NASA Gravity Probe Launched

ping pong writes "Forty-five years in the making and 24 hours late, NASA launched the $700 million satellite into orbit today to test Albert Einstein's general theory of relativity. The satellite, which was inserted into a polar orbit, will spend two months getting ready, then 16 months making measurements." NASA's mission news has more.

Comments

That's a lot of money to spend

[mindless4210]

It's a pretty fascinating experiment, although it seems like a lot of money to spend just for testing his theory. I think that recent missions to mars were a bit more interesting.

Stanford has a great overview of the mission. It's in pdf format.

Why has it been in the making for so long?

[Pi_0's don't shower]

This is an experiment designed to test the correction due to General Relativity of the thomas precession of a tiny spinning sphere.

The correction to the precession will be on the order of arcseconds (1/3600 of a degree) per year.

There are some very good general relativists who have very severe reservations about this project. If they do detect a signal, I suspect it will be more of a testament to the power of experimental precision rather than a test of GR, which practically every serious physicist believes to be correct.

It's also worth noting that if nothing is seen, it's more likely than not due to the difficulty of detecting such a small signal.

Re:Why has it been in the making for so long?

[Stuntmonkey]

The correction to the precession will be on the order of arcseconds (1/3600 of a degree) per year.

It's significantly smaller than that -- the precession due to frame dragging is predicted to be only 0.04 arcseconds over one year.

And I agree that the physics community is 99% confident that the Lense-Thirring effect is real. However, I also think this is more because of the aesthetic beauty of the theory, rather than actual measurements. If it were a less fundamental theory being tested I would call it a waste of money, but for something as fundamental as GR I think a confirming direct experiement is justified.

The real question is how many viable alternatives to GR are ruled out by this test, assuming it is successful. For example nearly all viable GR alternatives proposed have weak gravitational wave properties identical to GR, so detecting these waves provides little support for GR. I wonder if the Lense-Thirring frame-dragging effect is more discriminating.

Of course, by far the more interesting case is if the effect is not observed. They seem to have many sigma of signal to noise here, so a null result would be pretty compelling.

An experiment in inertia?

[David Hume]

That is, inertia in big science funding?

In 1995, the GP-B was described as the "only experiment ever devised to test [the existence of frame-dragging]."

However, in 1997 NASA announced that it had successfully tested frame dragging. See also here.

Not always hard

[Pi_0's don't shower]

The James Webb Space telescope, when launched, will be temperature controlled by simply putting a shield around it on the sun-side, keeping the telescope side cool and out of sunlight.

A pretty simple idea; as once it cools down to equilibrium temperature, there'll be nothing to heat it up.

Re:Absolute Zero?

[nrlightfoot]

They put all the gyroscopes in a dewar with 1500 liters of liquid helium to keep it cold. Plus they get to use the helium that evaporates for the stabiliztion thrusters.