Have you ever made a blunder you wish you could take back? One of the reasons we find time travel so intriguing is the possibility of correcting past mistakes. Nothing is permanent with a time machine, as depicted in science fiction, because you can always go back and change it. But, in our universe, is time travel possible, or is it just science fiction?
Although many people are fascinated by the idea of altering history or seeing the future before it happens, no one has ever demonstrated back-and-forth time travel or proposed a method of sending a person through significant periods without destroying him or her in the process. “The best evidence we have that time travel is not possible, and never will be, is that we have not been invaded by hordes of tourists from the future,” physicist Stephen Hawking wrote in his book “Black Holes and Baby Universes.”
However, science does allow for some time travel. The theory of special relativity, developed by physicist Albert Einstein, proposes that time is an illusion that moves relative to an observer. Time, with all its aftereffects (boredom, aging, etc.) will be experienced much more slowly by an observer travelling near the speed of light than by an observer at rest. That’s why, after a year in orbit, astronaut Scott Kelly aged slightly slower than his twin brother who remained on Earth.
Other scientific theories about time travel exist, such as some strange physics involving wormholes, black holes, and string theory. Time travel, for the most part, is the subject of an ever-growing number of science fiction books, movies, television shows, comic books, video games, and other media.
Time Travel Theories
Time travel via speed
Going fast is the simplest and most practical way to time travel into the far future.
When travelling at speeds approaching the speed of light, time slows down for you concerning the outside world, according to Einstein’s special relativity theory.
This is not just a guess or a thought experiment; it’s been proven. Physicists have demonstrated that the flying clock ticks slower due to its speed using twin atomic clocks (one flown in a jet aircraft and the other stationery on Earth.
In the case of aircraft, the impact is negligible. However, if you were on a starship travelling at 90% of the speed of light, time would move around 2.6 times slower than it would on Earth. The time travel becomes more intense as you get closer to the speed of light.
Protons whirling through the Large Hadron Collider at 99.9999991 per cent the speed of light are perhaps the fastest speeds achievable by any human technology. We can compute that one second for the proton is equal to 27,777,778 seconds, or nearly 11 months for us, using special relativity.
Time travel via gravity
Einstein also influenced the next method of time travel. The heavier the gravity you feel, the slower time advances, according to his general relativity theory.
The strength of gravity increases as you move closer to the Earth’s centre, for example. Your feet move at a slower pace than your head.
This effect has also been measured.
All we need is a location of incredibly intense gravity, such as a black hole, to go to the remote future. The closer you get to the event horizon, the slower time moves – but it’s a perilous business because once you cross the barrier, you’re stuck.
In addition, in any case, the effect is not particularly strong, so the trip is probably unnecessary.
If you had the technology to travel the immense distances required to reach a black hole (the closest is around 3,000 light-years away), the time dilation caused by travel would be substantially larger than the time dilation caused by orbiting the black hole.
Time Travel through Suspended Animation
Another approach to time travel to the future is to slow down or stop your physical processes and then restart them later.
Bacterial spores can exist in suspended animation for millions of years until the correct conditions of temperature, moisture, and food reactivate their metabolism. During hibernating, some mammals, such as bears and squirrels, can substantially reduce their cell’s need for food and oxygen by slowing down their metabolism.
Is it possible for people to do the same?
Though entirely shutting down your metabolism is probably beyond our current technological capabilities, some scientists are focusing on inducing a short-term hibernation state that lasts at least a few hours. This could be just enough time to get someone through a medical emergency like cardiac arrest before they can go to the hospital.
In 2005, American scientists demonstrated that exposing mice to minute quantities of hydrogen sulfide, which binds to the same cell receptors as oxygen, could impair their metabolism. The mice’s core body temperature dropped to 13 degrees Celsius, and their metabolism slowed tenfold. The mice might be reanimated without harm after six hours.
Similar trials with lambs and pigs were unsuccessful, implying that the procedure may not work for larger animals.
Another method, which involves replacing the blood with a cold saline solution to induce hypothermic hibernation, has been tested on pigs and is currently being tested on humans in Pittsburgh.
Time travel via wormholes
General relativity also allows for the creation of wormholes, which could span billions of light-years or different points in time.
Many physicists, including Stephen Hawking, believe that wormholes appear and disappear on a quantum scale far smaller than atoms. The trick would be to capture one and inflate it to human scales – a feat that would take a tremendous amount of energy but could theoretically be accomplished.
Attempts to demonstrate either side have failed due to the incompatibility of general relativity and quantum mechanics.
Time travel using light
Another time travel concept proposed by American physicist Ron Mallet is to twist space-time using a rotating cylinder of light. Anything dropped inside the whirling cylinder might conceivably be pulled around in space and time, much like a bubble on top of the coffee after a spoon swirl.
The appropriate geometry, according to Mallet, may lead to time travel into the past or future.
Mallet has been seeking to gather funds for a proof of concept experiment, which includes dropping neutrons through a circular array of spinning lasers, since announcing his theory in 2000.
Others, on the other hand, argue that one of his basic model’s assumptions is plagued by a singularity, which physics speaks for “it’s impossible.”
Recent Research
If our Universe allows multiple histories to coexist, time travel may be possible. So, can it?
If you believe Everett’s “many-worlds” interpretation, one history can “split” into multiple histories, one for each possible measurement outcome – for example, whether Schrödinger’s cat is alive or dead, or whether I arrived in the past.
Time travel and parallel timelines are usually associated with science fiction, but we now have proof that they must also be associated with real science. Time travel may be possible, according to general relativity and quantum mechanics, but if it is, multiple histories must also be possible.
Also published on Medium.