Time is not slower in space simpliciter. The effect depends on the relative gravity and velocity between two points of observation. The further you are from a gravitational source like Earth, the faster time passes relative to an observer on Earth. However, if you are moving at a high velocity relative to Earth, time passes slower for you relative to an observer on Earth.
Time Dilation Explained
Einstein's theory of relativity tells us that time is relative, not absolute. This means that the rate at which time passes depends on:
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Gravity (Gravitational Time Dilation): The stronger the gravitational field, the slower time passes. This is because gravity warps spacetime.
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Velocity (Special Relativistic Time Dilation): The faster an object moves, the slower time passes for that object relative to a stationary observer.
Time Dilation in Space: A Balancing Act
In space, the effects of gravitational and velocity time dilation are usually both at play.
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Lower Gravity: Spacecraft orbiting Earth experience weaker gravity than objects on Earth's surface. This causes time to pass slightly faster for astronauts. The higher the orbit, the weaker the gravity and the faster time passes.
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High Velocity: Spacecraft in orbit are also traveling at high speeds. This causes time to pass slightly slower for astronauts.
The net effect depends on the specific circumstances. For astronauts on the International Space Station (ISS), which orbits relatively close to Earth, the gravitational effect is greater than the velocity effect. As a result, time passes slightly faster for them (by about 0.01 seconds per year) compared to people on Earth.
Examples of Time Dilation
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GPS Satellites: GPS satellites experience both gravitational and velocity time dilation. Without accounting for these effects, the GPS system would quickly become inaccurate. The satellites' clocks would drift by about 7 miles per day.
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Black Holes: Near a black hole, gravity is extremely strong. Time slows down dramatically relative to someone far away from the black hole. Theoretically, at the event horizon of a black hole, time stops completely from the perspective of a distant observer.
Summary
Whether time is "slower in space" depends on the observer's relative velocity and gravitational potential. Generally, being farther away from Earth's gravity increases the rate at which time passes for an observer. However, the velocity of the observer has the opposite effect, slowing down the rate at which time passes. In low earth orbit the gravitational time dilation is stronger than the velocity time dilation, resulting in a net increase in the rate at which time passes.
