Strangelet

A strangelet (pronounced

The known particles with strange quarks are unstable. Because the strange quark is heavier than the up and down quarks, it can spontaneously decay, via the weak interaction, into an up quark. Consequently, particles containing strange quarks, such as the lambda particle, always lose their strangeness, by decaying into lighter particles containing only up and down quarks.

However, condensed states with a larger number of quarks might not suffer from this instability. That possible stability against decay is the "strange matter hypothesis", proposed separately by Arnold Bodmer^[3] and Edward Witten.^[4] According to this hypothesis, when a large enough number of quarks are concentrated together, the lowest energy state is one which has roughly equal numbers of up, down, and strange quarks, namely a strangelet. This stability would occur because of the Pauli exclusion principle; having three types of quarks, rather than two as in normal nuclear matter, allows more quarks to be placed in lower energy levels.

Relationship with nuclei

A nucleus is a collection of a large number of up and down quarks, confined into triplets (neutrons and protons). According to the strange matter hypothesis, strangelets are more stable than nuclei, so nuclei are expected to decay into strangelets. But this process may be extremely slow because there is a large energy barrier to overcome: as the weak interaction starts making a nucleus into a strangelet, the first few strange quarks form strange baryons, such as the Lambda, which are heavy. Only if many conversions occur almost simultaneously will the number of strange quarks reach the critical proportion required to achieve a lower energy state. This is very unlikely to happen, so even if the strange matter hypothesis were correct, nuclei would never be seen to decay to strangelets because their lifetime would be longer than the age of the universe.^[5]

Size

The stability of strangelets depends on their size. This is because of (a) surface tension at the interface between quark matter and vacuum (which affects small strangelets more than big ones), and (b) screening of charges, which allows small strangelets to be charged, with a neutralizing cloud of electrons/positrons around them, but requires large strangelets, like any large piece of matter, to be electrically neutral in their interior. The charge screening distance tends to be of the order of a few femtometers, so only the outer few femtometers of a strangelet can carry charge.^[6]

The surface tension of strange matter is unknown. If it is smaller than a critical value (a few MeV per square femtometer^[7]) then large strangelets are unstable and will tend to fission into smaller strangelets (strange stars would still be stabilized by gravity). If it is larger than the critical value, then strangelets become more stable as they get bigger.

Natural or artificial occurrence

Although nuclei do not decay to strangelets, there are other ways to create strangelets, so if the strange matter hypothesis is correct there should be strangelets in the universe. There are at least three ways they might be created in nature:

• Cosmogonically, i.e. in the early universe when the QCD confinement phase transition occurred. It is possible that strangelets were created along with the neutrons and protons that form ordinary matter.
• High-energy processes. The universe is full of very high-energy particles (cosmic rays). It is possible that when these collide with each other or with neutron stars they may provide enough energy to overcome the energy barrier and create strangelets from nuclear matter. Some identified exotic cosmic ray events, like the Price's event^{[clarification needed]} with very low charge-to-mass ratio could have already registered strangelets.^[8]
• Cosmic ray impacts. In addition to head-on collisions of cosmic rays,
  Earth's atmosphere
  may create strangelets.

These scenarios offer possibilities for observing strangelets. If there are strangelets flying around the universe, then occasionally a strangelet should hit Earth, where it would appear as an exotic type of cosmic ray. If strangelets can be produced in high-energy collisions, then they might be produced by heavy-ion colliders.

Accelerator production

At heavy ion accelerators like the

Space-based detection

The Alpha Magnetic Spectrometer (AMS), an instrument that is mounted on the International Space Station, could detect strangelets.^[12]

Possible seismic detection

In May 2002, a group of researchers at Southern Methodist University reported the possibility that strangelets may have been responsible for seismic events recorded on October 22 and November 24 in 1993.^[13] The authors later retracted their claim, after finding that the clock of one of the seismic stations had a large error during the relevant period.^[14]

It has been suggested that the

Impacts on Solar System bodies

It has been suggested that strangelets of subplanetary (i.e. heavy meteorite) mass would puncture planets and other Solar System objects, leading to impact craters which show characteristic features.^[15]

Potential propagation

If the strange matter hypothesis is correct, and if a stable negatively-charged strangelet with a surface tension larger than the aforementioned critical value exists, then a larger strangelet would be more stable than a smaller one. One speculation that has resulted from the idea is that a strangelet coming into contact with a lump of ordinary matter could over time convert the ordinary matter to strange matter.^[16][17]

This is not a concern for strangelets in

In the case of a neutron star, the conversion scenario may be more plausible. A neutron star is in a sense a giant nucleus (20 km across), held together by gravity, but it is electrically neutral and would not electrostatically repel strangelets. If a strangelet hit a neutron star, it might catalyze quarks near its surface to form into more strange matter, potentially continuing until the entire star became a strange star.^[27]

Debate about the strange matter hypothesis

The strange matter hypothesis remains unproven. No direct search for strangelets in cosmic rays or

Another argument against the hypothesis is that if it were true, essentially all neutron stars should be made of strange matter, and otherwise none should be.[28] Even if there were only a few strange stars initially, violent events such as collisions would soon create many fragments of strange matter flying around the universe. Because collision with a single strangelet would convert a neutron star to strange matter, all but a few of the most recently formed neutron stars should by now have already been converted to strange matter.

This argument is still debated,^{[29][30][31][32]} but if it is correct then showing that one old neutron star has a conventional nuclear matter crust would disprove the strange matter hypothesis.

Because of its importance for the strange matter hypothesis, there is an ongoing effort to determine whether the surfaces of neutron stars are made of strange matter or

• An episode of Odyssey 5 featured an attempt to destroy the planet by intentionally creating negatively charged strangelets in a particle accelerator.^[35]
• The BBC docudrama End Day features a scenario where a particle accelerator in New York City explodes, creating a strangelet and starting a catastrophic chain reaction which destroys Earth.
• The story A Matter most Strange in the collection Indistinguishable from Magic by Robert L. Forward deals with the making of a strangelet in a particle accelerator.
• Impact, published in 2010 and written by Douglas Preston
  , deals with an alien machine that creates strangelets. The machine's strangelets impact the Earth and Moon and pass through.
• The novel Phobos, published in 2011 and written by
  LHC
  and escape from it to destroy the Earth.
• In the 1992 black-comedy novel Humans by Donald E. Westlake, an irritated God sends an angel to Earth to bring about Armageddon by means of using a strangelet created in a particle accelerator to convert the Earth into a quark star.
• In the 2010 film Quantum Apocalypse, a strangelet approaches the Earth from space.
• In the novel The Quantum Thief by Hannu Rajaniemi and the rest of the trilogy, strangelets are mostly used as weapons, but during an early project to terraform Mars, one was used to convert Phobos into an additional "sun".

See also

• Grey goo
• Ice-nine

Further reading

• Holden, Joshua (May 17, 1998). "The Story of Strangelets".
  Rutgers. Archived from the original
  on January 7, 2010. Retrieved April 1, 2010.
• Fridolin Weber (2005). "Strange Quark Matter and Compact Stars". Progress in Particle and Nuclear Physics. 54 (1): 193–288.
  S2CID 15002134
  .
• Jes Madsen (1999). "Physics and astrophysics of strange quark matter". Hadrons in Dense Matter and Hadrosynthesis. Lecture Notes in Physics. Vol. 516. pp. 162–203.
  S2CID 16566509
  .