Note: This page is copied from http://hepweb.rl.ac.uk/ppuk/PhysFAQ/open_questions.html
because the link is unstable.
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
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
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
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