Rod wrote:
Why do scientists believe that if someone in a ship races at the speed of light to another galaxy, and gets there in 10 years, that humans on Earth would be any older than 10 years?
If you travel 1 million light years in 10 years, everyone on earth is still going to be 10 years older.By the time you got back they would be 2x10 years older.Sure you got there fast, but that doesn't change time.
It's because time and space are inextricably connected (space-time) and it's a proven, measurable fact. If you are accelerating in a spaceship to near the speed of light, time within the spaceship slows down for the astronaut, relative to an observer outside the space ship (
time dilation). Because of this phenomenon, a trip to the nearest solar system (Alpha Centauri) and back, would take ~10 years traveling near the speed of light, but relative to the outside observer on earth, the trip could take 100 years or more, depending on how close the astronaut came to the speed of light.
This phenomenon has been tested and observed many different ways. In one famous example, two atomic clocks were precisely synchronized. One clock was placed on an airplane and the other left on earth. The plane circled the earth several times at a high rate of speed. After the plane landed the clocks were compared and sure enough, just as the mathematics predicted, the clock on the plane had slowed down, relative to the clock left on the earth. There are numerous other observations that have proven this out as well.
Motion through space not only changes the distance between you and I, but it also changes time between you and I. As far as time is concerned, it's just too small of a difference to notice in our every day lives but there is no doubt that it changes. Even the act of me walking toward you slows down time for me as I cover the distance (space), but it's so small you just don't notice it.