Time-travel theories

There are many different theories for time travel. Steins;Gate uses the black-hole theory mostly, by compressing the data of the brain (2.5 Petabytes), to a size sendable by the phone microwave.

In the VN, Kurisu mentions that although there are many theories, 11 of them are widely accepted in the scientific world. Listed below:
 * 1) Neutron star theory
 * 2) Black hole theory
 * 3) Light Speed theory
 * 4) Tachyon theory
 * 5) Wormhole theory
 * 6) Exotic matter theory
 * 7) Cosmic String theory
 * 8) Quantum Gravity theory
 * 9) Cesium Laser theory
 * 10) Elementary particle ring and laser theory
 * 11) Dirac antiparticle theory

Neutron star theory
A neutron star is a type of stellar remnant that can result from the gravitational collapse of a massive star. These stars have a mass ranging from 449.482 to 665.900 times the mass of the earth in a diameter of 12 Km. That means they have a very high density, thus very high gravity ( 7 × 1012 m/s2, earth's is 9.81 m/s 2 ). As for the Gravitational time dilation, time would pass 30% slower on the surface of such a star, meaning that if someone could travel to such a star and withstand this gravity, they would have a way to "travel" to the future. This theory only serves for time travel to the future, to the past wouldn't be possible.

Black hole theory
Black holes are one of the most used resources in time-traveling novels, next to wormholes. The Kerr-black hole theory is the result of Roy Kerr's calculations for relativity. A Kerr-black hole is a singularity that possesses mass and angular momentum, but does not possess electrical charge. This hole spins around a central axis and has two event horizons, which contain a ring-formed singularity. Inside each of the two event horizons, time and space are reversed, so in a Kerr-black hole this swapping occurs twice. In theory, it's possible to escape the ring-formed singularity, although not by the same way you entered it, or simply avoiding it. Crossing the singularity would make you end up in a "negative space" (the definition is still unclear). Avoiding it would cause you to go back in time while you are crossing the first event horizon. Here's a Penrose diagram of two time travelers making their way through the black hole. The dark-blue one avoids the singularity and the light blue one crosses it:

Wormhole theory
Wormhole theory, as Cosmic String theory, is explained by Makise Kurisu during her press conference:

-"In that case, why don't we go to our next example, wormhole theory? It may be a little more realistic than cosmic string theory. By the way Hououin-san, do you know what wormholes are?"


 * -Kurisu Makise, chapter 1 of the VN

-"It's like a shortcut opened through space or something... right?"


 * -Okabe Rintarou, chapter 1 of the VN



-"Yes, that's correct. There are two holes, tied together by a tunnel. Transit time through the tunnel is zero, no matter how far away the second hole is."



-"But there's a catch. The tunnel suffers from super gravity, and collapses as soon as it opens. And that's why we need something to negate the effect of gravity."



-" The so-called 'exotic matter'. It's a substance with negative mass, which repels gravity."

-"The wormhole tunnel is all squished and squashed like this fist of mine. In order to pass through, you need something inside of my hand to oppose the 'grasping force' so that I can not squish anymore."

''-"If you stabilize a wormhole with exotic matter injection, teleportation becomes possible. For example, let's say there's a wormhole entrance here in Akihabara, and the exit is in LA. Now imagine the hole in LA goes all the way to the end of the universe at near-light speed. And once it reaches the end, it immediately pulls back to LA. According to the theory of relativity, time slows down for objects moving at the speed of light. Meaning the hole that returned to LA would be further in the past than the one in Akihabara. And with those co'ndition, Hououin-san can enter the wormhole and arrive at LA several years back. However, this cannot yet be called true 'time travel'. It is only 'pseudo-travel'. The so called Urashima effect. The important part is to return to Akihabara from LA through the wormhole once more. Having done that, transit time becomes zero, and Hououin-san would return to Akihabara several years back. Time travel complete. The prerequisites for wormhole theory are simpler than the ones for cosmic string theory. First: the wormhole itself. They may exist somewhere in the universe, yet nobody has ever seen one. Second: The energy required to move a wormhole to the end of the universe and back near-light speed. Third: Exotic matter, which by the way, has not been confirmed to exist.'"''


 * -Kurisu Makise, chapter 1 of the VN

Cosmic String theory
In the VN, Makise Kurisu explains the cosmic string theory at her conference:



-"A Cosmic String is a string-shaped crack which has an extreme mass. You can think of the crack as something the width of an elementary particle, and at least the length of a galaxy."



-"It has an immense mass, giving it the property of space-time distortion. If you were to travel through that distortion you could make a full rotation around the string in less than 360 degrees."



-"In short, you can do something resembling a warp. This is called Space-time angular deficit. When you pass through an area of angular deficit, transit time becomes zero."

-"Applying this, once the cosmic string moves approaching light speed, according to the theory of relativity, time will flow slower for the cosmic string in relation to its surroundings. Therefore, passing through the area of angular deficit would cause the zero transit time to become negative. In other words, it will be the 'past' after transit. So, if you use two cosmic strings, you can do a space deficit jump. If you revolve back to your original location, you can return to the same time you started revolving. And that, roughly speaking, is time travel by means of cosmic string theory."

''-"By the way, so nobody misunderstands, cosmic string theory is different from superstring theory. With that out of the way, you need three things in order to travel to the past with cosmic string theory. First: Cosmic Strings. Two of them are necessary. Ah, by the way, they are hypothesized to exist only where the universe was first formed, so they might be a little hard to find". Second: Even if you do find the strings, you need the energy to make them move at near-light speed. Just how much energy do you think you'd need to accelerate something as long as the milky way to near-light speed? I'm pretty sure it's a little bit more than 1,21 jigowatts. Third: a space ship able to go all the way to the cosmic strings. The time traveler must be on board. What do you think Hououin-san? Care to take on the challenge of cosmic string theory time travel?"