Think of an apple. In your mind, you can see its red color, feel its smooth surface, smell its sweet fragrance, take a bite and listen to its crunch. All the signals you perceive originate from the apple: visual, tactile, olfactory, gustatory and auditive. Combine all these senses, and you really know what an apple is. Now imagine you’re an astronomer. You’re no longer trying to understand a piece of fruit, but something far more mysterious, deep in the universe. You can look at this thing through a telescope, but light alone may not suffice to identify it. You need more information, and while you can’t necessarily smell, taste or touch this distant discovery, you can use other “messengers” – such as neutrinos or gravitational waves – coming from the same celestial source.
Ann Finkbeiner explains this so-called “multimessenger astronomy” in a fascinating Scientific American article.
- Astronomers use different methods to detect “messengers” that reach Earth from the cosmos.
- Multimessenger astronomy combines methods to observe light, particles and gravitational waves – originating from the same source – to better understand individual celestial events.
- New multimessenger detectors are under construction that will complement existing ones on Earth and in space.
Astronomers use different methods to detect “messengers” that reach Earth from the cosmos.
To study celestial objects and events, astronomers need to detect “messengers,” signals that reach Earth. For the longest time, people observed visible light from the stars using optical telescopes – with photons being the messengers. New methods then allowed astronomers to detect other messengers including neutrinos [electrically neutral elementary particles with very low mass]. In 1987, astronomers detected neutrinos originating from a supernova, the death of a star in a distant galaxy. In 2015, for the first time, the Laser Interferometer Gravitational-Wave Observatory (LIGO) detected gravitational waves – distortions in spacetime caused by accelerated masses.
“Two recent celestial events have ushered in the age of multimessenger astronomy – the technique of observing the same phenomena through light, particles and gravitational waves.”
It can be straightforward for scientists to locate the source of neutrinos and gravitational waves as they travel almost unhindered through any obstacle they might encounter following a direct path from the source. In contrast, it can be difficult to locate the source of light because obstacles may reflect, redirect or absorb light.
Multimessenger astronomy combines methods to observe light, particles and gravitational waves – originating from the same source – to better understand individual celestial events.
Astronomers can look at different messengers separately. However, using the full range of their tools to analyze different messengers (such as light, particles and gravitational waves) originating from one source, provides a more in-depth understanding.
“Astronomers want to know what happens after neutron stars merge. They want to see a neutron star merging with a black hole and to discover how jets arise and what powers them. They still do not know how the cores of stars collapse into supernovae, and they want to watch supermassive black holes in the centers of galaxies merging with other supermassive black holes in the centers of other galaxies.”
Two celestial events observed in 2017 mark important first “multimessenger astronomy” approaches.
- Astronomers observed the collision of neutron stars. Scientists had predicted so-called kilonovae but never seen them before. Fewer than two seconds after gravitational waves were detected by LIGO-Virgo, the Fermi space telescope picked up gamma waves from the same source. Other detectors followed the event by detecting light of different frequencies.
- Scientists observed what seems to be matter ejected by a supermassive black hole. Within a few days, sensors on Earth and in space received signals from the same region of the cosmos. First, the IceCube Neutrino Observatory, located deep in ice at the South Pole, detected a neutrino. Then an orbiting telescope detected X-rays, the Fermi space telescope detected gamma rays, and various Earth-based telescopes and receivers detected light and radio waves.
New multimessenger detectors are under construction that will complement existing ones on Earth and in space.
An improved version of the IceCube and KM3NeT, a neutrino detector located 3.5 km below the Mediterranean Sea, will be welcome additions to astronomers’ multimessenger arsenal. India and Japan are constructing additional gravitational wave observatories. The Laser Interferometer Space Antenna (LISA) will detect gravitational waves from the orbit after its planned launch in 2030.
About the Author
Science writer Ann Finkbeiner writes about astronomy and cosmology, and the relation between science and national security. Her recent book, A Grand and Bold Thing, is about the Sloan Digital Sky Survey, an attempt to map the universe.