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:hazaah

[adam mcmaster]

I believe that was to test the theory that changes in velocity affect time, whereas the current experiment is to test the theory that a rotating object affects time and space.

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.

Re:Scientists crossing fingers, pacing

[FatHogByTheAss]

That's special relativity. General relativity relates to gravity.

Re:Einstein...

[BlueUnderwear]

I think that Einstein would turn over in his grave

Nope, he was cremated. However, his brain could be spinning in its jar

Remember, this was the man that came up with some of the most complicated theories in modern physics,

... except that he plagiarized Dirac's works...

He used 'geddonken' experiments,

Gedankenexperimente, i.e. though experiments.

We already have a better understanding of gravity.

[I'm a racist.]

In all honesty, this probe won't tell us anything we don't already know. At the time the idea was proposed, it was useful. Since then, we've made more precise measurements of gravity and observed relativistic effects.

The only way this probe will really teach us anything (outside of the engineering that went into its construction) is if it fails, spectacularly. Sadly, those "eureka" moments don't happen very often, and I wouldn't hold out much hope for one here. Then again, the Hipparcos data has caused some debate, while its mission was somewhat routine (although highly precise).

We already know that relativity is wrong (in the same sense that classical mechanics is wrong). This experiment is not designed to figure out exactly how relativity is wrong, rather it is designed to tell us if relativity is wrong at all. Since we already know the answer to that question, it isn't very helpful.

I'm not blaming the guys that worked on this project. There were political/financial/logistical issues that made this launch 20+ years too late to be useful. The PhDs awarded during this project are good, they did some nice work, most notably in materials science and fabrication, but other areas as well. It's just not very meaningful in the areas of physics/cosomology.

Oh well, that's what happens when science is a slave to beauracracy.

Quick attempt at an answer

[Pi_0's don't shower]

Alright, disclaimer first:
Just a grad student, still learning stuff, apologies ahead of time if it's wrong.

Attempt at an answer:
"Frame-dragging", as I understand it, goes all the way back to an old theory of the aether, that the aether is all around us, but is dragged by masses so that some oddball features of special relativity is explained. I'm not sure how this applies to the problem here, so maybe people use frame-dragging to refer to something else.

This part, though, how gravity works, is easier. Einstein's theory relies upon the stress-energy tensor. All forms of energy, including energy due to angular momentum and relative motions, are included in this. Binary pulsars precess and their orbits evolve in time, as do their rotation rates, as energy is radiated away gravitationally. There is definitely a contribution to gravity due to what you call "velocity components". Gravitational signals only propagate at c, so don't worry.

You can look at my first 2 posts on this topic if you like, but basically GR predicts that there will be a precession of this little spinning sphere that's very small and hopefully detectable. If we don't detect it, it's probably due to the difficulty of the experiment, not to the failure of GR.