3 ms·
Note that in contemporary usage, the term 'mass' without qualifier is normally understood as a reference to invariant mass (aka rest mass). Photons have energy,
by cygx 6y ago
Note that in contemporary usage, the term 'mass' without qualifier is normally understood as a reference to invariant mass (aka rest mass). Photons have energy, but no mass. They can be used for propulsion due to having nonzero momentum.
- rssoconnor 6y agoWhile the 'working mass' in this case is massless, nevertheless the spacecraft still loses its own actual mass as it accelerates, which is something you might not expect from something labeled as "thrust without reaction mass".
- cygx 6y agoTrue. After sending out a photon of energy E, a spaceraft of mass m will have its (invariant!) mass reduced by a factor of sqrt(1 - 2E/mc²). In terms of energies, this is all rather trivial conceptionally: Rest energy (aka 'mass') gets converted into the kinetic energies of the spacecraft and the photon.