Building a faster-than-light spacecraft, causing entropy to decrease, entering and then exiting the event horizon of a black hole — these top the list of scientific impossibilities.
In the gray area though where science fiction and science fact meet lays a concept that has captured the imagination for centuries, and which is treated in my upcoming sci-fi work, "Thinwalker" — time travel.
Could it actually be accomplished?
The first half of the answer to that question is a resounding yes.
We are all time travelers; anyone is, who either moves or varies their altitude on Earth.
Time is relative, passing slower or faster based on the speed of the traveler and/or the strength of the gravitational field through which one passes.
We all engage in imperceptibly small bits of time travel every time we book passage on an airliner six miles above the ground where gravity is infinitesimally weaker. It’s been calculated that after racking up 10 million air miles at rarified heights a pilot will be 59 microseconds older than the rest of the world beneath him at ground level.
Our GPS only functions properly because corrections are made for what "time" it is in space for satellites orbiting 8,700 mph at some 12,000 miles above the surface, as opposed to the time of those positions they triangulate on the ground.
The speed of GPS satellites causes their on-board clocks to accumulate a daily 7 microsecond lag behind clocks on the ground, but because the satellite is also high above Earth where the effect of gravity is lessened, time for the satellite’s clock speeds up by 45 microseconds per day.
The combined and countervailing effect is that GPS satellites are moving into the future faster than we are by 38 microseconds per day.
In centuries to come, when humanity begins to reach for the stars in ships that will necessarily attain sizeable fractions of the speed of light, longer leaps into the future will accompany space-farers. For example, moving at 50 percent light speed a round trip to Alpha Centauri, our nearest stellar neighbor located 4.37 light years from Earth, would have the occupants of the spacecraft logging 15.13 years in transit, insofar as their clocks were concerned (simplifying greatly, ignoring acceleration/deceleration).
Upon their return to Earth though, they would discover that calendars around the world would all agree that 17.48 years had elapsed in the interim.
A starship is therefore also a time machine, but only one-way, only into the future. Cruising at speeds just below the velocity of light, a crew returning to Earth after what seemed to them like 10 or 20 years would find that many centuries had passed on the home planet, which presumably would be sufficiently disconcerting.
As concerns moving the other way — into the past — this is much more of a sticky subject. There are many physicists who believe this is inherently impossible, since a universe in which an effect precedes its own cause shouldn’t be able to exist. The well-known "grandfather paradox" treats the enigma of a time traveler going back to kill his ancestor. The mind-boggling ramifications of that single example are sufficient to cause some of the sharpest scientific minds to close the door on time travel to the past.
However, there is a crack left open, theoretically at least, yet only for super-advanced civilizations of perhaps a million years of age or older. An avenue to the past has been conceived and deemed at least plausible, but reserved for societies in command of technology of the highest order, almost beyond imagining. Indeed, there are some very tall orders that need be filled prior to customers lining up to purchase tickets to the past.
The first thing necessary is to either discover or create a "wormhole," and then manage to spin one end of it at close to light speed while the other end is left unmolested. If this process is continued for 10, 20, 100 years, etc., the time dilation will accrue sufficiently for the stationary end of the wormhole to have moved forward in time far more quickly than the terminus that was set in motion and then finally brought to rest. Hence, the wormhole now would be the means for voyagers to pass not so much to some other place, but to some other time in the past.
And here would be where science, religion, philosophy and metaphysics would collide spectacularly, in such a way as to test the very bounds of reality. Tourists passing through this gateway would be visiting ports-of-call where their great grandfathers might be called upon, or even more bizarre, when and where their younger selves may be found.
That might not be allowed in this universe. If it is, it probably shouldn’t be.
David Nabhan is a science writer, the author of "Earthquake Prediction: Dawn of the New Seismology" (2017) and three previous books on earthquakes. Nabhan is also a science fiction writer ("Pilots of Borealis," 2015) and the author of many scores of newspaper and magazine op-eds. Nabhan has been featured on television and talk radio all over the world. His website is www.earthquakepredictors.com. To read more of his reports — Click Here Now.
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