Back in ancient times, Astronomy was all about looking at the sky with the naked eye, trying to make sense of the tiny lights, what they were doing, how they were moving, and so on.

Then we discovered the laws of optics and used them to build telescopes so that we could see those small lights in the sky with a bit more clarity.

In the eighteenth century, William Herschel (him again!) discovered infra-red radiation and demonstrated that there was more to light than just the stuff that we can see. A century later and James Clerk-Maxwell showed (mathematically) that light and magnetism were closely related.

As our knowledge of the subatomic world grew, we were able to predict and then detect other forms of radiation - X-rays, gamma rays and cosmic rays for instance. And all of these different forms of radiation have some bearing on our knowledge of the sky.

In recent years, we've been able to detect gravitational waves, through which we've been able to witness events such as two Black Holes merging and later actually image a Black Hole itself.

These developments have given rise to a new branch of astronomy dubbed multi-messenger astronomy. We can use different phenomena and different messengers - be they gamma rays or neutrinos - to build a bigger picture of what's happening in space.

At the moment, there are four messengers:

Light - the stuff we can see
Cosmic Rays
Neutrinos
Gravitational Waves

Using two or more of these methods is called multi-messenger astronomy and it's based upon the coordinated observation and interpretation of disparate messenger signals, which are created by different astrophysical processes, and thus they can reveal different information about their sources.

It's also important to point out that detection via one messenger and non-detection from a different messenger can also be highly informative.

How does this work? Here's a good example.

On September 22nd 2017, scientists working at the the IceCube Collaboration in Antarctica announced that they had observed an extremely-high-energy neutrino event. They included a set of possible celestial coordinates within the alert.

This message was picked up by the Fermi-LAT Collaboration who announced that they had detected gamma rays above 100 MeV coming from the same region of the sky at exactly the same time.

The signals can be explained by ultra-high-energy protons accelerated in Blazar jets, producing neutral pions (decaying into gamma rays) and charged pions (decaying into neutrinos).

This was the first time that a neutrino detector had been used to locate an object in space and a source of cosmic rays had been identified.