Each photon of light bulb carries momentum. Why does the light bulb not recoil from conservation of momentum?
$\begingroup$
$\endgroup$
1
-
3$\begingroup$ Does the light bulb only emit photons in a particular direction? $\endgroup$– ACuriousMind ♦Commented Sep 7, 2015 at 22:45
Add a comment
|
2 Answers
$\begingroup$
$\endgroup$
There is a recoil when each photon leaves, but they radiate in all directions at once, as ACuriousMind intimates in his comment, so there is no collimated beam to concentrate the recoil. Even if the total recoil were concentrated, its effect is so small that the screw-in base of the bulb is more than sufficient to hold the bulb steady.
Theoretically, it would be possible to power a space craft from the recoil of photons.
It also might be possible to create thrust using amplified photons from a laser.
$\begingroup$
$\endgroup$
1
Even if it did, the energy-to-momentum ratio of light is $c$, a fantastically huge number.
For example, even if you house the light bulb in one big perfect mirror with a tiny collimated (all in one-direction) aperture that all of the light goes out of, and even if the light bulb is a perfectly efficient radiator which is using 100W of power, the corresponding force pushing on the light bulb is only $100\text{ W} / c = 3.33\cdot 10^{-7}\text{ N}$ of force, literally a third of a micronewton. A fly could handily beat that sort of force.
-
1$\begingroup$ Let's run with that. We will put the light bulb and mirror on a perfectly frictionless and level table in a vacuum and let it accelerate. We will assume that the bulb, mirror, fixture and all is 100 g. There is no force from the wires going to the bulb. It will take 13 minutes for it to go one meter from the momentum transfer of the light. That's actually surprisingly quick! This is why light sails and ion engines work -- you just need to be patient. $\endgroup$ Commented Sep 8, 2015 at 5:24