Everyone has a moment they’d love to redo, whether it’s a bombed job interview or something far bigger. Time travel stays firmly in science fiction for now, but physics itself doesn’t rule it out. In fact, the maths behind Einstein’s theories of relativity leaves the door open for travel into the future, and, in a much stranger way, possibly into the past too.

This isn’t speculation dressed up as science. It comes directly from equations physicists have been testing and refining for more than a century.

What Did Einstein Actually Prove About Time?

Einstein’s special theory of relativity (1905) and general theory of relativity (1915) overturned the assumption that time ticks at the same rate everywhere for everyone. Space and time, it turns out, can stretch or compress depending on speed, gravity, and acceleration.

That’s not the same as time simply feeling slow during a boring meeting and fast during a good conversation. Einstein was describing something measurable: an actual difference in how much time passes for one observer compared with another, under the right physical conditions.

Can You Really Travel to the Future?

Yes — and this part of relativity has already been confirmed experimentally. Special relativity shows that clocks moving at high speed run slower relative to a stationary observer.

Picture a five-year round-trip space voyage at 97 percent of the speed of light. The astronaut on board would return to find that roughly 20 years had passed back on Earth. Their friends would have aged two decades while they aged five.

You don’t need anything close to that speed to see the effect. NASA astronaut Scott Kelly spent 11 months aboard the International Space Station, and when he returned, he was about 13 milliseconds younger than his twin brother, who had stayed on Earth the whole time.

Gravity bends time too

General relativity adds a second way to shift time: gravity. Einstein reframed gravity not as a force but as geometry: mass and energy curve the fabric of spacetime itself, and that curvature is what we experience as gravitational pull.

The more massive an object, the more it warps spacetime around it, and the slower time passes nearby. This is the science behind a well-known scene in the film Interstellar, where a character spends only a few hours near a black hole while decades pass for his daughter back on Earth. The idea is dramatized for the movie, but the underlying physics is real.

Could Time Travel to the Past Actually Work?

This is where things get far stranger. General relativity’s equations technically permit something called a closed timelike curve — a path through spacetime that loops back to its own starting point, in both place and time.

Dutch mathematician Willem Jacob van Stockum identified this possibility in 1937, more than two decades after Einstein published general relativity. That gap wasn’t a matter of nobody looking. Einstein’s equations don’t describe just one universe; they describe an entire family of possible universes, and figuring out which solution matches our actual one is enormously difficult.

Why solving Einstein’s equations is so hard

General relativity is built on a set of differential equations that describe how the shape of space and the flow of time change depending on the mass and energy present. In compact form, physicists write this as a single relationship linking spacetime’s curvature to its matter and energy content, with a term for the cosmological constant that accounts for the universe’s accelerating expansion.

Solving that relationship means finding a specific geometry and mass distribution that satisfy it together, and both of those unknowns are still difficult to pin down for our own universe.

The Schwarzschild and Kerr solutions

Astronomer Karl Schwarzschild found one of the first solutions in 1916, describing the empty spacetime surrounding a stationary, spherical, massive object. That solution is still used today to calculate gravity near planets, stars, and non-rotating black holes.

In 1963, mathematician Roy Kerr extended that work to rotating masses, a far better model for real stars and black holes, which almost always spin. Kerr’s solution had a striking side effect: it technically allows for a closed timelike curve, meaning travel into the past isn’t mathematically forbidden inside it.

There’s a major catch. That path only exists in a region of Kerr spacetime that’s inherently unstable. The smallest disturbance causes it to collapse, which is a strong hint that nature doesn’t actually permit it, even where the math technically allows it.

So Is Backward Time Travel Actually Possible?

Not in any practical sense, and probably not in any sense at all. The Kerr solution is a mathematically valid answer to Einstein’s equations, but a valid equation isn’t the same as a physically realisable universe. Most physicists treat the instability of that region as strong evidence that the universe protects itself against actual paradox-generating time loops.

Forward time travel is a different story entirely; it’s been measured, confirmed, and even factored into the precision of GPS satellites, which have to account for time running at a different rate in orbit than on the ground.

Researchers haven’t stopped at Kerr’s solution, either. Other solutions to Einstein’s equations describe different, equally strange possibilities for time and space, and remain an active area of theoretical physics.

TL;DR

• Forward time travel is real and measurable: fast motion and strong gravity both slow time down relative to a stationary observer
• Astronaut Scott Kelly returned from 11 months in orbit measurably younger than his earthbound twin
• Backward time travel is only a theoretical possibility within general relativity, through structures called ‘closed timelike curves’.
• The Kerr solution (1963), which models rotating black holes, technically allows for one — but only in a region so unstable it likely collapses before anything could use it
• No experiment has ever observed backward time travel, and most physicists see the instability of the Kerr solution as a sign the universe rules it out in practice