Scientists May Have Proved a Century-Old Theory That the Vacuum of Space Isn’t Really Empty

3 min readHere’s what you’ll learn when you read this story:Astrophysicists used telescopes to study a type of neutron star called a fast-pulsing magnetar.They found that the magnetar shows evidence of vacuum birefringence, in which different light polarizations travel at different speeds.This specific magnetar has high X-ray and radio wave output, making it ideally measurable.A large, global team of scientists recently collaborated to study an unusual neutron star—known as a magnetar—in search of a quantum phenomenon called vacuum birefringence. Incredibly, they were able to find strong evidence for this long-predicted quantum electrodynamic phenomenon because of the combination of powerful X-ray radiation and an extra strong magnetic field on the star’s surface. The work appears now in the peer reviewed journal Nature.Birefringence is a Latin-derived term for the refraction (or bending) of light into two discrete beams that are parallel, but differently polarized. Vacuum birefringence, therefore, is when an analogous effect happens in an environment like outer space, where there are no particles to blame for the refraction. Legendary physicist Werner Heisenberg predicted vacuum birefringence 90 years ago and the study’s authors describe it as “[...] intense magnetic fields—comparable to those of magnetars—substantially modify[ing] the refractive indices of photon polarization eigenmodes and render[ing] them disparate: different polarizations propagate at different speeds. This property is a direct consequence of the polarization of the quantum vacuum, and its constituent virtual electron–positron pairs, by the electromagnetic field.”In other words, the strong magnetism causes light of different polarizations to travel at different speeds after passing through the strong magnetic field. And in the vacuum, where no “permanent” particles exist, it is reactive pairs of virtual particles that produce the magnetic effects on rays of light. This far-out idea follows from quantum principles, but has been extraordinarily difficult to observe directly.Quantum field theory holds that the electron field fills all of space and is never entirely at rest. The vacuum, in this picture, is filled with electron–positron pairs that appear and vanish too quickly for anything to catch them. In 1936, Werner Heisenberg and Hans Euler were the first to describe these elusive-yet-everywhere virtual particles. In most conditions, these strange pairs are ghosts that leave no detectable traces in our physical universe. But once a magnetic field nears a critical strength of about 4.4 × 10¹³ gauss, virtual particles begin to leave a mark on passing light, slowing one polarization more than the other.Enter the magnetar. We know of just 31 magnetars within our observed universe—nearly one for every year since magnetars were first theorized in 1992. NASA explains the extraordinary power of magnetars using magnetic credit cards:“[T]he strength of our planet’s magnetic field has a value of about one Gauss, while a refrigerator magnet measures about 100 Gauss. Magnetars, on the other hand, have magnetic fields of about a million billion Gauss. If a magnetar was located a sixth of the way to the Moon (about 40,000 miles), it would wipe the data from all of the credit cards on Earth.” Conveniently, the magnetic field density of a magnetar is several times stronger than that threshold needed for virtual particles to affect the polarization of light. And thanks to one magnetar in particular, we now have evidence that empty space has actually has properties—something like an index of refraction.In their newly published research, the physicists studied the magnetar named 1E 1547.0−5408, which is in a “persistently bright X-ray state,” emits radio pulses, and has a near-ideal viewing angle for scientists to observe its emissions. The magnetar rotates, effectively flashing, every 2.09 seconds. Scientists used three instruments at once to measure as much as possible about the magnetar: a radio telescope, an X-ray polarization observatory, and a neutron spectroscopy telescope.Among many observations, they found that the degree of polarization from the magnetar was far higher than has been seen in similar types of stars, and it held steady across the star’s rotation. This suggests that something in its magnetosphere is shaping the light: “Within this context, magnetospheric [vacuum birefringence] provides a natural physical mechanism to account for the observed polarization behaviour.”This work is only possible now because of the separate instruments that can observe one star in multiple ways, allowing scientists to build a fuller picture and do complex math to line up all the variables. Even so, this is just one step, the scientists concluded, “motivating further observational and theoretical studies concentrating on this domain.”Hopefully, our 31 known magnetars are ready for their closeups.Caroline Delbert is a writer, avid reader, and contributing editor at Pop Mech. She's also an enthusiast of just about everything. Her favorite topics include nuclear energy, cosmology, math of everyday things, and the philosophy of it all.