What Is Multi-Messenger Astronomy?

For most of the history of astronomy, scientists learned about the Universe by observing light.

Modern astronomy can detect several different types of signals arriving from astronomical events. These different carriers of information are called messengers.

The main astronomical messengers are:

• Electromagnetic radiation
• Gravitational waves
• Neutrinos
• Cosmic rays

Multi-messenger astronomy studies the same astronomical object or event using two or more of these messengers.

Each messenger provides different information. Combining them can give scientists a much more complete picture of what happened.

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Messenger 1: Electromagnetic Radiation

Electromagnetic radiation includes all forms of light.

The electromagnetic spectrum includes:

Radio→Microwave→Infrared→Visible→Ultraviolet→X-ray→Gamma ray

Different astronomical objects produce different wavelengths.

For example:

• Cool gas clouds can emit radio waves.
• Stars produce visible and infrared radiation.
• Very hot gas can produce X-rays.
• Extremely energetic events can produce gamma rays.

Astronomers therefore use many different kinds of telescopes rather than relying only on visible light.

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Why Observe Different Wavelengths?

Looking at the same object using different wavelengths can reveal different physical processes.

Consider a stellar explosion.

Visible light might show the expanding material.

X-rays could reveal extremely hot gas.

Radio waves could reveal interactions between the explosion and surrounding material.

Gamma rays could reveal extremely energetic processes.

Each observation contributes another piece of information.

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Messenger 2: Gravitational Waves

A gravitational wave is a travelling distortion, or ripple, in spacetime.

Einstein's theory of general relativity predicted their existence in 1916.

Accelerating masses can produce gravitational waves, but detectable waves generally require extremely massive objects undergoing dramatic motion.

Important sources include:

• Merging black holes
• Merging neutron stars
• Possibly some supernovae

Gravitational waves travel through space at the speed of light.

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How Are Gravitational Waves Detected?

Gravitational waves produce extraordinarily tiny changes in distances.

Scientists detect these changes using extremely sensitive instruments called laser interferometers.

The LIGO observatories in the United States use this technique.

Each LIGO detector has two long arms arranged at right angles.

Laser beams travel along both arms and reflect from mirrors.

When no gravitational wave passes through, the lengths of the two arms remain effectively unchanged relative to one another.

When a gravitational wave passes:

• One arm may become extremely slightly longer.
• The other may become extremely slightly shorter.
• The pattern then reverses as the wave passes.

The resulting change in the interference pattern of the laser light can be measured.

These changes are extraordinarily small — much smaller than the width of an atomic nucleus over the kilometre-scale detector arms.

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The First Direct Detection

On September 14, 2015, LIGO detected a gravitational-wave signal from the merger of two black holes more than a billion light-years away.

The signal was named GW150914.

This was the first direct detection of gravitational waves and provided a new way of studying the Universe.

Before this discovery, astronomers depended overwhelmingly on electromagnetic observations and particle detections.

Gravitational-wave astronomy allows scientists to study events that may produce little or no detectable light.

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Electromagnetic Waves vs Gravitational Waves

These two messengers are very different.

The two approaches complement each other.

Electromagnetic radiation often tells us about matter and radiation surrounding an event.

Gravitational waves can tell us about the motion and properties of massive compact objects involved in the event itself.

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Messenger 3: Neutrinos

A neutrino is an extremely light elementary particle that has no electric charge and interacts only very weakly with ordinary matter.

Huge numbers of neutrinos can pass directly through Earth without interacting with anything.

This makes neutrinos difficult to detect, but it also makes them extremely useful astronomical messengers.

Because neutrinos interact so weakly with matter, they can escape from regions that electromagnetic radiation may have difficulty leaving.

They can therefore carry information directly from energetic or dense environments.

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Where Do Astronomical Neutrinos Come From?

Neutrinos are produced by several important processes.

The Sun

Nuclear fusion inside the Sun produces enormous numbers of neutrinos.

Solar neutrinos allow scientists to investigate nuclear reactions occurring deep inside the Sun.

Supernovae

A collapsing massive star can release an enormous burst of neutrinos.

In fact, much of the energy released during a core-collapse supernova is carried away by neutrinos.

High-Energy Cosmic Sources

Scientists also detect extremely energetic neutrinos originating beyond the Solar System.

Their sources may include environments surrounding:

• Active galaxies
• Supermassive black holes
• Other extreme cosmic accelerators

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How Are Neutrinos Detected?

Because neutrinos rarely interact with matter, detectors need to monitor enormous volumes of material.

One example is the IceCube Neutrino Observatory.

IceCube uses thousands of light sensors embedded deep within Antarctic ice.

Occasionally, a neutrino interacts with matter in or near the detector and produces charged particles. These particles can generate a faint flash of light known as Cherenkov radiation.

By detecting this light, scientists can reconstruct information about the incoming neutrino.

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Supernova 1987A: An Early Multi-Messenger Event

In 1987, astronomers observed a supernova in the Large Magellanic Cloud.

It became known as SN 1987A.

Neutrino detectors recorded a burst of neutrinos shortly before the supernova became visibly bright.

This was extremely important.

The neutrinos provided direct evidence about what was happening deep inside the collapsing star.

Astronomers could therefore study the event using:

neutrinos + electromagnetic radiation

This became a major early example of multi-messenger astronomy.

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A Landmark Discovery: GW170817

One of the most important demonstrations of multi-messenger astronomy occurred on August 17, 2017.

Gravitational-wave detectors observed a signal called GW170817.

The signal came from two neutron stars spiralling together and merging.

Shortly afterward, telescopes detected a gamma-ray burst from the same general region of the sky.

Astronomers around the world then observed the event using many different wavelengths.

Scientists studied the event using:

• Gravitational waves
• Gamma rays
• X-rays
• Ultraviolet radiation
• Visible light
• Infrared radiation
• Radio waves

Together, these observations revealed far more than any single detector could have discovered.

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What Did GW170817 Teach Us?

The event provided evidence that neutron-star mergers can produce a kilonova and contribute to the production of heavy elements.

Processes associated with these mergers can create elements such as:

• Gold
• Platinum
• Other heavy nuclei

The gravitational waves also provided information about:

• The masses of the neutron stars.
• Their orbital motion.
• The merger itself.

Electromagnetic observations provided information about:

• Ejected material.
• Temperatures.
• Element formation.
• The aftermath of the collision.

This is the central advantage of multi-messenger astronomy:

Different signals reveal different parts of the same event.​

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Messenger 4: Cosmic Rays

Cosmic rays are high-energy charged particles arriving from space.

Most are:

• Protons
• Atomic nuclei
• Electrons

Some cosmic rays carry enormous amounts of energy.

However, because they are electrically charged, magnetic fields can change their direction as they travel through space.

This makes it difficult to trace many cosmic rays directly back to their original sources.

Neutrinos, gravitational waves, and light are often better directional messengers.

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Comparing the Astronomical Messengers

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Why Combine Different Messengers?

Imagine trying to understand a thunderstorm using only your eyes.

You could see lightning, but you would miss the sound of thunder, changes in air pressure, temperature, rainfall, and wind.

Using several measurements gives you a much better understanding.

Astronomy works in a similar way.

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More Information

Different messengers reveal different physical processes.

For example, during a neutron-star merger:

Gravitational waves tell us about the motion and masses of the neutron stars.

Gamma rays reveal extremely energetic radiation.

Visible and infrared light reveal material ejected during the collision.

Together, these observations create a more complete description of the event.

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Independent Evidence

Multiple messengers can also provide independent evidence for the same explanation.

Suppose gravitational-wave detectors identify a neutron-star merger.

If telescopes independently observe a flash of light from the same location at approximately the same time, confidence that the signals came from the same astronomical event increases greatly.

Science becomes stronger when different observations support the same conclusion.

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Testing Physics

Multi-messenger observations allow scientists to test fundamental theories.

For example, observations of GW170817 showed that gravitational waves and gamma rays travelled across roughly 130 million light-years and arrived only seconds apart.

This provided extremely strong evidence that gravitational waves propagate at essentially the speed of light, as predicted by general relativity.

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A Multi-Messenger Investigation

Imagine astronomers detect three signals from approximately the same region of the sky:

Signal 1: A gravitational-wave signal indicates two neutron stars merging.

Signal 2: A gamma-ray telescope detects a short gamma-ray burst.

Signal 3: Optical telescopes detect a rapidly changing source at the same location.

Taken separately, each observation provides useful information.

Taken together, they provide much stronger evidence that astronomers have observed a neutron-star merger and its electromagnetic aftermath.

This is multi-messenger astronomy in action.

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Challenges of Multi-Messenger Astronomy

Combining different astronomical signals is not easy.

Scientists must coordinate:

• Gravitational-wave observatories
• Ground-based telescopes
• Space telescopes
• Neutrino detectors
• Observatories in many countries

Some events occur without warning and may fade rapidly.

When a gravitational-wave or neutrino detector identifies a possible event, alerts can be sent to observatories around the world so that telescopes can quickly search the relevant region of the sky.

Modern multi-messenger astronomy therefore depends heavily on international cooperation and rapid data sharing.

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Did You Know?

A supernova can produce so many neutrinos that, although only a tiny fraction interact with detectors on Earth, scientists may still detect a sudden burst.

Neutrinos can escape from the collapsing stellar core before much of the visible light reaches the outside of the star.

A future nearby supernova could therefore produce a detectable neutrino signal before astronomers see the star dramatically brighten, providing an early warning for telescopes to begin observing it.

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Key Terms

• Multi-messenger astronomy – studying astronomical events using two or more different types of signals.
• Electromagnetic radiation – energy transmitted by photons across the electromagnetic spectrum.
• Gravitational wave – a travelling distortion in spacetime produced by accelerating masses.
• Interferometer – an instrument that uses interference, often of laser light, to make extremely precise measurements.
• Neutrino – an electrically neutral elementary particle that interacts very weakly with matter.
• Cosmic ray – a high-energy charged particle arriving from space.
• Cherenkov radiation – light produced when a charged particle moves through a medium faster than light can propagate through that medium.
• Neutron star – an extremely dense stellar remnant composed predominantly of neutron-rich matter.
• Kilonova – an electromagnetic event associated with the merger of neutron stars or a neutron star and black hole under suitable conditions.
• Gamma-ray burst – a short, extremely energetic burst of gamma radiation from an astronomical source.

Key Takeaways

• Modern astronomy uses several cosmic messengers: electromagnetic radiation, gravitational waves, neutrinos, and cosmic rays.
• Electromagnetic observations provide information across wavelengths from radio waves to gamma rays.
• Gravitational waves are ripples in spacetime produced by accelerating massive objects.
• Instruments such as LIGO detect gravitational waves using extremely precise laser interferometry.
• Neutrinos can escape from dense astronomical environments because they interact only weakly with matter.
• Neutrino detectors such as IceCube use enormous detection volumes to observe rare neutrino interactions.
• Events such as SN 1987A and GW170817 demonstrate the power of combining different astronomical messengers.
• Different messengers reveal different physical processes within the same event.
• Independent observations can strengthen the evidence supporting a scientific explanation.
• Multi-messenger astronomy gives scientists a more complete understanding of some of the most energetic events in the Universe.