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The Physics of Branes

Is an elementary particle really: (i) a particle? (ii) elementary?

Quantum mechanics tells us that particles are “fuzzy”:

Moreover, “elementary” and “composite” are just words..

In Quantum Field Theory this is true in a very precise sense.

Another distinction that is less basic than one would think:

In string theory, the fundamental string has a typical length scale.

This is why string theory is an approach to particle physics.

Extra dimensions:

Consider a string wrapped around a compact spatial dimension:

If strings can look like particles, can membranes look like strings?

In string theory, the fundamental string is really a membrane.

This means that string theory is not just a theory of strings!

At strong coupling, string theory turns into “M-theory”.

Consider the following membrane configuration in M-theory:

Let the vertical part be wrapped on a small compact dimension.

This observation turns out to be very profound and powerful.

Such membranes are called “D2-branes”.

A simple generalisation of D2-branes: “Dp-branes”.

We need to use our branes!

Some application of brane physics:

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Conclusions:

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