WEBVTT 00:00:06.716 --> 00:00:10.397 The coldest materials in the world aren’t in Antarctica. 00:00:10.397 --> 00:00:12.521 They’re not at the top of Mount Everest 00:00:12.521 --> 00:00:14.376 or buried in a glacier. 00:00:14.376 --> 00:00:15.897 They’re in physics labs: 00:00:15.897 --> 00:00:20.382 clouds of gases held just fractions of a degree above absolute zero. 00:00:20.382 --> 00:00:25.367 That’s 395 million times colder than your refrigerator, 00:00:25.367 --> 00:00:28.073 100 million times colder than liquid nitrogen, 00:00:28.073 --> 00:00:31.240 and 4 million times colder than outer space. 00:00:31.240 --> 00:00:35.901 Temperatures this low give scientists a window into the inner workings of matter, 00:00:35.901 --> 00:00:39.437 and allow engineers to build incredibly sensitive instruments 00:00:39.437 --> 00:00:41.292 that tell us more about everything 00:00:41.292 --> 00:00:43.130 from our exact position on the planet 00:00:43.130 --> 00:00:46.135 to what’s happening in the farthest reaches of the universe. NOTE Paragraph 00:00:46.135 --> 00:00:48.928 How do we create such extreme temperatures? 00:00:48.928 --> 00:00:51.989 In short, by slowing down moving particles. 00:00:51.989 --> 00:00:55.951 When we’re talking about temperature, what we’re really talking about is motion. 00:00:55.951 --> 00:00:57.716 The atoms that make up solids, 00:00:57.716 --> 00:00:58.458 liquids, 00:00:58.458 --> 00:00:59.338 and gases 00:00:59.338 --> 00:01:00.869 are moving all the time. 00:01:00.869 --> 00:01:05.616 When atoms are moving more rapidly, we perceive that matter as hot. 00:01:05.616 --> 00:01:09.147 When they’re moving more slowly, we perceive it as cold. NOTE Paragraph 00:01:09.147 --> 00:01:12.563 To make a hot object or gas cold in everyday life, 00:01:12.563 --> 00:01:15.960 we place it in a colder environment, like a refrigerator. 00:01:15.960 --> 00:01:20.498 Some of the atomic motion in the hot object is transferred to the surroundings, 00:01:20.498 --> 00:01:22.251 and it cools down. 00:01:22.251 --> 00:01:23.788 But there’s a limit to this: 00:01:23.788 --> 00:01:27.865 even outer space is too warm to create ultra-low temperatures. 00:01:27.865 --> 00:01:32.823 So instead, scientists figured out a way to slow the atoms down directly – 00:01:32.823 --> 00:01:34.204 with a laser beam. NOTE Paragraph 00:01:34.204 --> 00:01:35.751 Under most circumstances, 00:01:35.751 --> 00:01:38.464 the energy in a laser beam heats things up. 00:01:38.464 --> 00:01:40.533 But used in a very precise way, 00:01:40.533 --> 00:01:44.813 the beam’s momentum can stall moving atoms, cooling them down. 00:01:44.813 --> 00:01:49.403 That’s what happens in a device called a magneto-optical trap. 00:01:49.403 --> 00:01:51.954 Atoms are injected into a vacuum chamber, 00:01:51.954 --> 00:01:55.415 and a magnetic field draws them towards the center. 00:01:55.415 --> 00:01:58.090 A laser beam aimed at the middle of the chamber 00:01:58.090 --> 00:02:00.623 is tuned to just the right frequency 00:02:00.623 --> 00:02:06.170 that an atom moving towards it will absorb a photon of the laser beam and slow down. 00:02:06.170 --> 00:02:09.089 The slow down effect comes from the transfer of momentum 00:02:09.089 --> 00:02:11.108 between the atom and the photon. 00:02:11.108 --> 00:02:14.208 A total of six beams, in a perpendicular arrangement, 00:02:14.208 --> 00:02:18.375 ensure that atoms traveling in all directions will be intercepted. 00:02:18.375 --> 00:02:21.018 At the center, where the beams intersect, 00:02:21.018 --> 00:02:24.840 the atoms move sluggishly, as if trapped in a thick liquid — 00:02:24.840 --> 00:02:29.924 an effect the researchers who invented it described as “optical molasses.” 00:02:29.924 --> 00:02:32.315 A magneto-optical trap like this 00:02:32.315 --> 00:02:35.405 can cool atoms down to just a few microkelvins — 00:02:35.405 --> 00:02:38.785 about -273 degrees Celsius. NOTE Paragraph 00:02:38.785 --> 00:02:41.609 This technique was developed in the 1980s, 00:02:41.609 --> 00:02:43.913 and the scientists who'd contributed to it 00:02:43.913 --> 00:02:47.931 won the Nobel Prize in Physics in 1997 for the discovery. 00:02:47.931 --> 00:02:52.751 Since then, laser cooling has been improved to reach even lower temperatures. NOTE Paragraph 00:02:52.751 --> 00:02:55.990 But why would you want to cool atoms down that much? 00:02:55.990 --> 00:02:59.786 First of all, cold atoms can make very good detectors. 00:02:59.786 --> 00:03:01.530 With so little energy, 00:03:01.530 --> 00:03:04.961 they’re incredibly sensitive to fluctuations in the environment. 00:03:04.961 --> 00:03:09.562 So they’re used in devices that find underground oil and mineral deposits, 00:03:09.562 --> 00:03:12.203 and they also make highly accurate atomic clocks, 00:03:12.203 --> 00:03:15.093 like the ones used in global positioning satellites. NOTE Paragraph 00:03:15.093 --> 00:03:18.152 Secondly, cold atoms hold enormous potential 00:03:18.152 --> 00:03:20.243 for probing the frontiers of physics. 00:03:20.243 --> 00:03:22.662 Their extreme sensitivity makes them candidates 00:03:22.662 --> 00:03:27.300 to be used to detect gravitational waves in future space-based detectors. 00:03:27.300 --> 00:03:31.624 They’re also useful for the study of atomic and subatomic phenomena, 00:03:31.624 --> 00:03:35.894 which requires measuring incredibly tiny fluctuations in the energy of atoms. 00:03:35.894 --> 00:03:38.046 Those are drowned out at normal temperatures, 00:03:38.046 --> 00:03:41.090 when atoms speed around at hundreds of meters per second. 00:03:41.090 --> 00:03:45.265 Laser cooling can slow atoms to just a few centimeters per second— 00:03:45.265 --> 00:03:49.122 enough for the motion caused by atomic quantum effects to become obvious. 00:03:49.122 --> 00:03:53.599 Ultracold atoms have already allowed scientists to study phenomena 00:03:53.599 --> 00:03:56.150 like Bose-Einstein condensation, 00:03:56.150 --> 00:03:59.631 in which atoms are cooled almost to absolute zero 00:03:59.631 --> 00:04:02.200 and become a rare new state of matter. NOTE Paragraph 00:04:02.200 --> 00:04:05.791 So as researchers continue in their quest to understand the laws of physics 00:04:05.791 --> 00:04:07.925 and unravel the mysteries of the universe, 00:04:07.925 --> 00:04:12.161 they’ll do so with the help of the very coldest atoms in it.