Time Travel

Background

The prospect of revisiting the past or jumping to the future has captured people's imaginations for centuries. Our contemporary understanding of time travel is a marriage of theoretical physics and science fiction.

As far as we know, no one has successfully accomplished the feat. But the laws of physics don’t definitively rule out time travel, and novels and movies can’t resist exploring the possibility.

History and Popular Culture

Many early civilizations believed that time passed differently in the heavens than on Earth. An ancient Sanskrit epic poem, for example, recounted how 20 minutes for a god lasted over 100 million years for the humans waiting on him.

The time travel sub-genre of science fiction flourished alongside technological advancements in the late 1800s. The US adopted a standardized clock in 1883, which oriented people to time. A few years later, Edward Bellamy sent a man to the year 2000 in his 1888 novel "Looking Backward,” and Mark Twain sent a man to Medieval England in his 1889 satire, "A Connecticut Yankee in King Arthur's Court.”

Meanwhile, as cars and trains were making transit significantly faster, H.G. Wells popularized the idea of time travel by vehicle in his 1895 novella, “The Time Machine.”

Then, in the early 1900s, theoretical physicist Albert Einstein’s theories of relativity upended the way we think about time. He showed that, even though we measure time in standardized units (seconds, days, years, and so forth), it passes differently depending on speed (special relativity) and gravity (general relativity).

The variations in speed and gravity humans are capable of experiencing are too minute to have any real impact on how we experience time. But, technically, clocks on high-speed trains run slightly slower than stationary clocks, and time moves faster for satellites in orbit since they’re further from Earth’s gravitational pull.

How It Might (And Might Not) Work

From our human perspective, all events happen at a particular place at a particular moment. This relationship between where and when is called the space-time continuum, and time travel would hypothetically manipulate it.

If you could travel at the speed of light—which the laws of physics suggest is as fast as something can travel, at about 670,000,000 miles per hour—Einstein’s theory of special relativity says time would pass so slowly in relation to the rest of the universe that you would “arrive” in the future (this would be a one-way trip).

Scientists have proposed hypothetical particles known as tachyons that travel faster than the speed of light—and therefore back in time (these are strictly hypothetical for now).

Several other proposals have been put forward about how to warp the space-time continuum, including exploiting black holes’ strong centers of gravity.

Travel to the past is theoretically tricky. That’s because, at the most basic level, time is the rate of change in the universe and it moves in one direction: forward. We age, wood burns, food decays. None of that can be undone, and this trend toward disorder is captured in a physics concept known as entropy.

So, while the current laws of physics don’t preclude the possibility of time travel, renowned theoretical physicist Stephen Hawking’s “chronology protection conjecture” says that undiscovered laws likely make time travel impossible.

Paradoxes

We move through time understanding that a cause always comes before an effect. Present and past actions predicate future scenarios. Traveling back in time would create paradoxes that violate that law of causality.

This snag in time travel is commonly known as the grandfather paradox: If you traveled back in time to kill your grandfather, he’ll never have a son and that son will never have you, so you would never be able to kill your grandfather in the first place.

Physicists (and sci-fi writers) continue to theorize ways around the paradox, suggesting the existence of parallel universes and limitations on free will.