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.
If this theory is correct, dark matter should fill our galaxy, too, and as the sun makes its stately orbit round the galactic center, Earth should plough through a great ocean of dark matter.
There’s no obvious sign of this stuff, which makes physicists think it must interact very weakly with ordinary visible matter. But they hypothesize that if dark matter exists in small atomic-sized lumps, it might occasionally hit atomic nuclei head on, thereby transferring their energy to visible matter.
That’s why astrophysicists have built giant observatories in underground mines to look for the tell-tale energy released in these collisions. So far, they’ve seen nothing. Or at least, there is no consensus that anybody has seen evidence of dark matter. So other ways to look for dark matter are desperately needed.
Enter Roberts and co. They start with a different vision of what dark matter may consist of. Instead of small particles, another option is that dark matter may take the form of topological defects in space-time left over from the Big Bang. These would be glitches in the fabric of the universe, like domain walls, that bend space-time in their vicinity.
Should the Earth pass through such a defect, it would change the local gravitational field just slightly over a period of an hour or so.
But how to detect such a change in the local field? To Roberts and co, the answer is clear. According to relativity, any change in gravity also changes the rate at which a clock ticks. That’s why orbiting clocks run a little bit slower than those on the surface.
If the Earth has passed through any topological defects in the recent past, the clock data from GPS satellites would have recorded this event. So by searching through geophysicists’ archived records of GPS clock timings, it ought to be possible to see such events.
That’s the theory. In practice, this work is a little more complicated because GPS timing signals are also influenced by other factors such as atmospheric conditions, random variations, and other things. All these need to be taken into account.
But a key signature of a topological defect is that its influence should sweep through the fleet of satellites as the Earth passes through it. So any other kinds of local timing fluctuation can be ruled out.
Roberts and co study the data over the last 16 years, and their results make for interesting reading. These guys say they have found no sign that Earth has passed through a topological defect in that time. “We find no evidence for dark matter clumps in the form of domain walls,” they say.
Of course, that doesn’t rule out the existence of dark matter or even that dark matter exists in this form. But it does place strong limits on how common topological defects can be and how strong their influence is.
Until now, the limits have been set using observations of the cosmic microwave background radiation, which should reveal topological defects, albeit at low resolution. The work of Roberts and co improves these limits by five orders of magnitude.
And better data should be available soon. The best clocks in Earth laboratories are orders of magnitude more accurate than the atomic clocks on board GPS satellites. So a network of clocks on Earth should act as an even more sensitive observatory for topological defects. These clocks are only just becoming linked together in networks, so the data from them should be available in the coming years.
This greater sensitivity should allow physicists to look for other types of dark matter, which may take the form of solitons or Q-balls, for example.
All this is part of a fascinating process of evolution. The technology behind the GPS system can be traced directly back to the first attempts to track the Sputnik spacecraft after the Soviets launched it in 1957. Physicists soon realized they could determine its location by measuring the radio signals it generated at different places.
It wasn’t long before they turned this idea on its head. Given the known location of a satellite, is it possible to determine your location on Earth using the signals it broadcasts? The GPS constellation is a direct descendant of that train of thought.
Those physicists would surely be amazed to know that the technology they developed is also now being used as a planetary-sized astrophysical observatory.
Ref: arxiv.org/abs/1704.06844: GPS as a Dark-Matter Detector: Orders-of-Magnitude Improvement on Couplings of Clumpy Dark Matter to Atomic Clocks
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