Time travel: moving between different points in time has been a popular topic for science fiction for decades.
The reality 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.
Einstein’s theory of special relativity says that time slows down or speeds up depending on how fast you move relative to something else. Approaching the speed of light, a person inside a spaceship would age much slower than his twin at home. Also, under Einstein’s theory of general relativity, gravity can bend time.
Picture a four-dimensional fabric called space-time. When anything that has mass sits on that piece of fabric, it causes a dimple or a bending of space-time. The bending of space-time causes objects to move on a curved path and that curvature of space is what we know as gravity.
Both the general and special relativity theories have been proven with GPS satellite technology that has very accurate timepieces on board. The effects of gravity, as well as the satellites’ increased speed above the Earth relative to observers on the ground, make the unadjusted clocks gain 38 microseconds a day. (Engineers make calibrations to account for the difference.)
Through the wormhole
General relativity also provides scenarios that could allow travelers to go back in time, according to NASA. The equations, however, might be difficult to physically achieve.
One possibility could be to go faster than light, which travels at 186,282 miles per second (299,792 kilometers per second) in a vacuum. Einstein’s equations, though, show that an object at the speed of light would have both infinite mass and a length of 0. This appears to be physically impossible, although some scientists have extended his equations and said it might be done.
A linked possibility, NASA stated, would be to create “wormholes” between points in space-time. While Einstein’s equations provide for them, they would collapse very quickly and would only be suitable for very small particles. Also, scientists haven’t actually observed these wormholes yet. Also, the technology needed to create a wormhole is far beyond anything we have today.
Another possibility would be to move a ship rapidly around a black hole, or to artificially create that condition with a huge, rotating structure.
“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 traveling through time,” physicist Stephen Hawking wrote in the Daily Mail in 2010.
“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.”
However, he added, the crew would need to travel around the speed of light for this to work. Physicist Amos Iron at the Technion-Israel Institute of Technology in Haifa, Israel pointed out another limitation if one used a machine: it might fall apart before being able to rotate that quickly.
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.
It is generally understood that traveling 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.”
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.
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.
In the meantime, however, interested time travelers can at least experience it vicariously through movies, television and books.