A basic contradiction in physics

​Why does the process that takes place in nature develop in only one direction? For example, why can't we heat up a cup of coffee in the refrigerator or stop a drop of ink from spreading spontaneously in the water?

Why does the process that takes place in nature develop in only one direction? 

For example, why can't we heat up a cup of coffee in the refrigerator or stop a drop of ink from spreading spontaneously in the water? 

This is a problem that has perplexed many generations of physicists. It stems from the incompatibility of physical laws, especially the incompatibility between the laws that govern the behavior of macroscopic and microscopic systems. 

Macroscopic systems can be seen with the naked eye, they are made up of a large number of atoms and molecules. 

The microscopic system represents a very different world, which we can't see, but the behavior of each atom or molecule can be described. 

Physicists can simply explain why the process of macroscopic systems cannot be reversed spontaneously. 

This comes down to the second law of thermodynamics, the core of which is the energy properties of macroscopic systems. 

This law provides a standard for predicting the direction of spontaneous processes through the concept of entropy, a measure of material order. 

For example, liquids are less ordered than crystals, while gases are less ordered. 

Substances that are hotter or more dispersed have higher entropy. 

Simply put, entropy always increases, systems always become more disordered in the spontaneous process, and they cannot go backwards unless we provide energy. 

However, when looking at the individual atoms and molecules that make up the microscopic system, there is a different set of physical laws. 

But they don't explain where the process in the system is bound to go. 

Matter and process are the same, but when studying from a macro and micro point of view, the results may be contradictory. 

This is obviously a problem. 

Equilibrium and gradient. 

We can imagine an ideal pendulum. 

It can swing back and forth all the time without friction. 

If the motion is recorded and then played upside down, it looks no different. 

This is a time reversible process, and the motion of the pendulum is symmetrical in time reversal (that is, time reversal). 

But the heat lost from a cup of hot coffee never flows back. 

The heat inevitably flows from the hot coffee into the colder air, and when the coffee and the surrounding air have the same temperature, the heat flow stops. 

This final state is called equilibrium. 

Choose black sparkly prom dress that resonates well with your style of choice. Check out our gorgeous tailoring collections now!

Since it can not be reversed like a pendulum, the process is irreversible. 

If you rewind its video, it looks extremely unnatural. 

The direction of this spontaneous process that finally reaches balance is the famous arrow of time. 

Another concept is thermodynamic reversibility. 

Heat dissipation is an example, which is driven by a thermal gradient, always from hotter places to colder places. 

In fact, all spontaneous processes are driven by some type of gradient, whether it's temperature, concentration, or pressure difference. 

These processes develop along the gradient "downhill", from higher temperature to lower temperature, from higher concentration to lower concentration, or from higher pressure to lower pressure. 

Gradients provide the driving force for the process. 

Any process driven by some gradient in the universe is thermodynamically irreversible. 

Gradients dominate the process of events in small and large systems. 

The earth receives radiant energy from the hot surface of the sun and dissipates it into the cold background of the universe at a lower temperature. 

Life processes, including plants, animals and humans, as well as other organisms, are also driven by gradients. 

Their energy sources can eventually be traced back to tiny packets of light from the sun-photons. 

All living things dissipate energy in the form of colder photons and are eventually released into outer space. 

Molecular "memory" 

The two physicists believe that the essence of the current problem is that we usually default that thermodynamic irreversibility and time irreversibility have the same probabilistic origin, which is true in many cases, but not necessarily so. 

The two kinds of reversibility are actually essentially different. 

Time reversibility has nothing to do with entropy gradient. 

To put it simply, it's about memory. 

If all molecules can "remember" their position and speed at each point in time, so that the motion of each molecule can be reversed and restored to its original state, then the process is time reversible. 

If a system is not very large, this process can already be simulated by modern computers. 

With the development of computer technology, larger and more complex systems can be described at the level of individual atoms and molecules. 

In other words, the apparent incompatibility between micro and macro systems has nothing to do with the size of the system, but with the type of process and whether the process erases the "memory" of molecules. 

In the case of heat (or, more broadly, energy), the amount of energy used to synthesize a sugar molecule is released when the molecule provides fuel for a process in our bodies and decays into its original constituent molecules. 

This is the view of thermodynamics, but it ignores the aspect of time. 

If it takes five minutes to synthesize a molecule, that doesn't mean the molecule will decay after five minutes. 

We cannot predict the exact time of molecular decay because the decay process is dominated by a certain probability per unit time. 

And, importantly, probabilistic processes are never time reversible because they do not include memories of earlier states. 

The complete description of the probability process needs to take into account both energy and time. 

In this case, the synthesis and decay of sugar molecules are thermodynamically irreversible because a large amount of energy must be injected to reverse them. 

But this is very different from the time reversibility that involves "memory". 

In other words, in this case, thermodynamic reversibility and time reversibility do not have the same origin. 

The two physicists believe that a smooth transition between these two types of irreversibility will pave the way for a unified theory that can describe all physical states and processes based on a single set of principles. 

This is exactly what scientists expect.