Two Postulates
When Einstein began thinking about the problems of moving bodies, one question stuck in his mind: what does it mean when two events happen at the same time? He ran through many though experiments involving frames of reference traveling at the speed of light and relative time, and he arrived at two fundamental principles.

The first was the classic principle of relativity, which stated that the laws of physics remained the same for any frame of reference. This idea was not new, it was in fact an important part of Newtonian mechanics, but Einstein expanded this principle to include all phenomena, not just mechanical ones.

Einstein's second basic postulate assumed that the speed of light and all laws of physics remain constant in all inertial frames of reference, as stated by classical Maxwellian theory. Although both elements of special relativity had been proposed before, Einstein was the first to combine sections classical mechanics and Maxwellian electrodynamics into a more accurate description of the universe.

While in his first 1905 paper he described it in particles, he used the wave theory of light for Special Relativity, thus utilizing the dual particle-wave nature of light to shape his new worldview.

The scientific community immediately recognized special relativity as having profound implications for physics and cosmology. The application of both postulates simultaneously stirred enormous controversy in the physics community. This was not surprising, considering the incredible implications.

High Speed Travel
For a long time, people had observed that the velocity of sound seemed to be different in different directions when measured from the perspective of a moving object. For example, if an object, let's say, a train, traveled at one-half the velocity of sound while emitting sound waves, the waves going in the direction of travel will move with one-half the velocity of sound relative to the train, while the waves going opposite the direction of travel will move at one and one-half times the speed of sound. Notice that the sum of the velocity of the sound waves and that of the train remains constant, and thus an outside observer still measures the same velocity of sound going both ways.

Naturally, because of the similarities between light and sound waves, physicists assumed that the same phenomenon occurred with optical signals. But according to the second postulate and experimental results, the velocity of light remains constant regardless of the frame of reference.

Now, say we have a spaceship now. If the ship was traveling at nearly the velocity of light, say 99.99% of it, chasing a ray of light, we would expect that the passengers of the ship have almost caught up with the light, since the light ray travels only 0.01 % faster than they do. But, if they measured it, the light would still be moving at 277, 792 kilometers per second. No matter how fast the spaceship moves, the velocity of light remains absolutely constant. Since the spaceship is already moving at almost the velocity of light, and the velocity of light is still constant relative to it, we would then expect the velocity of light relative to an outside observer to nearly double. But again upon measurement, as Einstein's second postulate maintains, the velocity of light is still 277, 792 km/s as always.

In order for this to work, Einstein showed that the time intervals and/or lengths involved must change according to the speed of the system relative to the observer (in this case, the spaceship). Thus, while Newton's time and space were absolute and universal, Einstein's time and space were different for different frames of reference. And in the real world, the travelers of our spaceship experience time much slower than their friends back on earth, agreeing with Einstein's description of the universe. Although this defies common logic (everyone has his own unique space and time!), scientists have shown this effect in real world experiments. For example, an atomic clock travelling at high speed in a jet ticks slightly more slowly than its counterpart on the ground. The difference is extremely small when we're dealing with jets and cars and such, but at velocities approaching the speed of light, the effect is enormous.

Like atomic time, biological time (the rate of your bodily processes) is also relatively slowed down by high-speed travel. This time dilation (slowing down) is the basis of the famous Einstein "twin paradox," the most controversial and confusing idea in relativity. In this scenario, an astronaut flies off into outer space at very high speeds leaving his twin brother on Earth. When he returns home, he discovers that his twin brother is already an old man while he is still young. The clocks on board his spaceship as well as the biological clock inside his body recorded fewer hours and days and years than his brother's corresponding clocks on Earth. Agreeing with the Equivalence Principle, a strong gravitational field will produce similar time dilation .

If we accept the two postulates of Special Relativity, we must abandon Newton's concepts of absolute space and absolute time, and we must also abandon the idea of the luminiferous ether, which became unnecessary in Einstein's theory of physics. In fact, the differences in the doppler effect on light and sound effectively debunk the concept of the ether. Thus, with this one paper, Einstein destroyed three long-accepted concepts.

From the relativity of space and time, Einstein theorized the equivalence of mass and energy, a consequence of the special theory of relativity. The representation of this equation in its final form was the immortal equation E=mc².

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