Proving General Relativity with Crystal Balls

Gonzo, the Pirate King of the Underworld writes "It sounds like something out of one of those magazines that you might find at a grocery story checkout stand, but as is typical with news sensationalism, it is a play on words for what is really going on. Researchers at Stanford University, in cooperation with NASA, are preparing an experiment consisting of four extremely precise gyroscopes in the form of quartz crystal spheres. The Relativity Mission will last a year in an attept to measure the effects of frame-dragging and geodetic precession, and give scientists a means of testing General Relativity. "

Comments

Why does Einstein look so bummed?

[seldolivaw]

Why can't we have a picture of a smiling Einstein as the icon? He looks like he's just spent five hours installing NT and it's just flashed up a bluescreen.

Re:Why does Einstein look so bummed?

[esme]

If I'm not mistaken, it's from the famous photograph of Einstein taken by W. Eugene Smith. Smith and Einstein were discussing the ramifications of relativity and the nuclear bomb, and Einstein fell into something of a stupor as he pondered the horrible things that his science had made possible. It was then that Smith took this photo.

I think it would be hard to find a better icon of science: at once brilliant discovery and sobering consequences.

-Esme

Re:Wow!

[Money__]

Re:The manufacturing process of atomic bombs has required a similar degree of accuracy for decades.

40 Atoms? There was nothing in existance in the 40s or 50s to measure such deviations. While I would agree that perhaps 1 or 2 tenths would have been achievable, I doubt very much that +-20 atoms was achievable on anything 30 years ago.
___

The Project That Ate Stanford

[gilroy]

I was at Stanford for grad school back in the early nineties, and GPB occupied a special place in grad student lore. On the one hand, it's sounds really cool and obviously pushes the edge of the envelope on a host of technologies. (Something not mentioned in the article: They had to invent a way to screen magnetic fields, so someone came up with inflatable superconducting "balloons", since superconductors exclude the field within them. After twelve or so layers of balloons, the magnetic field was lower in the test chamber than the field in the great voids between superclusters...)

On the other hand, it seemed that they got a large share of resources for a project that had been in place for thirty (and now nearly forty) years. There are whole dynasties of physicists who have worked on essentially nothing else during that time. I'm not saying it is wrong, exactly, but it was odd to talk to GPB people while struggling to get a grant to keep your lab going for just one more year.

The article fails to mention the extended time that this experiment has been going on. After all, although 13 months sounds like a lot, it's really only 2.5% of the total project time -- well below most probes, I think. We used to joke that the launch date -- which I distinctly remember being announced as 1994 -- slips at a rate of slightly more than one year per year.

It'll be nice when they start getting the results they've been working towards for so long.

Those gyroscopes are cool

[Randy Rathbun]

There was a thing around the first of the year in one of the astronomy magazines talking about the gyroscopes this thing will be using. Apparently they are so precise that once they are spinning if you cut the power the things will continue to spin for the next 4000 years.

To me that is mind blowing....

Re:Gyroscope Design

[Anonymous Coward]

I worked for Lockheed, subcontracting to Stanford
on GPB.

As I recall, the gyros are spun up with streams
of compressed gas.

A couple of methods are used to keep the gyro from touching the walls:
- Charged plates establish an electric field
around the gyro and induce a charge on the gyro.
The E-field is modulated to induce a force on the
charge that serves to move the gyro around.

- The gyro positions are sensed and the spacecraft
actually maneuvers around to avoid hitting the
gyro.

Rotation rate is determined by bouncing light
off of the polished gyros and doing FFT's on
the reflected intensity.

The orientation of the gyros are determined by
measuring the dipole moment of the magnetic field
that comes from the spinning coated gyro. The
magnetic field is measured with SQUID's (Super
Conducting Quantum Interference Devices).

The part I worked on was the temperature
control system for the SQUID brackets; they had
to be stable to about 1uK over DC- 15mHz!

mks@pobox.com (the new user feature is broke!)

Sorta :-)

[tilly]

There are 3 classic tests of GR. One is the precession of the orbit of Mercury. One is the redshift of light coming out of a gravity well. And the last is how much gravity bends light.

Unfortunately the first and third effects are derivable to first-order as a necessary consequence if gravity moves at the speed of light. A German schoolteacher had come up with the first prior to Einstein. (A fact that the Nazis made an unfortunate amount of hay from.) The third was not shown until decades after. But neither of those is therefore a good test since pretty much any realistic theory would be likely to have the same first-order effects.

The second effect is derivable to first order from QM and potential energy. (Particles coming out of a gravity well lose energy, therefore lengthening their wavelength. Voila, red-shift. And it works out right to first order.) So that effect is again not a particularly amazing prediction in retrospect, even though it was when Einstein made it.

Unfortunately we cannot easily test the second-order correction for any of these effects from GR.

So all 3 classic tests actually didn't test as much as was thought at the time.

Cheers,
Ben