Simon Newcomb, an American mathematician and astronomer, said "Air flight is one of those problems that men will never have to face". Newcomb did not believe it. Yet the first plane has a precise date: December 17, 1903, when the two American brothers Orville and Wilbur Wright created the first vehicle capable of lifting off the ground and flying in a controlled way thanks to an engine. Newcomb had time to see him, he would have died a few years later, on 11 July 1909. And to think that NASA recently presented the first fully electric plane.
The New York Times, in 1920, released a long and detailed article explaining how and why no rocket could ever leave Earth's atmosphere. In June 1969 they had to retract such claims. But we must be clear: time travel, in the current state of things, based on our best understanding of the physics and laws that govern the universe, especially travel in the past, is absolutely impossible. Yet, as we have seen, our best understanding of physics and the laws governing the universe has proven wrong many times.
The man who believed it
Those who believed it (though not too seriously) were the famous physicist Stephen Hawking, which he organized on June 28, 2009 a party for all time travelers who wanted to meet him: nobody showed upalas. Of course, we would have liked to know if time travel will ever be a reality, and meeting a possible time traveler would have been an overwhelming test, but if this event had happened, you probably wouldn't have been reading this article: let's look at the glass half full.
But why didn't anyone show up? The possible answers are innumerable. Maybe in the future Stephen Hawking will not be such a popular person, or at least not enough to want to attend his parties, or maybe there will never be a time travel, or still, may have chosen not to participate debileratamente to avoid incurring any paradoxes (even if there are those who think it is possible without paradoxes). The possible answers are many. But what do physics think about time travel? To answer, we must necessarily consider Einstein's theory of relativity.
The journey into the future
All we know we owe to relativity, whose main merit was in fact to tell us "hey! See that the light moves at the same speed in every reference system!". A concept that in truth can even annoy us, that can be seen as a limit, because the speed of light is really very very small when compared with galactic or intergalactic distances. Just think that to reach Alpha Centauri, the star closest to our Sun, the light takes four years (NASA is planning to visit it). However, this fact that the speed of light is always the same has led to a long series of logical and mathematical consequences, to the elaboration first of special (or restricted) relativity and then of general relativity. Special relativity he says that time flows more slowly (or faster) depending on the speed at which we are traveling. Eg, the faster we go, the more time slows down.
That is: time is not absolute. This concept is counterintuitive for us, so much so that we struggle to believe it, sometimes we subconsciously think it is a matter of "perceived time"and not a real effect to deal with. But it is a real effect, there is no cosmic clock to which all stars and planets refer, for everything in the universe time flows differently (for us, too). This leads to what is known to most as "paradox of the twins".
There are two twins and one of them makes an interstellar journey at the speed of light. Go to a distant planet and go home. When it comes back it will be much younger of the twin who remained on Earth (that is, he will have traveled in the future compared to his brother who has remained still). However, despite the misleading name, this is absolutely not a paradox, but a real effect.
It really works, and the higher the speed at which you travel, the more the effect becomes evident. In other words, the faster you go and the more you travel into the future, the rest are all engineering issues, rather than physics. Obviously this at low speeds (like the ones we're used to) is negligible, but not absent. This means that technically, if you take your car and travel, slow down "your" time. However, the external time continues to flow normally. When you stop, for special relativity you will have traveled in the future of an infinitesimal and absolutely negligible fraction of time, but measurable (in fact, we measured it).
This means that the flow of our time is personal and unique, we are in equal time points, but we get there by following very different roads. How did we measure it? Through the Hafele-Keating experiment, performed in 1971 by Joseph C. Hafele and Richard E. Keating.
They used three atomic clocks perfectly identical and a very fast plane. Atomic clocks are the most accurate and powerful tool for measuring the time we currently have available. According to the theory of restricted relativity, for a clock located at the equator time is dilated and therefore flows more slowly than a clock stopped at one of the two earth poles, since the former has a speed due to the rotation of the earth which in the poles it is absent.
They then placed a third watch on a very fast plane, and time is expected to flow differently compared to the two watches mentioned above. In particular the differences in the passage of time of the clock on the plane will be due to two effects: on the one hand the dilation of times due to restricted relativity (of which we have already spoken) and on the other the opposite effect of acceleration of times compared to a clock on the ground due to lower intensity of the gravitational field terrestrial predicted by general relativity (of which we have not yet spoken).
By comparing the times, which are different in the three clocks, the predictions made by both general relativity and restricted relativity can be confirmed. We therefore know how time travel in the future, however surprising it may be, we know how to do it (we do it, small, daily) and the limits for making bigger jumps are only and exclusively technological, but this is a one-way ticket. There is no going back for now. Let's see what we can say about travel to the past.
The journey into the past
Mathematically we know that the faster we go the more time slows down. It slows down, slows down, slows down, until it stops completely at the speed of light. A photon has no time, does not age, in a single instant sees the entire universe pass in front of it and arrives immediately at the end of time (if it exists or will ever exist). Intuitively, with very crude and possibly wrong reasoning, it nevertheless comes to think that if we went then faster than light (like neutrinos? with cabbage!) time could flow backwards. The problem is that to push something (even without mass) beyond the speed of light you need ainfinite energywhich is neither practical nor realistic.
Special relativity is not enough, so let's see what we can say taking into consideration the second stroke of genius delivered by Einstein, general relativity. In fact, Einstein took Newton's theory of gravity and completely distorted it, stating that gravity is nothing more than space-time geometry, and not simply a "mysterious force".
A real geometric structure, which as such can be modified, curved, varied (all in a way that is anything but simple, as we will see). Everything moves within this structure. Recall that it was only 2018 when the Nobel Prize for the discovery of gravitational waves was awarded, another of the hundreds of correct predictions of this theory that manages to explain the world around us in a surprisingly accurate way.
So let's look at one of the first achievements, as regards time travel, of general relativity, the Tipler cylinder. The Tipler cylinder was discovered as a solution to the equations of general relativity by Willem Jacob van Stockum in 1936 and Kornel Lanczos in 1924, but it was only thanks to an analysis by Frank Tipler in 1974 that the true ones were understood theoretical implications regarding time travel. Tipler in his study showed "Rotating cylinders and the possibility of violation of global causality"He imagined a portion of space-time containing a huge cylinder, infinitely long and rotating along its longitudinal axis.
Such a cylinder would create a drag effect which would have the result of deforming the space-time in such a way that the light cones of the objects in the vicinity of the cylinder tilt, so that a part of the light cone therefore points backwards along the time axis on a space-diagram thunderstorm (of which I bring you an example, in photo).
To understand the concept (extremely complex) let's take a spiral staircase. We know that if we turn around 360 degrees we return to the starting point, but in a spiral staircase this does not happen, we are not in the exact same place, but on the upper floor. Similarly, in a rotating cylinder, if we choose the right direction and the right speed and make a 360 degree turn around the cylinder, we would find ourselves in the same spatial point, but at a different point in time. Future, or even past.
The problem with this methodology is: where do we find so much material, enough to create a sufficiently dense cylinder? Furthermore, Stephen Hawking pointed out that for such an idea it would be necessary to create or an infinitely long cylinder, or alternatively "negative energy". In fact, matter with a negative mass rather than a positive one. In short, nice in theory, but nothing functional.
What if instead of turning the space-time we created a hole through the space-time? The result would be what is commonly called Wormhole. In this way one could travel instantly from one point of the universe to another, but also to another universe or, possibly, to another point in time. Still, past or future. The problem with wormholes is that they are a great idea, equations can be written, solutions come back and predictions come from general relativity, which as we have seen it is extremely precise and accurate . Unfortunately, this also tells us that such a system, as far as possible, is hopelessly unstable. Even just a photon attempting to traverse a wormhole would instantly collapse it.
Again, you need something you own negative energy to be able to cross it. A negative mass. As you can see, the concept of negative mass is inextricably linked to time travel in the past.
However, the ideas presented are sufficiently old (decades have passed by now), and very few (serious) physicists today are concerned with finding a solution to the problem of time travel in the past on an experimental level. One of them is Ron Mallett, which through some articles and debates, has even built the prototype of a machine that rotates very powerful lasers with the intent of subsequently passing us a particle, which at the end of its path would find itself back in time.
The discussion is heated and there are many online sensationalist articles about. Obviously the results are not yet there and the scientific debate concerns once again the length (infinite?) That this vortex should have (which works in theory but not yet in practice) or the mass that must have the particle passing through it ( negative?). In short, there are recent research but they are almost all theoretical, and the very rare experiments done on them are still far from the realization of a "time Machine".
On the other hand we could ask ourselves: but if it will ever exist time travel because the world is not inhabited by thunderstorm tourists, people interested in visiting our era? The reasons can be varied, but the most interesting of these is that if we take a wormhole for example, this NEVER allows us to reach a point in time prior to its realization.
We create a wormhole today, and in a hundred years we can go back to today, but we will never be able to use it to reach yesterday, for example. That is the reason why we do not see thunderstorm tourists could trivially be "because time travel has not yet been invented"as stupid as such a phrase may sound in this context.
For time travel fanatics, I'm sorry; maybe we will be able to reach the technology needed to get to the future, but we will never know how to go back. I wonder: would you, having the opportunity, travel to the future, without being able to return?
