The Trouble with Physics

Introduction

The theory also has to be confirmable: it must be possible to verify a new prediction that only this theory makes.

[...] theories [...] in the last thirty years fall into two categories. Some were falsifiable, and were falsified. The rest are untested—either because they make no clean predictions or because the predictions they do make are not testable with current technology.

I. The Unfinished Revolution

1. The Five Great Problems in Theoretical Physics

Problem 1: Combine general relativity and quantum theory into a single theory that claim to be the complete theory of nature.

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Philosophers call this view realism. It can be summarized by saying that the real world out there (or RWOT, as my first philosophy teacher used to put it) must exist independently of us. It follows that the terms by which science describes reality cannot involve in any essential way what we choose to measure or not to measure.

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Problem 2: Resolve the problems in the foundations of quantum mechanics, either by making sense of the theory as it stands or by inventing a new theory that does make sense.

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Problem 3: Determine whether or not the various particles and forces can be unified in a theory that explains them all as manifestations of a single, fundamental entity.

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Problem 4: Explain how the values of the free constants in the standard model of particle physics are chosen in nature.

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Problem 5: Explain dark matter and dark energy. Or, if they don't exist, determine how and why gravity is modified on large scales. More generally, explain why the constants of the standard model of cosmology, including the dark energy, have the values they do.

3. The World As Geometry

So Einstein succeeded in unifying all kinds of motion. Uniform motion is indistinguishable fom rest. And acceleration is no different from being at rest but with a gravitational field turned on.

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For Newton, space and time constituted an absolute background. [...] We have a name for theories of physics that rely on such an absolute, fixed framework: We call them background-dependent theories.

Einstein's general theory of relativity is completely different. There is no fixed background. The geometry of space and time changes and evolves [...]

6. Quantum Gravity: The Fork in the Road

This doesn't mean that there is some other fixed geometry that characterizes space—that space is like a sphere or a saddle, instead of a plane. The point is that geometry can be anything at all, because it evolves in time, responding to matter and force.

II. A Brief History of String Theory

8. The First Superstring Revolution

The events of 1984 did not follow Kuhn's structure.

9. Revolution Number Two

There is a particle associated with every wave, including a particle associated with the sound wave travelling through the metal. It is called a phonon .

A phonon is not an elementary particle. It is certainly not one of the particles that make up the metal, for it exists only by virtue of the collective motion of the huge number of particles that do make up the metal. But a phonon is a particle just the same. It has all the properties of a particle. It has mass, it has momentum, it carries energy. It behaves precisely the way quantum mechanics says a particle should behave. We say that a phonon is an emergent particle .

IV. Learning From Experience

17. What Is Science?

We happen to live in a world hospitable to our understanding, and it has always been the case.

[...] we are masters at drawing conclusions from incomplete information.

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We agree to argue rationally, and in good faith, from publicly available evidence, to whatever degree of shared conclusions are warranted.

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The scientific community is thus both an ethical and an imaginative community.