Theories of Time Machine
“What if I told thou that I come from the future. A future beyond thy imagination, where the colonies rebel against the British and gain their independence, where slavery be outlawed, where immigrants stream into the land and manufacture refrigerator magnets by the millions?” ― Steve Bates
Relative: So what do you do now?
Me: I am a time traveler.
“Tick-tock. Tick-tock. Tick-tock…..Stop!!!, Time to talk”
Since the past century, Time travel has become one of the fascinating topics in physics. Books, articles, and even movies have fantasized about it so much that everyone wants to do over in their life. But what if I tell you that you are doing time travel right now? Yes, you heard it correctly. With each second of the clock, you are moving forward in time. Cheers!! Just like me, you are also a time traveler. Time travel simply means traveling through time. If that is the case, then can we go to the past where life on earth was about to take birth or to the future where human civilization will have colonization on other planets? To answer that question, we have to know about the theory of Time Machine and why we have not yet been able to build one. So, time to dive into physics.
Theories of Time Machine:
Since the discovery of Einstein’s relativity, the concept of time has been changed forever. The equations permit time travel and tell us how to do it. So, scientists are trying everything to build theories of time machines to make time travel possible. But before going to those brainstorming theories, let’s discuss some natural ways of time travel.
(i) Time Travel via crazy speed:-
The most practical way to travel time is to move at crazy speeds. Einstein’s special theory of relativity tells us that the faster the object, the slower the clock will run. Just imagine a situation where your friend(of the same age as yours) leaves the earth to visit a planet far away from earth with a speed 0.6 times the speed of light and returns. By the time your friend returns to earth, you will be more aged than your friend because of the time dilation. So, naturally, your friend travels into the future. We can also go backward in time. Time will stop flowing at the speed of light, and you can enjoy any book without worrying about returning it before the due date. And if we can move faster than light, the equations tell us that we can go back in time. Bingo!!!!!! You can have a selfie with Einstein or even with Isaac Newton(when there were no such as cell phones) or have a ride on Dinosaur.
(ii) Time travel via curved space-time:-
The second way of doing time travel is going under a higher gravitational field. According to the General Theory of Relativity, space and time are together; thus, if space can be bent, then time can also be bent. In short, the stronger the effect of gravity, the slower the time will pass. Once again, imagine you are on earth, and your friend is on a planet whose gravity is much greater than earth. So, naturally, your friend’s clock will tick slower than yours, and he will move into the future(this scenario is perfectly depicted in the Interstellar movie).
Our GPS considers these time dilations; otherwise, we would have errors in different time zones and locations. However, we can’t go back in time with this method.
(iii) Time travel via Wormhole:-
The equations of General relativity do predict the existence of Wormholes (a shorter route that bridges two different regions of space-time). A wormhole can be used for time travel. Here is the process. If we accelerate one end of the wormhole to some fraction speed of the light by some advanced propulsion system keeping another end stationary, or put one end in a stronger gravitational field and bring back it to its original point, then in both of the cases will cause time dilation to that end and result in a difference in time between the two ends of the wormhole. Now, if we enter one end and come out from another, we can travel into the past or future. The only limitation of this method is we can’t go back before the time of the creation of this wormhole. However, we have not discovered any wormholes yet, and even if we find out in the future, it will be pretty challenging to make it stable before we can use it as a time machine.
So, you see, natural time travel is not an impossible theory; it’s just our technology is not advanced yet. In many sci-fi movies and series, we may have seen many time machines used to travel through different timelines, but in reality, we are far from time travel. But physicists have not given up yet. Time travel opens the possibility of Parallel Universes. Quantum mechanics predicts the same. So, scientists are trying hard to propose many theories for time travel. Let’s discuss those theories and why they have failed.
(i) Tipler Cylinder:-
In 1937, W. J. Van Stockum found a solution to Einstein’s equations that permitted time travel. Later in 1974, Frank Tipler published a paper mentioning an infinitely long spinning cylinder. He computed that if such a cylinder starts rotating near the speed of light, it will drag the fabric of space-time(known as frame-dragging) along with it. Now, one can go back in time by making a complete trip around the cylinder. The faster the cylinder spins, the further one can go back in time. The only limitation is that one can’t travel further back in time than the creation of the cylinder itself.
For this mechanism to work, we need a matter of roughly ten times the sun’s mass, and it has to spin a few billion revolutions per minute. Just Imagine, even if such a cylinder could be built, the centrifugal forces on the cylinder would be so tremendous that it would cause the material that made up the cylinder to fly apart. Certainly, this mechanism would allow very, very advanced civilization for time travel, and that’s why the idea failed.
(ii) Gödel universe:-
In 1949, Kurt Gödel, a mathematician, found a different solution to Einstein’s field equations. His solution described another kind of space-time known as the “Gödel universe.” He assumed the universe to be rotating. More the gravity inside the universe, the more the rotation speed it will have to balance the gravity by its centrifugal force. Now, if anyone can make a round trip around this universe and return to the starting point, one can go back in time. But to make time travel, one must travel just below the speed of light. Einstein himself ruled out his solution because his assumption of the universe being rotating was wrong; the universe is expanding. And that’s the main reason why the idea did not work out.
(iii)Kip Throne’s Time Machine:-
In 1988, Kip Throne, with two other physicists, showed that it is possible to build a time machine. He mentioned a strange kind of matter and energy, known as “Exotic matter” and “negative energy.” And this theory represented a breakthrough in our understanding of time travel. Exotic matter can make a wormhole transversal, so one can make a two-way trip through it. The only problem with this type of matter is we have not discovered it yet. Exotic matter shows the opposite nature of ordinary matter. Where normal matter gets attracted by gravity, the exotic matter gets repelled. So, it’s pretty normal that it would have been floated away by now and hence difficult to find. But the good news is we have observed an effect of negative energy through the Casimir Effect. In 1933, Henrik Casimir showed that two uncharged parallel metal plates could create negative energy. It’s normal to expect two plates to remain stationary because they are uncharged. However, he showed a tiny attractive force between the two uncharged parallel plates. And in 1948, this tiny force was measured, indicating that negative energy was a real possibility. So, Kip Throne thought of two parallel plates with a minimal gap between them so that negative energy could take place. These plates then would be reshaped into a sphere with an inner and an outer shell. Then those two plates will somehow create a wormhole with one end enclosed in a sphere and the other end in another sphere. Time must be synchronized between the two ends. Now accelerate one end at some fraction of the speed of the light, keeping another end stationary and bringing it back to its original point; this will cause time dilation to the end and result in a difference in time between two ends of the wormhole. One can enter at one end and go back to the past. The limitation is that one can’t travel further back in time than the creation of the wormhole itself.
(iv)Gott’s time machine:-
In 1991, J. Richard Gott III of Princeton came up with a new theory. This time no spinning, no wormholes, or exotic substances. He put forward a new idea of cosmic strings. These are 1-dimensional topological objects with a width less than an atomic nucleus. Gott found a solution that allowed for cosmic strings. He noticed that if you take two cosmic strings and send them toward each other just before the collision, it is possible to use them as a time machine. Space gets contracted if one makes a round trip around these colliding strings. Because of the space contraction, one can exceed the speed of light( as viewed by a distant observer) without violating relativity(speed is not exceeding in the traveler’s frame) and go back to the past. But to make it happen, we need cosmic strings with a mass-per-unit length of about 10 million billion tons per centimeter. And there is no evidence of cosmic strings found till now; scientists are still searching.
So, you see, even the greatest minds of this civilization have invested their time and effort to make time travel happen. Many more have gone, many yet to come. But none of their efforts seemed very successful, but they didn’t fail either. There is no theory or any law of nature that forbids us from time travel. So, each time we made an effort, we got one step closer to reality. We have to keep trying until we hit the ground.
