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Virtual particles are particles described by quantum physics that exist for an extremely limited space and time. Specifically, less than one planck time. The term virtual should not suggest that these particles do not exist. They exist, and they have been measured -- or the effect they have on their environment has been. The very laws of physics prevent them from ever actually being directly seen or measured.
Virtual particles do have mass, even when they are part of massless forms, such as photons.
The vacuum of space (or, more correctly any "space") has an energy level. Nothingness is in fact something. Due to the uncertainty principle, virtual particles will always appear from the energy of a vacuum and always appear in pairs. These particles "borrow" energy from the vacuum and immediately collide and annihilate themselves, repaying the energy back into the vacuum so as not to violate the laws of thermodynamics. This process has implications for the development and eventual dissipation of black holes; when a virtual pair appears next to the event horizon of a black hole, one particle may fall in and if that happens the other will free itself. In order to maintain the first law of thermodynamics (energy cannot be created or destroyed) the black hole must then give up a little of its own energy to repay the lost energy - this is called "Hawking radiation."
 Why they matter to non-physicists
If a RationalTypePerson (TM) is ever out walking along the beach, minding his own damn business, and a loonie fundie creationist comes up to him, to try to convince our rational thinker of the very "irrational" position he holds in regards to his "faith" in the "religion" of "Darwinism", the Fundie type will almost always, at some point in his well reasoned argument, arrive at this comment: "Evolution does not work because of the Laws of thermodynamics".
Now such fundi will rarely understand what they are saying, they are just touting off a line as it was touted off to them, probably by the likes of Ken Ham or Kent Hovind. He will not really explain his position, he will just shout out "Violates Second Law".
The wise and thoughtful Rationalist will shout back "Virtual Particles".
Virtual Particles, by all accounts of modern quantum physics, do exist. They can, in fact,
appear and disappear, literally be self-created and self-destroyed in an instant, inside of a vacuum. In that sense, these very real particles are one of several very obvious violations of the "Law" (which is really a model anyhow) of Thermodynamics.
 See also
- ↑ In case you care, a Planck Time is the smallest unit of time that can be reasonably understood or described. It is roughly seconds.
- ↑ One way to think of nothing being something in this context, is of "nothing" being the sum of all possible positive and negative energy states, just as how 0 can be thought of as the sum of all positive and negatve numbers, as well as the layman context of it being actually nothing