Next book on the science and philosophy shelf:
The beautiful little physics book In Search of Schrödinger’s Cat: Quantum Physics and Reality
So here’s yet another excerpt from this book: This one has to do with alternative realities, and time travel.
EXCERPT FROM In Search of Schrödinger’s Cat: Quantum Physics and Reality
Cosmologists today talk quite happily about events that occurred just after the universe was born in a Big Bang, and they calculate the reactions that occurred when the age of the universe was 10-35 seconds or less. The reactions involve a maelstrom of particles and radiation, pair production and annihilation. The assumptions about how these reactions take place come from a mixture of theory and the observations of the way particles interact in giant accelerators, like the one run by CERN in Geneva. According to these calculations, the laws of physics determined from our puny experiments here on earth can explain in a logical and self-consistent fashion how the universe got from a state of almost infinite density into the state we see it in today. The theories even make a stab at predicting the balance between matter and antimatter in the universe, and between matter and radiation. Everyone interested in science, however mild and passing their interest, has heard of the Big Bang theory origin of the universe. Theorists happily play with numbers describing events that allegedly occurred during split seconds some 15 thousand million years ago. But who today stops to think what these ideas really mean? It is absolutely mind-blowing to attempt to understand the implications of these ideas. Who can appreciate what a number like 10-35 of a second really means, let alone comprehend the nature of the universe when it was 10-35 seconds old? Scientists who deal with such bizarre extremees of nature really should not find it too difficult to stretch their minds to accommodate the concept of parallel worlds.
In face, that felicitous-sounding expression, borrowed from science fiction, is quite inappropriate. The natural image of alternative realities is as alternative branches fanning out from a main stem and running alongside one another through superspace, like the branching lines of a complex railway junction. Like some super-superhighway, with millions of parallel lines, the SF writers imagine all the worlds proceeding side by side through time, our near neighbors almost identical to our own world, but with the differences becoming clearer and more distinct the further we move “sideways in time”. This is the image that leads naturally to speculation about the possibility of changing lanes on the superhighway, slipping across into the world next door. Unfortunately, the math isn’t quite like this neat picture.
Mathematicians have no trouble handling more dimensions than the familiar three space dimensions so important to our everday lives. The whole of our world, one branch of Everett’s many-worlds reality, is described mathematically in four dimensions, three of space and one of time, all at right angles to one another, and the math to describe more dimensions all at right angles to each other and to our own four is routine number juggling. This is where the alternative realities actually lie, not parallel to our own world, but at right angles to it, perpendicular worlds branching off “sideways” through superspace. The pciture is hard to visualize, but it does make it easier to see why slipping sideways into an alternative reality is impossible. If you set off at right angles to our world — sideways — you would be creating a new world of your own. Indeed, on the many-worlds theory this is what happens every time the universe is faced with a quantum choice. The only way you could gete in to one of the alternative realities created by such a splitting of the universe as a result of a cat-in-the-box experiment, or a two-holes experiment, would be to go back in time in our own four-dimensional reality to the time of the experiment, and then to go forward in time along the alternative branch, at right angles to our own four-dimensional world.
This might be impossible. Conventional wisdom has it that true time travel must be impossible, because of the paradoxes involved, like the one where you go back in time and kill your own grandfather before your own father has been conceived. On the other hand, at the quantum level particles seem to be involved in time travel all the “time,” and Frank Tipler has shown that the equations of general relativity permit time travel. It is possible to conceive of a kind of genuine travel forward and backward in time that does not permit paradoxes, and such a form of time travel depends on the reality of alternative universes. David Gerrold explored these possibilities in an entertaining SF book The Man Who Folded Himself, well worth reading as a guide to the complexities and subtleties of a many-worlds reality. The point is that, taking the classic example, if you back in time and kill your grandfather you are creating, or entering (depending on your point of view) an alternative world branching off at right angles to the world in which you started. In that “new” reality, your father, and yourself, are never born, but there is no paradox because you are still born in the “original” reality, and make the journey back through time and into an alternative branch. Go back again to undo the mischief you have done, and all you do is reenter the original branch of reality, or at least one rather like it.
