# 10 physical facts that you should have learned in school, but may not have learned

Original author: Sabine Hossenfelder
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### 1. Entropy measures probability, not disorder

The idea that entropy is a measure of disorder does not help at all in understanding the question. Suppose I make a dough, for which I break an egg and pour it into flour. Then add sugar, butter, and mix them until the dough is smooth. What state is more orderly - a broken egg and butter on flour, or the resulting dough?

I would say that the dough. But this is a state with greater entropy. And if you choose the option with an egg on flour - what about water and oil? Is entropy higher when they are separated, or after you shake them violently to mix? In this example, the entropy is higher in the variant with separated substances.

Entropy is defined as the number of “microstates” that give the same “macrostate”. Microstates contain all the details about the individual components of the system. The macrostate is characterized only by general information, such as “divided into two layers” or “on average homogeneous”. The ingredients of the dough have many different states, and all of them will turn into a dough when mixed, but very few states can divide into eggs and flour when mixed. Therefore, the entropy test is higher. The same works for the example with water and oil. They are easier to separate, harder to mix, so the separated version has higher entropy.

### 2. Quantum mechanics is applicable not only to small distances, it is simply harder to observe at large distances.

In the theory of quantum mechanics there are no restrictions, according to which it would work only at short distances. It just so happens that the large objects observed by us consist of many smaller ones, whose thermal motion destroys all typical quantum effects. This process is called decoherence, and it is because of it that we usually do not see the manifestations of quantum mechanics in everyday life.

But quantum effects were measured in experiments stretching hundreds of kilometers, and they can work at large distances in a fairly stable and cold environment. They can even extend to the entire galaxy as a whole.

### 3. Heavy particles decay not to the state with the minimum energy, but to the state with the maximum entropy

Energy is conserved. Therefore, the idea that any system is trying to minimize energy does not make sense. The reason why heavy particles disintegrate when they can is because they can. If we have one heavy particle (say, a muon), it can decay into an electron, a muon neutrino, and an electron antineutrino. The opposite process is also possible, but it requires that three decay products be gathered in one place. Consequently, the probability of it is small.

But it's not always the case. If you put heavy particles in a fairly hot “soup”, then the synthesis and decay can reach equilibrium, in which there will be a non-zero number of heavy particles.

### 4. The lines in the Feynman diagrams do not depict the paths of motion of the particles, they are just auxiliary drawings for complex calculations.

Periodically, I receive emails from people who notice that in many Feynman diagrams lines are assigned impulses. And since everyone knows that it is impossible to measure the location and momentum of a particle with arbitrary accuracy at the same time, there is no point in the lines of motion of the particles. From this it would seem that particle physics is wrong!

But particle physics is fine. Feynman diagrams are different, and those that denote impulses are designed for pulsed space. In this case, these lines are in no way connected with the paths of motion of the particles. At all. This is just a way to depict some kinds of integrals.

In some Feynman diagrams, the lines actually depict possible paths along which a particle can go, but in this case the diagram does not say what the particle actually does. For this it is necessary to carry out calculations.

### 5. Quantum mechanics is non-local, but it cannot be used for non-local information transfer.

Quantum mechanics gives rise to non-local connections, which are more quantitatively stronger than connections in non-quantum theories. This is what Einstein called “frightening long-range action”.

Alas, quantum mechanics is also essentially random. Therefore, even though we have these amazing non-local connections, they cannot be used to send messages. Quantum mechanics is actually fully compatible with Einstein's speed limit of light.

### 6. Quantum gravity begins to play a role in situations with high curvature, and not with short distances.

If we estimate the strength of the effects of quantum gravity, we can find that they cease to be negligible in the case when the curvature of space-time is comparable to the value inverse of the square of a Planck length. This does not mean that these effects can be seen at distances close to the Planck length. It seems to me that confusion arises because of the term “Planck length”. The Planck length is a unit of length, not the length of something concrete.

It is important here that the statement “approximation of curvature to the inverse square of a Planck length” does not depend on the observer. It does not depend on the speed of your movement. The problem with the idea that quantum gravity begins to play a role at short distances is that it is incompatible with the Special Theory of Relativity.

In SRT lengths can be reduced. For a fairly fast moving observer, the Earth will look like a pancake with a width less than the Planck length. And this will mean that we must either notice the effects of quantum gravity, or SRT is incorrect. The evidence speaks against both assumptions.

### 7. Atoms do not expand with the expansion of the Universe. Like Moscow

Expansion of the universe is incredibly slow and has a very weak effect. It is not affected by systems that are tied together with interactions that exceed expansion. The systems that the expansion can tear apart are larger than clusters of galaxies. The clusters themselves are held together by gravity. Like galaxies, solar systems, planets and, naturally, atoms. The latter are held together by atomic interactions, far more powerful than the expansion of the Universe.

### 8. Wormholes are science fiction, and black holes are not.

The evidence from black hole observations is extremely convincing. Astrophysicists confirm the presence of black holes in many ways.

The simplest way is to calculate how much mass you need to collect in a certain amount of space in order to get such a result of the movement of nearby objects, which is observed in reality. This in itself, of course, does not indicate whether a dark object that affects visible objects has an event horizon. However, you can see the difference between the event horizon and the solid surface, exploring the radiation emitted by a dark object. Also, black holes can be used as extremely powerful gravitational lenses to check their conformity with the predictions of Einstein's General Theory of Relativity. Therefore, physicists are waiting with great interest for data from the Telescope event horizon [a project uniting many radio telescopes around the world to study the central black hole of the Milky Way / approx. trans. ].

Perhaps the most important thing that we know is that black holes are a typical final state of the collapse of certain types of stars. In GR, they are easy to obtain and hard to avoid.

On the other hand, wormholes are deformations of space-time, the occurrence of which as a result of natural processes is unknown to us. Also, their presence requires negative energy that no one has ever seen, and about the existence of which many physicists have big doubts.

### 9. In a black hole can fall in a finite time. It will just look like it takes forever.

When approaching the event horizon, time slows down, but this does not mean that you end the fall before you reach the event horizon. This slowdown will be seen only by an observer located at some distance. You can calculate how long it will take to fall into a black hole by the hours of a falling one. The result is final. You can fall into a black hole. It's just that your friend outside will never see it.

### 10. In the Universe as a whole, energy is not conserved, but this effect is so small that it cannot be detected.

I said that energy is conserved - but this statement is true only in a certain approximation. It would be completely true in a universe in which space would not change over time. But we know that space expands in our Universe, and this expansion violates the law of conservation of energy.

However, this violation is so tiny that it will not be noticed in any experiment conducted on Earth. To notice it, you need to observe for very long over very long distances. If this effect were stronger, we would have noticed a long time ago that the Universe is expanding! So do not blame the Universe for your electricity bills, but simply close the window when you turn on the air conditioner.

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