Neutron Stars and Quantum Physics...

An electromagnetic wave traveling from left to right (positive x direction).
Image Credit: Supermanu (CC BY-SA 3.0)

Topics: Astrophysics, Electromagnetic Wave, Neutron Stars, Quantum Electrodynamics

Recently, scientists made some impressive measurements of light emitted by an isolated neutron star. The results support an 80-year-old prediction, made during the early days of quantum electrodynamics (QED), of a phenomenon known as vacuum birefringence.

Radio signals, microwaves, visible light, ultraviolet, X-rays, and gamma rays are all types of electromagnetic waves. All electromagnetic waves travel through empty space at the same speed, the speed of light (~300,000,000 m/s). More energetic electromagnetic waves have higher frequencies and shorter wavelengths.

In the diagram below above, the electric field is shown in blue. It points along the z axis, moving back and forth in the z direction as the wave travels to the right. Similarly, the magnetic field oscillates in the y direction. The changing electric field gives rise to the magnetic field, and the changing magnetic field gives rise to the electric field, so the two travel together.

When scientists say that light is polarized, they are referring to the direction of the electric field, depicted in the above diagram by the blue arrows. In the diagram, the electromagnetic wave is polarized in the z direction. That is to say: all of the electric field vectors are aligned (whether up or down) with the z axis.

When scientists make measurements on electromagnetic waves, they measure many waves. Most light is randomly polarized, so if you try to collect some light headed in the x direction, you’ll find just as many electromagnetic waves on the z axis as on the y axis, and at all angles in between. This type of light would be called unpolarized.

Most typical low- and medium-mass stars (anywhere from 0.1 to 3 times the mass of our Sun) use up their fuel in nuclear fusion then quietly cool off, usually forming a white dwarf. More massive stars have a lot more gravitational pull, so they burn up their fuel faster, resulting in a shorter life span and an explosive finale called a supernova. A supernova spews much of the material of the star outward, but what is left (which again depends on the initial mass of the star) becomes either a neutron star (if the initial mass was between 8 and 24 times the mass of our Sun) or a black hole (initial mass 25 or more times the mass of our Sun).

Although neutrons are neutrally charged, they are composed of charged particles that cause the neutron to have a magnetic dipole—that is, neutrons act like little magnets. Collectively, the number of neutrons that make up a 12- to 20-mile-diameter ball put out an incredible magnetic field. As a neutron star rotates, its rotating magnetic field creates radio waves that are emitted like beacons from the magnetic poles of the star. To our observatories, these signals appear to pulsate. As a result, neutron stars are sometimes called pulsars. Neutron stars don’t emit very much visible light, but they emit some.

Physics Central: Neutron Stars: Cosmic Laboratories for Quantum Physics, H.M. Doss