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This article sounds like it got written by a scientist but got butchered through a PR department. It talks about the "Mass of a photon" and weighing them direc

This article sounds like it got written by a scientist but got butchered through a PR department. It talks about the "Mass of a photon" and weighing them directly, rather than the energy-equivalent mass, which is presumably what they are intending to talk about?

fsh 7y ago
As the article correctly states, photons have a finite mass that could in principle be measured by putting an optical cavity on a scale.
- renoz 7y ago
  I know that zero is a finite number, but since the photon is a massless particle, wouldn't it be impossible to get a kilogram worth of photons bouncing around in an optical cavity?
  - bonoboTP 7y ago
    For some reason, zero is often considered not finite. I personally find this terminology quite unintuitive, but it is well established.
  - amluto 7y ago
    No, it’s entirely possible in principle. If you build a perfect optical cavity, weigh it, pump a bunch of light in, and weigh it again, you’ll find that the weight went up by ghf/c^2 summed over each photon added to the cavity. Divide by g and you get the sum of the masses of the photons. 1 kg of photons would be enough to destroy your lab and the rest of your city, so only a very advanced civilization would ever do the experiment on this scale. Photons indeed have no rest mass, but it’s impossible to ever find a photon at rest unless you yourself have no rest mass, in which case you also have no ability to do experiments.
    - phaemon 7y ago
      Actually, as a physicist with no rest mass, I'm perfectly capable of performing experiments. I just never seem to find the time...
  - ben_w 7y ago
    Only in practical terms, as that’s c^2 joules of photons, which is roughly equivalent to the Tsar Bomba detonation.
  - dzdt 7y ago
    Photons have zero rest mass but it so happens that they are always zipping around at the speed of light so their actual mass as observed is nonzero.
  - EliRivers 7y ago
    Something fun of a related nature; if you heat something up, zero particles added, it gets heavier :)
ahazred8ta 7y ago
If you had a perfectly reflective impervious box, put in 500g of matter and 500g of antimatter, and shook it, the box would then contain 1000g of gamma ray photons. Same gravitational and inertial mass as 1000g of inert matter. (Due to gravitational blueshifting, the photons hit the bottom of the box slightly harder than the top. This is not a halfbaked science blooper; your local physicist will tell you this is actually correct.) QM says photons have zero 'rest mass', but that's not the same thing as actual zero mass.

fsh 7y ago

As the article correctly states, photons have a finite mass that could in principle be measured by putting an optical cavity on a scale.

renoz 7y ago
I know that zero is a finite number, but since the photon is a massless particle, wouldn't it be impossible to get a kilogram worth of photons bouncing around in an optical cavity?
- bonoboTP 7y ago
  For some reason, zero is often considered not finite. I personally find this terminology quite unintuitive, but it is well established.
- amluto 7y ago
  No, it’s entirely possible in principle. If you build a perfect optical cavity, weigh it, pump a bunch of light in, and weigh it again, you’ll find that the weight went up by ghf/c^2 summed over each photon added to the cavity. Divide by g and you get the sum of the masses of the photons. 1 kg of photons would be enough to destroy your lab and the rest of your city, so only a very advanced civilization would ever do the experiment on this scale. Photons indeed have no rest mass, but it’s impossible to ever find a photon at rest unless you yourself have no rest mass, in which case you also have no ability to do experiments.
  - phaemon 7y ago
    Actually, as a physicist with no rest mass, I'm perfectly capable of performing experiments. I just never seem to find the time...
- ben_w 7y ago
  Only in practical terms, as that’s c^2 joules of photons, which is roughly equivalent to the Tsar Bomba detonation.
- dzdt 7y ago
  Photons have zero rest mass but it so happens that they are always zipping around at the speed of light so their actual mass as observed is nonzero.
- EliRivers 7y ago
  Something fun of a related nature; if you heat something up, zero particles added, it gets heavier :)

ahazred8ta 7y ago

If you had a perfectly reflective impervious box, put in 500g of matter and 500g of antimatter, and shook it, the box would then contain 1000g of gamma ray photons. Same gravitational and inertial mass as 1000g of inert matter. (Due to gravitational blueshifting, the photons hit the bottom of the box slightly harder than the top. This is not a halfbaked science blooper; your local physicist will tell you this is actually correct.) QM says photons have zero 'rest mass', but that's not the same thing as actual zero mass.