Scientists have found out why outer space cannot be considered empty
An international team of astronomers has obtained the most convincing evidence to date of vacuum double refraction, a quantum effect in which a strong magnetic field changes the properties of seemingly empty space and affects the propagation of light. Traces of the phenomenon were found when observing the magnetar 1E 1547.0−5408. This is stated in a study published in the journal Nature.
Why is the empty space not completely empty?
In everyday understanding, a vacuum is an area in which there is no substance. However, in quantum field theory, even a space with the lowest possible energy cannot be considered an absolute void. It is permeated by quantum fields, and their state can change under the influence of external conditions.
For a popular explanation of this process, physicists often use the idea of virtual particles that appear for an extremely short time and then disappear. They cannot be registered as ordinary stable particles, but the effects associated with quantum fields can influence the behavior of light and matter.
In 1936, physicists Werner Heisenberg and Hans Heinrich Euler predicted that a particularly strong magnetic field should change the optical properties of the vacuum. In such conditions, space begins to affect the components of a light wave with different polarizations in different ways. This phenomenon is called vacuum double refraction.
It is not yet possible to obtain the necessary field in the Earth laboratory. According to Marcus Lower, a researcher at Swinburne University of Technology, it should be more than 100 million times stronger than magnetic fields ever created by man.
"Fortunately, nature has provided us with magnetars, which are ideal space laboratories for searching for this effect," the scientist noted.
Space Laboratory for Quantum Physics
Magnetars are a rare type of neutron stars, the superdense remnants of massive stars formed after a supernova explosion. With a mass comparable to or greater than the mass of the Sun, the diameter of a neutron star can be only a few tens of kilometers.
The main feature of magnetars is considered to be exceptionally powerful magnetic fields. In the studied object 1E 1547.0−5408, they are more than a trillion times stronger than the Earth's magnetic field. It is precisely such conditions that make it possible to verify the predictions of quantum electrodynamics, which cannot be reproduced in the laboratory.
Magnetar 1E 1547.0−5408 also proved to be a particularly convenient object for observations. It makes a complete revolution in about two seconds and steadily emits both in the radio range and in the X-ray range. This allowed scientists to simultaneously monitor the polarization of different types of radiation and reconstruct the geometry of its magnetic field.
From March to April 2025, the NASA Imaging X-ray Polarimetry Explorer (IXPE) X-ray observatory observed the magnetar for more than 140 hours. The research was supported by the NICER telescope installed on the International Space Station and the Australian Murriyang radio telescope at the Parks Observatory. According to NASA, these were the first coordinated measurements of the radio and X-ray polarization of a magnetar.
Radio observations have shown that the axis of rotation of 1E 1547.0−5408 almost coincides with its magnetic axis. In addition, the object is visible from the Earth almost from the magnetic pole. This arrangement gave the researchers the opportunity to check whether the direction of polarization of the X-ray radiation remains associated with the magnetic field of the star during its rotation.
The polarization indicated the effect of vacuum
Polarization characterizes the direction of oscillation of an electromagnetic wave. If the directions of oscillation of the photons are randomly distributed, the degree of polarization of the radiation remains low. A high value means that a significant portion of the photons are aligned.
The resulting figure turned out to be almost three times higher than that of similar sources. Existing models of neutron star surface radiation, without taking into account changes in vacuum properties, could not explain this result. With the observed geometry of the magnetic field, the degree of polarization at some moments, on the contrary, should have approached zero.
The scientists also found that the directions of polarization of X-ray and radio emission changed consistently and remained associated with the large-scale magnetic field of the magnetar. According to Lower, within the framework of the quantum electrodynamic explanation, a strong field orders the state of the quantum vacuum along its direction.
As a result, the space around the magnetar acts as a kind of polarizing filter. As light waves with different orientations pass through this area, they experience different effects. This enhances the overall polarization of the radiation even before it leaves the vicinity of the star.
Computer modeling has shown that it is possible to simultaneously reproduce the observed X-ray polarization and observe the limitations obtained from the radio signal precisely when vacuum double refraction is included in the calculations.
"Our model shows that in order to reproduce the observed signs of X—ray polarization while taking into account the limitations of radio observations, the presence of vacuum double refraction in the vicinity of a neutron star is necessary," said Hoa Dinh Thi, one of the lead authors of the work, a researcher at Rice University.
Can the effect be considered definitively proven
Despite the loud statements that the space turned out to be "not empty," the authors of the study have not yet called the result definitive direct evidence. The article says that the propagation of radiation under the influence of vacuum birefringence naturally explains the detected polarization, whereas standard models face serious difficulties.
In other words, scientists have obtained the strongest candidate for observing the effect, but they have yet to rule out alternative processes that can affect X-ray radiation. The final signal may be affected by the structure of the magnetar's surface, the composition of its atmosphere, the location of hot areas, and processes in the magnetosphere.
The new data set will allow us to divide X-ray radiation into narrower energy ranges, more accurately track the change in polarization during the rotation of the star and compare the results with improved models. The researchers hope to test not only double refraction, but also the associated effect of vacuum resonance.
"With future data and updated models, we may finally be able to complete the search that Heisenberg began almost 90 years ago," Lower said.
Werner Heisenberg is a German theoretical physicist, one of the founders of quantum mechanics and a Nobel Prize winner in physics. In 1936, he and physicist Hans Heinrich Euler described the unusual behavior of a vacuum in an extremely strong electromagnetic field.
According to their calculations, a vacuum cannot be considered an absolute void: the quantum fields associated with it can affect the transmitted light. In particular, a strong magnetic field should have different effects on light waves depending on their polarization.
What the discovery can give to physics
The confirmation of vacuum birefringence will be a direct test of quantum electrodynamics in magnetic fields inaccessible to terrestrial experiments. This theory describes the interaction of light and charged particles and is considered one of the most precisely verified physical models. However, its behavior in extreme conditions is still insufficiently studied.
The results will also help to study magnetars more precisely. The polarization of radiation carries information about the geometry of their magnetic fields, the composition of the surface and the distribution of hot areas. Thanks to this, scientists will be able to better understand how neutron stars produce powerful bursts of X-ray and gamma radiation.
In addition, the work shows that distant space objects can be used to test the fundamental properties of space. In this case, the light from the superdense remnant of the star allowed us to get closer to the answer to the question of how vacuum works at the quantum level.
"The information obtained from studying the distant stellar core simultaneously gives us clues about the nature of the fabric of reality itself," concluded Rachel Stewart, the study's lead author and an associate at George Washington University.
Scientists have not yet proved that the "empty" space is filled with ordinary matter. Observations point to a more complex conclusion: even a vacuum has quantum properties and, in a sufficiently strong magnetic field, is able to directly change the behavior of light passing through it.
Переведено сервисом «Яндекс Переводчик»
