The 1905 publication of Einstein's special theory of relativity drastically altered how people understood science. Spacetime, mass-energy equivalency (E=mc2), and the idea that the speed of light remains constant for all observers were all introduced by this theory. The behavior of objects in inertial (non-accelerating) frames of reference is the main topic of special relativity. A sequence of thought experiments led to the development of the special theory of relativity.

The Michelson-Morley experiment, conducted in 1887, provided important new information by demonstrating that the speed of light in a vacuum remains constant independent of the observer's location or velocity. Einstein recognized this constant speed of light contradicts classical physics, specifically, Newton’s inertia. 

Acknowledging this, Einstein carried out a well-known thought experiment involving a fictitious scenario in which a train carrying a passenger travels at half the speed of light in a vacuum, while a light source is released in the direction of the train. Newton's relativity states that an observer outside the train should be able to see the accelerated light since the train's velocity, ½ c, and the speed of light, 1 c, should combine to produce 1.5 c. However, after observing, the train passenger and the stationary observer come to the same conclusion: light travels at a constant speed. 

Here, Einstein was able to discern an idea that, while seemingly unconventional, solves this conundrum. If the speed of light, which is around 300 million meters per second, remains constant, then meters and seconds must also alter. As a result, space and time itself dilate quickly, giving rise to the constant speed of light that we experience today. 

After publishing the theory of special relativity, Einstein was troubled yet again by the inconsistency between Newton's theory of gravity, an instantaneous force acting from a distance, and the principles of his special theory of relativity, which emphasized equivalence in the unity of space and time. He began to explore alternative explanations for gravity, ultimately drawing inspiration from the principle of equivalence, a principle that states gravitational and inertial forces are indistinguishable from an observer in free fall or in an environment that lacks gravity. By this logic, a person under Earth’s gravity or in a rocket in outer space that accelerates 9.8 meters per second squared would not be able to tell the difference, as it is not just similar, but the same. 

Einstein also wondered what would happen to light in these two scenarios. Light normally travels in a straight (the shortest line) to its destination, but if the rocket is accelerating, a beam of light that passes from one end of the rocket to another will bend slightly. 

After establishing that space must curve around huge masses, Einstein reasoned that light must also bend in the presence of large objects. This led to the observation of gravitational lensing, a phenomenon in which the gravitational field of massive objects, like stars or galaxies, bends light's course. This insight laid the groundwork for Einstein's theory of general relativity by demonstrating that mass and energy not only affect an object's motion but also distort spacetime itself.

In summary, Albert Einstein's theories of special and general relativity, born from insightful thought experiments and careful consideration of experimental evidence, have fundamentally transformed our comprehension of the universe. Einstein challenged and built something new upon classical physics. Both special and general relativity remain as one of the most accurate theories in the entirety of physics. Einstein has not only revolutionized physics but also laid the groundwork for our modern understanding of space, time, and gravity, leaving an enduring legacy that continues to shape the scientific field.

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