Empty space is not as empty as it looks.
That is the takeaway of a new study examining signals from a dead star, offering what researchers say is the strongest evidence yet that extreme magnetic fields can alter the properties of a vacuum, causing seemingly empty space to act like a prism and changing how light travels through it
The findings, led by Rachael Stewart, a graduate student in physics at the George Washington University, confirm a prediction of a strange effect first proposed 90 years ago.
"The information we obtained from looking at this distant star core also gives us clues about the nature of the fabric of reality as we know it, and I find that to be incredible," Stewart said in a statement.
The idea dates to 1936, when the German physicist Werner Heisenberg and his student Hans Euler proposed that space is never truly void. Instead, they argued, it is a simmering sea of "virtual particles" — electrons and their antimatter counterparts, positrons — that flicker in and out of existence, briefly interacting with their surroundings before vanishing.
This subatomic froth is a consequence of quantum mechanics, and it remains invisible under ordinary conditions. But the theory predicts that an extremely strong magnetic field, such as one around a magnetar, can change how light moves, causing the light's waves to become more strongly aligned in a particular direction in an effect known as "vacuum birefringence."
"Detecting vacuum birefringence requires a magnetic field that is over 100 million times stronger than any we've ever made on Earth," study co-author Marcus Lower, an astrophysicist at the Swinburne University in Australia, said in another statement. "Thankfully, nature has provided us with magnetars, which are the perfect cosmic laboratories to go looking for this effect."
Magnetars are dense, city-size remnants left behind after massive stars explode, hosting the most powerful magnetic fields known in the universe. They are among the rare celestial objects capable of generating fields strong enough to reveal vacuum birefringence, offering scientists an extreme environment to test physics under conditions impossible to replicate on Earth.
"We're not just studying astronomical objects anymore; we're using them to test the laws of nature," study co-author Michela Negro, an astrophysicist at the Louisiana State University, said in a statement.
Astronomers have caught glimpses of this elusive phenomenon before, but not conclusively. In 2017, researchers using the Very Large Telescope in Chile observed polarization hints around a faint neutron star called RX J1856.5-3754, located about 400 light-years from Earth. However, those optical measurements remained open to interpretation, partly due to the challenges of isolating the optical signal.
At the time, scientists noted that definitive proof would require space-based X-ray observatories, specifically NASA's then-forthcoming Imaging X-ray Polarimetry Explorer (IXPE). Launched in 2021, IXPE carries three identical telescopes designed to measure the polarization of high-energy X-rays.
"It's only in the last six or so years that we've actually had a telescope capable of detecting this effect around magnetars," Lower told Michael West Media, an independent news website in Australia.
In March and April 2025, the researchers pointed IXPE at 1E 1547-5408, a magnetar that spins once every two seconds and is unusual among its kind for steadily emitting radio waves. The team supplemented that data with observations from an X-ray telescope aboard the International Space Station, as well as Australia's Murriyang radio telescope and the South African Radio Astronomy Observatory.
According to the study, two findings pointed to vacuum birefringence at work.
First, the X-rays picked up by IXPE were nearly three times more polarized than in similar sources, far higher than standard models of a neutron star's surface emission could explain on their own. Second, the polarization pointed the same way as the star's magnetic field, matching the pattern already observed in its radio waves. The researchers concluded that this combination leaves vacuum birefringence as the only explanation that fits the data.
"It's a bit of a relief because it means that our theories still work and there's nothing broken with physics," Lower told the Michael West Media news site.
For Fernando Camilo, chief scientist at the South African Radio Astronomy Observatory and a co-author of the new paper, the discovery brings a long journey full circle. Camilo has been studying 1E 1547-5408 since 2007, when he first detected its radio waves using the Murriyang dish, revealing its two-second rotation rate.
"At the time 1E 1547 was only the second magnetar in the Milky Way known to emit radio waves, and we were confident that regular monitoring would unveil interesting behaviour," Camilo said in the statement. "However we could never have imagined that 20 years later it would contribute to investigating a fundamental, and particularly quirky, prediction of quantum mechanics."
The team hopes to confirm the finding with data from future missions, including a proposed orbital mission called GoSOX (short for Globe Orbiting Soft X-ray Polarimeter), alongside improved computer simulations to distinguish the vacuum birefringence signal from other processes around magnetars.
"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," Lower said in the Swinburne University statement.
This research is described in a paper published Aug. 5 in the journal Nature.
