Is String Theory Hanging by The Thread?

String theory, the fantastical 'theory of everything', is the physicist’s silver bullet to the current paradox of quantum gravity, due to how elegantly it proposes to solve the puzzle - so why does it appear so loose at the seams? With proclamations by scientists such as Brian Greene who claims that "String theory is the most developed theory… to unite general relativity and quantum mechanics in a consistent manner. ", why has not a strand of evidence been found to prove the theory? Boasting such profound applications, it's no wonder the theory continues to make waves in the physics community, but is it all beginning to look a little incoherent? 

The gravity of the situation

The story begins in 1687, with Newton’s ‘Principia’ which essentially establishes the field of classical physics, also known affectionately as Newtonian physics in respect to the major influence Newton and his laws have had over understanding how the world we see and perceive works. Notably, classical physics was also influenced by a scientist of the name James Clerk Maxwell, who discovered light is a wave composed of electric and magnetic fields propagating perpendicular to one another. Why is that any important? Because the discovery of EM (electromagnetic) waves proved for the first time that “There are not only objects that we can see with our eyes, but also intangible fields that we cannot see.” What the discovery of EM waves did not prove however were the true properties of light, with the discovery of such properties challenging the belief that classical physics was the be-all end-all theory for the field, and introducing a new, sturdy opponent in the ring along the way - quantum physics.

By consensus, the theory of quantum physics was established precisely on the 14th of December, 1900 when Max Planck had his light-bulb moment and proposed his explanation for the frequency of energy emitted by a glowing object, better known as Planck’s Law. Planck’s Law stated electromagnetic energy was emitted in packets of energy known as ‘quanta’, with the energy of each quantum being equal to the Planck’s constant multiplied by frequency. The discovery of Planck’s Law is significant because it provided an atomic explanation for the behaviour of particles and waves and paved the way for the later discovery of light’s particle-like properties (the photoelectric effect) by Einstein in 1905. These two discoveries proved quantum physics to be more thorough and effective than classical physics, leaving just one more fighter in the ring - one a little more down to earth - gravity.

If there’s one thing classical physics and quantum physics can’t take on however, it’s a cohesive explanation for gravity. While in classical physics, general relativity explains gravity as a result of the curvature in spacetime (the amalgamation of the three physical dimensions plus time as a fourth) due to mass and energy, quantum physics would argue gravity can be quantified (e.g. as graviton particles). Here, general relativity conflicts one of quantum physics' major principles, the Heisenberg Uncertainty Principle, which states the position and velocity of a particle cannot certainly be known at the same time, whereas general relativity describes gravity as being a geometric property afflicted by the set positions of everything in the universe which cause the fabric of spacetime to warp under the weight of it, or to put it briefly, "spacetime tells matter how to move, matter tells spacetime how to curve" famously as told by Wheeler. When it comes to explaining gravity, it's widely accepted that general relativity wins this round, but this doesn't excuse how effective and accurate quantum physics is, so why doesn't general relativity take the trophy? General relativity is not an entirely conclusive theory of gravity because of its inability to explain how gravity works on a small scale even though it obviously has to, or gravity could simply not exist, hence why physicists are so keen to introduce quantum physics to the mix, but have unfortunately failed to do so as the calculations are infinitely long (literally). Fortunately, some physicists have come up with a theory that’ll lift your spirits right up - a rather abstract theory known as string theory.

Stringing it together

String theory suggests quarks, the constituents of subatomic particles like electrons and protons, are made of string-like filaments of energy which have unique vibrations for each quark and thus the properties of every elementary particle.  These one-dimensional strings can stretch, merge and split; allowing particles to thus become different particles such as a graviton, which theoretically carries gravitational force. Therefore, string theory would unify both general relativity and quantum theory by allowing gravity to be defined at both a large and small scale and a theory of everything to be established. End of story, except not really.

Evidence (or lack thereof)

Unfortunately, the struggle does not end here as we reach multiple dead-ends to string theory, beginning with the glaringly-obvious lack of evidence. Firstly, though these strings would have to be one-dimensional, for string theory to work, the strings must vibrate in 9 spatial dimensions (plus time as a 10th), even though our universe only has 4, as stated before. Perhaps we humans lack the ability to perceive these other dimensions, but we may never be able to detect them given the limitations of our technology; supported by scientists’ belief that these other spatial dimensions are just far too small to be detected. Supposedly, the shape of strings depends on the shape of these extra dimensions (e.g. visualised by Calabi-Yau Manifold, fig. 1), 

The Calabi-Yau Manifold is particularly significant to string theory considering it aligns with the requirement of 6 imperceivable spatial dimensions of superstring theory - string theory with the added theory of supersymmetry which smooths our certain inconsistencies in the mathematics of regular string theory,

which also dictate the properties of our universe, so what if these strings are dictated by something else? Not necessarily another fundamental material, but if they essentially ‘make’ gravity, are strings then influenced by it also? Back to the topic at hand,  these supposed strings are so small our current technology simply cannot detect them, again supporting the belief there is insufficient proof for string theory. However, proof does lie in the fact that the mathematics for string theory does support the existence of gravity, perhaps suggesting the two are intrinsically linked, which could support some of mathematics for general relativity. However, as string theory mathematically can be used as a model for different universes in the multiverse theory (stemming from the previous idea of different dimensions affecting the properties of different universes), some believe the mathematics behind string theory is meaningless and too broad to prove anything in our universe. All hope is not lost however, as another theory, Kaluza-Klein theory is often described as “an important precursor to string theory” (Wikipedia, 2005) given it hypotheses a 5th intangible dimension also too small to be perceived by the naked eye, but its usefulness is debatable.

Conclusion

It may be thousands of years until we can construct the technology to prove or disprove string theory but until then, we can only continue postulating and pondering on what knowledge we do have, which I’ll admit, isn’t very much. It’s a little like life on Mars - we want life to be on Mars because it would be convenient for us given it’s within a closer proximity to Earth than most planets and it almost feels like home, but it’s about time we step out of our comfort zone isn't it? We aren’t simply just looking for life on Mars in passing, we’re staying until we do find it, and find exactly what we’re looking for (life like that on Earth) which I believe is going to be a fruitless endeavor unless we stop being so attached one idea of what life can look like (on Mars). I feel similarly about string theory too; I believe if string theory does turn out to be true, it would be absolutely magnificent and one of the most profound discoveries ever made, however, if things continue as they are, I believe it’d be best to move on to something else or at least consider something other theories. Of course, it's a very convenient theory so it's no wonder it hasn’t died yet, but until we know something for sure, it’s rather problematic to see string theory as the be-all end-all - I mean, look at the state of medicine in the middle ages! Maybe we don’t need to quantize gravity, maybe we’ll never be able to, but until we understand truly what gravity is beyond spacetime, I believe it unwise to try and look at it (and thus string theory) so narrowly.

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Destiny Towa Wontcheu is a sixth former from Leeds with a strong obsession of anything astrobiology. Science aside, she enjoys reading webtoons, drawing her own comics, learning languages and horse riding.

References

• https://global.canon/en/technology/s_labo/light/001/11.html

• https://en.wikipedia.org/wiki/General_relativity

• https://en.wikipedia.org/wiki/Electromagnetism

• https://www.space.com/17661-theory-general-relativity.html

• https://www.space.com/32147-why-is-gravity-so-hard-to-understand.html

• https://www2.oberlin.edu/physics/dstyer/StrangeQM/history.html#:~:text=He%20announced%20this%20result%20on,one%20found%20it%20particularly%20significant.

• https://www.forbes.com/sites/startswithabang/2020/02/26/why-string-theory-is-both-a-dream-and-a-nightmare/?sh=6fcb51eb3b1d

• https://en.wikipedia.org/wiki/Calabi%E2%80%93Yau_manifold#:~:text=Essentially%2C%20Calabi%E2%80%93Yau%20manifolds%20are,have%20not%20yet%20been%20detected.

• https://en.m.wikipedia.org/wiki/File:Calabi-Yau.png

• https://www.physicsforums.com/threads/string-theory-vs-superstring-theory.142799/

• https://www.dummies.com/article/academics-the-arts/science/physics/string-theory-and-supersymmetry-178720/