Einstein's Theory of Relativity, formulated by the renowned physicist Albert Einstein in the early 20th century, revolutionized our understanding of space, time, and gravity. Comprising two main theories—Special Relativity and General Relativity—Einstein's work fundamentally altered the fabric of theoretical physics and laid the groundwork for many technological advancements. To comprehend the essence of these theories, we must embark on a journey through the concepts of spacetime, the speed of light, and the profound implications of Einstein's groundbreaking ideas.

Special Relativity, introduced by Einstein in 1905, emerged from a desire to reconcile the observed constancy of the speed of light with classical mechanics. Classical physics, based on the works of Newton, treated space and time as separate and absolute entities, providing a fixed framework for understanding the motion of objects. However, the experimental results from the Michelson-Morley experiment and other studies challenged the prevailing notion of an absolute reference frame.

Einstein's stroke of genius was to discard the concept of absolute time and space and embrace a more flexible framework. Special Relativity introduced the concept of spacetime—a four-dimensional continuum combining three dimensions of space with one dimension of time. This unified approach allowed for a consistent description of physical events, incorporating the relative nature of motion.

One of the key postulates of Special Relativity is the constancy of the speed of light in a vacuum, denoted by ‘c.' Regardless of an observer's motion or the motion of the source emitting light, the speed of light remains the same. This principle shattered the classical idea of Galilean relativity, where the velocity of an object would simply add or subtract to the observer's velocity.

Einstein's theory introduced a series of mind-bending consequences. Time dilation, a phenomenon where time appears to pass more slowly for a moving observer compared to one at rest, arises from the constancy of the speed of light. The famous twin paradox illustrates this: a twin embarking on a high-speed space journey would age more slowly than the twin who remains on Earth.

Another consequence is length contraction, where an object's length appears shorter in the direction of its motion when observed from a different reference frame. Both time dilation and length contraction have been experimentally verified, confirming the validity of Special Relativity.

The theory also gave birth to the famous equation E=mc^{2}, expressing the equivalence of mass and energy. This groundbreaking formula signifies that energy (E) is proportional to mass (m) and the speed of light (c) squared. The practical implications of this equation have been harnessed in technologies like nuclear energy and have deepened our understanding of the interplay between matter and energy.

While Special Relativity dealt with objects moving at constant velocities, General Relativity, published by Einstein in 1915, expanded the scope to include gravity and accelerated motion. General Relativity redefined our understanding of gravitation, offering a new perspective that replaced Newton's gravitational theory.

In General Relativity, gravity is not a force transmitted between masses, as envisioned by Newton, but rather a curvature of spacetime caused by the presence of mass and energy. Massive objects, such as stars and planets, bend the fabric of spacetime around them, influencing the motion of nearby objects. This bending of spacetime is what we perceive as gravity.

The famous analogy often used to describe this concept is imagining spacetime as a rubber sheet. Placing a massive object on the sheet causes it to curve, and smaller objects nearby, like marbles or planets, will naturally move along the curved paths created by the curvature. This visualization captures the essence of how gravity operates in the context of General Relativity.

One of the early triumphs of General Relativity was the explanation of the anomalous precession of the orbit of Mercury. The precise predictions made by Einstein's theory matched the observed deviations from classical predictions, providing strong empirical support for General Relativity.

Another significant prediction of General Relativity is the gravitational time dilation, where time flows more slowly in regions of stronger gravitational fields. This phenomenon has been experimentally verified through precise measurements of time dilation near massive objects, such as Earth.

General Relativity also predicted the existence of gravitational waves—ripples in spacetime caused by the acceleration of massive objects. However, it took a century for technology to catch up with theory, and the direct detection of gravitational waves finally occurred in 2015, confirming yet another aspect of Einstein's visionary framework.

Black holes, predicted by General Relativity, are perhaps the most enigmatic objects in the universe. These gravitational behemoths result from the collapse of massive stars, and their intense gravitational fields are so strong that nothing, not even light, can escape beyond a certain boundary called the event horizon. The study of black holes has unveiled intriguing phenomena such as time dilation near the event horizon and the possibility of gravitational waves generated by the collision of black holes.

Einstein's Theory of Relativity has not only withstood the test of time but has also become an integral part of modern physics. Its predictions have been confirmed through numerous experiments and observations, and technologies like GPS explicitly incorporate corrections based on both Special and General Relativity to achieve accurate results.

The impact of Einstein's theories extends beyond the realm of pure science. They have influenced our philosophical understanding of the nature of space, time, and reality. The bending of spacetime, the interplay of matter and energy, and the relativistic effects on time challenge our intuitive perceptions, inviting us to reevaluate our fundamental notions of the universe.

Moreover, Einstein's ideas have found applications in various fields, from astrophysics to cosmology, and have contributed to the development of technologies that shape our daily lives. The exploration of the cosmos, the understanding of the Big Bang, and the quest for a unified theory of physics all draw inspiration from Einstein's enduring legacy.