- Quote: Once particles are imprisoned, their motion energy is trapped, too, and contributes to their prison’s rest mass even if they have no internal energy of their own.
- Endnote: Gravity provides another example of an object with rest mass made from objects that have none. A black hole is formed from objects whose mutual gravitational pull traps them in an embrace too tight for them to escape. In principle, a black hole could be made purely from photons, in which case the black hole’s rest mass would stem entirely from the photons’ trapped motion energy.
The only thing necessary to create a black hole is to put a great deal of energy in a relatively small space. Roughly speaking, as long as energy E is crammed into a region of diameter L, where
(here G is Newton’s gravitational constant and c is the cosmic speed limit), then a black hole will form. It does not matter where this energy comes from. In particular, the energy could be carried into this small region purely by photons.
In practice, however, this might not be so easy. For one thing, it would be far beyond our current technological abilities to aim photons with extreme precision to allow L to be very small, or conversely, to obtain the powerful laser beams needed to make E extremely big.
Still, in many universes this difficult experiment might be possible, using extreme technology. But it turns out that it might be completely impossible in our own! This was argued in a paper published in 2024, after my book was already in print. This paper argued that in our universe, where there are electrically charged particles such as electrons that have relatively low rest mass, any attempt to actually build a black hole from photons would instead lead first to the conversion of the photons’ energy to electron/positron pairs, thus diffusing their energy before they could be crammed into the small space required to make a black hole.
I haven’t yet had a chance to check this paper myself, as of August 2024, so I can’t confirm that it is fully correct and free of any loopholes. But I currently have no reason to think there’s any problem with it, either.
