Not empty space ESO/Digitized Sky Survey 2

The vacuum of space isn’t as empty as it seems. Astronomers using the Very Large Telescope in Chile have just watched virtual particles in space acting like prisms, aligning the light from a neutron star.

The observation confirms an 80-year-old prediction from the theory of quantum electrodynamics, a framework describing how light and matter interact at subatomic scales.

In the 1930s, German physicists Werner Heisenberg and Hans Heinrich Euler suggested that a strong magnetic field can give rise to a phenomenon called polarisation. The upshot is that like glare bouncing off a window, some of the light passing through a powerful magnetic field takes on a particular alignment.


In an apparent vacuum, this alignment is driven by virtual particles that share many properties with their “real” counterparts but are constantly popping in and out of existence, thanks to quantum uncertainty.

“If there is a magnetic field which is very strong, you don’t need a piece of glass or a prism to refract the light,” says team member Silvia Zane at University College London. “But you need a super-strong magnetic field, not just a simple magnet like one that you might find in regular life.”

So Zane and her colleagues looked to the stars. They used a series of filters, like polarised, glare-blocking sunglasses but bigger and more precise, to observe the light from a nearby, relatively dim neutron star – a dense stellar corpse with a colossal magnetic field – and compared it with light from ordinary nearby stars.

A faint signal

They discovered that the light from the neutron star had been polarised to about 16 per cent. It is the first demonstration of this phenomenon, called vacuum birefringence.

With their data, the team was even able to start inferring the star’s axis of rotation.

“This is a very important result, and it was very challenging to get it,” says Roberto Mignani at the National Institute for Astrophysics in Milan, Italy, who led the research. “We had to use the best telescope in the world under the best atmospheric conditions, and we had to set up the proper team – but most of all this measurement was really challenging because of the faintness of the source.”

This discovery may be the first crack in a window into the details of neutron stars, says Herman Marshall at the Massachusetts Institute of Technology. He’d like to see additional observations in the X-ray range, as neutron stars generally shine more much powerfully at these wavelengths.

“We could determine more about how these neutron stars are formed, how they change in time, their magnetic fields – regions of physics that are very poorly understood right now,” he says.

Although the result shows virtual particles and their quantum properties at work, whether it will have further implications for quantum mechanics or point to new physics is less clear.

“From the particle point of view, we don’t really know what’s next,” Mignani says. “When Einstein came up with the theory of general relativity 100 years ago, he had no idea that it would be used for navigational systems. The consequences of this discovery probably will also have to be realised on a longer timescale.”

Journal reference: Monthly Notices of the Royal Astronomical Society, DOI: 10.1093/mnras/stw2798