We know that time travel into the future is possible. Stephen Hawking says, “We do not have the technology today to do this, but it is just a matter of engineering; we know it can be done.” In Time Travel in Einstein’s Universe: The Physical Possibilities of travel Through Time, physicist J. Richard Gott says “if we can accelerate protons to greater than 99.995 percent of the speed of light, we could also send off an astronaut at the same speed. It’s just a matter of cost.”
The idea of time travel into the past is a far more exciting prospect though. Who hasn’t wanted to travel back into the past? Some would like to return to the past permanently to live in a simpler time and escape forever the stress, crime, pollution, and other unpleasant aspects of 21st century living. Some would like to be time tourists and actually be present in the crowd to hear the Gettysburg Address delivered by President Lincoln himself, to watch the pyramids being built, or to witness firsthand any number of historical events. Some would like to go back to do or undo things that affected their own lives.
There was a young lady called Bright
Who could travel far faster than light;
She set off one day,
In a relative way,
And returned home the previous night.
A.H. R. Buller
A cute limerick, but is it possible? Yes, says, Stephen Hawking, if one can travel faster than the speed of light, one can travel back in time in the same manner that one can travel into the future—by riding a really fast spaceship. The problem, he says, is that while we can accelerate particles to 99.99 percent of the speed of light, we have as yet been unable to exceed the speed of light. Nevertheless, Hawking says, travel to the past may be possible through the use of wormholes. The idea of wormholes is not new. In 1935, Einstein and Rosen wrote a paper saying that the Theory of Relativity allowed for the existence of wormholes and much has been written about them since, but as yet we have no conclusive evidence that they actually exist.
Some scientists theorize that time travel into the past may be possible through the use of cosmic strings, which are thin strands of high-density material left over from the early universe. Like wormholes, we don’t know if cosmic strings exist, but Gott says, “If cosmic strings exist, you could travel in a spaceship [at 99.9999999999 percent of the speed of light] and outrun a light beam by taking the shorter of two paths around a cosmic string. The door to time travel to the past begins to crack open.”
Doubts that travel into the past might become possible in the future are rooted not so much in terms of physics and technology, but in terms of intuition and what are perceived to be the use of logic or common sense. For example, if a man …