[nerd stuff] Proving Relativity

The coolest experiment ever: Was Einstein right about general relativity?

http://www.rednova.com/news/stories/1/2003/11/21/story104.html

[Guardian, The]

Scientists are about to launch the coolest experiment ever, to measure the most intangible thing in the universe - a warp in space- time. The experiment has been 40 years in the making, and it could settle a puzzle that is almost 90 years old: was Albert Einstein right about general relativity? Do massive objects create a gravitational "well" around them into which things fall? Does a spinning planet drag space-time around with it?
To answer these questions researchers have made gyroscopes out of the most perfect spheres ever, and the most exquisitely accurate telescope, sealed them inside a lead balloon in a 9ft-long vacuum chamber that is emptier than outer space, and dropped the whole thing into a flask holding 400 gallons of superfluid liquid helium.

The whole package is now being prepared for launch from Vandenberg air base, California, to orbit 400 miles above the Earth where the telescope will be focused on the dead centre of a distant star. So will the spinning gyroscopes.



After a year if Gravity Probe B, its formal name, performs to specification, and if the universe really does behave according to the predictions of Einstein's theory of general relativity, then the telescope will still appear to be pointing at the right spot. But the gyroscope should have shifted with any changes in local space- time by an amount so tiny that its angle would just about measure the height of a man across a distance of 4,500 miles. If it has shifted, it will confirm theories of space, time, gravitation and the behaviour of stars, which in turn underpin theories about the birth and death of the universe itself.

"This will be a new test of a subtle, twisting effect due to the Earth's spin, as if the planet's rotation sets up a sort of vortex in space that drags objects around. Newton's theory didn't do that," says cosmologist Paul Davies, of Macquarie University in Sydney. "If it does not yield the general relativity value, all hell will break loose."

Einstein's theory of special relativity - the one that begins with the speed of light as a constant that can never be exceeded and leads on to E=mc 2 and a mushroom-shaped cloud over Hiroshima - has been confirmed many times. His general relativity proposals have always been more troublesome. Isaac Newton, 340 years ago, worked out a theory of gravity that got Nasa astronauts to the moon and probes to the edge of the solar system, but Einstein's results in 1904 raised questions about Newton. If indeed the speed of light was limited to 300,000 kilometres a second, how could gravitational forces operate instantaneously on moving bodies across the whole of space?

So, in 1915, Einstein proposed another way of thinking: suppose space (and therefore space-time) was a kind of fabric that would be distorted by massive bodies: the heavier the body, the greater the distortion and therefore the faster other objects would roll down the slope towards it. This would also mean that when a star or a planet rotated, it would drag around with it the nearest region of space-time - and the nearer the object, the faster the drag. Nobody would see this effect. Light would follow the path curved by space- time and so would always seem to shine in a straight path.

Astronomers staged a number of not-entirely conclusive tests of Einstein's theory before 1930. In 1976, Gravity Probe A carried an atomic clock into space and confirmed that time did indeed change as gravity levels grew weaker. But direct proof that a massive object could warp or drag the space- time around it seemed impossible.

However in 1960, Leonard Schiff, one of the team at Los Alamos that tested the first atomic bomb, calculated that a gyroscope spinning in a vacuum and mounted in a spacecraft orbiting the planet's poles at an altitude of 400 miles should show that space- time was being "dragged" by the Earth's mass. The drag would be almost infinitesimal but at the end of a year it would add up to 42 milliarc-seconds. This is the angle thrown by a yardstick over a distance of 3,000 miles, or the thickness of a sheet of paper at a distance of a mile. The angle is so small that in 1960, there was no technology on Earth to measure it in space.

There is now. Gravity Probe B is a collection of superlatives. To detect a warp in space-time, its makers - scientists and engineers backed by Nasa and based at Stanford University in California - have had to eliminate or allow for just about every other heavenly and earthly influence that they know about.

The apparatus has to be kept cool -at 1.8 degrees K or less than 2 degrees above absolute zero. Hence the tank of superfluid helium. Its monitor, a superconducting quantum interference device, has to be shielded from Earth's magnetic field, which means it has to be enclosed in lead. To be sure of their results, the scientists had to make four 38mm [1.5in] gyroscopes out of quartz spheres so perfect that if they were scaled up to the size of the Earth, their biggest imperfection would be only 8ft high. Then they had to be coated with niobium - an element that is superconducting at low temperatures - to create magnetic fields that will move with the gyroscopes and can be monitored on Earth.

The gyroscope rotors must be kept in place in the vacuum by electrical levitation, and stay centred on their target while spinning at 10,000 revs per minute. The pressure inside the probe has to be one hundred million millionths of the Earth's atmospheric pressure: even the so-called "empty space" 400 miles up has more air than that. Its 14in telescope has to be made of fused quartz and must focus on the optical centre of its target star, called HR8703, in the constellation Pegasus, to an accuracy of one 10 millionth of an inch. And it must do these things for at least two years. Frank McDonald, former chief scientist of Nasa, called it "the most challenging experiment that Nasa will perform this millennium".

The cost of Gravity Probe B has been put, unofficially, at $650m (pounds 385m). If it doesn't produce the predicted result, theorists will have to do some serious tweaking to the theory of general relativity.

"The second and more dramatic possibility is that the entire theory would have to be abandoned and replaced by something else that is fundamentally different in conceptual structure. There are alternative theories of gravity waiting in the wings," says Davies. "But my prediction is: general relativity will pass the test with flying colours."

 

LOL

Mrs. Krabappel and Principal Skinner were in the closet making babies and I saw one of the babies and the baby looked at me

[/derail]