Is time travel possible?

How do we know time runs at different rates?

Start with speed. The Max Planck Institute’s Einstein Online explains it with a “light clock”: a pulse of light bouncing between two mirrors, one tick per round trip. Watch that clock fly past you, and its light has to travel a longer, zigzag path. Since light always moves at the same speed, each tick takes longer. At about 86.7 percent of the speed of light, the moving clock runs at half the rate of yours. This is called time dilation.

Gravity does it too. Einstein’s general theory of relativity says clocks deeper in gravity tick slower. In 2010, physicists at the National Institute of Standards and Technology (NIST) compared two of the world’s best atomic clocks. Raising one by 33 centimeters, about a foot, made it tick faster. As NIST puts it, you age faster when you stand a couple of steps higher on a staircase. Over a 79-year lifetime, NIST says, that adds up to about 90 billionths of a second. The same clocks also showed the speed effect at about 20 miles per hour.

The most famous test used airline seats. In October 1971, J. C. Hafele and Richard Keating flew four cesium atomic clocks around the world on regular passenger flights, once eastward and once westward. Compared with clocks at the U.S. Naval Observatory, the flying clocks lost 59 billionths of a second on the eastward trip and gained 273 billionths on the westward trip. Their theory had predicted a loss of about 40 and a gain of about 275, with error ranges that overlap the results.

Does the GPS in your phone really depend on this?

Yes. GPS works by timing signals from atomic clocks on satellites. The physicist Neil Ashby, in a review of relativity in GPS, writes that these clocks have shifts due to gravity and motion so large that, without carefully accounting for relativity, the system would not work. The satellites are higher up, where gravity is weaker, so their clocks tick faster. Their motion matters too, but Ashby explains that the gravity effect is the main one, so on balance the satellite clocks run fast. To fix this, the satellite clocks are set to run slightly slower on the ground before launch.

This was tested in space. The NTS-2 satellite, launched on June 23, 1977, carried the first cesium atomic clock placed in orbit. Ashby writes that, at the time, some people doubted the relativity correction would be needed at all. Measured in orbit, the clock ran fast by 442.5 parts in a trillion compared with clocks on the ground. General relativity had predicted 446.5.

So could a crew reach a star and come back younger?

In principle, yes. The nearest star to the Sun, Proxima Centauri, is just over four light-years away. Imagine a ship cruising at 86.7 percent of light speed, ignoring the time spent speeding up and slowing down. By Earth clocks, the trip takes a little under five years. On board, the crew’s clocks run at half that rate, so they age a little under two and a half years. The catch is getting there. As our story Can Anything Travel Faster Than Light? explains, pushing anything with mass close to light speed takes enormous energy.

Gravity can do the same job. Near a black hole, clocks slow down dramatically as seen from far away. NASA’s Imagine the Universe describes watchers who see a falling explorer’s clock run slower and slower, while for her time runs normally. Our story What Is a Black Hole, Really? has more.

What about going back in time?

Here the science turns from “measured” to “possible on paper.” The Stanford Encyclopedia of Philosophy explains that general relativity allows shapes of spacetime with closed timelike curves: paths an observer could follow to return to their own past, without ever going faster than light. In 1949, Kurt Gödel found one in a model of a rotating universe.

Wormholes are another route. A wormhole is a tunnel through space linking two distant places. In 1988, the physicists Michael Morris, Kip Thorne and Ulvi Yurtsever argued that if an advanced civilization could build and keep open a wormhole, it could turn it into a time machine. But in an earlier paper that year, Morris and Thorne showed that holding a wormhole open needs a material with a strange property: a pull, or tension, greater than its own energy. No known material has it, they wrote, though it could not be firmly ruled out. Physicists call such stuff exotic matter. Morris and Thorne also noted that Carl Sagan’s novel Contact treated wormhole travel in a way that fit the physics known at the time.

The strongest objection comes from Stephen Hawking. In his 1992 paper “Chronology protection conjecture,” he argued that as a time loop started to form, quantum effects would pile up energy and stop it. He wrote that his results strongly support the idea that the laws of physics do not allow time loops to appear. With a wink, he called this a “chronology protection agency” that keeps the universe safe for historians. But it is a conjecture, not a proof. The Stanford Encyclopedia’s entry on time machines says there are still no convincing arguments that Einstein’s theory, even with quantum effects added in the usual approximate way, contains such protection.

What about the grandfather paradox, and the missing tourists?

The grandfather paradox asks: what if you went back and stopped your grandfather from meeting your grandmother? Then you would never be born, so you could not go back. Many people think this proves backward time travel is impossible. Philosophers disagree. In 1976, David Lewis argued that the traveler would simply fail for ordinary reasons: the gun jams, or she slips on a banana peel. The Stanford Encyclopedia says his solution has been widely accepted. The past would not be changed, because it already includes the visit. Physicists who build simple models with time loops find the same pattern: consistent stories exist.

And why haven’t we met any time travelers? One reason is simple: there is no evidence that anyone has built a time machine. Another comes from the physics itself. The Stanford Encyclopedia notes that the wormhole-style machines in physics papers cannot carry anyone back to a time before the machine existed. So even if one were built someday, nobody could use it to visit today.

How can you check this yourself?

Read the short abstract of the 1972 Hafele–Keating paper in Science, which gives the numbers above. Read NIST’s 2010 news release about its “personal scale” clocks. Look up the NTS-2 story in Neil Ashby’s free review of relativity in GPS. For the harder questions, the Stanford Encyclopedia’s entries on time travel are free and carefully sourced. A good test for any time travel claim you see online: does it describe something measured, or something that is only allowed on paper?