According to quantum theories, whose many predictions have now been verified with great precision, empty space is not empty but is teeming with particles such as photons (particles of light) which pop into and out of the vacuum and are referred to as virtual photons or zero point photons. The presence of these zero point photons has been verified in experiments such as those which measure the "Lamb shift," a small but significant shift in the energy levels of hydrogen atoms (Nobel Prize 1955). Virtual particles are not just concepts of esoteric interest to physics but are central to biological life and chemistry as they play a role in the van der Waals forces that are responsible for the strength of cell walls and determine the preferred structure of proteins, among many other effects. They also contribute to a large component of the frictional and adhesive forces between neutral objects. While the importance of the virtual photons is clear, exactly how they interact with real objects and lead to such forces is not fully understood. In this project the principal investigators study this directly by comparing precision measurements and theory of the Casimir force, the macroscopic long distance version of the van der Waals force which predicts a force between two uncharged ideal metal plates placed in empty space. The Casimir force can be thought of as the net force from the photons bouncing off the plates on reflection or as an interaction of the charge and current fluctuations induced by the same photons. If real photons such as the thermal photon emission from the interacting objects are also present, they will contribute to the Casimir force in the same manner. One of the most basic questions is how zero point photons interact with real material objects and whether the interaction is different from that of real photons. For one, it has to be different with regard to the net energy absorption. Real photons interact with all materials to transfer energy and heat (for example, heating of an object left in the sun). But zero point photons cannot do the same, as that would lead to a net creation of energy. This project will look for potential differences between real and zero point photons by performing precision measurements of the Casimir force with different materials and at different temperatures. Different materials have different reflections and thus different Casimir forces. Also all materials at non-zero temperature emit real photons which can be related to their temperature and material properties. By using different materials, the principal investigators will vary the ratio of contribution of the real photons to the zero point photons and thus be able to tease out any differences in their interaction. The scientific impact is a fundamental understanding of the nature of zero point photons. The technological impact will be in the design of novel micro electromechanical (MEM) devices. As the Casimir force exceeds normal electromagnetic and gravitational effects in MEMs operating with submicron scale features, there is a real need to understand these effects.
At room temperature and plate separations below 1 micron, the Casimir force comes overwhelmingly from zero point photons. As the peak of the Planck thermal emission spectrum is around 7 microns, one intuitively expects that the additional thermal photon contribution adds to the force as separation increases, but with material absorption, the thermal photon contribution is surprisingly repulsive up to 6 microns. The case of two objects at different temperatures is fascinating, as the force can be repulsive, oscillatory or zero. In this case the critical contribution is from the near-field thermal emission, which is many orders of magnitude larger than that expected from the Planck thermal emission spectrum. The principal investigators will also develop a new experimental configuration to improve the precision of the experiments: instead of using two plates, a sphere-plate arrangement will be used to avoid issues with keeping two plates perfectly parallel. Two methods will be used: (i) measurement of the Casimir force gradient through the resonance frequency shift of a cantilever; and (ii) difference Casimir force measurement between a surface and vacuum using periodically patterned deep trenches which drive a cantilever into resonance with a large amplitude which can then be detected. Measured forces will be compared to the developed scattering theories relevant to the different experimental configurations to understand the roles of the zero point and thermal photons.
