Virtual Particles

(Created prior to 1993)

Question:  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?
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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.

Sam Bowen
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