NASA's experiment with a gyroscope featured in Fabric of the Cosmos

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In Brian Greene's documentary The Fabric of the Cosmos, the thesis of the documentary is that empty space has physical properties even if there are no atoms or molecules in it. As I recall, most of the documentary shows the story of how NASA had a goal of sending a gyroscope up into space to see if gravity moved the gyroscope which would prove that empty space has physical properties. Most of the documentary is about the obstacles that NASA faced to send this gyroscope into space and to monitor the gyroscope. To make a long story short, NASA eventually got the money and sent the gyroscope into space, and gravity caused the gyroscope to turn slightly just as people who said that empty space has physical properties predicted.

I don't really see what the point of it was, and why NASA spent so much taxpayer dollars on something that I thought was already proven. As I recollect, the NASA experiment with the gyroscope happened in 2004. Einstein published his Theory of Relativity in 1915. Didn't Einstein's Theory of General Relativity that gravity is just curves on the fabric of spacetime already prove that empty space had physical properties? I mean, hadn't Einstein's Theory of General Relativity been proven by countless falsifiable experiments long before NASA launched the gyroscope in 2004?
 

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Orodruin
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That particular feature of general relativity had not been tested. Of course, there might have been good reason to expect the prediction of GR to hold, but that is not the same as actually having tested it in experiments. This is how science is pushed forward, you keep testing a theory until you find a domain where it is not applicable and you have to revise it.
 
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Ibix
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To add to Orodruin's response, we know general relativity is not completely correct because we can't reconcile it completely with quantum theory. We keep testing in the hope that our predictions will be wrong one day (or, more precisely, they'll be wrong enough for us to see the difference) and we'll have some clues where to go to make a better theory.

I don't think anyone expected Gravity Probe B to show anything other than what it did. But even so we have gained experience in making such measurements and we have slightly widened the known areas on our map of conditions where GR holds good.
 
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pervect
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I assume you are talking about gravity probe B? As I recall there was a fair amount of debate on whether the experiment was worth the expense - some argued that an unexpected result was unlikely, the counter-argument to this was that unexpected results are never seen as likely until they happen, otherwise they wouldn't be unexpected. To complicate the issue, the expense was, as you point out, very high, and of course the budget for science experiments is limited. In the end, it was decided to proceed with the experiment, and the experiment did confirm the predictions of General Relativity.

I personally have some doubts that characterizing the results of the experiment as "prove that empty space had physical properties" is really the best description, but I suppose it has less jargon than the alternative that comes to mind, "proving the existence of frame dragging. Regardless, it is a description, and if it's been published, we'll just have to deal with any confusion that may or may not result...
 
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I assume you are talking about gravity probe B? As I recall there was a fair amount of debate on whether the experiment was worth the expense - some argued that an unexpected result was unlikely, the counter-argument to this was that unexpected results are never seen as likely until they happen, otherwise they wouldn't be unexpected. To complicate the issue, the expense was, as you point out, very high, and of course the budget for science experiments is limited. In the end, it was decided to proceed with the experiment, and the experiment did confirm the predictions of General Relativity.
I'm not 100% sure, but I think that the gyroscope was called Gravity Probe B. I don't think (I'm also not 100% sure about this. It's been a few months since I watched the documentary.) that the documentary covered the debate on whether the experiment was worthwhile. The documentary made it sound like everyone at NASA thought it would be worthwhile, but it was just a matter of not enough money in the science budget. There are a lot of other worthwhile experiments they could have done with that fortune. But I'm glad that other people agreed with me that the experiment was not worthwhile. I imagine such people still think it was not worthwhile since the experiment confirmed the predictions of General Relativity: "See, I told you so."



I personally have some doubts that characterizing the results of the experiment as "prove that empty space had physical properties" is really the best description, but I suppose it has less jargon than the alternative that comes to mind, "proving the existence of frame dragging. Regardless, it is a description, and if it's been published, we'll just have to deal with any confusion that may or may not result...
NASA should have called the experiment "Frame Dragging Probe B" then.

Edited to add this: I just looked up Gravity Probe B on Wikipedia. According to Wikipedia, the entire Gravity Probe B experiment cost $750 million dollars. We could have saved the lives of hundreds of homeless people from dying of exposure during the winter with that money. We could have used that money for other science projects. We could have used that money to fund research into medical therapies for cancer and other diseases. We could have used that money to house more violent criminals in our overcrowded prison systems. There are a lot of more worthy things that could have been done with that money.
 
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To add to Orodruin's response, we know general relativity is not completely correct because we can't reconcile it completely with quantum theory. We keep testing in the hope that our predictions will be wrong one day (or, more precisely, they'll be wrong enough for us to see the difference) and we'll have some clues where to go to make a better theory.

I don't think anyone expected Gravity Probe B to show anything other than what it did. But even so we have gained experience in making such measurements and we have slightly widened the known areas on our map of conditions where GR holds good.
Was there really any reasonable chance of this experiment reconiling General Relativity with Quantum Theory though?
 
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Ibix
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No. See my last paragraph and Pervect's response.
 
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Vanadium 50
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I mean, hadn't Einstein's Theory of General Relativity been proven by countless falsifiable experiments
Nothing is ever proven. As an example, people thought atoms were eternal until radioactivity was discovered and then they weren't.

In the case in question, is there a theory of gravity that makes the same predictions as GR except for frame dragging? (Or makes predictions within the measurement errors to be the same as GR, but again, except for frame dragging?) The answer is "yes", e.g. the vector-tensor theory of Will and Nordtvedt or Bekenstein's TeVeS. TeVeS was in fact devised to solve another problem, so this is in theory a test.

There are two criticisms one could level at Gravity Probe B. One is that it wasn't worth doing at all. In that, the dozens of scientists who were on various peer review panels disagree with you. The other is that it didn't do as well as hoped. That's certainly true - it ended up comparable to LAGEOS when it hoped to do an order of magnitude better.
 
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