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It has recently been mooted that micro-satellites could be propelled to nearby stars through the use of powerful, ground based laser systems. However, given that light has no mass, I don't know how to calculate the force exerted by a laser on an object and Google has been relatively little help.

Consequently, is there an easy way to calculate the force exerted on an object by a laser given the power, frequency, distance, albedo of the impacted material etc.

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  • $\begingroup$ The immediate place to go is the Wikipedia page on radiation pressure. Is there some specific part of that which you find confusing? $\endgroup$
    – Emilio Pisanty
    Commented May 2, 2017 at 16:15
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    $\begingroup$ Hi James. The question I've linked is essentially a duplicate of yours since both questions ask about the force exerted by light. $\endgroup$
    – John Rennie
    Commented May 2, 2017 at 16:32

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Relativistic energy is:$$E^2 = (m_0c^2)^2 + (pc)^2,$$ For light, rest mass is obviously $0$, so $E^2 = (pc)^2$ and $E = pc$. You can substitute $E = hf$ here, and that gives you the momentum of light. Combine this with the number of emitted photons per second from a LASER, and Using this and some simple maths you should be able to work out the rest.

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  • $\begingroup$ Substituting $E=hf$ will give you the momentum transfer corresponding to a single photon, without saying anything at all about the total force. $\endgroup$
    – Emilio Pisanty
    Commented May 2, 2017 at 16:21
  • $\begingroup$ @EmilioPisanty Oh, sorry, I thought you could take that and since they're bosons, just add up the momentum using the number of photons per second that you'd get from the LASER calculations? $\endgroup$
    – user95137
    Commented May 2, 2017 at 16:23
  • $\begingroup$ Could you correct my logic? $\endgroup$
    – user95137
    Commented May 2, 2017 at 16:24
  • $\begingroup$ You can, but you actually need to do so. At present this answer is not useful to the level the OP's question is pitched at. More generally, relying on the photon picture to explain radiation pressure is misleading - radiation pressure is a classical phenomenon and does not need any sort of underlying energy discretization to work. $\endgroup$
    – Emilio Pisanty
    Commented May 2, 2017 at 16:27
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    $\begingroup$ At the end of the day this solution works, thank you. I'm working on a potential, laser based, solution for space-junk disposal but I've only got A-Level physics which doesn't get into enough detail with regards to this sort of thing. $\endgroup$
    – ScottishTapWater
    Commented May 2, 2017 at 16:55

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