What Lies Beneath the Void

Professor Chris Binns (Physics and Astronomy) on his exciting project connected to the 'zero-point energy' of space.

Three thousand years ago the Greek philosophers Leucippus and his student Demokritos proposed the concept of the atom, as a fundamental building block of materials, in order to circumvent a paradox that arises with continuous elements (such as earth fire air and water). They pointed out that if matter was really a continuum then you could cut it into smaller and smaller pieces ad infinitum and, in principle, cut it out of existence into pieces of nothing that could not then be reassembled. Thus, they reasoned, there must be a smallest piece of matter that could not be further divided the a-tomon (uncuttable) from which the word atom is derived. To complete the picture you also need a void in which the atoms move, a concept that produced fervent debate, for example, is the void a ‘nothing’ or a ‘something’ and is it a continuum or does the void itself have an uncuttable smallest unit.

While the atom, the legacy of Leucippus and Demokritos, is now a familiar part of the scientific landscape, the true nature of the void remains a mystery. In classical Physics the void is a ‘nothing’, a simple absence of all matter and energy. Quantum theory tells a different story and states that the void is definitely a ‘something’. It is a seething mass of ‘virtual’ particles that fleetingly appear into and then disappear from our observable universe. This activity, known as quantum fluctuations, corresponds to an intrinsic energy of the void, the ‘zero-point energy’, which, if the void were a continuum, would be infinite. It is generally believed that there is a smallest piece of void, which makes the zero-point energy finite but still colossal beyond the imagination. Each cubic millimetre of empty space contains more than enough zero-point energy to create a new universe.

In a sense the actual value of the zero-point energy is not important because everything we know about is on top of it. According to quantum field theory every particle is an excitation (a wave) of an underlying field (for example the electromagnetic field) in the void and it is only the energy of the wave itself that we can detect. A useful analogy is to consider our observable universe as a mass of waves on top of an ocean, whose depth is immaterial. Our senses and all our instruments can only directly detect the waves so it seems that trying to probe whatever lies beneath, the void itself, is hopeless. Not quite so. There are subtle effects of the zero-point energy that do lead to detectable phenomena in our observable universe. An example is a force, predicted in 1948 by the Dutch physicist, Hendrik Casimir, that arises from the zero-point energy. If you place two mirrors facing each other in empty space they produce a disturbance in the quantum fluctuations that results in a pressure pushing the mirrors together. Detecting the Casimir force however is not easy as it only becomes significant if the mirrors approach to within less than 1 micrometre (about a fiftieth the width of a human hair). Producing sufficiently parallel surfaces to the precision required has had to wait for the emergence of the tools of nanotechnology to make accurate measurements of the force.

In the last decade this has happened and a spate of measurements using atomic force microscopes has confirmed Casimir’s prediction to a precision of about 5% and the zero-point energy of the void is an experimental reality. This is just the beginning however as the force has only been measured in very simple geometries such as flat parallel plates. More recent calculations show that the force is sensitive to geometry and by changing the materials and the shape of the cavity you can alter the magnitude of the Casimir force and possibly even reverse it. This would be a ground-breaking discovery as the Casimir force is a fundamental property of the void and reversing it is akin to reversing gravity. Technologically this would only have relevance at very small distances but it would revolutionise the design of micro- and nano-machines.

The srif2 and srif3 investment by the University of Leicester in the Virtual Microscopy Centre and the Nanoscale Interfaces Centre has put the University in a key position to take a lead in Casimir force measurements in novel geometries. It has led to the award of an 800,000€ grant (NANOCASE) from the European framework 6 NEST (New and Emerging Science and Technology) programme to lead a consortium from three countries (UK, France and Sweden). The programme will use the ultra-high vacuum Atomic Force Microscope installed in the Physics and Astronomy department under srif2 to make very high precision Casimir force measurements in non-simple cavities and assess the utility of the force in providing a method for contactless transmission in nano-machines.

The new instrumentation to be installed soon following the srif3 investment will enable researchers to extend the measurements to yet more complex shapes and, for the first time, to search for a way to reverse the Casimir force.

This new wave of measurements will enable an unprecedented level of probing of the void and will provide important information on new theories of gravity and with sufficient precision will even put limits on the true number of spatial dimensions. Knowing how zero-point energy varies with the shape of an enclosure may also give clues to the origin of so-called ‘dark energy’, discovered recently.

* Professor Chris Binns, Physics and Astronomy.
http://www2.le.ac.uk/ebulletin/features/2000-2009/2005/08/nparticle-82w-fqr-2cd