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Strange matter (or strange quark matter) is quark matter containing strange quarks.

In extreme environments, strange matter is hypothesized to occur in the core of neutron stars, or, more speculatively, as isolated droplets that may vary in size from femtometers (strangelets) to kilometers, as in the hypothetical strange stars. At high enough density, strange matter is expected to be color superconducting. Ordinary matter, also referred to as atomic matter, is composed of atoms, with nearly all matter concentrated in the atomic nuclei. Nuclear matter is a liquid composed of neutrons and protons, and they are themselves composed of up and down quarks. Quark matter is a condensed form of matter composed entirely of quarks. When quark matter does not contain strange quarks, it is sometimes referred to as non-strange quark matter.

In particle physics and astrophysics, the term "strange matter" is used in two different contexts, one broader and the other more specific and hypothetical: In the broader context, our current understanding of the laws of nature predicts that strange matter could be created when nuclear matter (made of protons and neutrons) is compressed beyond a critical density. At this critical pressure and density, the protons and neutrons dissociate into quarks, yielding quark matter and potentially strange matter. A more specific hypothesis is that strange quark matter is the true ground state of all matter, and thus more stable than ordinary nuclear matter. This idea is known as the "strange matter hypothesis", or the Bodmer–Witten assumption. Under this hypothesis, the nuclei of the atoms we see around us are only metastable, even when the external critical pressure is zero, and given enough time (or the right stimulus) the nuclei would decay into stable droplets of strange matter. Droplets of strange matter are also referred to as strangelets.

In the general context, strange matter might occur inside neutron stars, if the pressure at their core is high enough to provide a sufficient gravitational force (i.e. above the critical pressure). At the sort of densities and high pressures we expect in the center of a neutron star, the quark matter would probably be strange matter. It could conceivably be non-strange quark matter, if the effective mass of the strange quark were too high. Strange matter comes about as a way to relieve degeneracy pressure. Neutrons consist of twice as many down quarks (charge −⁠1/3⁠ e) as up quarks (charge +⁠2/3⁠ e), so the degeneracy pressure of down quarks usually dominates electrically neutral quark matter. The higher rest mass of the strange quark costs some energy, but by opening up an additional set of energy levels, the average energy per particle can be lower, making strange matter more stable at such high pressures than non-strange quark matter. A neutron star with a quark matter core is often called a hybrid star. One major area of activity in neutron star physics is the attempt to find observable signatures by which we could tell whether neutron stars have quark matter (probably strange matter) in their core. During the merger of two neutron stars, strange matter may be ejected out into the space around the stars, which may allow for the studying of strange matter. However, the rate at which strange matter decays is unknown, and there are very few binary pairs of neutron stars nearby to the Solar System, which could make the official discovery of strange matter very difficult.

If the "strange matter hypothesis" is true, then nuclear matter is metastable against decaying into strange matter. However, under this hypothesis there should be strange matter in the universe: Quark stars (often called "strange stars") consist of quark matter from their core to their surface. Strangelets are small pieces of strange matter, perhaps as small as nuclei.

Quick Facts

• If the "strange matter hypothesis" is true, then nuclear matter is metastable against decaying into strange matter.
• In extreme environments, strange matter is hypothesized to occur in the core of neutron stars, or, more speculatively, as isolated droplets that may vary in size from femtometers (strangelets) to kilometers, as in the hypothetical strange stars.
• Ordinary matter, also referred to as atomic matter, is composed of atoms, with nearly all matter concentrated in the atomic nuclei.
• When quark matter does not contain strange quarks, it is sometimes referred to as non-strange quark matter.
• At high enough density, strange matter is expected to be color superconducting.

Source material: Wikipedia - "Strange matter". Adapted and summarized for DiscoverScroll. Original contributors are credited through the linked Wikipedia article. Read original on Wikipedia. CC BY-SA 4.0. Changes were made from the original.