The perfect planet-sized dark matter detector is already in orbit: GPS satellites :- Physicists are always trying to own their field’s shortcomings, pointing to deficiencies in modern physics as a reason for big investment in continuing physical research. That’s been the strategy when pitching dark matter investigations: As far as we know, this stuff outweighs conventional matter by more than five to one, its gravity dictated the distribution of visible galaxies, and we know basically nothing about it. Modern physics has given us some concrete idea of just how ignorant we really are, and this fact is used to argue strongly both for and against costly event experiments like the Large Hadron Collider. A paper published this week in the journal Nature may offer a way out: Our constellation of GPS satellites, which have been orbiting Earth for decades, might be able to function as a planet-sized dark matter detector. The lead author of this paper is a physicist, as you might expect, but his specialty has always been atomic clocks; keeping time with the accuracy of nuclear decay allows everything from global communications to ultra-profitable high-frequency trading on the stock market. Atomic clocks also power GPS satellites by doing incredibly precise time-of-flight measurements on GPS signals — the system is only accurate enough when you can make measurements so precise that you can account for the difference in relative speed of multiple GPS satellites moving in different directions. However, velocity is only one of the ways that time can be warped, with the other being gravity. Just as objects moving at different speeds will measure time differently, objects in different gravitational fields will measure time differently as well; anyone who’s seen the recent sci-fi epic Interstellar will recognize this principle. Right away, we see a thematic connection: if dark matter makes up most of the mass in the universe, and mass creates gravitational fields, and gravitational fields affect the subjective speed of time, and we’re very good at measuring time… it seems we’re almost done! In fact, that does seem to be the argument on display in this paper, that if (and only if) certain theories of dark matter are true, then we might be able to detect dark matter with the network of GPS satellites that already cages our planet. Seen from this perspective, we’ve already paid something like $5-6 billion for our Distributed Orbital Dark Matter Detector — only a little less than the Large Hadron Collider itself cost. Read: MIT’s DarkLight project prepares to finally create dark matter in the lab If scientists could watch the effects of gravitational time dilation ripple through the GPS clocks, knocking time readings off of expectations according to some sensible pattern, they might be able to figure out what invisible dark object must have passed near to them to create those errors. In this way, the physical separation of the orbiting GPS clocks is as important to the experiment as the accuracy of each one individually; since they’ll likely be affected by the same gravity source, they need to be as far from one another as possible so they’ll experience it measurably differently. Of course, it’s not enough to just say that dark matter has gravity, and then go out looking for gravity. One of the things that has made dark matter so difficult to study is that, for some reason, it doesn’t seem to clump together like regular matter does. If dark matter were collecting into dark planets and dark stars, we’d see characteristic gravitational hotspots as we do when regular matter collects into regular planets and stars. And wouldn’t that be fun, mapping a shadow universe through its eerily familiar gravitational signatures? Instead, dark matter doesn’t seem to be able to compress enough to form things like stars and black holes, and flies around at high speeds, leading it to be distributed around the universe like a three (or more?) dimensional web. That would be difficult to measure with a detector as small as the Earth. Given the scale of these forces, this GPS dark matter hunt will only have anything meaningful to detect if a certain theory of dark matter is true: the topological defect model. The topological defect model basically says this: as the universe cooled in the very early period after the Big Bang, dark matter went through a phase change and settled into an almost matrix-like structure, with some flaws. Think of the surface of water as it cools into ice, and how the perfect lattice of stationary water molecules always ends up being marred by defects. As dark matter went through a similar phase transition in three (or more?) dimensions, it may have formed similar “topological defects” of various types — and that’s what this study proposes to search for. The most famous type of defect is a cosmic string (not be confused with a superstring), a long thin dark matter defect that may be millions of light years long and just an atom thick, but which would still be incredibly massive. If a cosmic string or any other major feature of the dark world did pass near enough to affect us, the atomic clocks on GPS satellites might be able to pick it up. Like so much astronomical work, this study would rely on events beyond the experimenters’ control — the cosmic movement of dark matter structures we’re not even sure really exist. Even if the GPS dragnet sees no evidence for dark matter defects, that won’t necessarily mean that the defects don’t exist — but given the relative lack of expense, this still seems like a bargain of a study.
Posted on: Thu, 20 Nov 2014 03:29:11 +0000