As is common knowledge, Einstein’s theory of relativity states that the flow of time is not absolute. The rate is inversely proportionate to velocity. That is, as velocity increases, the rate of time decreases. Furthermore, a series of tests of scientific rigor, the most archaic involving synchronized analog clocks, have proven that machines also slow down to reflect the deceleration of the flow of time. Therefore, since a complex organism is a conglomeration of natural machines, it would be reasonable to conclude that the aging process decelerates as well.
Thus, anybody who has travelled has time travelled, aging a fraction of a femtosecond less than those who were at rest during the trip, meaning they’ve essentially travelled that far into the future. When astronauts go into orbit, their aging slows by approximately 0.01 second per year. The current record for a single trip is held by Russian cosmonaut Sergei Avdeyev who travelled 0.02 seconds into the future over the course of a 748 days (2.05 years) orbiting the Earth.
As we approach the speed of light, the time dilation factor – the number by which the normal rate of time is divided – approaches infinity. That is, at light speed, machines and the aging process stop. If a person could travel to Proxima Centauri, the second nearest star to Earth, at the highest velocity proven possible by matter, roughly 4.5 years would pass on Earth while the traveller would only age a few minutes The trouble though is that a solid mass travelling at light speed would need to have infinite mass to compensate for mass compression.
However, if it were possible – or made possible through scientific efforts – to travel at the speed of light, exceeding the barrier would become a strong likelihood. Thus, if time halts at light speed, we can reasonably postulate that time reverses beyond light speed. What’s bizarre though is that this doesn’t mean the flow of time reverses outside. That is, the traveler won’t emerge in the year 2000 if they were to travel fast enough to go back a decade on a trip to Proxima Centauri. Rather, since the aging process stops at light speed, it would theoretically reverse beyond light speed. In other words, in the aforementioned scenario, the traveler would leap forward 14 years on a 4 year trip, or go back 10 years as the world moves forward by 4 years.
Einstein’s theory of relativity presents the possibility of closed time loops. As the name implies, if a person were to find a way onto one, the trip would end at the start while still spanning a perceived length of time. Since his publication of the world famous theory, many a mathematician and physicist – from Gott to Godel – have theorized ways to travel back in time such that aging is not reversed and the destination is our universe at an earlier age.
In the early years of research, a handful of eventually rejected theories were presented, laying the groundwork for our currently most accepted likelihood. One such theory stated that if the universe were spinning, travelling against the spin quickly enough would result in arriving at an earlier date upon the completion of a revolution. Unfortunately, no evidence has been found to show that the universe is spinning. Yet another, an upgrade to the previous, postulated that if one were to travel around a spinning pole of infinite length, they would eventually arrive at an earlier date. In addition to the pole needing to be infinitely long, the trouble is that it would need to spin too quickly for any known materials to withstand. Another still theorized that travelling through the point of impact at the moment of the collision of large cosmic strings would send the traveler back in time. This was deemed impractical since it would require causing the strings to collide in an extremely precise manner.
The most widely accepted possibility involves the use of wormholes. Wormholes link two distant points in space. Since space and time are intimately intertwined, it has been theoretically calculated that wormholes also link two points in time. Thus, if we could find a way to journey through a wormhole and survive the trip, we would, according to the calculations, travel back in time. A series of problems have been presented, yet all have been dismissed.
First of all, we have the possibility of paradoxes. A traveller could prevent their own birth somehow, as there would be nothing to physically stop them from, let’s say, killing their grandparents. However, building on the theory of the multiverse, it was concluded that travelling back in time would open an alternate timeline. That is to say, if the traveller killed their grandparents in the past, said grandparents would still be alive in the original timeline. This also dismissed the argument that the same photon of radiation would repeatedly travel through the wormhole, accumulating infinitely.
Also presented as an obstacle is the instability of the wormhole. According to the theory of relativity, a wormhole is likely to collapse with reckless spontaneity and absent of warning. However, at the event horizon, the border of the wormhole, the theory of relativity becomes moot and quantum physics take the helm. Therefore, though we perceive instability, this is likely not the case once we’re in the event horizon.
The only way to know for certain is through a unified theory, which physicists and mathematicians draw ever closer to reaching with every advancement in String Theory research. Therefore, time travel into the past is possible with our current knowledge. Absolute verification may come with the completion of String Theory, assuming the researchers’ predictions are correct.
But, even if alternate timelines and the stability of wormholes at the event horizon are disproven therein, this does not entirely disprove the possibility of traveling back in time. After all, it was once concluded that the universe is not spinning, which discredited one of the most popular theories of the day, and another theory emerged soon after. If String Theory ends up discrediting our current wormhole theory, contrary to the expectations of researchers of the matter, yet another theory will soon arise to take up the stead. But it looks quite promising that we got it right this time.