD), which describes how the force between the quarks is carried by the massless gluons.
In QCD, it is the vacuum that imprisons the quarks. While it may sound like a barren place, the vacuum of QCD is a complex, dynamic arena. It writhes with virtual particles that appear in pairs, then annihilate and disappear again. It is haunted by strange creatures of various kinds, too, topologically complex knots and twists that are relatives of wormholes, places where space turns in on itself and seems treacherous. These knots and twists carve out paths for the gluons to travel along, thereby keeping the quarks together. These strange ideas have credence because of the success of QCD in predicting the reactions of fundamental particles.
The only way to unglue quarks is to 鈥渕elt鈥� the vacuum between them. But the vacuum doesn鈥檛 give in easily. To raze its jagged terrain requires enormous amounts of concentrated energy, found only in powerful nuclear collisions, or the fireball at the earliest moments of time.
鈥淭he vacuum is haunted by strange creatures, complex knots and twists, where space seems treacherous鈥�
Melting the vacuum is like returning to the state of the universe at the time it first existed. The RHIC at Brookhaven was built to do just that, and its experiments were designed to allow physicists to study what happens when the vacuum is heated so much that quarks and gluons are freed and matter reverts to a fundamental state.
Beginning in 2000, RHIC has repeatedly sent two beams of gold nuclei, each containing hundreds of protons and neutrons, speeding in opposite directions around a 4-kilometre track. Steered by superconducting magnets and achieving energies of 100 billion electronvolts, they collide, producing a fireball 300 million times hotter than the surface of the sun. Inside the fireball over a thousand quarks are unleashed.
When, by chance, two quarks hit each other head on, the extreme energy of their collision is turned into matter. A pair of virtual particles from the vacuum are given enough energy that they become real, and fly apart in opposite directions. Each of them goes on to drag further pairs of particles out of the vacuum, and the process repeats again and again, creating streams of particles called jets. The jets rush out of the collision site and are eventually captured by detectors.
These jets reveal the presence of the quark-gluon plasma. The plasma鈥檚 lifetime is a mere fraction of an instant, roughly 10鈭�23 seconds. But that is long enough for it to block particles from the jets as they stream out.
Jets are always produced in pairs and stream off in opposite directions. But they rarely originate in the exact centre of the collision site, so one jet usually has further to go before it hits the detector than the other. If the quark-gluon plasma has formed, it gets in the way of particles from one jet more than from the other. So when researchers on all four of the RHIC鈥檚 detectors saw uneven jets emerging, they knew immediately that the quark-gluon plasma could be causing this effect.
A further experiment provided more evidence for this idea, yet RHIC as a whole has held off making an official announcement. 鈥淢any theoretical physicists, and some number of the experimenters, say it鈥檚 pretty clear to us that this is the discovery,鈥� says Thomas Kirk, the associate laboratory director for high energy and nuclear physics at Brookhaven. But some experimenters at the laboratory remain wary of claiming it for certain. This is partly because of a controversy that erupted in 2000 when researchers at CERN, the European centre for particle physics in Geneva, claimed (many experts said wrongly) to have seen a hint of the quark-gluon plasma there. But Brookhaven鈥檚 case is far stronger, and most researchers tend to refer to the RHIC quark-gluon plasma as fact. 鈥淭here鈥檚 no doubt. It is quite evident,鈥� says University of Arizona and CERN physicist Johann Rafelski.
Much of the reason for Brookhaven鈥檚 hesitancy is to be found in the bizarre and perplexing effects they have seen in their quark-gluon plasma. 鈥淚t is different than we thought it was going to be,鈥� says Kirk. 鈥淚t鈥檚 brand new.鈥� Prior to the RHIC experiments, researchers had assumed that the vacuum structure would melt away easily once the energy exceeded 170 million electronvolts 鈥� the energy at which the plasma forms. At this energy, calculations suggested the plasma would be like a weakly interacting gas, with quarks and gluons floating haphazardly about, barely bothering each other. The researchers had figured that the jets streaming through the fireball would encounter only mild resistance on their way out.
But measurements confirmed last year shocked experimenters. Not only were the jets uneven, but their absorption by the plasma was 10 times as high as anyone had expected. 鈥淏ecause the quark-gluon plasma is so opaque, these quarks simply can鈥檛 get through to make a jet,鈥� explains Kirk. The jet particles appear to be getting stuck in the plasma like flies trapped in honey.
鈥淧articles appear to be getting stuck in the plasma like flies trapped in honey鈥�
This means the quark-gluon plasma is extremely dense 鈥� 30 to 50 times as dense as predicted 鈥� which suggests the quarks in the plasma are exhibiting incredibly synchronised group behaviour and interacting strongly with each other and the surrounding gluons. This makes the plasma more similar to a liquid than a gas. 鈥淚nstead of flying past each other, as in a gas, the whole liquid moves more coherently,鈥� says physicist Edward Shuryak, director of the Center for Nuclear Theory at Stony Brook University, New York.
In fact, the strength of interactions in the quark-gluon plasma make it the most ideal liquid ever observed 鈥� 10 to 20 times as liquid-like as water. 鈥淭hat was surprising,鈥� says Shuryak. 鈥淚f you could take a few thousand water molecules, make a tiny drop and let it explode, they would not flow, they would kind of move individually. But these thousands of particles actually move coherently.鈥� It seems that from the time of the big bang until 10 microseconds later, the universe was liquid.
Armed with this knowledge, Shuryak and others have run new computer simulations of the gold-nucleus collisions, superseding previous calculations that suggested the quark-gluon plasma would be only weakly interacting. Their results suggest that bound quark states are persisting in the quark-gluon plasma even at energies twice as great as 170 million electronvolts,where no particles had been expected to remain bound together. One such bound particle is called charmonium, because it is made of a charm quark and an anti-charm quark. 鈥淚t seems like charmonium can physically survive in the quark-gluon plasma in RHIC conditions,鈥� says Shuryak. 鈥淭hat led to a complete revelation, and the question, are there other particles surviving? And the answer seems to be yes.鈥�
It is the vacuum that holds quarks together to form particles, so the survival of particles in the plasma, as well as its liquid-like nature, reveals the unexpected resilience of the vacuum 鈥� a tendency not to give up, but to linger, and structure matter more strongly than researchers thought it could.
What鈥檚 more, the vacuum seems to create a whole variety of new particles built of different combinations of quarks in the plasma. None could ever exist in our everyday world, but they can exist in the quark-gluon plasma at RHIC 鈥� and may well have been around in the early universe, too.
As the collision fireball quickly expands and cools, the vacuum 鈥渇reezes out鈥�, going from the less-structured state of the plasma to the intricate vacuum structure of our cooler world, like amorphous water freezing into orderly ice crystals. As this happens, the vacuum grabs hold of the quarks, locking them into the familiar protons and neutrons of our world. This entire sequence of events from the hot plasma to the formation of ordinary matter replicates the formation of the universe from the fireball that followed the big bang.
Figuring out exactly how the quark-gluon plasma turns into the low-energy world in which we live is crucial if we are to answer some of the fundamental questions about the history of the universe. It will almost certainly shed light on one of the biggest mysteries in cosmology: why it is that our universe seems to be expanding faster and faster as it ages.
The expansion of the universe is driven by some kind of repulsive energy, or 鈥渄ark energy鈥�, lurking throughout space. Its origin may well be within the vacuum itself. 鈥淭he physical vacuum is not empty by any means,鈥� says Kirk. 鈥淚t is a complicated structure, and I鈥檓 not sure theorists or experimenters are far enough along to know what it is like.鈥�
This is how large the discrepancy between our theoretical understanding of the vacuum and our observations of it is: the repulsive energy of the vacuum as calculated from theory is as much as 10120 times as large asthe energy needed to produce the observed acceleration of our universe鈥檚 expansion. The gulf is a source of serious dismay for physicists. 鈥淚 have called it the biggest fudge factor in the history of science,鈥� says physicist Murray Gell-Mann, who won the 1969 Nobel prize for the development of QCD theory and coined the name quark.
Experimenters at RHIC hope to try and bridge the gap between theory and observation by measuring the properties of the quark-gluon plasma more carefully. One important quantity to pin down is its latent heat. This is the amount of energy it takes to dissolve the vacuum and create the plasma. 鈥淭he universe came out of the quark-gluon plasma into our current vacuum,鈥� Rafelski says. 鈥淪o at some early time, at 10 microseconds, the universe was in a melted state. At that time, the cosmological constant was maybe 1080 times bigger than today. And then the vacuum froze. When the vacuum froze, for some miraculous reason the cosmological constant went down by 80 orders of magnitude.鈥�
鈥淚t now seems that for 10 microseconds after the big bang the universe was liquid鈥�
The secret to dark energy probably lies somewhere in the transition between the quark-gluon plasma and ordinary matter. 鈥淭he discovery of the quark-gluon plasma is evidence for a change in the vacuum,鈥� says Rafelski. Because the vacuum is more robust at higher temperatures than anyone had thought it would be, the plasma too is a much more complicated place than expected 鈥� a highly organised, dense liquid in which strange new particles swim about.
To summarise the situation physicists now find themselves in, Shuryak draws a comparison to Columbus鈥檚 discovery of America. 鈥淐olumbus had a correct theory that if you go west you get to India. But he made an accidental discovery along the way that in the middle something else exists. We had the correct theory that if you increase temperature, you get a very dilute and simple gas of quarks and gluons. But it turns out that before you get there, when you cross the phase boundary, you again get something complicated.鈥� It鈥檚 this unexpected middle ground that provides tantalising clues to the history and construction of the universe.
Continuing experiments at RHIC and future experiments at the Large Hadron Collider which is being built at CERN should be able to measure the latent heat of the plasma, and help us learn more about its behaviour. For now one thing is certain: the plasma is not the simple, easygoing place it was once thought to be, and that鈥檚 all down to the vacuum. In the end, it seems we can鈥檛 know everything about something until we know more than a little about nothing.
