Hey Photon, That Black Hole Makes You Look Rather Svelte
Physicists’ Use of Black Hole Studies to Measure Photon Mass Published in Physical Review Letters
“Ultralight photons with nonzero mass would produce a ‘black hole bomb’: a strong instability that would extract energy from the black hole very quickly,” said Pani, the paper’s lead author. “The very existence of such particles is constrained by the observation of spinning black holes. With this technique, we have succeeded in constraining the mass of the photon to unprecedented levels: the mass must be one hundred billion of billions times smaller than the present constraint on the neutrino mass, which is about two electron-volts.”
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Can you explain this to me? I thought that it was necessary for photons to have no rest mass, due to their always moving at the speed of light. Why would there be an “upper bound” on zero?
Measurements always have an uncertainty, so you can never measure precisely zero. All you can do is find that an answer agrees with zero, with as small an uncertainty you can manage, which gives an upper bound on the value.
Ok, so measurements have uncertainty. I thought that since particles that travel at c *must be massless*, that there was no need to measure it. If they weren’t massless, they wouldn’t travel at c. Hasn’t the speed of light been measured to an astounding degree of accuracy?
No, you always do the experiment to check the theory. And then you try and do a better experiment to get more precise and accurate results.