Oxford Literary Festival: 25 March 2017: Talk in Blackwell’s Marquee
Telescopes: A Very Short Introduction
There are one hundred billion galaxies in the observable Universe, and each of them has a hundred billion stars. We know this because of one instrument: the telescope.
I am going to tell you 10 interesting things about telescopes …
- Who invented the telescope?
No one knows. The Romans knew about the focusing properties of water-filled glass spheres because Seneca wrote about them two thousand years ago. 400 years ago the availability of lenses in Europe lead to the discovery of the telescope. Two lenses are needed. The first lens collects the parallel rays of light from the object and focuses it. The second produces a parallel beam.
- What’s the most important property of a telescope?
It is its light collecting power. As the light from a star spreads out into larger volumes of space, it gets diluted. A star 10 times further away is 100 times dimmer.
Larger telescopes can see the most distant objects. Galileo’s spyglass could gather 64 times as much light as the naked eye. The largest 10-metre telescopes today collect 10 million times more.
The further away an object is, the longer it takes light to reach us. We can now observe galaxies 13 billion light years away and see back in time, almost to the Big Bang. Galaxies then looked very different from those today.
- Why is Galileo and his telescope so important?
In 1609 Galileo took a Dutch spyglass and increased the magnification from three to 20 times. The military and commercial significance of being able to see ships approaching Venice, before they became visible to the naked eye, was not lost on him. He gave a telescope to the doge, who rewarded him with a tenured professorship.
Galileo raised his telescope to the heavens and made discoveries of historic importance. His book “The Starry Messenger” lies in the Bodleian Library.
Galileo’s greatest discovery was of the four largest moons of Jupiter. It provided evidence for the Copernican heliocentric model of the solar system, and got him into a great deal of hot water with the church.
Although Galileo improved the telescope’s power, what was really important was what he did with it. He contributed to the overturn of a 2000-year old cosmology that had placed the Earth at the center of the Universe.
- Why do big telescopes have mirrors?
Big glass lenses are heavy and sag badly. The largest refractor ever made was the 1-metre telescope at Yerkes observatory of the University of Chicago.
Isaac Newton made the first 2½-inch reflecting telescope in 1668. There’s no size limit to a reflector, which uses a dish-shaped mirror to collect light.
William Herschel built many telescopes. But it was with a 6-inch reflector that he discovered Uranus, from his garden in Bath. There is an example of such a telescope in Oxford’s Museum of the History of Science.
The big 10-metre Keck telescopes use mirrors segmented into hexagons. They are located on a volcanic summit in Hawaii 4 km high.
They have looked into the heart of the Milky Way, and traced the orbits of stars, haplessly caught in the thrall of the supermassive black hole lurking there. Telescopes perform better high up, where the air is thinner, and less infrared is blocked.
- Why do we need space telescopes?
Our own atmosphere limits what we observe from the Earth. First the atmosphere blocks all the high-energy light from the cosmos. Second the atmosphere is turbulent. While turbulence makes the stars twinkle prettily, it blurs telescope images.
In space there is no atmosphere to limit the performance of telescopes, and they can realize their full potential. This applies across the whole of the spectrum: from gamma rays to radio.
- What can you see with a radio telescope?
The atmosphere is transparent to radio waves. Radio wavelengths are a million times longer than light, so the detail that a single dish can see is limited. The solution is to assemble many dishes as elements of a much larger telescope.
This opened up the Radio Universe – viewing the radio emission from Active Galaxies, spewing out vast quantities of energetic particles and radiation, and pulsars.
Pulsars are spinning neutron stars – matter so dense, that it is only one step away from a black hole.
A neutron star is the size of a city, and a teaspoonful weighs as much as a mountain!
Just by picking up this paper, I have used more energy than has ever been picked up by all the radio telescopes of the world!
- What are today’s big questions about the Universe?
- What is dark matter?
- What is dark energy?
- How did the first galaxies form?
- Are there habitable Earth-like exoplanets?
This includes the search for biomarkers – the fingerprints of extra-terrestrial: oxygen, carbon dioxide, and water.
- What is the next radio telescope?
It is the Square Kilometre Array, or SKA, which will have an aperture of one million square metres, and soon be the world’s largest scientific instrument.
It will bridge two continents, from South Africa, to Australia. Hundreds of radio dishes will be linked together, acting like a ‘zoom’ lens on a camera.
The SKA will observe gas clouds from the Cosmic Dawn. Its antennas will be sensitive enough to detect airport radar on a planet 50 light years away, and so could detect any extra-terrestrial signals. Within this distance there are over 100 stars similar to the Sun, many of which may possess Earthlike planets.
- What is the next space telescope?
It is the infrared, James Webb Space Telescope (or ‘Webb’ for short). It will be launched next year. Webb will have 100 times the resolution of the Hubble Space Telescope.
The light from the earliest galaxies has been redshifted by the expansion of the Universe into the infrared part of the spectrum.
How do you squeeze Webb’s huge 6.5-metre mirror into a rocket?
Webb’s mirror is made from a mosaic of beryllium hexagons folded like origami. When the telescope is released, a million miles from the Earth, the segments will unfurl, like a butterfly emerging from a chrysalis.
Webb will peer inside cosmic nurseries where stars are born, search for planetary discs, and look for organic molecules and biomarkers. It will also search for the reflected light from exoplanets, which is difficult. It is like trying to see the dim light of a firefly next to a dazzling searchlight.
- What about the data?
In 400 years, the size of telescopes has doubled, every 30 years. Since the 1980s, the number of picture elements in light detectors has doubled every 2 years.
For the next telescopes therefore, we are talking about Big Data. The data rate will compete with the WWW – equivalent to 36 million year’s worth of high definition video!
The needles in the data haystack are the science discoveries. The big question is how to find them.
Two possibilities are:
- ONE: Harnessing the eyes and brains of millions of volunteer citizen scientists to classify the data (e.g. the Zooniverse projects)
- TWO: using autonomous pattern-recognition programmes to look for unusual features
To conclude: in addressing the key questions, the next telescopes will strain at the limits of what is technically possible.
Will telescopes show us that we are not alone in the Universe?