By Celine Teh

You’re inspecting an apple and you wonder what it is made of. If you zoom into it, you should see a collection of **atoms**. Magnifying further, an atom is made of **subatomic particles **(which are protons, neutrons, and electrons), and beyond that, protons and neutrons are made of quarks, a type of **elementary particles**, which as far as we know is indivisible.

So it goes like this: Apple > Atom > Subatomic particles > Elementary particles > ?

Atoms may be the smallest unit of matter, but not the smallest unit of everything. If not, then what could it be?

**Vibrating Strings **

** **What if...elementary particles are vibrating strings? This is what the **superstring theory** proposes. Just like the string of a violin produces a variety of notes when vibrating differently, strings vibrate in different frequencies -- hence looking and acting like various types of elementary particles. For example, a group of strings that vibrates in one way makes up photons, while another group of strings that vibrates in another acts like electrons.

There may be open strings (like a thread) or closed strings (like a rubber band), according to different theories. Currently, there are 5 types of string theories (I, IIA, IIB, HO, HE), with the M-theory (also a string theory) that lies in the prospect of unifying them.

*For example, electrons and quarks are open-strings, but they vibrate at different frequencies, resulting in two different elementary particles. *

**Extra dimensions**

We live in a world of four-dimensionals, which are three dimensions of space (length, width, height) and one dimension of time. **The theory of relativity** proposed by Albert Einstein works well in the four-dimensional world, but string theory requires **extra dimensions** to make sense. Superstring theory consists of 10 dimensions, which are the 3 dimensions we live in, 6 extra dimensions, and 1 dimension of time. The geometry of each dimension affects how the strings vibrate.

You might be wondering, where are all those extra dimensions?

Good question asked. To answer this, you need to understand what does the first 3 dimensions look like. 1D looks like dots and straight lines (only length), 2D represents shapes and cartoons on paper (length and width), while 3D appears as cubes, cylinders, and spheres (length, width, and height).

Extra dimensions are compacted in tiny tiny spaces where we can’t see, even though they could be around us all the time. Imagine looking at an ant on the ground, all you see is just a little dot moving around; now using a magnifying glass to enlarge it, you suddenly see its head, body segments, and tentacles, which all appear in 3D. Similarly, extra dimensions are folded and curled up so small until we can’t see them, not even to detect them with the most advanced and sensitive searches.

*A rolled paper may look like a mere line (one dimensional), but it turns into a rectangular paper (two dimensional) when you roll it out. Dimensions are compacted similarly, it is ‘rolled’ so that it looks like a lower dimension. *

**The Theory of Everything**

The superstring theory aims to integrate all known forces (including electromagnetic force, gravitational force, weak force, and strong force) together and unify the theory of relativity and quantum physics. For decades, the theory of relativity deals with the macroscopic space-time world, while **quantum physics** describes at an atomic scale. They are rarely used together, but when they do, the equation falls apart.

**Gravity** is one of the most controversial topics among theoretical physicists. The theory of relativity believes that gravity exists as a smooth fabric that warps and curves when the matter is present, while quantum physics states that a chaotic mess of gravitons makes up gravity. When superstring theory enters the stage, it suggests that gravitons exist as vibrating strings, and lots of strings spread out to create enough space for gravitons to interact, hence unifying two theories.

**Lack of Evidence**

So far the superstring theory sounds great, yet its backlash is lack of experimental evidence. It is because those strings are stupendously small, each of **Planck’s length** (about 10−33 cm), which is 100 000 smaller than an atom and is believed to be the shortest possible length in the universe. Additionally, superstring theory is still incomplete and has several variations, so it is difficult to make tests.

However, there are ways to prove without designing a test. In 2012, the only possible way was to smash particles together in the CERN machine, or the Large Hadron Collider, which is very expensive to operate. Now, physicists can search for supersymmetric particles, or partner particles for each elementary particle, since superstring theory depends on it. Another way is to measure the geometry of the extra dimensions, calculate the property of the particle produced by the vibrating string, and compare the values to the known ones.

*You might have seen this in the Spiderman animation. Located on the border of Switzerland and France, this is the Large Hadron Collider that accelerates particles almost to the speed of light and smashes them together to get the elementary particles and other exciting unknown bits. *

Superstring theory is exciting, but the unknown ahead of us stretches beyond our perception. Humans have made tremendous progress in those 30 years, but this is just the beginning.

**References**

Greene, B. (2021, May 27). string theory. Encyclopedia Britannica. __https://www.britannica.com/science/string-theory__

Brockmeier, E. K. (1970, January 1). Making sense of string theory. Penn Today. Retrieved September 20, 2021, from __https://penntoday.upenn.edu/news/making-sense-string-theory__.

Sutter, P. (2020, March 20). Putting string theory to the test. Space.com. Retrieved September 20, 2021, from https://www.space.com/putting-string-theory-to-test.html.

Science and Cocktails. (2019, August 29). String Theory and the End of Space and Time with Robbert Dijkgraaf [Video]. Youtube. __https://www.youtube.com/watch?v=0T--WC4D1C0__

TED [TED-ed]. (2013, August 11). String theory - Brian Greene [Video]. Youtube. https://www.youtube.com/watch?v=kF4ju6j6aLE