Why absolute zero remains just out of reach

HostIt's pretty amazing to think that right now, on the International Space Station, there are spots that are a billion times colder than the deepest parts of dark space. Scientists have built these high-tech chillers to see what happens when things get as cold as possible, but they always seem to hit a wall before they reach the very bottom. Why is that final step to absolute zero so impossible to take?

GuestTo understand that, we have to look at what temperature actually is. We usually think of it as a feeling, like hot or cold, but in physics, it's really just a way to measure chaos. Think of the atoms in a piece of metal like a huge crowd of people. When it's hot, everyone is running around, jumping, and crashing into each other. As things cool down, the crowd slows to a walk, then a crawl. Absolute zero is the point where every single atom should just stop. No shaking, no vibrating, just total stillness. But there's a rule called the Third Law of Thermodynamics that basically says you can never reach that point in a limited number of steps. It's like a game where every time you try to cool something down, you get halfway to the goal, but you never actually touch it. The closer you get to perfect order, the harder it's to squeeze out that last little bit of messiness.

HostThat's a bit frustrating. If we have the tech to get to a billionth of a degree, it feels like we just need one more really good push to get the rest of the way. What's actually stopping us from just building a bigger, better freezer?

GuestWell, that brings up the problem of power. To make something cold, you have to grab the heat from inside it and dump it somewhere else. That's how your fridge at home works, and it takes electricity to do that job. But as the temperature drops lower and lower, the laws of the universe make your cooling machine work a lot less well. The cost of removing heat starts to shoot up. By the time you get near absolute zero, the amount of work you need to do to pull out one more tiny sip of heat becomes massive. To get all the way to zero, you would basically need a fridge with never-ending power and an infinite supply of energy. You would essentially need more energy than exists in the entire universe just to make those last few atoms stand still. It's not just about having better tools; it's a limit built into the way energy itself works.

HostSo even if we had all the power in the stars, we're still fighting the way the universe is wired. But let's say we did have that power. If we could somehow force everything to be still, would the atoms finally just stop?

GuestEven then, you would run into a problem with the rules of the subatomic world. There's a famous idea called the Heisenberg Uncertainty Principle. It says there's a limit to how much we can know about a particle at one time. Specifically, we can't know exactly where a particle is and exactly how fast it's going at the same moment. If an atom reached absolute zero and stopped moving entirely, we would know both of those things. We would know it's right there, and we would know its speed is zero. Nature basically forbids that. To keep from breaking that rule, every particle has to keep a tiny, permanent shake that we call zero-point energy. It's like a baseline hum that never goes away. Atoms are required to jitter forever, so even at the coldest state possible, there's still a tiny bit of motion. True stillness just isn't allowed.

HostIt's wild that the universe has a built-in rule to keep things from being too quiet. But if we're doing these tests in a lab, can't we just shield the experiment from everything else? If we block out all the light and heat, doesn't that help us get closer?

GuestWe try, but the universe is a very noisy, warm place. Even the empty parts of space aren't actually empty. They're filled with something called the Cosmic Microwave Background radiation. This is a faint glow of heat left over from the Big Bang, and it keeps the entire universe at about three degrees above absolute zero. When scientists run these experiments, they have to build massive shields to block out that warmth, but no shield is ever perfect. Heat is always trying to move from a warmer place to a colder one. It leaks through the walls of the containers or travels in on tiny particles we can't see. It's like trying to keep a single ice cube from melting while you're standing in the middle of a massive bonfire. The heat from the rest of the world is constantly trying to get in and ruin the stillness.

GuestThat constant leak of energy means we're always fighting a losing battle against the warmth of the entire universe just to keep things slightly moving.

HostThe scientists on the space station are doing some of the most advanced work in history, but they're still just trying to find a quiet moment on a floor that's always shaking.