Is Time Travel Possible? – Part 2
If the speed of light was universal, (in an empty space vacuum), however, it would need to be measured the same – 299,792,458 metres per second – no matter the speed we move at. Einstein worked out that the consequence of the speed of light being absolute is that time and space cannot be. It also turns out that a clock in motion must tick at a slower speed than a stationary one.
If someone were to fly at a super-fast speed in a spaceship and return to Earth, they would have experienced less time passed than those they left on Earth. Everyone at home would perceive that the time traveller’s life had appeared to run in slow motion. The time traveller would have agreed upon the difference in perceived time but would have concluded that the life of those on earth had appeared to be in fast forward.
So, if we could indeed travel faster than the speed of light, would time run backwards as we’ve been led to believe by science fiction? The problem lies in the fact that it would take an infinite amount of energy for an individual to accelerate at the speed of light, never mind any further than that. Even if it was possible, time wouldn’t run backwards. Rather, it would make no sense to even discuss time going forwards and backwards. The causality law would be violated and there would no longer be any meaning to ‘cause and effect’.
Einstein told us something else: the force of gravity is an effect of how mass warps time and space. Essentially, the more amount of mass that is squeezed into an area of space, the more space-time is warped and the lower that nearby clocks tick.
If we were able to squeeze in sufficient mass, spacetime would become warped to the degree that light would’ be able to escape its gravitational pull and would form a black hole. If someone were to go near the edge of the hole (the event horizon), their clock would tick at an infinitely slow speed relative to anyone far away.Is it possible that spacetime could be warped in a way as to close back on itself and travel in time? Maybe. The warping we could need would be a traversable wormhole. We also require regions of negative energy in order for it to be stabilised and the 19th-century classical physics prevents this from being the case. The modern quantum mechanics theory might not, however. Quantum mechanics suggests that empty space isn’t actually empty. Rather, it asserts that it’s filled with particle pairs that sometimes, but not always, appear in existence. Should we be able to make a region with fewer pairs popping in and out than everywhere else, the areawould contain negative energy density. However, one of theoretical physics’ biggest challenges is uncovering a consistent theory that combines Einstein’s theory of gravity with quantum mechanics. And so, the mystery continues.