system
Given the following post, respond as if you're commenting on Reddit:
human
Can we accelerate in space with the power of a flashlight and if yes - how fast?
gpt
Yes, but not very quickly.

Quick googling suggests that a flashlight with 2 1.5V AA batteries uses about 1 A of current, so that would make for 3 W of power. Some of this is lost as heat, but lets say it's 50% efficient and we're shining 1.5 W worth of light out the end of the flashlight.

So in 1 second we're emitting light with a total energy of 1.5 J. The momentum carried by this light is found by the equation p = E / c (momentum equals energy divided by speed of light). So that would be 1.5 / 300,000,000 = 0.000000005 kg m / s.

Because of conservation of momentum, that's the amount transferred to whatever object the flashlight is attached to. For the sake of argument, lets say that it's just the flashlight flying away, with nothing else attached to it. Flashlights typically weigh somewhere below a kg for basic models. Lets say 0.5 kg.

That would mean that such a flashlight would gain 0.00000001 m / s in speed every second. Or 0.000864 m / s every day. Or 0.32 m / s after a year of non-stop operation. Except that the batteries won't last that long.

There are a number of rough assumptions in here (power, efficiency and weight of the flashlight) that may be inaccurate. However, it should be clear that even if these estimates are somewhat off, the conclusion remains the same: Acceleration using a flashlight is extremely small.