In Interstellar, in order to save humans on Earth, Cooper and his party embark on an interstellar journey to distant extragalactic galaxies in search of habitable planets. When Cooper left Earth, his daughter Murphy was just a little girl. When Cooper finally returned to Earth, his daughter was old, but Cooper was still not much different from when he left Earth.
So why did Cooper, a father, end up being much younger than his daughter Murphy?
This question involves Einstein's theory of relativity. According to relativity , time in the entire universe is not synchronized , and the rates of time elapsed in different frames of reference are not consistent. The time in the frame of reference passes more slowly, and the time in the strong gravitational field passes more slowly, which is the time expansion effect.
Physicists have confirmed the effect of time expansion through comparative experiments with atomic clocks, and they have also been used for time calibration of navigation satellites. For navigation satellites operating in orbits at an altitude of 20,000 km, their flight speed is about 3.89 km / s, which needs to take into account the relativistic effect.
Because the navigation satellite flies very fast, the atomic clock on it will go 7 microseconds slower than the atomic clock on Earth every day. Navigation satellites, on the other hand, are farther from the center of the earth than the ground and are subjected to weaker gravitational effects, so the time on the satellite will pass faster than on the ground, about 45 microseconds faster per day.
In general, the time on the navigation satellite will be 38 microseconds faster than the ground per day, that is, 0.000038 seconds. Such a small time difference may seem insignificant, but if you multiply the speed of light, the cumulative positioning error per day exceeds 10 kilometers, so it cannot be ignored that the time of heaven and earth must be synchronized.
In Interstellar, spaceships don't fly fast, and intergalactic travel relies on wormholes. The other end of the wormhole connects a galaxy 10 billion light-years away, and although this distance is very far away, the wormhole is a shortcut in space-time, and the time it takes for the spacecraft to pass through the wormhole is very short. Therefore, the clock-slow effect brought about by speed can be ignored. What really causes time to slow down significantly is gravity, caused by Kagantuya's supermassive black hole.
Cooper and his party first visited Planet Miller, which orbits the edge of a supermassive black hole with a massive gravitational pull that slows time on Miller's planet, where 1 hour is equivalent to 7 years on Earth, or 1 second is equal to 17 hours. Cooper was on planet Miller for more than 3 hours, during which time 23 years had passed on Earth.
Although time on The Planet Miller passes relatively slowly, Cooper, who is on the planet, cannot directly perceive it. For Cooper, time is normal, and spending 1 hour there feels the same as spending 1 hour on Earth.
However, for observers on Earth and outside the region of time distortion, time on the planet Miller is very slow. If the people on board the Eternity spacecraft were to observe The Miller planet with an astronomical telescope, they would see that things on the planet were barely moving, or that everything would look like slow motion. If Cooper waved his hand on the planet Miller, it would seem to outside observers that the wave would take more than a dozen hours to complete. The relativity of time is relative to others, not to oneself.
After that, when Cooper left Mann's planet, the spacecraft slid into the edge of the black hole and used the black hole's gravitational slingshot effect to travel to the third planet. During this time, Cooper experienced time that was once again much slower than Earth's, this time by 51 years. Later, Cooper fell into the black hole, and time slowed down another 13 years.
As a result, 89 years have passed since Cooper left Earth and returned to the solar system. According to Cooper's revelations on the planet Miller, Murphy was 10 years old when Cooper set out, while Cooper was 35 years old. Because 25 years later on Earth, Murphy sent a message to Cooper saying that she was now the same age as Cooper when he left Earth.
Finally, when Cooper returned to the solar system, Cooper was 124 years old and Murphy was 99 years old for the humans there. For Cooper, however, the entire space mission took only two years, and his biological age is actually only 37 years old, but he is 62 years younger than Murphy. Of course, the calculations here ignore the time for low-temperature sleep.