What's Wrong with String Theory?

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String theory postulates that we live in a universe of 11-dimensions. Up/down, left/right, forward/backward, time, and... well... seven more dimensions that are curled up and impossible to see. All of the smallest particles that we can see are not actually little dots or spheres but actually tiny strings with vibrational modes. It lives on its reputation of mathematical beauty. How does the development of this theory compare to the development of some of the great accepted theories of physics?

Late in the 17th century, Isaac Newton built the first comprehensive mathematical models of the universe. He based them off of his own experiments and the experiments of others. He calculated that the pull of the earth upon the moon is 3600 times weaker than the pull of the earth on the apple on its surface. Combined with his knowledge that the moon is roughly 60 earth radii away, this supported his law that the strength of gravity weakens as the distance between objects grows, at a rate of that distance squared! He experimented with prisms, buckets of water, lead weights and even invented the world's most ubiquitous telescope to inspire and support his theories. For roughly 200 years, his work dominated physics.

By about 1900 however, more clever experiments began finding problems with Newton's theory. Over the next 30 years, two entirely revolutionary branches of physics were founded to explain this. Einstein's relativity was one of these. The other was quantum mechanics. The very first cornerstone of quantum mechanics was laid by Planck, simply because his data could only be explained by a totally new and at the time off-the-wall theory. (The topic under consideration was a phenomenon known as blackbody radiation.)

"I was so desperate..." he explained. This alluded to how crazy he thought it was to invent a new theory, but how he felt it was necessary because the old theory worked so poorly and the new one worked so well.

Planck's theory (solid line) fits the data (circles) perfectly, while the old theory (dotted line) is completely wrong! (source)

Bohr and Einstein added to this fledgling discipline by again forcing a model that no one previously believed in to the forefront, strictly because it was the only way to explain the radical results their experiments (and everyone else's) were beginning to see.

For the next 50 years or so, theoretical physics grew drastically stranger. No one would have believed that the things contained in the Standard Model could possibly be true except that, somehow, they worked incredibly precisely and accurately. Similarly, in every other field of physics from semiconductors to superconductors, new models were developed and accepted because and only because they explained actual experimental results better than previous theories. If they didn't they were eventually abandoned more (plum-pudding model) or less (luminiferous aether) quickly.

The Standard Model is neither particularly concise nor beautiful. Everyone in the field would love to see something new supersede it. However, for more than 40 years now, there has never been found a single piece of experimental data that can only be explained by string theory. String theory cannot explain anything better than the SM. A proposed experiment to test string theory would cost more than the annual GDP of the entire planet to construct. Another hypothetical experiment which can perhaps check the validity of the math (but can't determine whether the theory holds for everything) has even been attacked by string theorists! This is a strange state of affairs and one that worries many people in the field. Freeman Dyson, one of the greatest physicists and polymaths alive eloquently states his own fears here.

In short, a new theory needs to be able to explain data that an old theory cannot. So far, this hasn't been the case with string theory.
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