Much Ado about Nothing - Exploring the Vacuum with the LHC

Empty space is anything but. Remove everything you can from an area of space and it will still bustle with activity. A veritable abundance of particles and all-pervasive fields fill space with energy. Empty space even weighs something. Indeed, studying 'nothing' can tell us almost everything about the universe we live in.

Setting the stage

The 54 km of LHC beam pipes are pumped down to one of the best vacuums humankind can produce. Air pressure is higher on the moon than inside the LHC. This engineering feat is worthy of articles in itself, but the kind of vacuum we ask you to imagine here is something altogether different. It is quite simply the emptiest the laws of Nature allow.

The vacuum is defined as the physical state with the lowest possible energy. Lowest possible... but not zero. This is because both particles and fields exist in the vacuum and both can be thought of in terms of energy.  While some components are constant, others fluctuate wildly due to the indistinctness inherent in quantum theories. Together these different contributions combine to make the vacuum a surprisingly busy place.

The cast of particles

The laws of quantum mechanics allow particles to pop in and out of existence for undetectably small fractions of time. The more massive these "virtual" particles, the shorter the amount of time they can exist. This quantum fuzziness animates the vacuum with a constant buzz of particles and anti-particles.

In addition, quantum chromodynamics, the underlying theory of the strong interaction, brings something altogether more tangible to the vacuum: an effect that allows quark-antiquark pairs to exist in what is known as a chiral condensate. This condensate is one of the phenomena that contributes mass to particles and, by doing so, it also adds energy to the vacuum.

The chiral condensate is studied in lead ion collisions at the LHC where the high temperature and density allows the ALICE experiment to explore the conditions when the effect switched on in the early universe.

Waiting in the wings - Supersymmetry

Whatever the findings this year for the Higgs, it will certainly not be the last surprise the vacuum has in store. One unsolved mystery arises from the incessant activity of virtual particles, because although they may not be directly detectable, they do interact with the Higgs field. Being virtual, quantum mechanics allows all kinds of interactions to take place.  In fact, the sum of all possible interactions of heavy virtual particles with the Higgs field should contribute an infinite energy to the vacuum.

Theories such as Supersymmetry (SUSY) attempt to resolve this problem. In SUSY, particles interact on a multi- dimensional stage called superspace. This has consequences at higher energies where the theory excludes infinite contributions from virtual particles to the vacuum.

Evidence for this may be uncovered at the LHC. Experiments are looking out for signs of a whole family of new particles that are predicted by SUSY. The lack of any such signs in LHC data to date only means that a certain subset of models has been ruled out, not that the theory has been disproved.

A full house - Dark energy

The power of nothing is not restricted to the minute world of particles, it can also be seen on cosmic scales. The energy in the vacuum, although tiny on laboratory scales, becomes considerable on astronomical ones, where great voids of space are filled with mere pin pricks of matter.  Indeed, it is the energy of the vacuum - collectively known as dark energy - that causes the expansion of the universe to get faster and faster. Last year's Nobel prize in physics was awarded to the astronomers who made the first large scale measurements of this acceleration by studying the light emitted from supernova explosions.

These measurements led to one of the greatest mysteries in physics today. The rate of acceleration of the universe does not correspond to what we can calculate about the vacuum. And it's no small discrepancy! The supernovae observations suggest that the vacuum energy is over 20 orders of magnitude smaller than what is expected from known particles and fields.  The missing piece of the puzzle will be inextricably linked to our understanding of the universe on both small and very large scales. 

Such is the large cast of particles and fields that comprise the vacuum. And the LHC may yet uncover more. So, just as in the Shakespeare play, 'nothing' is a source of much agitation also at CERN. Whereas Shakespeare made his play a comedy, here at CERN it is more a question of drama at its most thrilling. As LHC data taking starts again, expect a year of highs and lows, intrigues and suspense, as audacious theories are slain and new particles take centre stage.

Source: Cern