Applying Einstein's Work to the Mysteries of the Universe (String Theory)

Einstein’s work in developing the theory of relativity had shown amazing results, unifying key concepts and clarifying important symmetries in the universe. Still, there are some cases where relativity predicts strange behavior, such as singularities, where the curvature of space-time becomes infinite and the laws of relativity seem to break down. String theory today continues this work by trying to extend the concepts of relativity into these areas, hoping to find new rules that work in these regions.
With relativity in place, physicists could look to the heavens and begin a study of how the universe evolved over time, a field called cosmology. However, Einstein’s field equations also allow for some strange behavior — such as black holes and time travel — that has caused great distress to Einstein and others over the years.
If you haven’t read about relativity before, this topic may seem like a whirlwind of strange, exotic concepts — and these new theories certainly felt so to the physicists of the time. Fundamental concepts — motion, mass, energy, space, time, and gravity — were transformed in a period of only 15 years!
Motion, instead of being just some incidental behavior of objects, was now crucial to understanding how the laws of physics manifested themselves. The laws don’t change — this was key to all of Einstein’s work — but they
can manifest in different ways, depending on where you are and how you’re moving — or how space-time is moving around you.
In topic 9, I cover the ideas of modern cosmology arising from Einstein’s work, such as the black holes that can form when massive quantities of mass cause space-time to curve infinitely far and similar problems that come up when trying to apply relativity to the early universe. Or, as you see in topic 16, some solutions to Einstein’s equations allow time travel.
Einstein himself was extremely uncomfortable with these unusual solutions to his equations. To the best of his ability, he tried to disprove them. When he failed, he would sometimes violate his own basic belief in the mathematics and claim that these solutions represented physically impossible situations.
Despite the strange implications, Einstein’s theory of general relativity has been around for nearly a century and has met every challenge — at least when applied to objects larger than a molecule. As I point out in topic 2, at very small scales quantum effects become important, and the description using general relativity begins to break down. The equations make no sense, and space-time becomes an exotic, tumultuous mess of energy fluctuations. The force of gravity explodes to an infinite value. String theory (hopefully) represents one way of reconciling gravity at this realm, as I explain in topics 10 and 11.