High-energy physics

An emptier emptiness?

Temperatures similar to those reached an instant after the Big Bang can be created in collisions of gold atoms. The resulting fireballs may allow us a glimpse of a world that is more symmetrical than our own.

The concept that what we ordinarily perceive as empty space is in fact a complicated medium is a profound and pervasive theme in modern physics. This invisible, inescapable medium alters the behaviour of the matter that we do see. Just as Earth's gravitational field allows us to select a unique direction as up, and thereby locally reduces the symmetry of the underlying equations of physics, so cosmic fields in ‘empty’ space lower the symmetry of these fundamental equations everywhere. Or so theory has it. For although this concept of a symmetry-breaking aether has been extremely fruitful (and has been demonstrated indirectly in many ways), the ultimate demonstration of its validity — cleaning out the medium and restoring the pristine symmetry of the equations — has never been achieved: that is, perhaps, until now.

In a new paper, Cramer et al.1 claim to have found evidence that — for very brief moments, and over a very small volume — experimentalists working at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory in New York have vaporized one symmetry-breaking aether, and produced a more perfect emptiness. This pioneering attempt to decode the richly detailed (in other words, complicated and messy) data emerging from the RHIC experiments is intricate2, and it remains to be seen whether the interpretation Cramer et al. propose evolves into a consensus. In any case, they've put a challenge on the agenda, and suggested some concrete ways to tackle it.

But what exactly is this underlying symmetry of nature that is broken by the aether? How is it broken, and how might it be restored? The symmetry in question is called chiral symmetry, and it involves the behaviour of quarks, the principal constituents of the protons and neutrons in atomic nuclei (among other things).

Chiral symmetry is easiest to describe if we adopt the slight idealization that the lightest quarks, the up quark (u) and down quark (d), are massless. (In reality their masses are small, on the scale of the energies in play, but not quite zero.) According to the equations of quantum chromodynamics (QCD), the theory that describes quarks and their interactions via the strong nuclear force, the possible transformations among quarks are very restricted. One rule is that u-quarks and d-quarks retain their ‘flavour’ — that is, a u never converts into a d, nor a d into a u. Quarks also, like the more familiar photons, have an intrinsic spin. If the spin axis is aligned with the direction of motion, then the sense of the rotation defines a handedness, known as chirality, rather like a left- or right-handed screw. The two possible states of chirality of a quark, left and right, are essentially the same concept as left and right circular polarization for photons. The fundamental interaction between quarks and gluons, to which we ultimately trace the strong nuclear force, conserves chirality as well as flavour. Thus a u-quark with left-handed chirality (written uL) never converts into a right-handed uR, and so on. But these extra conservation laws, which follow from the symmetry of QCD's equations, are too good to be true. In reality, one finds that although the rule forbidding changes of flavour holds true, there is no additional conservation law for chirality — chiral symmetry is broken.

The accepted explanation for this mismatch blames a form of aether. The idea is that there is such a powerful attractive interaction between uL-quarks and ūR-antiquarks (every quark has an antiquark with the opposite charge), and likewise between dL-quarks and d̄R-antiquarks, that the energy gained from their attraction outweighs the cost of creating the particles in the first place. Thus, perfectly empty space, devoid of quarks, is unstable. One can lower the energy of the vacuum by filling it with bound uL–ūR and dL–d̄R pairs (and their antiparticles, ūL–uR, d̄L–dR). Physicists call this process the formation of the chiral condensate. In the stable state that finally results, the conservation of chirality is rendered ineffective, as space itself has become a reservoir containing, for example, an indefinite number of uL-quarks. (Because each uL–ūR pair contains both a quark and an antiquark, net conservation of flavour still holds good.) This extraordinary picture has experimental consequences: the lightest strongly interacting particles, the π-mesons, can be identified as collective oscillations of the chiral condensate. This identification provides clues to the unusual properties and interactions of the π-mesons, notably their small masses compared with those of other strongly interacting particles.

At the RHIC, collisions between heavy ions — gold nuclei with a total of 197 protons and neutrons each — create a fireball in which temperatures exceeding 1.5 × 1012 kelvin are achieved (Fig. 1). Impressive evidence has accumulated that a qualitatively new state of matter has been created, a liquid-like plasma of quarks and gluons3,4,5,6. Could something even more dramatic — a qualitative change in the properties of empty space — be occurring as well? Theoretical calculations indicate that at such temperatures the pairs that make up the chiral condensate will break apart. When the condensate vaporizes, the full underlying chiral symmetry of QCD becomes operative. This change in the properties of ‘empty’ space last occurred throughout the Universe in the early moments after the Big Bang, when temperatures were as high as those reached in the RHIC fireball. This and similar vaporizations of other condensates at higher temperatures play an important role in modern cosmological thinking. Such an event might, for example, have triggered an epoch of inflation — a period of accelerated growth in which the horizon of the Universe expanded, temporarily, much faster than the speed of light.

Figure 1: Gold dust.
figure1

A side view of one of the first high-energy collisions captured by the Solenoidal Tracker of the STAR detector at the Relativistic Heavy Ion Collider (RHIC). The initial head-on collision of two gold ions, each consisting of a total of 197 protons and neutrons, occurs at the mid-point of the central tube (running across the image from right to left). The tracks indicate the paths taken by thousands of subatomic particles created in the fireball of energy set free in these collisions. Several layers of detectors, arranged concentrically around the central tube, and encased in a powerful magnet, allow the identification of these particles. (Courtesy of Brookhaven National Laboratory, STAR collaboration.)

Vaporization of the chiral condensate affords by far our best opportunity to access a phase transition of empty space in a controlled terrestrial experiment. The difficulty arises not so much in creating the necessary extreme conditions, but in reconstructing from the ashes available to us what happens during the initial stages of the experimentally created fireballs. Cramer et al.1 use correlations between observed π-mesons to reconstruct properties of the medium through which they travelled. Previous models have had difficulty in dealing with these correlations, resulting in what has been called the ‘HBT puzzle’7. Only by allowing for the possibility that the medium significantly alters the properties of the π-mesons, in the way expected if that medium were free of the chiral condensate8,9, do Cramer et al. achieve a satisfactory fit to the data they consider. Thus, they may both resolve an old puzzle and open a new vista. Whether their model can be extended successfully to cover additional phenomena, and whether models based on other ideas can be equally successful, are questions sure to receive considerable attention in the near future.

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Wilczek, F. An emptier emptiness?. Nature 435, 152–153 (2005). https://doi.org/10.1038/435152a

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