Last month, Albert Einstein’s Theory of General Relativity celebrated its 100th birthday. One hundred years later, scientists still recognize Einstein’s model as the standard. Despite the dynamism that usually characterizes science, general relativity has withstood countless efforts from modern physicists to disprove it, and for that feat alone it deserves to be understood.
Before the twentieth century, Isaac Newton’s Law of Universal Gravitation was the accepted definition of gravity; he explained the strange phenomenon as being the result of attractions between massive bodies. While his mindset was effective when applied to the motion of “real world” objects, Newton’s theories lost some validity on the microscopic and macroscopic orders. For example, his theory regarding interactions between two massive bodies contrasted with experimental calculations of Mercury’s orbit around the sun; his predictions did not account for the exceptional gravity that Mercury experienced as the planet closest to the sun.
Einstein’s 1905 Theory of Special Relativity was the small but necessary first step away from tradition. He abandoned the consideration of absolute space and time and favored the notion of a four-dimensional universe that included the three cardinal directions (x, y and z) as well as spacetime, a new parameter that interwove space and time. He also determined that for all inertial, or non-accelerating, reference frames, the laws of physics and the speed of light were constant.
Unfortunately, even Einstein’s theory had its limitations: he still had yet to provide a concrete solution to the problem Newton’s Laws offered. In fact, by introducing a completely new perception of the universe, he only complicated the issue. To address this, Einstein conducted a “thought experiment” through which he further developed general relativity.
His first realization stemmed from the concept that objects in free fall do not “feel” a force, despite accelerating. That is, if a person on Earth was in freefall, he experiences a sensation of weightlessness. Similarly, if the person was on a rocket in deep space moving at a constant velocity but with no quantifiable gravitational force, he would also not be able to feel his own weight. This notion convinced Einstein that Newtonian reference frames such as massive objects that exist in gravitational fields could not be distinguished from those that are merely accelerating. In 1911, he formally developed the Equivalence Principle, which equates the effects of gravity and acceleration.
After this thought experiment, Einstein considered incorporating gravity into his theory for special relativity, but he still had yet to create a formal definition for what gravity actually was. Eventually he concluded that gravity was the warping of spacetime. Einstein theorized that massive bodies like the sun have the ability to mold spacetime around them, and that free-falling objects will follow the straightest path through spacetime, which leads to massive bodies. Picture a stretched rubber sheet, and the effect that placing a bowling ball on its center has on the rest of the surface-in a crude sense, the reaction of the material around the bowling ball models the effect that the sun has on the space around it. At last, years of work yielded a set of differential equations that elaborated on exactly how mass and energy tend to warp spacetime. Four years later, general relativity was confirmed when scientists measured the bending of starlight due to the sun’s gravity.
However, such a theory wouldn’t be considered so magnificent if it did not have any practical applications, and general relativity, even a century after its conception, holds many consequences for the modern world around us. For instance, the accuracy of the global positioning systems (GPS) that we use everyday rely heavily on the concept of general relativity. Because satellites in space experience only one-fourth of the gravity that receivers on Earth do, general relativity suggests that because spacetime is warped by gravity, the clocks on the satellites actually move faster than clocks on the ground, by about 38 microseconds per day, which would tabulate to daily navigational errors of up to ten kilometers. This is not to say that clocks as we know them fail to tell time, but without correction for general relativity, GPS would fail within minutes.
In the early 1900s, very few were as daring as Albert Einstein to disregard all that was accepted about a field of science and synthesize what would later be called a revolution. One hundred years later, Einstein’s “most beautiful theory” persists as one of the most important discoveries ever achieved in modern physics.