With all the ancient châteaux, city walls and really old buildings that one finds everywhere in France, it is easy, for those of us who ponder such things, to wonder if travel to the past will ever be possible. Yes, Henry Warner is speaking of time travel. À la “Back to the Future”.
Henry Warner’s personal position is that “time travel” to the future is possible, but impossible into the past. Despite the appeal to our imaginations of traveling back in time to watch dinosaurs roaming the earth, the pyramids being built, Roman ships sailing into the port of Villefranche-sur-mer, or meeting your grandfather when he was a boy, my belief is that these possibilities will forever be limited to only our imaginations. Henry Warner’s position is that the past is forever gone – like a dream or a memory. Sure, the past was real at one time, but it no longer exists.
Travel to the future, however, is another thing. We are all traveling to the future right now at the rate of 60 seconds per minute. It is theoretically possible, and actually scientifically proven, that an object can travel to the future at other than 60 seconds per minute. Per the theory of relativity, as objects speed up, the rate of time slows for that object relative to an external reference. For example, it has been measured that atomic clocks in GPS satellites orbiting the earth at 9,000 miles per hour lose approximately 8 microseconds per 24 hours relative to time on the earth’s surface.
In theory, one could travel vast times into the future by going for a very long ride in space traveling just under the speed of light. You could theoretically go to sleep inside such a space capsule and wake up 1,000 years into the future. I am sure there are many practical problems that would have to be overcome in order to achieve this, but it is theoretically possible and supported by our current knowledge of science. Traveling to the future violates no principle of causality – that is – an effect cannot occur before its cause.
Traveling to the past is another matter. It is easy to speculate on all sorts of causality-violating scenarios. For example, you travel to the past and accidentally or intentionally change something that precludes your birth (e.g. kill your grandfather). Therefore, if you were never born, how could you ever have gone into the past in order to change it?
Another scenario that Henry Warner likes to point out, is to imagine a time machine that could be set to any time in the past. Most of us faced with this prospect might opt for some exciting prehistoric or historical time such as mentioned in the second paragraph above. But, what if, instead, you set the dial on the time machine to just one second earlier? There would now be two of you. Two of you virtually identical, except for the fact that the original you would be one second older than the other you. But your mother only gave birth to the original you! Where did your twin come from? Is your head swimming yet?
And where would the older you actually appear when popping one second into the past? Right next to the younger you? Sitting in the same chair as you? Keep in mind that in that one second, the earth has rotated a bit on its axis, the earth has moved in its trajectory around the sun, the sun has continued hurtling away from the center of the universe. In short, where you were one second ago is not the same place you now are in an absolute space reference.
And why stop at just one? Keep pressing that button once per second and in hardly any time at all the room will be full of “yous”.
Lastly, travel to the past is at odds with entropy – the universe’s movement from order to disorder. To go back in time, the decayed corpses would need to be be whole again, the fallen apple would need to re-attach to the tree, the silt washed downstream would need to end up back where it started. No, the past is gone. Gone forever.
That’s enough time travel to think about for today. Best to just admire all the lovely and awesome reminders of the past (and present)!