Time travel has been a subject of great interest in science fiction for a long time. Whether it’s films like Planet of the Apes (1968) or modern franchises like “Doctor Who” and “Star Trek” ; the concept is attracting a lot of eyes. Not just movies and shows, but even some mythological tales like Mahabharata and the Japanese story of Urashima Tarot support the evidence that time travel exists. We often see stories where characters use time machines to travel through the years, but the reality is much more complex and inexplicable.
Understanding the concept of time travel
Time Travel is defined as the phenomenon of movement between different points in time using a hypothetical device called a “Time Machine”. Despite being predominantly related to the field of philosophy and fiction, it is somehow supported, to a small extent, by physics in conjunction with quantum mechanics. However, before we get into the argument of how real it is, let's understand the fundamental meaning of time.
Basically, the whole idea of Time Travel is managed by the concept of time. Normally people believe that time is constant, but the famous Physicist Albert Einstein introduced the “Theory of relativity” according to that, time is relative. In other words, time slows down or speeds up depending on how quickly the observer moves relative to something else. According to him, a person traveling inside a spacecraft at the speed of light would age much more slowly than his twin brother at home.
Time is relative
After Einstein's Theory of Relativity, his teacher Herman Minkowski emphasized space-time, a mathematical model that unites space and time in a continuum. This implies that time and space cannot exist without each other. Space is a three-dimensional arena consisting of length, width and height. To this is added Time with the fourth dimension called direction. Therefore, everything that happens in the universe occurs in this space-time continuum. While this validates that space travelers are slightly younger than their twins when they return to Earth, a large leap into the past or future is still not possible with current technology.
Time Machines
It is believed that to go back or forward in time you would need a device called a Time Machine . Research into such a device would involve bending spacetime to such an extent that the timelines rotate on themselves to form a loop, which is termed a “closed time curve.” Such an action requires the use of an exotic form of matter with “negative energy density” that has the unique property of moving in the opposite direction of normal matter when pushed. Even if it exists, the quantity would be too small to build a machine.
Pictorial representation of time travel through a closed time-like curve
However, some other research suggests that time machines could also be built by constructing a donut-shaped hole surrounded by a sphere of normal matter. Inside this vacuum-filled donut-shaped hole, gravitational force can be used to bend spacetime to form a closed time-like curve. After running inside this donut, the traveler will be able to go back in time with each lap. But in reality, it is quite complex because the gravitational fields have to be very strong and would require precise manipulation.
Time travel approaches in physics
After studying and researching Time Travel, several physicists have created approaches that can support its possibility, at least theoretically. Let's take a look at these concepts to understand how Time Travel might actually work someday.
Time dilation
Explanation of time dilation
An important aspect of Einstein's theory of relativity is the term “time dilation” , which is defined as the difference in time elapsed between two events, measured by observers who are moving relative to each other or are situated at different locations in the world. gravitational mass. According to the theory, time dilation can be summarized as a phenomenon that occurs due to the difference in gravity or relative speed.
In special relativity, the effect of time dilation is reciprocal, that is, when two clocks are in motion relative to each other, for both observers, the other will be dilated in time or the other clock will move more slowly. However, in general relativity, an observer at the top of the tower will find that the clock closer to the ground is slower and the other observer will agree on the direction and magnitude of this difference.
Due to the concept of time dilation, the current human record for time travel is held by Russian cosmonaut Sergei Krikalev . Due to the high speed (7.66 km/s) of the ISS and the time spent in space, it is believed that the cosmonaut actually arrived 0.02 seconds into the future while returning to Earth.
Cosmic String
Diagram representing cosmic strings
In 1991 J Ricardo Gott gave the idea of Cosmic Strings , which are believed to be remnants of the early cosmos. They are defined as string-like objects or narrow tubes of energy that stretch the entire length of the universe. Due to the enormous amount of mass and enormous gravitational attraction, it would allow objects attached to the Cosmic Strings to travel at the speed of light.
Therefore, if two strings are pulled close to each other or one of them is stretched close to the black hole, this could warp spacetime to such an extent that it would lead to the creation of a closed time-like curve and thus the possibility of time. trip. Theoretically, the gravity generated by these two cosmic strings would help propel a spacecraft into the past.
However, coming to reality, it requires the loop of strings to contain half the mass-energy of an entire galaxy to travel back in time one year. This implies that to power a time machine it would be necessary to split half of the atoms present in the entire galaxy.
Black holes
Kerr Hole Illustration
When stars (with a mass greater than four times that of our Sun) reach the end of their life and all their fuel is burned, they collapse under the pressure of their own weight, creating “Black Holes” . The boundary of a black hole, called the Event Horizon , has such a strong gravitational pull that it doesn't even allow light to pass through it. Because light travels at the fastest speed, everything else that travels through a black hole is also dragged behind it. A non-rotating black hole is called a Schwarzschild black hole .
However, traveling to a parallel universe is possible through a rotating black hole called the Kerr Hole . It was proposed in 1963 by a mathematician named Roy Kerr . According to his theory, if dying stars collapsed into a rotating ring of neutron stars, it would produce enough centrifugal force to prevent singularity formation.
Note: Singularity can be perceived as the point at which the black hole narrows, like an ice cream cone. At this point, the laws of Physics cease to exist and all matter is crushed beyond recognition.
Since there will be no singularity, it would be safe to pass through a black hole without being crushed and exit a black hole. “White Hole” . A white hole is believed to be the exhausting end of a black hole that pushes everything away from it. Therefore, we can travel to another time or even to another universe.
Although Kerr holes are only theoretical, if they exist, we could find our way to a one-way trip to the past or future. However, physicist Kip Thorne believes that such a black hole does not exist and that it would suck up everything before anyone even reaches the Singularity.
Wormholes
Diagrammatic representation of the wormhole
Wormholes, also known as Einstein-Rosen Bridges , are considered the most potential means of time travel. It could allow us to travel several light years from Earth and in much less time compared to conventional methods of space travel. The possibility of wormholes is based on Einstein's theory of relativity, which says that any mass curves space-time. The following example is used to explain this curvature.
If two people are holding a sheet taut and a baseball is placed on the sheet, its weight will cause it to roll to the middle of the sheet, creating a curve at that point. Now, if a marble is placed on the sheet, it will travel towards the baseball because of the curve. Here space is represented as a two-dimensional plane rather than the four dimensions that actually constitute space-time.
Now, if this sheet is folded leaving a space at the top and bottom, placing the baseball on top would form a curvature. If an equal mass were placed at the bottom at a point corresponding to the baseball's location, the second mass would eventually meet the baseball. Likewise, wormholes can develop.
In space, masses that exert pressure on different parts of the universe combine to form a tunnel. Theoretically, this tunnel unites two distinct times and allows passage between them. However, it is possible that certain unforeseen physical properties could prevent wormholes from occurring, and even if they do exist, they could be truly unstable.
Possibly someday humans may learn to capture, stabilize and enlarge these tunnels, but according to Dr. Hawking, prolonging the life of a tunnel through warped space-time could lead to a radiation feedback loop that destroys the time Tunnel.
Paradoxes of time travel
If we were ever to develop a theory for time travel, we would give rise to certain complexities known as paradoxes. A paradox is something that contradicts itself. In other words, time travel is not believed to be a practical concept due to certain situations that are likely to arise as consequences. These are broadly classified as -:
1. Closed Casual Loops: The cause and effect run in a circle causing a loop and are also internally consistent with the timeline history.
Diagram representing the time loop
• Predestination Paradox
It is defined as the situation in which a traveler who goes back in time causes the event he is trying to avoid. It implies that any attempt to prevent any event from occurring in the past would simply lead to the cause itself. The paradox suggests that things are destined to happen as they did and anyone who tried to change the past would be trapped in the repeating cycle of time. For example, if you travel back in time to prevent your lover from dying in a car accident, you will discover that you were the one who accidentally ran her over.
• Bootstrap Paradox
A bootstrap paradox, also known as the Ontological Paradox, where an object, person, or information sent back in time leads to an infinite loop where the object has no discernible origin and is believed to exist without ever being created. It implies that the past, present and future are not defined, making it complicated to identify the origin of anything. It raises questions such as how objects were created and by whom.
2. Consistency Paradox: Generates a series of inconsistencies in the timeline related to the possibility of altering the past. It can be divided into the following categories.
• The Grandfather Paradox
Grandfather Paradox
This paradox speaks of a hypothetical situation in which a person travels through time and kills his paternal grandfather at a time when his grandfather did not even know his grandmother. In such a situation, his father would never have been born and neither would the traveler himself. So if he was never born, how would he travel to the past to kill his grandfather?
The paradox also talks about self-infanticide, where a time traveler goes to the past to kill himself as a child. Now, if he killed himself when he was a child, how would he exist in the future to go back in time? Some physicists say that you would be able to go back in time but not be able to change it, while others suggest that you would be born in one universe but not born in another universe.
• Hitler's Paradox
Similar to the grandfather paradox, the Hitler assassination paradox erases the reason why you would want to go back to the past and kill Hitler. Furthermore, killing the grandfather could have a “butterfly effect”, but killing Hitler would have a far-reaching impact on History, as it would change the entire course of events. If you managed to kill Hitler, there would be no reason why you would want to go back in time and kill him.
This paradox was explained very well in a Twilight Zone episode called “Cradle of Darkness”, as well as in a Dr. Who episode “Let's Kill Her”.
• Polchinski paradox
American physicist Joseph Polchinski proposed a paradox where a billiard ball enters a wormhole and emerges on the other side in the past just in time to collide with its younger self and prevent it from entering the wormhole in the first place. In proposing this scenario, Joseph had in mind Novikov's Principle of Self-Consistency, which states that time travel is possible but time paradoxes are prohibited.
A number of solutions have been suggested to avoid these inconsistencies, such as the billiard ball delivering a blow that changes the course of the younger version of the ball, but would not prevent it from entering the wormhole. This also explains that if you go back in time to kill your grandfather, something or other will happen to stop you from making it happen, thus preserving the consistency of History.
Solutions to the paradoxes
In order to find a solution to these above-mentioned paradoxes, scientists have proposed some explanations which are listed below
The solution: Time travel is impossible because of the paradoxes it creates.
Self-healing hypothesis: If we can change the events of the past, this will trigger another set of events that will keep the present unchanged.
The Multiverse: Each time an event in the past is altered, an alternative parallel universe or timeline is created.
Erased timeline hypothesis: A person traveling to the past would exist in the new timeline, but their own timeline would be erased.
Is time travel possible?
Is time travel possible?
No one seems to have a definitive answer for or against the existence of Time Travel. On the one hand, Einstein suggested traveling at the speed of light to wander into the future, but that would mean that an unimaginable amount of energy would be required. Furthermore, the centrifugal force on the body would be fatal. Although it has been observed that space travelers age a little slower compared to their identical twins on Earth, some believe there is no definitive answer to traveling back to space.
Theoretical physicist Brian Greene of Columbia University says that “No one has given definitive proof that you cannot travel to the past. But every time we look at the proposals and the details, it seems clear that they are at the limit of the known laws of physics.” Furthermore, Prof. Hawking feels that “today’s science fiction is tomorrow’s science fact.”
However, the paradoxes, especially the grandfather paradox, have imposed a big question mark on the possibility of Time Travel. Basically, with the current laws and knowledge of Physics, humans will not be able to survive the Time Travel process. Therefore, we need certain developments in quantum theories until we are sure how the paradoxes can be resolved.
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