Why does a constant speed for light mean that a moving clock will run slower?
A constant speed of light means the same speed of light for anybody anywhere moving at any speed. It’s from that “any speed” part that the whole time slowing thing comes in, but is only noticeable if you move closer to the speed of light. Suppose you shine a light past a space ship moving really fast. You see the light pass by about 300,000 km every second. The people in the space ship, watching your light beam, also see that light pass by 300,000 km every second. But you, watching the space ship would see it pass, say 100,000 km in a second. So from your point of view, the light that was beside them one second should be 200,000 km away from the ship 1 second later, they should see a speed of 200,000 km/s for the light. But light has a constant speed for everybody. If the ship has more time (from your point of view) for the light to travel, then they can see the light go 300,000 km in one second like they are supposed to. Their time gets slowed so that light can make it the full distan
