The clock beats slowly
![[NASA/JPL] Proposed position (arrow) of the atomic clock aboard the ISS](iss-clock.jpg) Q: Since time and speed is relative, according to Einstein's theory of relativity, will an atomic
clock on the International Space Station be slower than a synchronized atomic clock on the
ground? --Yiu Wai Chan
A: Yes, the clock on the International Space Station (ISS) will tick slower than the earthbound clock
because of relativistic effects. However, two effects compete and complicate the picture.
[NASA/JPL] Proposed position (arrow) of the atomic clock aboard the ISS
The dominant effect on the ISS clock is, as you surmised, a slow down because the space station zings
around Earth. Einstein figured out, back in 1905, that the speed of light is a constant for all frames of reference. Given that, an observer
on Earth peering through a telescope at the ISS clock would see it running slow compared with his own clock. This is the same effect
(the"time dilation" effect) that allows a returning space farer twin to be much younger than her earthbound sister, after the astronaut
spends years blasting through space at near light speeds.
Our genius of the last century, Einstein, however, didn't stop with the Special Theory of Relativity. He went on to develop his General
Theory of Relativity, which includes gravity effects. In 1915, he predicted that clocks run slower in high gravity fields. A clock on the
Sun would run slower than an Earth clock, for example. Thus, the gravity effect causes the ISS clock to speed up, since the gravity field is
slightly less for an object in orbit (more distant) than at the Earth's surface. We have competing effects.
The question is: which effect wins out-the speed or the gravity effect? Interestingly enough, the two effects cancel if you orbit about Earth
at a radius of 1.5 times Earth's radius
(about 5,900 miles, 9500 kilometers). This is pretty far
out. If the space station were to orbit
this high, an observer on Earth peering through his telescope would see the ISS clock and his clock agreeing.
The lower the ISS, the faster it must orbit. Increased speed means a slower clock. Thus, orbiting below that
5900-mile height, speed
dominates: the ISS clock ticks slower. Orbiting above 5900 miles high, gravity dominates: the ISS clock ticks faster. The ISS actually
orbits only 255 miles (407 kilometers) high so the ISS clock does run slow: About 0.0000000014 % slower.
(Answered by April Holladay, science correspondent, Apr. 3, 2002)
Further Surfing:
NASA/JPL: Primary Atomic Reference Clock in Space
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