Time travel i.e., moving between different points in time has been a popular topic for science fiction for decades. We have seen humans get in a vehicle of some sort and arrive in the past or future, ready to take on new adventures in many sci-fi movies.
The reality, however, is more muddled. Not all scientists believe that time travel is possible. Some even say that an attempt would be fatal to any human who chooses to undertake it.
What is Time?
While most people think of time as a constant, physicist Albert Einstein showed that time is an illusion; it is relative, it can vary for different observers depending on your speed through space. To Einstein, time is the "fourth dimension." Space is described as a three-dimensional arena, which provides a traveler with coordinates, such as length, width and height showing location. Time provides another coordinate, direction, although conventionally, it only moves forward. Time joins the party as that most crucial fourth dimension. Time can't exist without space, and space can't exist without time. Any event that occurs in the universe has to involve both space and time.
Time travel theories
Wormholes are hypothetical, warped space-time which are also permitted by the Einstein field equations of general relativity. The method is to take one entrance of the wormhole and move it to within the gravitational field of an object that has higher gravity than the other entrance, and then return it to a position near the other entrance.
Astronomer Frank Tipler proposed a mechanism (sometimes known as a Tipler Cylinder) where one would take a matter that is 10 times the sun's mass, then roll it into a very long but very dense cylinder. After spinning this up a few billion revolutions per minute, a spaceship nearby following a very precise spiral around this cylinder could get itself on a "closed, time-like curve."
Another possibility would be to move a ship rapidly around a black hole or to create that condition with a huge, rotating structure artificially.
"Around and around they'd go, experiencing just half the time of everyone far away from the black hole. The ship and its crew would be travelling through time," -physicist Stephen Hawking
“Imagine they circled the black hole for five of their years. Ten years would pass elsewhere. When they got home, everyone on Earth would have aged five years more than they had.”
Another theory for potential time travelers involves something called cosmic strings, narrow tubes of energy stretched across the entire length of the ever-expanding universe. These thin regions, leftover from the early cosmos, are predicted to contain huge amounts of mass and therefore, could warp the space-time around them.
Cosmic strings are either infinite or in loops, with no ends, scientists say. The approach of two such strings parallel to each other would bend space-time so vigorously and in such a particular configuration that might make time travel possible, in theory.
It is generally understood that travelling forward or back in time would require a device — a time machine — to take you there. Time machine research often involves bending space-time so far that time lines turn back on themselves to form a loop, technically known as a “closed time-like curve.”
To accomplish this, time machines often are thought to need an exotic form of matter with so-called “negative energy density.” Such an exotic matter has bizarre properties, including moving in the opposite direction of normal matter when pushed. Such a matter could theoretically exist, but if it did, it might be present only in quantities too small for the construction of a time machine.
Also, humans may not be able to withstand time travel at all. Travelling nearly the speed of light would only take a centrifuge, but that would be lethal.
Using gravity would also be deadly. To experience time dilation, one could stand on a neutron star, but the forces a person would experience would rip you apart first.
So, is time travel possible?
Stephen Hawking once suggested that the absence of tourists from the future constitutes an argument against the existence of time travel. Hawking notes elsewhere that time travel might only be possible in a region of space-time that is warped correctly, and that if we cannot create such an area until the future, then time travelers would not be able to travel back before that date.
While time travel does not appear possible, at least in the sense that the humans would survive it, with the physics that we use today, the field is constantly changing. Advances in quantum theories could perhaps provide some understanding of how to overcome time travel paradoxes.
No one has given a definitive proof that you can't travel to the past. But every time we look at the proposals and details, it seems clear that they're right at the edge of the known laws of physics.
One possibility, although it would not necessarily lead to time travel, is solving the mystery of how certain particles can communicate instantaneously with each other faster than the speed of light.
But as you've probably noticed, we're all constantly engaged in the act of time travel. At its most basic level, time is the rate of change in the universe -and like it or not, and we are continually changing. We age, the planets move around the sun, and things fall apart.
(Inspired from Interstellar)
A version of this article appears on our Medium Page.
First published on Jan 19, 2017.