How to build a dark matter detector - Jenna Saffin

More than two kilometers below the surface of northern Ontario, 00:12 suspended in 345,000 liters of ultra-pure water, 00:18 there's a perfect sphere. 00:20 It contains 3600 kilograms of liquid argon, 00:24 cooled to -180 degrees Celsius. 00:28 Scientists continuously monitor this chamber from above ground, 00:32 looking for a glimmer of light in the darkness. 00:35 Because down here, 00:36 deep beneath the Earth's surface and cocooned in a watery shield, 00:39 that light would indicate the presence of one of the universe's greatest mysteries: 00:44 dark matter. 00:46 All the matter we can see, planets, stars and galaxies, 00:50 doesn't create enough gravitational pull 00:52 to explain the universe's larger structure. 00:55 It's dark matter, which is estimated to make up 25% of the known universe. 01:01 But despite its prevalence, 01:02 so far we haven't been able to detect it directly. 01:05 It's no small challenge. 01:07 Dark matter was so named because it doesn't interact with any type of light, 01:11 visible or otherwise, 01:13 which means our usual observation tools 01:15 simply don't work when trying to observe it. 01:18 But while dark matter may not be visible in the electromagnetic spectrum, 01:21 it's still matter, 01:23 so we should be able to measure its interactions with other matter. 01:26 And if our current model of physics is correct, 01:29 billions of sub-atomic dark matter particles 01:32 are passing through the Earth every second. 01:34 Despite the prevalence of dark matter, 01:36 its interactions are predicted to be rare and extremely weak. 01:40 To detect these interactions, 01:41 dark matter experiments need to be incredibly sensitive. 01:45 With such sensitive equipment, 01:47 the ever-present background radiation on Earth's surface 01:50 would create so much noise in the data 01:52 that any dark matter particles would be completely overwhelmed. 01:56 It would be like trying to hear a pin drop on a busy city street. 02:00 To solve this problem, 02:01 scientists have had to dig deep into the Earth. 02:04 Dark matter experiments are set up in specialized underground labs, 02:08 either in mines or inside mountains. 02:11 The rock that makes up the Earth's crust works like a filter, 02:15 absorbing radiation and stopping disruptive particles. 02:18 The ultra-pure water in which the detector is suspended 02:22 adds an additional layer of radiation filtering. 02:25 This shielding ensures that only the particles scientists are looking for 02:29 can make their way into the detectors. 02:31 Once these particles reach an experiment's inner vessel, 02:34 scientists have a chance of detecting them. 02:37 The detector media are chosen because they're exquisitely sensitive detectors 02:41 that can be purified extremely well. 02:43 These could be a liquid noble gas, 02:45 germanium 02:46 and silicon crystals, 02:48 a refrigerant, 02:49 or other materials. 02:50 When radiation interacts, it leaves tell-tale signs, 02:53 such as light or bubbles, 02:55 which can be picked up by the sensors inside the detector. 02:58 The detector media are held in a central chamber made of glass 03:01 or a special type of acrylic. 03:03 These chambers have to be able to hold the substance inside 03:06 without interacting with it 03:08 while withstanding incredible pressure from the water outside. 03:12 The inner vessel is surrounded by powerful sensors 03:15 designed to detect even the tiniest blips of light, 03:18 or the sound vibrations caused by a single bubble. 03:21 Each sensor records data 24/7, 03:25 and experiments run for months and years at a time, 03:28 generating terabytes of data every day. 03:31 Building dark matter detectors is as much a feat of engineering 03:35 as it is a feat of physics. 03:37 By the time an experiment is ready to start collecting data, 03:40 years or decades of work and investment have already gone into it, 03:44 to the tune of tens of millions of dollars. 03:47 As of 2017, no dark matter particles have been directly detected. 03:53 That's not entirely surprising. 03:55 Physicists expect these interactions to be incredibly rare and difficult to detect. 04:00 In the meantime, 04:01 scientists continue to develop new technologies 04:04 and increase detector sensitivity, 04:06 closing in on where dark matter is hiding. 04:09 And when they find it, 04:10 we'll finally be able to bring the universe's darkest secrets into the light.