Taylor Made: It's all relative
In the movie Interstellar, Joe Cooper leaves his teenage son and 12-year-old daughter on earth to navigate through space with the hopes of finding a habitable planet. He experiences different gravitational pulls along the way, most notably spending minutes on a water-logged planet and returning to his orbiting space ship to find that the crew member who had stayed on board has aged 23 years. Cooper eventually hurtles through a black hole before being recovered to a space station orbiting Saturn that had been built and launched in his absence. He is told he is now 124 years old in earth time, and is able to reunite with his elderly daughter before her death, appearing to her at basically the same physical age as he was when he left.
While real-life astronauts have not experienced such an extreme bending of time, Einstein’s relativity theories tell us that the phenomenon exists. His theory of special relativity has two postulates: that the speed of light in a vacuum is the same to all observers regardless of their motion, and that all non-accelerating bodies experience the same laws of physics. These postulates led him to discover the space-time continuum. His general theory of relativity found that space-time has curvature, saying that the gravitational pull of massive bodies (like Earth) can warp and twist the continuum.
The space-time continuum says that the physical constructs of space and time must be combined mathematically for accurate depiction. It therefore regards 3D space and 1D time as fused into a 4D entity. Recall that a continuum describes a set of elements in which the extremes are very different from each other, but the differences between adjacent elements are imperceptible. Additionally, all elements can be endlessly divided and have no missing points.
The concept of gravity warping time, known as gravitational time dilation, says that time slows down as gravity increases (i.e. distortion in the continuum). For an observer, to increase gravity would mean to get physically closer to its pull. This means that people living thousands of feet above sea level are actually experiencing less gravity than those below them, i.e. they are living in faster time. This was proved using atomic clocks—of a pair of identical clocks, the one closer to the earth’s surface moved slower than the one at a higher altitude. So yes, mountain dwellers are technically aging faster than people in Florida—by a fraction of a second over their lifetime, that is.
When Cooper spent a few minutes on the surface of that watery planet, its gravity would have been such that time slowed way down for him relative to the space ship. However, it is not always true that those on the surface are aging slower than those in space, because relativity also says that an object in motion experiences time at a slower rate than one at rest.
When you observe a satellite, you can see it move across the sky fairly quickly. This means that the satellite is orbiting the earth faster than the earth is rotating, so the satellite has faster angular velocity than you, the observer, do. By the theory of relativity, the clock of the satellite is moving slower compared to yours. This was again proven through the use of atomic clocks. GPS units sending and receiving signals from stations on earth have to account for the relativity between them and the orbiting satellites—it has been calculated that, without accounting for relativity, a GPS map showing that you are 0.8 km away from a destination based on satellite information could show you as being 8.8 km away after one day of that satellite being in space.
As the reach of space travel and exploration continues to expand, with rocket projects from companies like SpaceX and the increased exploration of Mars through upcoming missions like NASA’s 2020 rover launch, Einstein’s theory is becoming more and more relevant to the work of new generations of engineers.