Vacuum is 'a space absolutely devoid of matter'. But according to Quantum ElectroDynamics (QED), particles can still be created in this emptiness of vacuum through light-light interactions. This property follows directly from the quantum nature of the sub-atomic world, to be specific, from the Heisenberg Uncertainty Principle which states that the uncertainty in the position of a particle and the uncertainty of the momentum of a particle are related. A consequence of this principle is that even though there is nothing in the vacuum (no matter or radiation at all), there is still an uncertainty in the amount of energy which can be contained in the vacuum. On average, the energy is constant, however, there is always a slight uncertainty in the energy, which may allow a nonzero energy to exist for short intervals of time. Because of the equivalence between matter and energy, these small energy fluctuations can produce matter (particles) which exists for a short time and then disappears.
In a paper entitled "Using High-Power Lasers for Detection of Elastic Photon-Photon Scattering" published in March 3 issue of Physical Review Letters (Vol.96), Physicists from Umeå University, in Umeå, Sweden, and the Rutherford Appleton Lab, England, propose an experiment to explore the vacuum by aiming three powerful laser streams at each other in 3-dimensional space of the Laboratory (This is important because such proposals mooted earlier had the beams all in a single plane). These three beams will merge to produce a fourth stream with a wavelength shorter than any of the input beams.
The actual experiment is planned to be carried out over the next year at the Rutherford Appleton Lab near Didcot, England. By carefully polarizing the incoming light beams, the number of photons in the output beam can be controlled. This would be an important tool for investigating the parameter space of such a complex experiment, thus providing valuable information about the interactions that took place in the vacuum.
Besides providing good insight into QED itself, this experiment would also be used for testing theories that propose the existence of minor departures from Lorentz invariance which is an important proposition in special relativity that there is no preferred frame of reference. Light-light interactions may also be used to explore various hypotheses related to dark energy that is a hot topic of cosmology nowadays and may provide some clue about the rate and nature of the expansion of the universe.