Errant Satellites Provide Test Case for General Relativity

[COUNTING DOWN IN GERMAN] IRA FLATOW: There you go– rocket launch. Hang on. Room is shaking.

They are captivating, aren’t they? And sometimes, if you follow them over the years, they are unpredictable. Some explode on the launch pad. Others get lost in space. In this one we were just listening to, that was the launch of the European Space Agency’s two Galileo V and VI satellites. The satellites made it into space, but a technical glitch pushed them into the wrong orbit. Details, details.

Ah, of course, you can’t let though a good satellite go to waste. So two groups of physicists have repurposed these satellites to make the best of a bad situation– repurposed them to test general relativity– a fitting idea for this year, the 100th birthday of Einstein’s famous idea.

My next guest is on one of those teams. Sven Herrmann is an experimental physicist at the Center of Applied Space Technology and Microgravity. That’s out of the University of Bremen in Germany. Welcome to Science Friday.

SVEN HERRMANN: Yeah. Hello. Thanks for having me.

IRA FLATOW: So you’re now going to use these satellites. How do you use them to test general relativity?

SVEN HERRMANN: Well, so as you maybe– I don’t know if you mentioned that, but these satellites are now on a very eccentric orbit. So they were supposed to go in a circular orbit. Now they are on eccentric orbits. And that means with these clocks on board of the satellites, they are changing the altitude in the gravitational potential.

And that gives us an excellent opportunity to test the gravitational redshift or the gravitational time relation. So that’s a very basic prediction of general relativity, that a clock that is closer to a massive object, like Earth, will seem to tick a bit more slowly than a clock that is located further away. And the clocks onboard of those satellites, they are now just exactly doing this. They are approaching Earth and going back and forth with every revolution on the orbit that they do about twice a day.

IRA FLATOW: How much in seconds, in fractions of a second, would you see to justify or to back up Einstein’s theory?

SVEN HERRMANN: So that’s on the order, I think, of– we put it in a shift in the frequency. The shift in the frequency is 10 to the minus 11, or one part in 10 to the 11. And that’s caused by a change in altitude on the order 8,000 kilometers per orbit. So if you put that into seconds, I think you end up with something on the order of microseconds– so several ten or hundred microseconds.

IRA FLATOW: I’m Ira Flatow. This is Science Friday from PRI, Public Radio International. We’re talking about this interesting experiments with these satellite. See you have to see– so you had to find satellites, though, that had clocks on them, right? Now all the satellites have the kind of clocks that you would need, but these two did.

SVEN HERRMANN: So the clocks in the Galileo program, they are all rubidium atomic clocks and they are passive hydrogen masers, and they are all the same model that is used in the global satellite navigation system in if Galileo, like in GPS. So these are pretty good clocks. They are maybe not the world’s best clocks. But they are designed for the purpose of positioning and navigation, and I think they are sufficient for the test that we want to do.

IRA FLATOW: Why do we continue to need to test Einstein’s theories of relativity?

SVEN HERRMANN: That’s a good question. So I think general relativity is just such a very basic theory. It’s essentially defining our understanding of space and time, so it’s the very basic of physics. I think we are obliged, more or less, to really test this fundamental theory to the best what technology can provide us with. And we should use, I think, every chance to look at the fundamentals and the principles that this theory is built on.

And especially if you look at the problem that is– something many physicists try to tackle today is a unification of gravity, a gravity theory that’s general relativity and quantum theory on the other hand. There was two big theories in physics. They are seemingly incompatible. And if you want to formulate a theory of quantum gravity, a unified theory of those two, you might have to change some of the basics that these theories are built on. And so maybe at some level of precision, you might find small deviations in the fundamentals of general relativity.

And so unfortunately, we don’t know at what precision we find the deviation. So it might be around the corner. It might be orders of magnitude away. But as I said, my philosophy is just do the best test you can with the technology you have at hand. With this satellite, I think we have a good chance or a good opportunity to improve these tests.

IRA FLATOW: Well, we’ll await your results. It sounds quite exciting, and we wish you the best of luck.

SVEN HERRMANN: Well, the results– we have scheduled now to collect the data that these satellites provide us with, more or less for free, over the course of one year. So we want to have one year of data that may help us to disentangle the signal that we are looking for from some systematic errors or systematic effects.

IRA FLATOW: All right. Well, we’ll check back in a year. We’ve got to say goodbye, Sven. Sven Herrmann, experimental physicist at the Center of Applied Space Technology and Microgravity at the University of Bremen.

One last thing before we go. Late last week came word that Amir Aczel had passed away. And he wrote a slew of books, many dealing with math on topics as diverse as Fermat’s last theorem, monks and evolution, ancient cave art, uranium, religion, the hunt for the Higgs Boson. He joined me many times on this program over the years, and I was always struck by his humor and his ability to describe complexity in simple terms, as in this description of Hilbert’s paradox and infinity.

AMIR ACZEL: There’s a better way of seeing it, and that’s my all-time favorite about infinity, and that’s called the infinite hotel, or Hilbert’s Hotel. So you go to this hotel someplace, and you need a room. You’re very tired. And it’s called the Infinite Hotel, so you think certainly we’ll find a room here. But the person at the desk says, I’m sorry, we have infinitely many rooms, but they’re all full. There’s no vacancy.

And so you say, OK, I’m a mathematician. I should be able to solve this problem. And you tell the person at the desk, take the person in Room 1, move him or her to Room 2, the person in Room 2 to Room 3, Room 3 to Room 4, and you have Room 1 for me right now. So that’s that the idea of infinity– that you can really do very strange things. You can always extract a finite quantity from an infinite one.

IRA FLATOW: Amir Aczel passed away too soon last week at the age of 65.

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