Much of the cosmos goes undetected by our eyes. All About Space peers beyond visible light to reveal the explosive and catastrophic behaviour of the universe

Written by Gemma Lavender

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If we only relied on our eyes, we would be blind to most of what the universe has to offer. There would be so much that we I would completely miss, from the explosive nature of gamma-ray bursts to the dusty skeletons of galaxies and even the radiation left behind by the Big Bang. This is because the universe emits light that goes well and truly beyond what our eyes alone can see. Visible light, which is the light that we see making up our everyday surroundings, is just one small part of the electromagnetic spectrum. This runs all the way from low-energy radio waves through to astonishingly high-energy gamma rays.

Human eyes are unable to see anything that’s outside of the visible part of the spectrum, so you might be surprised to learn that we produce this hidden light on Earth, too. Think of infrared night-vision goggles, the X-rays you might get to see a broken arm, ultraviolet security tags or radio waves transmitting music and telephone calls across the planet. But in space these wavelengths of light are everywhere, coming from all kinds of cosmic objects. It’s like a hidden universe, but luckily for us, astronomers are able to tune in to these other wavelengths to make the invisible visible.

Light is a funny thing – it’s able to act like both a wave and a particle at the same time, and this is why we say that a photon of light can have a wavelength. The longest wavelengths are radio waves, which range from a millimetre in length to many kilometres. Meanwhile, the shortest wavelengths on the electromagnetic spectrum belong to gamma rays and can be as small as a trillionth of a metre. To reveal this hidden light, astronomers need a variety of tools, from giant bowl-shaped radio dishes that can be seen from miles around to detectors in orbit above Earth’s obscuring atmosphere.

This other light remained hidden until its discovery at the beginning of the 19th century. It was the year 1800 when William Herschel used a prism to split light into a spectrum of colours and measured the temperature of each shade. He came to the conclusion that there must be an extra unseen colour beyond red, because his thermometer was responding to light in the spectrum that he could not see – the infrared. A year later, German scientist Johann Ritter conducted a similar experiment, but this time found that unseen light beyond violet had the ability to darken paper soaked with silver chloride – ultraviolet had been discovered.