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Strange Stars Collide 
Essay posted by The Centaur ~ June 15, 2002

Fragments Strike Earth at Million Miles an Hour 

In the past ten years science has been on a hunt for "strangeness": physicists made "strange" nuclei,astronomers found "strange" stars, and seismologists detected the trails of "strangelets" ripping through the Earth at just under a million miles an hour. This isn't merely a hunt for the peculiar: on the lips of a scientist, 'strange' means far more than out of the ordinary. Strange refers to strange quark matter , and even though it has been seen only in tiny quantities or on distant stars, there is vast significance to at last finding this special kind of matter flavored with particles not found in the normal fabric of the Earth.

Astronomers Claim Sun In Danger of Going Out!

All the matter on the Earth was produced in the deaths of stars. This modern belief goes against millenia of traditional thought that held that the Sun and stars were perfect, unchanging, and separate from the flawed and inconstant Earth. But military research in the Renaissance into a new device called the telescope revealed spots upon the Sun and mountains on the Moon, and with these discoveries the floodgates opened to a host of nontraditional ideas --- speculations that the Earth might move, the planets might be worlds ... and that the Sun might be burning.

But scientists realized that if the sun burns, then one day it must burn out. Initially scientists speculated that the sun might be literally burning, as if it was giant lump of coal on fire. But burning releases energy by transforming chemical compounds, like carbon and oxygen, into more stable compounds, like carbon dioxide, liberating energy from the electrical field that holds the molecules together in the process. When all available fuel compounds have been transformed, burning stops and all that is left is a cinder of compounds in their most stable, or "ground" states. If the Sun burnt chemically, it would consume all its fuel in only a few thousand years, not even enough to light the skies for the duration of recorded history.

Then scientists developed a greater understanding of gravitation, and speculated that the Sun was lit by its own tremendous gravity as it collapsed in on itself. There is potential energy in gravity: a falling object is just another system seeking its most stable ground state, extracting energy from the gravitational field in the form of increasing speed. But an object falling onto itself, such as a collapsing cloud of gas and dust, cannot continue to accelerate: its substance collects and resists the collapse, turning kinetic energy into heat. The heat generated by a dust cloud large enough to collapse into a star is tremendous, more than enough to make the Sun shine: but it could only heat the Sun for a few million years, far less than the geological history of the Earth. Clearly a hotter fire was required.

This fire was nuclear fusion.

Scientists Say Sun Essentially a Giant Fusion Bomb!

Like chemical reactions and gravitational collapse, nuclear fusion generates energy by transforming matter into more stable states. But in fusion, energy is liberated not from the electrical field that makes atoms lump into molecules or the gravitational field that makes molecules lump into planets, but from the nuclear force that holds atoms together in the first place. The discoveries of particle physicists at the dawn of the 20th century revealed that the vast variety of chemical elements which make life interesting, not to mention possible, are all due to the immense strength of the nuclear force binding protons and neutrons together in the core of the nucleus --- and like electrical and gravitational fields, energy is released as more and more particles agglutinate into larger and larger nuclei. The vast amount of energy released from fusing hydrogen to helium alone is enough to light a star for billions of years.

Stars Die in Terrific Explosion - All Life On Earth at Risk! Understanding this - understanding that the Sun is lit by its vast mass crushing its tiniest components - was a crucial first step on a path to marry our understanding of the very very large and the very very small. And just as particle physics provide the first hints about how stars might live, so it gives the first clues about what happens when stars die. For stars do die. Sooner or later, the usable hydrogen in the core of a star will run out. Fusion can proceed for a while by agglutinating matter into heavier and heavier nuclei, but this process, too, is ultimately limited: the electrical repulsion of protons resists their nuclear attraction, reaching a balance at Iron 54 - the most stable element in the universe, the so-called "ground state" of normal matter. When this point is reached fusion continues, creating heavier and heavier nuclei - but now the reactions steal heat from the core of the star, causing it to shrink in on itself and rapidly collapse, releasing enough energy to blow the outer shell of the star off in an explosion called a supernova.

The death of a star is spectacular, a display of interstellar fireworks visible thousands of light years away and leaving colorful, intricate clouds of gas which remain visible for centuries - as in the Crab supernova, first seen in broad daylight by Chinese astronomers in 1054 and still visible through telescopes a thousand years later as a beautiful nebula. But astronomers can tell that within this tracery of gas the core of the star still remains: a small, dense cinder no longer supported by fusion. For a small star, this core is simply normal matter, a giant pile of atoms compressed to an ultradense state by its own gravity, a "white dwarf" slowly cooling over time. But in the case of the Crab supernova, the star was too heavy: the force of gravity ultimately overcame the electrical field supporting the atoms and caused them to collapse.

Atoms Collapse! Democritus Spins in Grave. The very idea of the collapse of an atom seems to contradict Democritus' original definition of atoms as the fundamentally indivisible building blocks out of which matter was made. This "atomic hypothesis" was qualitative and approximate - and impossible to prove with the techniques available to the Greeks - and so fell in and out of favor over the next two thousand years, until finally the atomic idea was refined enough to gain predictive power - first at the hands of chemists explaining chemical reactions in terms of combinations of different chemical elements, then at the hands of Einstein explaining the jiggling of small objects in a liquid in terms of atomic bombardment. But as physicists studied these tiny objects they labeled in Democritus' ancient language, they found they were not indivisible or immutable: atoms had dense positive cores surrounded by hazes of negative charge; that negative charge could be knocked away from the core in the form of free electrons; and even the cores themselves could be blasted apart or could fly apart on their own into smaller, "subatomic" particles.

Understanding that atoms were made of protons and neutrons bound together by a strong nuclear force was the key to understanding fission and fusion, but physicists soon found that the subatomic particles themselves could split, fuse or change form. Just as chemists found a way to explain the transformation of physical substances in terms of combinations of the chemical elements, soon physicists found a way to explain the transformation of subatomic particles in terms of even more fundamental particles, a new kind of chemistry based on the interaction of particles called quarks .

Quarks are odd, cliquish folk: they have fractional electric charges,and are bound together so tightly that if you try to rip them apart, the energy required will spontaneously create new quarks from empty space - thus ensuring that quarks are always safely packaged into the groups with integral charges that we recognize as subatomic particles.  In the "standard model" there are six different kinds of quarks: up and down, strange and charm, and top and bottom - but only the least massive up and down quarks normally appear. According to this model, the bulk of visible matter in the universe is combinations of up and down quarks, surrounded by a sea of photons and electrons.

This standard model enabled Chandresakar to predict in a too-massive star, gravity would force the protons and electrons in its atoms to collapse and merge into neutrons, leaving a small, dense object only a few miles across made entirely of pure neutrons in the form of neutronium. If a star was still heavier, not even the strong nuclear force could overcome gravity's pull, and the neutrons themselves would collapse on each other - perhaps creating a "gravastar", a thin shell of matter surrounding a region where gravity's pull becomes so distorted that it becomes repulsive, or perhaps even creating a "black hole" where the matter itself collapses into a singularity - a "single point" where the mathematics behind physics breaks down.

Strange Quarks Infect Normal Matter The controversy surrounding the ultimate fate of superheavy stars is a tale we will tell another time; for the purpose of our discussion, neutron stars are the end of the line - in more ways than one. Neutronium is the end of the line for normal matter: within a deep gravity well, neutronium is effectively the ground state, the most efficient way to package any material under extreme pressure. But neutronium is not the ultimate ground state: if we take the pressure away, we find that for free matter Iron 54 is the ground state - the most energy-efficient, least-massive way to package up and down quarks into a given number of subatomic particles.

But what about matter made of other kinds of quarks? Quarks can change form from one to the other if the energy is right, and tantalizingly, matter made of three kinds of quarks is potentially more energy efficient than normal two quark matter - if it was not for the cost of the quarks themselves. Top, bottom and charm quarks are all massive - too hard to make from up or down quarks, and too massive to make stable matter: like a boulder perched on a stack of pebbles, matter made of heavy quarks is all too likely to spontaneously disintegrate into configurations of lighter quarks. But strange quarks break this pattern: they are not much heavier than up and down quarks - close enough for up or down quarks to spontaneously transform from one to the other. This form of matter, incorporating up, down, and strange quarks, would be more stable than neutronium - and, perhaps, even more stable than Iron 54. If some scientists are right, strange quark matter is the true "ground state" of matter - the ultimate cinder that all matter strives to reduce itself to.

However, there's no need to worry that your bagel will suddenly collapse into strange quark matter if you leave it in the microwave too long (which is a good thing; the energy released would vaporize everyone in the kitchen in the process). In normal matter, an up or down quark that changes into a strange quark is just as likely to change right back. To be stable, a substantial proportion of quarks in an atom would have to change to strange quark matter all at once, and that is unlikely. So it is difficult to make strange quark matter out of normal matter in precisely the same way that it is difficult to make graphite out of diamond: graphite is a more efficient way to package carbon than diamond, but once a diamond is formed the strength of its bonds makes it unlikely to shift into the form of graphite, making it effectively stable.

For neutronium, however, introducing a strange quark leads to a different fate. Neutronium blurs the distinction of separate subatomic particles found in normal nuclei - and a "bag" of three kinds of quarks is far more efficient than a bag containing only two. If enough strange quarks form in a neutron star, the process will not stop: with a vast release of energy, the entire body of the star will convert from neutronium into strange quark matter, leaving a strange quark star. What's more, once formed strange quark matter is completely stable: a nugget of strange quark matter knocked off a strange quark star would remain strange quark matter, appearing as a "strangelets" that act like normal chemical elements or subatomic particles but mass hundreds or thousands of times greater.

This story sounded so outlandish that scientists didn't initially take it completely seriously. The theory behind strange quark matter is not merely difficult, it is approximate: the best models available can only sort of predict what strange quark matter would be like. Worse, there was no experimental way to test the theory: scientists have no way to convert the bulk of the quarks in an atom into strange quarks, and no samples of neutronium to attempt a more direct transformation. So scientists refined their models, trying to predict where we might see evidence of strange quark matter - to predict how a strange quark star might differ from a neutron star, or what it might look like if a strangelet impacted the Earth.

Impact in Antarctica! And the evidence was found.  It was just traces at first - scientists made "strange" nuclei, containing one or two strange quarks. These strange nuclei were stable for only the briefest instants, but were tantalizing hints that strange quark matter was possible. Then, astronomers found evidence of "strange" stars: stars that looked like neutron stars, but were smaller and colder than expected for their age and mass. This was more promising: strange quark matter is potentially denser than neutronium, and may cool more rapidly. Other astronomers disputed these results, but this was the first hint that strange quark matter was not just a transient laboratory curiosity, but a stable form of matter. But even if strange stars existed and strange quark matter was stable, it would remain an astronomical curiosity if strange quark matter was only stable under the fantastic pressure of a collapsed star.

But if strange quark matter is stable ... really stable ... then it is far more than an astronomical curiosity, even if it was only made in the deaths of stars.  Statistically, if strange quark stars exist then eventually two will collide. This would be rare - stars do not often collide, and a collision between strange stars or a strange star and a neutron star would be rarer still. But when it happens, it would be lethally spectacular, sending out gravity waves and gamma rays ... and fragments of strange quark matter travelling close to a million miles an hour.

If you were struck by one of these tiny particles, you would find it packed as much punch as a Volkswagen traveling at a tenth of a percent of the speed of light. That would be quite a fender bender if all that mass was packaged as a Volkswagen - but in a strangelet, the same number of quarks would be compressed to the size of a grain of dust. In that collision, the strangelet wouldn't just go through you - it would go through the person behind you, the wall behind them, and even the mountain range behind that. In fact, strangelet striking a planet would barely be slowed down: the sharp explosion at its point of impact would be followed almost immediately by another explosion as it burrowed its way out the other side.

A planet such as the Earth, for example.

Scientists studying earthquake data have found precisely this telltale hint of a strange quark matter strike: matching pinpoint impacts on opposite sides of the Earth, separated by less than half a minute - precisely the "entry-exit" signature one would expect if the Earth was struck by a fast-moving strangelet. It's hard to say for sure - the offender didn't stick around to give us his license number - but the Earth may have been a victim of a hit and run by strange quark matter.

Scientists Create Miniature Big Bang! All this evidence is persuasive, but not conclusive: after all, there could be other explanations. Our stellar measurements could be off, or our seismographs mistaken. But enough independent evidence has accumulated for scientists to start taking strange quark matter seriously. If strange quark matter exists in the corpses of stars and skips through the spaces between them, could we make it right here on the Earth? Surprisingly, attempts have already been made for another purpose - the attempt to detect "quark-gluon plasma" in particle accelerators. Scientists believe that near the beginning of the universe all matter was hotter than the core of the sun and denser than the heart of a neutron star. In this intense, violent state, quarks did not clump into particulate matter (of either two-quark or strange quark varieties) but existed as an undifferentied mass of quarks in a sea of "gluons" binding the quarks together. In this quark-gluon plasma, the laws of physics would be very different from what we experience everyday, and so studying this plasma will help us determine whether our understanding of matter is correct.

Scientists at Brookhaven rammed heavy atomic nuclei at each other at close to the speed of light in an attempt to create this plasma; later, scientists at CERN tried the same thing and claimed they succeeded. Work continues to analyze this data, but if they were successful the creation of this quark-gluon plasma could be the first step in making a sample of strange quark matter large and stable enough to be captured and manipulated. And a seed of strange quark matter could be used to create more strange quark matter - growing by accreting normal matter with a tremendous release of energy in the process. If this is possible, strange quark matter one day could be a new energy source, far more efficient than burning, fission or fusion.

Statistically Challenged March on Brookhaven! Many people were disturbed at the Brookhaven experiment - disturbed by the possibilities inherent in this unusually energetic experiment. Some worried that strange quark matter would be generated and would consume the crust of the Earth in a tremendous explosion. Others worried that the impacts would create microscopic black holes that would fall into the Earth and eat away at its core. And perhaps most improbably, some worried that the impact might cause a "mini Big Bang", again ending all life. The slightest possibility of these dangers led some to question why anyone would attempt such at thing at all.

But what these questioners do not realize is that if such a thing as strange quark matter is possible, then the laws of physics and probability dictate that the Earth must be interacting with strange quark matter all the time. The Brookhaven experiment was unusually energetic only by human standards: in our upper atmosphere Nature regularly does similar "experiments" vastly more powerful without the benefit of safety goggles or OSHA standards. Particles thousands of times more energetic than any produced in any human particle accelerator called "cosmic rays" strike the Earth daily, showering radiation down upon the surface of the Earth without ever recreating the Big Bang, producing microscopic planet eaters, or otherwise wiping out all life.

Even if we believed that strange quark matter only forms in neutron stars, Earth would not be safe from strange quark matter if Man turned off all his particle accelerators. As we said  befofre, if strange stars form, then one day strange stars will collide, spattering nuggets of strange matter across the sky - in a distribution that scientists can guesstimate.  The tantalizing possibility of finding strange matter right here on Earth that led scientists to those telltale dual signatures in earthquake records also led scientists to look for smaller strangelets, which would act like superheavy atoms - and some scientists believe that they've found these too.

Raiding the Strange Corpses of Stars So perhaps the best bet for studying strange matter is not to try to create it in the laboratory (thus needlessly worrying those with a weak grasp of the laws of probability) but instead to look for strange matter in nature. Many of the discoveries I have mention occurred during the writing of this very article. The hunt for strange matter has been going on for years, but at times it still seems that physicists had no sooner realized that strange matter might exist than they began to find it everywhere.

Regardless, now that we believe strange matter exists, the next step is to capture a sample for study. Who knows where it might be found: whether an strangelet made in an an accelerator, or a superheavy atom winnowed out of normal matter, or a nugget of strange matter dug from the core of an asteroid, strange quark matter holds the potential to change our world. If it behaves as scientists predict, it could serve as an amazing source of power. Perhaps more importantly, whether it behaves as scientists predict will put to the test our most fundamental theories of matter and energy. Strange quark matter may confirm everything we think we know - or throw our entire knowledge of the subatomic world in doubt ... perhaps opening vistas on hitherto unforseen possibilities that will make strange quark matter seem ... normal.

-The Centaur
Renaissance Engineer

Sighting SQM

- Strange Stars I
- Strange Stars II
- Strange Impacts
- Strange Atoms

Making SQM

- At Brookhaven...
- The Fears
- Debunking Them
- More Fears
- More Debunking
- The Results
- Quark-Gluon Plasma at CERN
- The Results at CERN


- Neutron Stars
- Strange Stars
- Gravastars
- Black Holes
- Or, Take a Trip


- Strange Quark Matter
- The Standard Model
- More Readings
- Or, Hurt Your Head

The Library of Dresan
The Library of Dresan ~ (C) Copyright 2002 Dr. Anthony G. Francis, Jr. ~ All Rights Reserved
Writer / Artist / Producer: Anthony Francis