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Is Time Travel (going backwards) Possible?
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<blockquote data-quote="freyar" data-source="post: 6044726" data-attributes="member: 40227"><p>Actually, the weak nuclear force interacts (roughly) equally with protons and neutrons, so that won't work. DM and normal matter must interact through two different forces. I'll give you a simplified model that illustrates the idea but wouldn't actually work (I think). Imagine that DM interacts with both protons and neutrons equally via the weak force but that DM also carries an electric dipole moment (still electrically neutral, so ok). The dipole interacts with protons (charged) but not neutrons. If the sign of the two interactions is different, then the scattering can depend a lot on the target nucleus. I wouldn't say that working models are more complex but do require a bit more background to explain. These ideas were pretty popular a year or so ago but have dropped off a bit recently partly because none of the experiments are coming out with new data right now --- it's kind of a wait-and-see phase.</p><p></p><p></p><p></p><p>The collisions are shockingly low energy, actually. The earth's orbital speed is only about 1/1000 the speed of light, which is similar to what we believe is a typical DM speed in the galaxy. But that means the kinetic energy available to excite the DM in a collision with something on earth is only about (1/2) mv^2, or about 1 part in a million compared to the mass of the DM particle (energy mc^2). So only a small relative gap between DM states is enough to prevent the scattering. (For typical WIMP masses of around 100 GeV, you're only talking about a 100 keV splitting, which is a fraction of an electron mass).</p><p></p><p></p><p>Since we've been talking dark matter so much (in a time travel thread, oddly enough), I figure you all might be interested in the latest news. The big news is that, about 6 months ago, analysis of data from the Fermi gamma ray telescope discovered a signal of high energy gamma rays with a set energy coming from the galactic center. People have for years thought this would be a "smoking gun" for dark matter annihilating very rarely into photons just because there aren't astrophysical mechanisms that can do this easily. So there's quite a lot of work going on right now to (1) figure out if the signal is real (not a problem with the instrument or how the data is analyzed), (2) see if it has other characteristics consistent with what we'd expect from dark matter, and (3) what models of dark matter can create this signal. Very exciting times right now in the field; if this holds up, it will mark the first known non-gravitational detection of dark matter and effectively the discovery of a particle outside the Standard Model. There are of course the usual caveats that this is being scrutinized very carefully as indicated above and that there's also a lot of thought going into whether some kind of standard astrophysics can produce this. And everyone agrees that more data is needed. So we'll see how this plays out over the next year or two.</p></blockquote><p></p>
[QUOTE="freyar, post: 6044726, member: 40227"] Actually, the weak nuclear force interacts (roughly) equally with protons and neutrons, so that won't work. DM and normal matter must interact through two different forces. I'll give you a simplified model that illustrates the idea but wouldn't actually work (I think). Imagine that DM interacts with both protons and neutrons equally via the weak force but that DM also carries an electric dipole moment (still electrically neutral, so ok). The dipole interacts with protons (charged) but not neutrons. If the sign of the two interactions is different, then the scattering can depend a lot on the target nucleus. I wouldn't say that working models are more complex but do require a bit more background to explain. These ideas were pretty popular a year or so ago but have dropped off a bit recently partly because none of the experiments are coming out with new data right now --- it's kind of a wait-and-see phase. The collisions are shockingly low energy, actually. The earth's orbital speed is only about 1/1000 the speed of light, which is similar to what we believe is a typical DM speed in the galaxy. But that means the kinetic energy available to excite the DM in a collision with something on earth is only about (1/2) mv^2, or about 1 part in a million compared to the mass of the DM particle (energy mc^2). So only a small relative gap between DM states is enough to prevent the scattering. (For typical WIMP masses of around 100 GeV, you're only talking about a 100 keV splitting, which is a fraction of an electron mass). Since we've been talking dark matter so much (in a time travel thread, oddly enough), I figure you all might be interested in the latest news. The big news is that, about 6 months ago, analysis of data from the Fermi gamma ray telescope discovered a signal of high energy gamma rays with a set energy coming from the galactic center. People have for years thought this would be a "smoking gun" for dark matter annihilating very rarely into photons just because there aren't astrophysical mechanisms that can do this easily. So there's quite a lot of work going on right now to (1) figure out if the signal is real (not a problem with the instrument or how the data is analyzed), (2) see if it has other characteristics consistent with what we'd expect from dark matter, and (3) what models of dark matter can create this signal. Very exciting times right now in the field; if this holds up, it will mark the first known non-gravitational detection of dark matter and effectively the discovery of a particle outside the Standard Model. There are of course the usual caveats that this is being scrutinized very carefully as indicated above and that there's also a lot of thought going into whether some kind of standard astrophysics can produce this. And everyone agrees that more data is needed. So we'll see how this plays out over the next year or two. [/QUOTE]
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