Momentum of photons

Question

We know that photons (light) are massless but they have momentum. Now suppose I am in the space far away from planets/stars that there is no external force exerts on me, if:

1- I turn on a flashlight (torch), would I be pushed in the opposite direction which the flashlight is facing (Newton's third law)?

2- If a star is shedding light on me (its photons hit my body), would I start moving due the impact of the photos?

See How can a red light photon be different from a blue light photon? — mmesser314, Aug 12 '20 at 13:12
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Philip Wood · Accepted Answer · 2020-08-12T11:23:27.230

14

Yes in both cases. In fact there is no need to invoke the concept of photons; electromagnetic radiation consistent with Maxwell's equations carries momentum.

You might care to search online for articles on photon propulsion and solar sails.

edited Aug 12 '20 at 11:23

answered Aug 12 '20 at 11:08

Philip Wood

35,641

1

Note that it takes a lot of energy. For light, $E=pc$, so (for example) to give 1 kg an impulse of 1 m/s takes almost 300 megajoules. – PM 2Ring Aug 12 '20 at 11:51
which is why there are many proposals for practical applications but few if any actual implementations. – Philip Wood Aug 12 '20 at 12:00
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@PM2Ring "lot of energy" -- everything is relative to what you expect :-) . A supernova will produce enough photons to accelerate the crap out of your General Products Number 4 Hull. – Carl Witthoft Aug 12 '20 at 12:47
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@Carl Sure, but if you're that close to a supernova, even the neutrino (& antineutrino) flux will make things "interesting". On the topic of supernova energies, please see https://physics.stackexchange.com/a/455549/123208 ;) – PM 2Ring Aug 12 '20 at 12:51

Momentum of photons

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