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PH V SICS OF THE IMPOSSIBLE
proposed a "Chronology Protection Conjecture" to ban time travel
from the laws of physics in order to "make history safe for historians."
The embarrassing thing, however, was that no matter how hard
physicists tried, they could not find a law to prevent time travel. Apparently time travel seems to be consistent with the known laws of
physics. Unable to find any physical law that makes time travel impossible, Hawking recently changed his mind. He made headlines in the
London papers when he said, "Time travel may be possible, but it is
not practical."
Once considered to be fringe science, time travel has suddenly become a playground for theoretical physicists. Physicist Rip Thorne of
Cal Tech writes, "Time travel was once solely the province of science
fiction writers. Serious scientists avoided it like the plague-even when
writing fiction under pseudonyms or reading it in privacy. How times
have changed! One now finds scholarly analyses of time travel in serious scientific journals, written by eminent theoretical physicists ...
Why the change? Because we physicists have realized that the nature
of time is too important an issue to be left solely in the hands of science
fiction writers."
The reason for all this confusion and excitement is that Einstein's
equations allow for many kinds of time machines. (Whether they will
survive the challenges from the quantum theory, however, is still in
doubt.) In Einstein's theory, in fact, we often encounter something
called "closed time-like curves," which is the technical term for paths
that allow for time travel into the past. If we followed the path of a
closed time-like curve, we would set out on a journey and return before we left.
The first time machine involves a wormhole. There are many solutions of Einstein's equations that connect two distant points in space.
But since space and time are intimately intertwined in Einstein's theory, this same wormhole can also connect two points in time. By
falling down the wormhole, you could journey (at least mathematically) into the past. Conceivably, you could then journey to the original starting point and meet yourself before you left. But as we
PH V SICS OF THE IMPOSSIBLE
proposed a "Chronology Protection Conjecture" to ban time travel
from the laws of physics in order to "make history safe for historians."
The embarrassing thing, however, was that no matter how hard
physicists tried, they could not find a law to prevent time travel. Apparently time travel seems to be consistent with the known laws of
physics. Unable to find any physical law that makes time travel impossible, Hawking recently changed his mind. He made headlines in the
London papers when he said, "Time travel may be possible, but it is
not practical."
Once considered to be fringe science, time travel has suddenly become a playground for theoretical physicists. Physicist Rip Thorne of
Cal Tech writes, "Time travel was once solely the province of science
fiction writers. Serious scientists avoided it like the plague-even when
writing fiction under pseudonyms or reading it in privacy. How times
have changed! One now finds scholarly analyses of time travel in serious scientific journals, written by eminent theoretical physicists ...
Why the change? Because we physicists have realized that the nature
of time is too important an issue to be left solely in the hands of science
fiction writers."
The reason for all this confusion and excitement is that Einstein's
equations allow for many kinds of time machines. (Whether they will
survive the challenges from the quantum theory, however, is still in
doubt.) In Einstein's theory, in fact, we often encounter something
called "closed time-like curves," which is the technical term for paths
that allow for time travel into the past. If we followed the path of a
closed time-like curve, we would set out on a journey and return before we left.
The first time machine involves a wormhole. There are many solutions of Einstein's equations that connect two distant points in space.
But since space and time are intimately intertwined in Einstein's theory, this same wormhole can also connect two points in time. By
falling down the wormhole, you could journey (at least mathematically) into the past. Conceivably, you could then journey to the original starting point and meet yourself before you left. But as we