Galileo's magnifico measurement: 1976 redshift test updated Clocks on a pair of Galileo satellites have given physicists the first refinement of gravitational redshift since 1976. The experiment was only possible because in 2014, a launch went ever-so-slightly wrong: two Galileo satellites known as Doresa and Milena were launched into elliptical orbits, rather than circular orbits. …
I think 'precision of 0.007%' is fine: 'precision' usually means some parameter which represents the amount by which measurements vary, and the number you quote is that parameter. 'Accuracy', importantly, is different & is a parameter which tells you how far the centre of the distribution of values you measure is from the true value. Measurements can sometimes be very precise but very innacurate if there is some systematic bias.
This seems to be a more recent report. As of Aug 15, they are only being used for emergency beacons. But it looks as if they might get used for navigation once there have been software upgrades to the satellites and the ground station.
since the satellites' orbits take them in and out of Earth's gravity just a little
Aren't they always in Earth's gravity hence orbit otherwise known as perpetually falling? Otherwise they would be quite happily flying on to Andromeda or some such other far flung place.
That is unless another gravity hog pulled them in.
While this is another matter of language I think the wording is fine: the satellites do indeed move in (lower / into a stronger field) and out (higher / into a weaker field). I'd be completely happy saying that, as a physicist. Indeed you can't move completely 'out' of any gravitational field: since gravity is a long-range force there is no point at which Earth's field is not felt. You can reach escape velocity which means your distance from Earth will keep increasing for ever, but that's very different.
"since gravity is a long-range force there is no point at which Earth's field is not felt."
Technically, there are such places, loads of them. Anywhere outside of a 4.6 milliard light-year wide sphere centered on Earth will not, yet, have felt the gravitational pull of the planet, merely that of a lot of bits and gases.
At about ten Millys, even the nebula that produced the Earth probably does not have a coherent effect as it, too, probably didn't exist that far back in time as a separate entity.
Space is big. Really, really big. You may think it's a long walk home from the pub on a wet Saturday morning but ...
"...Thus was born the ESA's GREAT experiment..."
A guy with three kids and a van did something very similar in 2005. Same "GREAT" name, same basic concept. With a van. And three kids. In 2005.
Ref: http://leapsecond.com/great2005/
Project GREAT: General Relativity Einstein/Essen Anniversary Test
Clocks, Kids, and General Relativity on Mt Rainier
"In September 2005 the kids and I took several very accurate cesium atomic clocks from home and parked 5400 feet up Mt Rainier (the volcano near Seattle) for a full two days. The goal was to see if the clocks actually gained time, even if billionths of a second, as predicted by Einstein's general theory of relativity. Does gravity really alter time and can this weird phenomenon be detected with a family road trip experiment? ..."
Yes, it did test something: it tested an effect predicted by GR to a precision not previously achieved. That's important: we know GR is correct to quite high accuracy in many regimes and we therefore need to do two sorts of tests: increase the precision our measurements for the regimes where we know it makes good predictions to see if we can find places where those predictions break down, and try to test it in new regimes. This experiment is the first sort (testing to higher precision predictions we have already seen): LIGO / VIRGO are the second (testing GR in the strong field regime where it has not really been tested before). The first sort of experiments tend to be seen as rather boring, but they're really not.
So far GR passes everything which is cool, but means we're not getting clues to new physics.
Well I haven't decided. I think the two options are GIANT FRICKING ROCKETS (a bit obvious I feel, also that other guy is all over them), and LOWERING THEM FROM SPACE ON A SPACE ELEVATOR TETHERED TO THE MOUNTAIN. I kind of prefer the second option. Obviously you understand that all options must be in CAPS, and you are not to ask how I got them into space in the first place.
...what's it for? The article doesn't actually mention any examples of what this greater accuracy does to help us in any other area than "mine is better than yours". I'm sure there are very good reasons for doing this, but it's well out of my field so some hints in the article would have helped.
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It's a test of General Relativity, and although this test has been done before it's here being done with greater precision than previous cases.
This is important because that's how science proceeds: people put up theories which make predictions, and other people (or the same people) go out and test the predictions. Ultimately we hope the theory will fail one of these tests so we can find its limits and move on to some new theory. In the case of GR we pretty much know it must fail in some regimes, and it would be very interesting if it failed in others although we don't have any strong reason to believe it will: this is a test of one of those places it might fail. (It would be interesting if it failed in one of these places as it might mean the standard models of dark matter or dark energy are wrong.)
But if you're looking for an answer to the question 'what practical use is this?': probably none, it's 'just' scientists doing science.
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