Astronomers may have just confirmed one of the strangest aspects of quantum mechanics: that apparently empty space can alter the behavior of light. This phenomenon, known as vacuum birefringence, was first predicted nearly 90 years ago by Werner Heisenberg, one of the founders of quantum mechanics. He proposed that even a perfect vacuum should be filled with “virtual particles” that constantly appear and disappear almost instantaneously.
Discovery of “Vacuum Birefringence”
Astronomers, including Dr. Marcus Lower of Swinburne University of Technology, used the properties of a magnetar—a rare type of neutron star with some of the strongest magnetic fields in the Universe—to investigate this long-standing quantum mystery.
Their observations revealed what may be the first detection of vacuum birefringence occurring in the extremely powerful magnetic field of a magnetar and could open new avenues for studying the quantum Universe. The results were published on August 5 in the journal Nature.
Magnetars as Quantum Laboratories in Space
Under the influence of an extraordinarily strong magnetic field, Heisenberg’s sea of virtual particles is expected to refract light in a particular way, producing vacuum birefringence. Only magnetars have magnetic fields strong enough to make this quantum effect observable.
Dr. Lower is part of an international team that observed the magnetar known as 1E 1547.0–5408, or simply 1E1547, using NASA’s Imaging X-ray Polarimetry Explorer (IXPE), with support from the NICER X-ray telescope aboard the International Space Station and the Murriyang radio telescope, also known as the Parkes radio telescope, operated by CSIRO, Australia’s national science agency.
Dr. Lower’s observations with Murriyang, followed by analysis performed using Swinburne University’s Ngarrgu Tindebeek supercomputer, may represent the first direct detection of this quantum effect, which had previously remained purely theoretical.
Dr. Lower noted that although the phenomenon was predicted as far back as the 1930s, a definitive detection of vacuum birefringence had remained elusive.
“To detect vacuum birefringence, you need a magnetic field more than 100 million times stronger than anything we have ever created on Earth. Fortunately, nature has given us magnetars, which are perfect cosmic laboratories for searching for this effect.”
Nearly Perfect Magnetic-Field Geometry
By carefully tracking how the direction of the oscillations of the radio waves emitted by the magnetar—their “polarization state”—changed as it rotated, the team found that the magnetic and rotational axes of 1E 1547 are almost perfectly aligned, while the object is viewed at nearly a right angle to its pole. This combination of magnetic geometry and viewing geometry makes 1E 1547 an ideal object for searching for vacuum birefringence.
The team then identified two characteristic signs that vacuum birefringence may be occurring around the magnetar. They found that the X-ray emission generated by the magnetar and detected by IXPE has an extremely high degree of polarization, and that the direction of this polarization aligns with the magnetic field of 1E 1547 in the same way as the radio waves do.
“Because of the strength of the magnetic field, Heisenberg’s virtual particles become aligned with the direction of the field,” Dr. Lower said. “By carefully tracking the oscillation direction of the radio waves and X-rays as the magnetar rotated, the team found that the alignment of 1E1547’s magnetic and rotational poles was ideal for detecting vacuum birefringence.”
The Data Still Require Confirmation
If confirmed, the discovery will open a path toward understanding how our theories of quantum physics operate in one of the most extreme environments in the Universe.
Dr. Lower noted that this important finding could soon be confirmed with additional data and improved computer simulations, which will make it easier to distinguish the vacuum-birefringence signal from other processes occurring around magnetars.
“With these future data and our improved simulations, we may finally be able to complete the search that Heisenberg began nearly 90 years ago.”
