How does matter terminate? That is, at the microscopic level, how
does nature make the transition from a densely packed material
surface (the skin of an apple, say) to the nothingness that lies
above? This issue is especially dramatic for collapsed stars, where
the matter density gradient marking the star-to-vacuum transition
can be as great as 1026 g/cm4
(grams per cubic centimeter per centimeter of displacement).
A new model, proposed by
physicists at Los Alamos and Argonne National Labs, claims that the
prevailing theory of what happens at quark-star surfaces is wrong.
These quark stars are characterized by interiors which consist of
quark matter from the center all the way to the surface. For quark
matter to exist in the low-pressure environment near the surface,
matter containing nearly equal numbers of up, down and strange
quarks must be preferred over neutrons and protons.
speculated about this possibility (often called the Strange Quark
Matter Hypothesis) since the early 1980's. A star made in this way,
a quark star, is thought to be the densest possible type of matter.
Any denser than this, and the star must become a black hole.
In the ordinary kind of matter prevailing in our solar system,
matter consists of up (u) and down (d) quarks. A proton, for
example, consists of two u quarks and one d quark. A neutron
consists of two d quarks and one u quark.
Converting u or d quarks
to strange (s) quarks in neutrons or protons is typically unstable.
In the high-density environment of quark stars, however, matter
containing up, down, and strange quarks might be stable. The reason
for this is that when thousands of quarks are together (unlike the
ordinary twosome or threesome quark combinations we see on Earth in
the form of protons, neutrons, and mesons) u-d-s matter is likely to
be a more
energy-efficient form of packaging than the u-d form of matter we
have on our planet if the strange quark matter hypothesis is
This process really comes into play in collapsed stars, where
strange quarks could roughen the surface of the stars. Such a
surface, says Los Alamos scientist Andrew Steiner
(email@example.com, 505-667-0673), can be compared to a liquid
surface. On Earth, liquid surfaces are generally flat. Because of
surface tension, too much energy would be required to overcome the
tension and form additional facets above the surface. At a quark
star, by contrast, surface tension may not be large and the crust of
the star could form extra surfaces, nugget-like
objects without any undue energy cost. The positively charged quark
lumps would be surrounded by a sea of electrons, as required to make
the crust electrically neutral (see figure at
Physics News Graphics).
Where did the electrons come
from? They're left over from the atoms that were crushed in the
collapse phase; some electrons are pressed into protons to make
neutrons, but some would have survived.
What would be the test of the hypothesis of an inhomogeneous
termination at a quark-star surface? Again, the Los Alamos group is
at odds with the prevailing model, which says that quark stars
should be more luminous than neutron stars. Au contraire, they
say. Just as
foam on the surface of a water surface clouds our view into the
water, so the quark bumps on an otherwise smooth surface at a quark
star would enhance the scattering of photons and neutrinos, lowering
the quark star's luminosity.
Jaikumar et al., Physical Review
Letters, upcoming article
See movie at the Los Alamos Web site