# Light travels faster than time

### Question:

When you sit in an aircraft ..., ... which flies at the speed of light, what happens when you turn on the headlights?

Everyone has heard the loud thunder that is caused by an airplane breaking the sound barrier. The plane compresses the air in front of it. This change in air pressure, which we perceive as sound, propagates through the air at the speed of sound, around 330 meters per second. As the aircraft approaches the speed at which the pressure waves move away from it, they condense until they finally overlap to form a large pressure wave when the aircraft reaches the speed of sound. We hear this pressure wave as a sonic boom.

For a long time it was thought that light, like sound, needed a medium in which it could propagate. At the end of the 19th century, however, experiments were carried out that refuted this assumption. In contrast to sound, light can also propagate through empty space.

Let us remember the supersonic aircraft: If this moves relative to the still air at more than the speed of sound and generates sound in front of it, the aircraft will immediately overtake the sound because it is linked to the speed of sound. But if the aircraft flies through the empty space and switches on its headlights, no dormant medium prevents the light from moving away from the aircraft at the usual speed of light! Considerations of this kind led Albert Einstein to claim that light spreads equally quickly for every observer, no matter how fast he moves relative to another observer. The consequences of this assumption are formulated in the theory of relativity, which states, among other things, that no particle with mass can ever reach the speed of light. But one can imagine accelerating a spaceship from Earth until it has almost reached the speed of light. If you turn on the headlights while sitting in the spaceship, you can still see the light flying away from you at the speed of light.

Incidentally, the fact that you can never reach the speed of light only applies in a vacuum. In a medium, e.g. B. water, the light moves more slowly. If you accelerate a particle to almost the speed of light and let it immerse in the medium, it will move faster than the light in the medium for a short time (but never faster than light in a vacuum). The particle is slowed down by the medium and can emit light, similar to a spaceship that turns on its headlights. Now something similar happens as with a supersonic aircraft: Like the pressure waves there, the light waves now overlap to form a "supersonic boom" of light, which can be observed as a flash of light. This so-called Cherenkov effect is z. B. used in measurements in particle physics.