Imagine you are a scientist. You are working on the science behind time travel. You encounter a beautiful woman and fall in love with her. You marry her. After few years you realize that your wife was actually a boy, who underwent gender transformation operation. After sometime you discover another shocking thing that your wife is actually YOU from the future. Shocking right?This was one of the many scenarios in which time travel could prove to be hazardous, shocking and dangerous as well. I am going to share with you the science of time travel. How will it be travelling in time? As shown in Avenger: end game? Or it would be like the movie Looper? Or like something we haven’t seen yet? Will it affect our past? Can we actually travel in past or just future or both?
Let’s get started…….
What is time?
According to Newton, time is absolute. In other words,1 sec passed on Earth is 1 sec passed for every part of the universe, on Jupiter, on Pluto, on M67 everywhere. Time is like an arrow, once shot never returns back. But Einstein proved that time is relative. It slows down when we travel with the speed close to the speed of light or when we travel near a massive object like the sun. Einstein proved that time is relative, it means that 1 sec passed for you may not be 1 sec for me. Interesting right? There is a concept based on this, called as Twin Paradox….If two twins, Fahim and Fujal, right after their birth get separated. Fahim stays on Earth and Fujal travels in a spaceship, with speed near to that of light. When Fujal returns back to Earth after 5 years, from Earth’s frame (i.e. 5 years are passed on Earth), Fahim’s age
will be 5 years but Fujal will be Younger than Fahim. As time got slowed in his spaceship, his age is less than 5 years. In short, we can say that Fujal has come to
the future. According to Twin paradox, travelling to future means being present in a period of time that you were to see after some period of time. Like in the above case Fuzal would have seen those things, which he would see just after the instant he returns back to Earth, when he would have become 5 years old like his Twin brother Fahim.So, travelling or staying in space also gets you to the future time. The world record for this is created by Russian astronaut, Sergei Avdeyev. He spent 748 days orbiting around Earth. So according to relativity, he travelled 0.02 seconds into the future. With this illustration you must have got a brief idea of how time travel would work in real world. Right?
What does it mean to travel in time?
It is an obvious, but complex, question. There are some theories that claim we can travel both in past as well as future. There are also some theories that tell us that we can travel only in future and not in the past. Doestravelling in time mean that we can go in other generation or another phase of time? It is a complex question with complex answers. Some theories suggest that travelling in past would open a gateway to new parallel universe, which would be same as ours in all kind except that would be different in terms of the consequence of travelling in the past. Any changes in the past would open a new timeline (i.e. a new parallel universe) that won`t affect our universe, as we saw in the Avengers: end game.
Some theories also suggest that we can change the past events. That means we can let Hitler win 2nd world war or we can stop Britishers from taking Kohinoor diamond to England. But if this happens historians would lose their jobs, as anybody can rewrite past. This could lead to biggest destruction, we could kill the first human on Earth and eventually wipe out human civilization from Earth. Like it
happened in the movie Looper, the main character killed himself hence killed his future version too.
Closed time-like curve
Before understanding the theories of time travel, we need to understand Closed Time-like curve. There are many solutions to Einstein’s field equation from his famous Theory of Relativity. One of those solutions, Closed time-like curve, predicts a path to travel in the past, actually it’s a 2-way path, that means we can go to past and come back to our original time. It is a mathematical way for time travelling. This is the science used by Tony Stark in Avengers: End Game to travel into past.
Theories of time travel: -
There are a number of theories telling us how can we travel in time. Some of those theories look absurd, some of them look impossible today but can be possible in near future. We are going to look at 2 theories, from many theories, which make more sense than the other: -
1. Transvers able wormhole Theory
Wormhole is a gateway which connects 2 points in time. It is one of the solutions of Einstein’s field equation. Transvers able wormhole allows faster than light travel. It allows 2-way trip to the past. The key concept behind it is negative energy.
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| Wormhole |
How to build transvers able wormhole?
We need 2 chambers each made up of 2
concentric spheres separated by a tiny distance. When the outer sphere is made to implode (i.e. come in contact) with the 1st sphere, Casimir effect (a small interactive force which acts between two
close parallel uncharged conducting plates, due to the vacuum fluctuation of Electromagnetic field) is developed. This Casimir effect generates negative energy needed for time travel. After all this is done the 1st chamber is sent to space, travelling at the speed near to the speed of light. Time slows down in the 1st chamber. The time traveler is present in the 2nd chamber. After 1stchamber is sent to space, the time traveler is transferred to 1st chamber though the wormhole connecting both chambers. In this way the time traveler can go into the past.
Problems with transvers able wormhole
i. Wormhole would be small, even smaller than an atom.
ii. The spheres could squeeze to Plank’s length (i.e. 1.616 * 10 -35) which is too small.
iii. The time traveler could go to the past only up to the instant when time machine was built.
2. Gigantic Cosmic String Theory
This theory is proposed by Richard Gott in 1991. The theory predicts about the presence of cosmic strings which would be left over from the big bang. If a person could travel around these strings, when those strings are about to collide, then he can go into the past.
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| Gigantic cosmic string |
Problems with this theory
i. It is difficult to find such strings.
ii. It is even more difficult to make these
strings collide.
iii. In order for the time traveler to travel
around these strings, the spacecraft must
have energy more than the half mass
energy of entire galaxy.
iv. If all the problems are overcome, there
lies the biggest problem, i.e. this would
exist for very short time.
v. It is a one-way trip option.
Some more theories which are not as practical as those discussed above: -
1. Spinning Universe: -
Kurt Gödel, in 1949, gave the theory of spinning universe. He said that if we spin faster and faster along with the spinning universe, we would travel into the past. This theory doesn’t make sense with our present-day knowledge of the universe. We know that universe is expanding and not spinning. So, the spin of the universe is zero. Hence, this theory is not practical.
2. Infinitely long cylinder: -
This theory was proposed by W.J. van Stockum, back in 1931. He said that if we construct an infinitely long cylinder and make it spin fast, we would travel in time. The cylinder needs to spin so fast that every material on it must fly apart. The cylinder needs to be infinitely long which is impossible to build.
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| Tipler Cylinder |
3. Train around the globe: -
This theory was proposed by Stephen Hawkings. This theory tells that if we successfully build a train which would travel with or near light speed, we can travel to future. The train must cover the whole circumference of earth. It needs to keep making several trips around the Earth. It is impossible as of today. We know that near to light speed travel is impossible. If we make it possible somehow than also, we would need lots of energy to make those trips. Hence, this is also an impractical idea.
Time travel has been the main center in sci-fi movies but its soon going to be a reality. But we can’t say with certainty that we would be alive after that or not. When time travel is made possible there would be no distinction among the past, the present and the future. I can bet it would be way different from as shown in the many of the movies. Our technology needs to evolve 100 time more than it is today. It will be one of the biggest achievements of science.
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