Virtual Particles

Virtual Particles a mathematical trick used to make some calculations in Quantum Field Theory work. One example of this is when calculating interactions between particles. When we idealize these interactions (reduce them to their simplest forms) Virtual particles are used to transfer energy. They are essential to  our approximations of the behavior of quantum fields. (We get to quantum fields later on. For now, think about them as fields that particles come from.)

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Because they aren’t physical, they can do things like travel faster than the speed of light, move backward in time, and have any mass we want. They also disobey mass-energy equivalence as described by Einstein’s

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E^2 - |p|^2 * c^2 = m^2 * c^4.

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Virtual particles also have a role in vacuums. In a vacuum, the vibrational mode of a field should have zero energy. In other words, vacuums should have no energy. However, due to the Heisenberg uncertainty principle, there is a slight chance that when a vacuum is observed, a given mode has non-zero energy. This leads to a non-zero average energy called zero point energy. This causes the vacuum to appear to have energy, and because of that, it looks like they contain virtual particles that come in and out of existence. 

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Ultimately, it is important to know that virtual particles do not exist and are purely mathematical tools.