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Paul FendleyIn physics, understanding requires both experiments and a quantitative theoretical description. Neil Fendley

In a 2006 book that garnered much press for its silly attacks on string theory, author and physicist Lee Smolin provides a list of “The Five Great Problems in Theoretical Physics.” There are many offensive things about this list, starting with the use of the definite article in the title, which implies that people not working on these problems (the majority of theoretical physicists) are working on less-than-great problems. But to me the most offensive thing is that only one of the five problems, I believe, could eventually be resolved by experiment.

Most physicists don’t consider a phenomenon to be understood until there are both repeatable experiments displaying it and a quantitative theoretical description. The only physics problems without both aspects are those unrelated to experiment. We have a name for such problems: mathematics.

The book’s list, however, did inspire me to come up with my own list. Here are my “Five Great Problems in Theoretical Physics,” without the definite article:

1. Explain the dark matter and energy in the universe

This problem is the one of Smolin’s five that stands a shot at being resolved in my lifetime. It’s actually two related problems. Astronomers have observed that the gravity we theoretically understand does not describe how galaxies rotate — unless there’s a lot of matter out there that we don’t see. This is known as dark matter. Similarly, at staggeringly long-distance scales, astronomers observe that light is overall not bent, even though gravity does indeed bend light. The only way this is consistent with Newton and Einstein is for the universe to possess a precise energy density. Dark energy is our name for this extra energy. For both dark matter and energy, we need to figure out what this stuff is or we need to figure out how to extend the work of Newton and Einstein.

2. Explain high-temperature super-conductivity

Even ignoring possible real-world applications, superconductivity is one of the coolest (literally and figuratively) phenomena in quantum physics. It’s hard not to be impressed with experiments that let current flow for years without a battery. We understand theoretically what characterizes a superconductor: Electrons of opposite momentum form an unusual quantum state of zero energy called a Cooper pair. But this long happened only at excruciatingly low temperatures, hard to achieve outside a lab. Thus the physics version of mass hysteria occurred in the late 1980s when materials that superconduct at high temperatures were found. “High” here is still pretty cold but is above the temperature of liquid nitrogen, which means it’s easy to get that cold. For the high-temperature superconductors, we theorists are embarrassed to admit that after more than 20 years, we still aren’t sure how these Cooper pairs form.

3. Explain the “Higgs” phenomenon in the standard model

Billions (in any currency unit) have been spent to build the Large Hadron Collider, a gigantic accelerator in Switzerland and France. Explanations for why usually start with “we’re trying to understand what gives particles mass.” According to the successful standard model describing quantum physics at the shortest observable distances, at even shorter distances (or higher energies) there’s a symmetry that requires particles to be effectively massless. Since we know particles do have mass, something must break this symmetry. The simplest candidate is a particle called the Higgs boson, which may or may not exist. The LHC is built to go to short enough distances to find it, or find whatever else breaks this symmetry. Theorists are hoping for the latter — it would be much more interesting.

4. Figure out how to make a quantum computer

There are many simple-to-state problems unsolvable by even the fastest computers. For example, encryption on the Web relies on computers’ inability to factor very large numbers into prime numbers. A radical proposal to solve this and other computationally intractable problems is to build a quantum computer, where each bit obeys the laws of quantum mechanics. A quantum bit is very hard to build and manipulate, so current quantum computers have only a handful of bits. Building a suitable quantum computer sounds like an experimental problem. But we’re currently so far from our goals that brilliant new ideas from theorists and experimentalists will be required to further advance this field.

5. Say nice things about your own work without slamming others

Your list will probably be different from mine. Diverse priorities in any science are a strength — the study of one problem often helps solve another. The theory of the Higgs phenomenon, for instance, was first understood in a completely different context: superconductivity.

Paul Fendley is a theoretical physicist at the University of Virginia in Charlottesville.

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