Down on Quantum Street, you can get something for nothing, Norman Dombey finds some electrifying news in a vacuum
And, as imagination bodies forth
The forms of things unknown, the poet's pen
Turns them to shapes, and gives to airy nothing
A local habitation and a name
MODERN PHYSICISTS and cosmologists not only give to nothing a name - the vacuum - but the shapes and structures of nothing in quantum theory soar well beyond the imagination of Shakespeare's poet.
Two fundamental principles of modern quantum physics distinguish the subject sharply from classical physics. First, it is not always possible to measure two different physical quantities with precision at the same time. The most famous example of this is Heisenberg's Uncertainty Principle: there are limits on the accuracy with which the position and momentum of a particle can be simultaneously measured. This is because a measurement of the position of a microscopic particle will disturb the particle and give it an unknown momentum.
The second principle is a generalisation of the first: it is only possible in general to give probabilities for the result of measurements - there is an intrinsic uncertainty about the description of a physical system.
So consider a vacuum from the point of view of quantum physics. Suppose a metal box contains nothing - that is to say, there is a perfect vacuum there. (This is not really possible as the walls of the box and any joins will leak a little, but ignore that for a moment.) Since there is nothing in the box, we must define the vacuum as a physical state with no particles (electrons, protons, neutrons and so on) present. Correspondingly, there are no electric charges in the box. But just because the particle number is zero everywhere in the box, it does not mean that the electric field there is zero. For particle number is a distinct physical quantity from electric field in in quantum theory. Calculations have since shown that there are indeed non-zero and fluctuating electromagnetic fields in the box.
The walls of the box are metal and conduct electricity, and the electric field vanishes in a conductor. So, whatever the electric field is within the box, it must vanish at the walls. A calculation of the energy stored in the box due to these fluctuating electromagnetic fields is possible - it was first done by the Dutch physicist Casimir in 1948, and shows that for a long thin cavity, the energy depends on the distance between the walls of the cavity.
MATHEMATICALLY, the description of the quantum vacuum in this case is just like that of a stretched surface of a rectangular drum in which only vibrations of certain wavelengths are possible. Since the energy depends on the width of the box, there must be a force between its walls. Just as for the drum, in the case of the quantum vacuum, there is an attraction between the walls of the box. This tension arises literally from the physical properties of nothing. The Casimir force, although electromagnetic in nature, is not proportional to electric charge since there is no electric charge in the problem. It is just proportional to the two fundamental constants of modern physics: Planck's constant, h, and c, the velocity of light in free space.
It is an exceedingly small effect. The pressure difference, due to the Casimir effect, between two metal plates one millimetre apart is 10-20 of atmospheric pressure. There have been sporadic experimental attempts to measure the Casimir force, but, not surprisingly, they have been only qualitatively successful.
In the past two years, however, a group of experimental physicists at Yale, led by Professor Ed Hinds, have made an astonishing precision measurement of a directly-related phenomenon: the deflection of a beam of neutral atoms that pass through a narrow cavity with conducting walls. Even though there is nothing in the cavity and there is no external electromagnetic field applied to it, the innate vacuum electric field which Casimir calculated pushes the sodium atoms towards the walls.
The cavity was made of gold and was 3cm high, 8mm long and was adjustable in width from 0.5 to 8 microns (1 micron is one millionth of a metre). Finely-tuned lasers detect the sodium atoms as they emerge. By varying the width of the cavity, the vacuum electric fields are changed. The results are in complete agreement with the predictions of the theory. Hinds concludes that "quantum mechanics has been fully of surprises for most of this century. People resist the idea that this (vacuum) fluctuating is happening - that the energetic vacuum is real. What makes this experiment fun is that it helps us accept that reality."
There are other more direct manifestations of these effects. The fluctuating electric fields that are always present within a vacuum help to explain an old puzzle of atomic physics: why do atoms radiate spontaneously? This is no longer mysterious when we realise that the vacuum itself is in ceaseless motion. The vacuum electric fields push an atom about randomly in a sort of Brownian motion and stimulate it to radiate if it is not in its most stable state.
The most surprising feature yet discovered about the quantum vacuum is that the speed of light is not necessarily constant in a vacuum. It is well known, that the speed of light in a medium, say water or glass, is less than the speed of light, c, in a vacuum. Since Einstein's Special Theory of Relativity in 1905, c has been considered to be one of the most fundamental physical constants in nature, but the East German physicist, Klaus Scharnhorst, showed recently that the velocity of light travelling in a narrow evacuated cavity perpendicular to the cavity walls (which are assumed to be conducting) is larger than c. The Scharnhorst effect is incredibly small (but so was the Casimir effect when first discovered). It is due to the light in the cavity scattering off the fluctuating electromagnetic fields of the quantum vacuum, just as the sodium atoms in the Yale experiment were deflected by the same vacuum fluctuations.
Scharnhorst was unable to stay in Leipzig as a result of German unification: he is now in Swansea. Ed Hinds and his colleague, Malcolm Boshier, are going to move to Sussex from Yale this October. These are somewhat rare examples of a reverse brain drain and we can therefore hope for further experimental and theoretical demonstrations in Britain of the literally extraordinary behaviour of the quantum properties of noting.
Norman Dombey is Professor of Theoretical Physics at the University of Sussex.