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Published on August 13th, 2013 | by Emily Corbett


A Braney Summer

By Mark Bugden, Australian National University

This student took part in the 2012/13 AMSI Vacation Research Scholarship program. For more information on this years program please click here

Physics. It describes what we know about the universe on a fundamental level. Physics describes the motion of planets, the production of Nuclear power, the efficiency of engines, the reflection and refraction of light and countless other everyday phenomena.


The two pillars of modern physics are Quantum Mechanics and General Relativity. Quantum Mechanics describes the very small – it tells us how subatomic particles move and interact. On the other hand, Einstein’s General Theory of Relativity describes the massive – it describes the curvature of space-time around heavy objects like black holes and neutron stars. Together, these two theories completely describe the known universe.



There is a slight problem. Quantum Mechanics and General Relativity are incompatible theories! General Relativity is what physicist’s call a ‘Classical Theory’, as opposed to a ‘Quantum Theory’. In order to unify Quantum Mechanics with General Relativity, we would need to turn General Relativity into a Quantum Theory – In physics terms we would be quantizing gravity. This is where we run into a problem. Whenever we try to quantize gravity using the standard method, quantities become infinite and everything starts to break. Clearly, another approach is needed.

Enter String Theory. String Theory is a branch of theoretical physics which attempts to unify Quantum Mechanics with General Relativity by modelling subatomic particles inside atoms as tiny vibrating strings. A standard analogy to describe string theory is musical, referring to the strings on a guitar. On a guitar string, different modes of vibration give rise to different musical notes. By analogy, the different vibrational modes of the string give rise to different particles, with different mass and electric charge. Poetically, String Theory describes the  ‘Cosmic Symphony’ of the Universe.

My project over the summer was based around some of the mathematics relating to String Theory. Specifically, the project focused on objects in String Theory called D-Branes. To understand what a D-Brane is, imagine that I hold one end of a rope in my hand, and tie the other end to something solid, like a tree.


Although the rope moves in the middle, at the endpoints to rope is fixed. One end has to stay in my hands (provided I hold on tight enough), and the other end has to stay tied to the tree. Now to mix things up a bit, lets tie the rope to a ring, and put that ring around apole so that the endpoint of the string can slide up and down the pole.

Now the endpoint of the string is free to move up and down along the pole – it’s not just fixed in the one position.

A string stretched between two parallel two-dimensional D-Branes.

In String Theory, the endpoints of the strings are confined onto certain objects we call D-Branes. The ends of the string can move around on the D-Branes, but can’t break off it.

A zero-dimensional D-Brane would be a point, which the endpoint of the string must stay on (like the tree example). A one-dimensional D-Brane would be a line to which the endpoint of the string is confined (like the example with the ring and the pole), and so on.

D-Branes are interesting objects with a variety of cool properties. My project was focused on investigating some of these.



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