A panel of US physicists and astronomers has formulated a list of eleven
fundamental questions about the nature of the universe which may be
answered in the 21st century. The questions are in the first report from
the committee on the physics of the universe set up by the National
Academy of Sciences on January 18, 2001.
The eleven questions are:
1. What is dark matter?
2. What are the masses of the neutrinos, and how have they shaped the
evolution of the universe?
3. Are there additional spacetime dimensions?
4. What is the nature of the dark energy?
5. Are protons unstable?
6. How did the Universe begin?
7. Did Einstein have the last word on gravity?
8. How do cosmic accelerators work and what are they accelerating?
9. Are there new states of matter at exceedingly high density and
temperature?
10. Is a new theory of matter and light needed at the highest energies?
11. How were the elements from iron and uranium made?
I shall try to comment upon these questions:
1. Dark matter is certain to exist. The most prominent evidence of its
existence is as follows. Relict radiation discovered 35 years ago is a
face of the universe as far back as 10 billions of years ago. This face
seems to be uniform and isotropic to within 0.001 per cent. Apparent
universe was uniform within such a huge exactness! We can see, however,
planets, stars, galaxies, clusters all around us... Therefore, there is
some dark matter, "uneven" and "rough". Now, there is one point to be
clarified: what is this dark universe made of? The question could be
answered in the 21st century.
2. If neutrinos having finite masses were discovered, they could be
responsible for dark matter. Such neutrinos should have masses at least
5-10 eV or more. Searching for these neutrinos is quite a possible task
for the decade to come.
3. This is a much more fundamental question. Is there the fifth dimension
in nature, the sixth, or the seventh? The idea draws upon Kaluza and
Klein's paper appeared in the 1920s. The paper was mentioned by Einstein
as a new word in physics. It was suggested to introduce additional
spacetime dimensions in order to make unification of physical fields and
gravitation possible. These additional spacetime dimensions are not
available now, and this problem is unlikely to be solved in the century to
come.
4. The difference between dark matter and dark energy is in the proposal
that the former should remind of "light" matter; that is, it has to be
"lumpy", to "crawl about", to "attract", the latter being rather a pure
massless field alike the illustrious lambda-term in the Einstein's
equation, vacuum energy, etc. Such a massless field is distributed
absolutely uniformly along the universe. Dark energy possesses properties
of antigravitation and makes the universe extend slightly. It is most
likely to exist and can be confirmed before long.
5. Are protons stable or unstable? Experiments with great amount of water
in Japan (Kamiokande) have shown protons to live for a long time. How
long?
That is the really question for 21 century.
6. How did the Universe begin? The deeper one attacks the problem,
the longer time it will take him to solve it. In a sense, we know how it
began, to a certain extent. If one wants an absolute precise answer, he
will never receive it. The question seems to imply a part of "the birth"
connected with post-Plankian time, i.e. some time later the situation when
there had been no time at all. One may hope to succeed, to a slight
extent, in solving this problem and success would be due to detection of
gravitational waves because the Universe is opaque to all other fields and
ways of observation. I think that physicists are going to be greatly
disappointed in this most fundamental area of knowledge. In fact, the
whole body of physics is based upon experimental data which may be
repeated and reproduced. As for the Universe, we don't have but one
object for
our investigation. Alas! And we could "give birth" to the other,
artificial one in, say, 500 years. During this period of time science is
likely to put up with a kind of scholasticism and medieval ignorance. By
the way, such stagnation periods were known to happen already in physics
(there was "a blank" of 2000 years from Aristotle to Halileo).
7. It is classical rather than quantum mechanics that is most likely meant
here. In this case the answer is positive. As a matter of fact, this
question is not of great significance now. A theory of quantum gravity may
not appear soon enough, it is not a problem of the 21st century.
8. Neither this question is of general interest. It may well be solved in
one or two decades to come. One can hardly anticipate any overturns along
this line.
9. Are there new states of matter at high temperatures? Of course, there
are. At moderately high temperatures, however, discovering an extremely
new state would be a great surprise.
10. The answer is: such new theory is actually needed.
11. The answer is more or less known: the heavy elements were made
during
the explosions of supernovae, gravitational energy of collapsing stellar
cores being expended on the formation of heavier atomic nuclei. Though
details of the process are not quite clear for the moment, the problem is
not of a fundamental character and might well be solved in the 21st
century.
Completing the review of the questionnaire I would like to point out that
the questions are not homogeneous in structure. They seem to have been
influenced by a kind of "democratic voting". That is why the list, besides
having "eternal" questions, contains "routine" ones. Unfortunately, the
most
important questions concerning detection of slightly evolved civilizations
near other stars have been omitted. It is the area where a true overturn
may happen. From the other hand, it would be the most naive to wait for
the discovery of highly developed civilizations in the 21st century. If
those had existed somewhere, they would have been discovered so far.
Vladimir Lipunov
