The uncertainty principle also predicts that there will be similar virtual pairs of matter particles, such as electrons or quarks.

One can regard the member of the virtual pair that fell into the black hole (say, the antiparticle) as a particle traveling backward in time out of the hole.

If, instead, it were the particle member of the virtual pair that fell into the hole, one could regard it as an antiparticle traveling back in time and coming out of the black hole.

They are known as 'virtual' pairs (Figure 5.6).

Other virtual particles may contribute to the summation as well; for example, two photons may interact indirectly through virtual electron-positron pairs.

In all, Herschel discovered over 800 confirmed double or multiple star systems, almost all of them physical rather than virtual pairs.

Even in the void between galaxies, virtual pairs of positive and negative electrons, mu mesons, and baryons are continually being created and annihilated.

That's because if there's a point failure..." Now a virtual pair of scissors appeared and snipped one of the blue strands.

Paul Dirac was the first to propose that empty space (a vacuum) can be visualized as consisting of a sea of virtual electron-positron pairs, known as the Dirac sea.

It is an example of quantum foam and is the gravitational analog of the virtual electron-positron pairs found in quantum electrodynamics.