This old thread appeals both to the modern physics and the Star Trek fan!
Some info about the fictional photon torpedo from the official and fan lore of Star Trek:
Photon torpedoes are just physical objects, round-edged capsules about the size of a funeral casket. Inside the weapon versions (some versions don’t explode, but rather contain scientific instruments, or, you might recall from the movie Wrath of Khan, even the bodies of dead crew members) is some antimatter, with a mechanism that keeps it separate from its surrounding ordinary matter until the torpedo reaches it target, where it releases the antimatter to annihilate with the ordinary matter, making a really big bang. The torpedo glowing in flight is just a side-effect due to a short-lived “warp field” that the ships launching system surrounds it in – you might recall an episode of Voyager where the starship Voyager, it’s torpedo launchers damaged, just dump its last photon torpedoes in its trail, no glowiness, where they still blow up her pursuing enemies nicely.
I’m pretty sure, the original Star Trek writers and special effects makers, and the early fan “technical” writers and game developers, didn’t have this explanation, but rather thought that photon torpedoes were simply, torpedoes made out of photons, physically nonsensical as that may sound. Trek writers and fans have never been known for their strict adherence to the known laws of physics.
Enough STrek fan lore. The question at hand is, how could you actually make a torpedo out of photons?
I think feeztar’s on the right track here
What has not been discussed is how you get a discrete package of light to travel as a concentrated cluster. Photons are omnidirectional. That is the first hurdle. They might be contained within some kind of gravitational field, but there is never any hardware shown when torpedos fly. Hmm...
An object that contains photons with gravity is a black hole. A black hole that is made of nothing but photons is a kugelblitz
. (John Wheeler wrote about them in 1955)
The problem with a kugelblitz photon torpedo is, since light can’t escape from it, it wouldn’t glow. You can get around that by imagining that the glow of a photon torpedo isn’t from the photons making its kugelblitz, but from hot matter in an accretion disk, which also explains its glowing tail, as accretion disk matter stripped away due to friction with the interstellar medium.
With a bit of Hawking radiation
science, you can even come up with a plausible way that the kugelblitz torpedo can be made with just the right mass-energy to evaporate explosively at a preset range.