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Strange matter is a particular form of quark matter, usually thought of as a 'liquid' of up, down, and strange quarks. It is to be contrasted with nuclear matter, which is a liquid of neutrons and protons (which themselves are built out of up and down quarks), and with non-strange quark matter, which is a quark liquid containing only up and down quarks. At high enough density, strange matter is expected to be color superconducting.

Contents 1 Two meanings of the term "strange matter" 2 Strange matter that is only stable at high pressure 3 Strange matter that is stable at zero pressure 3.1 Danger of strangelets: catalyzed conversion to strange matter 3.2 Is the "strange matter hypothesis" true? 4 Strange matter in popular culture 5 Further reading 6 References

In particle physics and astrophysics, the term is used in two ways, one broader and the other more specific.

1. The broader meaning is just quark matter that contains three flavors of quarks: up, down, and strange. In this definition, there is a critical pressure and an associated critical density, and when nuclear matter (made of protons and neutrons) is compressed beyond this density, the protons and neutrons dissociate into quarks, yielding quark matter (probably strange matter).
2. The narrower meaning is quark matter that is more stable than nuclear matter. The idea that this could happen is the "strange matter hypothesis" of Bodmer ^[1] and Witten ^[2]. In this definition, the critical pressure is zero: the true ground state of matter is always quark matter. The nuclei that we see in the matter around us, which are droplets of nuclear matter, are actually metastable, and given enough time (or the right external stimulus) would decay into droplets of strange matter, i.e. strangelets.

Under the broader definition, strange matter might occur inside neutron stars, if the pressure at their core is high enough (i.e. above the critical pressure). At the sort of densities 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. Charm and heavier quarks would only occur at much higher densities.

A neutron star with a quark matter core is often called a hybrid star. However, it is hard to know whether hybrid stars really exist in nature because physicists currently have little idea of the likely value of the critical pressure or density. It seems plausible that the transition to quark matter will already have occurred when the separation between the nucleons becomes much smaller than their size, so the critical density must be less than about 100 times nuclear saturation density. But a more precise estimate is not yet available, because the strong interaction that governs the behavior of quarks is particularly intractable, and numerical calculations using lattice QCD are currently blocked by the fermion sign problem.

One major area of activity in neutron star physics is the attempt to find observable signatures by which we could tell, from earth based observations of neutron stars, whether they have quark matter (probably strange matter) in their core.

If the "strange matter hypothesis" is true then nuclear matter is metastable against decaying into strange matter. The lifetime for spontaneous decay is very long, so we do not see this decay process happening around us. However, under this hypothesis there should be strange matter in the universe:

1. Quark stars (often called "strange stars") consist of quark matter from their core to their surface. They would be several kilometers across, and may have a very thin crust of nuclear matter.
2. Strangelets are small pieces of strange matter, perhaps as small as nuclei. They would be produced when strange stars are formed or collide.

If there are strangelets flying around the universe, then occasionally a strangelet should hit the planet Earth, where it would appear as an exotic type of cosmic ray. This raises the question whether a strangelet from space would convert the whole planet to strange matter. The disaster scenario is this: one strangelet hits a nucleus, catalyzing its immediate conversion to strange matter. This liberates energy, and sends pieces (more strangelets) flying in all directions. These merge with other nuclei and convert them, leading to a chain reaction, at the end of which all the nuclei of all the atoms have been converted, and earth has been reduced to a hot cloud of strangelets.

The general belief is that this would not happen, because most models predict that strangelets, like nuclei, are positively charged, so they are electrostatically repelled by nuclei, and would rarely merge with them.^[3] However, concerns of this type were raised at the commencement of the Relativistic Heavy Ion Collider (RHIC) experiment at Brookhaven, which could potentially have created strangelets. A detailed analysis ^[4] concluded that the RHIC collisions were comparable to ones that naturally occur as cosmic rays traverse the solar system, so we would already have seen such a disaster if it were possible.

In the case of a neutron star, however, the conversion scenario seems much more plausible. A neutron star is in a sense one giant (20 km across) nucleus, held together by gravity. If a strangelet hit a neutron star, it could convert a small region of it, and that region would grow to consume the entire star.^[5]

The strange matter hypothesis is generally regarded as a radical idea. Because one strangelet can convert a neutron star to a strange star, it seems likely that if the strange matter hypothesis were correct, all the objects we observe as neutron stars would actually have to be strange stars. But there is good evidence that at least some of them are not strange stars, and have fairly thick crusts of nuclear matter. There is an ongoing debate among experts on this question.^{[6] [7]}

• The Doctor Who serial Time and the Rani (1987) concerned the destruction of an asteroid composed of strange matter.

• The docu-drama End Day dramatizes a scenario wherein strange matter created in a particle accelerator leads to the end of the world.

• The TV show Odyssey 5 episode 'Trouble With Harry' hypothesized the end of the earth due to strange matter.

• The comic book newuniversal features a vast web of strange matter, each strand several light years across. The Earth's laws of physics alter once the solar system enters the web.

• J. Madsen, "Physics and astrophysics of strange quark matter" Lect. Notes Phys. 516:162-203 (1999)

1. ^ A. Bodmer "Collapsed Nuclei" Phys. Rev. D4, 1601 (1971)
2. ^ E. Witten, "Cosmic Separation Of Phases" Phys. Rev. D30, 272 (1984)
3. ^ J. Madsen, "Intermediate mass strangelets are positively charged" Phys. Rev. Lett. 85 (2000) 4687-4690 (2000)
4. ^ W. Busza, R. Jaffe, J. Sandweiss, F. Wilczek, "Review of speculative 'disaster scenarios' at RHIC", Rev. Mod. Phys.72:1125-1140 (2000)
5. ^ C. Alcock, E. Farhi and A. Olinto, "Strange stars", Astrophys. Journal 310, 261 (1986)
6. ^ A. Balberg, "Comment on 'strangelets as cosmic rays beyond the Greisen-Zatsepin-Kuzmin cutoff'", Phys. Rev. Lett. 92:119001 (2004)
7. ^ J. Madsen, "Strangelet propagation and cosmic ray flux" Phys. Rev. D71, 014026 (2005)

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