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