Is it possible to travel through time. In fact we already have. Repeatedly. Only that perhaps the trips do not seem impressive enough as some think and it is not as seen in movies or science fiction books. The question is purely physical.
We have already done so. Although it may seem a typical issue of science fiction films, and although there are those who get their hands on the head when reading these lines, the truth is that it is undeniable that we have already traveled to the future. But how can it be? Why had not anyone told you so far? Well, maybe no one is aware of it. Or simply because the trip made is so small that it does not mean too much. In any case, the truth is that time travel can be done. Although it is not so clear in which direction.
Traveling in time into the future
The most seasoned traveler in this time is none other than Sergei Krikalov. Yes, the cosmonaut who spent 803 days at the Mir space station. And its longest journey (probably the longest journey of all humanity) counts 1/48 seconds into the future. But before we go any further, it’s best if we stop to explain some necessary concepts. This time travel is due to general relativity (a case of special relativity) that defined Einstein back in the early twentieth century. In this theory, Einstein explains the relativity of certain concepts that to date were absolute. Among them, space and time.
According to the formulation, the passage of time depends on the speed at which a body moves. Thus, the faster it is, the slower time will pass around it. But not in itself, where time will normally pass. This is exemplified very well with the well-known parable of the twins. It explains how two identical twins separate. One of them is heading for a space trip that will take you to the confines of the solar system for several years traveling at speeds close to those of light. The other one stays at home. When the twin cosmonaut returns, it is found that while for him it has only been a few years, for his brother have been decades. He may even have his entire immediate family dead.
The relativity of time has been proven in more than one occasion directly. This theoretical concept, which is quite difficult to understand, has actually been tested on more than one occasion, including a test made with ultra precise atomic clocks. As we see, what happens in the end is that the twin who traveled to space, also did to the future. It also happened to Krikaliov, who spent almost three years at 27,000 km per hour and traveled to the future a tiny fraction of time. But he did. Because traveling forward in time is just a matter of money and technique. And it is possible.
Travel into the past
If you can travel to the future, is it possible to travel back in time to the past? Well, depending on who you ask. In fact, as Kurt Gödel long ago demonstrated, physical laws do not prohibit travel to the past. It’s just that we have not discovered that there is such a possibility. To do so, as he demonstrated with a Gödel model, it would only take travel into the future long enough to meet our past. Something similar to what would happen if we began to travel the world. When we turned it around we would reach the same point and even go back to the same places we had already passed.
The possibility of traveling to the past made Einstein deeply uncomfortable. For this to happen with the time of our universe, it should be in rotation. In such a case, closed curves, infinite time lines, could be given as circles in time. Physicists have described and discovered piles of such time lines. But only in theory. So far no one has managed to travel back in time. Not at all. Gödel also failed to demonstrate that the universe is in rotation, a necessary premise. Now, if we ever succeeded, it would not mean that we were violating the physical laws we know, much less. Which bothered Einstein deeply.
Now, based on this aspect, traveling to the past «turning around» is neither practical nor convenient. In order to do so we should use a shortcut. And what would this be? Here physicists engage in intense debates between advocates and detractors of time travel. A plausible option would be a singularity. Like a wormhole. A singularity is an event whose physical properties vary until it «breaks» the physical laws we know. In the case of the wormhole, where two points of space become a distance of zero, not only space transport would be possible, but perhaps also the temporal.