Proof That Einstein Got It Right
IRA FLATOW, host:
This is SCIENCE FRIDAY. I'm Ira Flatow.
After almost 100 years, Albert Einstein has been proven right, but that proof did not come easily. The experiment took five decades and about $750 million to design and conduct. The results are in from NASA's Gravity Probe-B, a sort of gyroscope sent into orbit around the Earth to measure how the Earth and other massive bodies like it warp the space and time around them and how they actually - it's hard to believe - they actually drag it. They drag the space and time around them as they rotate it.
And the father of that experiment is here to talk about it, and our number is 1-800-989-8255 if you'd like to discuss it. You can tweet us @scifri, @-S-C-I-F-R-I.
Francis Everitt is the principal investigator of the Gravity B lab experiment and professor at the Hansen Experimental Physics Lab at Stanford University. Welcome to SCIENCE FRIDAY.
Dr. FRANCIS EVERITT (Principal Investigator, Gravity Probe-B): I'm very glad to be with you. Francis Everitt, here.
FLATOW: Tell us about the concept. The concept was that Einstein said in his general theory of relativity that you have a massive object, and that warps space around it, and that's what creates gravity.
Dr. EVERITT: Yes. Our experiment involves putting a gyroscope in orbit around the Earth and asking what happens. The gyroscope is a ping-pong ball, which is a sphere the size of a ping-pong ball, and one spins it and points it at a remote star and asks what happens.
If one lived in Newton's universe, where space and time are absolute, and you made a really perfect spinning sphere, then it would just go on pointing in the same direction for all time.
Einstein's universe is different, and as you said, there are two different effects: one the distortion of space by the mass of the Earth, and the other is the dragging of space by the rotation of the Earth.
Now, when people talk about the distortion of space, this sounds pretty mystical, but it really isn't mystical at all. So let me just state what happens. Supposing you were to draw a circle in empty space, the circumference of that circle would be two pi times the radius. And I think most of us will remember that result; it goes back to the Greeks.
What Einstein says is that when you put an object, for example the Earth, inside the circle, so we've got a 4,000-mile-radius circle, then the circumference will be a little bit less than two pi times the radius. So obviously you want to know how much less.
Remember the circumference of the Earth is 25,000 miles, and the amount that Einstein's theory predicts it'll be less is just 1.1 inches. So that's a pretty small difference.
Now, our spinning gyroscope measures that difference indirectly but very accurately. And in fact, we managed to measure it to a fraction, a small fraction of a percent, which is equivalent to measuring the difference of 1.1 inches to about two or three thousandths of an inch. That's the accuracy we had to achieve with this Gravity Probe-B mission.
FLATOW: Were you able to measure the concept of the Earth dragging space with it, also?
Dr. EVERITT: We also were able to separately measure the concepts of the Earth's dragging. My picture of this, which is a good picture and is a helpful one, is imagine the Earth immersed in honey. Then as the Earth rotates, it'll drag this honey around with it, and likewise, it will drag a gyroscope around with it.
This effect is very much smaller than the first one. It's about a factor of 150 smaller. We were able to measure that to somewhat better than 20 percent, about 19 percent accuracy.
So the experiment ended up by being a star, a telescope, a very accurate telescope, pointing at the star and four spinning spheres supported electrically. And we, with all four spheres, we measured both effects, and that's a powerful cross-check between the different spheres of how we were doing the experiment.
FLATOW: You know, it's easy for us to imagine the honey analogy, with the Earth sitting in the honey, but we know because the honey is sticky, and it would stick to, let's say, the Earth or whatever was stuck to it. But what is the sticky part of gravity that the Earth is sticking to with the space that it's in? How is it sticking to the space?
Dr. EVERITT: It is an analogy, but it is a very nice analogy that actually works. And you say: Well, how does it do? At one level, one has to say this is what Einstein's equations predict. But another sense, you can say they predict the kind of variation analogous to that.
You know, there's an amusing aspect to this because one is told that the Victorians' simpleminded earlier physicists pictured the Earth as a kind of ether, and Einstein destroyed that. The picture that I have given you of the honey almost seems like restoring that idea of an ether, and this is not a completely silly point of view.
FLATOW: And do you think - and this is basically Einstein was saying that this is geometric, that gravity is a geometric distortion, it's a distortion of geometry - thinking that, now taking that analog and going to the quantum world, one of the great challenges is uniting a geometrical world with the world of particle physics.
Do you think that's ever going to happen, given the geometrical idea that we're talking about here?
Dr. EVERITT: That is a beautiful question, and one asks whether it's ever going to happen. One would certainly like it to happen. We may remember that Einstein, who was rather an intelligent man, worked on trying to unify understanding of the atom and his theory of gravity for 40 years and failed.
Other people have managed to unify other parts of physics. Still we don't know how to unify gravity with the rest of physics. This is one of the great mysteries. If one makes the belief that somehow all of physics will get unified, that's a beautiful belief.
It hasn't happened, and we've been waiting and trying for something like 90 or 100 years or maybe a little bit less because the quantum picture of the atom came up in the 1920s. So that's only 80-odd years ago.
And we've all been struggling since then, and we've made some progress, but we don't know the answer.
FLATOW: I was reading about how round you had to make your gyroscopes, those little ping-pong balls up there. They are almost perfectly - how perfectly round can you describe them?
Dr. EVERITT: Okay, if you take one of our spheres, they're one and a half inches diameter. That's the size of a ping-pong ball. They're round to about two-millionths of an inch.
If you were to imagine taking this gyroscope and blowing it up to the size of the Earth and keeping everything in proportion, the highest mountain would be nine or 10 feet high, and the deepest ocean would be nine or ten feet deep. So that's the degree of roundness that we eventually made our gyroscopes to have.
FLATOW: And I understand that things went somewhat awry with the instruments once they were up in space, correct?
Dr. EVERITT: Well, you see, when you're doing experimental physics, you always try to make everything perfect, but if you think ahead, you also think of checks that need to be done.
And we had thought ahead enough. So after gathering all the science data, we allowed ourselves 46 days to check things. And one of the ways you check is you deliberately make things worse. I mean, you think a certain disturbance might have a certain size. Is that true? Crucial thing to do: deliberately, by a known amount, make it bigger.
So one of the things we were worrying about or aware of - not worrying about but aware of - was that you point at a star, but you don't quite perfectly point. So is there a little effect because you are not quite perfectly pointing? And we knew there would be, but we had a calculation of how big it is.
What do you do? You point off to other stars, which are not a few arc seconds, a few very small angle, which we were most of the time, you point them off to a whole degree, which is a factor of a couple of hundred times bigger.
And in fact, we ended up by pointing off to several stars, both real and conceptual stars, and we found we had a disturbance of a hect, or a hundred, bigger than we were expected.
We were able to understand the cause of this disturbance and actually of two other disturbances, and the whole rigor of the data in our systems is a really interesting detective story where there were three connected criminals: one that misalignment effect, 100 times bigger than we were expecting; and then a couple of others. And finally, one understands what's going on.
Now, I just told you how round the sphere is mechanically. The interesting thing is that both the sphere and the housing it was in mechanically were wonderfully round, but electrically, because of little charge effects on the surface of the sphere and the housing, were not so round.
So you can imagine that electrically, it was, so to speak, as if you had electrical Mount Everests on it, the housing of the thing, and it was the indirection between those, these things, that caused the problem.
But we found a very beautiful way of mapping and determining what was actually happening here, magnetically, and this is how we were able to pursue our detective story. When we did it, we were able to check the size of those calibrations by a completely different method and found that it came out with the same answer.
And this is why one can believe the result, finally, that and the fact that all the gyroscopes ended up by agreeing. But it was a tremendous effort by our team, on which I was in some sense just overlooking, watching the two ways that the able team found of identifying these disturbances and removing them.
FLATOW: Well congratulations to you and your team, Dr. Everitt, and good luck to you in your future research. Thank you for taking time to be with us today.
Dr. EVERITT: It's a great pleasure to be with you, and know you have a good science program here.
FLATOW: Thank you very much. Francis Everitt is principal investigator of the Gravity Probe-B experiment and professor at the Hansen Experimental Physics Lab at Stanford University.
We're going to take a short break, and when we come back, something more mundane, but a mystery nonetheless: fat. Stay with us.
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FLATOW: I'm Ira Flatow. This is SCIENCE FRIDAY, from NPR.
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