How Does Matter Terminate?

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.

Theorists have 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 correct.

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 (asteiner@lanl.gov, 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