The one about Absolute Zero
Our universe is infused with various fundamental limits that we are not allowed to break. The most famous of all limits is the speed of light which states that nothing (with mass) can move through space faster than light or 186,000 miles per second. The smallest possible length is called Planck length at 1.616 x10^-35 meters. The smallest possible time is Planck time at 10^-44 seconds. And the coldest possible temperature is Absolute Zero in Kelvin scale or –273.15°C or –459.67°F. Now, you may think that -459F isn’t that small of a number compared to the other fundamental limits. Surely, there must be colder temperatures in the emptiness of space somewhere. And the answer is no. In fact, the coldest temperature in the known universe is right here in labs on Earth. And just like other fundamental limits, reaching a temperature of 0 K is not a limit of our technology but in all sense a true fundamental limit of our universe. This is why we have cooled things down to a billionth of Kelvin above absolute 0 and we can keep getting colder but never actually reach 0 K. To understand why, we need to first understand what temperature is.
HOT and COLD
When we sense hot or cold after touching something, what exactly is going on at the molecular level? All molecules, atoms, protons, and electrons have an intrinsic vibration. And it’s this intrinsic vibration aka kinetic energy that radiates heat. The faster the molecules move, the warmer the substance they make up. Even a jug of water harbors so much molecular motion that if you gently place a drop of ink on the surface, it will diffuse evenly throughout. Faster moving particles hit your skin with more force than colder particles that vibrate less. In theory, absolute zero is a point where all motion — and hence all heat — ceases to exist. Quantum physics says that absence of motion is impossible.
This kinetic energy of particles is also what defines the state of matter. Particles vibrate less in a solid than in a liquid but they do still vibrate. Even if you take a cold block of iron and zoom in, the individual iron atoms will be moving within their fixed structure.
As you heat up ice, the molecules of water gain more energy and break off from their crystal structure. If you continue to heat the liquid water, the molecules will gain even more kinetic energy till they escape as vapor. There’s actually another state after gas where individual electrons break off from atoms and you have ionized plasma or what’s found inside stars.
There’s a fifth state of matter called Bose-Einstein Condensate. This is a collection of atoms that have been cooled very close to 0 K so that they are barely moving. At this point, the atoms enter the same energy state and the whole group starts behaving as one single atom. When atoms fall in the same quantum states, they cannot be distinguished from one another. At that point the atoms start obeying what are called Bose-Einstein statistics, which are usually applied to particles you can’t tell apart, such as photons. At this state of matter we get cool effects like superfluid, superconductivity, and zero-resistance. There are two things that are special about Bose-Einstein Condensate:
1) It’s the only state of matter that so far has only been created in labs
2) It took scientists decades to figure out how to create this matter which won three physicists nobel prize in 2001. They trapped a gas of atoms and cooled it down using lasers. It seems counterintuitive but lasers pointed from multiple directions can be used to slow down atoms. Then in 2016, an AI created it from scratch in under and hour. “It did things a person wouldn’t guess, such as changing one laser’s power up and down, and compensating with another,” said ANU’s Paul Wigley, co-lead researcher.
SUPERCONDUCTIVITY
One result of chilling atoms past a tipping point is superconductivity. Chilled to extraordinarily cold temperatures — colder than -321F — certain metals conduct electricity with absolute no resistance. The current can flow undiminished in a loop forever. An electrical current consists of electrons streaming through a medium toward a positive charge. There are snags along the way. The conductive medium is like a lattice of atoms that may have irregularities and that vibrates with the energy of heat. What’s more, the atoms in the lattice, having given up loose electrons to the stream, are now positively charged and attracts the electrons tumbling past. Akin to the friction that stops a sliding object, these snags result in a loss of electrical energy. Resistance typically diminishes as a conductive material’s temperature drops. But superconductivity is different; when susceptible materials reach a critical cold threshold, resistance abruptly vanishes. To rush toward their destination with utmost efficiency, the streaming electrons organize themselves into pairs. As one electron passes near the positively charged atoms of the lattice, the atoms bend inward toward the electron, temporarily increasing the positive charge around it. This creates a weak attraction between the electron and the one just behind it, and they are drawn together through the lattice. Locked together, the electrons simply ride roughshod over the lattice, losing no energy. Because heat energy readily breaks them apart, such unusual pairings of electrons form only at a critical cold threshold — the tipping point for superconductivity.
In the 1980s scientists discovered that certain ceramics become superconducting at unexpectedly high temperatures — a leap forward that could make this ultra-efficient technology far more practical. Ordinary superconductors work only in the extreme cold achieved by applying liquid helium which is costly to produce. They are used in such sophisticated and expensive technologies are medical magnetic resonance imaging (MRI) but can’t say if and when we will be able to use is to transmit power to homes or run electronics. Elon Musk’s Hyperloop will be using two superconducting magnets. The new superconductors do their thing at temperatures as high as -211F. That’s still ultracold, but it’s achievable using liquid nitrogen, which is cheaply derived from liquid air. Of course, the field’s holy grail is the material that could eliminate resistance at room temperatures.
But even atoms chilled to the point of superconductivity or Bose-Einstein condensate state are still moving, however slow. If we keep cooling the matter, there will eventually come a point (theoretically) where the atoms are not vibrating anymore. That temperature is same for all objects and it’s 0 K (unlike melting point and boiling point which is different for individual objects). So, if you take all elements, compounds, molecules in the universe and create a chart that shows the temperature at which the individual atoms vibrate at 100%, 75%, 50%, 25%, up to 0 vibrations, you will see different temperatures for everything except 0 vibrations. That is the same for everything — the aptly named, 0 Kelvin. But we can never reach the point where there’s 0 vibrations.
In fact, it’s these very vibrations that give off infrared radiation. Since the primary source of infrared radiation is heat or thermal radiation, any object which has a temperature radiates in the infrared. Even objects that we think of as being very cold, such as an ice cube, emit infrared. When an object is not quite hot enough to radiate visible light, it will emit most of its energy in the infrared. For example, hot charcoal may not give off light but it does emit infrared radiation which we feel as heat. A light bulb emit visible as well as infrared light which is why it is hot to touch. The warmer the object, the more infrared radiation it emits. And that’s why we can see things even in dark with infrared vision — it’s our intrinsic light. Unless something is at 0 K, it will radiate in infrared frequency.
So, basically, every matter in the universe radiates energy. Some less, some more, but they all do. This is because all fundamental particles vibrate. Higher temperature = higher vibration; lower temperature= lower vibration. In fact, it is impossible to stop this vibration. Before we try and understand why particles can’t stop vibrating, there’s an important distinction between temperature and heat energy that we should clarify. Ever wondered why you can put your hand in a 400F oven without burning yourself when putting your hand in 100F boiling water would burn you instantly? Even though the oven has a higher temperature, it has a lower heat energy than the boiling water. And that’s because along with temperature, heat energy is also dependent on the number of particles and their density. The individual particles hiting your skin in the over carry more energy than the water particles. But the sheer number of water particles hitting your skin every second far exceed the particles in oven and as result carry much more energy. Temperature measures the average kinetic energy of particles and heat energy is represented by collective kinetic energy of all particles. For this reason, an iceberg has more heat energy than a hot cup of coffee.
GETTING TO ABSOLUTE ZERO
Now we know that heat is nothing but a result of intrinsic vibrations of individual atoms and molecules. This also means that there is no such property as “cold”. Cold is nothing but absence of heat. We can keep adding energy to a closed system (constant volume and pressure) and its temperature will keep rising. Given we have enough energy we can just keep doing this. There’s no theoretical upper limit to temperature. In contrast, to make something colder, we have to take energy away from the system. And eventually there will come a point where there isn’t any energy left to take out. What’s the point called? You guessed it — Absolute Zero or 0 K.
THIRD LAW OF THERMODYNAMICS
How do you take heat energy away from something? The simplest way would be to keep something colder next to it. The third law of thermodynamics states that heat always flows from hotter objects to colder objects. Though there’s some heat in the kitchen countertop on which rests a piping hot pan, it’s the pan that will heat the countertop and not vice versa. Heat will never transfer by itself to another hotter object unless we use energy to make it happen. This is how refrigerators work. The air in the fridge is colder than the food so it draws heat out of the food. When the fast moving particles of the food collide with the slow moving particles of cold air, they transfer some of their momentum and heat energy. As a result the particles of food get slower and colder and particles of fridge air become faster and warmer. This heat is then released from the back which is why the back of fridges are always hot. This heat was once inside your food.
If you think about how we cool things down, you will soon see the problem with cooling something to coldest possible temperature. Since we always need something colder to draw the heat out, we can’t cool to absolute 0 as there is nothing colder. No part of the universe could end up absolutely without heat energy and molecular motion because it would take heat from elsewhere. But if a pure substance that’s absolutely regular in its atomic structure could reach the theoretical temperature of absolute zero, it would also be devoid of all entropy. One definition of entropy is the number of configurations the jostling and twitching atoms and molecules of a substance may take — randomness. Because of this random and unpredictable behavior by molecules within a substance, energy transfer in the real world is not perfectly efficient; some energy invariably remains unavailable for useful work, another definition of entropy. But in absolute cold, molecules would cease their random jiggling; there’d be no heat to flow from one place to the other; activating molecules along the way. The crux of the third law of thermodynamics is that, as the temperature of a pure, perfectly regular substance falls to absolute zero, its entropy also approaches absolute zero. In a nearly fixed, inert substance randomness subsides. And quantum physics prohibits this randomness or entropy from being zero. But atoms in superconductive state come pretty close to zero entropy.
But there is another way. Temperature can also be controlled by PRESSURE and VOLUME. Increase pressure and temperature increases. Decrease volume and temperature increases. So, let’s say we have helium gas cooled at 1K. Instead of finding a colder object to draw the rest of the heat out of we can actually expand the gas and it should get colder. This is the exact physics behind the naturally occurring coldest place in the known universe — Boomerang Nebula. The nebula has a temperature of 1 K. The gas in the nebula is moving as extreme speeds explaining how it got so cold.
Lastly, when objects we are trying to cool are at nanoscales then we can use lasers like we discussed earlier. And that’s why the coldest place on Earth or in entire known universe, is in MIT and CERN labs in forms of Bose-Einstein Condensate. We have managed to cool atoms down to 1 billionth of Kelvin above Absolute 0. But it would take infinite amount of energy to cool something to absolute 0. And the reason for that lies in quantum mechanics. So, remember heat is the vibration of these particles. As a particle is cooled, its vibration slows down.
Absolute 0 or -273.16 C is that temperature where all intrinsic vibration of all particles comes to a complete stop.
Heisenberg’s Uncertainty principle states that the position and the velocity of an object cannot both be measured exactly, at the same time, even in theory. And if a particle is not moving then we can know exactly where and how it is. That’s prohibited according to the laws of quantum physics and I guess in a way, the absolute limit of Absolute 0 ensures that.
And that’s all that you would ever need to know about Absolute 0. Hope you found this post interesting as well informative. :)
