*The speed of light is the same to all observers.*

From these principals, the basic equations of the special theory of relativity follow logically (refer to time dilation section). These equations relate measurements by observers moving at different speeds. They ensure, for example, that both you on the Earth and a friend in a spacecraft traveling near the speed of light agree on the same laws of physics, unaffected by any pitfalls or paradoxes caused by how you and your friend are moving relative to each other.

In developing the special theory of relativity, Einstein found that he had to abandon old-fashioned, rigid notions of space and time. For example, imagine a friend whizzing across or solar system in a spacecraft while you remain here on the Earth. Einstein proved that your friend's clock would seem to tick more slowly than your own. In addition, your friend's rulers when held parallel to the direction of motion will seem shorter than yours. In brief, *moving clocks are slowed* and *moving rulers are shortened in the direction of the motion*. These strange results are direct consequences of the speed of light being an absolute constant.

**(a)As seen by the outfielder, ball is approaching her at (30 m/s) + (10 m/s) = 40 m/s**

**(b)Incorrect Newtonian description:**

As seen by astronaut in spaceship, light is approaching her at (3x108 m/s) = 4x108 m/s

As seen by astronaut in spaceship, light is approaching her at (3x108 m/s) = 4x108 m/s

**Correct Einsteinian description: As seen by astronaut in spaceship, light is approaching her at 3x108 m/s**

*The Speed of Light is the Same to All Observers.*

(a) in Newtonian physics, the speed of light of any object depends on how the observer is moving.

(b) Einstein showed that this commonsense principle does not apply to light. No matter how an observer is moving, he will always measure light to have the same speed. This remarkable fact about light, which goes completely against intuition, is at the heart of Einstein's special theory of relativity. This theory has other bizarre consequences. For example, the astronaut with the flashlight will see the flying astronaut's spaceship as being shortened along the direction of motion and will see the flying astronaut's clocks (including his wristwatch and his heartbeat) as ticking slowly. Furthermore, the special theory of relativity says that the laws of physics are the same no matter how fast you are moving, so both astronauts see the same effects of relativity. Here the flying astronaut will see the astronaut with the flashlight (who is moving relative to him) as shortened along the direction of motion and as having slowly ticking clocks.

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