Cosmic Crystal: How a Magnetar Bends the Void ✛
In 1936, two physicists, Werner Heisenberg and Hans Euler, poring over the equations of quantum electrodynamics, found something strange. In a sufficiently strong magnetic field, vacuum should cease to be empty. It should become like a crystal: light of one polarization would travel slightly faster, light of the other slightly slower. This phenomenon was named vacuum birefringence. For nearly ninety years, it remained a beautiful theory. There was no place in the Universe where the field was strong enough for the effect to become noticeable.
Such a place turned out to be the magnetar 1E 1547.0-5408. It is a neutron star with a magnetic field of about 2.2×$10^{14}$ gauss, quadrillions of times stronger than Earth's. In such a field, vacuum should indeed behave as an anisotropic medium, refracting X-rays depending on their polarization. In 2023, the space polarimeter IXPE was pointed at the star. The result exceeded expectations: in the 2–3 keV range, the degree of polarization reached 59%, and at certain rotation phases, 82%. This is a record among all six magnetars ever observed by IXPE.
To understand what the telescope actually saw, physicists compared the data with a computer model of radiation transfer in a magnetized atmosphere. The model was run twice: once with vacuum birefringence included, and once without. Without the effect, the theoretical polarization curves oscillated so wildly that they diverged from observations by tens of times. With the effect included, agreement became almost perfect. This is not just a pretty analogy: the mathematics showed that vacuum indeed refracts light like a gemstone.
The discovery changes our view of emptiness. Vacuum no longer looks like nothing: in extreme fields, it acquires the properties of a material medium. For strong-field physics, this is a long-awaited confirmation of a fundamental prediction. For astrophysics, it is a new tool: the polarization of X-ray emission can reveal the structure of neutron star magnetic fields. And for future missions, such as GoSOX, it is a guide: observations in even softer X-rays, where polarization may exceed 80%, will provide a direct test of quantum electrodynamics. The cosmic crystal has been found. Now it needs to be cut.
The magnetic field of magnetar 1E 1547.0-5408 is so strong that at a distance of 1000 km from the star, it still exceeds the field of the most powerful laboratory magnets by millions of times.
In Stanislaw Lem's novel "Fiasco", stars with extreme magnetic fields that affect the properties of vacuum are mentioned. The idea that emptiness can behave like a crystal echoes the concept of vacuum birefringence.