Notes:
I hesitate to explain things that are so well known to everyone present here, but please humor me because I am a physicist, and therefore must start at the beginning.
The rocket equation, derived from the requirement that the change in the total momentum must be zero, demonstrates that the change in velocity of the rocket is the product of the exhaust velocity times the logarithm of the final to initial mass. The exhaust velocity is traditionally given in terms of specific impulse (Isp) times the acceleration of gravity. Rating the various types of rockets that have been invented by their specific impulse tells us how “efficient” the rocket uses its fuel supply. On this basis, photons or light provide the most efficient use of fuel, because they travel at the maximum exhaust velocity permitted by physics—the speed of light. (If the rocket doesn’t carry its own fuel, say, by reflecting photons from the Sun, then some would say that the Isp is infinite.) From this brief survey it is apparent that photon propulsion is the best we can do. The importance of Isp is shown by the next slide.