0:00:06.716,0:00:10.397 The coldest materials in the world [br]aren’t in Antarctica. 0:00:10.397,0:00:12.521 They’re not at the top of Mount Everest 0:00:12.521,0:00:14.376 or buried in a glacier. 0:00:14.376,0:00:15.897 They’re in physics labs: 0:00:15.897,0:00:20.382 clouds of gases held just fractions[br]of a degree above absolute zero. 0:00:20.382,0:00:25.367 That’s 395 million times colder [br]than your refrigerator, 0:00:25.367,0:00:28.073 100 million times colder [br]than liquid nitrogen, 0:00:28.073,0:00:31.240 and 4 million times colder[br]than outer space. 0:00:31.240,0:00:35.901 Temperatures this low give scientists a[br]window into the inner workings of matter, 0:00:35.901,0:00:39.437 and allow engineers to build [br]incredibly sensitive instruments 0:00:39.437,0:00:41.292 that tell us more about everything 0:00:41.292,0:00:43.130 from our exact position on the planet 0:00:43.130,0:00:46.135 to what’s happening in [br]the farthest reaches of the universe. 0:00:46.135,0:00:48.928 How do we create such [br]extreme temperatures? 0:00:48.928,0:00:51.989 In short, by slowing down [br]moving particles. 0:00:51.989,0:00:55.951 When we’re talking about temperature,[br]what we’re really talking about is motion. 0:00:55.951,0:00:57.716 The atoms that make up solids, 0:00:57.716,0:00:58.458 liquids, 0:00:58.458,0:00:59.338 and gases 0:00:59.338,0:01:00.869 are moving all the time. 0:01:00.869,0:01:05.616 When atoms are moving more rapidly,[br]we perceive that matter as hot. 0:01:05.616,0:01:09.147 When they’re moving more[br]slowly, we perceive it as cold. 0:01:09.147,0:01:12.563 To make a hot object [br]or gas cold in everyday life, 0:01:12.563,0:01:15.960 we place it in a colder environment, [br]like a refrigerator. 0:01:15.960,0:01:20.498 Some of the atomic motion in the hot[br]object is transferred to the surroundings, 0:01:20.498,0:01:22.251 and it cools down. 0:01:22.251,0:01:23.788 But there’s a limit to this: 0:01:23.788,0:01:27.865 even outer space is too warm[br]to create ultra-low temperatures. 0:01:27.865,0:01:32.823 So instead, scientists figured out a way [br]to slow the atoms down directly – 0:01:32.823,0:01:34.204 with a laser beam. 0:01:34.204,0:01:35.751 Under most circumstances, 0:01:35.751,0:01:38.464 the energy in a laser beam [br]heats things up. 0:01:38.464,0:01:40.533 But used in a very precise way, 0:01:40.533,0:01:44.813 the beam’s momentum can stall [br]moving atoms, cooling them down. 0:01:44.813,0:01:49.403 That’s what happens in a device [br]called a magneto-optical trap. 0:01:49.403,0:01:51.954 Atoms are injected into a vacuum chamber, 0:01:51.954,0:01:55.415 and a magnetic field [br]draws them towards the center. 0:01:55.415,0:01:58.090 A laser beam aimed [br]at the middle of the chamber 0:01:58.090,0:02:00.623 is tuned to just the right frequency 0:02:00.623,0:02:06.170 that an atom moving towards it will absorb[br]a photon of the laser beam and slow down. 0:02:06.170,0:02:09.089 The slow down effect comes from[br]the transfer of momentum 0:02:09.089,0:02:11.108 between the atom and the photon. 0:02:11.108,0:02:14.208 A total of six beams, [br]in a perpendicular arrangement, 0:02:14.208,0:02:18.375 ensure that atoms traveling [br]in all directions will be intercepted. 0:02:18.375,0:02:21.018 At the center, where the beams intersect, 0:02:21.018,0:02:24.840 the atoms move sluggishly, [br]as if trapped in a thick liquid — 0:02:24.840,0:02:29.924 an effect the researchers who invented it [br]described as “optical molasses.” 0:02:29.924,0:02:32.315 A magneto-optical trap like this 0:02:32.315,0:02:35.405 can cool atoms down [br]to just a few microkelvins — 0:02:35.405,0:02:38.785 about -273 degrees Celsius. 0:02:38.785,0:02:41.609 This technique was developed in the 1980s, 0:02:41.609,0:02:43.913 and the scientists [br]who'd contributed to it 0:02:43.913,0:02:47.931 won the Nobel Prize in Physics in 1997[br]for the discovery. 0:02:47.931,0:02:52.751 Since then, laser cooling has been [br]improved to reach even lower temperatures. 0:02:52.751,0:02:55.990 But why would you want [br]to cool atoms down that much? 0:02:55.990,0:02:59.786 First of all, cold atoms can make[br]very good detectors. 0:02:59.786,0:03:01.530 With so little energy, 0:03:01.530,0:03:04.961 they’re incredibly sensitive [br]to fluctuations in the environment. 0:03:04.961,0:03:09.562 So they’re used in devices that find [br]underground oil and mineral deposits, 0:03:09.562,0:03:12.203 and they also make [br]highly accurate atomic clocks, 0:03:12.203,0:03:15.093 like the ones used [br]in global positioning satellites. 0:03:15.093,0:03:18.152 Secondly, cold atoms hold [br]enormous potential 0:03:18.152,0:03:20.243 for probing the frontiers of physics. 0:03:20.243,0:03:22.662 Their extreme sensitivity [br]makes them candidates 0:03:22.662,0:03:27.300 to be used to detect gravitational waves [br]in future space-based detectors. 0:03:27.300,0:03:31.624 They’re also useful for the study [br]of atomic and subatomic phenomena, 0:03:31.624,0:03:35.894 which requires measuring incredibly [br]tiny fluctuations in the energy of atoms. 0:03:35.894,0:03:38.046 Those are drowned out [br]at normal temperatures, 0:03:38.046,0:03:41.090 when atoms speed around [br]at hundreds of meters per second. 0:03:41.090,0:03:45.265 Laser cooling can slow atoms to just [br]a few centimeters per second— 0:03:45.265,0:03:49.122 enough for the motion caused by[br]atomic quantum effects to become obvious. 0:03:49.122,0:03:53.599 Ultracold atoms have already [br]allowed scientists to study phenomena 0:03:53.599,0:03:56.150 like Bose-Einstein condensation, 0:03:56.150,0:03:59.631 in which atoms are cooled almost [br]to absolute zero 0:03:59.631,0:04:02.200 and become a rare new state of matter. 0:04:02.200,0:04:05.791 So as researchers continue in their quest [br]to understand the laws of physics 0:04:05.791,0:04:07.925 and unravel the mysteries of the universe, 0:04:07.925,0:04:12.161 they’ll do so with the help [br]of the very coldest atoms in it.