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

Original title: Halloween is still cos traditional ghosts? Let's see the physical version of "demons"!

It's Halloween again,

All kinds of demons and ghosts are also stirring.

We do physics,

Of course, they didn't believe this.

We have a physical version of "boogeyman."

Demon/ DEMON First of all, I would like to introduce to you two members of the "Demon" clan representative team.

One of them can understand the past and the future, and the other can control a single molecule. As members of the famous "Four Divine Beasts" of physics, they are well-known to women and children. They are Démon de Laplace and Maxwell's demon!

Laplace demon is a hypothetical creature proposed by French mathematician Laplace. It knows the position and momentum information of all particles in the universe. In classical mechanics, it can calculate the information of any particle in the universe at any time through Newton's equations of motion, that is, it knows the past and future of the universe.

Laplace... Sorry for the confusion.| Source: Baidu Image

Laplace demon| Maxwell demon is also a hypothetical creature proposed by British physicist Maxwell. It is special in that it can control individual molecules, separating molecules in an insulated container according to their motion speeds, thus reducing the entropy of the whole system. Maxwell's demon power comes from his own "demon power", which consumes no energy in the process of controlling molecules, so the whole system realizes spontaneous entropy reduction, which violates the second law of thermodynamics.

Of course, physicists now have cages ready for each of the two demons, called Heisenberg uncertainty principle for Laplace demon and information entropy theory for Maxwell demon. Limited by space, this is not to be expanded, interested friends can learn by themselves.

Magic/ MAGIC is going to introduce the representative of the "magic" family. It has been popular in the field of materials in recent years. Related articles have been published in top journals such as Nature and Science for many times. A simple deflection opens the door to a new world. It is magic-angle multilayer graphene!

18 years of Nature magazine top ten people cover picture, ripple shape is a hint of magic angle graphene Before understanding magic angle graphene, we first start with two-dimensional materials.

We can use points, lines, surfaces, and volumes to represent zero-dimensional, one-dimensional, two-dimensional, and three-dimensional materials, respectively. That two-dimensional material is a material with only one surface and zero thickness. Of course, the actual material can not be zero thickness, usually a material only a few atomic layers thick (a few nanometers), can be called a two-dimensional material. Graphene is a typical two-dimensional material.

multilayer graphene structure| What are the characteristics of @ graphene big health two-dimensional materials? Atoms in the same layer are usually covalently bonded to each other, while layers are bonded to each other only by weak van der Waals forces. So it takes only a small amount of energy to break the bonds between layers and peel them off into a few layers of material. Geim et al. of the University of Manchester, England, who first began to study graphene, used tape to tear graphene directly from graphite crystals.

Think of two-dimensional material as a pie stacked together. We can easily remove one or more layers, but it takes some force to separate a sheet.

stacked pie| Under normal circumstances, the positional relationship between the atoms of the upper and lower layers is fixed, and the angle between the layers is also fixed. However, when we get single-layer graphene, we can artificially stack it to get double-layer and multi-layer graphene.

Five-layer corner graphene In the process of "stacking big cakes," if we turn it around, so that there is an angle between the upper and lower layers, and then add a low temperature, good guy, the stacked multi-layer graphene superconducting, and there is hope to become a high temperature superconductor! Not only superconductivity, but also correlated insulation states, quantum anomalous Hall states, and many other novel states appeared!

A simple twist angle, like magic, introduced the study of two-dimensional materials into a new world, which is probably the origin of "magic horn."

Ghost/ GHOST In quantum field theory, there are a series of states that are given the name "ghost": ghost, ghost field, ghost particle, and gauge ghost. Although they have "physical" names such as "particle" or "field," they are themselves non-physical and only generated for computational convenience.

Ludwig D. Fadeyev, a theoretical physicist of the former Soviet Union Faddeev illustrates the meaning of "ghost" with an example: in electrodynamics, the electromagnetic field, the photon field, has four components, but there are actually only two photon polarizations, so two of these four components are non-physical, and the effects of non-physical terms can be cancelled by introducing "the ghosts."

For example in electrodynamics, in order to maintain manifest Lorentz invariance, one uses a four component vector potential, whereas the photon has only two polarizations. Thus, one needs a suitable mechanism in order to get rid of the unphysical degrees of freedom. Introducing fictitious fields, the ghosts, is one way of achieving this goal.

In fact, in physics there are not only known "ghosts," but also divided into "good ghosts" and "bad ghosts."

A ghost is a good ghost if it makes a given theory self-consistent, and a bad ghost if it doesn't. A "good ghost" is virtual, whereas a "bad ghost" requires theory to admit an unwanted non-virtual state, e.g. Pauli-Villars ghosts introduce particles with negative kinetic energy, which are clearly "bad."

Among the many "ghosts," one of the more classic is the ghost field, also known as Faddeev-Popov ghosts, whose name "Faddeev" is partly derived from the theoretical physicist exemplified above. The introduction of ghost field solves the problem of consistency in path integration of quantum field theory, and it is also virtual, not an actual field.

Strange/ STRANGE After "demon,""devil" and "ghost," it is finally the turn of "strange."

If the scope is not limited to physics, there is a saying of "strange number" in various programming questions. However, the physical noun with "strange" has been thought about for a long time, and it seems that there is really no such thing. For this reason, Xiao Bian also tried to search with possible terms such as "strange field" and "strange force" until he got the following picture when searching with "strange son".

OK, give everyone a science popularization, clam (chīng) son! It's delicious.

After a useless search, Xiaobian also gave up the idea of looking directly.

Let's just change our thinking.

In fact, many nouns have a variety of translations, such as "strange particle," which is commonly translated as a strange particle in China, but it is not impossible to translate it into a strange particle. Along this, there is also a singular number "strangeness" in quantum numbers, which can be translated into strange numbers, right? For example, the devil particles "The devil particles" can also be called monsters. From this point of view, there are still many "strange" in physics.

(There are special rules for the translation of physical nouns. The above is purely a joke. Please don't take it seriously.)

Let's open our minds again. Is a ghost such an inconvenience?

The long history of physics is full of strange and unexpected discoveries.

Poisson himself calculated to refute the wave theory of light and found that the light source would form a bright spot behind a baffle, which was obviously counterintuitive; but people found that such a bright spot would really appear through experiments! This argument, which refuted the wave theory, became a powerful proof of the wave theory, and from then on the name "Poisson bright spot" will always be left in textbooks.

Poisson bright spot Rutherford bombarded gold foil with alpha particles, originally intended to observe the jujube cake model of atoms, but unexpectedly discovered the existence of atomic nuclei, overturned the jujube cake model and established the classical nuclear structure model of atoms.

Planck, the nuclear structural model of the atom, for the first time postulated that energy was absorbed and emitted in a discontinuous manner in order to explain blackbody radiation, although he himself found it hard to believe and even spent years trying to disprove the idea. But it was this discovery that opened the door to the quantum world.

After Planck, quantum mechanics flourished, and this is the biggest "strange" in physics. Energy is discontinuous, strange; particle position and momentum cannot be determined at the same time, strange; matter is both waves and particles, strange. Such a theory from head to tail is actually one of the two cornerstones of modern physics. Even various "quantum products" can be seen in daily life. Isn't this also a kind of "strange"?

Physics, in fact, itself is a subject of discovering "strange" and studying "strange." After study and explanation, the old "strange" gradually became known and was no longer "strange"; the new "strange" was discovered and attracted people to study. So repeat, keep exploring, keep seeking truth.

In addition to physics, there are many fields where "demons and ghosts" exist. What do friends know?

References:

1. https://encyclopedia.thefreedictionary.com/Ghost+(physics)

This article comes from Weixin Official Accounts: Institute of Physics, Chinese Academy of Sciences (ID: cas-iop), author: Le Zi Superman

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