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@johncarlosbaez@mathstodon.xyz
Post #4423255
2026-08-06 09:46 UTC
Wow! You may have looked through polaroid glasses and seen how the Sun reflecting off water looks different depending on how you turn your head. That's because the electric and magnetic fields in light are able to wave back and forth in different directions, so there are different kinds of light: different 'polarizations'.
Now for the cool part:
In 1971 the physicist Stephen Adler predicted that in extremely strong magnetic fields, the speed of light depends noticeably on its polarization. And now this effect may have been seen near a 'magnetar': a young neutron star that's spinning around and has an extremely strong magnetic field - a hundred million times stronger than any man-made magnet.
You might be shocked, because you know that the speed of light in a vacuum is constant. But an extremely strong magnetic field is far from a vacuum! A magnetar's magnetic field has an energy density that corresponds to a mass density over 10,000 times that of lead. The magnetic field of a magnetar would be lethal even at a distance of 1,000 kilometers - and at a distance of halfway from Earth to the Moon, a magnetar could wipe information from the magnetic stripes of all credit cards on Earth.
https://arxiv.org/abs/2509.19446
https://www.nature.com/articles/s41586-026-10859-z?linkId=63101551
Replies (2)
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@johncarlosbaez@mathstodon.xyz
Those numbers are sort of underwhelming when considering the effect of a "starquake" on a magnetar's surface - a massive burst of power that could fry entire planets if they happened to be in the line of fire.
cf. "SGR 1806-20, located around 50,000 light-years away from Earth. The light and energy reached Earth in the infamous event in 2004. In merely one-tenth of a second, the magnetar hit Earth with 1040 watts of energy, which is more energy than the Sun produces in 150,000 YEARS."
Open ##4423254
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Indeed!
Experimental physicists on the Earth must be very envious 🙂.
By the way, the very example of polarized light reflected off water was used by Feynman to illustrate the difference between being able to recite a definition of a phenomenon and being able to recognize the phenomenon on seeing it.
He wrote about teaching in Brazil and encountering students who could do the former, but not the latter.
@johncarlosbaez@mathstodon.xyz
Open ##4423461