872. What is a vacuum? Is it filled with charges with no mass? — AW, Karachi, Pakistan
In principle, a vacuum is a region of space containing no real particles (no atoms, molecules, electrons, or other subatomic particles). Because the universe is filled with particles that pass easily through lots of matter (neutrinos, for example), it's very hard to obtain a true vacuum. But let's suppose that you could actually obtain a region of space with no real particles in it. That region of space would still contain large numbers of virtual particles at any given moment. These virtual particles are temporary quantum fluctuations of the vacuum; brief excursions of the quantum fields associate with various subatomic particles. These excursions are permitted by the Heisenberg uncertainty principle, which allows temporary violations of the conservation of mass/energy as long as those violations are extremely brief. While the presence of these virtual particles can only be detected indirectly, they are not massless. Except for their short lifetimes, these particles have characteristics similar to those of normal particles. In fact, if enough energy is used in the process of looking for a virtual particle, that virtual particle can be converted from virtual to real so that it can be detected directly. The energy of detection serves to "pay" for the mass of the particle so that it can leave the virtual realm and become a real, permanent particle.