You have probably heard how time is slower for someone on a spaceship traveling super fast, and if someone leaves from earth on a light speed spaceship and comes back in 20 earth years, everyone on earth will be twenty older but he will have barely aged. The following is a simple explanation of Albert Einstein’s theory of relativity and how time for that man on the spaceship really is slower than for the person standing on earth.
The first thing to know is the speed of light is constant.
Also, remember that when you are sitting on an airplane, within the enclosed atmosphere of the insides of the airplane, it feels as if you are sitting still. Even though the aiplane is flying at 350 miles per hour, you don’t feel a 350 mile per wind on your face. If you did, you’d blow away. If you get up from your seat, stretch, read a magazine and walk around, it seems the same as if you are stretching, reading a magazine and walking around just as if you are the ground on earth. You can throw a ball up and catch it, even though it is moving laterally at 350 miles per hour.
It is the same for the guy inside the spaceship flying at, let’s say, half the speed of light.
Let’s say the guy inside the spaceship going half the speed of light has a flash light and is standing directly underneath a mirror facing directly down from the ceiling. If he shines a beem of light from the flashlight directly up to the mirror, the light will go up and be reflected back down in the same line. Or, at least, that’s the way it appears to him. To the man, and anyone else on the spaceship, the light will go up and back down in a straight line. Just as with when you toss up and catch that ball on the airplane.
However, to a person (with really, really good eyesight) standing on earth, the same beam of light on that spaceship speeding will appear to take a different, longer path. From his vantage point and with the spaceship moving super fast left to right as the light beam shines, the light will appear to move upwards at a rightwards angle (hitting the mirror) and be reflected back down at a rightwards angle. This means, to the person on earth, the beam of light will appear to move further or take a longer distance path. And with the speed of light being constant that means, to the person on earth, it will take longer in time for the light to travel to up to the mirror and back down. In other words, more time passes for the person on earth for the same light beam to travel up and down than passes for the man on the space ship.
This is similar to how the doppler effect works, where a car’s constant horn (how annoying) seems to sound higher in pitch as it appoaches you and lower after it passes you and moves further away. The car’s horn sounds constant in pitch when the vehicle is stationary and to the driver or rider in the vehicle while the vehicle is moving. However, to the person standing still with the car approaching, the moving car shortens the sound’s wavelength (and raises the pitch) and the car moving away lengthens the wavelength (and lowers the pitch). The car’s speed adds or subtracts to the speed of the wavelengths, but only to the persons standing still.