Why Astronauts on the ISS Are Aging Slightly Slower Than You Right Now
Right now, while you are reading this, there are astronauts living aboard the International Space Station who are aging just a little bit more slowly than you are. Not in a sci-fi movie sense — in a real, measurable, scientifically verified sense. The effect is tiny, but it is absolutely genuine, and the same physics behind it is what makes the GPS on your phone work. If that sounds like two completely unrelated things, keep reading, because the connection is one of the coolest ideas in all of science.
The ISS orbits Earth at roughly 17,500 miles per hour, completing a full lap around the planet approximately every 90 minutes. At that extraordinary speed, something strange happens to time itself. According to Albert Einstein's special theory of relativity — a set of equations he published in 1905 that describe how the universe actually behaves — objects moving quickly through space experience time more slowly compared to objects that are stationary. This is called time dilation, where "dilation" means stretching or slowing. It is not a metaphor or a rough approximation. It is a precise, tested prediction that has been confirmed by experiments many times over.
There is actually a second effect working at the same time, and it pulls in the opposite direction. Einstein's later theory, general relativity, tells us that gravity also affects time: clocks deep inside a gravitational field — like here on Earth's surface — tick more slowly than clocks farther away from a massive object. The ISS is about 400 kilometres above the surface, slightly farther from Earth's centre, so that effect would make the station's clocks tick faster. The two effects compete, but the speed effect wins. The net result is that ISS astronauts age a small fraction of a second slower for every six months they spend in orbit.
This was tested beautifully with real people. Astronaut Scott Kelly spent nearly a year on the ISS between 2015 and 2016. His identical twin brother Mark — also an astronaut — stayed on Earth during that time. Because they share the same DNA, they were the perfect biological comparison pair. When Scott returned, scientists could calculate the precise time difference between the two brothers based on the station's known orbital speed and altitude. Scott came back measurably younger than Mark, not in the way you might feel after a good holiday, but in the strict physical sense that his clock had simply ticked fewer times. The Kelly twin study became one of the most discussed experiments in modern space research.
Now here is where this stops being just a space curiosity and starts being part of your everyday life. GPS satellites orbit much higher than the ISS — around 20,200 kilometres above Earth. At that altitude, they are far enough from Earth's gravity that the general-relativistic effect dominates, and their clocks actually run faster than ground clocks by about 38 microseconds every single day. That might sound tiny, but if GPS engineers did not correct for it, your phone's navigation would drift off by several kilometres within just one day and become completely useless almost immediately. The engineers who designed GPS had to build relativistic corrections directly into the system. Every time you get accurate directions somewhere, Einstein's equations are quietly doing their job in the background.
It is important to be clear about what time dilation does and does not mean for the astronauts themselves. From inside the station, time feels completely normal. The crew do not experience a slow-motion life. They do not feel younger. The time difference only becomes apparent when you compare their clock against a clock on the ground from a separate reference point. Meanwhile, the physical effects on their bodies from living in microgravity — reduced bone density, changes in vision and cardiovascular function, and other adaptations — are far larger and more medically significant than the relativistic time difference. The universe is making two very different things happen to them at once, for entirely different reasons.
What makes all of this genuinely mind-bending is what it says about time itself. Time is not a fixed, universal background that ticks at the same rate for everyone everywhere. It is a local property — it depends on how fast you are moving and how strong the gravity is around you. The ISS clock is not broken or inaccurate. It is running at exactly the right rate for where it is and how fast it is going. The same is true for your clock on the ground. They are both correct. They simply do not agree with each other. That is not a quirk or a rounding error — it is how the universe is built.
Source: Space Daily