system
Given the following post, respond as if you're commenting on Reddit:
human
Do lights produce thrust?

I'm at my desk pretending my flashlight is a rocket. Does it actually produce any (super tiny) thrust if I left it going from the photons it emits?

Bonus question: if the answer is yes, how big of a flashlight would I need to leave earth?
gpt
Yeah, the basic principle of rocketry is that momentum is conserved and since the emitted photons have momentum then the flashlight must be gaining equal and opposite momentum.

The force is actually very easy to calculate, since the momentum of a photon is simply p=E/c then the force is just F=P/c so, for a 100W LED flashlight (pretty hefty), the force would be ~3x10^-6 N. (this assumes all the light is shining in one direction and all the power is going twoards light not heat)

So for your bonus if we assume a person weighs 80kg then the force of gravity is ~800N which means we need approximately a light 10^9-10 times stronger than the 100W bulb  above, i.e. 1 tera-watt. A rough rule of thumb for terawatts is that 1TW is the total installed capacity of the USA (as in peak power production capacity). So you would need all that electricity to send you up (and that doesn't include the weight of the flashlight itself).