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The Global Positioning System consists of 31 Earth-orbiting satellites, each carrying an atomic clock that sends a highly accurate timing signal to the ground. Anybody with an appropriate receiver can work out their position to within a few meters by comparing the arrival time of signals from three or more satellites.
And this system can easily be improved. The accuracy of GPS signals can be made much higher by combining the signals with ones produced on the ground. Geophysicists, for example, use this technique to determine the position of ground stations to within a few millimeters. In this way, they can measure the tiny movements of entire continents.
This is an impressive endeavor. Geophysicists routinely measure the difference between GPS signals and clocks on the ground with an accuracy of less than 0.1 nanoseconds. They also archive this data providing a detailed record of how GPS signals have changed over time. This archival storage opens the possibility of using the data for other exotic studies.
Today Benjamin Roberts at the University of Nevada and a few pals say they have used this data to find out whether GPS satellites may have been influenced by dark matter, the mysterious invisible stuff that astrophysicists think fills our galaxy. In effect, these guys have turned the Global Positioning System into an astrophysical observatory of truly planetary proportion.
The theory behind dark matter is based in observations of the way galaxies rotate. This spinning motion is so fast that it should send stars flying off into extra-galactic space.
But this doesn't happen. Instead, a mysterious force must somehow hold the stars in place. The theory is that this force is gravity generated by invisible stuff that doesn't show up in astronomical observations. In other words, dark matter.
-- submitted from IRC
(Score: 5, Funny) by VLM on Monday May 08 2017, @05:53PM (2 children)
Or is it possible that the black hole thought to be at the center of every galaxy is exerting a large force somehow that makes up for the apparent lack of gravity?
Gravity is non-linear so stars in close would tend to fall in more than we see and stars at the edge would continue to floop out and away.
Some of your other thought experiments have a similar problem where the math has already been run (although I have no links sorry) such that if there was a fifth force to get the observed behavior it would have to act just like gravity (not like strong force that drops off quick etc).
One interesting way to troll astrophysicists is to ask if the gravitational constant is varying over time. I say troll because its only about 50% serious and I think theres a math model disproving it something to do with distant vs near galaxies. It sounds tasty though, so if we're seeing light from a galaxy 3 billion years in the past and its acting like gravity was stronger 3 billion years ago, well, why not change the value of G over time? From what I remember that turns into a variant of the old "if ancient glass flows in windows, then eyeglasses and telescopes and microscopes would flow today and eyeglasses would only have a constant prescription for five minutes or whatever... So if G varied enough to get rid of dark matter supposedly we can measure the moon's orbit accurately enough to detect its change, which it isn't...
Historically a lot of interesting physics comes from WTF is that or F this I'm sick of it, so I propose the best way to explain there being too much stuff in the universe is to accept that there's just a lot more stuff in the universe. Maybe we're out in the galaxy suburbs and its just unusually not-dense out here. Right now there's a galaxy asking its partner galaxy "does this model of astrophysics gravity make my butt look fat?" "No dear it hasn't sagged a bit in 4 billion years" "Oh thanks honey I don't want any dark matter wrinkles"
(Score: 1) by Demena on Tuesday May 09 2017, @03:14AM (1 child)
A couple of observations. We get these 'missing mass' problems when relative accelerations are very small. They appear to start at a particular low value for acceleration. Newton fails at high velocities, Einstein fails at low acceleration. My first reaction would be "That is interesting!", not "invent more mass".
The further away a galaxy is in space, the 'older' it appears. There is some correlation between age and spin rate. So your 50% troll may actually have some substantial truths buried in it.
There is an interesting blog "Physics From the Edge"
(Score: 2) by VLM on Tuesday May 09 2017, @12:42PM
Thats where the math gets rough because a billion-years is only 1e9 years but EE and astronomer stuff easily measures stuff like time to 12 decimal places so if we can easily see a change in G from a billion years away, then up close bouncing radar and lasers off the moon we should see a moon orbital change of equally detectable magnitude in a thousandth of a year or certainly over the course of a day or so. Like the "glass is a liquid" analogy.