Section 1.1: First Principle of Relativity

First Principle: Absolute Motion Cannot Be Established

The concept of absolute motion suggests that an object can be considered to be in motion without any reference point. Imagine floating in a boundless, featureless space. If you attempted to move, you would be unable to determine your motion without something to compare it against. Only with a second object present could you ascertain whether you were moving closer to or further from it.

If you had a floor beneath you, that would serve as a reference point, allowing you to claim you were moving across it. However, that wouldn't denote absolute motion, as there would be two reference frames—the floor and you—each in motion relative to the other.

For instance, if I'm seated at my desk and a car drives past outside, is the car moving relative to me, or am I moving relative to it? Given that I am stationary in a room with solid furniture while the car moves away from us all, it is reasonable to state that the car is in motion relative to me.

If the car stops, then we are all stationary relative to one another. However, since we are on Earth, which is traversing space at approximately 11 miles per second, both of us are still moving in relation to the sun. Thus, Einstein's first principle emphasizes that all motion is indeed relative.

Section 1.2: Second Principle of Relativity

Second Principle: The Speed of Light Remains Constant

In essence, the speed of light in a vacuum is uniform for all observers, regardless of their motion or that of the light source. Initially, this notion may seem trivial, but its implications are profound.

The speed of light is approximately 299,792,458 meters per second, commonly rounded to 300,000 kilometers per second. To put this in perspective, if you could travel at light speed, you could circle the Earth over seven times in just one second, reach the moon in 1.3 seconds, Mars in three minutes, and the sun in about 8.2 minutes.

Light consistently travels at this "cosmic speed limit" denoted as 'C.' Importantly, this speed does not change, regardless of the observer's own velocity. For instance, if you are stationary, light moves away from you at 300,000 km/s. If you were traveling at 100 mph, light still departs at that same speed. Even if you approached light speed, light would not seem to slow down in comparison to you.

How can this be? The concept of speed is defined by the formula: speed = distance/time. Let's illustrate this through a thought experiment.

Imagine two mirrors facing each other, with a light particle (or photon) bouncing between them. If we set the distance between the mirrors to 1 meter, the photon travels 1 meter in one time unit.

Now, if the top mirror shifts while the photon is en route, the distance increases, but the speed of light remains constant. Although the photon has traveled a longer distance, it still takes the same amount of time to reach the top mirror, resulting in a fascinating scenario where time itself must adapt.

Einstein's theory reveals that time is not absolute; it is flexible and differs based on relative motion. This leads to the phenomenon known as time dilation, where time can stretch or contract based on speed and gravitational influence.

As the speed of an object increases, time dilates for it, meaning that hypothetically, the faster you travel, the slower you age. However, the speeds humans can achieve are far from enough to notice significant time dilation effects.