Virtual Reality

The so-called Standard Model of physics, which culminated in its modern form about three decades ago, represents the triumph of quantum field theory, itself conceived about 80 years ago. Quantum field theory embodies quantum wave particle duality in a mathematically satisfying way, but doesn’t exactly settle the thorny question of interpretation. Quantized fields are the underlying reality, but particles are what we detect.

Most doable calculations in quantum field theory are done by a mathematical technique called perturbation theory, and Feynman and others showed that perturbation theory has an intuitively appealing visualization in terms real and virtual particles – real particles being the ones that show up in our detectors. Interactions between particles are really interactions between quantum fields, but in perturbation theory, those interactions are visualized in terms of the exchange of so-called virtual particles. This visualization is not just a handy mnemonic – the particles and their Feynman diagrams correspond in a mathematically precise way to the terms in the perturbation series. Thus, to lowest order of perturbation theory, the scattering of two charged particles can be conceived as the exchange of a (virtual) photon between them.

One way to read this diagram is to say the electron on the left was moving along and spit out a virtual photon, which was absorbed by the electron on the right. The momentum carried by the photon deflected both electrons, and hence responsible for their change of path. So what’s the difference between exchanging a real photon and a virtual one? Ignoring any subtleties, let’s just say that the virtual photon doesn’t have to obey relativistic energy momentum rules - E is not necessarily equal to pc, or as physicists (at least those of my day) say, it is off the mass shell. Virtual particle's violation of energy-momentum conservation is strictly short term. The vacuum is sort of a central banker to particle world – need some energy or momentum to complete a scattering deal? You can borrow it from the vacuum – interest free – but the rules on quick repayment are very strict (they are governed by the time-energy uncertainty relation). Very small loans can last for a while, but the big ones are super short term. Particles are allowed to “borrow” from the vacuum, but the deal is that all energy accounts must be settled at the end of the transaction – absorption of the virtual particle.

In quantum field theory, virtual particles form a kind of ubiquitous ether in which everything else happens. The virtual particles are constantly winking in and out of existence.

The pass virtual particles get on energy conservation doesn’t apply to charge conservation, so charged particles need to be created in pairs – an electron with a positron, for example. Among other things, this fact creates a path for virtual particles to the citizenship of the real, if circumstances happen to be just right. Suppose a virtual electron-positron pair is created in a very strong electric field. The field will tend to pull the electron one direction and the positron the other, doing work on them and creating energy of separation. If the work done becomes equal to the energy needed to create a real electron positron pair (about 1.022 MeV), the pair can pay off their loan to the vacuum and escape to real existence.

Why doesn’t this happen in every electric field? Well, the field needs to be large, strong, and long lasting enough to create the necessary energy of separation, but it also needs to be able to do it quickly – the vacuum field loan service won’t tolerate long term loans. This process was first described by Schwinger in 1951, but has apparently not yet been tested.

http://arxiv.org/abs/0708.1471

One reason this is important is that there seems to be a relation between the Schwinger effect, the Unruh effect, and the Hawking effect, so quantum gravity could be in the crosshairs too.

http://prd.aps.org/abstract/PRD/v55/i6/p3603_1