Time is not absolute; its passage is relative and dependent on an observer’s state of motion and location within a gravitational field. This phenomenon is known as time dilation. It means that one hour spent on Earth will not necessarily equal one hour experienced by someone in space. The differences are governed by the two major theories of relativity, which describe how both immense speed and the presence of mass can alter the rate at which time flows.
Time Dilation Caused by Velocity
Time passes differently based on an object’s speed, a concept explained by Albert Einstein’s theory of Special Relativity. This theory establishes that the speed of light is a universal constant, meaning every observer measures light traveling at the same speed, regardless of their own motion. A consequence of this constant is that when an object moves at a high velocity, time slows down for that object relative to a stationary observer. The term “time dilation” specifically describes this difference in elapsed time between a clock that is moving and one that is at rest.
The effect is negligible at everyday speeds, but it becomes pronounced as an object approaches the speed of light. For instance, if an imaginary clock were traveling at 95% of the speed of light, an hour on a stationary Earth clock would correspond to only a little over 18 minutes on the moving clock. This relativistic effect has been confirmed in experiments involving high-speed subatomic particles, which are observed to exist for a longer duration when traveling near the speed of light.
Time Dilation Caused by Gravity
The second mechanism influencing the rate of time is gravity, described by Einstein’s theory of General Relativity. This theory posits that massive objects, like Earth, curve the fabric of spacetime around them, often conceptualized as a “gravity well.” The closer one is to this massive object, the stronger the gravitational field they experience.
A direct result of this spacetime curvature is that time passes more slowly for an observer who is nearer to the center of a strong gravitational field. For example, a clock at sea level ticks marginally slower than an identical clock placed on top of a mountain. Conversely, an object in high orbit, where gravity is weaker, experiences time passing faster relative to the clock on the planet’s surface. Unlike the velocity effect, the gravitational effect on time is not reciprocal; all observers agree that the clock in the stronger gravitational field is running slower.
Measuring Time Differences in Earth Orbit
The actual, measurable time difference for objects in Earth orbit, such as the International Space Station (ISS) or Global Positioning System (GPS) satellites, is a combination of both velocity and gravity effects.
International Space Station (ISS)
The ISS orbits Earth at an altitude where it experiences a reduced gravitational field, which works to speed up the astronauts’ time. However, the ISS also travels at approximately 17,500 miles per hour, and this high velocity works to slow down their time. The velocity-induced time dilation effect slightly outweighs the gravitational effect at the ISS altitude. The net result is that astronauts on the ISS age a tiny bit slower than people on Earth, with the total difference amounting to only a few milliseconds after a year in orbit.
Global Positioning System (GPS) Satellites
GPS satellites orbit at a much higher altitude of about 12,550 miles, providing a striking example of combined relativistic effects. At this height, the gravitational effect that speeds up time is significantly greater than the velocity effect that slows it down. Specifically, the reduced gravity causes a GPS clock to gain approximately 45 microseconds per day compared to an Earth-based clock. Simultaneously, the satellite’s speed causes it to lose about 7 microseconds per day. Engineers must account for the net effect, which is that the satellite’s atomic clocks run faster by about 38 microseconds every day. Without this built-in correction, the system’s timing errors would quickly accumulate, rendering all GPS navigation inaccurate by several miles within a single day.
Clarifying the Scale of Time Differences in Space
The answer to how long one Earth hour lasts in space depends completely on the specific location and speed of the observer. For any space travel currently undertaken by humans, the time difference is exceptionally small and accumulates over long periods. For an astronaut in low Earth orbit, one hour on Earth is essentially one hour for them, with the difference being measured in tiny fractions of a second over many months. The dramatic time differences often imagined, where one hour on Earth equals days or years in space, would require conditions far beyond current human capability. Such significant dilation would only occur if a spacecraft were traveling at an extreme velocity very close to the speed of light or if it were positioned immediately next to a massive object like a black hole.