Understanding Temporal Dilation: The Time Vs. Velocity Equation - Part 2
The same principle applies to numerous other situations. If an avalanche begins at a popular ski resort or a rabid dog starts chasing a mailman, the skiers and mailman will both run from the source of the danger. That is because the approaching speed relative to them will become less, since they are also moving.
Light appears to behave in a way that is completely unlike anything ever seen. Johnny and Jenny are now standing on a long, wide-open street. Jenny pulls out a flashlight and quickly turns it on and off, sending quick pulses of light down the street. Imagine those pulses of light as being made of bundles of photons, which are the tiny particles that make up light. This is very similar to the way that Einstein first developed his theory. He wondered what would happen if someone chased after a beam of light at the speed of light. Logically, just as if Johnny had been able to run the same speed as the bus, the light should appear to be standing still. Einstein wondered if someone who was chasing after light at the speed of light would be able to reach out and grab a clump of the photons. In a sense, he wondered if it would be possible to hold light in his hand.
Johnny has just bought a new rocket and he wants to see if it can keep up with the light. Jenny quickly flashes the light and it shoots down the street at 186,000 miles per second (mps). The instant that the light takes off, Johnny hops on the rocket and chases after it at 100,000 mps. Common sense tells us that the light would be going (186,000 mps - 100,000 mps) 86,000 miles per second faster than Johnny. Just like the bus, avalanche, and rabid dog, the speed of the light should be slowed down relative to Johnny.
But, according to Einstein's second postulate, the speed of light is constant in all inertial frames of reference. This means, from Johnny's point of view, the light must still be moving away from him at 186,000 miles per second. No matter how fast Johnny tries to run to catch up with the beam of light, it is always going to be traveling 186,000 mps faster than him. If he were to stand at the far end of the street and run towards Jenny and the beam of light, no matter how fast, the beam of light would still be approaching him at 186,000 mps. Light travels a constant 186,000 miles per second at all times, from everybody's frame of reference.
I know many of you are shaking your heads right now. This concept flies directly in the face of all logical thought. It defies common sense. The bottom line is that this effect has been verified by countless scientific experiments. It is completely counter-intuitive but it is very real nonetheless.
Think of light as an object in an elusive dream, always just out of reach no matter how much effort is put into catching up with it. From Jenny's point of view, after one second has elapsed, the light will be 186,000 miles ahead of her. But after that same one second, from Johnny's point of view, the light will also be 186,000 miles ahead of him. If both of them are right, then the light has to be in two places at the same time, and that would be impossible.