Why does time slow down for somebody traveling at near light speed?

In the late 1800s scientests tried to measure the speed of light and got a strange result.  The speed of light was a constant.  

Think of a train with a transparent boxcar.  There is a Major League pitcher throwing baseballs at 180 km/hr toward the front of the train.  The train is going 120 km/hr.  To the pitcher the balls are going 180 km/hr.  To somebody standing next to the train with a radar gun the balls are going 120 + 180 = 300 km/hr.  The scientest expected the same result with light, but they got strange results.  Both the pitcher and the guy with the radar gun got the exact same speed.  If the baseballs behaved like light, both would get a speed of 180 km/hr.  All very strange.  The scientests were sure their was something wrong with their data.  But, every thing they tried they got the same result, the speed of light was a constant regardless of the motion of the person observing.

Einstein saw the way out of this mess:  Make the speed of light a constant (The experiments were correct!).  To do that something must give to make it work.  What has to give is Space and Time.  The motion of an observer causes Time and Space to shrink or expand so that the speed of light remains constant.  A weird thought, but it works.  The universe works like Einstein says it must.




Imagine a "photon clock"

The person traveling at c is in a transparent space ship exactly 1 light second long. There is a perfect mirror at each end of the ship with a photon bouncing back and forth between the 2 mirrors. Each time the photon hits a mirror the clock ticks one second. Now take the ship to c.

The observer can see the photon just as it bounces off the rear mirror. Since the ship and the photon are traveling at the same speed (c) the photon appears to be motionless with respect to the ship (and going c with respect to the observers frame of reference. Think of 2 cars side by side on the highway both going exactly 60 km/hr. Relative to each other they are motionless. Relative to the highway both are going 60 km/hr). In effect, time has stopped, the clock will never tick again.

That clock is dead accurate for the frame of reference it is in.

Whether or not time is passing depends on the frame of reference. For the Photon on the ship time has stopped and it is not moving with respect to the ship. For the Photon as observed from outside the ship, time is still passing and it is moving in that frame of reference.

this is time dilation

imagine this: two spaceships flying parallel to each other, one above the other, with each having a mirror pointing at the other. Now imagine a beam of light shot across the mirrors. what would a passenger on the ship see? he would see a straight path that the light traveled, a vertical path.

now imagine this: what if someone saw the same beam of light but was stationary and was watching the ships pass horizontally by? he would see the same beam of light travel not only vertically but horizontally. meaning he saw a zigzag path which is LONGER. a longer path means the light will take MORE time to reach the finish line

thus time is relative

why is it like this? even einstein wouldn't be able to explain it well because even he doesnt know

u just have to accept it for now until someone can offer a broader theory of relativity

Please read all of my answer. When the first particles appeared, gravity, space, and motion came into existence, gravity caused motion, motion only required space. When man arrived on the scene and invented a clock to help him organize his day he found that with the clock he could now measure speed. Time is a man made thing. I am convinced that at near light speed the only thing that slows is motion. If you subject a clock to near light speed the moving parts gain mass, the clock does not have enough energy to overcome the inertia of the added mass, the moving parts no longer move at the same rate and the clock shows time lost, it is motion that slowed to cause this condition. Using an atomic clock would make no difference, this clock uses the vibration of a crystal to monitor time, but vibration is a form of motion, the end result will be the same.