Why Scientists Can't Find a 'Theory of Everything'
Scientists have been trying to create a unified theory of everything for the past couple centuries, but it seems like the best contender is always dethroned a couple decades later, after everyone has already declared that yep, we figured it all out.
Newton's theory of gravitation was unseated by Einstein's general relativity, which was undermined by the discovery of quantum physics, which was supposedly explained by string theory, which everyone started laughing at after the 1980s.
To understand why each of these theories failed, you'll have to know something about them.
Here's a short run-down on each one.
Newton's Theory of Gravitation
You've probably heard the third of Newton's 'Three Laws of Motion,' which is usually quoted as "For every action, there is an equal and opposite reaction." The First Law is that "every object in a state of uniform motion tends to remain in that state of motion unless an external force is applied to it." But it's the Third Law that changed the game—it was the idea that the relationship between and object's mass, its acceleration, and its force looks like this: force = mass x acceleration.
Along with gravity, Newton had seemingly found the mechanics that explained the way the universe worked, at least on a physical level. His work could be applied to the motion of planets, to apples falling off trees, and anything else scientists could imagine.
Of course, we future people of the 21st century realize that Newton's view of 'everything' was way too simplistic. Enter: Einstein.
Einstein's Theory of Relativity
Einstein's theory of general relativity explained some of the more persistent holes in Newton's supposed 'laws' and ended up delving much deeper into how gravity worked. It turns out that gravity's mechanics depended on space and time being woven together—usually compared to something like a rubber sheet. Massive objects warp the sheet, creating wells around themselves that cause other objects to be attracted to them.
Einstein's general relativity gave a much deeper insight into what we now see as reality: instead of space and time being separate entities, they're actually closely linked. But when looking at micro-scale physics, it all started to break down.
Quantum physics is still the Wild West of science. At the level of subatomic particles, things like electrons and quarks seem to operate on rules we still don't understand (and which certainly don't jive with Einstein's relativity): their locations can't be predicted with accuracy, their properties seem to change when observed, and things like quantum entanglement (what Einstein called "spooky action at a distance") starts to show up.
We're still unraveling quantum physics, which means we've got a long way to go until we've got a 'theory of everything.' But that may be the problem—Stephen Hawking claimed in his 2010 book The Grand Design that there may be no theory of everything—it's just an illusion.
Hawking and the End of Theories
Stephen Hawking is still one of the chief authorities on incredibly weird, complex physics, but even he apparently doubts that there's a final answer to all the universe's questions. He claims that scientists should instead adopt a point of view called "model-dependent realism":
"This radical theory holds that there doesn't exist, even in principle, a single comprehensive theory of the universe. Instead, science offers many incomplete windows onto a common reality, one no more "true" than another. In the authors' hands this position bleeds into an alarming anti-realism: not only does science fail to provide a single description of reality, they say, there is no theory-independent reality at all."
You could claim that the reason that scientists can't find a theory of everything is because our knowledge of the universe is still growing, or you could side with Hawking and say there is no 'theory' to find. Either way, we've come to a point where we've started making theories about the theory of everything. As you can imagine, that causes some problems.