The Ghost Particle - Explained!

The Ghost Particle: What Does it Mean?

The discovery of the source of a "ghost particle" from beyond our solar system created a huge stir in the scientific community when it was announced in mid-2018. But what exactly is a ghost particle, and what does this discovery mean?

Otherwise known as neutrinos, ghost particles are elementary particles with no electric charge and almost no mass. This “neutral” electric charge is where it gets the name “neutrino” from. Due to their lack of charge and mass, they don’t interact much with their surroundings, making them extremely difficult to study - like trying to find a black object in a dark room . . . with your eyes closed while wearing a blindfold. Even after the existence of ghost particles was first postulated, back in the 1930s, it took two decades for scientists to successfully detect them. The ironic thing is that these particles are all around us; in fact, our bodies are exposed to trillions every second.

So where do these mysterious non-interactive particles come from? Most come from our very own sun, but the origin of other, high-energy neutrinos is more mysterious, and seems to be somewhere out in deep space.

Neutrinos are closely connected to cosmic rays, high energy particles whose source has also remained elusive, and whose interactions with other atoms can produce neutrinos. Unlike neutrinos however, whose path across the universe follows a direct line, cosmic rays interact with magnetic fields and their path can be erratic.

The first signs of cosmic rays were encountered in 1909 by German physicist Theodor Wulf, who developed a device to detect energetic charged particles and found that there were higher levels of radiation at the top of the Eiffel Tower than at the bottom. Three years later, in 1912, Austrian-American physicist Victor Hess ascended into the atmosphere in a hot air balloon with modified Wulf electrometers, and confirmed that there’s more radiation higher up than near the ground. By conducting his experiment during a near-total eclipse, he concluded that there must be another source beside the Sun.

While scientists have had theories as to their origins of these non-solar neutrinos and cosmic rays, none could be proven until specialized equipment was made to study them - such as the detector at the IceCube Neutrino Observatory.

Completed in 2010 at the Amundsen-Scott South Pole Station in the Antarctic, this highly specialized installation is designed to detect and study ghost particles. The IceCube’s sensors consist of optical spheres called Digital Optical Modules, which are connected via strings and placed up to 2,450 meters into the ground. Once deployed, two staff members monitor the equipment for any signs of the elusive particle.

One of the few ways that a neutrino interacts with anything is by creating a secondary charged particle after colliding with an atom, and in 2017 one such interaction was detected and dubbed IceCube-170922A. Once detected, an alert was sent out to several other observatories to help trace the place of origin of this particle.

A total of eighteen observatories – including the Major Atmospheric Gamma Imaging Cherenkov Telescope and NASA’s Fermi Gamma-ray Space Telescope – collected data on the source of the neutrino, and measured the whole electromagnetic spectrum from radio waves to gamma rays. After some number crunching, it was determined that the source of the ghost particle was a blazar known as TXS 0506+056 - one of the brightest objects in the universe.

Blazars were first thought to be irregular stars from our galaxy but are instead large galactic nuclei which emit powerful beams of gamma rays and irregular beams of electromagnetic radiation from outside our galaxy. It’s believed that blazars are powered by black holes in their centre. Black holes are born when massive stars come to the end of their life cycle and collapse. As such, they have a strong gravitational pull, and the accretion disc around them generates a huge amount of energy. Thanks to this process, they act like giant particle accelerators, beaming particles across the universe. These pass through us daily, bathing the Earth in a constant stream of energy from distant galaxies.

In tracking the neutrino IceCube-170922A back to its origin, scientists killed two birds with one stone, discovering something about cosmic rays as well as neutrinos. We now know that neutrinos and cosmic rays can come from blazars - a huge step in understanding how the universe works. By understanding neutrinos, we have another piece in the puzzle for multi-messenger astronomy - a type of astronomy that interprets four main types of extrasolar energies: neutrinos, electromagnetic radiation, cosmic rays, and gravitational waves. Gravitational waves are generated from the curving of space-time by massive objects. Electromagnetic radiation is generated by accelerating electrically charged particles.

Combined, these sources can tell us a lot about our universe. Our cosmic equation is far from complete, but now it makes a little more sense. With this new information about ghost particles and cosmic rays, we can put to rest old questions and begin formulating new ones. It’s an exciting time for astrophysics. What will these lessons lead us to in the future? And what new discoveries are waiting just around the corner?