Date: Around 1993
In the book/movie "A Brief History of Time," Steven Hawking
mentions that black holes can be detected by looking at virtual particle
creation and annihilation events near a black hole event horizon. What are
virtual particles? How is it speculated they are created? What are the
products of the annihilation?
This is difficult, but here goes. Particles and anti-particles
if they come too close, combine and disappear in the creation of two photons
(light quanta) of generally high energy. This is called annihilation. Energy
and momentum are conserved, but mass disappears and light energy appears.
Virtual particles is an idea that represents pairs of particles and anti-
particles that appear for very short times. If these particles appear for
very short times it appears that they can violate energy conservation a
little. Since we expect real particles to obey energy conservation we call
such things virtual. They have real effects. Their presence effects the
energy levels of atoms by a very small amount which was calculated accurately
and that is why physicists believe these things exist and are important.
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Update: June 2012