Reversal and irreversible: Why can't we go back to the past?

When you see this title, many friends may think of relativity, but you need to know that this problem can also be explained from another perspective.

Let's look at an animation first.

Can you find anything special about this animation?

Obviously, the process of this animation violates life experience. Smoking always spreads and it is impossible to gather automatically, so the animation above must be upside down.

The following process is true. Smoke particles continue to spread to four weeks, in line with observation experience.

The above is the concept formed by all of our daily observations. We all believe that proliferation is spontaneous, and re -gathering is impossible.

So, how to explain this problem from the perspective of physics?

Physics believes that if there is a way to eliminate the impact caused by the original process and fully restore the system, then the process is reversed; Then the process is irreversible.

Practice shows that once the smoke particles diffuse, it is impossible to re -gather and recover.

Therefore, the diffusion process is irreversible!

However, why is the diffusion irreversible?

The diffusion phenomenon is experienced by a macro system containing a large amount of micro particles. In physics, this system is a thermodynamic system. According to molecular motion, a large number of molecules inside the thermodynamic system do no regular thermal exercise in the non -stop.

Since it is an irregular exercise, as the name suggests, you can't regulate the movement of each molecule, and you cannot return each molecule to the initial state, so you cannot restore the system.

Even if you are God himself, you do n’t have so many hands, eyes, and brains to order so many elements to act in accordance with your instructions! In other words, as long as there are a large number of particles -such as Afgardro's constant particles, you can only let them move in chaos.

It is not only the process of spreading the process, but any process that contains a large number of individual systems. Once there is no process, there is no process to restore it. In other words, the actual process of all macro system experience is irreversible!

In short, all macro processes are irreversible!

Warm reminder: "The actual process of the macro system experience" is often referred to as the "macro process".

The characteristics of the macro system that are giving a long time giving people passing the passage of time. Therefore, irreversibleness indicates that time is unidirectional, just like the arrow shot out, there is no return.

The first two sentences of Li Bai's "Will into the Wine": "The monarch is not seen, the water of the Yellow River comes up, and runs to the sea without returning. The irreversibility of this time of time.

So, what are the physical laws behind this? The answer is: the second law of thermodynamics.

There are two versions of thermodynamics in the second law of thermodynamics. Among them, the version proposed by Clausis said: heat cannot be automatically transmitted from low -temperature objects to high -temperature objects without other impacts.

You may ask: What is said here is only an irreversible heat transfer process. How can it explain that all macro processes are irreversible?

Indeed, the expression of the second law of thermodynamics is a bit strange, and its two versions are explained to specific phenomena. This is rare in physical laws.

This seems very unreasonable, a bit like the legal provisions in Zhang Tianyi's call story "Dalin and Kobayashi". Its Article 38864 stipulates that if Pippi picked up Kobayashi on the ground, Pippi can pick up Kobayashi on the ground. Kobayashi is owned by Pippi.

However, for the second law of thermodynamics. Although its expression lacks generality, it is reasonable. Because the irreversibility of all macro processes is equivalent to each other!

In other words, as long as you admit that a macro process is irreversible, then all other macro processes are irreversible. For example, from the "irreversibility of heat conduction", "the irreversibleness of the heat" can be launched, and vice versa.

Therefore, saying that a macro process is irreversible, it is equivalent to saying that all macro processes are irreversible. Because of this, Clausus's expression is good, Kelvin's expression, or even tens of millions of other expressions, in fact, it shows a basic fact: all macro processes are irreversible. This is what the Second Law of thermodynamics wants to say.

Of course, if you ask again: How did the second law of thermodynamics come from?

The answer is: it is a basic assumption of physics, summarized from physical experiments and experience, and accepted full tests, so it is correct.

If you explore the second law of thermodynamics from a micro perspective, you will find that it can be explained by another physical concept, it is entropy! But in this way, you will face another assumption -wait for the probability.

The probability assumption tells us that the micro -state it contains any macro system appears at the same probability. Therefore, for a state of a macro system (that is, macro state), the more micro -state it has, the greater the chance it appears.

The so -called micro -state refers to a specific distribution of all members in the system.

For example, 30 people are surrounding the side of a square lake to see the scenery, which is a macro state. Where is the position of 30 people, corresponding to a micro -state. For example, at a certain moment, there are 3 east, 8 west, 10 south, and 9 on the north; and another moment, there are 10 east, 2 on the west, 8 south, and 10 on the north. These different moments distribution is a micro -state.

Boltzmann believes that all micro -states are equal rights, and a macro state always traverses all the micro -states it has at the same probability. Therefore, the more macro -state in the micro -state, the greater the probability of appearing.

This makes it always evolved in the direction of any macro system in any macro system.

Based on this micro -state, the entropy defined by Boltzmann is based on this, and the entropy is positively related to the micro -state number. Therefore, the greater the macro state of entropy, the greater the opportunity. In other words, the system in the change is always evolved in the direction of the increasing entropy. This is the principle of entropy increase.

Therefore, the irreversible constraints of the second law of thermodynamics on the macro process can also be understood as the role of entropy increase. In other words, the second law of thermodynamics is essentially the principle of entropy increase.

Well, now I know, the macro phenomenon is irreversible, which is the result of the domination of a physical law.

So, what about the reversible process? Is there no reversible process? If this is the case, isn't this word unnecessary?

The answer is: Generally speaking, the reversible process only exists in theory for the macro system, that is, the quasi -static process without dissipation. The straightforward thing is: without friction, infinite slow process. However, this topic is a bit far away, and this article is temporarily pressed.

But if you don't pay attention to the macro system, in fact, the reversible process exists!

So the question comes, not the system of the macro system?

As long as the system does not have a micro component, or it does not need to consider it is composed of a large number of micro particles, then the system is not a macro system.

For example, if a few particles, or a mass or rigid body without internal structure, then it is not a macro system, and its movement is reversible.

When you observe a flea jumping, its movement looks reversible, because you do not pay attention to the details of the energy consumption in its body, it is a quality point without structure in your eyes.

In fact, a major or rigid in Newtonian mechanics, if we shoot its movement into a video, and then reverse, you will not have any special feeling.

Like the following, is it upside down? You can't know.

Or like the following, you probably wo n’t doubt whether the video is upside down.

Similarly, if the exercise of the electromagnetic effects of the electromagnetic effect is carried out in reverse, you will not feel that there is anything wrong.

Not to mention optics, since the light path is reversed, the light reflection is not bad at all. If it is used as an incident light, is there any problem.

This means that in these circumstances, you can't realize whether the time is in that direction -because the direction of the positive and negative direction is not good or bad for you. If you look at these phenomena fully, you can experience the illusion of a time backflow.

Therefore, the processes experienced by the mass or charged particles meet the time counter -degeneration, which you can see from those famous equations. For example, in the fluctuation equation of Newton's equations and electromagnetic waves, the equation remains unchanged if the time is reversed. We often say that force is the cause of changing the state of the object. Without external forces, the motion status of the mass point will not change. This shows that the mass point does not change its spontaneous change. Because it cannot change spontaneously, the process it experiences does not matter -if you want it, it will be there. It has no tendency itself.

But the macro system is different, you do not use it, it will also change spontaneously in one direction! In other words, it does not need to accept any instructions at all, it has its own choice! If you want to change its direction, then you have to pay an additional price.

Of course, you may say that in that swinging experiment, due to resistance, the range of swinging may only be smaller and smaller. If it is found that it is getting larger and larger in the film, it must be unreasonable. Determine the film at this time.

That's right! But the above examples do not involve changes in the width. In other words, frictional resistance is small or neglect.

What is the frictional resistance? It is a kind of scattered force that can cause heat to cause heat. Therefore, as long as the friction is considered, it involves heat science, and it is no longer pure Newtonian mechanics, electromagnetics and optics, and the process is no longer reversible. On the contrary, as long as it does not involve thermalology, there will be no problems in all processes.

In fact, according to the aforementioned, a system composed of a large amount of particles is a macro system, or thermodynamic system. The friction involves the interaction of a large amount of particles at the contact surface, so the friction work itself is a macro -thermodynamic process.

In short, as long as it is a system composed of a large number of particles, it will inevitably lead to heat phenomenon, and all the processes you have experienced are irreversible. This is the case, whether biological or non -creature.

Now you know, why can't you go back to everything around you, including yourself?

Simply put, because the things you and you see are too big! If you can shrink to several molecules, you can return to the past. But in that case, your structure is too simple. Don't think that you will become a Maxwell little demon, because you will be unable to think about it, and nothing to do.

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Source: University Physics

Edit: lychee jelly

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