Updated June 1997 by JCB.
Updated Jan. 1997 by PEG.
Updated 1993 by SIC.
Original by John Baez.

Open Questions in Physics

While for the most part a FAQ covers the answers to frequently asked questions whose answers are known, in physics there are also plenty of simple and interesting questions whose answers are not known. Before you set about answering these questions on your own, it's worth noting that while nobody knows what the answers are, there has been at least a little, and sometimes a great deal, of work already done on these subjects. People have said a lot of very intelligent things about many of these questions. So do plenty of research and ask around before you try to cook up a theory that'll answer one of these and win you the Nobel prize! You can expect to really know physics inside and out before you make any progress on these.

The following partial list of "open" questions is divided into three groups; Condensed Matter and Non-linear Dynamics, Cosmology and Astrophysics, and Particle and Quantum Physics. However, given the implications of particle physics and non-linear dynamics on cosmology, and other connections between the groups, the division is somewhat artificial, and, consequently, the categorization is somewhat arbitrary.

There are many other interesting and fundamental questions in other fields and many more in these fields than those listed here. Their omission is not a judgement about importance, but merely a decision about the scope of this article.

Condensed Matter and Non-linear Dynamics

What causes sonoluminescence? Sonoluminescence is the generation of small light bursts in liquids caused by sound. Bubbles form in the liquid at low pressure points of the sound wave, then collapse again as a high pressure wave passes. At the point of collapse a small flash of light is produced. The exact cause has been the subject of intense speculation and research.
How can turbulence be understood and its effects calculated? One of the oldest problems of them all.
What causes high temperature superconductivty? Is it possible to make a material that is a superconductor at room temperature? Superconductivity at very low temperatures has been understood since 1957 in terms of the BCS theory, but high temperature superconductors discovered in 1986 are still unexplained.

Cosmology and Astrophysics

What happened at or before the Big Bang? Was there really an initial singularity? Of course, this question might not make sense, but it might. Does the history of the Universe go back in time forever, or only a finite amount?
Will the future of the universe go on forever or not? Will there be a "big crunch" in the future? Is the Universe infinite in spatial extent?
Why is there an arrow of time; that is, why is the future so much different from the past? If the universe is finite and it recollapses, will the thermodynamic arrow of time be reversed during the collapse towards the big crunch?
Is spacetime really four-dimensional? If so, why--or is that just a silly question? Or is spacetime not really a manifold at all if examined on a short enough distance scale?
Do black holes really exist? (It sure seems like it.) Do they really radiate energy and evaporate the way Hawking predicts? If so, what happens when, after a finite amount of time, they radiate completely away? What's left? Do black holes really violate all conservation laws except conservation of energy, momentum, angular momentum and electric charge? What happens to the information contained in an object that falls into a black hole? Is it lost when the black hole evaporates? Does this require a modification of quantum mechanics?
Is the Cosmic Censorship Hypothesis true? Roughly, for generic collapsing isolated gravitational systems are the singularities that might develop guaranteed to be hidden beyond a smooth event horizon? If Cosmic Censorship fails, what are these naked singularities like? That is, what weird physical consequences would they have?
Why are the galaxies distributed in clumps and filaments? Is most of the matter in the universe baryonic? Is this a matter to be resolved by new physics?
Why does it seem like the gravitational mass of galaxies exceeds the mass of all the stuff we can see, even taking into account our best bets about invisible stuff like brown dwarfs, "Jupiters", and so on? Is there some missing "Dark Matter"? If so, is it baryonic, neutrinos, or something more exotic? If not, is there some problem with our understanding of gravity, or what?
What is the origin of the Cosmic Gamma Ray bursts? There are literally hundreds of theories for these mysterious bursts which are thought to originate from some cataclysmic astronomical events.
What is the origin and nature of the highest energy Cosmic rays? The record is an event detected by the Fly's eye detector in the US that recorded a shower from a cosmic ray of about 300 EeV. A similar event was detected by the Japanese scintillation array AGASA. When first detected these events were far higher than what had been expected. So far only a few very speculative theories have been proposed.

Particle and Quantum Physics

Why are the laws of physics not symmetrical between left and right, future and past, and between matter and antimatter? I.e., what is the mechanism of CP violation, and what is the origin of parity violation in Weak interactions? Are there right-handed Weak currents too weak to have been detected so far? If so, what broke the symmetry? Is CP violation explicable entirely within the Standard Model, or is some new force or mechanism required?
Why are the strengths of the fundamental forces (electromagnetism, weak and strong forces, and gravity) what they are? For example, why is the fine structure constant, that measures the strength of electromagnetism, about 1/137.036? Where did this dimensionless constant of nature come from? Do the forces really become Grand Unified at sufficiently high energy?
Why are there 3 generations of leptons and quarks? Why are their mass ratios what they are? For example, the muon is a particle almost exactly like the electron except about 207 times heavier. Why does it exist and why precisely that much heavier? Do the quarks or leptons have any substructure?
Is there a consistent and acceptable relativistic quantum field theory describing interacting (not free) fields in four spacetime dimensions? For example, is the Standard Model mathematically consistent? How about Quantum Electrodynamics? Even the classical electrodynamics of point particles does not yet have a satisfactory mathematically rigorous formulation.
Is QCD a true description of quark dynamics? Is it possible to calculate masses of hadrons (such as the proton, neutron, pion, etc.) correctly from the Standard Model? Does QCD predict a quark/gluon deconfinement phase transition at high temperature? What is the nature of the transition? Does this really happen in Nature?
Why is there more matter than antimatter, at least around here? Is there really more matter than antimatter throughout the universe?
What is meant by a "measurement" in quantum mechanics? Does "wavefunction collapse" actually happen as a physical process? If so, how, and under what conditions? If not, what happens instead?
What are the gravitational effects, if any, of the immense (possibly infinite) vacuum energy density seemingly predicted by quantum field theory? Is it really that huge? If so, why doesn't it act like an enormous cosmological constant?
Why doesn't the flux of solar neutrinos agree with predictions? Is the disagreement really significant? If so, is the discrepancy in models of the sun, theories of nuclear physics, or theories of neutrinos? Are neutrinos really massless?

The Big Question (TM)

This last question sits on the fence between the last two categories above:

How do you merge Quantum Mechanics and General Relativity to create a quantum theory of gravity? Is Einstein's theory of gravity (classical GR) also correct in the microscopic limit, or are there modifications possible/required which coincide in the observed limit(s)? Is gravity really curvature, or what else--and why does it then look like curvature? An answer to this question will necessarily rely upon, and at the same time likely be a large part of, the answers to many of the other questions above.