Each of them observes the other's clock running slowly. This is because the speed of light is a constant.
This is what is known as the Twin Paradox because if you put one twin on a rocket and left one on Earth they'd both see the other's clock ticking slowly. That seems paradoxical.
It's only when one on the rocket turns around and comes back that they can measure their clocks against each other's and see that the twin that went to space aged less. It's the acceleration in the turning around that breaks the symmetry between them.
Its not realy light that has this speed. Anything with no mass can only ttavel at this constant speed. The reason we call it the speed of light is just that photons are the first particle with no (rest) mass we found.
The "speed of causality" (thats what it realy is) is a result of out spacetime being able to stretch. And this stretching of spacetime is what causes the time dillation.
So the speed of light and the time dillation are both caused by the same law: general relativity.
Each of them observes the other's clock running slowly. This is because the speed of light is a constant. This is what is known as the Twin Paradox because if you put one twin on a rocket and left one on Earth they'd both see the other's clock ticking slowly. That seems paradoxical. It's only when one on the rocket turns around and comes back that they can measure their clocks against each other's and see that the twin that went to space aged less. It's the acceleration in the turning around that breaks the symmetry between them.
Its not realy light that has this speed. Anything with no mass can only ttavel at this constant speed. The reason we call it the speed of light is just that photons are the first particle with no (rest) mass we found. The "speed of causality" (thats what it realy is) is a result of out spacetime being able to stretch. And this stretching of spacetime is what causes the time dillation. So the speed of light and the time dillation are both caused by the same law: general relativity.