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As I understood it,

the reason I cannot stick my hand through a metal block is due to the repelling force between electrons in my hand and in the block. QED depicts two electrons repelling with a photon exchange, but it's not clear to me what determines which electron emits the photon and which one absorbs it. Does it for instance mean that my hand emits photons to the metal block? Or does it go both ways equally? What if me pushing the block makes it move, intuitively I would say the kinetic energy added to the block should be equal to the difference in energy between the photons emitted by my hand to the block, and the photons emitted back by the block. Let's say my hand pushes a block of 1kg that is resting on a table. I move it for a distance of 1meter, with friction of 1N. The amount of work my hand does is 1J. So I would think my hand has emitted 1J more photons than it has absorbed. But if I shine a flashlight onto the block and emit 1J of photons onto it, why does it not move at all?

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  • $\begingroup$ the electron electron force is due to "virtual photons" ... these virtual photons are just a mechanism or placeholder for how the EM field works for forces ... there is no further understanding possible by science. Real photons called photons are true energy exchange of the EM field, not forces. Once an object moves because of EM forces the virtual photons are no more and real photons are exchanged. That's just how they've defined it. $\endgroup$
    – PhysicsDave
    Commented Feb 20 at 22:10
  • $\begingroup$ So if photons are exchanged in both cases, why does my flashlight not move the mass? $\endgroup$
    – twmen
    Commented Feb 20 at 23:53
  • $\begingroup$ sorry ... charge difference/force is the the virtual photons with virtual ones becoming real ones if there is motion. Yes a solar sail involves real photons and will move the object upon absorption or reflection due to conservation of momentum. $\endgroup$
    – PhysicsDave
    Commented Feb 21 at 0:33

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