But a virtual electron can have any mass - in fact it can have any mass-squared. A true particle has a definite rest mass: every real electron, and every real positron, always has a rest mass of 0.000511 GeV/c 2. It is a general disturbance in a field, and abides by far fewer rules than a true particle, which is a steady, regular ripple in a field. The precise statement is that, via the uncertainty principle, “a photon can be a virtual electron/ virtual positron pair part of the time.” This word “ virtual” makes a world of difference.Īs I’ve written elsewhere, a “virtual particle” is not a particle.

It’s harmless shorthand in a physics class, where the math quickly makes clear what is and isn’t meant by it, but it’s misleading otherwise. In fact, the statement “ A photon can be an e+ e− pair part of the time” is shorthand. But if that were true, then either its overall speed would be slightly slower than c, or, to make up for lost time, it would occasionally have to travel above the speed limit!įortunately, this distressing line of argument is wrong-headed. This would also then seem to imply that a photon, too, can travel below the speed limit, as long as it does so for a very short time. This implies that a massless particle can turn into particles with mass as long as the time involved is short enough. The well-meaning but incorrect answer, found in many books and articles, is that the electron and positron can come into existence as long as they do so only for a very short time, via the uncertainty principle. So how can a photon, traveling at exactly the speed of light, spend any time at all as a pair of particles with non-zero rest mass? Wouldn’t this force it to slow down, or something? But an electron has a positive rest mass, and a positron has the same rest mass as an electron, so they must travel slower than the cosmic speed limit.A photon, like any particle with zero rest mass, has to travel at the cosmic speed limit, a.k.a.Many non-experts have problems with this mysterious statement, and rightly so. It’s also common to find it mentioned in scientific journalism and in most books about particle physics. This phrase is often heard in particle physics classes, such as in this one, where you’ll see it explicitly stated that “ A photon can be an e+ e− pair part of the time“. So what do people mean when they say “A photon can be an electron-positron pair part of the time”? This statement comes with a fancy “Feynman diagram”, in which the photon is shown as the wavy line, time is running left to right, and the loop represents an electron and a positron created from the photon. Meanwhile electrons and positrons both have positive rest mass.

Feynman diagram suggesting a photon is sometimes an electron-positron pair.īut in the meantime, here’s a related question: how can a particle with zero mass (zero rest mass, to be precise) spend part of its time as a combination of objects that have positive mass? For instance, a photon has zero rest mass.

Art of charm time of uncertatinty