May 10 2009
It seems extraordinary that a device on the nanoscale, smaller than a human hair, could hold clues to the fundamental driving force shaping the entire universe.
But a series of experiments at the London Centre for Nanotechnology (LCN), led by Dr Paul Warburton and Dr Jon Fenton, centred around a new generation of devices only nanometres in size, are working towards producing data with enormous consequence
This strange coupling of the massive with the minuscule is a consequence of speculation that the vacuum of space is not quite as empty as we would believe. Instead it is bristling with zero-point fluctuations, a phenomenon predicted by quantum mechanics, the area of physics modified to account for the unusual behaviour of matter on the small scale. The Heisenberg uncertainty principle, well known for the assertion that it is impossible to know both a particle's position and momentum at the same instant,. It also demands that even the vacuum of space must possess some form of residual energy. Due to the assertion of equality between energy and mass, this vacuum energy is believed to take the form of 'virtual particles' continually popping in and out of existence.
Suggestions that this vacuum energy is in fact the enigmatic substance referred to perplexingly as 'dark energy' have caused quite a stir. Dark energy, one of the unsolved mysteries of modern cosmology, is believed to make up over 75% of the universe and be responsible for the acceleration of the universe's expansion. It is proving difficult to define; there have been many theories put forward to describe dark energy - quintessence, phantom fields, chaotic scalar fields to name a few - all rather unfamiliar concepts and all as of yet unsubstantiated. As an explanation zero-point energy is appealing in its reassigning new meaning to a concept that has already been around for decades and is well grounded in other theory.
While the physical basis behind this theory of dark energy is phrased in a language we understand, the experimental difficulties Drs Warburton and Fenton encounter in the course of their work are substantial. They use devices that may hold some answers, devices based on Josephson junctions. The Josephson junctions themselves are tiny electronic devices consisting of strips of superconducting material, in which a current experiences no resistance, and separated by a thin insulating gap. In such a setup it is another consequence of the Heisenberg uncertainty principle that there is a probability for the electron to be present in the insulating layer and, under the right circumstances, it can effectively 'tunnel' across the insulating gap, resulting in the flow of a current.
As in any electronic set-up the effects of background noise must be taken into account, but whereas in most experiments noise is a nuisance to be minimised as far as possible, in these investigations one component in the spectrum of noise is the area of real interest.
At low temperatures, when the thermal excitations in the superconducting material are somewhat suppressed, the current due to noise is dominated by zero-point fluctuations. The potential for proof, or at least compelling evidence, as to whether vacuum energy is in fact dark energy lies in a limit placed on the vacuum energy by astronomical observations of the amount of dark energy in the universe. The finite value of dark energy density dictates that the team should see no zero-point fluctuation current generated above a critical frequency of around 1.7 THz.
The biggest challenge now is to develop experimental equipment capable of measuring the background noise up to such high frequencies. By exploring the properties of high-temperature superconductors the team at the London Centre for Nanotechnology, in collaboration with scientists at the University of Cambridge, hope to take measurements across the critical THz frequency range.
The implications of verifying the upper limit and connecting vacuum energy with dark energy would be huge, throwing light onto the formation of the current universe and fuelling predictions for its future. But in the atmosphere of uncertainty and even controversy surrounding the theory any answers either way to the question of whether we can measure dark energy in a laboratory will be a welcome step in moving towards long searched for answers.