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One day, lasers may be propelling craft into space.The momentum of photons is too low to lift a sizeable object against Earth's gravity, but the photon has far more energy than is absorbed by a solar sail. If I understand it correctly, the kinetic energy transferred to the spacecraft is contributed by the slight redshift of photons reflected from the solar sail.The idea is to build a space elevator, but you don't want the elevator to be carrying its own fuel, so you could use ground-based lasers to power the elevator.(I would expect that you could also power it by electricity from the ground - only a conductive space elevator is likely to be destroyed by lightning...)
Sorry if this is a gormless question but if a photon loses energy by pushing something - its wavelength increases and its frequency falls - what happens when it does this several times and the frequency falls virtually to nothing and wavelength approaches infinity? Is it absorbed long before then?