PhysicsIn Peter Howitt's movie, Sliding Doors, two tales unfold depending on whether our heroine (Gwyneth Paltrow) just catches, or misses, an underground train. We are treated to both stories in parallel and are reminded how our lives can evolve very differently depending on chance circumstances, such as making a tube connection, never mind our own conscious decisions.
Of course you might say this is just pure Hollywood: in the real world our heroine either did or did not catch the train and suffered the consequences of what happened next. Michio Kaku argues, however, in his new book, that modern physics almost forces us to believe in the existence of parallel worlds: that the world in which she catches the train exists alongside the one in which she does not. According to some interpretations of quantum mechanics, for example, the universe is constantly splitting and giving rise to additional universes where all of the various options are acted out. For each universe in which Schrödinger's famous cat lives, there is another in which it dies. As observers, we move through this labyrinth of universes, following a path that depends as much on the vagaries of quantum mechanics as on our own free will. Quantum mechanics, Einstein's General Theory of Relativity, inflationary models for the Big Bang, as well as modern string theory, all support the "many worlds" hypothesis to varying degrees. General Relativity, for example, through the concept of wormholes, allows for the possibility of time travel. Time travel, on the surface, however, seems absurd. How could nature permit it, since it allows for the possibility of you going back in time and preventing your parents ever meeting? As Kaku eloquently explains, however, the "many worlds" or multi-verse, theory is a way out of the logical mess that time travel would cause.
Heady stuff, you might say, and more reminiscent of Alice in Wonderland than Leaving Cert physics, but eminent astronomers, such as Sir Martin Rees, from Cambridge, are on the record as believing in the existence of the multi-verse. According to this theory many different types of universe exist, some like our own and others with physical laws that are so different from ours that even stars cannot form. The concept of the multi-verse allows us to escape, after a fashion, the rather daunting conclusion that our physical laws are unique. It is daunting because if our laws were altered, even by a small amount, you and I would cease to exist. On the other hand, if all options exist, universes that give rise to intelligent life like ours are not so surprising after all, even though they may be rare.
Kaku is normally extremely good at explaining what can be very complex concepts and theories. His brilliance as a writer shines through every now and then, but I could not escape the conclusion that this volume is a hodgepodge of chapters thrown together without a strong underlying narrative. The introduction is a rather breathtaking ride (mile wide and inch deep . . . or should that be kilometre wide and centimetre deep?) through modern astrophysics and cosmology. This might put some readers off. While many of his explanations are lucid, others are confusing even for those of us who are familiar with the science.
Kaku is perhaps most entertaining and enlightening when discussing the "ultimate" fate of mankind. He points out that our universe will eventually die and that if we are to escape doom, we will have to find some way of entering a suitable parallel universe (there are, of course, many unsuitable ones). The death of our universe, however, will not happen for many billions of years to come, but Kaku has come up with a number of ingenious ways by which we might enter these other worlds, such as the use of wormholes. Shakespeare once said "All the world's a stage"; well, wormholes, as Kaku points out, may well be the trapdoors.
In any event, we have some time to work this problem out and, besides, the death of the sun will occur before the death of the universe. Modern astronomy confidently predicts that the sun will expand and engulf the earth five billion years from now. Personally, I reckon that if we haven't learnt how to leave the earth by then, we deserve to fry. Really, what I want to know is how do I enter that universe where I win the Lotto?
Tom Ray is Professor of Astrophysics at the Dublin Institute for Advanced Studies. He is currently helping to design the James Webb Space Telescope, NASA and ESA's replacement for the Hubble Space Telescope, which is due for launch in 2011
Parallel Worlds By Michio Kaku Allen Lane, 428pp. €20