5.2 Entropy

summary

What has string theory taught us about black holes?

In the 1970s, Bekenstein and Hawking discovered the existence of microstates within black holes. That is to say, a black hole could store vast amounts of information, or entropy, proportional to the area of its event horizon. But they could not explain where these microstates came from.
This led to an amazing principle known as holography. Normally the entropy of an object would scale with volume, but in this case it was dependent on area. Somehow the degrees of freedom behaved as though they were projected onto a lower dimension.
In 1996, Cumrun Vafa and Andrew Strominger discovered that a black hole could be expressed as wrapped branes in the extra internal dimensions of string theory. The entropy that emerged from black holes in this context perfectly matched what Bekenstein and Hawking had discovered.

Is our universe unique?

The answer is a resolute “no.” By this, we mean that the number of allowed solutions that could represent our universe is far from unique.
The “string theory landscape,” as it is known, which refers to the number of theoretically allowed configurations that could represent this universe, is incredibly large—somewhere between $10^{10}$ and $10^{100}$.
Nevertheless, we have evidence that we live in an exceptional corner of the landscape with exceptional forces and properties needed in order to explain all of the features we have discovered.

We do not yet have the final formulation of string theory.

String theory is leading to a revolutionary revision of many fundamental and long-held principles of physics.
There are many hints for a new principle of relativity where duality frames play the role of reference frames.
Although string theory has changed our understanding of many things we believed to be fundamental, this is not a first-time occurrence in science.
By the time the dust settles, modern physics will look very different.