Quick Answer
Astronauts on the International Space Station age slower than us on Earth. This is because their high speed and slightly weaker gravity cause time to pass for them at a different rate, a real-world effect of Einstein's theories of relativity. It's a mind-bending demonstration that time isn't absolute, but can be influenced by motion and gravity.
In a hurry? TL;DR
- 1Astronauts on the ISS age slightly slower due to time dilation, experiencing a net time gain of about 0.005-0.007 seconds every six months.
- 2Time dilation on the ISS results from a tug-of-war between Special Relativity (speed slows time) and General Relativity (weaker gravity speeds time).
- 3The speed of the ISS (17,500 mph) is the dominant factor, causing time to tick slower for astronauts compared to Earth.
- 4NASA's Kelly Twin Study confirmed astronauts age less, with Scott Kelly returning technically younger than his twin Mark on Earth.
- 5Time dilation effects are negligible in daily life but crucial for technologies like GPS, which require relativistic corrections.
- 6The subtle age difference demonstrates time is relative, influenced by velocity and gravitational forces rather than being a universal constant.
Why It Matters
It's surprising that travelling faster in space means you actually age less, proving time isn't fixed for everyone.
Time dilation is a physical reality that causes astronauts aboard the International Space Station (ISS) to age approximately 0.01 seconds less than people on Earth for every six months spent in orbit.
Key Statistics: Time in Orbit
- Daily Time Gain: Approximately 28 microseconds (due to gravity) vs loss of 7 microseconds (due to speed)
- Total Net Difference: Roughly 0.005 to 0.007 seconds per six months
- Orbital Speed: 17,500 miles per hour
- Record Holder: Sergei Krikalev has technically lived 0.02 seconds into the future
Why It Matters
This phenomenon proves that time is not a universal constant but a relative experience dictated by your specific location and velocity in the universe.
The Twin Stakes of Relativity
The time difference experienced by astronauts is the result of two competing forces described by Albert Einstein: Special Relativity and General Relativity.
On the ISS, these two theories actually work against each other. According to Special Relativity, the faster you move through space, the slower you move through time. Because the ISS travels at 7.66 kilometres per second, an astronaut’s clock ticks slower than a clock on the ground.
However, General Relativity introduces the impact of gravity. Gravity curves space-time, meaning time actually moves slower the closer you are to a massive object like Earth. Because the ISS is 400 kilometres above the surface, gravity is slightly weaker, which causes time to speed up relative to the surface.
The Kelly Twin Study
NASA conducted a landmark study involving identical twins Scott and Mark Kelly to observe these effects. Scott spent nearly a year on the ISS while Mark remained on Earth as a control subject.
According to researchers at NASA’s Human Research Programme, the mission provided a unique biological dataset. While the time dilation effect was mathematically confirmed (Scott returned technically younger than his twin), the study focused heavily on the genetic expression and telomere changes caused by the environment of space.
Why We Don't Feel It
The reason humans do not notice time dilation in daily life is that the effects are only significant at extreme speeds (approaching the speed of light) or near extreme gravitational wells (like black holes).
Compared to the speed of light, the ISS is moving at a snail's pace. While 0.01 seconds sounds trivial, it is a massive hurdle for technology. GPS satellites, which sit further away and move faster than the ISS, must Have their internal clocks programmed to account for a 38-microsecond daily discrepancy. Without these relativistic adjustments, your phone's GPS location would be off by several kilometres within a single day.
Real-World Implications
- Satellite Synchronisation: Global telecommunications rely on correcting for time dilation to keep networks aligned.
- Deep Space Travel: If we eventually send humans to Mars, the time discrepancy will grow more pronounced, impacting communication logs.
- Particle Physics: Subatomic particles called muons live longer when accelerated to high speeds in labs like CERN, providing tiny proofs of the same concept.
Does time dilation make you live longer?
Technically, yes, but only relative to someone else. You would not feel like you have extra time; your heart would beat and your cells would age at a normal rate from your perspective. You simply bypass a fraction of a second that everyone else experiences.
Is this basically time travel?
In a strictly physical sense, yes. Astronauts who return from long-term missions have travelled a few milliseconds into the future relative to the people they left behind.
Does this happen on aeroplanes too?
Yes, but to a much smaller degree. A passenger on a long-haul flight around the world will end the trip a few nanoseconds younger than those who stayed home.
Key Takeaways
- Velocity: Speeding up slows your clock down relative to a stationary observer.
- Gravity: Being further from a heavy mass (Earth) speeds your clock up.
- Net Effect: On the ISS, speed has a greater impact than altitude, making astronauts age slower.
- GPS Necessity: Your phone works only because it accounts for these identical physics.
Time is a fabric we weave through, not a drumbeat we all follow in unison. While an astronaut’s 0.01-second advantage won't win them a longer life, it serves as a permanent reminder that the universe does not keep a single set of books.
