# NASA Collects Gravity Data to Test Einstein's Theory

Is Earth ina vortex of space-time?

We'll soonknow the answer: A NASA/Stanford physics experiment called Gravity Probe B(GP-B) recently finished a year of gathering science data in Earth orbit. Theresults, which will take another year to analyze, should reveal the shape ofspace-time around Earth--and, possibly, the vortex.

Time andspace, according to Einstein's theories of relativity, are woven together,forming a four-dimensional fabric called "space-time." The tremendousmass of Earth dimples this fabric, much like a heavy person sitting in themiddle of a trampoline. Gravity, says Einstein, is simply the motion of objectsfollowing the curvaceous lines of the dimple.

If Earthwere stationary, that would be the end of the story. But Earth is not stationary. Our planetspins, and the spin should twist the dimple, slightly, pulling it around into a4-dimensional swirl. This is what GP-B went to space to check

The ideabehind the experiment is simple:

Put aspinning gyroscope into orbit around the Earth, with the spin axis pointedtoward some distant star as a fixed reference point. Free from external forces,the gyroscope's axis should continue pointing at the star--forever. But ifspace is twisted, the direction of the gyroscope's axis should drift over time.By noting this change in direction relative to the star, the twists ofspace-time could be measured.

Inpractice, the experiment is tremendously difficult.

The fourgyroscopes in GP-B are the most perfect spheres ever made by humans. These pingpong-sized balls of fused quartz and silicon are 1.5 inches across and nevervary from a perfect sphere by more than 40 atomic layers. If the gyroscopesweren't so spherical, their spin axes would wobble even without the effects ofrelativity.

Accordingto calculations, the twisted space-time around Earth should cause the axes ofthe gyros to drift merely 0.041 arcseconds over ayear. An arcsecond is 1/3600th of a degree. Tomeasure this angle reasonably well, GP-B needed a fantastic precision of 0.0005arcseconds. It's like measuring the thickness of asheet of paper held edge-on 100 miles away.

GP-Bresearchers invented whole new technologies to make this possible. Theydeveloped a "drag free" satellite that could brush against the outerlayers of Earth's atmosphere without disturbing the gyros. They figured out howto keep Earth's penetrating magnetic field out of the spacecraft. And theyconcocted a device to measure the spin of a gyro--without touching the gyro.

Pulling offthe experiment was an exceptional challenge. A lot of time and money was on theline, but the GP-B scientists appear to have done it.

"Therewere not any major surprises" in the experiment's performance, saysphysics professor Francis Everitt,the Principal Investigator for GP-B at Stanford University. Now thatdata-taking is complete, he says the mood among the GP-B scientists is "alot of enthusiasm, and a realization also that a lot of grinding hard work isahead of us."

A careful,thorough analysis of the data is underway. The scientists will do it in threestages, Everitt explains. First, they will look atthe data from each day of the year-long experiment, checking forirregularities. Next they'll break the data into roughly month-long chunks, andfinally they'll look at the whole year. By doing it this way, the scientistsshould be able to find any problems that a more simple analysis might miss.

Eventuallyscientists around the world will scrutinize the data. Says Everitt,"we want our sternest critics to be us."

The stakesare high. If they detect the vortex, precisely as expected, it simply meansthat Einstein was right, again. But what if they don't? There might be a flawin Einstein's theory, a tiny discrepancy that heralds a revolution in physics.

First,though, there are a lot of data to analyze. Stay tuned.

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