how to cool things to absolute zero using only a laser

We’re used to lasers being either death beams, or harmless little points of light. But we also use lasers to cool matter to absolute zero and probe the limits of physics.
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If someone were to tell you that you can use a laser to literally freeze clouds of gas, you’d be forgiven for incredulously raising your brow. Lasers are just focused beams of particles used to measure or cut things, right? If you shine a light on gas, how exactly do you cool it down? Hit that cloud with a bunch of electrons and you’ll probably only disturb it more. How can you chill everything from rubidium to antihydrogen by using something normally associated with sci-fi weapons? Well, as it turns out, physics does some very strange stuff at atomic and subatomic scales, and once you can see the difference between temperature and heat, a lot of seemingly bizarre and confusing experiments suddenly become possible.

For example, consider the thermosphere where the International Space Station orbits. This atmospheric layer can register temperatures in the thousands of degrees but the ISS hasn’t been completely vaporized and the planet below is not a molten hellscape. Why? Because a temperature tells you the average rate of motion of particles while heat is the ability to transfer that energy. The thermosphere has such a low density, there simply aren’t enough particles to exchange heat, creating a realm similar to the surface of a red dwarf star. And with that in mind, we can consider how particle colliders smash atoms to generate temperatures in the trillions — yes, you read that right, trillions — of degrees.

The hottest known place in the universe since the Big Bang is a chamber inside CERN’s giant ion smasher because that’s where atomic nuclei move so quickly and have so much energy, they effectively melt into what’s known as a quark-gluon plasma, a frictionless, zero viscosity substance we think lies at the heart of neutron stars. Extrapolating the average entropy of this violent state of matter into something meaningful on a human scale gives us roughly 5.5 trillion C, the hottest we think matter can be and still actually exist. Conversely, slowing the motion of particles reduces entropy, cooling the affected matter. Stop motion altogether and you can say that the substance is now at absolute zero.

why you need lasers to reach absolute zero

Usually, when we want to cool something, we refrigerate it and over the decades, we’ve gotten very good at it. We can even turn flowers into porcelain-like icicles that can shred our skin cells if we touch them without protection by dipping those flowers in liquid nitrogen. So, why couldn’t we just dip whatever we want into a super-chilled gas or liquid? Well, while the kinetic energy of the atoms inside the objects would escape into the far less energetic coolant and significantly slow their motion, the system would eventually reach equilibrium above absolute zero. We can come close with this approach, but there will still be that pesky few extra degrees caused by quantum instabilities in and around atoms.

Keep in mind that all of these experiments are happening in time spans on which seconds may as well be decades, if not millenia, and on scales that see specks of dust barely visible to the human eye looming as large as planets. Such tiny systems are inherently unstable because the very oscillations of the fabric of space and time inject entropy into them. This is why scientists don’t think it’s possible for absolute zero temperatures to exist in nature and use 0 K as an ideal limit. Even the coldest known place in the universe, the Boomerang Nebula, still hovers one or two degrees above our entropy-annihilating goal because it takes real dedication to stop even subatomic particles in their tracks.

And that’s why we need to bring out lasers. Only direct pulses of photons and electrons firing for just a billionth, if not a quadrillionth, of a second, can interfere with the motion of electrons and quarks. In a way, these highly specialized lasers are like daycare workers trying to block unruly, running children and corral them in designated spots. If particles of a gas in our experimental chambers are like our hypothetical kids who were rounded up, sitting exactly where they’re told without making a peep, we’ve achieved absolute zero. But we’re not done yet. With this level of control, we can create some weird and exciting paradoxes, brand new arrangements we’re all but certain can’t occur naturally.

how to probe beyond absolute zero and why

Let’s say we have our hypothetical subatomic children run and jump in place. This creates an interesting paradox. While the kids wouldn’t be moving, maintaining absolute zero, the level of entropy among them would increase as they jump, which moves us past absolute zero into the artificial domain of negative temperatures. This is very different from temperatures below zero, which are arbitrary values in Celsius and Farenheit because those negative numbers refer to temperatures below the freezing point of water and below those most humans are likely to see in daily life, respectively. Our zeros and negatives refer to the amount of entropy at the most fundamental levels of existence as we understand it.

And this isn’t just a theoretical exercise, it’s been done with a gas of potassium atoms for just a tiny fraction of a moment to show us that it’s possible to use lasers and magnetic fields to create an arrangement in which entropy works backwards. If that sounds like it breaks physics, it really doesn’t. It’s just taking advantage of a loophole in the laws governing quantum mechanics and messing with our simplistic definition of how atoms and molecules should act at extreme limits of temperatures and electromagnetism. In other words, it’s not that we managed to use lasers to blow a hole in reality as much as we put our understanding of absolute zero and entropy to the test and found that it came up short.

All right, you may be thinking, this is all well and good but why go through all this trouble to cool things to absolute zero and beyond? What possible benefit would we have from these abstract experiments? Well, this is how we get faster computers, a safer internet, and better treatments for cancer just to give three examples. Quantum computing for better security and solutions to complex problems we can’t currently solve or automate due to the limits of our machinery rely on understanding how to keep complex quantum systems stable. If we can reverse entropy with lasers, we can run quantum algorithms and collect the answers before they decohere into noise, and more precisely target beams that kill cancerous tumors.

Manipulating matter at such degrees of precision and understanding why it behaves the way it does gives us more accurate and useful tools enabling everything from better GPS navigation, to faster download speeds, to more durable materials, to better medical imaging and treatment options. We can talk about how cool it is to use lasers as freeze rays a Batman villain would be envious of, and how weird atoms behave at the limits of known physics, say “damn, that’s crazy” and move on with our lives, but when we do that, we’re ignoring that we live at a time when we genuinely need to run these experiments and understand what happens to move forward as a civilization, and we sure as hell need to start thinking and acting like it.

# science // absolute zero / laser / particle physics

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