ESO, DIGITIZED SKY SURVEY 2
A nearby star is helping scientists study a bizarre effect of physics that occurs on the smallest of scales. The effect is called “vacuum birefringence.” It was predicted 80 years ago. But until now, scientists had never seen signs of it.
A magnetic field is what allows magnets to attract or repel one another. Vacuum birefringence describes how light interacts with the emptiness of space — when that space is under the influence of a strong magnetic field.
Researchers think they observed this long-anticipated effect in the light of a neutron star. That’s a star that exploded in a fiery death, leaving behind only a dense core. That stellar remnant sits about 400 light-years from Earth. Light from this star has been warped in a way that scientists now think points to vacuum birefringence. They described their finding in a paper to be published in the February 11 issue of the Monthly Notices of the Royal Astronomical Society.
“It’s the most natural explanation,” says Jeremy Heyl. He is an astrophysicist at the University of British Columbia in Vancouver, Canada. Heyl was not involved with the new study. It showed that the light of the neutron star is slightly polarized. That means all of the light waves wiggle on the same plane. This is the first time scientists have seen polarized light from a star that could be a sign of vacuum birefringence. The detection is helping scientists to study quantum physics.
What excites scientists
Quantum physics describes how matter and energy behave at the level of atoms or even smaller particles. With the light of the neutron star, scientists can test their ideas about a theory in quantum physics known as quantum electrodynamics. This theory describes how light interacts with charged particles such as electrons. The theory states that empty space isn’t really empty. It is filled with a roiling soup of particles, which constantly blip into and out of existence. As light passes through space, its wiggling electromagnetic waves may interact with those particles.
Strong magnetic fields influence how the waves move. Light waves that wiggle along the direction of the magnetic field will travel slightly more slowly than light that wiggles perpendicular to the direction of the magnetic field. This difference in speed polarizes the light coming from the star.
A similar effect can happen in a more familiar situation. It occurs in what are known as birefringent materials. The liquid crystals used for displaying images on some computer monitors and TVs offer one example. They can rotate the polarization of light. Horizontally polarized light, for example, is sent to each pixel on the display screen. But a filter lets only vertically polarized light escape. To switch on a pixel, the liquid crystals twist the light waves 90 degrees. That allows the waves to pass through.
That’s a classical version of the birefringent effect.
Evidence for the quantum version has not been so easy to come by. Observing it requires a magnetic field stronger than those that can be produced in the laboratory, says Roberto Mignani. He is an astrophysicist at the National Institute for Astrophysics in Milan, Italy. The magnetic field around the neutron star that he and his colleagues studied is about 10 trillion times the strength of Earth’s. But the star is incredibly faint. “
A neutron star of this kind is about as bright as a candle halfway between the Earth and the moon,” Mignani explains. That makes detecting its light difficult. And it makes measuring the polarization of its light even more of a challenge.
To detect the star’s light, Mignani and his team used the Very Large Telescope in Chile. The scientists found that visible light from the neutron star was about 16 percent polarized. This result is consistent with scientists’ theories of vacuum birefringence. But other things could twist the light, too, Heyl notes. Plasma, for example.
Plasma is a gaseous form of matter. There are lots of positively and negatively charged particles floating about in that form of gas. An unexpectedly large amount of plasma surrounding a star could polarize its light. And, the plasma could do it in a way that looks like vacuum birefringence, Heyl notes.
As a result, scientists need more airtight evidence to prove the quantum effect. They could study X-rays from neutron stars. In those wavelengths of light, the polarization effect should be even stronger. No telescope currently exists to make such measurements. But there are several proposed X-ray satellites that may soon be able to clinch the case for vacuum birefringence.
Scientists might want to keep their fingers crossed. Future measurements could overturn the evidence for vacuum birefringence. That would cause problems for the theory of quantum electrodynamics. Vacuum birefringence is one basic prediction of the theory, Heyl says. “So to fix it, you'd really have to rip the theory all the way back down to the foundations and rebuild it.”
astrophysics An area of astronomy that deals with understanding the physical nature of stars and other objects in space. People who work in this field are known as astrophysicists.
atom The basic unit of a chemical element. Atoms are made up of a dense nucleus that contains positively charged protons and neutrally charged neutrons. The nucleus is orbited by a cloud of negatively charged electrons.
coauthor One of a group (two or more people) who together had prepared a written work, such as a book, report or research paper. Not all coauthors may have contributed equally.
colleague Someone who works with another; a co-worker or team member.
crystal (adj. crystalline ) A solid consisting of a symmetrical, ordered, three-dimensional arrangement of atoms or molecules. It’s the organized structure taken by most minerals. Apatite, for example, forms six-sided crystals. The mineral crystals that make up rock are usually too small to be seen with the unaided eye.
degree (in geometry) A unit of measurement for angles. Each degree equals one three-hundred-and-sixtieth of the circumference of a circle.
electromagnetic The science of sounds and hearing.
electron A negatively charged particle, usually found orbiting the outer regions of an atom; also, the carrier of electricity within solids.
field (in physics) A region in space where certain physical effects operate, such as magnetism (created by a magnetic field), gravity (by a gravitational field) or mass (by a Higgs field).
filter (in physics) A screen, plate or layer of a substance that absorbs light or other radiation or selectively prevents the transmission of some of its components.
light-year The distance light travels in one year, about 9.48 trillion kilometers (almost 6 trillion miles). To get some idea of this length, imagine a rope long enough to wrap around the Earth. It would be a little over 40,000 kilometers (24,900 miles) long. Lay it out straight. Now lay another 236 million more that are the same length, end-to-end, right after the first. The total distance they now span would equal one light-year.
magnetic field An area of influence created by certain materials, called magnets, or by the movement of electric charges.
neutron star The very dense corpse of what had once been a star with a mass four to eight times that of our sun. As the star died in a supernova explosion, its outer layers shot out into space. Its core then collapsed under its intense gravity, causing protons and electrons in its atoms to fuse into neutrons (hence the star’s name). Astronomers believe neutron stars form when large stars undergo a supernova but aren’t massive enough to form a black hole. A single teaspoonful of a neutron star, on Earth, would weigh a billion tons.
perpendicular An adjective that describes two things that are situated approximately 90 degrees to each other. In the letter “T,” the top line of the letter is perpendicular to the bottom line.
physics The scientific study of the nature and properties of matter and energy. Classical physics is an explanation of the nature and properties of matter and energy that relies on descriptions such as Newton’s laws of motion. Quantum physics, a field of study which emerged later, is a more accurate way of explaining the motions and behavior of matter. A scientist who works in that field is known as a physicist.
pixel Short for picture element . A tiny area of illumination on a computer screen, or a dot on a printed page, usually placed in an array to form a digital image. Photographs are made of thousands of pixels, each of different brightness and color, and each too small to be seen unless the image is magnified.
plasma (in chemistry and physics) A gaseous state of matter in which electrons separate from the atom. A plasma includes both positively and negatively charged particles.
polarization The scattering of light in a specific pattern, with all light waves traveling on the same plane.
quantum (pl. quanta) A term that refers to the smallest amount of anything, especially of energy or subatomic mass.
quantum theory A way to describe the operation of matter and energy at the level of atoms. It is based on an interpretation that at this scale, energy and matter can be thought to behave as both particles and waves. The idea is that on this very tiny scale, matter and energy are made up of what scientists refer to as quanta — miniscule amounts of electromagnetic energy.
quantum physics A branch of physics that uses quantum theory to explain or predict how a physical system will operate on the scale of atoms or sub-atomic particles.
remnant Something that is leftover — from another piece of something, from another time or even some features from an earlier species.
satellite A moon orbiting a planet or a vehicle or other manufactured object that orbits some celestial body in space.
star The basic building block from which galaxies are made. Stars develop when gravity compacts clouds of gas. When they become dense enough to sustain nuclear-fusion reactions, stars will emit light and sometimes other forms of electromagnetic radiation. The sun is our closest star.
stellar An adjective that means of or relating to stars.
telescope Usually a light-collecting instrument that makes distant objects appear nearer through the use of lenses or a combination of curved mirrors and lenses. Some, however, collect radio emissions (energy from a different portion of the electromagnetic spectrum) through a network of antennas.
theory (in science) A description of some aspect of the natural world based on extensive observations, tests and reason. A theory can also be a way of organizing a broad body of knowledge that applies in a broad range of circumstances to explain what will happen. Unlike the common definition of theory, a theory in science is not just a hunch. Ideas or conclusions that are based on a theory — and not yet on firm data or observations — are referred to as theoretical. Scientists who use mathematics and/or existing data to project what might happen in new situations are known as theorists.
trillion A number representing a million million — or 1,000,000,000,000 — of something.
X-ray A type of radiation analogous to gamma rays, but of somewhat lower energy.
vacuum Space with little or no matter in it. Laboratories or manufacturing plants may use vacuum equipment to pump out air, creating an area known as a vacuum chamber.
vacuum birefringence An effect of quantum physics that describes how light interacts with the emptiness of space in a strong magnetic field.
wave A disturbance or variation that travels through space and matter in a regular, oscillating fashion.
wavelength The distance between one peak and the next in a series of waves, or the distance between one trough and the next. Visible light — which, like all electromagnetic radiation, travels in waves — includes wavelengths between about 380 nanometers (violet) and about 740 nanometers (red). Radiation with wavelengths shorter than visible light includes gamma rays, X-rays and ultraviolet light. Longer-wavelength radiation includes infrared light, microwaves and radio waves.
Journal: R. P. Mignani et al. Evidence for vacuum birefringence from the first optical-polarimetry measurement of the isolated neutron star RX J1856.5−3754. Monthly Notices of the Royal Astronomical Society. Vol. 465, February 11, 2017, p. 492. doi: 10.1093/mnras/stw2798.