Seeing the invisible, from the largest to the strangest telescope on Earth.

Of course, astronomers now make telescopes that can see the invisible.

“How can a telescope see things that aren't visible?”

Light is just a small part of a much larger spectrum, the electromagnetic spectrum.

“Why is it called that?”

It's called the electromagnetic spectrum because a Scotsman, called James Clerk Maxwell, in 1873 discovered that magnetic and electrical fields both behave as waves and travel at precisely the same speed as light. Maxwell then went on to produce a theory that encompassed light, electrical waves and magnetic waves: hence electromagnetic. The range of all electromagnetic waves is huge; from x-rays and gamma rays with wavelengths of one hundred trillionth of a metre to radio waves with wavelengths of hundreds of miles. Do you remember what a wavelength is?

“I think so, it's the distance between the peaks a wave.”

That’s right. For light, the whole range of colours is squeezed into a surprisingly small range of wavelengths. Even though light and sound are completely different types of waves, if you think of light in musical terms, the whole range of visible light, every colour, would fit into a single octave. Do you know what an octave is?

“An octave is the eight notes of a scale isn’t it? Doe a deer a female deer, Ray a drop of golden sun, Me a name I call myself and all that stuff until you get back to Doe.”

Exactly. Now from one end of an octave to another the wavelength of sound changes by a factor of two. Going up it halves and going down in doubles. . If you think of a piano keyboard, which only covers a fraction of the range of possible pitches of musical notes, then this can give you a range of frequencies for sound which is eight times bigger than that for the whole of visible light. In other words, if you could construct a light keyboard the long wavelength light on this keyboard would look red and be like the low pitched notes in sound terms. The short wavelength light would be blue and be like the high pitched sounds with the whole of the rainbow in between. But all the light you can see would only cover eight white keys or thirteen if you count the black ones too.

“Every colour would fit into thirteen keys on a light piano? But that's impossible there are millions of different colours.”

To a completely colour blind man there are only three.

“What are you talking about Albert?”

You may see million of colours but someone with no colour vision at all will see only white, black and gray. There appears to be so many colours because our eyes is much better at detecting fine variations in light than our ears are at detecting differences in the pitch of a sound. If we were bats we might have decided to put a thousand notes in an octave and thirteen keys for light might be too many to describe all the colours a bat can see.

“OK. So where does this electromagnetic stuff fit in?”

If there was an electromagnetic piano its keyboard would be almost eight times longer than a normal keyboard and would have over 750 keys. Visible light would still represent only thirteen of these keys (that's counting the black ones as well), spanning only one octave in a range of over 63 octaves. A normal piano keyboard covers only 7 octaves or so.

“So what does this have to do with telescopes?”

Astronomers can now make telescopes that can detect the whole of this electromagnetic spectrum from x-ray telescopes which are good for detecting black holes and exploding stars in distant galaxies to radio telescopes which can detect pulsars. The biggest single telescope in the world is a radio telescope.

“How big is it?”

It is 305 metres across and in Arecibo, Puerto Rico. The most interesting thing about this telescope is that it was made inside the crater left by a meteorite.

“There are craters on Earth? I thought they only existed on the moon?”

Oh they exist here too but most meteorites burn up in the atmosphere. There is one in Arizona, the Barringer Crater, which is almost a mile across. It looks just like a crater on the moon.

“So why are there so few craters on Earth?”

There have been lots of craters on Earth but the atmosphere burns up smaller meteorites and the weather hides evidence of the rest. The moon doesn't have an atmosphere or any weather so all the craters for the last few billion years are piled one on top of the other. Remember the voyager probes and their golden discs with information about Earth? Well when the Arecibo telescope was upgraded in 1974 a radio message was sent into space towards a tightly packed group of stars in the constellation Hercules. This message, that included coded information about the structure of DNA, the shape of human beings and the solar system.

“Are these scientists crazy, giving away all our secrets to aliens?”

Well even though radio waves travel at the speed of light, the message won't reach its intended target until the year 26,000. So it will be a little while yet before anyone hears the message and decides to pay us a visit. But that's not the most unusual type of telescope. The strangest telescopes look particles called neutrinos which are nothing to do with light or the electromagnetic spectrum.

“Neutrinos? Never heard of them."
Well you have now. There are more neutrinos in the universe than any other particle, knocking us photons into second place. A neutrino could not only pass straight through your body but go through the whole earth. A neutrino telescope has to be the strangest type of telescope in the world. In 1964 a man called Raymond Davis junior decided to go in search of neutrinos.

"Why? Just because they exist?”

When he started no one was sure if they existed or not. They had been invented by theoretical physicists who worked out that they ought to exist before anyone had even detected one.

“So why bother try to find particles that someone has made up?"

They are important because scientists' best theory of how nuclear fusion works inside the sun predicted that the sun should produce a lot of them.

"I thought you said that Earth scientists had already worked all that out, even if they were a bit slow about it."

Well they had come up with what seemed like the right idea, but in science you need to start with the right theory, or hypothesis, and then do experiments or observations to check that the theory is correct. If they could measure that the sun was releasing the right amount of neutrinos is would help to show their theories were right. The problem with neutrinos is that they will fly straight through a normal telescope, through the head of the astronomer and nearly all of them will even pass straight through the earth

"So how do you detect something that can do that?"

It's not easy but this was Raymond Davis's approach. He found a deserted gold mine almost a mile underground and built a huge tank filled with 400,000 litres of dry cleaning fluid. The sun is thought to pump out almost 20 million trillion trillion trillion neutrinos every day and of these only one neutrino a day was expected to interact with one of the chlorine atoms in the fluid and change it into an atom of the gas argon. Having pumped the best part of a million dollars into this old gold mine they only found one neutrino every two or three days. Finding one argon atom in tank of 400,000 litres should have impressed most people, but theoretical physicists aren't impressed by any experiment that gives an answer that disagrees with their theories like this one did. Allowing for the difficulty in detecting neutrinos, this machine and others like it only found a third of the expected amounts of neutrinos. Lots of neutrino detectors have been built but most of the neutrinos are still missing.

“What happened to them?”

Well once they realised it was a problem with current ideas about neutrinos rather than the telescopes they changed their theory. The new theory is that the sun makes the expected amount of neutrinos but on their way to earth they change type to a sort that can't be detected by neutrino detectors.

“Is that cheating?”

No, it's science. You start with a theory that predicts that you ought to find a particular thing. You go looking for that thing and if you find it you start believing that theory but if you find something different it is time to create a new theory.