100 Years of Einstein's General Theory of Relativity

By Julia Zeh
Edited by Ashley Koo

Exactly one hundred years ago, in November of 1915, Albert Einstein solidified his field equations of the general theory of relativity. Despite the passage of a century, both theories still stand and are still incredibly relevant in modern research, making November 2015 a month to celebrate Einstein’s work.

Einstein’s theories of special and general relativity have been fundamental in advancements in physics and cosmology over the past century. These theories were revolutionary in the way they broke from classical Newtonian physics, changing the way scientists think about and define time, space, and mass.

Part of what makes Einstein’s theories so incredible is how they came to be. Einstein used thought experiments to look at scientific problems; by imagining certain situations he was able to uncover vital physical ideas about the universe.

One of Einstein’s most famous thought experiments involved the constancy of the speed of light. At age 16, an age when most people are spending their time thinking about the perils of high school rather than the implications of Maxwell’s equations, Einstein was imagining traveling at 300 million meters per second (the speed of light) alongside a beam of light. (Maxwell’s equations describe the electromagnetic behaviors that lead to the production of light.) According to classical Newtonian mechanics, light would appear to be standing still. Experimental results and Maxwell’s equations, however, suggest otherwise. The teenage Albert Einstein then concluded that the speed of light must be constant, and not relative to anything. This thought experiment led him to further thought experiments and to the special theory of relativity.

Light is particularly interesting because its velocity is not actually relative to anything else. Einstein’s theories of relativity operate on the basis of the fact that all velocities are relative, an idea that dates back to Galileo. For example, if someone were to run while inside a bus in motion, they would be running relative to the frame of reference of everyone else on the bus. The velocity of the runner is observed differently by people also on the moving bus and people at rest on the street outside the bus.

Special relativity is derived from the idea that all velocities are relative except the speed of light, which is always constant. The theory of special relativity describes a special case of motion where velocity is constant and there is no acceleration. Simply put, special relativity proposes that space and time are not absolute, as was previously thought; they vary from observer to observer. In certain situations, observers see objects in motion getting smaller in their direction of motion and time running slower on clocks in motion. These effects stem from the constancy of the speed of light. Therefore, everything is relative -- there is no “correct” observation of things in the universe. Time dilation and length contraction are real effects, but because human observations concern a very small range of speeds, lengths, and times within the scale of objects in the universe, we don’t see the effects of time dilation and length contraction in our daily lives.

Space and time are not absolute and different observers see different lengths and times depending on their motion. Thus velocity, mass, and distance all impact time and space, as the theories of special and general relativity imply, and so scientists describe a single fabric of the universe, namely spacetime. Briefly put, time is fundamental in describing events and thus is considered a fourth dimension, which is described alongside space.

Einstein also came up with general relativity, the theory which we celebrate this month. General relativity is a more general form of his special theory (special relativity), and it applies to acceleration and to gravity. Gravity is a unique force: it is vastly weaker than the other physical forces (electromagnetic, strong nuclear, and weak nuclear) and acts at huge distances. Einstein was able to discover that gravity is also unique because it warps spacetime, and this is described in his general theory of relativity.

The easiest way to think about gravity’s effects and the warping of spacetime is by imagining a large sheet with a bowling ball at the middle. If the sheet is held up off the ground, the bowling ball pulls it down in the middle, changing its shape. If other objects are then placed on the sheet, they are attracted to the bowling ball at the middle because of the shape of the sheet. Any other round objects placed on the sheet would roll towards the middle because of this warping effect. This is analogous to the effect that matter has on the fabric of space and time.

​General relativity has a multitude of implications for the Big Bang, for black holes and for other aspects of the universe. Although the theory is now one hundred years old, it is still remembered and honored. What is particularly remarkable is that Einstein generated ideas that nobody else could at the time. These ideas also may not have seemed entirely rational, given how radically different they were from the established laws of classical physics. However, Einstein’s ability to think creatively and innovatively and to challenge preexisting notions of space, time and classical mechanics is an ability that is essential to scientific discovery.