The internal problems of the theory are even more serious after another decade of research. These include the complexity, ugliness, and lack of explanatory power of models designed to connect string theory with known phenomena, as well as the continuing failure to come up with a consistent formulation of the theory.
String theory is perhaps the most controversial big idea in all of science today. On the one hand, it’s a mathematically compelling framework that offers the potential to unify the Standard Model with General Relativity, providing a quantum description of gravity and providing deep insights into how we conceive of the entire Universe. On the other hand, its predictions are all over the map, untestable in practice, and require an enormous set of assumptions that are unsupported by an iota of scientific evidence.
For perhaps the last 35 years, string theory has been the dominant idea in theoretical particle physics, with more scientific papers arising from it than any other idea. And yet it has not produced even one testable prediction in all that time, leading many to decry that it hasn’t even risen to the standard of science. String theory is simultaneously one of the best ideas in the entire history of theoretical physics and one of our greatest disappointments. Here’s why.
When a meson, such as a charm-anticharm particle shown here, has its two constituent particles pulled apart by too great an amount, it becomes energetically favorable to rip a new (light) quark/antiquark pair out of the vacuum and create two mesons where there was one before. This is not a successful approach towards creating a free quark, but this realization did give rise to the string model of the strong interactions. (THE PARTICLE ADVENTURE / LBNL / PARTICLE DATA GROUP)
The story begins in the late 1960s when particle accelerators were just entering their heyday. After the discovery of the antiproton in the 1950s, larger and more energetic particle accelerators began to be constructed, leading to an enormous suite of new particles that arose from colliding charged particles into other charged particles. The newly discovered particles came in three types:
- baryons, like the proton, neutron, and their heavier cousins,
- anti-baryons, like the anti-proton, anti-neutron, and heavier ones that matched 1-to-1 with the baryons,
- and mesons, which came in a variety of masses and lifetimes, but which all were unstable and quickly decayed away.
But one interesting thing to note was that mesons, before decaying, were like bar magnets. If you break a bar magnet (with a north and south pole), you don’t get an independent north and south pole, but rather two magnets each with their own north and south poles. Similarly, if you try to pull a meson apart, eventually it “snaps,” creating two separate mesons in the process.
Magnetic field lines, as illustrated by a bar magnet: a magnetic dipole, with a north and south pole bound together. These permanent magnets remain magnetized even after any external magnetic fields are taken away. If you ‘snap’ a bar magnet in two, it won’t create an isolated north and south pole, but rather two new magnets, each with their own north and south poles. Mesons ‘snap’ in a similar manner. (NEWTON HENRY BLACK, HARVEY N. DAVIS (1913) PRACTICAL PHYSICS)
This was initially where string theory began: as the string model of the strong nuclear interactions. If you envision a meson as a string, then pulling it apart increases the tension in the string until you reach a critical moment, resulting in two new mesons. The string model was interesting for this reason but predicted a number of strange things that didn’t appear to match reality, such as a spin-2 boson (which wasn’t observed), the fact that the spin-1 state doesn’t become massive during symmetry breaking (i.e., there’s no Higgs mechanism), and the need for either 10 or 26 dimensions.
Then the idea of asymptotic freedom was discovered and the theory of quantum chromodynamics (QCD) came to be, and the string model fell out of favor. QCD described the strong nuclear force and interactions extraordinarily well without these pathologies, and the idea was abandoned. The Standard Model, now complete, didn’t need this new, esoteric, and simultaneously ineffective framework.
At high energies (corresponding to small distances), the strong force’s interaction strength drops to zero. At large distances, it increases rapidly. This idea is known as ‘asymptotic freedom,’ which has been experimentally confirmed to great precision. (S. BETHKE; PROG.PART.NUCL.PHYS.58:351–386,2007)
But a decade or so later, this idea was reborn into what’s now known as modern string theory. Instead of working at the energy scales where nuclear interactions are important, the idea was put forth to take the energy scale all the way up to the Planck energy, where the spin-2 particle that made no sense could now play the role of the graviton: the theoretical force-carrying particle responsible for a quantum theory of gravity. That spin-1 particle could be the photon, and other excited states could be associated with the known Standard Model particles.
All of a sudden, a long-sought dreamed seemed within reach in this new framework. For one, string theory suddenly made it plausible that the Standard Model of particles and interactions could be reconciled with General Relativity. By viewing each of the elementary particles as either an open or closed string that vibrated at specific, unique frequencies, and the fundamental constants of nature as various states of the vacuum in string theory, physicists could finally hope to unify all the fundamental forces together.
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