27-Jan-2007: Time Out
I like to think about time. Time is a very mysterious thing. Physicists tell us it is the fourth dimension of our universe and talk about
the space-time continuum. We travel along
the time dimension in only one direction. What is behind us is no longer accessible; and what is
before us is a blank.
If one sets out from Halifax and
drives to Vancouver, one can describe the length of the trip in two ways: by the number of kilometres
covered, or the amount of time it took to get from one place to the other. Both are equally valid ways of
measuring what is, essentially, the same thing: how far something is. We mix the two dimensions when we talk of
light-years. A light-year is the distance that a beam of light travels in one year. Length (distance) and time (year)
are used as a tool.
We can always reverse our route from Vancouver to Halifax. We will pass places that we saw
when going to other way, except that we will see them reversed. The east-facing side of objects was seen
first when we were travelling west; and the west-side face was seen first in the east-bound trip. Makes sense, we
all agree. So, why can't we travel the other way in time? Because of that same reversal that we saw when travelling distances in
opposite directions. Travelling through time in reverse, we will see the end of something before we see its beginning. Just
like running a reel of film backwards. We would see elderly people get younger until they were in their twenties, teens, childhood,
babyhood, as a foetus, as an egg, as a sperm cell and egg cell meeting. So, even if we could, we would not be able to go back further
in time than our own existence. As a single cell we would not
have much opportunity for observing events. That is, we can not go back except as some sort of passive observer.
We can time travel in one sense. When we look out into the universe, we are looking back in time. The further away something is, the older it is. Why? Because it took the
light from that star or galaxy a long time to get to us. Time and distance mixing it up again. Another curious fact about our universe is that the older (further away) that
something is, the faster it is receding from us. However, nothing can travel faster than the universal speed-limit of the speed of light. So, if anything were receding from us faster
than the speed of light, it would not exist in our universe. No sense thinking about it: it is gone irretrievably. That's a part of the meaning of the phrase "observable universe."
How far can we see in our universe? We can see as far back as time has existed. The background
radiation of the Big Bang is still all around us and is measurable. In other words, if our universe is 13 billion years old, which is about the average of estimates, then we can see a sphere
about us with a radius of 13 billion light years. Impressive, but we, ourselves, can never get there. If we travelled to a star that is one light year away, we are initially seeing it as it was
a year ago. As we approach the star time appears to speed up to us, relative to the star, so that by the time we finally get there, our "time" and the star's "time"
are in synch. When we look back at earth, a light year away, we are now seeing it as it was a year ago. As far as we are concerned, we have been travelling at a steady pace the entire trip.
To sum up: the further we travel away from something the further it is from our present time; as we approach an object, its past moves into our present.
Time is a slippery subject once you start thinking things through. Unfortunately, the makers of space and time science fiction ignore the tricks of time.
The crew of the Enterprise talk to the Vulcans and to the High Command on Earth simultaneously in real time. And, when the Enterprise returns to its
base on earth, after having travelled many, many light years, only the time that the crew experienced on their trip has passed on Earth. That is as
amazing as seeing someone walk through a brick wall. It just does not happen in our universe.
Space travel will be a very lonely business. The further away from Earth you are, the more it recedes into your past. If you could, somehow, communicate with Earth in "real time,"
it will appear that time is speeding up back home. Eventually, when establishing your communications link with Earth the person you are communicating with will have aged and
died before the transmission is completed. (That is making the huge assumption that we will be able to
build a faster-than-light communications system.) Sorry to sound like such a Trekkie spoil-sport.
We know that no one has stepped back in time with his corporal form intact because it has not happened. If time travel were as easy as the movies
make it out to be, then our existence would have collapsed into chaos long ago. You cannot put a body, or some other object, into the past because it will
alter the future from that time forward, making our present unstable. Despite the non-interference regulations that time travellers are supposed to honour,
just their existence alone in the past alters the present.
I haven't ruled out the possibility that some day we will be able to view the past from a completely passive observer position, but, even that,
will take some pretty profound revolutions in our understanding of physics.
Meanwhile, I love reading novels and watching good movies involving time travel, suspending my disbelief for a while.