Time Travel: Is It Possible? What Physics Really Says

A phone’s GPS works because its satellites correct, every day, for a time difference predicted by Einstein’s theory of relativity. That’s a useful place to start for understanding how physics looks at time travel.

Time That Runs at Different Speeds

According to NASA, the GPS system depends on corrections predicted by relativity. Two effects, both described by Einstein, act together on satellite clocks and pull in opposite directions. General relativity says that weaker gravity in orbit makes time run faster for the satellites than for the ground below; special relativity says that an object moving much faster than another experiences time running more slowly. According to Ohio State University, speed alone costs the satellite clocks roughly 7 microseconds a day against clocks on the ground, while the weaker gravity alone adds back about 45 microseconds a day; combined, the two effects leave GPS satellite clocks running about 38 microseconds a day ahead of clocks on Earth. A microsecond looks negligible for everyday life, but it builds up every day across a network of satellites used by billions of devices, and the system has to correct for it to stay accurate.

Practical example: according to Ohio State University, a position calculated by GPS without the relativistic correction would become unreliable after just 2 minutes, with an error growing to around 10 kilometers a day. Every time a navigation app gets the turn right, a calculation that accounts for relativity is behind it.

The same family of effects shows up outside the GPS system too. According to NASA, the gravitational redshift predicted by Einstein has been observed in a system of two stars about 29,000 light-years from Earth: a clock under stronger gravity runs slower than a clock observed from a region with weaker gravity.

Another confirmation comes from a subatomic particle. According to HyperPhysics, the muon has a half-life of just 1.56 microseconds measured at rest, yet counting them at sea level turns up more than a calculation that leaves relativity out would predict.

Why Going Back Is a Different Problem

Speed and gravity settle the forward trip, and the measurement is already in hand. Going toward the past changes nature: the Stanford Encyclopedia of Philosophy notes that time travel isn’t just a science-fiction theme, because with general relativity it has been taken seriously by physicists too.

The classic problem has a name: the grandfather paradox. A traveler goes back in time to kill their own grandfather before he becomes a parent. Succeeding at that would strip the traveler of the very possibility of being born to attempt it. It’s a logical contradiction, not a technical obstacle.

For a long time, these causal paradoxes were used as an argument for dismissing the general relativity solutions that allow closed timelike curves, accused of violating logic as well. In its 2023 revision, the Stanford Encyclopedia of Philosophy describes a consensus that has largely reversed after deeper analysis of the paradoxes, to the point that those solutions are no longer dismissed outright.

Strictly speaking, general relativity does not mathematically rule out backward time travel: its equations have been solved in ways that allow trajectories to loop into the past. A path that locally always moves forward in time can, taken as a whole, close back on itself, forming what the source calls a closed timelike curve. Among the theoretical solutions discussed are wormholes, hypothetical tunnels that would connect different points in spacetime: the Stanford Encyclopedia of Philosophy presents them as mathematical results, far from an engineering project ready to build.

H.G. Wells’s novel The Time Machine popularized the idea of moving through time before relativity existed as a theory.

Two Trips, Two Different Questions

The same label, time travel, hides two separate questions. Forward travel has an experimental answer: different clocks, under different conditions of speed or gravity, measure time differently, and the device that proves it every day sits in the reader’s pocket. Backward travel has, for now, only a mathematical answer, with a logical problem that physics has been debating for decades.

Anyone asking whether time travel is possible is usually thinking of the past, of going back to fix a mistake. Physics answers that direction with more caution.

For spacetime curvature at its extreme, see what black holes are.