Gravity Probe B was launched in 2004 Nasa’s Gravity Probe B has produced remarkable new confirmation of some key predictions of Albert Einstein.
The satellite’s observations show the massive body of the Earth is very subtly warping space and time, and even dragging it around with it.
Scientists were able to see these effects by studying the behaviour of four perfectly engineered spinning balls carried inside the probe.
The results are published online in the journal Physical Review Letters.
“We’ve completed this landmark experiment, testing Einstein’s Universe – and Einstein survives,” said Francis Everitt, the mission’s principal investigator at Stanford University.
Gravity Probe B was launched in 2004, but it has taken seven years for researchers to assess the data and to be sure of their observations.
Part of their difficulty has been in showing that some fantastically small deviations are real and not biases introduced by flaws in the experimental set-up.
Gravity Probe B was looking to confirm two important consequences stemming from Einstein’s Theory of General Relativity that describe the way space and time are distorted by the presence of huge objects such as planets and stars.
One, known as the geodetic effect, is the amount by which the Earth will warp the local space-time in which it resides.
The other, which physicists refer to as frame-dragging, is the phenomenon that sees the Earth drag local space-time around with it as it rotates.
Gravity Probe B sought to observe both these effects by measuring tiny drifts in the spin axes of four gyroscopes relative to the position of a star called IM Pegasi (HR 8703).
To ensure accuracy, the balls had to be chilled to near absolute zero and flown inside a vacuum flask to isolate them from any disturbance.
The quartz gyros (left) were coated with niobium (right). They were described as the most perfect spheres ever engineered Over the course of a year, the anticipated drift in the spin axes of the balls due to the geodetic effect was expected to reveal itself on the scale of a few thousand milliarcseconds. The frame-dragging effect was predicted to be even smaller.
“A milliarcsecond is the width of human hair seen at a distance of 10 miles. It really is a rather small angle, and this is the accuracy Gravity Probe B had to achieve,” explained Professor Everitt.
“For the geodetic effect, the predicted relativity effect is 6,606.1 of these milliarcseconds, and the measured result is a little over a quarter of a percent of that. The frame-dragging we’ve measured to a little better than 20%.”
The idea of the mission was first proposed in 1959, but the project had to wait until the technologies to carry it through could be invented.
“GP-B, while conceptually simple, is technologically an extremely complex experiment,” said Rex Geveden, the president of Teledyne Brown Engineering from Huntsville, Alabama.
“The idea came about three to four decades before the technology was available to test it. Thirteen novel technologies were created for GP-B. The quartz balls were thought to be the roundest objects ever manufactured. The diametric variation across the spheres is about two-tenths of a millionth of an inch.”
Some 100 students achieved their PhDs by working on some aspect of the mission during the many years it took to develop, build and then fly the probe. Most of these PhDs were earned at Stanford, and at the universities in Huntsville; and in Aberdeen, UK.
More than 350 undergraduate students also worked on GP-B including one who later became the first female American astronaut in space, Sally Ride. Another was Eric Cornell, who won the Nobel Prize in Physics in 2001.
1. The spin axes of the gyroscopes were initially aligned with a guide star. The gyroscopes were then monitored for changes in their angle of spin caused by general relativity effects2. The disturbance due to frame-dragging was expected to cause the spin axes of the super-smooth gyroscopic spheres to change by an angle of just 0.041 arcseconds per year3. For the geodetic effect, the spacecraft expected to see a bigger signal – for the gyroscopes’ spin axes to change by an angle of 6.6 arcseconds over a year of observations4. Gravity Probe B’s gyroscopes were held inside a vacuum container5. The US space agency (Nasa) satellite was launched on 20 April, 2004
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