The Beginning of Time
Depending on what you believe, the Universe began with a bang or was summoned into existence by some great force, or perhaps both or neither. But it did have a beginning. How can I be so certain? Well, there are two very convincing arguments that leave little doubt that universe (at least the one we know) must have had a beginning. The first is a simple application of the Second Law of Thermodynamics, which states that entropy must always increase. Technically this means that the possible energy states that a system can occupy always increases. But a more simple explanation is that the total amount of disorder increases. If the universe is eternal and had no beginning then it has certainly had enough time (forever seems sufficiently long) to have reached a state of complete disorder. However, this is not the case. The universe is observed to have a great deal of order. Stars have structure, and they conglomerate into larger structures which we call galaxies. Galaxies cluster into more immense structures and there are even superclusters of galaxies observed in the universe. Additionally human beings are very ordered systems; in an eternal universe we could not live to ask these questions. Still not convinced? Then consider another simple argument. In an eternal and static universe every line of sight would end on the surface of a star. Every photon of light emitted in the universe would find its way to everywhere else in the universe, over the span of forever. This would make the night sky as bright as the surface of the sun. Again this is clearly not the case. Thus, the universe had a beginning. But did time also have a beginning? In the current cosmological view of the universe time and space are linked together in a continuum, the spacetime continuum. The spacetime of our universe is currently being observed to expand. This effect was first seen by Astronomer Edwin Hubble in the 1920's. No matter which direction one looks, one will see distant galaxies moving away from us and each other. The current theory of the universe's beginning is known as the Big Bang. If all of the galaxies are now moving apart from each other in a uniform way then at some point in the past they must have all been in the same place. The densities and energies of the moment of creation were so great that no current theory of physics can describe what the universe must have been like at that singular point, called a singularity for obvious reasons. An explosion started the universe in motion as long as 20 billion years ago. At that moment of creation the spacetime continuum began, consisting of the spatial dimensions and a temporal dimension. So it would seem that time also had a beginning.
But what is time? Everyone talks about it, everyone feels that they can sense its passing, and there are also those who talk about traveling through it. We of course all travel through time at rate of 60 seconds per minute and 60 minutes per hour and 24 hours per day and 365 days per year. What I am speaking of is the kind of travel that has only been a realm of science fiction for over 100 years, but in the last 10 years has become a theoretical possibilty. I mean traveling to the past to see events long gone or jumping to the distant future to see the progress of humanity. Who hasn't thought of returning to the past to relive a sweet moment or jumping ahead to learn what the outcome of an important decision might be? Until recently that was only possible in the imagination. Now it seems that modern physics holds yet more surprises. I hope to lead the reader through a discussion of the nature of time and space and then to show the physics involved in creating a time machine. Then I shall discuss the rather outrageous consequences that time travel holds for the nature of the universe. At the end we may be lost in a whirl of paradoxes, equations, and parallel universes, but the journey should be fun.