Engraving by an unknown artist first appearing in Camille Flammarion's L'atmosphère: météorologie populaire (1888). The image depicts a man crawling under the edge of the sky, as if it were a solid hemisphere, to look at the mysterious Empyrean beyond. The caption (not shown here) translates to 'A medieval missionary tells that he has found the point where heaven and Earth meet...'
We’ll get to the meaning (and purpose) of life later, but first we need to talk about guts, toes and strings.
Physicists tie guts and toes with string. Or to put it less simply, string theory is seen as a promising way to integrate grand unifying theories (GUTs) with theories of everything (TOEs).
GUTs are about unifying three of the four forces that physicists call fundamental. “Fundamental” means there are no others. "Fundamental" means that across the whole of the universe are no forces, powers or energies other than the fundamental four: electromagnetism, the weak force, the strong force, and gravity.
Every force is one of the four fundamentals or is based on one of the four. Centripetal and centrifugal forces, for example, are just gravity in the round. Nuclear energy is the strong force flexing its muscles. Radioactivity is the weak force at work.
Electricity and magnetism have been unified: they are different aspects of the same underlying force, electromagnetism.
Electromagnetism and the weak force have been unified into the electroweak. To put it less simply: electricity, magnetism and the weak force are thought to have once been the same thing, or at least, different aspects of the same thing.
And you're not wrong if you think this is all weird, strange or abnormal. In fact, renormalizing the electroweak was how three nerds (Glashow, Salam and Weinberg) earned their Nobel Prize.
The strong force keeps it all together. What is it? It is ordinary matter (as opposed to non-ordinary matter, called dark matter, which is believed to exist but about which we know very little). Specifically, the strong force binds protons and neutrons into nucleons, and confines quarks to their hadrons. (Jeez! It’s got to be true, you just can’t make this stuff up!)
It is thought that the strong and the electroweak were once unified but went their separate ways shortly after the big bang.
GUTs are about unifying three of the four fundamental forces. TOEs are about unifying all four. Unfortunately, the fourth fundamental force, gravity, has a mind of its own and continues to go its own way. Relatively speaking, of course!
TOEs aim to wrap all four fundamental forces into a nice, neat, tight, little parcel. And the best way to wrap a parcel is with string.