After deriving equations and theorems for some of physic's most intractable problems (the photoelectric effect, special and general relativity), Einstein spent the last 20 or so years of his life trying to come up with a Unified Field Theory, also dubbed a theory of everything. The prospect was to marry seemingly, and demonstrably, conflicting "fields," that of gravity, electromagnetism and later quantum physics, into one that could accurately and eloquently describe the nature of reality. He didn't succeed, but that doesn't mean he failed, either.
James Clerk Maxwell came up with the first of the field theories when he put forth electromagnetism ”” the force that deals with electricity, magnetism and light. In this field theory, there are two charges (positive and negative) and attractions between dissimilar and repulsions between similar.
Then Einstein proposed the general theory of relativity, which basically proffered that gravity is the outcome of large objects in spacetime altering the fabric of the universe, thus "pulling" objects nearer them. It's thought of as a weak force, whereas electromagnetism is a strong one.
Then quantum theory hit the scene, and that didn't rub Einstein the right way at all. The uncertainty, randomness and probabilistic nature of quantum theory irked him, and he believed that it was a poor derivation of another more complete, more unified theory still out there to be found. So he set out to find it.
Around the middle of the 20th century, Einstein and Erwin Schrodinger, working tangentially, came to a general theory that took general relativity and incorporated a metric tensor ”” a way to coordinate a system or space ”” that they assumed might also be able to account for electromagnetism. It didn't get picked up.
However, later scientists presumed a large cosmological constant that refines the metric tensor and actually picks up where Schrodinger's individual equations left off. There is some hope that this is approaching a unified theory. And yet, there's another conjecture that has captured the attention of physicists.
That potential solution is today agreed upon as string theory. Though the idea of an embedded universe that can account for (now) four forces of nature ”” gravity, electromagnetism, and strong and weak nuclear attraction ”” had origins before 1968, that was the year CERN researcher Gabriele Veneziano first used "strings" to describe and model strong forces. Essentially, the theory has it that there are 11 (or between 10 and 26 depending on which theoretical physicist you talk to) dimensions, and they happen to be tucked into each other...like strands of string. The forces that govern and the particles that make up our universe emanate from "vibrations" of these strings. Trippy, right?
See, the idea of more than just four dimensions (three for space, and one for time) had been floated past Einstein while he was still in the early part of his study. In 1918, some of Einstein's contemporaries had written him letters with their own proofs that increased the number of workable dimensions to five, in order to fit in Einstein's complex and daunting equations. He wrote back to them, "The idea of achieving unification by means of a five-dimensional cylinder world would never have dawned on me...At first glance I like your idea enormously." Einstein published his first paper on unified field theory a few years later.
On his site, theoretical physicist Dr. Michio Kaku has challenged and solicited from his readers to submit their unified field theories. There are plenty of stipulations meant to weed out the wacky and senseless, groundless and mystical and encourage the grounded, mathematical and provable. Central to any theory, he states, will be Einstein's tensor equations and the Standard Model of particle physics (which are geometric in nature). He also goes onto state another crucial attribute of any unified theory...and it gets to the heart of the matter.
The problem is one of "renormalizability," meaning that when General Relativity and the Quantum Theory are combined into one coherent model, the resultant equation malfunctions. String theorist proponents try and get around this by bumping out to not just five but 10 dimensions. You can imagine that this is hard for the layperson, and even the seasoned scientist, to come to grips with.
The other rope burn String Theory leaves with grappling physicists is the presumption that though the calculations of each step yield finite sums, all in all the entire sequence yields an infinite sum...What? String theorists are emboldened by the inconsistency, as it erases some of the weirdness that would arise from finite (and dissimilar) results, like having more than one gravity and not know what to do with it. But it still doesn't make a whole lot of sense.
12. The Vacuum: Full of Nothing or Empty of Everything?
String theorists do posit one, somewhat concrete, assertion about the nature of the universe that they believe can be derived numerically, and it goes back to equations Einstein and Schrodinger were working on. And that's the cosmological constant, which is the measurement of stuff in a vacuum. Wait: what stuff? Ah. String theorists believe that because of these tucked and spiraled strings that undergird reality, a vacuum isn't evacuated in the true sense, and there's a >0 mass there. But, who knows what it is.
Remember in the past year everyone was talking about how maybe our universe is just a hologram? Well, that idea stems from one interpretation (or redress) of string theory known as the Maldacena conjecture. Back in 1997, Juan Maldacena did some fantastic work reconciling and translating Einstein's equations with quantum theory in a way that was mathematically and physically kosher. Well, in 2013, some mathematicians following in Maldacena's footpath came up with some serious papers that shook the science world.
The papers, from Japan, prove numerically that the thermodynamic energy measured in a black hole could have been produced by a lower dimension force, granting credence to string theory as well as Maldacena, who introduced the idea of holography in his original '97 paper. If modes of existence can be "bumped up" or represented in higher or lower dimensions, then you could make the case for 10 or so intertwined dimensions tucked away inside each other. Maldacena concedes that the notions presented in the newest papers are completely paradigm shifting and without their peer: but maybe that will change. Or maybe it will need to.
Einstein may have died before he could successfully solve what is largely considered the "Holy Grail" of physics, but that doesn't mean that his work died with him. Scientists all over the world are still attempting to bring to fruition his quest for an elegant, singular, and internally consistent answer to the nature of the universe. But it's the hardest question we have to answer.