Shulou(Shulou.com)11/24 Report--

Original title: "0K, not OK"

Our universe is full of basic boundaries that cannot be broken. For example, like the most famous one: the speed of a mass substance cannot exceed the speed of light, which is about 3 × 108 meters per second. Of course, not only that, the Planck length is the smallest possible length, the value is 1.616 x 10-35 meters, the Planck time is the minimum possible time: 10-44 seconds, and the coldest temperature that can exist is absolute zero degrees Celsius (Kelvin, the unit of temperature), that is,-273.15 ℃ (degrees Celsius), or-459.67 ℉ (degrees Fahrenheit). -273 ℃ does not look very small compared to the previous figures. Is there no place in such a large universe where the temperature is lower than this?

In the knowable universe, the lowest temperature exists in a laboratory on this planet; and like other basic boundaries, a temperature of 0K is theoretically-not technically, impossible. At present, we can keep lowering the lowest temperature we can reach to 1/1000000000 Kelvin or less, but we can never reach 0K.

Before we explain why, let's talk about what temperature is.

Cold or hot what happens at the micro level when we feel cold or hot by touch?

All molecules, atoms, protons or electrons have an intrinsic vibration, which is also called kinetic energy and radiates heat. The faster the particles move, the warmer the matter becomes. When you drop a drop of ink into a pot of water and the ink spreads evenly, you will find that the pot of water also carries so many molecular movements. Particles that vibrate faster will hit your skin harder than particles that vibrate more slowly. In theory, absolute zero is a state in which all motion, that is, all heat, does not exist.

But quantum mechanics denies the possibility of such a situation.

In addition to temperature, the kinetic energy of particles also determines the properties of matter. Particles in solids vibrate less than liquids, but they are still vibrating-even in cold iron, a single iron atom vibrates in its fixed structure.

When heating ice, the molecules in water gain energy and break away from the crystal structure (to become liquids); as they continue to heat liquid water, water molecules can gain more kinetic energy to escape into steam (gas); when electrons are separated from atoms, matter is ionized into plasma, a state that can be found in cosmic stars.

In addition, matter has a fifth state, Bose-Einstein condensed matter, which consists of a mass of atoms that cool very close to zero and barely move; at this point, these atoms reach the same energy state and behave like an atom. When all atoms are indistinguishable from each other in the same quantum state, they follow the Bose-Einstein statistical law and apply to indistinguishable particles, including photons. In this state, we will observe amazing phenomena such as supercurrent, superconductivity and non-resistance.

There are two characteristics about Bose-Einstein condensates:

1. This is a state that currently exists only in the laboratory.

two。 It took decades for scientists to acquire this state of matter, and it also won three physicists the 2001 Nobel Prize. Counterintuitively, physicists use lasers in multiple directions to capture and cool a cluster of atoms.

Superconducting

A phenomenon occurs when atoms are cooled below a critical value-superconductivity. For example, when the temperature is below-196 ℃, the resistance of some metals decreases to zero: electrons flow in the medium to form an unattenuated current in a cycle.

Usually, electrons are hindered after they form an electric current. The atoms in the conductive medium are arranged into a lattice and move in an irregular thermal motion; the atoms in the lattice release electrons to the current and are positively charged, attracting electrons to roll past. Just as the friction on the plane acts on the slider, such hindrance (represented by resistance) loses the energy of the current, and when the temperature decreases, the resistance of the conductor decreases significantly.

But when superconductivity occurs, the phenomenon is completely different: when a material reaches a certain degree of cold, its resistance disappears immediately. In order to rush to the end point immediately, the electrons in the current will combine in pairs-when an electron flows through the lattice, it will cause the surrounding positively charged atoms to bend to it, which in turn will attract neighboring electrons to combine with it, and when the two electrons are locked together, they will travel freely through the lattice without losing energy. Such pairs of electrons can only be formed below the critical temperature of superconductivity, and once beyond this temperature, the heat is enough to separate them.

Ordinary superconductors can only work at very low temperatures and require very expensive liquid helium. Superconductivity is generally used in very complex and expensive devices, such as medical nuclear magnetic resonance imaging (MRI). But it is hard to say when and whether we can come into our daily life. Two superconducting magnets will be used in Elon Musk's super transport device hyperloop.

In the 1980s, scientists found that some special ceramics could become superconducting at very high temperatures, and this technique could become very useful with only a small step forward. The new superconductor can operate at temperatures as high as-135 ℃, which is still very low, but can be achieved with cheaper and readily available liquid nitrogen. Of course, superconductivity at room temperature is still the ultimate goal for scientists.

But even in superconductivity or Bose-Einstein condensed matter, atoms are still moving slowly. In theory, if we continue to cool matter, there will be a point where the atom no longer vibrates, and the temperature of this point is different from the melting point or boiling point, and it is the same for all materials, which is 0K. That is to say, for all elements, compounds, or molecules in the universe, create a temperature-vibration-related table for them, corresponding to 100%, 75%, 50%, 25%, etc. As long as the vibration is above 0, the temperature of different materials is different; but 0 vibration is the same for all matter, and they have a common definition-0 Kelvin, a state that we can't reach.

In fact, the vibration of this limit state (Bose-Einstein condensed matter) also radiates infrared waves. Infrared waves come from thermal radiation, so all objects with a temperature of not zero will emit infrared waves, including ice cubes that we think are already very cold, and objects with temperatures not high enough to produce visible light are emitting infrared radiation. The higher the temperature of an object, the more infrared waves it radiates-- that's how night vision works, because infrared radiation is our "intrinsic light"-- unless its temperature is 0K.

Infrared thermal imaging: different colors correspond to different temperatures. All matter in the universe is radiating energy, either more or less, but it must exist, which stems from the vibration of all elementary particles. High temperature is high vibration frequency, low temperature is low vibration frequency, it is impossible to prevent such vibration.

Before we understand why particles cannot stop vibrating, we need to distinguish an important difference between temperature and thermal energy.

Have you ever wondered why putting your hand in a 200 ℃ stove is less harmful than putting it in 100 ℃ boiling water? This is because even if the temperature in the furnace is very high, the heat energy is lower than that of boiling water. The heat energy of an object is not only related to temperature, but also related to the number and density of particles contained. A single particle in the furnace has more energy than boiling water, but the number of water molecules hitting your skin per second is much larger than the particles in the furnace. It also carries more energy. Temperature represents the average kinetic energy of molecules, while thermal energy reflects the sum of the kinetic energy of all particles in matter, so an iceberg has more heat than a cup of coffee.

Although the air in the stove is hotter than the boiling water in the pot, the water has a higher energy density and carries more heat to absolute zero. We now know that heat is only the result of the intrinsic vibration of some atoms and molecules. This means that in fact, "cold" is not a property of matter, but reflects less heat of matter.

We can raise the temperature of a closed system by adding energy to it (constant volume and pressure), as long as we have enough energy. In theory, there is no upper limit of temperature. Conversely, when we need to cool a system, we need to "take" energy from it until at some point the system has no energy left. This is the absolute zero point.

How does the third law of thermodynamics take away the heat of an object? The easiest way is to put a colder object next to it. The third law of thermodynamics states that heat always flows from objects with higher temperatures to objects with lower temperatures. For example, if there is a relatively hotter pan on the countertop of the kitchen, the pan will heat the table instead of the pan. Heat never flows to hotter objects-unless we use energy to do so. That's what the refrigerator does. The air in the refrigerator is colder than food to get energy, and when faster-moving food particles hit slower particles in the air, they exchange some momentum and energy. The movement of food particles slows down and cools, while air particles move faster and hotter, and the heat of the air is released on the back of the refrigerator, which is why the back of the refrigerator is usually hot, and part of the heat comes from your food.

But when we want to minimize the temperature of the object in this way, the problem arises. What if there is nothing colder? There is no place in the universe without motion and heat, because it always absorbs heat from somewhere else. If every atom of a pure substance is completely kept in its crystal structure, then it can reach the theoretical absolute zero, when its entropy is zero.

One of the definitions of entropy is to describe it as the number, or randomness, of atoms and molecules that are crowded with each other in matter. Because of the random and unpredictable nature of matter, the transformation of energy is not completely effective, and entropy determines that part of the energy can not be "useful". However, when it is absolutely cold, the molecules will stop vibrating randomly, the flow of energy will disappear, and there will be no corresponding excited molecules. The key to the third law of thermodynamics is that when the temperature of a perfect, pure crystal drops to 00:00, its entropy is zero. The confusion of a nearly stable, inert substance will also subside.

But quantum mechanics does not allow chaos or entropy to be zero-but it can be very close to zero, such as the entropy of atoms in a superconducting state.

In addition to heat transfer, temperature can also be controlled by pressure and pressure: increasing pressure and decreasing volume can increase temperature. Let's assume that there is already helium cooled to 1K. In addition to looking for cooler media to drain energy, we can also cool it by expanding it. This process is taking place in the coldest place in the known universe, the Darts Nebula (Boomerang Nebula), which has a temperature of 1K. The gas in the nebula explains why it has become so cold at an extremely high speed.

Closest to Earth's Darts Nebula, 5, 000 light-years away, MIT and CERN labs are trying to use the lasers we mentioned earlier to minimize the cooling of nanoscale objects, which are in the form of Bose-Einstein condensation. This may be the lowest temperature in the whole earth or even in the known universe-1/1000000000 Kelvin above zero. According to the theory of quantum mechanics, it takes an infinite amount of energy to reduce it to 0K.

Absolute zero or-273.15 ℃ is the temperature at which all intrinsic vibrations of all particles stop.

The Heisenberg uncertainty principle states that the position and momentum of an object cannot be accurately determined at the same time, even in theory. But when a particle stops moving, we know its exact position and state, which is not allowed by the principle of quantum mechanics, which in turn proves that absolute zero cannot be achieved.

Original text link:

Https://rashmi-singh1789.medium.com/the-one-about-absolute-zero-30f1c1f78318

This article comes from the official account of Wechat: Institute of Physics, Chinese Academy of Sciences (ID:cas-iop), author: The Basic of Everything, translator: zhenni, revision: Dannis

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