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:Why does Einstein look so bummed?

[gad_zuki!]

He's still angry about quantum mechanics. Nice guy, but can sure hold a grudge.

This one is guaranteed to succeed

[Brento]

Here's an overview of the mission: Nasa is putting four gyroscopes up in orbit for a year. If, after a year, they end up pointing in a slightly different direction, then they believe they've proven the theory of relativity. (Omitting a lot of the details here.)

Here's the catch: this relies on Nasa designing four absolutely perfect gyroscopes. A quote from the site:

"We've tried very hard to design an absolutely perfect gyroscope," said Dr. Francis Everitt, the Principal Investigator at Stanford University. Even in an age of exquisite measurements, nothing is perfect. The GP-B gyros, though, are about as close as humans can get. The gyros and their support system are so precise that non-relativity effects will cause them to drift by no more than 1/3 milli-arc-second during a year.

So basically, if the gyros were NOT made perfectly, they will drift. Nasa making something that isn't perfect is pretty well a guaranteed bet these days. That leads me to predict that in 2003, when the year is over, Nasa will be celebrating jubilantly that they've "proven" the theory of relativity. Whoop-dee-doo.

Re:This one is guaranteed to succeed

[ZanshinWedge]

It doesn't work that way. The gyros are supposed to drift. They will be pulled by a very very slight effect called frame dragging (which occurs due to the rotation of a massive body). The precise amount of the drift of the gyros from frame dragging has been calculated and if the end result fits that amount within the expected error margin, then it will be another confirmation that Einstein's theory of relativity (and, for example, not some other forumulation of relativity that is very close) is most likely correct. If the data shows something else, then it means we gots some old fashioned new science on our hands.

Re:This one is guaranteed to succeed

[QuantumET]

I worked on this project last summer (granted, it wasn't exactly at the high levels) but I picked up a decent amount of background information.

First, the gyros are tested extensively, for smoothness and consistency. They're not that hard to test, really. The hard part was making the darn things. If you blew up one of the gyros to be the size of the earth, the largest height difference would be around 16 feet. They're smooth.

Besides, NASA didn't make these. Stanford's laboratories did, if that makes any difference. The whole project is an effort between Stanford and Lockheed (with NASA funding, to be sure).

And 4 gyroscopes is for redundancy. They only really need two, I believe, and the gyros are not measured off each other. Instead, they are all referenced to a guide star which is tracked by a telescope on satellite.

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.

Re:The Project That Ate Stanford

[Animats]

Yeah. I remember GPB from the mid-80s. A friend of mine did some of their machine shop work. Some of the GPB people were talking about using genetic algorithms as part of the data reduction process, because the signal to noise ratio of their data is so low. That bothered me; if they're working that close to the noise, the validity of the results is iffy.

I notice that NASA still hasn't announced a launch date.

Re:The Project That Ate Stanford

[gilroy]

Quoth the poster:

You mentioned many of the other strugles the project had to overcome, was the fabrication of the spheres to such a tight tolerance among these challanges?

Disclaimer: I never worked on GPB, I just knew grads in my department who did. And I sat through a handful of colloquia on the subject.

Making the gyros to the right tolerance was definitely one of the big challenges. I think they did some pioneering work in computer aided design and fabrication because they needed such precision. Making quartz pure enough was a big problem for the chemists on the project. Designing a bus vehicle that would cushion the gyros enough so that the launch didn't damage them consumed a lot of effort, too, I think. They also advanced the state of the art in cryonics, because they needed a stable, light, small system.

My earlier comment might have sounded like I didn't like GPB. Actually, I've generally been impressed with them. If you ever want to see how "basic research" can benefit the larger economy, look at the diverse areas of research needed for GPB to fly. They have "spin-off" written all over them.

Unclear on the scientific method are you?

[tilly]

They have a clear estimate of the actual error tolerances they believe that they are achieving. They likewise have a clear estimate of the exact drift they expect to see.

If they mess up on the first the odds of their accidentally getting a measurement because of that agrees with the second is miniscule (to say the least).

So while right now the best bet is indeed that they will confirm Einstein, this is by no means a sign of incompetence or a foregone conclusion.

Sincerely,
Ben

Re:Unclear on the scientific method are you?

[gilroy]

Quoth the poster:

You have to agree that any incompetence involved with fabricating the gyros will be a sign in their favor, unlike the other missions

Um, no, you don't have to agree to that at all. You see, there's this little process called "calibration", which will allow the mission scientists to quantify how precise and how stable the gyros are. I'm not sure what the procedure will be, but you can bet they'll check these things in a way that minimizes or negates any real signal. Then, if there's drift, they'll know it comes from fabrication error or other noise.

I don't know if the poster is a scientist or not, but that kind of misconception is very common. Nothing is made perfectly, so if science depended on that, we'd all be in trouble. Science is not about what you know -- it's about quantifying what you don't know. The important thing is never the number by itself but the number with its error bars.

Indeed, one complaint levelled at GPB over the years is that their signal-to-noise ratio is so low, it'll be hard to believe anything that comes out. Personally I believe they've got a handle but honest opinion, for now, can easily differ.

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....

Gyroscope Design

[Detritus]

Does anyone have any information on the design of the gyroscopes? They are crystal spheres, spinning in a vacuum. How do they spin them up, keep them from floating into the side of the enclosure, measure the rotation rate and direction of the rotational axis?

The Stanford web site appears to be broken.

Re:Gyroscope Design

[Boli]

Way back in high school (summer 1986), I was fortunate enough to tour some of the GPB labs and talk to a grad student or two. One of them wore a rugby outfit ('cause he just got back from practice) and explained how his PhD was based on solving this problem: When a perfectly spherical metal-plated ball is spinning at a few 1000 rpm in a vacuum, static electricity between the chamber & ball will pit the surfaces. Thus the perfect gyroscope is damaged and the whole experiment is trashed. How can you prevent the static discharges?
He mentioned that they rev-up the gyros using bursts of nitrogen gas. The rest of the satellite is designed to follow the gyro in orbit. So whereever the gyro wants to travel, the satellite moves to keep the zero-particle/zero-EM field centered around the gyro. Thus eliminating all forces except gravity. One of us asked if the gravity of the satellite would affect the gyro. "Yes" was the answer. "The gyro must remain at the center-of-mass of the satellite..."
I think the measurements were performed by bouncing lasers off the gyros.

- boli

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!)

Re:Gyroscope Design

[QuantumET]

Worked at GPB last summer, wrote some code for the systems that keep the gyros levitated.

The gyros are free-floating quarz spheres, coated with a layer of niobium. They are surrounded by four electrode plates, which keep the gyros centered in their housing in case of micrometeorite, etc impacts. (they can handle up to about 1 kg*m/s of impulse)

The gyros are spun up by running high-velocity helium gas by them (helium because there's a lot of it onboard, the whole assembly is cooled with liquid helium) to a speed of around 10,000 rpm. This is a one-time thing, they can't respeed them up (would ruin the data)

Because of the liquid helium cooling, the niobium will superconduct, and a spinning superconductor creates a magnetic field precisely aligned with its spin axis. This is measured by a SQUID (Superconducting Quantum Interference Device)

I don't know if they measure the spin rate or not.

The satellite follows a free-floating proof mass in its center to maintain a pure gravitational orbit, so the gyros won't drift offcenter much, unless there are micrometeorite impacts or other forces.

Re:NASA??

[HeghmoH]

Two days ago.

Jerk.

I could have sworn a similar expirement has al...

[tcd004]

ready been done with atomic clocks.

tcd004

Here's my Microsoft Parody, where's yours?

"Proving Einstein" is a little misleading

[Ledskof]

The experiment will give reasonable proof whether Einstein's general relativity elements of space time distortion and so on exist, but it does not prove all of general relativity the way that everyone including that space news site are talking about. This experiment will not disprove a current belief among many scientists that Einstein is wrong in the areas of his physics that the math blows up and creates black holes, infinite masses, and so on. Einstein puts a speed limit on the universe, but he doesn't put a limit on his physics. Here's a good article to read: http://mist.npl.washington.edu/AV/altvw100.html this article talks about a modified version of GR. I think it was posted on slashdot recently. Also, this modified version of GR is quantizable, which Einsteins GR is not.

Re:Don't they already have a way?

[gilroy]

No, actually, that was true.

Of course, I am a physics teacher , too, so you might lump me in on the conspiracy. :)

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

Re:I could have sworn a similar expirement has al.

[coyote-san]

IIRC, the atomic clock experiments demonstrated that time passes slower on the surface of the earth than in orbit, and by the factor predicted by GR. This theory, taken to extremes, is why black holes are "black" -- the time dilation is so severe that that *all* wavelengths are red-shifted into oblivion.

"Frame dragging" is a far more subtle effect that says that a rotating mass will actually "drag" spacetime around with it. That means that a full circle is less than 360 degrees if you go in the same direction as the rotation. This effect is far more subtle than time dilation, and far harder to measure.

To test for this effect, you set something pointing at a known distant point, let it orbit once, then measure the angle it's been deflected. A gyroscope will keep pointing in the same direction, but only if you remove all other influences. Even in orbit that's not easy - there's the earth's magnetic field and its interaction with the solar wind, the thin atmosphere, gravitational anomolies, tidal forces from the moon and sun, etc. You can't stay too close to the earth, yet if you go out too far the "frame dragging" effect becomes immeasurable. And if you make the gyroscrope *totally* immune from outside influences, how do you determine how it's spinning?

One of the pop science magazines, possibly Discover, had an in-depth article on this mission a year or so ago.