Wednesday, November 30, 2005

Time Part Three

One of the stock answers to the question of time travel is to suggest that if time travel were possible it would have already happened. By way of explanation let's say that at some time in the future a clever scientist invents a time machine that can travel through time in any direction, just like the fictional time machine created by H. G. Wells. So where is our intrepid time traveller? History shows that no time traveller has ever visited us from the future, therefore it's never going to happen. This appears at first glance to be a good solid argument against the possibility of time travel, but the argument is flawed. It may be that time travellers from the future have visited us, but have not revealed themselves in order to avoid changing the future. Alternatively, time travel may be possible, but only into the future, the past already having been determined. Finally, time travel into the past may be possible, but only into a different (alternative) universe, thus avoiding paradoxes. The fact that we have no record of having been visited by time travellers does not exclude its possibility. Having dealt with that little problem we can now move on the fun part, paradoxes.

This is where we start to run into some of the problems posed by time travel. The most commonly posed paradox is known as the 'grand parent' paradox. This states that if you could travel back in time you could murder your grand parents and thus prevent your existence, thus rendering it impossible for you to have gone back in time and killed them... no need to draw you a picture. Even more to the point perhaps, you could travel back in time and kill the person responsible for time travel before they discover it! However, popular though the grand parent paradox is, it only reveals the tip of the iceberg. Let's examine a theoretical time travel situation in more detail in order to highlight some of the problems involved.

Imagine that today you travel back in time to August 2001 and warn the authorities that the World Trade Centre in New York is going to be attacked on September 11th. They take the necessary action and as time unfolds the disaster is eventually averted. You are still in August 2001 at this point and the disaster has not yet been prevented, but it will be eventually because of the chain of events that you have put into motion, and you now wish to return to the time you came from. Here comes the Big Question - can you return to where you came from? Where you came from the attack had taken place, therefore where you came from no longer exists! If you do return to your original time, the World Trade Centre will still be standing (because you prevented the attack), so it cannot be where you came from. You will have changed the course of history, not just for yourself, but for the entire world. The question is, will the World Trade Centre still be standing when you return to your starting time? This is a major point of contention. According to one theory it both will and it won't! This is because your actions will have created an alternative possibility and in the process an 'alternative' universe, where we now have one universe with the World Trade Centre intact, and one with it destroyed, and we will all exist in both. The theory is firmly rooted in quantum theory that states that ALL alternative outcomes are possible. However, there is no evidence at this stage that the strange phenomenon found in quantum theory can be applied to the larger world.

Is Time Travel Possible?

More on time travel here, here and here.


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