system
Given the following post, respond as if you're commenting on Reddit:
human
If I put a flashlight in space, would it propel itself forward by "shooting out" light?
gpt
Yes, very slowly.

Light has momentum, even though it is massless, so if you shoot a beam of light in one direction, conservation of momentum will push you in the opposite direction.

A reasonably powerful LED flashlight will use about 1-3 Watt, lets say 3 W. The efficiency of a LED is somewhere between 25% and 40%, so for sake of ease of computation lets make that 33% and we get a net amount of light output of 1 W. 

The ratio between the momentum and energy of light is 299,792,458 (Which is also the speed of light). So in 1 second, the flashlight produces 1 J worth of light, which is equal to 0.33 * 10^-8 kg m/s. If the flashlight is not too heavy, say 100 gram or 0.1 kg, that means that 1 second of light would propel the flashlight to a velocity of 10^-7 m/s. This assumes that all light is directed in straight line. The more cone-shaped the bundle of light is, the lower the momentum transfer is.

Leaving the light on for one day would propel the flashlight to about 0.009 m/s or almost 1 cm per second. Unfortunately, operating a 3 W LED for a day uses about 260 kJ of energy. Regular AA batteries have somewhere around 10 kJ of energy (depending on the type). And at a weight of 20-30 grams per battery, you can't carry put more than 2-3 in the device without violating our original assumption of a 100 gram device.