[intro music] Hank Green: Absolute zero is the Holy Grail of temperatures, but even though we know exactly how cold that is (-273.13°C (-459.670°F)), reaching it has eluded us for centuries and will continue to elude us, probably forever. For this frustration, we have to thank Sir William Thomson, 1st Baron Kelvin, who, in the mid 1800s, tested a new theory that heat is just molecules moving around in a substance. So, he wanted to get stuff as cold as he could get it, so he conducted experiments that drew heat from a warm substance toward a cooler one, and found that at some point, all the kinetic energy could be drained from the warm substance -- it could no longer be cooled any further. This temperature wasn't like melting points or boiling points, which change for every substance. It was the same for everything. So Kelvin created a thermodynamic temperature scale that measured the amount of kinetic energy within any given material, and we still use his Kelvin scale today. But ever since Kelvin's day, scientists have been trying to chill stuff to absolute zero, and no one has succeeded -- all a bunch of failures, because it turns out quantum mechanics is involved, which means it's really complicated. Physicists know that absolute zero does not mean a complete absence of motion in a substance. Instead, zero degrees Kelvin marks the state of minimal motion of a substance's particles. That's because of Heisenberg's uncertainty principle, which says that for any particle in the universe, it's impossible to know both its momentum and its exact position at the same time. So, suppose you chill a lump of lead down to the point where there is no motion going on within it, even at a subatomic level. If you could do that, you'd know both the particles' positions and their momentum, which would be zero, but measuring this is impossible -- it's forbidden by the uncertainty principle, so it cannot be done. So you can't reach true 0°K but you can get pretty darn close, like a billionth of a degree away. When you get that cold, some pretty weird stuff starts happening. Below about 30°K, some substances can become superconductive, meaning that they carry an electrical current with no resistance, which is super useful when you're making particle accelerators or really powerful electromagnets to put in your MRI machines, and those superconductors have been discovered that operate at much warmer temperatures. The development of the field is thanks to work at very, very cold temperatures measured on the Kelvin scale. And you might be wondering, because I was, how cold is the coldest place in the universe? You'd think, like, deep space, right? Well, yeah, space is cold, but it's pretty uniformly filled with microwave radiation left over from the big bang. This actually heats up space to a balmy 2.73°K. The coldest natural place in the known universe is the boomerang nebula, which has been spitting out gas for so long that it's cooled down to only about one degree Kelvin, and all of this means that, in fact, the coldest place in the known universe is in laboratories right here on planet Earth. Pretty cool. Thank you for watching this episode of SciShow. I hope that you smarter. If you wanna continue getting smarter, you should go to youtube.com/scishow and subscribe. If you have any questions, ideas, or comments, leave them in the comments below or get in touch with us on Facebook or Twitter. We'll see you next time. [outro music]