According to theoretical physicist Michio Kaku, “in string theory, all particles are vibrations on a tiny rubber band; physics is the harmonies on the string; chemistry is the melodies we play on vibrating strings; the universe is a symphony of strings, and the ‘Mind of God’ is cosmic music resonating in 11 dimensional hyperspace” [1]. This post will not attempt to cover the physics behind the need for an 11-dimensional hyperspace. Instead, it will provide an insight into the beauty of a remarkable theory, which has the potential to be the theory that physics has strived for since its inception: the theory of everything.

The Edge of Knowledge

Currently, there are two core theories upon which the entirety of modern physics is built. The first is quantum mechanics, which allows us to understand the universe at the smallest scales, giving us an insight into the behaviour of atoms; and their even smaller constituents, electrons and quarks. The second is Einstein’s theory of general relativity, which allows us to understand the universe at the largest scales, giving us an insight into the behaviour of stars and galaxies. Both of these theories have been tested rigorously and the accuracy of the predictions made by these theories is incredibly precise. We have used these theories to make remarkable advancements in technology and the world has derived great benefit from them. However, quantum mechanics and general relativity cannot both be right. Our best understanding of the movement of the heavens and of the building blocks from which our universe is created directly contradict each other. 

This seems like it should create an urgent problem which needs to be solved immediately, but the contradiction has not caused many issues in most physics research, as each theory applies to very different extreme circumstances which do not overlap much. Quantum mechanics is used when we would like to study things that are small and light (like atoms) and general relativity is used when we study things that are large and heavy (like stars and galaxies). However, when we encounter things that have a mix of these properties, we get into trouble. When we’d like to study the centre of a black hole or the beginning of the universe, we encounter situations where immense masses have been crushed into a tiny scale (small and heavy).  Which theory do we use here? It would appear that we need a combination of the two. However, when we try to bring these two theories together, we get chaotic and nonsensical results. The laws of physics break down. Clearly, if we want to advance our understanding of the centre of black holes, we need something more.

Superstring Theory

Physicists have discovered that through the lens of superstring theory, the conflicts between quantum mechanics and general relativity are resolved. In superstring theory (or string theory for short) we no longer need to change the theory we use depending on the situation. One theory of the universe fits all situations. For the first time in human history, we have a theory that has the capacity to explain the entirety of the known universe. We have a candidate for the theory of everything. 

The Fabric of The Universe

Figure 1    [2]

String theory states that the elementary particles in our universe (the smallest known building blocks of our universe) are not actually indivisible little balls, but instead tiny loops called strings. In Figure 1, the apple is shown on increasingly smaller scales, starting off with the whole apple, then the atoms, the protons and neutrons, the elementary particles (electrons and quarks) and then finally, strings.

Musical Strings

Figure 2     [2]

In order to understand how the strings in string theory operate, it’s helpful to first think about more familiar strings, such as those on a violin. Each string on a violin can experience a huge number of different vibrational patterns called resonances. Examples of these vibrational patterns are shown in Figure 2. Each different vibrational pattern will create a different musical note. The resonance patterns consist of a number of peaks (top of the wave) and troughs (bottom of the wave) that are equally spaced across the length of the string. Each different resonance pattern will have a different number of waves and troughs that fit between the two ends of the string.

The strings in string theory operate in a similar way in that different resonance patterns have different numbers of peaks and troughs that can fit across a given length but now instead of the peaks and troughs fitting inside a straight line, they now fit inside a loop as shown in Figure 3. 

Figure 3     [2]

Mass Visualised Through Strings

Figure 4     [3]

Let us cast our minds back to the violin strings. The energy of a given vibrational pattern will depend on the amplitude (vertical distance between the peaks and troughs) and wavelength (the horizontal distance between 2 peaks) as shown in Figure 4. The energy of the vibrational pattern increases as the amplitude increases and the wavelength decreases. This makes sense intuitively, because we can see that more frantic vibrational patterns have higher energy and the calmer vibrational patterns have lower energy. In Figure 2, the vibrational patterns increase in energy as you move downwards, as their wavelengths decrease and they become more frantic in appearance. We can also imagine plucking a violin string more vigorously (supplying more energy) will cause the string to vibrate more frantically and plucking a string less vigorously (supplying less energy) will cause the string to vibrate more calmly.

From special relativity, we know that energy and mass are equivalent (E=mc2); which means that if the mass of an object increases, its energy increases, and vice versa. Therefore, the mass of an elementary particle is determined by the vibrational pattern of its internal string.  Whilst a heavier particle will have an internal string vibrating more energetically, a lighter particle will have an internal string vibrating less energetically. The forces of the universe are explained by detailed aspects of the string’s vibrational pattern.

So, we can see that in string theory, the properties of matter can be determined by investigating the vibrations of the fundamental strings that make up our universe. This is a sharply different perspective from pre-string theory physicists, who claimed that each of the elementary particles that make up our universe were “cut from a different fabric”. Each particle was viewed as being made up of different “stuff”, for example, electrons were made up of “stuff” with negative electric charge and neutrinos were made up of “stuff” with no electric charge. String theory breaks this notion and declares that all “stuff” is the same, tiny vibrating strings. Different elementary particles are strings vibrating at different notes, joining together in enormous numbers to form planets, stars and galaxies – creating a cosmic symphony.

Definitions

Further Reading

The Elegant Universe ~ Brian Greene

The Cosmic Landscape ~ Leonard Susskind

References

[1] Lubin, G., 2014. String Field Theory Genius Explains The Coming Breakthroughs That Will Change Life As We Know It. [online] Business Insider. Available at: <https://www.businessinsider.com/michio-kaku-talks-about-coming-breakthroughs-2014-3?r=US&IR=T> [Accessed 12 May 2022].

[2] GREENE, B. (1999). The elegant universe: superstrings, hidden dimensions, and the quest for the ultimate theory.

[3] Courses.lumenlearning.com. 2022. Waves and Wavelengths | Introduction to Psychology. [online] Available at: <https://courses.lumenlearning.com/atd-bhcc-intropsych/chapter/waves-and-wavelengths/> [Accessed 12 May 2022].