I had to look it up (I'm no medical student!), but the process is called positron emission tomography. It's used to detect tumours in oncology and, and also to detect some dementias.
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Matter/antimatter imbalenc - forked from AMA ask a physicist
- Thread starter Scott DeWar
- Start date
I had to look it up (I'm no medical student!), but the process is called positron emission tomography. It's used to detect tumours in oncology and, and also to detect some dementias.
Scott DeWar
Prof. Emeritus-Supernatural Events/Countermeasure
It sounds like using an electron scanning microscope without the destructive x-rays. An electron beam like that used with a CRT or electron microscope creates some nasty radiation. I guess the positrons . . . I don't know. I am stuck on this.
freyar
Extradimensional Explorer
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!
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!
As others have noted, we've observed, and outright used, anti-matter. So, no, we aren't just making stuff up.
Well, there are some issues with that. Mainly, where did the matter and anti-matter come from, why were they colliding, and why was the mix uneven? Moreover, the Big Bang is more than just about energy release - it is also about the origin and development of spacetime.
Every fundamental particle has an anti-particle. For electrons there are positrons. For quarks (the things that make up protons and neutrons) there are anti-quarks. Antimatter is made up of these anti-particles.
Well, you aren't getting a PET scan without it. So it may matter if you have brain cancer.
There is always a possibility. But if you want a percentage chance, I can't give you one.
We have observed and experimented with anti-electrons, anti-protons, and anti-neutrons, and measured their masses. They have the expected mass.
You probably read such, but it doesn't mean what you probably think it means. I'll try to avoid going too much into terminology, as when we start talking about quantum mechanics and "spin", there are a lot of terms.
There are a bunch of numbers we use to describe a particle, often called "quantum numbers". Speaking broadly - an anti-particle will have all its quantum numbers reversed from a matter particle produced in the same way.
So, say we have a *really* energetic photon. Rather than just fly along forever, it may at some point spontaneously create a particle-antiparticle pair out of its energy. Say it creates a positron and electron. The electron has some angular momentum intrinsic to it. In order to conserve angular momentum at the moment of creation, the positron must be created with the opposite angular momentum. In that sense, we may say it has the opposite spin.
In astrophysics, "visible" universe and "known" universe are synonymous. Yes, there is a limit to the volume of space we know anything about. It is possible that nearly anything could be going on outside that visible volume of space. However, I note that the laws and constants we see are apparently *extremely* consistent across what space we can see. You'd need a particular reason to justify why those laws then become different somewhere else. Moreover, you'd have to deal with how, *just by coincidence* that weird thing happens to be out where we cannot see it.
All around you!
We are talking about events at a time when the universe was extremely dense, just a plasma of particles. Lots of particles and antiparticles tooth-by-jowl, so to speak. When the particles annihilate, what you typically get* is photons**. With things so dense, those photons get immediately absorbed by particles of matter***, and re-emitted.
Eventually, as the universe expanded, atoms formed, and the universe dropped to a density where photons could fly free without running into stuff. What photons were left that didn't run into stuff to be absorbed became what we now call cosmic microwave background radiation.
*You sometimes get particle-antiparticle pairs instead, but then those annihilate, too, because everywhere you go there's stuff to annihilate with, so eventually you get back to photons.
** Freyar may correct me if I am wrong - this may be happening before the electromagnetic force and weak nuclear force decoupled, so we would then be looking at W and B bosons, carrying the electroweak force of the time, but the rest is about the same.
***We note there that "matter" in this case may also include so-called "dark matter" - while today it doesn't interact much, back in the day or high density it may have been a major contender- so some of that energy may be tied up in stuff you can't see now.
Note that "someone filed a patent" does not mean what they propose actually works. The Patent Office is not a good place to go looking for how physical laws actually work. The patent office are engineering applications of the laws, often for specific cases, such that they don't give you insight into the general case.
There is a great deal of hand-waving in the words "suitable". Yes, you can manipulate moving charged particles with magnetic fields. But doing so is complicated - it generally requires all the particles to be sorted to be moving in the same direction, with known momentum (speed and mass), have known charge, and sometimes also be spinning in a known direction.
In the early universe, the matter and antimatter are of mixed type (some heavy some light), mixed charge (some of the antimatter is positively charged, some negatively charged, some electrically neutral), moving in random directions (not a beam) with a random distribution of spins. There no simple way to sort a random mix like that with the known forces available.
Moreover, we are talking about a time with densities such that the universe was not "transparent". Right now, space is basically empty, so photons, carrying the electromagnetic force, right now can go pretty much forever without hitting anything. But back then things were so dense that they couldn't reach anything but their very nearest neighbors. Reaching out to a distant particle of antimatter, and getting it from point A to point B without hitting anything along the way? Not realistic.
Basically, the universe can't sift though all the particles in itself as if they were M&Ms, and you want all the green ones in a separate bowl.
tomBitonti
Adventurer
I thought your idea of expanding bubbles arising from local statistical concentrations of matter or anti-matter was interesting.
There are a couple of problems though: Relative to a single bubble, points are distinguished by how far they are from the edge of the bubble. Unless the bubble immediately detaches from the larger universe. Also, that makes for a much larger universe than we can observe, so it seems hard to test.
Of course, "local statistical concentrations" could be quite large, if the universe is much much larger than the billions of light years which we can observe.
Thx!
TomB
freyar
Extradimensional Explorer
Umbran's covered a lot of stuff, so I will be relatively brief compared to my usual rambles. Just wanted to go through a couple more basic issues right now.
There's a question upthread about the difference between matter and antimatter and whether antiparticles have the opposite spin than normal particles. I'll just add a couple of things to what Umbran has said, which is first of all that each particle has an antiparticle. In some cases, the antiparticle is the same as the particle itself; this is the case for photons (quantum particles of light). As Umbran said, the "charges" of antiparticles are always opposite of the corresponding particles. That includes electric charge but also more mathematically abstract things. For example, the neutron is electrically neutral but has "weak charge," so the antineutron has opposite "weak charge." (I am grossly oversimplifying that.) Quarks have three types of "strong nuclear force charges" a.k.a. colors, known as red, green, and blue, but antiquarks have anticolors, which you might colloquially call cyan, magenta, and yellow.
Spin is a little more complicated in that it's a more technical definition. But, using suitable definitions of "spin" and "antiparticle," yes, they have opposite spins. This isn't too big of a deal, though, as most types of particles can spin in either direction, which means their antiparticles can too. Neutrinos are a bit weird, however. Neutrinos can only "spin left" (technical definition), which means antineutrinos can only "spin right." Theoretically, "right-handed neutrinos" could exist (and could explain some puzzles about neutrinos), but they don't interact with any types of particles we know about, so we've never been able to observe them.
There's a question upthread about the difference between matter and antimatter and whether antiparticles have the opposite spin than normal particles. I'll just add a couple of things to what Umbran has said, which is first of all that each particle has an antiparticle. In some cases, the antiparticle is the same as the particle itself; this is the case for photons (quantum particles of light). As Umbran said, the "charges" of antiparticles are always opposite of the corresponding particles. That includes electric charge but also more mathematically abstract things. For example, the neutron is electrically neutral but has "weak charge," so the antineutron has opposite "weak charge." (I am grossly oversimplifying that.) Quarks have three types of "strong nuclear force charges" a.k.a. colors, known as red, green, and blue, but antiquarks have anticolors, which you might colloquially call cyan, magenta, and yellow.
Spin is a little more complicated in that it's a more technical definition. But, using suitable definitions of "spin" and "antiparticle," yes, they have opposite spins. This isn't too big of a deal, though, as most types of particles can spin in either direction, which means their antiparticles can too. Neutrinos are a bit weird, however. Neutrinos can only "spin left" (technical definition), which means antineutrinos can only "spin right." Theoretically, "right-handed neutrinos" could exist (and could explain some puzzles about neutrinos), but they don't interact with any types of particles we know about, so we've never been able to observe them.
Just want to correct this quickly. A photon can never just split into an electron and positron because that would violate energy and momentum conservation. A photon that hits something, though, can create an electron and positron. Photons also have intrinsic angular momentum (spin), and total angular momentum coming in always has to equal total angular momentum coming out. So keeping track of spin is important in calculating particle interaction possibilities.Umbran said:
Not so much correct, as complete. The fact that this transition must be mediated was something I was leaving out for sake of brevity. Concept first, then details!
Same reason I left out discussion of "spin" vs "helicity".
