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Seeing the invisible 00:00:06:08,00:00:09:02, When you listen to your favourite|piece of music, your ears pick up 00:00:09:03,00:00:12:20, on a very wide range of frequencies,|from the deepest rumblings of the 00:00:12:21,00:00:16:03, bass to the very highest|pitched vibrations. 00:00:16:04,00:00:18:24, Now imagine your ears were|only sensitive to a very limited 00:00:19:08,00:00:20:23, range of frequencies. 00:00:20:24,00:00:23:13, You'd miss out on most|of the good stuff! 00:00:23:14,00:00:27:00, But that's essentially the situations|that astronomers are in. 00:00:27:01,00:00:30:04, Our eyes are only sensitive|to a very narrow range 00:00:30:05,00:00:33:00, of light frequencies:|visible light. 00:00:33:01,00:00:35:14, But we are completely blind|to all other forms of 00:00:35:15,00:00:37:15, electromagnetic radiation. 00:00:37:16,00:00:40:16, However, there are many objects|in the Universe that do emit 00:00:40:17,00:00:43:24, radiation at other parts of|the electromagnetic spectrum. 00:00:44:00,00:00:47:19, For example, in the 1930s it|was discovered by accident 00:00:47:20,00:00:51:06, that there are radio waves|coming from the depths of space. 00:00:51:07,00:00:53:24, Some of these waves have the|same frequency as your favourite 00:00:54:00,00:00:57:04, radio station, but they are|weaker and of course there's 00:00:57:05,00:00:59:07, nothing to listen to. 00:01:00:12,00:01:03:24, In order to "tune in" to the radio|Universe, you need some sort 00:01:04:00,00:01:06:14, of receiver: a radio telescope. 00:01:06:16,00:01:10:24, Now for all but the longest wavelengths,|a radio telescope is just a dish. 00:01:11:00,00:01:14:02, Much like the main mirror|of an optical telescope. 00:01:14:04,00:01:18:10, But because radio waves are so|much longer than visible lightwaves 00:01:18:10,00:01:21:06, the surface of a dish doesn't|have to be nearly as smooth 00:01:21:08,00:01:23:00, as the surface of a mirror. 00:01:23:02,00:01:25:16, And that's the reason why it's|so much easier to build a 00:01:25:16,00:01:30:20, large radio telescope than it is to|build a large optical telescope. 00:01:30:20,00:01:34:24, Also, at radio wavelengths, it is|much easier to do interferometry. 00:01:34:24,00:01:38:02, That is, to increase the level|of detail that can be seen 00:01:38:02,00:01:41:24, by combining the light from two|separate telescopes, as if 00:01:42:00,00:01:45:14, they were part of a|single, giant dish. 00:01:45:14,00:01:48:16, The Very Large Array in New Mexico,|for example, consists of 00:01:48:16,00:01:53:18, 27 separate antennas, each|measuring 25 metres across. 00:01:53:18,00:01:56:24, Now each antenna can be moved|around individually, and in 00:01:57:00,00:02:00:10, its most extended configuration,|the virtual dish mimicked by the 00:02:00:12,00:02:04:20, array measures 36|kilometres across. 00:02:04:22,00:02:07:14, So what does the Universe|look like in the radio? 00:02:07:16,00:02:12:00, Well, for a start our Sun shines|very brightly at radio wavelengths. 00:02:12:02,00:02:14:18, So does the centre of our|Milky Way Galaxy. 00:02:14:18,00:02:16:10, But there's more. 00:02:16:12,00:02:20:12, Pulsars are very dense stellar|corpses that emit radio waves 00:02:20:12,00:02:22:16, only into a very narrow beam. 00:02:22:16,00:02:25:20, In addition, they rotate at speeds|of up to several hundred 00:02:25:20,00:02:27:18, revolutions per second. 00:02:27:18,00:02:31:20, So in effect, a pulsar looks like|a rotating radio lighthouse. 00:02:31:22,00:02:35:08, And what we see from them is|a very regular and fast 00:02:35:08,00:02:38:08, sequence of very|short radio pulses. 00:02:38:10,00:02:40:16, Hence the name. 00:02:40:16,00:02:43:08, The radio source known as|Cassiopeia A is in fact 00:02:43:10,00:02:47:16, the remnant of a supernova that|exploded in the 17th century. 00:02:47:16,00:02:52:06, Centaurus A, Cygnus A and Virgo A|are all giant galaxies that 00:02:52:06,00:02:54:16, pour out huge amounts|of radio waves. 00:02:54:16,00:02:59:24, Each galaxy is powered by a|massive black hole at its centre. 00:03:00:00,00:03:04:00, Some of these radio galaxies|and quasars are so powerful that 00:03:04:02,00:03:09:08, their signals can still be detected|from a distance of 10 billion light-years. 00:03:09:08,00:03:12:22, And then there's the faint,|relatively short-wavelength radio hiss 00:03:12:24,00:03:15:08, that fills the entire Universe. 00:03:15:08,00:03:18:04, This is known as the cosmic|microwave background 00:03:18:04,00:03:20:10, and it is the echo of|the Big Bang. 00:03:20:10,00:03:24:14, The very afterglow of the hot|beginnings of the Universe. 00:03:26:02,00:03:30:10, Each and every part of the|spectrum has its own story to tell. 00:03:30:10,00:03:33:24, At millimetre and submillimetre|wavelengths, astronomers study 00:03:33:24,00:03:37:02, the formation of galaxies in the|early Universe, and the origin 00:03:37:04,00:03:41:06, of stars and planets|in our own Milky Way. 00:03:41:06,00:03:45:10, But most of this radiation is blocked|by water vapour in our atmosphere. 00:03:45:12,00:03:48:10, To observe it, you need|to go high and dry. 00:03:48:10,00:03:51:08, To Llano de Chajnantor, for example. 00:03:51:10,00:03:54:24, At five kilometres above sea|level, this surrealistic plateau 00:03:54:24,00:03:57:24, in northern Chile is the|construction site of ALMA: 00:03:58:00,00:04:00:22, the Atacama Large Millimeter Array. 00:04:00:22,00:04:05:22, When completed in 2014, ALMA|will be the largest astronomical 00:04:05:22,00:04:08:08, observatory ever built. 00:04:08:20,00:04:13:24, 64 antennas each weighing|100 tonnes, will work in unison. 00:04:13:24,00:04:17:22, Giant trucks will spread them out|over an area as large as London to 00:04:17:24,00:04:20:20, increase the detail of the image,|or bring them close together to 00:04:20:22,00:04:23:00, provide a wider view. 00:04:23:02,00:04:27:06, Each move will be made|with millimetre precision. 00:04:28:16,00:04:32:04, Many objects in the Universe|also glow in the infrared. 00:04:32:06,00:04:35:24, Discovered by William Herschel,|infrared radiation is often also called 00:04:36:00,00:04:40:18, "heat radiation" because it is|emitted by all relatively warm objects 00:04:40:18,00:04:43:02, including humans. 00:04:45:20,00:04:49:06, You may be more familiar with|infrared radiation than you think. 00:04:49:08,00:04:52:06, Because on Earth, this kind|of radiation is used by 00:04:52:08,00:04:55:04, night vision goggles|and cameras. 00:04:55:06,00:04:59:04, But to detect the faint infrared glow|from distant objects, astronomers 00:04:59:06,00:05:02:24, need very sensitive detectors,|cooled down to just a few degrees 00:05:03:00,00:05:08:00, above absolute zero, in order to|suppress their own heat radiation. 00:05:10:22,00:05:15:18, Today, most big optical telescopes are|also equipped with infrared cameras. 00:05:15:18,00:05:19:08, They allow you to see right through|a cosmic dust cloud, revealing the 00:05:19:10,00:05:24:06, newborn stars inside, something that|just cannot be seen in the optical. 00:05:24:06,00:05:29:02, For example, take this optical image|of the famous stellar nursery in Orion. 00:05:29:04,00:05:31:10, But look how different it is|when seen through the eyes 00:05:31:12,00:05:34:02, of an infrared camera! 00:05:34:04,00:05:37:08, Being able to see in the infrared|is also very helpful when studying 00:05:37:08,00:05:39:24, the most distant galaxies. 00:05:39:24,00:05:45:00, The newborn stars in a young galaxy|shine very brightly in the ultraviolet. 00:05:45:02,00:05:49:00, But then this ultraviolet light has to|travel for billions of years across 00:05:49:02,00:05:50:16, the expanding Universe. 00:05:50:18,00:05:54:14, The expansion stretches the lightwaves|so that when they are received 00:05:54:14,00:05:59:06, by us, they've been shifted all|the way into the near-infrared. 00:06:00:14,00:06:04:06, This stylish instrument is the|MAGIC telescope on La Palma. 00:06:04:08,00:06:06:24, It searches the sky for|cosmic gamma rays 00:06:06:24,00:06:10:20, the most energetic form|of radiation in Nature. 00:06:12:08,00:06:14:24, Lucky for us, the lethal gamma|rays are blocked by the 00:06:14:24,00:06:16:08, Earth's atmosphere. 00:06:16:08,00:06:20:00, But they do leave behind footprints|for astronomers to study. 00:06:20:02,00:06:23:00, After hitting the atmosphere,|they produce cascades of 00:06:23:02,00:06:24:16, energetic particles. 00:06:24:18,00:06:29:08, These, in turn, cause a faint|glow that MAGIC can see. 00:06:30:22,00:06:34:16, And here's the Pierre Auger|Observatory in Argentina. 00:06:34:16,00:06:37:02, It doesn't even look|like a telescope. 00:06:37:02,00:06:42:24, Pierre Auger consists of 1600|detectors, spread over 3000 00:06:42:24,00:06:44:06, square kilometres. 00:06:44:08,00:06:48:14, They catch the particle fallout of|cosmic rays from distant supernovas 00:06:48:14,00:06:50:12, and black holes. 00:06:51:16,00:06:56:10, And what about neutrino detectors,|built in deep mines or beneath the 00:06:56:12,00:06:59:18, surface of the ocean,|or in the Antarctic ice. 00:06:59:20,00:07:01:22, Could you call those telescopes? 00:07:01:24,00:07:03:10, Well, why not? 00:07:03:12,00:07:07:20, After all, they do observe the Universe,|even if they don't capture data from 00:07:07:20,00:07:10:02, the electromagnetic spectrum. 00:07:10:02,00:07:13:22, Neutrinos are elusive particles|that are produced in the Sun 00:07:13:24,00:07:16:06, and supernova explosions. 00:07:16:08,00:07:19:20, They were even produced|in the Big Bang itself. 00:07:19:22,00:07:24:16, Unlike other elementary particles,|neutrinos can pass through regular 00:07:24:16,00:07:29:16, matter, travel near the speed|of light and have no electric charge. 00:07:29:18,00:07:34:06, Although these particles may be|difficult to study, they are plentiful. 00:07:34:06,00:07:38:04, Each second more than 50 trillion|electron neutrinos from the Sun 00:07:38:04,00:07:40:14, pass through you. 00:07:40:16,00:07:44:20, Finally, astronomers and physicists have|joined forces to build gravitational 00:07:44:22,00:07:46:16, wave detectors. 00:07:46:16,00:07:50:16, These "telescopes" do not observe|radiation or catch particles. 00:07:50:16,00:07:55:06, Instead, they measure tiny ripples in|the very structure of space-time - 00:07:55:06,00:08:00:24, a concept predicted by|Albert Einstein's theory of relativity. 00:08:01:00,00:08:05:04, With a stunning variety of instruments,|astronomers have opened up the full 00:08:05:04,00:08:10:24, spectrum of electromagnetic radiation,|and have even ventured beyond. 00:08:11:00,00:08:15:06, But some observations simply|can't be done from the ground. 00:08:15:06,00:08:16:20, The answer? 00:08:16:22,00:08:19:06, Space telescopes.