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Message: <blockquote data-quote="freyar" data-source="post: 6687296" data-attributes="member: 40227">SO MANY GOOD QUESTIONS!  Sorry for shouting, just got a little carried away...Let me pick up a couple of the fast ones, then I'll work through the others.  I think I'll kind of work backwards, as in starting with how we know there is antimatter and why you care and eventually getting to the more complex and abstract stuff.So, first, how do we know antimatter exists?  As several people have said already, we see it quite a bit.  For example, there have been a number of satellite experiments with the capability to detect cosmic rays --- particles from outer space --- including antimatter.  There's a pretty sensitive one called the Alpha Magnetic Spectrometer (AMS) on the International Space Station right now, and the numbers of antiprotons and positrons (antielectrons) that it finds at each energy are of great interest for astrophysics and also dark matter physics.  We also see the results of antimatter annihilating with normal matter in astrophysics; the extreme environments around pulsars (highly magnetized, rapidly spinning cores of stars that are leftover after the rest of the star goes supernova) can produce positrons, which annihilate as they spread through the galaxy.There are also some radioactive nuclei on earth that decay by producing positrons.  As noted, this is useful for medical imaging in positron emission tomography (PET).  The way PET works is that you stick some of those radioactive atoms in what's basically sugar otherwise, so it preferentially goes to highly metabolic areas of the body, like a tumor.  Then the nucleus decays, which releases the positron. The positron doesn't have to go very far to find an electron, so it then annhilates, producing 2 gamma rays that (usually) fly out of your body to a detector.  This lets you build up a nice clean picture of the tumor.  It's a powerful technique.  The cost of it is partly to do with producing the radioactive elements needed for it, since the one we use decays pretty quickly.  Right now, it's done at relatively few nuclear reactors around the world, but there's ongoing research into producing these nuclei using small particle accelerators that could be more common.  Anyway, PET does expose the patient to some radiation, but (a) usually it's only used when the disease is bad enough that the radiation isn't a concern and (b) we're fairly transparent to gamma rays.  It is true that PET gives you more of a radiation dose than other imaging techniques, though.And, on the fun side, there is an experiment called ALPHA at CERN (home of the Large Hadron Collider experiment) that produces anti-hydrogen and tests some of its properties.  I know one of the scientists in charge of it a bit (his home institution is TRIUMF, the Canadian national particle physics lab).  As you might expect, one of the biggest challenge is keeping the antihydrogen atom away from regular matter long enough to make a measurement on it because otherwise it will just go POOF!</blockquote>

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