In the BBC TV series "Dr. Who," the Tardis -- the Doctor's machine for traveling through time and space -- is disguised as a blue police box. As a Time Lord, The Doctor lives outside of time -- his life has no clearly definable past, present and future.

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How would time travel affect life as we know it?

Science fiction has thoroughly covered the topic of time travel, starting with H.G. Wells' "The Time Machine" in 1895 and continuing right up to modern movies like "Déjà Vu" starring Denzel Washington. But physicists have also explored the nature of time and the plausibility of time travel for more than century, beginning with Albert Einstein's theories of relativity. Thanks to Einstein, scientists know that time slows as moving objects approach the speed of light. Gravity also slows time. This means that, in one sense, all of us can already consider ourselves time travelers in a limited way because we experience a tiny time warp (a difference of only nanoseconds) when we, for example, take a flight on an airplane. But physicists who study time travel today search for plausible ways to create a time warp large enough to allow noticeable travel into the past or future.

In his book "How to Build a Time Machine," physicist Paul Davies writes, "The theory of relativity implies that a limited form of time travel is certainly possible, while unrestricted time travel -- to any epoch, past or future -- might just be possible, too." This astonishing statement begs an important question: If time travel did indeed become a reality, how would it affect our world as we currently experience it?

First, it's important to realize that building a time machine would likely involve enormous expense, and the sheer complexity of such an apparatus would mean only a limited group of time travelers would have access to it. But even a small group of "astronauts" traveling through time and space could conceivably have a tremendous impact on life as we know it today. The possibilities, in fact, seem almost infinite.

Let's begin by assuming that it's possible to create a complete loop in time travel -- that time travelers could travel back into the past and then return to the future (or vice versa). Although scientists view traveling to the future as a much less problematic proposition than traveling to the past, our daily lives wouldn't change much if we could only send time travelers backward or forward in time, unable to recall them to the present. If we could, in fact, complete this loop of time travel, we can conjure up an incredible array of possible effects.

Consistent Versus Inconsistent Causal Loops

Physicist Paul Davies gives a good example of a consistent causal loop in his book "How to Build a Time Machine." A mathematics professor uses a time machine to travel forward in time, where he discovers a new theorem. He returns back to the time he came from and gives one of his particularly gifted students the idea for that theorem. The student goes on to publish the theorem, and it turns out that it was this very student's work that the professor perused during his journey to the future. The narrative here is consistent.

On the other hand, with the grandfather paradox, a time traveler goes back in time and kills his grandfather. But if the time traveler's grandfather dies before the time traveler is born, how can he or she exist at all? And if the time traveler doesn't exist, how could he or she travel back in time to kill granddad?

Possibilities and Paradoxes of Time Travel

Imagine sending a time traveling astronaut 100 years into the future. The time traveler could witness technological advancements that we can only dream of today, much as people at the turn of the 20th century likely couldn't imagine the items we take for granted in 2010, such as iPods or laptop computers. The time traveler could also gain insight into medical advancements, such as new medicines, treatments and surgical techniques. If the time traveler could bring this knowledge backward in time to the present, the time from which he or she came, society could effectively leap forward in terms of its technical and scientific knowledge.

The futuristic time traveler could also bring back knowledge of what lay ahead for the world. He or she could warn of natural disasters, geopolitical conflicts, epidemics and other events of worldwide importance. This knowledge could potentially change the very way we operate. For example, what if a time traveler journeyed into the future and literally saw the effects that automobiles would eventually have on our planet? What if the time traveler witnessed an environment so polluted and damaged that it's unrecognizable? How might that change our willingness to use alternative forms of transportation?

Imagine that time travel became less restricted and more available to a larger population. Perhaps travel into the future would be exploited for personal gain. A futuristic time traveler could draw on knowledge of the stock market to guide his or her investment decisions, effectively using the granddaddy of all insider information to amass a fortune. Militaries might rely on time travel to gain valuable knowledge about the enemy's positioning and resources in future battles. Terrorists could use time travel to scout out the scenes of future attacks, allowing them to carefully plan with precise knowledge of future conditions.

The potential effects seem equally limitless in terms of the less likely possibility of time travel into the past. History books would no longer be based solely on exhaustive research and interpretation of ancient materials. Time travelers could resolve historical debates and verify how things did or didn't happen in the past. Imagine how different our understanding of the world might be if we could say definitively, for example, whether Moses actually parted the Red Sea or whether Lee Harvey Oswald acted alone in killing John F. Kennedy. A journey into the past could prove or disprove religious beliefs or result in face-to-face encounters with people such as Jesus, Buddha, Napoleon or Cleopatra -- or even the time traveler's former self. Perhaps time travelers could even bring back from the past things that had been lost, such as extinct species or dead and long-forgotten languages.

But here it's very important to raise the issue of self-consistent narratives and paradoxes. The concept of self-consistent narratives tells us that anything a time traveler would alter or affect in the past would have to remain consistent with the future from which he or she journeyed. Changing the past would effectively change the future, creating a causal loop. But such causal loops would only pose inherent problems if changes to the past resulted in a future different from the one the time traveler came from.

But perhaps the question of how time travel would affect life as we know it goes deeper than even a discussion of potential paradoxes and causal loops. Perhaps a discussion of specific effects of consequences on life as we know it makes little sense when faced with something that could change everything about the way in which we perceive our world.

Time Travel Turned Total Mayhem

As physicist Paul Davies describes it, unrestricted time travel -- meaning time travel that could form a complete loop to both the past and future -- would ultimately lead to total mayhem. In his words, "Time travel opens a view of the world that is a sort of madhouse where the rational order of things would no longer work. Under those circumstances, it's very hard to see how ordinary human life could continue."

In a world where the relationship between past, present and future is turned on its head, we would transcend the things that define our lives today. We would lose our notion of how time works, which could be so fundamentally damaging to our worldview that we would no longer care as much about the things that matter to us today: work, finances, making plans with friends and family, shopping -- you name it. These things just wouldn't be relevant in this crazy new world because we'd have a newfound preoccupation with simply making sense of a world without a set chronology -- we wouldn't know the order in which things occur.

It may be beside the point, then, to talk about resolving historical debates, saving endangered species or gaining technological, financial or military insight because those things might very likely fall by the wayside in the strange world that would follow the advent of unrestricted time travel.

As Davies makes clear, none of this fallout would occur from one-way travel. Hitching a one-way ride to the future or even the past (assuming we stick with self-consistent narratives) wouldn't cause this kind of profound reordering of the world as we currently experience it. But closing that loop of travel could be, in a word, disastrous.

Davies points out that science fiction normally focuses on the novelty aspect of time travel. But according to him, "It's not a novelty or a curiosity, it's something that strikes at the very rational basis of how we live and function. It's really hard to imagine that anything could be the same again." In his view, unrestricted time travel could change life as we know it so dramatically that we wouldn't even recognize it. Because chronology would have no meaning, we couldn't easily tell if something happened before or after, was a cause or an effect, and we would lose the ability to predict rationally the outcomes of our actions. In essence, it would be as though we had all gone insane.

These sobering potential effects of time travel have caused some scientists to wonder whether a principle exists in nature that would actually prevent unrestricted time travel, such as Stephen Hawking's "chronology protection hypothesis." This type of "theory of everything" might provide a scientific explanation as to why we could never unhinge the universe as we know it by making unrestricted time travel a reality. Scientists have yet to discover such a theory, but hearing Davies' take on the frightening effects of time travel makes one hope that they find it soon -- even if it means that we won't ever know for sure who killed JFK.

Lots More Information

Related ArticlesMore Great LinksSources
  • ABC Science Online. "The Big Questions: The Riddle of Time." Jan. 17, 2002. (Oct. 7, 2010) http://www.abc.net.au/science/bigquestions/s460740.htm
  • Davies, Paul. "How to Build a Time Machine." Penguin Books. 2002.
  • Davies, Paul. Personal interview. Oct. 13, 2010.
  • PBS Nova. "Sagan on Time Travel." October 1999. (Oct. 7, 2010) http://www.pbs.org/wgbh/nova/time/sagan.html
  • Pickover, Clifford. "Time: a traveler's guide." Oxford University Press. 1999.