In the last article, we talked about the fact that time travel, in principle, does not contradict the laws of physics that we know about. Moreover, the theory of relativity directly says that time flows differently in different regions of our Universe, and under certain circumstances we will definitely move forward or backward in relation to observers who are in a different frame of reference. However, we still do not know if time travel is possible in our world that is not related to relativistic effects. Kurt Gödel proved in 1949 that Einstein’s equations allowed the existence of such worlds, and his followers derived several more solutions describing such universes. But it has not yet been possible to prove that we live in such a place. However, even in this case there are loopholes.

**Zero method. Passive and not very interesting**

The first way of time travel will be denoted as zero, because, strictly speaking, this is not quite a journey. In order to look into the past, it is enough just to lift your head at night and look at the sky. Here again we recall the theory of relativity, which says that the speed of light in vacuum is constant and amounts to almost 300 thousand km/s. It would seem a lot, but the distances in the Universe are so huge that the light of stars reaches us for hundreds, thousands, millions of years. Even from the Sun, light reaches the Earth in 8 minutes and 20 seconds. There is even a unit for measuring distance in space – the light year (that is, the path that light travels in a year, approximately equal to 9.5 billion kilometers). One of the brightest stars in the Northern Hemisphere, Arcturus, is 37.3 light years away. So, we see her as she was almost 40 years ago.

So the starry sky is a map from a more or less distant past. But, of course, this is not a way of traveling to the past, but rather of contemplating it.

**Method 1. Accelerate to near-light speed**

So, we take into action the effects of the special theory of relativity. Starting from Earth on a spaceship and accelerating to near-light speed, we will experience the effect of time dilation. It is described in the Soviet duology “Moscow – Cassiopeia” and “Youths in the Universe.” The crew of the ship, recruited from teenagers, was supposed to fly to Alpha Cassiopeia for 27 years, but due to a malfunction, the starship accelerates to the speed of light. While a few hours have passed for people on board, several decades have passed for an observer on earth.

Thus, the recipe for traveling to the earthly future is simple: we fly somewhere at near-light speed and return back. Depending on the flight time, we will move into the future for tens or even hundreds of years. The problem is that so far we cannot achieve such a speed. The fastest man-made object, the Parker Solar Probe, reached a speed of 163 km / s thanks to gravitational maneuvers. Very fast, but negligible compared to 300,000.

There is another reason to try to reach the speed of light in the context of time travel. If we find a way to travel faster than the speed of light (and this, despite what was said earlier, is in principle possible), then this will lead to the creation of a closed time-like curve – in fact, a trajectory along which travel into the past is possible. All further methods that will be described in this article assume that such curves either exist in our Universe or can be created. However, some physicists, such as Stephen Hawking, believe that their existence is impossible. Hawking even came up with a hypothesis about the security of the chronology – however, he did not manage to prove it.

**Method 2. Jump into the hole!**

We are talking about wormholes, or wormholes. These are hypothetical tunnels in space-time, which, as it were, directly connect two regions of the Universe. Thanks to them, you can instantly move between distant coordinates both in space and in time. Such objects were predicted by Einstein himself – in 1935, he, together with Nathan Rosen, proposed a solution to equations that imply a connection between regions of space. This connection was called the Einstein-Rosen bridge. In the context of our problem, its only drawback is that it is impassable. The bridge will collapse before the hypothetical time traveler can get from one mouth of the wormhole to the other.

But not all is lost! In 1988, Kip Thorne and Mike Morrison gave us a traversable wormhole, which they called the Morrison-Thorne wormhole. True, there is also a nuance here: in order to keep the mouth of this passage open, exotic matter is needed – such a substance that has a negative mass and, as a result, is not attracted, but repelled under the influence of gravity. However, a year later, Matt Visser showed that such wormholes are also possible, the path to which does not lie through an area with exotic matter.

Be that as it may, wormholes still remain hypothetical objects. It is likely that we simply have not been able to find any of them yet.

**Method 3. Rotate around an infinite cylinder**

Although Karl Gödel is most often credited as the pioneer of closed timelike lines, twelve years before his work, the Dutch physicist Willem van Stockub published a paper in which he described one of the simplest solutions to Einstein’s equations. In fairness, it should be noted that he was not a pioneer either, but independently rediscovered the solutions deduced by Cornelius Lanczos back in 1924.

Be that as it may, the Lanczos-Stokub equations describe the gravitational field created by dust rotating around the axis of cylindrical symmetry. These are the first solutions in history that allowed the creation of a closed time-like curve. Thus, if we rotate around a sufficiently long (at least several light-years) cylinder, we will fall into this curve and return to the past.

The American physicist Frank Tipler proved that the Lanczos-Stokub solutions create a closed time-like curve. In honor of him, the hypothetical object was named Tipler’s cylinder. All we need to do is find such a spinning cylinder, fly towards it, make a few spins (how many depends on how far back in time we want to go), and return to Earth. But where can we find such an object?

Cosmic strings, hypothetical deformations of space-time that arose at the moment when fundamental interactions were separating in the newly born Universe, can claim the role of Tipler’s cylinder. This separation caused a phase transition that did not occur simultaneously in all parts of the universe. Because of this, defects formed in space-time, which we call cosmic strings.

Strings are predicted by so many theories, so their existence is practically proven. It remains only to find them – and use them to travel to the past.

**Method 4.** Create a bubble around you

Above, we said that in order to create a closed timelike line, it is necessary to develop superluminal speed. But what about the fact that it’s impossible? Alcubierre’s bubble comes to the rescue, which we wrote about in detail when we analyzed the possibility of a warp drive . In short, the essence is to compress the space in front of the ship and stretch behind. Then the ship itself will be in a kind of bubble that can move through space-time at any speed, because the starship itself will be at rest. This does not violate the principles of the theory of relativity. Alas, there is a catch here too: creating such a bubble requires a huge amount of energy, and we do not have enough of the whole Sun to get it.