# Black Holes, Relativity And Time Travel

Stories in which people have traveled through time and experienced the effects of time dilation have been around for centuries, with one of the earliest found in the ancient Indian epic the Mahabharata, in which King Revaita travels to another world only to find on his return entire ages have passed.

But is time travel really possible? Certainly, as we all travel forward in time by one hour per hour, but can we actually go faster or slower than that rate? Scientists now realize we can after Albert Einstein shook the foundations of physics by demonstrating the flexible and relative nature of time.

An atomic clock placed on top of a mountain, for instance, will experience a weaker gravitational force and so go faster than one placed in a valley, in accordance with Einstein’s theory of General Relativity. Also, as predicted by Special Relativity, an atomic clock on an airplane would run slower than a stationary one back on Earth as time slows down as speed increases.

## Time Travel and Special Relativity

Consider the equation: Speed = Distance ÷ Time.

Einstein’s theory of special relativity suggests nothing can travel faster than the speed of light, so as the speed of an object approaches the maximum limit set by the speed of light (c), the other variables in the equation start to change with distances becoming shortened relative to time which becomes lengthened.

It actually works out that for a person traveling at 99% the speed of light, time is stretched and slows down by a factor of roughly 7, such that a return journey to a star system 7 light-years away would take the astronaut 2 years to complete, while to an observer on Earth over 14 years would have passed. If, however, our astronaut was able to reach a speed equal to 99.999% the speed of light, then 1 year on board his craft would pass for every 223 years back on Earth.

## Time Travel and General Relativity

In addition to speed affecting time, Einstein discovered that gravity, too, causes time to slow down, with the effect more pronounced near a massive object. Therefore, a black hole with its intense gravitational field could potentially provide a fantastic means to travel through time by getting close enough to its Event Horizon without being swallowed up. In principle, by maintaining this “safe” distance you could travel centuries into the future relative to outside observers, although for you just a few hours or days would seem to have elapsed.

## Time Travel to The Past

All the scenarios discussed so far involve traveling to the future, but is time travel to the past possible? Yes, in theory at least, as general relativity does allow for a tunnel through space-time linking two distant points in time, which then form a circular time loop called a wormhole or Einstein-Rosen bridge. Similar to a black hole, a wormhole would have a massive gravitational field capable of bending space-time, but in contrast to a black hole’s single point singularity which collapses matter, a wormhole may have a central spinning singularity-shaped ring, known as a Kerr black hole. It may then be possible to pass through its “empty” middle to a white hole on the other side, from which matter is subsequently expelled.

That said, many scientists question the theory of stable, traversable wormholes which are able to be crossed in both directions. Further concerns about time travel to the past are thrown up by temporal paradoxes, such as the Grandfather Paradox, which seem to imply their own negation. Nevertheless, the principles involved in using closed timelike curves to travel and return to one’s own spacetime may be perplexing but still remain theoretically possible.