As we know, a "wormhole" is the name given to an area of warped spacetime through which light and matter might pass. The term was coined in 1957 by the American theoretical physicist John Archibald Wheeler, although the idea itself had already been theorised in 1921 by the German mathematician Hermann Weyl in connection with his analysis of mass in terms of electromagnetic field energy. Stephen Hawking describes wormholes as 'tiny shortcuts through space and time [that] constantly form, disappear, and reform within this quantum world [which] actually link two separate places and two different times.'
Evidence of wormholes opening up on earth has been recorded on innumerable occasions throughout history – the most recent of which took place on a residential street in Brighton just a couple of years ago.
The first point of consideration for any time traveller is to make certain that their chosen wormhole bridge is actually traversable, by which I mean that both its entrance and exit are and will remain open, and that its walls are not in danger of collapsing. A bit like kissing (only in the inverse), both mouths need to be open for tongues to find their fullest footing, else it gets a bit embarrassing.
Once the traversability of the wormhole has been established, a human would need two things to proceed: (1) a device to increase the diameter of the wormhole to a size which could physically accommodate the human body (and its vessel) – in their natural state they are much too small to do so, being only a billion-trillion-trillionths of a centimetre across, and (2) a vessel that could travel at the speed of light (or somewhere close to it – let's say for now at least 50,000,000 mph). An alternative to the first consideration would be to create new larger sized wormholes ourselves. Obviously this would take an enormous amount of power, as well as extremely advanced technology, but the successful completion of the Large Hadron Collider (LHC) indicates that such a thing is possible.
With these factors in place, Hawking theorises that the trajectory of the time machine would play out thus:
In one week it would have reached the outer planets. After two years it would reach half-light speed and be far outside our solar system. Two years later it would be travelling at 90 per cent of the speed of light. Around 30 trillion miles away from Earth, and four years after launch, the ship would begin to travel in time. For every hour of time on the ship, two would pass on Earth
After another two years of full thrust the ship would reach its top speed, 99 per cent of the speed of light. At this speed, a single day on board is a whole year of Earth time. Our ship would be truly flying into the future.
The key element of this description for me is its final phrase "into the future". According to Hawking, time travel is only possible in a forward direction. Going back in time is impossible because it violates the fundamental rule that governs the Universe: that cause must come before effect and not the other way around. As a result, he says, 'as soon as the wormhole expands, natural radiation will enter it, and end up in a loop. The feedback will become so strong it destroys the wormhole.'
An alternate theororetical method, put forward by Morris, Thorne and Yurtsever in 1988, is this: that one does not attempt merely to move through wormholes at the speed of light (thus limiting oneself to forward-facing travel only), but to actually manipulate and reposition the wormholes themselves. In this way, travelling backwards in time is possible, but within specific limits – it is impossibe to travel back to a time before the technology you use to do so was created, because that is the point at which the earliest static wormhole exit point will have been established. So if I set such a thing up this afternoon for example – and tether one end of my chosen wormhole here in my St Petersburg hostel room, then nobody who goes on to use my technology will be able to go back to yesterday, regardless of when and where they position the other end of this wormhole bridge – today is necessarily the ultimate backwards destination of their travel.
This limitation has been disputed however, and I too am in disagreement. Yet again it comes back to looking at what we perceive to be possible right now instead of investigating what might be possible with further research and development. For example, to suggest that time travel before the point of invention is impossible assumes that the technology used is static (in both time and space). If the technology could be developed as a mobile device which would allow specific points to be plotted out for each individual journey – both forwards and backwards – and from any mobile location, then I see no reason why the traveller must be limited to specific routes. It is like using your own car rather than taking a bus. If I am in Brighton and I take the no. 7 bus to the marina I know that I will only go as far as the marina, and that even if I stay on the bus as it goes back through its route, I will only go as far as its starting point: George Street in Hove. With my own car I can stretch out as far as I like in either direction. All I would need to do is learn to drive, buy a street map of Brighton and Hove and buy a car: Experience, knowledge and a means of transportation. This point is connected to my disagreement with Hawking's theory that any attempt to go back in time would cause a collapse in the wormhole. Using something like the LHC to develop new wormholes (rather than seeking to expand pre-existent ones) would preclude this kind of failure. Of course, an attempt to travel backwards in time might take the traveller to a parallel universe whose history would begin to diverge from their original history after the moment they arrived in the past (we need only watch Back to the Future to realise that!), but that is another concern for a different day, and perhaps a risk one must simply be prepared to take...