In the early 20th century, Albert Einstein was a young patent clerk working in Switzerland. It was during this time that Einstein had much time to think deeply about the problems of physics of the day. Einstein was very different from his fellow peers. He never truly excelled in school, except for physics and mathematics. He was ridiculed and called "lazy" by his professors. He never saw the true value for it, but instead focused his time on learning more and more about physics. Eventually, in 1905, Einstein had his annus mirabilis, or his miracle year. It was during this time that he launched two new branches of physics: relativity and quantum mechanics. Relativity will be discussed here, and the latter will not. First, I will explain relativity, its differences from classical mechanics, and finally how only a creative genius could have seen relativity through the clouds of classical mechanics.
To begin, we need some background onto the leadup of the creation of the theory of relativity. At the beginning of the twentieth century, physics was considered to be complete. There was classical mechanics, which was able to accurately predict the motions of planets and everyday object, and also electrodynamics, which was being used to its fullest ability in the creation of usable electricity such as alternating current. The incompatibility between classical mechanics and electrodynamics was in the Galilean theory of relativity. According to the theory of electrodynamics, the speed of light is a constant. By solving Maxwell's equations, the critical equations that explain all off electrodynamics, one can prove that the speed of light is a constant, traveling at 3,000,000 meters per second. In classical mechanics, the Galilean theory of relativity can be illustrated by this example. Suppose a car is traveling at 10 m/s, and a person throws a baseball at a speed of 5 meters per second out of the moving car in the same direction it moves. How fast does the ball move? It moves 10 m/s plus 5m/s, which equal 15m/s. That is Galilean relativity, when one object is moving relative to another, the velocities are simply added. But what about light? Suppose the car is moving straight at 10 m/s and then it turns its headlights on. Is the light traveling at 3,000,000 m/s+10 m/s according to Galilean relativity? Or is it still moving at a constant 3,000,000 m/s according to electrodynamics? The answer to this was very subtle and swept under the rug for many years by physicists, until Einstein solved it.
The answer is the light still moves at 3,000,000 m/s according to electrodynamics. Galilean relativity, and classical mechanics along with it, was wrong. The truth is Newton and Galileo, the founders of classical mechanics, did not have access to electrodynamics that Einstein did, and therefore produced incorrect theories. Their theories did work for macroscopic objects moving at slow speeds, but failed at velocities close to the speed of light. Through the use of Lorentz Transforms and the concept of spacetime, Einstein proved two implications of a universal light constant speed. First time dilation and length contraction, which can be summed up as faster object, when moving at a significant fraction of the speed of light, will experience time slower and will contract in length. The second implication was that he showed that the fastest speed any object can travel in the universe is the speed of light. By proving these two implications mathematically as well, Einstein provided the correction to classical mechanics and put it on equal footing with electrodynamics.
The creativity of Einstein is seen in his gedanken experiments, or thought experiments. Here is one of his famous thought experiments that illustrates the relativity of time. The twin paradox: two identical twins, Alice and Bob, synchronize their clocks. Alice stays on Earth, while Bob travels away from Earth in a spaceship moving close to the speed of light. When Bob returns, they compare their clocks. According to special relativity, Bob—the traveling twin—will have aged less than Alice, who remained stationary. This seemingly paradoxical result arises because each twin sees the other's clock moving slower during the journey. However, the paradox resolves when we recognize that the traveling twin undergoes acceleration (turnaround), breaking the symmetry and leading to genuine differences in elapsed time. Ultimately, the paradox is resolved by understanding that accelerated frames of reference differ fundamentally from inertial frames, clarifying why one twin genuinely ages less than the other. It is in thought experiments like the twin paradox, and recognizing the fatal flaw in classical mechanics, that can be attributed to Einstein's creative genius.