I have recently finished Brian Greene’s The Fabric of the Cosmos. Greene is one of the leading cheerleaders for string theory and much of the second half of this book is made up of an explanation of how it works. I’ll leave off commenting on this aspect of the book until I’ve read Lee Smolin’s The Trouble with Physics which criticises string theory and complains it gets way too much attention. Smolin himself is a supporter of another and equally unproven theory called quantum gravity.
For the record, I thought The Fabric of the Cosmos was quite good although I couldn’t get as carried away as some of the reviewers quoted on the cover. In particular, I found the frequent references to characters from the Simpsons to be rather grating. (This finally persuaded me that I would have to take the passage about Wile E. Coyote out of my discussion of Aristotelian dynamics in God’s Philosophers.)
In this post, I wanted to share with you one idea of Greene’s that derives from good old relativity theory. An old philosophical saw is the question “Does the past (or the future) exist?” Greene says yes, definitely, and relativity proves it. The argument goes like this:
If two people are standing still relative to one another then they both occupy the same time. For both of them, ‘now’ is at the same instant. But suppose that one of them, who we’ll call Albert, starts to walk away from the other, who we’ll call Bertie. Relativity tells us that Albert’s personal clock no longer runs in synchrony with Bertie’s. In fact, Albert’s clock gets slightly ahead of Bertie’s so that, in effect, Albert has travelled back in time relative to Bertie by a tiny amount.
However, if Albert and Bertie are now on opposite sides of the universe from each other, 10 billion light years apart, the divergence of their personal clocks becomes quite marked. Albert moving away from Bertie at a walking pace means that his personal ‘now’ becomes the same moment as a hundred and fifty years before Bertie’s. This doesn’t cause a problem because there is no way that Albert could get to Bertie’s area of space before Bertie’s now had had a chance to catch up with Albert’s. Because nothing can exceed the speed of light, travel back in time in a particular place is not possible.
But that does not alter the fact that we each carry a personal clock and its relationship with the rest of space-time can alter both backwards and forwards. Since we cannot claim that any clock is privileged, the future and past must exist because we can become contemporaneous with them simply by going for a walk. I’m open to thoughts about what this means for creation, divine intervention and freewill. I’ll be posting my own thinking soon.
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