Vacuum Birefringence Possibly Detected

A little background...

Birefringence is when the velocity of light (refractive index, n) varies between two perpendicular polarizations (fast and slow axis). A good example is calcite crystals.

https://en.wikipedia.org/wiki/Birefringence

When light is linearly polarized at 45 degrees to these axes, the result is that it becomes elliptically polarized due to the difference in speed of propagation.

Theoretically there should be birefringence in a vacuum due to the existence of virtual electron-positron pairs, but the effect is extremely small. The refractive index for light polarized parallel to the magnetic field is different from the light polarized perpendicular to the field. The difference in refractive index Δn is:

https://indico.in2p3.fr/event/7399/session/6/contribution/179/material/slides/0.pdf

where B is in Teslas. With the strength of magnetic fields available in laboratories (max 100T), the effect is well below the noise level of sensors and the effect has not been measured, AFAIK.

However, neutron stars have magnetic fields that may be as high as a million to 100 million Teslas, far higher than anything that can be reproduced on earth.

https://en.wikipedia.org/wiki/Orders_of_magnitude_(magnetic_field)

No wonder I can't get rid of being cross-eyed....

RX J1856.5-3754 is a magnetar, a rare type of neutron star with an extremely powerful magnetic field. The fields around these stars utterly defy imagination. A typical magnetar's field strength is about 10¹⁰ Tesla (quadrillions of times greater than Earth's). The energy density in a field of this magnitude exceeds 10⁴ times the E/c² energy density of lead. In other words, a cubic meter of such a magnetic field has an equivalent mass of 110-120 million kilograms, or about 125,000 short tons. That's a lot of energy.

Putting it another way, were the energy in that cubic meter suddenly released on Earth, it would be enough to remove a substantial portion of Earth's crust. In terms of nuclear weapons, about 2.5 billion megatons' worth. And that is just one cubic meter. The energy stored in the entire field? Unimaginable. While it is convenient for a magnetar to be this close for study, a big starquake and we just might be up the creek without a paddle. It depends on whether the crack is facing in our direction at the time.

The surface of a magnetar is under tremendous stress not only from the star's enormous gravity but also from its magnetic field. As the field diminishes the stresses in the crust reconfigure and can rupture the surface resulting in a 'starquake.' While these cracks may be only a few kilometers long (the entire star is less than 20 km in diameter), they can release enough energy to outshine the entire galaxy for a few milliseconds at gamma wavelengths and, voila! A gamma-ray burst, or GRB. One such GRB ripped through our Solar System a few decades ago, nearly fried and blinded a few our space probes, and measurably heated Earth's upper atmosphere on the facing side. That star was 50,000 light years away. This one is 400.