Humans are fond of dreaming and look forward to understanding the past and the future. Time travel seems to be the simplest way to vividly show us history and destiny. However, the manufacture of the time machine is still in the stage of fantasy, and even fantasy is mostly produced by novelists and film special effects artists. Few people know scientists' specific ideas about the time machine. Paul davis is a world-famous theoretical physicist. He decided to be the first person to eat crabs and try to make a time machine, even if it was just "theoretical manufacturing".

The first question in making a time machine is whether you want to go back to the past or fly to the future. In Davis's view, time travel to the future is easy. If you are close to the speed of light or in a strong gravitational field, you will feel that time passes slower than others-you have entered their future.

Travel to the future is also called "limited time travel". At the beginning of How to Make a Time Machine, paul davis affirmed that "limited time travel" is feasible, but "unlimited time travel" is only "possible and feasible" to return to any era.

Wormhole: the key to going back to the past

Einstein's theory of relativity allows this trip to take place in a specific space-time structure: a rotating universe, a rotating cylinder, and a very famous wormhole-a tunnel that runs through time and space. In other words, as long as a stable wormhole can be created, it can cross time and space. So, what is a wormhole? What does it have to do with black holes?

In Stephen Hawking's A Brief History of Time, there is such an explanation: wormholes are time-space tubules connecting distant regions in the universe, which can connect parallel universes or baby universes and provide the possibility of time travel. A black hole is a space-time region, where gravity is so strong that nothing, even light, can escape from it.

In Davis's plan, building a wormhole is divided into three steps:

The first step is to find or build a wormhole and open a tunnel to connect two different areas in space.

The second step is to stabilize the wormhole. With the negative energy generated by quantum, wormholes allow signals and objects to pass safely. Negative energy will resist the wormhole becoming infinite or close to infinite density. In other words, it prevents the wormhole from becoming a black hole.

The third step is to drag the wormhole. A very advanced spaceship separates the entrances of wormholes from each other. If both ports are placed in proper places in space, the time difference will remain unchanged. Suppose the difference is 10 year. If the astronaut crosses the wormhole from one direction, he will jump into the future in 10 years. On the contrary, if the astronaut crosses the wormhole from another direction, he will jump to the past 10 years ago.

Paradox of time machine

If many technical problems are overcome, the production of time machine will open Pandora's box full of paradoxes. For these, Davis also showed some concern. He said: "I personally don't want to distinguish the time travel I described from other behaviors that control nature. All technologies are interfering with nature in some way. In some science fiction, some people change the status quo by going back to the past. This kind of thing is harmless in the novel, but if it happens in reality, it will bring serious ethical problems. Who gave you the right to go back and change history? "

Since the past, the present and the future are closely related, and the past can influence the present, it makes logical sense for the present to influence the past. If you can't do whatever you want, people finally have the freedom to travel through time and space, but they lose the freedom to act. Isn't it a great loss to watch history slip away without being able to change anything?

In this regard, the solution given by Davis is the "multiverse" theory-there is not only one world, but also many parallel worlds. You go back to the past, but it's not your own world, but a world similar to your history. In this way, even if you kill your mother, she did die in that world, but when you return to the future, she is still alive and well.

The idea is crazy, but many famous physicists believe in the existence of parallel worlds except Davis. Hawking said in A Brief History of Time: Another possible way to solve time travel is to choose the historical hypothesis. The idea is that when a time traveler goes back to the past, he enters another history that is different from the historical record. In this way, they can move freely and are not bound by the original history. If so, what's the difference between going back to the past and dreaming?