The Vacuum Field
The common sense notion of vacuum is surely that it is just an empty, featureless, quiescent void - the ultimate nothing. Most branches of science share this notion, mainly because the phenomena they are dealing with are independent of what the vacuum may be like. Nevertheless, common sense is not of much relevance when it comes to the two sciences that are directly confronted with the peculiarities of the vacuum - Quantum Field Theory and cosmology. Concerning the vacuum, the latter has to rely on the findings of the former because the prevailing cosmological theory of our time, General Relativity, has no means to handle the microscopic levels where the immanent qualities of the vacuum play a crucial role - in experiments as well as in theoretical considerations.
What is vacuum?
In Quantum Field Theory the universe, and everything in it, is regarded as a system of interacting quantum fields. Such a system can exist in different energy states - the ground state and various excited states. The latter correspond to the various appearances (matter, force, etc.) and are characterized by the presence of quanta. The vacuum is defined as the ground-state of the system - i.e. the situation where all fields are in their unexcited ground-state and no quanta are present. However, according to the mathematics of Quantum Field Theory, this vacuum is not empty - it is a field of infinite energy. In principle, one can start from a situation where nothing exists except the vacuum and then, by means of mathematical entities called emission-operators, create all physical phenomena in the present universe. Likewise, one can start with the present universe and then, by means of absorption-operators, end up with nothing but the vacuum. This concept is confirmed repeatedly by experimental high-energy physics which regularly encounters phenomena that can only be explained by quanta popping out of the vacuum or by quanta vanishing into the vacuum respectively.
The Higgs Field
Currently, physics is in lack of a general theory of what the vacuum and the vacuum energy-field actually is. However, since the nineteen-sixties the consensus has gradually emerged that a certain field, the Higgs field, resides in the vacuum. The Higgs field differs from the ordinary fields in the respect that even in its ground-state there are quanta present. These quanta, the Higgs bosons, are regarded as actual physical manifestations of the vacuum energy-field. It is generally believed that the Higgs field is the entity which is responsible for turning the vacuum energy into matter. According to Quantum Field Theory the ordinary quanta require their mass by interactions with the Higgs field (or rather, with the vacuum expectation value of the Higgs field). Prior to this interaction the electrons, quarks, force-carriers, etc. are the massless quanta of some assumed fields - the socalled gauge fields. In their massless shape the gauge field quanta are undetectable - so it is currently an open question whether the gauge fields actually are of physical relevance or just mathematical entities. Nevertheless, it is the consensus that the theory about the Higgs mechanism is the one theory that fits best with everything else known to quantum physics.
Vacuum Field Fluctuations
An essential issue in relativistic quantum physics is the notion of virtual quanta. The reason for the label 'virtual' is that these quanta cannot be detected directly. Nevertheless, their existence can be inferred by energy considerations and virtual quanta can become real under certain conditions and thereby be detected indirectly. The characteristic property of virtual quanta is that they can only exist for a limited and very short period of time. Any quantum - real or virtual - is subject to continuously emit and absorb virtual quanta. Consequently, every real quantum is generally surrounded by a cloud of virtual quanta. Due to the short-livedness of the virtual quanta, these might as well be regarded as rapid energy-fluctuations in the surrounding field or interactions between the real quantum and the field of which it is a part. Vacuum energy fluctuations are special in the respect that here the virtual quanta are emitted from and absorbed by the vacuum itself without the presence of real quanta or any other known phenomenon. - Below, the Feynman diagram to the left represents a real photon which emits a virtual electron-antielectron pair and later absorbs it again. The diagram to the right is an example of a vacuum field fluctation where three virtual quanta are emitted from the vacuum and instantly reabsorbed (bear in mind that the lines are not quantum trajectories but just symbolic):
Actually, the existence of vacuum fluctuations is the strongest evidence that the vacuum is not empty but is a field - or rather, fields. The pondering of the vacuum lies right at the ultimate frontiers of quantum physics, but the quest is troubled by the lack of sufficient equipment. One problem of physics is that the more fundamental the level (i.e. the smaller the scale) the more energy is required to investigate it. Currently, quantum physics is merely scratching the surface of the vacuum and it is the consensus that, in order to dive deeper, physics has to await the facilities of the next generation of high-energy remedies - expected to be ready in 2010-2020.
