



A group of researchers have identified two seismic events that they think provide the first evidence of a previously undetected form of matter passing through the Earth.



We can't prove that this was strange quark matter, but that is the only explanation that has been offered so far

Eugene Herrin

The so-called strange quark matter is so dense that a piece the size of a human cell would weigh a tonne.

The two events under study both took place in 1993.

Other scientists are tantalised, saying that while these seismic disturbances are unlikely to have been caused by strange quark matter, they do not as yet have alternative explanations.

Out of the fireball

Strange quark matter could have arisen after the Big Bang, according to a theory by physicist Edward Witten of the Institute for Advanced Study in Princeton, US.

The primordial fireball may have produced dense, heavy particles made of three types of quarks, which are fundamental particles.

Whereas so-called "up" and "down" quarks form protons and neutrons, the addition of "strange" quarks might result in a stable form of matter that could grow far more massive than ordinary atoms.

There is some evidence that strange quark matter does exist in the cosmos. In April 2002, two different teams of scientists reported that they had identified collapsed stars that might be composed of the ultra-dense material.

In 1984, Harvard physicist and Nobel Laureate Sheldon Glashow suggested that physicists should team up with seismologists to search for traces of the strange matter that might have passed through the Earth at supersonic speed.

'Unassociated events'

He calculated that strange quark particles would dash through Earth with dramatic effect: a one-tonne spec would release the energy of a 50-kilotonne nuclear bomb, spread along its entire path through the Earth.

In 1993, Vidgor Teplitz, Eugene Herrin, David Anderson and Ileana Tibuleac, all of the Southern Methodist University in the US, began looking for such events.

They searched the world's seismographic records for so-called "unassociated events". They looked at more than a million records collected by the US Geological Survey between 1990 to 1993 that were not associated with traditional seismic disturbances, such as earthquakes.

Previously, Herrin and Teplitz speculated that it would be possible to search for seismic events that might indicate passage of strange quark matter (also known as nuclearites) through the Earth because such events would have a distinct seismic signal - a straight line.

This seismic signature would be caused by the large ratio of the nuclearites speed to the speed of sound in the Earth. It was estimated that the strange quark matter might pass through the earth at 400 km per second (250 miles per second), 40 times the speed of seismic waves.

Data collection halted

The team also determined that the minimum requirement for detection of a nuclearite would be detection of its signal by seven monitoring stations.

The researchers latest findings single out two seismic events with the linear pattern they were looking for.

In two cases, the arrival times and forms of seismic waves at nine far-flung stations pointed to linear bursts of energy. The ruptures ripped through the planet at hundreds of kilometres per second rather than fracturing only near the surface, as typical earthquakes do.

One event occurred on 22 October 1993, when, according to the researchers, something entered the Earth off Antarctica and left it south of India 0.73 of a second later.

The other occurred on 24 November 1993, when an object entered south of Australia and exited the Earth near Antarctica 0.15 of a second later.

The first event was recorded at seven monitoring stations in India, Australia, Bolivia and Turkey, and the second event was recorded at nine monitoring stations in Australia and Bolivia.

"We can't prove that this was strange quark matter, but that is the only explanation that has been offered so far," Herrin says.

Unfortunately, scientists may not be able to find any more events that suggest the passage of strange quark matter through the Earth.

In 1993 the US Geological Survey stopped collecting data from "unassociated events."