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Gravitational Time Dilation
Our lesson talked about time dilation being caused by things moving a different speeds relative to each other. However, there is also another form of time dilation that we didn't get to – gravitational time dilation. Albert Einstein came up with this too, this time in his general theory of relativity (which came after the special theory). In this work he shows that if a clock is placed near to an object with very strong gravity then that also slows down time. Move further away from the massive object and time speeds up (relative to before, of course).
Because astronauts and satellites orbiting the Earth are slightly further away from the centre of the planet (compared to people on the ground) they actually experience less gravitational time dilation. On its own this would mean astronauts' time would run faster. However, this effect is quite small because Earth's gravity is quite weak and so the time dilation due to their speed wins out and astronauts really do travel a tiny amount into their futures.
Time travel to the past
We only had time to talk about time travel to the future – which, as we've seen, has already been achieved. However, time travel to the past would also be a popular option. Imagine instead of learning about Henry VIII, JFK and Michelangelo in a classroom, you could travel to Tudor England, the White House of the 1960s, or the Sistine Chapel during its painting and experience it first hand!
Scientists love to argue about whether time travel to the past is possible. The short answer is that it probably isn't. In any case it is a lot harder than going forwards in time. Some physicists have concocted an elaborate way that might one day achieve it, however. Their scheme uses a hypothetical tunnel in space called a wormhole, that is turned into a time machine by using the effects of time dilation. But, there is a catch. Time traveling this way, it is impossible to go back to a time before you have built the time machine. Having not built it yet, you couldn't go back to before today.
Such a machine also throws up some rather thorny questions. Firstly, the moment you build your time machine you might end up inundated with visitors. This is because once it has been built it exists at all future times (unless someone subsequently destroys it). So someone from the future might use it to go back to meet the person who first built it. You probably wouldn't get a chance to use the machine yourself as you'd be so busy meeting people who will use it in the future!
There are also problems with paradoxes. Say a time machine that can take you to the past is one day invented. 100 years after it is invented, a time traveler uses it to go back in time and then shoots his grandfather when he was a teenager. Now dead, his grandfather cannot grow up to marry his grandmother and have his father. If his father was never born, then how was he born? How can someone who was never born travel back in time and shoot someone? These problems are used by some physicists to argue that time travel to the past is therefore impossible.
If you're interested in some of these questions then there are plenty of books out there to read. I have written a book myself called “The Big Questions in Science: The Quest to Solve The Great Unknowns”. The final chapter is called “Is time travel possible?” and looks at both forms of time dilation, as well as how to build a time machine to the past using the wormhole technique.
Other good books on the subject include:
Time Travel in Einstein's Universe: The Physical Possibilities of Travel Through Time by J. Richard Gott