Time Travel: Theories, Paradoxes & Possibilities
Time travel — moving between different points in time — has been a popular topic for science fiction for decades. Franchises ranging from “Doctor Who” to “Star Trek” to “Back to the Future” have seen humans get in a vehicle of some sort and arrive in the past or future, ready to take on new adventures. Each come with their own time travel theories. 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.
While most people think time is a constant parameter, physicist Albert Einstein showed that time is an illusion; it is relative and it can vary for different observers depending on your speed through space. Einstein’s theory of special relativity says that time slows down or speeds up depending on how fast you move relative to something else — this effect is called Time Dilation. Time dilation can be caused due to various factors like relative velocity or gravitational potential between their locations.
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. Approaching the speed of light, a person inside a spaceship would age much slower than his family at home. Also, under Einstein’s theory of general relativity, gravity can bend time. The object travelling at the speed of light will face less gravitational pull with respect to the surrounding objects and hence time slows down for it.
Through the wormhole
Wormholes are tunnels connecting points in space-time in such a way that mass travelling through the wormhole would take much less time than the trip through the normal space. Wormholes are predicted by the theory of general relativity. But be wary: wormholes bring with them the dangers of sudden collapse, high radiation and dangerous contact with exotic matter, and even loss of memory.
Wormholes were first theorized in 1916. While reviewing another physicist’s solution to the equations in Albert Einstein’s theory of general relativity, Austrian physicist Ludwig Flamm realized another solution was possible. He described it as “White Hole,” a theoretical time reversal of a black hole.
In 1935, Einstein and physicist Nathan Rosen used the theory of general relativity to elaborate on the idea, proposing the existence of “bridges” through space-time. These bridges connect two different points in space-time, theoretically creating a shortcut that could reduce travel time and distance. The shortcuts came to be called Einstein-Rosen bridges, or wormholes.
Einstein’s theory of general relativity mathematically predicts the existence of wormholes, but none have been discovered to date. A negative mass wormhole might be spotted by the way its gravity affects light that passes by. However, a naturally occurring black hole, formed by the collapse of a dying star, does not by itself create a wormhole.
Alternate time travel theories
While Einstein’s theories appear to make time travel difficult, some groups have proposed alternate solutions to jump back and forth in time.
Infinite cylinder
Astronomer Frank Tipler proposed a mechanism known as a Tipler Cylinder where one would take matter that is 10 times the sun’s mass, then roll it into 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”, according to the Anderson Institute. There are limitations with this method, including the fact that the cylinder needs to be infinitely long for this to work which in practical world is not possible.
Cosmic strings
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, left over 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 they’re 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.
Alcubierre drives
The Alcubierre drive or Alcubierre warp drive, or Alcubierre metric is a speculative idea based on a solution of Einstein’s field equations in general relativity as proposed by theoretical physicist Miguel Alcubierre, by which a spacecraft could achieve apparent faster than light travel if a configurable energy density field lower than that of vacuum (with negative mass) could be created.
Rather than exceeding the speed of light within a local reference frame, a spacecraft would traverse distances by contracting space in front of it and expanding space behind it, resulting in effective faster-than-light travel. Objects cannot accelerate to the speed of light within normal spacetime; instead, the Alcubierre drive shifts space around an object so that the object would arrive at its destination faster than light would in normal space without breaking any physical laws.
So is time travel possible?
While time travel does not appear possible (at least, possible 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. Even if time travel is made possible it’s questionable whether you can feel it, who knows you might already be in a world where somebody somewhere is using the time machine.Whether or not the future has time machines in store, we can comfort ourselves with the knowledge that when we climb a mountain or speed along in our cars, we change how time ticks.
“People like us, who believe in physics, know that the distinction between past, present and future is only a stubbornly persistent illusion.” — Albert Einstein.
