The question of how a day in space compares to a day on Earth involves time dilation, a real physical phenomenon where the passage of time is not uniform for observers in different locations or states of motion. This difference in elapsed time between two clocks is typically minuscule for human space travel but is measurable. Time is not absolute; it is directly influenced by both motion and mass. The exact duration of a day in space depends entirely on the observer’s location and speed.
Relativity: The Core Principle Governing Time
Albert Einstein’s theories of Relativity provide the scientific framework for understanding why time flows differently for a space traveler. His theories fundamentally changed the understanding of time and space, demonstrating that they are not separate entities but are interwoven into a single continuum called spacetime.
Relativity is divided into two parts that describe the two ways time can be stretched or compressed. The Special Theory of Relativity addresses the effects of constant velocity on time, while the General Theory of Relativity describes the effects of gravity. Both theories rely on the observation that the speed of light in a vacuum is a constant value for all observers, regardless of their own motion. This constant speed of light is the ultimate factor that forces time to adjust for different observers.
Time Dilation Caused by Velocity
The first mechanism for time dilation arises from high-speed movement, as described by Special Relativity. This effect dictates that the faster an object moves relative to a stationary observer, the slower time will pass for the moving object. From the perspective of someone standing on Earth, an astronaut traveling rapidly in a spaceship would appear to have their clock ticking more slowly.
This phenomenon is best illustrated by the classic thought experiment known as the Twin Paradox. Imagine two twins: one remains on Earth while the other embarks on a long journey in a rocket traveling at a speed close to the speed of light. The twin who stays on Earth is considered to be in an inertial frame of reference for most of the trip.
The traveling twin, however, must accelerate away from Earth, turn around mid-journey, and then decelerate to return home. Because the traveling twin undergoes acceleration and changes their frame of reference, there is a fundamental asymmetry between the two paths through spacetime. When the traveling twin returns to Earth, they will have aged less than their Earth-bound sibling.
This result is an intrinsic property of spacetime; the clock on the fast-moving craft simply records less elapsed time. The effect has been confirmed using high-speed subatomic particles in accelerators and even with atomic clocks flown on commercial aircraft. The difference is only noticeable at speeds nearing the speed of light, but the principle holds true for any relative motion.
Time Dilation Caused by Gravitational Mass
The second way time is affected relates to the presence of mass and is described by General Relativity. This theory states that massive objects, like planets and stars, cause a curvature in the fabric of spacetime. This curvature is what is experienced as gravity.
In a strong gravitational field, time passes more slowly than it does in a weaker field. A clock located near a massive object will tick slower than an identical clock located farther away from that mass. This is often described as time running slower the deeper you are in a “gravity well.”
Therefore, time passes slightly faster in space, far from Earth’s mass, than it does on Earth’s surface. For instance, a clock at the top of a mountain runs infinitesimally faster than a clock at sea level. Both gravitational and velocity time dilation are independent effects that must be accounted for simultaneously when calculating time in space.
Calculating the Actual Time Difference
For an astronaut aboard a spacecraft like the International Space Station (ISS), both velocity and gravitational effects influence the flow of time. The ISS orbits Earth in Low Earth Orbit (LEO) at an altitude where gravity is only slightly weaker than on the surface, which would tend to make time run faster. However, the ISS is also moving at a tremendous speed of roughly 7.66 kilometers per second (17,100 miles per hour), which tends to make time run slower.
For the ISS, the time-slowing effect of its high velocity is slightly greater than the time-speeding effect of its reduced gravity. The two effects do not cancel out completely, and the velocity effect dominates. As a result, one day for an astronaut on the ISS is slightly shorter than one day on Earth, by a margin of a few microseconds.
An astronaut spending six months aboard the ISS will age approximately 0.005 to 0.007 seconds less than a person remaining on Earth. This difference accumulates over time, making an astronaut who has spent over 800 days in space about 0.02 seconds younger than they otherwise would be. This combined calculation of velocity and gravitational time dilation is a necessity for modern technology. For example, the clocks on Global Positioning System (GPS) satellites must be constantly adjusted by engineers to account for a net time gain of about 38 microseconds per day to ensure accurate navigation.