8 - Horizons

Summary

Minkowski space

At the beginning of the twentieth century, Albert Einstein replaced the aether theory by relativity, but a twenty-first century aether is still puzzling physicists today. This modern aether is the quantum vacuum. The aether was thought to be an all-penetrating mysterious substance that carries light through space like air carries sound. Take away all light, and the aether would still be there, defining a universal frame of reference. Now, according to quantum field theory, the state of absolute darkness, the vacuum state, is still a physical state filling space throughout, similar to the aether. There is an important difference though: one does not notice motion at uniform speed relative to the quantum vacuum, but, as we describe in this chapter, during acceleration the vacuum glows, although slightly, causing friction. Furthermore, as Stephen Hawking predicted in 1974 (Hawking, 1974), the quantum vacuum should also cause black holes to evaporate, because at the event horizon particles are created from nothing, at the expense of the black hole's mass. In this chapter we also describe this creation of radiation at horizons. None of these fascinating phenomema have been observed in astrophysics yet, but they can be demonstrated in laboratory analogues (Philbin et al., 2008a).

The quantum vacuum may also account for the dark energy that, according to astronomical data, constitutes the lion's share of the energy of the Universe (Hogan, 2007).