Comment by Borg3

1 day ago

Photons? They cannot be stopped, because they have no "static" mass? As far as they move, they react to gravity, not sure if they can provide its own gravity field. Might be a case as far as they move. Not expert here :)

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Borg3

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raattgift  1 day ago

Photons have momentum. They can drive solar sails. They can kick electrons out of atomic orbitals. Lots of them streaming outwards hold up the outer layers of stars, keeping them from collapsing. Radiation pressure.

Momentum-density is represented in the stress-energy tensor, and so is directly related to spacetime curvature. Light feels and generates gravity.

A sibling comment points out the hypothetical kugelblitz as an extreme example. In cosmology textbooks you can read about the "radiation-dominated era" of the first roughly 50000 years of the universe where the energy density working strongest against the expansion was dense photons (and neutrinos).

  • jjtheblunt  16 hours ago

    I had a radiometer when i was a kid ... is that momentum what makes it spin, because i thought the light was heating the black surfaces more than the white surfaces, so the air molecules bounced more vigorously away from the black.

    i didn't think about photons having momentum when i was 8, though.

    https://a.co/d/00nIWNpB

    • antonvs  4 hours ago

      Photon momentum is a negligible factor in the case of a Crookes radiometer. On a typical vane, it amounts to nano-Newtons of force which wouldn’t move the vane, and certainly wouldn’t overcome friction. Your other explanation is roughly correct for a first approximation.

      To make a radiometer that runs on photon momentum, you’d need much lighter vanes, probably larger, a harder vacuum, mirrored surfaces, a stronger source of photons, like a laser, and probably most importantly, some way to reduce friction - like a torsion balance.

      NASA does solar sail experiments along these lines, e.g. at the Space Power Facility at Plum Brook Station, Ohio - see e.g. https://www.sciencedirect.com/science/article/abs/pii/S00945...