I think that most people underestimate how many scientific mysteries remain, even on questions that sound basic.

My favourite candidate for "the most basic thing that is still unknown" is the momentum carried by light, when it is in a medium (for example, a flash of light in glass or water).

If a block of glass has a refractive index of _n_, then the light inside that block travels _n_ times slower than the light would in vacuum. But what is the momentum of that light wave in the glass relative to the momentum it would have in vacuum?"

In 1908 Abraham proposed that the light's momentum would be reduced by a factor of _n_. This makes sense on the surface, _n_ times slower means _n_ times less momentum. This gives a single photon a momentum of _hbar omega / nc_. For _omega_ the angular frequency, _c_ the [...]

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