World's biggest microscope to peer into the Big Bang

Scientists have completed an extraordinary instrument designed to follow pieces of atoms traveling at nearly the speed of light. The device will become part of one of the largest machines ever constructed and will be used to investigate questions reaching back to the earliest moments of the universe.

The final one of four cylindrical measuring units made from carbon fiber and silicon was assembled at Oxford University and sent to CERN, the European nuclear research center in Geneva. The components were manufactured to micrometer-level accuracy, with measurements precise to millionths of a meter.

At CERN, the devices will be installed inside a huge detector positioned within a tunnel approximately 17 miles long and about 100 meters beneath the ground. The project forms part of an international effort to recreate conditions that existed when the universe was roughly the size of a grapefruit. Nearly every aspect of the experiment presents a new technical and scientific challenge.

Engineering at an extraordinary scale

Oxford physicist Tony Weidberg is part of the international group developing the semiconductor tracker located at the center of the ATLAS detector, which will itself form a crucial component of the Large Hadron Collider experiment. The collider was expected to begin operating at CERN in 2007.

Weidberg explained that engineers routinely construct enormous buildings, while other specialists work with microscopes and measurements at the scale of microns. What makes this project unusual is the requirement to maintain micrometer-level precision across a cavern large enough to accommodate something comparable in size to Canterbury Cathedral. Achieving that combination of scale and accuracy represents one of the project's many engineering difficulties.

Pushing particle physics to new extremes

The Large Hadron Collider is built around a technique physicists have refined for roughly four decades. Scientists accelerate extremely small pieces of matter to speeds approaching that of light, collide them with other particles, and then examine the resulting showers of subatomic fragments.

The Large Electron Positron Collider was the most recent major experiment before the new machine. It brought matter and antimatter together, allowing researchers to break apart the structure of atoms and investigate how that structure was assembled. The enormous volume and speed of information generated by these experiments even contributed to the creation of the World Wide Web by a CERN scientist as a way to distribute scientific data.

Yet some of the most fundamental questions remain unanswered. Scientists want to understand the true nature of matter, determine why matter possesses mass, and explain why it has the structure observed today. Addressing these mysteries requires recreating conditions from an extraordinarily early stage of cosmic history, when the universe was less than a billion billion trillionth of a second old.

Recreating the earliest universe

To approach these conditions, the scientists planned to accelerate protons, the positively charged particles found at the center of atoms, to 99.99999% of the speed of light. They would then collide these particles with other protons traveling in the opposite direction at nearly the same speed.

Collisions would occur every 25 nanoseconds, equivalent to 40 million collisions or interaction intervals per second. The process would generate roughly a billion rapidly moving fragments of raw matter every second.

Hidden among this enormous amount of debris could be rare and extremely short-lived phenomena that have not naturally appeared since the earliest moments following the Big Bang. Discovering these particles or effects could provide evidence supporting existing theories or force physicists to reconsider some of their most established ideas. The central difficulty is identifying these unusual events among the overwhelming number of ordinary collisions.

Inside the ATLAS detector

One of the primary instruments responsible for searching through these collisions is the ATLAS detector, one of the largest scientific instruments ever constructed. The detector weighs approximately 7,000 tonnes, measures about 44 meters in length, and rises roughly 22 meters high.

At its center is the semiconductor tracker. It contains around 30 million silicon-strip channels arranged within carbon-fiber structures. These components must tolerate radiation exposure equivalent to approximately one billion chest X-rays.

The tracker operates at temperatures below freezing and depends on an extremely advanced cooling system. Cooling tubes only about 70 microns thick are used to remove approximately 40 kilowatts of heat generated by the detectors. The entire structure must also remain highly stable, with the silicon channels positioned to an accuracy of about 10 millionths of a meter.

A giant three-dimensional puzzle

Maintaining such precision throughout assembly is itself a major undertaking. Weidberg described the detector as an enormous three-dimensional jigsaw puzzle. Its components are assembled in layers, with one detector section fitting inside another in a configuration resembling a set of Russian dolls.

The integration process was expected to continue through the end of the year. Once assembled, the tracker would remain inside another detector structure at the surface for additional testing. The following March, it was scheduled to be lowered into the underground facility and moved into its permanent position.

A microscope for the smallest pieces of matter

In a sense, the scientists have created the world's largest microscope in order to search for some of the most elusive particles and phenomena in the universe.

The need for such a massive machine may initially seem surprising. Weidberg pointed out that investigating new physics requires particle accelerators capable of reaching increasingly higher energies. Increasing the accelerator's power produces particles with greater energy, but those more energetic collisions also generate larger and more complicated showers of debris.

As a result, higher-energy beams require not only a more powerful accelerator but also a larger and more capable detector. The detector must be able to contain the resulting particle activity and accurately record what happens during each collision.

Searching through an overwhelming stream of data

The detector will follow the paths taken by particles produced in every collision, determine their momentum, and identify roughly 100 potential fragments or events that could represent physics beyond current understanding.

Separating these rare signals from the enormous amount of ordinary collision data is an immense technical challenge. The detector must perform this process continuously because a new collision occurs every 25 nanoseconds.

Light itself travels only about one foot during a nanosecond, while the detector is considerably larger than that distance. Because no information can travel faster than light, the system cannot simply wait for information from every part of the detector before deciding whether a particular collision is worth recording.

The challenge was compared to trying to locate a single needle in a haystack while traveling on a train moving at 100 miles per hour. The experiment therefore depends not only on the enormous size and precision of the detector, but also on extremely sophisticated systems capable of identifying potentially significant events almost instantaneously.