Space-time crossing has always been a subject of constant research by people and scientists! But Hawking once pointed out that it is impossible to return to the past through time and space in the same space and time. He gave an example: Suppose there is a scientist who has created a time machine, this machine can let him go back a minute ago, and then He shot and killed himself on the bullet. Then ask: When his pistol hasn't been shot, he's been killed. Who then shot it? Therefore, there is a great paradox when you cross the time and return to the past! Can't go back to the past. Can we go to the future? Hawking thinks this answer is yes. There are two ways to reach the future! First: Make a spaceship and fly to the center of the Milky Way. Because in the center of the galaxy, there is a supermassive super black hole hidden in the middle of the galaxy. If this spaceship is not sucked in by a black hole and moves along its edge, it will take 16 minutes for them to walk around the black hole on the earth. However, people on the spacecraft only need to spend half the time. If they fly around the black hole for five years, it has been ten years on earth. The astronauts on this spaceship are making journeys to the future. The second method is to make a train that is infinitely close to the speed of light. This speed is equivalent to 7 seconds around the earth in 1 second. Assume that the train departed on March 15, 2018, and stopped on March 15, 2118. For people on Earth, it has been a hundred years, but for people on the train, it is only spent. It's only 7 days! The people on this train are real time travelers!
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