When astronauts orbit Earth at extremely high speeds, something extraordinary happens: time flows differently for them. This effect, known as time dilation, is a real and measurable consequence of Einstein’s Theory of Relativity [1]. On the International Space Station (ISS), astronauts travel at about 28,000 km/h, and at this velocity, time for them slows down ever so slightly compared to people on Earth [2].

Einstein showed that time is not fixed. It changes depending on speed and gravity:

• Special Relativity: When a person moves very fast, time slows for them relative to someone who remains still [1].
• General Relativity: When someone is farther from a strong gravitational field—such as being in orbit—time runs faster [3].

Astronauts in orbit experience both effects:

• High speed → slows time
• Weaker gravity → speeds time
  The two effects don’t cancel out perfectly. The speed factor is stronger, so astronauts age slightly slower overall [2].

The difference is tiny (milliseconds), but measurable with modern atomic clocks [4]. Over years of flight, it accumulates—meaning astronauts return to Earth just a little younger than if they had stayed on the ground.

This remarkable phenomenon shows that time is flexible—a dynamic quantity shaped by motion and gravity. Every astronaut becomes a living demonstration of Einstein’s equations in action.

References

[1] Einstein, A. Relativity: The Special and General Theory.
[2] NASA. “Time Dilation Observed in the International Space Station.”
[3] Misner, C., Thorne, K., & Wheeler, J. Gravitation.
[4] Chou, C. et al. “Optical Clocks and Relativity.” Science Magazine.

Date of Input: 08/12/2025 | Updated: 08/12/2025 | emma