The Trouble with String Theory

Whether you’ve looked into it or not, it’s probably true to say that you have at least heard of string theory.  It’s strange, a bit crazy, and challenges our intuitions on the most basic of levels – just like a good physical theory should do.  It comprises a number of slightly altered versions — collectively known as ‘superstring theory’ — but all ultimately deal with the same fundamental principles.

The need for a theory such as this is down to the complete and utter incompatibility of our two current best theories that describe the Universe: Einstein’s revolutionary paper on general relativity and the mind-bending theory of quantum mechanics.  Both do fantastically well at describing and predicting their respective scales (general relativity for things we can see; quantum mechanics for the sub-atomic world), but both are useless when combined together to attempt to describe everything.  This, of course, means that both theories are incomplete, and to some extent, wrong.  What string theory, and other concepts like it, try to do is explain everything in just one single overriding theory.

String theory

String theory is based on the idea that every particle in the Universe is composed of unimaginably tiny vibrating strands of energy — or strings — and the frequency of which determine the type of particles that exist.  Much like piano keys playing certain allowed notes, these strings can only vibrate at certain ‘harmonics’, with each harmonic corresponding to a different type of elementary particle.

Now this is all well and good – it is certainly an elegant and compelling theory.  However, the maths doesn’t work.  That is, unless you introduce 10 dimensions into the mix.  Once you use 10 (yes, TEN) dimensions, the equations work perfectly and generate all the correct values for things like the mass of an electron (that, without the dimensional additions, came out wrong before).

But this is the inherent problem with string theory: the introduction of a further 6 dimensions more than we are used to in our every day lives.  How can there be more than up/down, forwards/backwards, left/right, plus one dimension of time?  These 3 spatial dimensions encompass everything around us, to suggest there can be a further 6 is tantamount to hallucinogenic drug use and requires a leap of faith more associated with religious types than scientists!

There is also the slightly convenient snag in string theory, so to speak – it can’t be disproved.  The cardinal ingredients — the minute strings of vibrating energy — are so small that they are billions upon billions of times smaller than protons, neutrons and electrons (on a scale known as the Planck length, or 10−35 m), and are likely to be veiled from view forever.  The theory is also unable (at its current state) to make any testable predictions about the Universe.

So an elegant theory, yes; but one with enormous hurdles to overcome if it is to be taken seriously. Introducing 6 additional dimensions and claiming that resonating strings (that can never be seen) populate every particle in the Universe without some sort of evidence other than complex mathematics is asking a lot from even the most open of scientific minds, but we must remember that Copernicus and Einstein started down roads quite similar to this one…

The ant on the telephone pole

The analogy often used to entice people into the extra-dimensional viewpoint of string theory is one of ants on telephone cables.  Imagine you are in a third-storey flat, looking out of a window at a distant telephone pole.  From your frame of reference you see the pole and the telephone cables running out into the various buildings, and if you didn’t know any better you’d say that the pole appears as a flat brown object and the cables as thin black lines, both obviously appearing from your distance, respectively, as 2-D and 1-D objects (you can see the movement up/down and left/right is possible on the pole, but you can’t infer that forwards/back is an option.  And the cable is so thin from your vantage point that you can only assume that left/right movement is possible.)  Now zoom in and imagine yourself as an ant climbing the telephone pole and scurrying around the cables; from this frame of reference the cables appear anything but 1-D, they are very much round and the ant can move along in all 3 dimensions.

The point of this analogy, then, is to show that if something is small enough then it can easily go unnoticed.  Moreover, something can appear to be very different from how it really is — simply from not being observed in enough detail — and such might be the case with the proclamations of string theory; dimensions so small that we can never notice them.  Whereas our 4-dimensional universe is suggested to be ‘flat’, these other dimensions are proposed to be curved and form tiny loops.

This is a very hard concept to grasp, even for just one extra spatial dimension.  But to visualise another 6 is likely to make one’s brain explode.  This again addresses an intrinsic flaw, if you like, with the human species.  I think it might have been Richard Dawkins who quipped — possibly in reference to another author —  ‘We evolved to avoid lions on the Great Plains of Africa; we didn’t evolve to do quantum physics’.  Our deep-rooted common sense is perhaps the greatest limitation of our understanding of the Universe; our brains aren’t built to comprehend such distinctly counter-intuitive and bizarre concepts.

“We evolved to avoid lions on the Great Plains of Africa; we didn’t evolve to do quantum physics”

To simply dismiss the wacky claims of string theory is to be foolish and somewhat naive; it is very possible that there could be more ‘space’ to the Universe other than up/down, left/right and forwards/backwards, we just find it excruciatingly difficult to visualise.  It wasn’t so long ago that Ernest Rutherford proved the existence of atomic nuclei, and Arthur Compton the electron.  More recently the discovery of quarks, dark matter and dark energy.  All seemed silly, strange and unlikely before they were proved, and yet how strange or silly does the idea of an electron seem in the twenty-first century?  Arthur C. Clarke wrote that ‘Any sufficiently advanced technology is indistinguishable from magic’, and this same line of reasoning can be applied to physical theories, in the sense that concepts so strange do seem indistinguishable from make-believe at first glance.

The trouble with string theory, then, is the task of accepting these extra dimensions.  Thought experiments often help with visualising scientific ideas, but I can’t help but feel the one about the ant doesn’t really suffice.  After all, whether or not we are too far away to safely say a distant object is 3-dimensional, it is still just 3-dimensional.  The cable doesn’t behave any differently than anything in the overlooking flat – it still operates in the same 3 dimensions as we do.  Yes, the ant has more of a claim to this realisation than us, but all that it proves is that there is greater detail the smaller something gets (and the associated observational blindness one has when one is much larger!).

Flatland

Perhaps more effective would be Edwin A. Abbott’s 1884 novel Flatland.  In the story the world is 2-D, existing on what is akin to a flat piece of paper.  Everything on this 2-dimensional world can move either forwards/back and/or side-to-side, but cannot move up/down (much like a graph drawn on some A4 paper displaying the X- and Y-axes; the same sheet of paper cannot literally show the Z-axis which effectively would rise right out of the paper and into the air).  Now imagine that a 3-D object — such as a ball — somehow descended upon Flatland and passed completely through it.  From the frame of reference of someone confined within the limitations of this 2-D world they would notice a small line materialise out of nowhere (as the bottom of the ball began to penetrate through) that gets progressively longer (as the ball nears halfway) and then progressively smaller again (as the other half of the ball goes through) before finally disappearing.

This would be a very mystifying experience indeed for a resident of Flatland – their comprehension of space is limited to the 2 dimensions they exist in.  To suggest there is a third would be too much for them to understand, and even by demonstrating a 3-dimensional object results in a 2-dimensional translation.  The only way for a ‘flatlander’ to grasp 3 dimensions would be to lift them out of 2 dimensions and into 3, and showing them the ball passing through Flatland.  Analogous to this are humans not being able to appreciate further spatial dimensions whilst existing in the 3 we can comprehend – so programmed are we to see and process our world in 3 dimensions that contemplating any more defies every ounce of common sense that we possess.

A 'calabi-Yau' shape. A proposed arrangement of the extra dimensions that warp the strings into their allowed harmonic frequencies. Image credit: Brian Greene
A ‘Calabi-Yau’ shape. A proposed arrangement of the extra dimensions that warp the strings into their allowed harmonic frequencies. Image credit: Brian Greene

If we have learned one thing from dark matter, dark energy, and quantum mechanics it is not to discount something based solely on how odd and outlandish it is.  This isn’t to say that the necessary extra dimensions of string theory are real, far from it, but it is this willingness to think outside the box that has spawned many of the greatest scientific achievements.  Had the ancient Greeks not argued that the Earth could be spherical, Eratosthenes might never have made his calculations to estimate its circumference and Magellan might never have proved it by circumnavigating the planet.  Had Copernicus not announced the heretical notion that the Sun did not go round the Earth — but in fact was the exact opposite — Galileo and Newton might never have made their respective breakthroughs in astronomy and physics.  Maxwell’s work on electromagnetic fields; Einstein’s special and general theories of relativity; the spooky reality-defying quantum theory as developed by Heisenberg, Schrödinger and Bohr (amongst many others) – all required brave steps into the unknown and were initially met with scepticism and, quite often, ridicule from others that understandably felt threatened by the strangeness and scope of that which at first appeared so extraordinary.

by Chris Phoenix Clarke

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3 thoughts on “The Trouble with String Theory

  1. “We evolved to avoid tigers on the Great Plains of Africa; we didn’t evolve to do quantum physics”

    There are lions in Africa, but no tigers. As far as I am aware there have never been tigers on the plains of Africa. Are you sure he said that?

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