The concept of a time machine conjures up images of an implausible device that is too often used in science fiction stories. However, according to Albert Einstein's theory of general relativity, which explains how gravity works in the universe, time travel is not just a figment of the imagination. And if time travel is a plot twist in movies, what about reality?
Time travel, according to Einstein's theory, is absolutely possible. Essentially, physicists have succeeded in sending tiny particles called muons, very similar to electrons, forward in time by manipulating the gravity around them. This does not mean that the technology to send people forward into the future will be possible in the next 100 years, but still.
1. Wormholes
Astrophysicist Eric Davis of the EarthTech International Institute for Advanced Study in Austin believes this is possible. All you need is a wormhole or wormhole, a theoretical passage through the fabric of spacetime as predicted by the theory of relativity.
Wormholes have not yet been proven, and if they are ever found, they will be so small that even a person, let alone a spaceship, cannot fit in them. That being said, Davis believes that wormholes can be used to travel back to the past.
Both general relativity and quantum theory offer several possibilities for travel - for example, a "closed timelike curve" or a path that shortens spacetime, that is, a time machine.
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Davis argues that the modern scientific understanding of the laws of physics "is teeming with time machines, that is, numerous solutions to the geometry of space-time that allow time travel or possess the properties of a time machine."
As you can imagine, a wormhole would allow a ship, for example, to travel from one point to another faster than the speed of light - almost like in a warp bubble. This is because the ship will arrive at its destination earlier than the beam of light, following a short path through space-time. The transport, therefore, will not violate the universal speed limit rule that light imposes, since the ship itself does not travel at that speed.
Such a wormhole can theoretically lead not through space, but through time.
"Time machines are inevitable in our physical space-time," Davis writes in the paper. - "Passage wormholes include time machines."
Still, Davis adds, turning a wormhole into a time machine won't be easy. It will take a titanic effort. This is because once the wormhole is created, one or both of its ends will need to be accelerated in time to the destination, which follows from general relativity.
2. Time machine: Tipler cylinder
To use a Tipler cylinder based time machine, you need to leave Earth in a spaceship and travel into space to the cylinder that rotates there. When you get close enough to the surface of the cylinder (the space around it will be mostly "warped", deformed), you will need to go around it several times and return to Earth. You will arrive in the past.
How far back in time depends on how many times you orbit the cylinder. Even if it seems to you that your own time is moving forward as usual while you circle the cylinder, outside of the distorted space you will inevitably move into the past. It's like walking up a spiral staircase and finding yourself one step down with each full circle.
3. Donut vacuum
According to Amos Ori of the Israel Institute of Technology in Haifa, space can be twisted enough to create a local gravitational field that resembles a donut of a certain size. The gravitational field forms circles around this donut, so space and time are tightly twisted.
It is important to note that this state of affairs negates the need for any hypothetical exotic matter. Although how it will look in the real world is difficult to describe. Ori says mathematics has shown that at regular intervals, a time machine will form inside the donut in a vacuum.
All you need to do is get there. In theory, it would be possible to travel at any point in time since the time machine was built.
4. Exotic matter
In physics, exotic matter is matter that is somehow different from normal and has some "exotic" properties. Since time travel is considered non-physical, physicists believe that so-called tachyons (hypothetical particles for which the speed of light is at rest) either do not exist or are unable to interact with normal matter.
But when negative energy or mass - the very exotic matter, or matter - twists space-time, all incredible phenomena become possible: wormholes that can act as tunnels connecting distant parts of the universe; a warp drive that will allow travel faster than the speed of light; time machines that will travel back in time.
5. Cosmic strings
Cosmic strings are hypothetical 1-dimensional (spatially) topological defects in the fabric of space-time that have remained since the formation of the universe. With their help, in theory, fields of closed time-like curves can be formed, allowing one to travel into the past. Some scientists suggest using "cosmic strings" to build a time machine.
If you bring two cosmic strings close enough to each other or one string to a black hole, in theory this could create a whole array of "closed timelike curves." If you make a carefully calculated figure eight in a spaceship around two infinitely long cosmic strings, in theory you can be anywhere, anytime.
6. Through a black hole
The black hole has an incredible effect on time, slowing it down like nothing else in the galaxy. In fact, it is a natural time machine. If the mission to fly around the black hole were operated by a ground agency, it would take them 16 minutes to fly around the black hole. But for the brave people aboard a ship that is close to a massive object, time would pass very slowly. Much slower than on Earth. Time for the team would slow down in half. For every 16 minutes, they would only experience 8.