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Can anyone suggest existing technologies that must account for electromagnetic momentum? I believe one example to be confinement in nuclear fusion; but momentum there appears in an averaged form, as pressure.

Just to give an idea of what I mean with "existing technologies": general-relativistic time dilation has to be concretely taken into account in any phone's GPS.

(Edit: many excellent answers have been given, it's really difficult and partly meaningless to choose one as "the" answer! I chose JEB's one mostly because it is about a macroscopic effect and about a directed form of electromagnetic momentum, rather than more "averaged" forms closer to electromagnetic pressure.)

pglpm
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Laser cooling techniques do rely on the fact that the absorption and emission of photons is accompanied by a transfer of momentum from the absorbed photon to the electron (and from there to the atom).

Quantumwhisp
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I'm not sure if there's a particular mass scale you're interested in, but any technology that uses EM radiation to communicate is going to rely on momentum from those waves to accomplish anything.

Just like the railway car in the example you linked, an electron that appears to spontaneously move in one direction after an incoming radio wave must have gotten its momentum from the wave, otherwise the conservation of momentum would be violated.

Señor O
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uh....solar sails?

Radiation pressure is also important in precision satellite GNC. It affects orbits, and to some degree, pointing.

And of course, any asteroid deflection technology relying on the Yarkovsky Effect.

Any mission to intercept a comet...needs to know where the tail(s) are going to be.

JEB
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Optical tweezers are quite a well-developed technology and use the force generated when you divert laser light through a dielectric object to manipulate small objects. See for example https://en.wikipedia.org/wiki/Optical_tweezers#Ray_optics for a description of how the light exerts a force on the particle by changing its momentum (direction).

ProfRob
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