[buildbot]

Generating matter somehow via E/c^2=m?

(I don't think you can do this but I'm not a physicist...)

[jfengel]

You don't need to go that far. Light has momentum, and you can use that directly. It has the maximum possible specific impulse.

https://en.wikipedia.org/wiki/Photon_rocket

However, it requires a lot of energy and we are nowhere near a practical model. It's also not "propellant-less"; the photons are the propellant.

[BurningFrog]

The practical point of being "propellant-less" is that you don't use up some finite supply of propellant.

If the "propellant" is electricity that our solar panels can generate, that's functionally propellant-less!

[philipkglass]

Known physics permits creating matter from energy:

https://en.wikipedia.org/wiki/Pair_production

If you have a source of energy on the spacecraft, like a solar panel, you could theoretically convert some of that energy to particle pairs with mass. But this is such an inefficient process (and so inherently low-mass with any practical energy source) that I doubt the claimed thruster could work this way.

[buildbot]

100% this would never be practical, more wondering if that technically, it’s possible. Which it seems to be, thanks for the reference!