Empty space sounds like the last place where light should encounter resistance. Yet astronomers have now found evidence that, under some of the most extreme conditions in the universe, even a vacuum may influence the way light travels. By studying a rare, intensely magnetic neutron star, they have identified what could be the clearest sign yet of a quantum effect that has remained beyond direct reach for nearly nine decades.
Nearly 90 years ago, quantum mechanics pioneer Werner Heisenberg proposed that a perfect vacuum is not truly empty. Instead, it should be filled with fleeting “virtual particles” that rapidly appear and disappear.
According to that prediction, an extraordinarily powerful magnetic field should cause those virtual particles to alter the way light moves through otherwise empty space. The effect, known as vacuum birefringence, has remained one of quantum physics’ longstanding unsolved experimental challenges.
Now, observations published on August 5 in the journal Nature suggest astronomers may have detected this phenomenon around a magnetar, an exceptionally magnetic type of neutron star.
Why magnetars make the perfect natural laboratory
Detecting vacuum birefringence requires magnetic fields more than 100 million times stronger than any created on Earth. That makes laboratory experiments extraordinarily difficult.
Fortunately for astronomers, nature provides another option.
Magnetars possess the strongest magnetic fields known in the universe, making them the only known environments where this quantum effect is expected to become visible.
An international research team, including Dr. Marcus Lower of Swinburne University of Technology, focused on the magnetar 1E 1547.0–5408, also known as 1E1547.
The researchers combined observations from NASA’s Imaging X-ray Polarimetry Explorer (IXPE) with data from the NICER X-ray telescope aboard the International Space Station and Murriyang, CSIRO’s Parkes radio telescope in Australia. Dr. Lower also analyzed the observations using Swinburne’s Ngarrgu Tindebeek supercomputer.
According to the team, the observations could represent the first direct detection of this once purely theoretical quantum phenomenon.
“Detecting vacuum birefringence requires a magnetic field that is over 100 million times stronger than any we’ve ever made on Earth,” Dr. Lower said. “Thankfully, nature has provided us with magnetars, which are the perfect cosmic laboratories to go looking for this effect.”
A rare viewing angle made the difference
The researchers first examined how the polarization of the magnetar’s radio waves changed as the star rotated.
From those measurements, they determined that the magnetic axis and the rotational axis of 1E1547 are nearly aligned and that Earth views the star almost directly along one of its poles.
That unusual combination of geometry made the magnetar particularly well suited for searching for vacuum birefringence.
The team then identified two signatures they say point toward the effect.
The first was that X-rays detected by IXPE displayed extremely high levels of polarization. The second was that the direction of that polarization remained locked to the magnetar’s magnetic field in the same way as the observed radio waves.
Dr. Lower explained that the magnetar’s immense magnetic field is expected to align Heisenberg’s virtual particles.
“Because of the magnetic field’s strength, Heisenberg’s virtual particles become aligned with the direction the field is pointing,” he said. “By carefully tracking the direction the radio waves and X-rays oscillate as the magnetar rotates, the team found that the alignment of 1E1547’s magnetic and rotational poles was ideal for detecting vacuum birefringence.”
The case is compelling, but not yet closed
Although the observations provide what the researchers describe as a possible first detection of vacuum birefringence, they emphasize that confirmation is still needed.
The team says additional observations and improved computer simulations will help separate the expected quantum signal from other physical processes occurring around magnetars.
If future evidence supports the current interpretation, the result would provide a new way to investigate how quantum physics operates under some of the universe’s most extreme conditions.
Dr. Lower believes that next step may finally settle a question that has remained open since Heisenberg first proposed it nearly 90 years ago.
“With these future data on hand and our updated simulations, we may finally be able to complete the quest started by Heisenberg nearly 90 years ago.”
