Where time doesn't fly: How the speed of light can slow down ageing for astronauts

What may have seemed a fanciful Hollywood movie plot is grounded in scientific fact

Nasa astronaut Chris Cassidy takes a photo of the Earth from the International Space Station. Reuters / Nasa

If an astronaut on a deep space mission returned to Earth years later, only to find their loved ones had aged decades, it would sound like something out of a sci-fi blockbuster.

This is, in fact, one of the most memorable scenes from Interstellar, where an astronaut’s journey near a black hole causes him to age much slower than his family back home.

But this effect, known as time dilation, is not just science fiction. It is a real, measurable phenomenon described by Einstein’s theory of relativity.

While today’s missions do not reach speeds anywhere close to what would be needed to create significant time dilation, future high-speed space travel could bring this effect into play.

“If an astronaut were on a high-speed journey, they’d feel time passing normally on-board,” Jonathan Ward, an astronomer and author of several highly acclaimed space history books, told The National.

“But when they returned and compared their clock to one back on Earth, they’d see that much more time had passed on Earth. The effect of time dilation becomes stronger the faster you go.”

Understanding time dilation

He said that time dilation at its core means that time does not pass at the same rate for everyone, especially under extreme conditions of speed or gravity.

“When you travel close to the speed of light, you experience time very differently than people who stay behind,” said Mr Ward, a fellow of the UK’s Royal Astronomical Society.

“The closer you get to light speed, the slower time moves for you relative to Earth.”

The speed of light is the fastest speed in the universe, travelling at about 300,000km/sec, which means light could circle the Earth more than seven times in just one second.

To illustrate time dilation, Mr Ward described a hypothetical journey to Alpha Centauri, the closest star system to our solar system, 4.37 light years away.

He said that if an astronaut travelled at 1 per cent of the speed of light to the star system, the journey would take about 874 years from Earth’s perspective, and the astronaut would experience the vast majority of this, 873.98 years to be exact.

“But at 90 per cent of light speed, while nearly 10 years would pass for those on Earth, the astronaut would experience only about 4.24 years of travel,” he said.

“The closer you get to light speed, the more dramatic the time difference. If you were a photon, a particle of light, no time would pass for you at all, regardless of how far you travelled.”

Why time dilation is already a factor in space missions

Time dilation might seem far off for future space missions, but its effects are already impacting the technology we use in space today.

Sahith Reddy Madara, an aerospace engineer and founder of Paris advisory firm Bumi and Space, told The National that engineers must keep time dilation in mind when operating satellites around Earth and nearby planets.

“Engineers account for every minute relativity effects daily, especially with GPS satellites in low-Earth orbit,” he said.

GPS satellites orbit the planet at 28,163kph, so their on board clocks run slower than Earth's clocks by about seven microseconds a day due to their speed, and 45 microseconds due to Earth’s gravitational pull, according to Mr Ward.

“In deep space missions, these adjustments would scale up, potentially affecting mission timelines and data accuracy the farther out we go,” said Mr Madara.

He said that time dilation becomes crucial if interstellar travel becomes possible in future.

Higher speeds would mean greater differences in time flow between the spacecraft and Earth, which could impact communication, potentially creating problems when monitoring an astronaut’s health.

“Special systems would definitely be needed – possibly AI-based time-adaptive systems – to help bridge the communication gap created by these relativistic differences,” said Mr Madara.

“In terms of health data, time dilation could add a twist. Medical research already faces challenges comparing data between astronauts and Earth-based controls due to the space environment itself.

“If an astronaut’s ‘time’ flows slower, syncing health data would become more complex.

“We might have to adjust the data to account for their slower biological ageing relative to Earth’s time, potentially reshaping how we interpret long-term space mission health outcomes.”

Time dilation has also been observed in astronauts on the International Space Station, as seen in Nasa’s twin study with Mark and Scott Kelly.

The research demonstrated a tiny but measurable effect of time dilation. While Scott was on the station, travelling at 28,000 kph, time for him passed slightly slower than it did for Mark, who remained on Earth.

This difference, though extremely small, resulted in Scott ageing a few milliseconds less than Mark over the year he spent in orbit.

The time dilation effect is a consequence of both the station’s speed and its position in Earth’s gravitational field.

Could future missions unlock new physics?

Beyond navigation and practical concerns, time dilation offers scientists a valuable tool for unlocking new findings in physics.

Prof Matthew Griffin from the University of Cardiff told The National that time dilation helps explain certain high-speed particles that reach the Earth’s surface.

One example is cosmic muons, particles created high in Earth’s atmosphere with very short lifespans.

“Muons decay in a characteristic time that is very short. They are detected on the ground even though their lifetimes (in their own frame of reference) are too short for them to get to the ground before they decay,” he said.

“Distance-dependent time-dilation effects due to the varying expansion rate of the universe since the Big Bang are also seen in astronomical observations of very distant galaxies and supernovae.

“It has also been verified by comparing ultra-precise time measurements using atomic clocks that have been in orbit with clocks that have stayed on the ground.”