Back and forth with time
If it is possible to travel in time, where are the travelers? It is clear that at this moment in history we have no time machine. Technology will have to go a long way. On the other hand, technological advancement only takes time. It is logical to think that if physically possible, at some point physicists or engineers of the future will make a time machine. And if you have that machine, why have you not come to visit us in history?
This question was posed for the first time by physicist Enrico Fermi, but not as a question, but as an argument against the possibility of making trips over time, and affirmed that, not coming from the future, it is clear that you cannot travel in time.
Is it true? No one knows. And that is why this question of time travel admits all hypotheses. Maybe the travelers came from another time, but we didn't distinguish them. Maybe we can't see them. It may in time be possible to travel forward and not backward, and then no one can reach the time before the invention of the time machine. Perhaps you cannot travel to any time by the structure of space time. Perhaps from the practical side it is difficult to travel, few trips have been made and travelers have not reached our time. Perhaps the trip requires a lot of energy. Or a lot of money. Or travel involves violent transformations of matter and a human being could not survive on the journey. There are many hypotheses. And no one knows if anyone is right.
Many hypotheses have a solid scientific basis. Some are proposed by prestigious scientists. For example, Stephen Hawking proposed that only very small things can travel. Microscopic things. It could be the supposed natural protection of the universe so that paradoxes do not arise. This idea was published in 1992, but then expanded its proposal. Hawking believes that man will ever travel forward because the theory of relativity "allows" this journey.
Protection of Physics
There is no reason to think that there are errors in the theory of relativity. All the experiments carried out confirm Einstein's work. It can be said, in short, that science makes it clear that a journey in time is possible. At least it is possible to advance in time.
To travel forward you have to extend the time, the seconds have to be longer than the seconds. Thus, when a second has passed for him, for example, a year has passed for others, so the traveler will jump forward in the time of others.
The general theory of relativity offers two options to do so: increase speed and increase gravity. As the speed increases, the seconds are getting longer, but it is very difficult to reach an appreciable speed, since the energy needed to accelerate also increases with speed. However, astronauts at ISS station suffer this effect. The station moves very fast from the Earth, covering 27,000 kilometers per hour. This makes time for them a little slower with those on Earth. They are only a few nanoseconds throughout the stay, but it happens.
The second option of lengthening time seems simpler, even a very large gravity zone. Therefore, it is enough to pass by a large mass (for example, a large star) to influence the scale of time. The truth is that with current technology it is very difficult to approach the environment of a big star; in order for a person to make that journey before he dies, he should travel at high speed, which brings him back to the first option. Physically it is possible but not simple.
But all this is applicable to meet who finds the cancer vaccine, but not to see it in the battle of Napoleon Waterloo. Travelling in time is more difficult than moving forward, at least from a conceptual point of view.
Einstein's theory did not announce the journey back. Not at all. The most logical option to go back is to use a matter that moves faster than light, which is impossible. However, in scientific hypotheses much has been said about this matter, also giving name to the particles that compose it: the tachyons. It seems the name of serious physics, but only hypothetical particles. A fantasy.
However, mathematician Kurt Gödel developed the field equations of relativity one step beyond Einstein and found an opportunity to travel back in time. It was very limited, equation solutions only work within closed paths. That is, these solutions do not allow travel at any time and place. Among other things, you cannot travel to a time before the time machine. According to Gödel's work, at least, Napoleon cannot be visited. But the equations offer a small opportunity worth analyzing.
Who knows? Perhaps Einstein and Gödel have not described all physical reality. These limits may be exceeded. But then where are the travelers in time? New initial question.
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