[Physics FAQ] -
[Copyright]
updated 4-Jun-1997 by JCB
updated 11-JAN-1997 by PEG
updated 01-JUN-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
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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 which 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?
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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?
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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?
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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?
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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
which 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, which 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.