- A collection of scientific articles on various aspects of mirror matter theory
- Mirror matter article on h2g2
- R. Foot. "Mirror matter type dark matter". http://arxiv.org/abs/astro-ph/0407623.
- L.B. Okun. "Mirror particles and mirror matter: 50 years of speculation and search". http://arxiv.org/abs/hep-ph/0606202.
Antimatter sounds like the stuff of science fiction, and it is. But it's also very real. Antimatter is created and annihilated in stars every day. Here on Earth it's harnessed for medical brain scans.
"Antimatter is around us each day, although there isn't very much of it," says Gerald Share of the Naval Research Laboratory. "It is not something that can be found by itself in a jar on a table."
So Share went looking for evidence of some in the Sun, a veritable antimatter factory, leading to new results that provide limited fresh insight into these still-mysterious particles.
Simply put, antimatter is a fundamental particle of regular matter with its electrical charge reversed. The common proton has an antimatter counterpart called the antiproton. It has the same mass but an opposite charge. The electron's counterpart is called a positron.
Antimatter particles are created in ultra high-speed collisions.
One example is when a high-energy proton in a solar flare collides with carbon, Share explained in an e-mail interview. "It can form a type of nitrogen that has too many protons relative to its number of neutrons." This makes its nucleus unstable, and a positron is emitted to stabilize the situation.
But positrons don't last long. When they hit an electron, they annihilate and produce energy.
"So the cycle is complete, and for this reason there is so little antimatter around at a given time," Share said.
The antimatter wars
To better understand the elusive nature of antimatter, we must back up to the beginning of time.
In the first seconds after the Big Bang, there was no matter, scientists suspect. Just energy. As the universe expanded and cooled, particles of regular matter and antimatter were formed in almost equal amounts.
But, theory holds, a slightly higher percentage of regular matter developed -- perhaps just one part in a million -- for unknown reasons. That was all the edge needed for regular matter to win the longest running war in the cosmos.
"When the matter and antimatter came into contact they annihilated, and only the residual amount of matter was left to form our current universe," Share says.
Antimatter was first theorized based on work done in 1928 by the physicist Paul Dirac. The positron was discovered in 1932. Science fiction writers latched onto the concept and wrote of antiworlds and antiuniverses.
Antimatter has tremendous energy potential, if it could ever be harnessed. A solar flare in July 2002 created about a pound of antimatter, or half a kilo, according to new NASA-led research. That's enough to power the United States for two days.
Laboratory particle accelerators can produce high-energy antimatter particles, too, but only in tiny quantities. Something on the order of a billionth of a gram or less is produced every year.
Nonetheless, sci-fi writers long ago devised schemes using antimatter to power space travelers beyond light-speed. Antimatter didnt get a bad name, but it sunk into the collective consciousness as a purely fictional concept. Given some remarkable physics breakthrough, antimatter could in theory power a spacecraft. But NASA researchers say it's nothing that will happen in the foreseeable future.
Meanwhile, antimatter has proved vitally useful for medical purposes. The fleeting particles of antimatter are also created by the decay of radioactive material, which can be injected into a patient in order to perform Positron Emission Tomography, or PET scan of the brain. Here's what happens:
A positron that's produced by decay almost immediately finds an electron and annihilates into two gamma rays, Share explains. These gamma rays move in opposite directions, and by recording several of their origin points an image is produced.
Looking at the Sun
In the Sun, flares of matter accelerate already fast-moving particles, which collide with slower particles in the Sun's atmosphere, producing antimatter. Scientists had expected these collisions to happen in relatively dense regions of the solar atmosphere. If that were the case, the density would cause the antimatter to annihilate almost immediately.
Share's team examined gamma rays emitted by antimatter annihilation, as observed by NASA's RHESSI spacecraft in work led by Robert Lin of the University of California, Berkeley.
The research suggests the antimatter perhaps shuffles around, being created in one spot and destroyed in another, contrary to what scientists expect for the ephemeral particles. But the results are unclear. They could also mean antimatter is created in regions where extremely high temperatures make the particle density 1,000 times lower than what scientists expected was conducive to the process.
Details of the work will be published in Astrophysical Journal Letters on Oct. 1.
Though scientists like to see antimatter as a natural thing, much about it remains highly mysterious. Even some of the fictional portrayals of mirror-image objects have not been proven totally out of this world.
"We cannot rule out the possibility that some antimatter star or galaxy exists somewhere," Share says. "Generally it would look the same as a matter star or galaxy to most of our instruments."
Theory argues that antimatter would behave identical to regular matter gravitationally.
"However, there must be some boundary where antimatter atoms from the antimatter galaxies or stars will come into contact with normal atoms," Share notes. "When that happens a large amount of energy in the form of gamma rays would be produced. To date we have not detected these gamma rays even though there have been very sensitive instruments in space to observe them."
From studying antimatter for over 100 years, antimatter, which is sometimes called mirror matter and can be a solid, liquid, gas, or plasma. Antimatter is mirror image of matter and like matter is composed of antimatter elements that have been incorporated in the Periodic Table of Matter-Antimatter Elements. Each antimatter element's nuclear, chemical and physical characteristics have been defined to such an extent that scientists know almost as much about antimatter as matter. Using the government’s estimate of $10 billion to find the Higgs bosons, the intellectual value of the Periodic Table of Matter-Antimatter Elements is estimated to be over $1 trillion.
In 1898, Arthur Schuster, British physicist, coined the name "antimatter." Schuster believed that there were entire antimatter solar systems that were indistinguishable from our solar system. In 1905, Einstein unveiled his special relativity and his famous equation, E=mc2. Erwin Schrödinger and Werner Heisenberg apply the concept to the atoms and invented quantum theory of physics. In 1928, Paul Dirac combined quantum theory and special relativity, which resulted in a solution containing an electron with positive energy, and a positive electron (positron) with negative energy, which was confirmed by Carl Anderson in his study of cosmic particles.
When antimatter or mirror matter enters our solar system, mirror matter is called comets. The quantity of mirror matter in our solar system is a million times less than scientists had estimated. In 2002, Norm Hansen announced the discovery that comets are composed of antimatter to the joint meeting of American Physical Society and American Astronomical Society. The solar dust particles blast antimatter off the comet to create the comet's plasma coma; and the solar wind pulls the coma's plasma into the comet's tails. People are able to see comets from the conversion of matter-antimatter into Mirror Energy, which produce a spectrum of radiation that includes light, x-rays, and gamma rays.
Antimatter sungrazers comets have collided with the Sun and produced enormous explosions. According to the National Geographic magazine, July 2004 issue, solar bursts were equivalent to billions of megatons of TNT. When Earth passes through these enormous solar storms, communication satellites have been damaged and electrical power on Earth has been disrupted. Comets are not only colliding with the Sun, but also have colliding with stars throughout the Universe and are the source of gamma-ray bursts that scientists have been studying for 40 years. http://www.energyusa.net/antimatter.htm
Mirror Matter Mystery
Two Australian scientists believe they have found evidence of a parallel universe of strange matter within our own Solar System.
Dr Robert Foot and Dr Saibal Mitra report that close-up observations of the asteroid Eros by the Near-Shoemaker probe indicate it has been splattered by so-called "mirror matter".
Mirror matter is not anti-matter, it is altogether weirder. It is somehow a "reflection" of normal matter, a sort of parallel series of particles required to restore the balance of the Universe.
Sounds far-fetched - some believe so. However, experiments are underway to confirm or deny the existence of this strange, potentially significant but as yet undetected component of the cosmos.
Mirror matter is a hypothetical form of matter that restores nature's flawed left-right symmetry.
Laws of nature, such as the rules that govern the interactions of fundamental particles, show a high degree of symmetry except that some laws are not the same when reflected in a hypothetical mirror.
This means that elementary particles display a preference for left over right. In a way, the Universe is left-handed. Why? Nobody knows.
Many physicists are happy with this idea believing that in the first instants of the Big Bang everything was perfectly symmetrical. Only when the cosmos cooled did it become asymmetric, with a difference emerging between left and right.
But some scientists do not accept this. They maintain that the Universe has a left-right balance because there exists "mirror matter" - for every known particle there is a mirror particle that restores the cosmic balance.
Mirror matter would produce its own light but we would not be able to see it because mirror matter only interacts with our matter via gravity.
Dr Robert Foot believes that mirror matter would have been made in abundance in the Big Bang and that it is all around us but we can't see it.
"There could be mirror matter stars, planets and galaxies out there," he told BBC News Online.
"In fact, some think that the unseen so-called "dark matter" of the Universe could actually be mirror matter," he adds.
"Mirror matter is perfect to explain dark matter. It's dark and can only be detected through its gravity."
Dr Foot believes he has found evidence that it is here, closer than we believed, and that it had had a measurable effect on our spaceprobes.
In October 2000, the Near-Shoemaker spacecraft lightly touched down on the 13-by-13-by-33-km (8 by 8 by 20 miles) Eros asteroid. It was the first time a probe had landed on an asteroid.
Its close scrutiny of Eros revealed many strange features - such as flat-bottomed craters filled with a peculiar bluish dust, and a puzzling lack of small craters.
Unexplained by conventional understanding, Dr Foot believes that mirror matter provides an answer.
He calculates that small objects containing mirror matter could have struck the asteroid and left behind precisely the same scars that are seen. Indeed, he says there is no other credible explanation.
He also calculates that mirror matter may explain the mysterious force that acts on both the Pioneer 10 and 11 deep spaceprobes.
Launched in 1972, the Pioneers are leaving the Solar System in opposite directions. Detailed analysis of their trajectory indicates that they are both subject to a tiny, unexplained force that is slowing them down.
Dr Foot believes that mirror matter exerting a drag on the Pioneers could be to blame.
"How else can you explain that both Pioneers, on opposite ends of the Solar System, experience the same force pushing in the same direction?" Dr Foot asks.
In a research paper to be published shortly, Drs Foot and Mitra suggest that mirror matter may even have struck the Earth.
He singles out three possible events: the 1908 Tunguska impact in Siberia and low-altitude, low-velocity fireballs seen in Spain in 1994 and in Jordan in 2001.
"Mirror matter could also explain these events," he told BBC News Online.
Many scientists dismiss mirror matter as wild speculation but even the sceptics will have cause for thought if the latest experiments from the European Centre for Nuclear Research (Cern) are to be believed.
Experiments involving so-called ortho-positronium - an arrangement in which an electron orbits a positron (its antimatter equivalent) - show that it decays slightly faster than can be explained.
This could be due, says Dr Foot, to the electrons changing fleetingly into mirror matter and then back again.
Experiments at Cern and in Moscow hope to determine in the next year or so if mirror matter really does exist.
Dr Robert Foot is from the University of Melbourne; Dr Saibal Mitra is from the University of Amsterdam.
50 years ago it was discovered that the fundamental particles, <br>
such as the electron and proton, have 'left-handed' interactions -- they <br>do not respect mirror symmetry. This experimental fact motivates the idea <br>that a set of 'mirror particles' exist. The left-handedness of the ordinary <br>particles can then be balanced by the right-handedness of the mirror particles. <br>In this way mirror reflection symmetry can exist but requires something <br> profoundly new. It requires the existence of a completely new form of matter <br> called 'mirror matter'.
<P> The mirror symmetry requires that the masses of the mirror particles<br>
should be the same as their ordinary counterparts. However, ordinary and <br>
mirror particles do not interact with each other by any of the known <br>
fundamental forces except via gravity. Einstein's theory suggests that<br>
anything with mass should couple to gravity, however the other fundamental<br>
forces can act completely separately on the ordinary and mirror sectors.<br>
Mirror matter should not be confused with anti-matter which has quite <br>
different properties (in fact, some people have the unfortunate habit
of <br> using the term 'mirror matter' when they really mean anti-matter).
In anycase, <br> mirror matter is still hypothetical
although there is a large range of astrophysical<br> and experimental evidence
for its existence.
The evidence ranges from observations suggesting that most of the matter <br> in the Universe is invisible to unexpected properties of ghostly particles <br>called 'neutrinos'.<P><UL><Li>It predicts mirror matter stars which are invisible -- and there is a large<br> body of evidence for such invisible </span></font></b> <a href="http://www.sciam.com/specialissues/0398cosmos/0398rubin.html"><font size=4>dark matter.</font></A><font size=4>
There is also some specific <br>evidence that mirror stars have been
observed from their gravitational effects <br> on the bending of
light (</span></font></b> <a href="http://www.cosmiverse.com/space01090102.html"><font size=4>MACHO</font></A><font size=4>
<Li>If mirror matter exists then mirror planets should also exist.
In fact, <br> there is remarkable evidence that these planets have
actually been detected <br> orbiting around nearby ordinary stars.<P><Li>
The opposite type of system, with an ordinary planet orbiting a mirror
<br> star, also apparently exits, but has been misidentified as an
</span></font></b> <a href
'isolated' planet!</font></A><font size=4>
<P><Li>Perhaps most remarkable of all, is the evidence that mirror matter <br> not only exists
in our solar system, but mirror matter asteroid or comet sized <br> objects are
frequently colliding with our planet. There may even be fragments <br> of mirror
matter at various impact sites around the world (such as the one <br>
in </span></font></b> <a href="http://www.jas.org.jo/mett.html"><font size=4>Jordan</font></A>)<font
size=4>, which could potentially be found. Nobody has looked!</UL>On the
microscopic level two types of forces or interactions can connect ordinary <br> and mirror matter.
That is, by small transition forces connecting photons with <br> mirror photons and by small mass mixing terms between neutrinos and mirror <br> neutrinos.
The observational consequences of these effects are actually observed:<UL><Li>The photon-mirror
photon transition force implies a shorter effective lifetime <br>for orthopositronium (a
type of atom made from an electron and a positron) <br> in vacuum experiments.
A shorter lifetime is seen!
<P><Li>Neutrino-mirror neutrino mass mixing implies
that each ordinary neutrino <br> transforms (oscillates) into its mirror
Remarkably, solar, <br> atmospheric and other neutrino experiments
actually require the existence of <br> new types of neutrinos, oscillating with
the ordinary neutrinos in a way <br> which is
</span></font></b> <a href
consistent with the mirror matter theory.
<P><Li>Any mirror matter hydrogen gas left over in our solar system
would not have <br> been swept out by the solar wind. Remnant mirror
matter particles striking <br> spacecraft would
lead to a drag force slowing them down.
<br> precise measurements of the velocities of the
<font size=4> Pioneer
spacecraft </font></A><font size=4> indicate <br> that
they are indeed slowing down faster
than expected due to known forces!
In physics, mirror matter, also called shadow matter or Alice matter, is a hypothetical counterpart to ordinary matter. Modern physics deals with three basic types of spatial symmetry: reflection, rotation and translation. The known elementary particles respect rotation and translation symmetry but do not respect mirror reflection symmetry (also called P-symmetry or parity). Of the four fundamental interactions electromagnetism, the strong interaction, the weak interaction and gravity, only the weak interaction breaks parity.
Parity violation in weak interactions was first postulated by Tsung Dao Lee and Chen Ning Yang  in 1956 as a solution to the τ-θ puzzle. They suggested a number of experiments to test if the weak interaction is invariant under parity. These experiments were performed half a year later and they confirmed that the weak interactions of the known particles violate parity.  
However parity symmetry can be restored as a fundamental symmetry of nature if the particle content is enlarged so that every particle has a mirror partner. The theory in its modern form was written down in 1991 , although the basic idea dates back earlier . Mirror particles interact amongst themselves in the same way as ordinary particles, except where ordinary particles have left-handed interactions, mirror particles have right-handed interactions. In this way, it turns out that mirror reflection symmetry can exist as an exact symmetry of nature, provided that a "mirror" particle exists for every ordinary particle. Parity can also be spontaneously broken depending on the Higgs potential.  While in the case of unbroken parity symmetry the masses of particles are the same as their mirror partners, in case of broken parity symmetry the mirror partners are lighter or heavier.
Mirror matter, if it exists, would have to be very weakly interacting with ordinary matter. This is because the forces between mirror particles are mediated by mirror bosons. With the exception of the graviton, none of the known bosons can be identical to their mirror partners. The only way mirror matter can interact with ordinary matter via forces other than gravity is via so-called kinetic mixing of mirror bosons with ordinary bosons or via the exchange of Holdom particles. These interactions can only be very weak. Mirror particles have therefore been suggested as candidates for the inferred dark matter in the universe.
Observational effects of mirror matter
If mirror matter is present in the universe with sufficient abundance then its gravitational effects can be detected. Because mirror matter is analogous to ordinary matter, it is then to be expected that a fraction of the mirror matter exists in the form of mirror galaxies, mirror stars, mirror planets etc. These objects can be detected using gravitational microlensing. One would also expect that some fraction of stars have mirror objects as their companion. In such cases one should be able to detect periodic Doppler shifts in the spectrum of the star. There are some hints that such effects may already have been observed.
What if mirror matter does exist but has (almost) zero abundance? Like magnetic monopoles, mirror matter could have been diluted to unobservably low densities during the inflation epoch. Sheldon Glashow has shown that if at some high energy scale particles exist which interact strongly with both ordinary and mirror particles, radiative corrections will lead to a mixing between photons and mirror photons. This mixing has the effect of giving mirror electric charges a very small ordinary electric charge. Another effect of photon-mirror photon mixing is that it induces oscillations between positronium and mirror positronium. Positronium could then turn into mirror positronium and then decay into mirror photons.
The mixing between photons and mirror photons could be present at tree level or arise as a consequence of quantum corrections due to the presence of particles that carry both ordinary and mirror charges. In the latter case, the quantum corrections have to vanish at the one and two loop level, otherwise the predicted value of the kinetic mixing parameter would be larger than experimentally allowed.
An experiment to measure this effect is currently being planned.
If mirror matter does exist in large abundances in the universe and if it interacts with ordinary matter via photon-mirror photon mixing, then this could be detected in dark matter direct detection experiments such as DAMA/NaI. . In fact, it is one of the few dark matter candidates which can explain the positive DAMA/NaI dark matter signal whilst still being consistent with the null results of other dark matter experiments . Mirror matter may also be detected in electromagnetic field penetration experiments  and there would also be consequences for planetary science.
Mirror matter could also be responsible for the GZK puzzle. Topological defects in the mirror sector could produce mirror neutrinos which can oscillate to ordinary neutrinos.  Another possible way to evade the GZK bound is via neutron-mirror neutron oscillatons.    
The phrase "mirror matter" was also introduced by physicist and author Dr. Robert L. Forward as an alternative term for what is commonly called antimatter, in an attempt to emphasize that antimatter is identical to ordinary matter, except reversed in all possible ways (i.e., CPT). (Forward was apparently not aware of the use of the word "mirror particles" by Russian physicists to mean parity reversed matter that does not interact strongly with "ordinary" matter). This is elucidated in his book Mirror Matter: Pioneering Antimatter Physics (1988), and his editing the small review journal Mirror Matter Newsletter (1986-1990). However, this use of the term "mirror matter" for antimatter was never widely picked up by others and is not currently in common use.
The mirror photon is the mirror counterpart of the photon. It may be massless or massive in theory. Mirror photons and other mirror matter particles have been proposed as a candidate for dark matter. The mirror photon is also invisible and undetectable, except for their gravitational effects. According to Bob Holdom, of the University of Toronto, says that photons and mirror photons, (along with other particles and their mirror counterparts) can change into each other through the exchange of a "Holdom force" particle, or H particle. Mirror photons, in theory, can interact with regular photons. However, the mirror photon can not interact with any charged particle, it can only interact with its neutral counterpart, the regular photon. A mirror photon can decay into two separate particles, a mirror electron and a mirror positron. These two particles quickly combine to form mirror ortho-positronium. Since mirror particles and mirror photons have the same gravitational properties of regular matter, they could form mirror planets or mirror stars. A mirror star would continually emit mirror photons, and not only would the mirror photons be undetectable, the mirror star, being made out of mirror matter, would be undetectable too, except for its gravitational pull.
A mirror photon would have the same speed of a regular photon. Mirror photons and mirror matter both feel the force of gravity, as gravity is part of spacetime. Aside from the aforementioned characteristics, a mirror photon is the same as a regular photon.
New Theory Asserts The Existence Of Mirror Matter
Melbourne - May 06, 2002
Invisible asteroids and other cosmic bodies made of a new form of matter may pose a threat to Earth, asserts Australian Physicist Dr. Robert Foot.
In a revolutionary new theory, Dr. Robert Foot of the University of Melbourne argues that meteorites composed of `mirror matter' -- a candidate for the invisible dark matter that astronomers say is necessary to explain their observations -- could impact with the Earth without leaving any ordinary fragments.
Indeed, the theory seems to provide a simple explanation for the puzzling Tunguska event - the blast which destroyed a huge area of Siberian forest in 1908.
While scientists have attributed this explosion to an ordinary meteorite, no traces of such an object have ever been found. Moreover, there are frequent smaller such events, occurring on a yearly basis, which are even more puzzling.
The idea of mirror matter comes from the established fact that the interactions of the known elementary particles, such as the electrons, protons and neutrinos, violate mirror symmetry -- they have left-handed interactions.
This experimental fact motivates the idea that a set of `mirror particles' exist. The left-handedness of the ordinary particles can then be balanced by the right-handedness of the mirror particles.
In this way mirror reflection symmetry can exist but requires something profoundly new -- a new form of matter called `mirror matter'.
In a recently published book -- Shadowlands, quest for mirror matter in the Universe -- the scientific case for the existence of mirror matter is given.
At the very least, there is a range of fascinating evidence for its existence including: astronomical observations suggesting that most of our galaxy is made from a new form of matter - dark matter, puzzling Jupiter sized planets only a few million miles from their host star, and the mysterious slowing down of spacecraft in our solar system. Remarkably, it is also possible that Pluto -- the most distant planet in our solar system -- might even be a mirror world, which can explain various anomalous features of its orbit.
Perhaps, the most important consequence of all this -- if true -- is the possibility of actually extracting the mirror matter from the Tunguska impact site and other such sites around the world.
The mirror matter idea has not attracted a huge following among physicists. In a recent UPI article, Howard Georgi of Harvard University says: "Foot's ideas have not attracted a huge following in the community that cares about these things, perhaps because the problems they solve, while interesting, are not the most critical puzzles that we are wrestling with."
Nevertheless, mirror matter, if it exists, would be a completely new type of material with a potentially huge commercial value.
Its scientific value would be of no less importance.
What's New with My Subject?
"Antimatter: Matter consisting of atoms which are composed of positrons, antiprotons, and antineutrons. Also loosely refers to the antiparticle corresponding to a particle -- the antiparticle may be regarded as the particle traveling backward in time, or "conjugated." In present physics, the photon is recognized as its own anti-particle. However, since physicists do not differentiate between positive and negative energies in a photon, then they assume that there is no difference between a photon and an antiphoton. This is not true prior to the photon's interaction, where the photon is a 4-space entity, but it is essentially true after the interaction when one looks or observes only 3-spatially. The antiphoton, however, acts quite differently from the photon in 4-space, since it is time-reversed, as is clearly shown in optical phase conjugation. The tentative hypothesis of scalar electromagnetics for the final photon emission interaction with a charged particle in an atom is this: In the expression CPTEGS (where C is charge and normally negative, P is normal parity, T is time and normally positive, E is energy and normally positive, G is gravity and normal is positive, S is entropy and normally positive or increasing). if one of the components is reversed in sign, then each of the remaining components is also individually reversed in sign. This "first order rough hypothesis" will probably need much further refinement, but there is a nugget of real truth therein, once it is more properly sorted out. The phase conjugate photon (antiphoton: negative energy, negative time), e.g., is produced by the photon interaction with the positively charged nucleus, producing a time-reversed wave having negative time-energy, negative time, reversed parity, causing gravitational repulsion, and causing negentropy. Note that, if the rough CPTEGS hypothesis holds, then time-reversal (phase conjugation) is intimately involved with antigravity. At least we have rigorously shown that the common dipole produces a giant negentropy from its mere formation, and this giant reordering of a fraction of the vacuum energy spreads at the speed of light in all directions. This giant negentropy continues so long as the dipole remains intact. Dipoles in matter that has been here since the beginning, have been pouring out 3-spatial EM energy (real EM energy) continuously for some 15 or so billion years. They have been receiving reactive power (phase conjugate energy) from the active 4-vacuum all that time as well." --Tom Bearden
Stable baryonic matter in the Universe currently appears to be composed of up and down quarks. However, a system of 3A up, down, and strange quarks would have a lower energy per baryon than normal nuclear matter. The energy savings comes from the decreased Fermi levels of a system with 3 different flavors instead of only two. The quarks in this case would not form individual baryons, but would have wave functions ranging over the entire size of the system. Color must still be confined, so it is still possible to talk about baryon number when discussing such a system. This would represent a new ground state of matter.
In 1984 it was suggested that such strange quark matter, or `strange matter,' might be both stable and bound at zero temperature and pressure. Normal nuclear matter does not decay to this true QCD ground state due to the high order weak process necessary to produce the strange quarks in abundance. The radically different wave functions between nuclear and strange matter would also greatly inhibit such decay. However, certain specific processes can be envisioned that would produce strange matter.
It is natural to divide the spectrum of strange matter into 3 categories by size. Each requires a different model. Bulk strange matter is sufficiently large that no surface effects need to be considered. The concepts needed to explain bulk strange matter are useful in more detailed models of `strangelets,' nuggets of strange matter with a baryon number . Very small strangelets would resemble superheavy isotopes of known elements. A model of very small strangelets requires a different approach, as these systems are too small to simply apply a bulk model with surface corrections.
Naturally occuring strangelets may have formed in the early stages of the Universe. Strange matter may also be formed in the present under high pressure in the dense interiors of neutron stars. In order to detect naturally occuring strange matter, or to produce it in the lab, it is necessary to determine its stability. An understanding of the interactions between strange and normal matter is also important.
Several searches have been conducted for natural sources of strangelets, both astronomical and terrestrial. Accelerator experiments have attempted to produce strangelets artificially, and more are planned as new facilities become available. Occasionally, speculations on the properties of and uses for strangelets have resembled science fiction.
Finally, there are many details which must be thoroughly investigated in order to make meaningful specific predictions from the models I will discuss. (For example the choice of renormalization scale.) Rather than attempt this, I will be interested in revealing the general properties of strange matter predicted by these models.
It is not clear that strangelets exist naturally, can be produced, or are stable enough to be detected. Searches have so far not found evidence of them. But while it is not clear that they must exist, they cannot be ruled out. The allowed range of parameters for stability is fairly wide, and the values are not unreasonable. The arguments that motivated the first discussions of strange matter remain valid reasons to expect it. The existence of strangelets would provide possible explanations for some observed phenomena (e.g. dark matter). Fascinating new phenomena would also be available for study. Further theoretical exploration of the possibilities of strangelets is fundamentally limited by the inability to use QCD for detailed calculations of the many-quark system, and by a lack of specific knowledge of the critical parameters. For these reasons, the experimental search must continue. -- Joshua Holden http://www.physics.rutgers.edu/~jholden/strange/node1.html#SECTION00010000000000000000
A strangelet is a hypothetical particle consisting of a bound state of roughly equal numbers of up, down, and strange quarks. Its size would be a minimum of a few femtometers across (with the mass of a light nucleus). Once the size becomes macroscopic (on the order of meters across), such an object is usually called a quark star or "strange star" rather than a strangelet. An equivalent description is that a strangelet is a small fragment of strange matter. The term "strangelet" originates with E. Farhi and R. Jaffe. Strangelets have been suggested as a dark matter candidate .
Strange matter hypothesis
The known particles with strange quarks are unstable because the strange quark is heavier than the up and down quarks, so strange particles, such as the Lambda particle, which contains an up, down, and strange quark, always lose their strangeness, by decaying via the weak interaction to lighter particles containing only up and down quarks. But states with a larger number of quarks might not suffer from this instability. This is the "strange matter hypothesis" of Bodmer  and Witten. According to this hypothesis, when a large enough number of quarks are collected 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.
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 order a few femtometers, so only the outer few femtometers of a strangelet can carry charge .
The surface tension of strange matter is unknown. If it is smaller than a critical value (a few MeV per square femtometer ) then large strangelets are unstable and will tend to fission into smaller strangelets (strange stars are still 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 which 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.
- Cosmic ray impacts. In addition to head-on collisions of cosmic rays, ultra high energy cosmic rays impacting on 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 we might make them at heavy-ion colliders.
At heavy ion accelerators like RHIC, nuclei are collided at relativistic speeds, creating strange and antistrange quarks which could conceivably lead to strangelet production. The experimental signature of a strangelet would be its very high ratio of mass to charge, which would cause its trajectory in a magnetic field to be extremely straight. The STAR collaboration has searched for strangelets produced at the Relativistic Heavy Ion Collider, but none were found. The Large Hadron Collider (LHC) is even less likely to produce strangelets, but searches are planned for the LHC ALICE detector.
Possible seismic detection
In May 2002, a group of researchers at Southern Methodist University reported the possibility that strangelets may have been responsible for a seismic event recorded on October 22 and November 24 in 1993 . 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.
It has been suggested that the International Monitoring System being set up to verify the Comprehensive Nuclear Test Ban Treaty (CTBT) may be useful as a sort of "strangelet observatory" using the entire Earth as its detector. The IMS will be designed to detect anomalous seismic disturbances down to 1 kiloton of TNT's equivalent energy release or less, and could be able to track strangelets passing through Earth in real time if properly exploited.
If the strange matter hypothesis is correct and a strangelet comes in contact with a lump of ordinary matter such as Earth, it could convert the ordinary matter to strange matter . This "ice-nine" disaster scenario is as follows: one strangelet hits a nucleus, catalyzing its immediate conversion to strange matter. This liberates energy, producing a larger, more stable strangelet, which in turn hits another nucleus, catalyzing its conversion to strange matter. In the end, all the nuclei of all the atoms of Earth are converted, and Earth is reduced to a hot, large lump of strange matter.
This is not a concern for strangelets in cosmic rays because they are produced far from Earth and have had time to decay to their ground state, which is predicted by most models to be positively charged, so they are electrostatically repelled by nuclei, and would rarely merge with them.  But high-energy collisions could produce negatively charged strangelet states which live long enough to interact with the nuclei of ordinary matter. 
The danger of catalyzed conversion by strangelets produced in heavy-ion colliders has received some media attention, and 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  concluded that the RHIC collisions were comparable to ones which naturally occur as cosmic rays traverse the solar system, so we would already have seen such a disaster if it were possible. RHIC has been operating since 2000 without incident. Similar concerns have been raised about the operation of the Large Hadron Collider (LHC) at CERN  but such fears are dismissed as far-fetched by many scientists. 
In the case of a neutron star, the conversion scenario seems much 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 so does not electrostatically repel strangelets. 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, creating a quark star.
It should be remembered all the issues discussed above relating to the conversion of ordinary matter to strange matter only arise if the strange matter hypothesis is true, and its surface tension is larger than the aforementioned critical value.
Debate about the strange matter hypothesis
The strange matter hypothesis remains unproven. No direct search for strangelets in cosmic rays or particle accelerators has seen a strangelet (see references in earlier sections). If any of the objects we call neutron stars could be shown to have a surface made of strange matter, this would indicate that strange matter is stable at zero pressure, which would vindicate the strange matter hypothesis. But there is no strong evidence for strange matter surfaces on neutron stars (see below).
Another argument against the hypothesis is that if it were true, all neutron stars should be made of strange matter, and otherwise none should be. Even if there were only a few strange stars initially, violent events such as collisions would soon create many strangelets flying around the universe. Because one strangelet will convert a neutron star to strange matter, by now all neutron stars would have been converted. This argument is still debated, but if it is correct then showing that one 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 nuclear matter. The evidence currently favors nuclear matter. This comes from the phenomenology of X-ray bursts, which is well-explained in terms of a nuclear matter crust, and from measurement of seismic vibrations in magnetars.[