EDIT: LRS
Nostalgic empty chanting flute fu, to the countryside to turn like rotten Ke people.
"Rotten KeRen" is about a man in the Jin Dynasty named Wang Qiang who went up the mountain to cut firewood, watched others play chess for a while, and found that the axe had rotted, and it had been hundreds of years since he returned home, and the people of his time were gone.
But there are many doubts about this mythical story, where have hundreds of years gone? Did the king enter the space-time wormhole? If you watch a little more chess, won't you all cross over to 2021? Is there a scientific truth to a day in heaven and a year on earth?
Many people also sigh and fantasize about "time and space travel" in the story of the Rotten Ke people.
The good news is that Einstein's theory of relativity offers a theoretical possibility of traveling through space-time, simply by going beyond the speed of light or into a black hole capable of distorting time.
Of course, whether it can be realized is an engineering problem, not a problem that theoretical physicists want to solve.
The bad news is that the three dimensions of time and space are actually mixed together, and while we can move forward and backward in space, we can't reverse the flow in the time dimension, which is also the rule of the universe. Whenever we want to build a time travel machine, the universe will relentlessly add a rule to you.
Want to travel faster than the speed of light for time travel? Sorry, you can't exceed the speed of light in the universe.
The debate about time travel has gone from physics to philosophy.
French science fiction writer René Barjavel famously proposed the "grandfather paradox" in 1943.
If you go back in time and kill your grandfather before your father was born, then this act will create a paradox: you go back in time and kill your young grandfather, and when your grandfather dies, there will be no father, and there will be no father without you, so who killed your grandfather? Or if your presence says that your grandfather didn't die because of you, then how did you kill your grandfather?
But physicists say that if the existing theory can't be explained, I will make another one, and "parallel universes" will appear.
When you go back in time and kill your grandfather, you are actually killing people from another universe, or your actions can also create a new parallel universe, and your grandfather's death will only make the people of that parallel universe no longer exist, and the people of this parallel universe will be safe.
Another explanation is that you may be able to travel through time, but what you can do is actually limited by the universe, the past can't change after it happens, you can't kill your grandfather, but you "play" a stranger to arrange a blind date for your grandfather and grandmother!
Some philosophers believe that the concept of time exists only in human consciousness, but is only a form used to synchronize human senses. Perhaps the fault lies in us, for imposing this subjective concept on the universe, and the study of time is meaningless.
Einstein first explained the gravitational field as the curvature of space-time in 1916 in General Relativity. Imagine the universe as a "big mattress" where the heavier the ball, the larger the depression is, and the faster the nearby ball rolls toward him, it looks as if it is drawn to the past. In addition to the spatial dimension, the depression on the mattress also distorts the time near the ball.
That is to say, with general relativity, we can know the degree of depression of the mattress, and according to the degree of depression, we can know how the ball on the mattress moves.
In order to create a time machine, physicists have proposed the concept of closed time-like curve (CTC), trying to find a closed trajectory of material particles in four-dimensional space-time.
That is to say, if there is a trajectory that can make the particle walk around in space-time and then return to the origin, and the speed of the particle does not exceed the speed of light in the whole process (nor can it exceed the speed of light), will it not complete the passage through time and space, and the time machine will not be created!
Scientists have been trying to solve the problem of time travel for decades, and the weirdest thing is that CTC is theoretically real!
The famous mathematician Gödel found that if a universe is filled with slowly rotating uniform dust, due to the centrifugal force and gravitational force generated by rotation canceling each other out, we can find orbits entangled in our past time in the universe, such a universe is also called Gödel rotating universe.
Wormholes, first proposed in 1916, could also act as time machines. Theoretically, wormholes are space tunnels connecting two distant space-time spaces, created by the combined action of star rotation and gravity, and although wormholes are ubiquitous but fleeting. To date, scientists have not observed evidence of wormholes.
In 1974, physicist Frank further found that if there is an infinitely long cylinder, when the cylinder rotates to near the speed of light, its outer space-time is already a CTC, and if you walk along this column, you will find yourself back to the original point, and you will encounter another self who is just beginning to prepare to circle. This cylinder is also known as the Tipler column.
The interior of a black hole is also a fairly magical place, with extremely strong centrifugal forces and gravitational forces, and it is also possible to produce CTC.
Theory exists, practice begins! (Mistaken)
Within the framework of general relativity, there are more CTC solutions that have been studied, proving the possibility of the existence of time machines again and again, but these solutions have extremely special requirements, so special that they cannot meet the requirements in the real world!
With the further development of physics, under the observation of the cosmic microwave background, we can only desperately declare that the universe is not rotating, infinitely long cylinders do not exist, singularities (common laws of physics) appear inside black holes (common laws of physics all fail at singularities), and the instability of wormholes cannot even withstand the passage of a photon.
Since it is so difficult to build a time machine, then I can't prove that I can't build a time machine.
Coincidentally, similar to the various solutions of the CTC above, physicists can always find evidence of time travel in general relativity, but each time they can find some unattainable conditions in these methods, it seems that God has not yet decided whether to open up the "time travel" permission for human beings.
Since no progress has been made in CTC research, researchers need to think differently to explain "why can the arrow of time only go forward"?
Physicists' first reaction is to study the fundamental particles! But the more I studied, the more confused I became that the interaction of microscopic particles had "time inversion symmetry", that is, the motion process of a particle was the same whether it was looked at positively or backwards.
It's like holding an apple in your hand and throwing it upwards, taking the throw as the starting node, and recording a video for the terminating node with the catch, even if the video is upside down, we can't see it.
Of course, macroscopically there is no time inversion symmetry.
Due to the second law of thermodynamics, all isolated systems evolve towards the maximum entropy state, that is, no matter what is done macroscopically, as long as there is a temperature, the microscopic particles are constantly moving and consuming energy, resulting in the entropy (degree of confusion) of the system increasing over time, and the two systems cannot be inverted.
Someone in the room a powder, gradually the room is full of the smell of snail powder, in the room all the air molecules are constantly moving, entropy gradually increased and irreversible, but the collision between molecules in the air is symmetrical in time.
The arrows of thermodynamics and the arrows of time seem to suddenly coincide, so entropy is time? The concept of time that humans have always thought is nothing more than a process of entropy increase? Why is the universe entropy increasing?
At present we have not observed a violation of the second law of thermodynamics, but the thermodynamic explanation of time only "describes the irreversibility of time" and does not explain "how time arises".
Although the concept of entropy was proposed in the mid-eighteenth century, no one has yet proven a clear correlation between the passage of time and the increase in entropy, and physicists are still looking for inspiration in various concepts to explain time, and perhaps in the future it may really be possible to create an infinitely long cylinder, or to create a wormhole for time travel.
At least the universe hasn't explicitly banned time travel!
The discussion about "traveling through time" is always exciting, but all I can do is sit back and wait for the time machine to appear.
Resources:
https://arstechnica.com/science/2021/11/why-the-expletive-cant-we-travel-back-in-time/