Time travel subtly intertwines with our measurement of time through clocks and calendars â artificial constructs weâve put up. But how do we know time exists? How can we say for sure that time is passing by? Say, it did actually exist. When did the Universeâs clocks start ticking? It is generally accepted that the concept of time came into existence after the Big Bang. But who put the batteries in that clock?
Time has continued to confound physicists and philosophers for centures. Oxford defines time as an indefinite, continued, progress of existence and events in the past, present and future, regarded as the whole. And weâll take their word for it. What about time travel, though? What if you could go back in time and change your past? Or what if you could travel hundreds of years into the future and meet your great-great-great-something grandchildren? How exciting would that be?
Early stories of time travel feature a character goes away or sleeps for some time, only to be reawakened in a future society. The idea of time travel first appeared in the Hindu Text, the Mahabharata, in the 9th century BC. The Mahabharata mentions the story of a King called Raivata Kakudmi, who travels to heaven in order to meet the Ultimate Creator Brahma. When he returns to Earth, he is shocked to have learnt that many years have passed. The Japanese tale of Urashima Taro is one of a fisherman who visits an undersea palace, and on returning home, realises that 300 years have passed.
The term âTime Machineâ was coined by the author H.G. Wells, who penned a novel with the same name. It introduced the concept of forward as well as backwards time travel. The novel went on to gain cult status, and has been adapted into comic books, television series, as well as two feature films. Since then, time travel (or manipulation) has been the topic of many popular movies such as the X-men series, Source Code, Back to Future (I, II &III), The Edge Of Tomorrow, and The Terminator,  among many others.
In the 1985 hit Back To The Future, a boy named Marty travels back in time using the time machine invented by an eccentric scientist. He meets his parents there and accidentally attracts his motherâs attention, putting his existence at stake. He must make his parents fall in love. In the Edge of Tomorrow, Major William Cage (Played by Tom Cruise), has to save Planet Earth from an alien species, after being caught in a time loop. All in all, the genre of science fiction itself has espoused the idea of time travel with great enthusiasm.
Before jumping into the answer, we need to understand a few things, including the Theory of Relativity. According to Einsteinâs theory, time passes differently â that is, at different rates â for people who are moving relative to one another. What does this tell us?
First of all, there is no âabsoluteâ frame of reference. Whenever you measure an objectâs velocity, momentum, etc, it is always with respect to something else. Secondly, the speed of light stays constant no matter who measures it or how fast the person measuring it is going. Thirdly, nothing can go faster than the speed of light.
Related: A Brief History Of Life
The Twinsâ Paradox
Letâs take a look at the Twinsâ Paradox. Take two twin brothers. Say one twin travelled to space, while the other stayed back here on Earth. Consider that the twin in space travels for 10 years at the speed of light. When that twin comes back to Earth, he seems to have hardly aged at all, while his twin brother has aged 10 years.
In fact, he has technically jumped 10 years into the future! A concept called time dilation. Although it is noticeable at velocities comparable to the velocity of light, this concept exists all around us. This shows that all of us are moving into the future, at different rates! According to Caltech Physicist Kip Thorne, replacing the twins in the earlier scenario with corresponding mouths of a wormhole, you could, in fact, created a time machine!
A popular concept called The Grandfather Paradox poses a challenge to the concept of time travel. According to it, in a theoretical scenario,  if you were to go back in time and kill your grandfather before your father is born, your would prevent your own birth, thus making it impossible to go back in time and kill your grandfather in the first place.
But in fact, there is a loophole in that one too. A proposed âmany universesâ interpretation of quantum mechanics by British physicist David Deutsch puts forward another theoretical scenario. It proposes that alternate, or parallel universes exist. You would therefore go to another universe, kill your grandfather there â thereby losing your existence in that universe only. This gives an interesting twist to the Paradox.
Technically, yes. Russian Cosmonaut Sergei Avdeyev is considered Earthâs first-time traveller. Across 3 visits to the Mir station in space, he spent around 747 days in low Earth orbit. Sergei circled Earth a whooping 12 thousand times, travelling a total of 515 million kilometres. Sergei Avdeyev is one-fiftieth of a second âyoungerâ than if he had stayed on Earth. This means that he has travelled one-fiftieth of a second into the future, experiencing 0.02 seconds less than the rest of us here on Earth. This is of course, nearly nothing. But itâs obvious that if we can somehow attain exponentially higher speeds, the time dilation factor could end up being significant.
All these concepts are on the brink of reality, tipping into the realm of science fiction. It does make you wonder â does time travel go against the laws of nature? But then, isnât everything that humans have done on this Earth doing just that? According to Hawking.org, Professor Stephen Hawking proposed the Chronology Protection Conjecture. According to the conjecture, on a macroscopic scale â the laws of physics have actually conspired to prevent us from travelling through time! We humans though, keep chanting âNothing is impossible!â on a regular basis. There is nothing wrong in that, but in cases such as time travel, it is a little far-fetched.
To travel into the future, we will need to travel at a speed greater than the speed of light. Think of travelling back into the past, and we are faced with a lot of paradoxes. Letâs face it â the odds donât seem to be in our favour. But then again, a hundred years ago, the concept of man flying seemed impossible. We can continue to do what we humans do best-hope!