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36

So, what is antimatter? Even from the name it is obviously the "opposite" of ordinary matter, but what does that really mean? As it happens there are several equally valid ways to describe the difference. However, the one that I think is easiest to explain is that in antimatter, all of the electrical charges on all of the particles, at every level, have ...


36

To the best of my knowledge, most physicists don't believe that antimatter is actually matter moving backwards in time. It's not even entirely clear what would it really mean to move backwards in time, from the popular viewpoint. If I'm remembering correctly, this idea all comes from a story that probably originated with Richard Feynman. At the time, one of ...


34

A sophisticated, yet easy way to see that this the answer must be "No." is to recall that velocity is relative — that there is no absolute notion of velocity. You said the matter was moving and the antimatter still, but that point of view (AKA frame of reference) is not privileged in any way. An observer at rest with respect to the matter has just as much ...


25

Well, they do and don't. Depends on your point of view. Here's the story. Quantum field theory requires for consistency reasons that every charged particle has its antiparticle. It also tells you what properties will the anti-particle have: it will have the same characteristic from the point of view of space-time (i.e. Poincaré group) which means equal mass ...


25

Antimatter has the same mass as normal matter, and its interaction with gravity should be the same according to GR and QM. That said, antimatter has only been created in tiny amounts so far and only few experiments have been performed to confirm there is no new physics involved. The gravitational interaction of antimatter with matter or antimatter has ...


24

One cannot tell by the light spectra. Hydrogen and antihydrogen would give the same lines in the spectrum. The prevalence of matter over antimatter from other evidence indicates matter is predominant in the observable universe, and here is a nice review. How do we really know that the universe is not matter-antimatter symmetric? The Moon: Neil ...


24

To be a little pedantic, nobody has yet done precision spectroscopy of antihydrogen, though the recent success in trapping it at CERN (all over the news this week, paper here) is an early step toward that. It's possible that there are small differences in the spectrum of antihydrogen and hydrogen, though these differences can't be all that large, or they ...


21

If I ruled the world, I would ban the phrase "pure energy" in contexts like this. There's no such thing as pure energy! When particles and antiparticles annihilate, the resulting energy can take many different forms -- one of the basic principles of quantum physics is that any process that's not forbidden (say, because of violation of some sort of ...


19

The only experiment I know of was done by the ALPHA team at CERN. The results are published in this paper. The error bounds are huge - all the team were able to say is that the upper limit for the gravitational mass of antihydrogen is no greater than 75 times its inertial mass! However I believe an updated version of the experiment, ALPHA2, is in progress ...


19

This is really just an extended comment on CuriousOne's answer. You probably know that there are just a few elementary particles: six quarks, three electron-a-likes (electron, mu and tau), three neutrinos and various assorted bosons. All matter is made up from various combinations of these particles. The problem is that the heavy particles decay into the ...


18

According to http://en.wikipedia.org/wiki/Antihydrogen, anti hydrogen has been produced - so the answer to your question is "yes, it is real".


17

The basic tragedy of space travel is expressed by the Tsiolkovsky rocket equation, which says that the amount of reaction mass you need grows exponentially with your $\Delta v/v_e$, where $v_e$ is the exhaust velocity. The advantage of antimatter propulsion is high energy density, but energy density doesn't have any direct, major effect on the amount of ...


17

I am assuming that by "energy" you mean photons. So you want to transform protons into photons. It is not possible. It would violate several conservation laws - mainly the charge conservation (protons are positively charged), but also baryon number conservation. The antiparticle is necessary to cancel these quantum charges to make the transition possible.


16

There are other neutral particles with antiparticles, such as the neutron and the $K^0$ meson. In those cases we have a microscopic theory that says those particles are made of quarks: for instance, the $K^0$ is made of a down quark and an anti-strange quark, while its antiparticle the $\bar K^0$ is made of a strange quark and an anti-down. The neutrino is ...


15

The definition of an antiparticle is dependent on having the opposite quantum numbers of the particle so that they can annihilate, i.e. the sum of the conserved quantum numbers are zero. Thus the answer by @mpv is adequate. The implication of your question is then: is baryon number conservation a strict law or an emergent law that may be violated at some ...


15

The ultimate goal is to be able to do precision spectroscopy of antihydrogen, to make sure that the energy states are the same as in ordinary matter. If there are differences between the energy levels of ordinary hydrogen and antihydrogen, that would violate "CP" symmetry, which says that if you change the sign of all the charges in some system, and invert ...


14

Dear Chad, you misinterpret the statement that "the known sources of CP-violation are not enough to explain the matter-antimatter asymmetry in the Universe." You seem to think that the statement means that the known CP-violating parameter (namely the CP-violating phase in the CKM matrix) and the processes based on it are qualitatively insufficient to ...


14

Well, the "new" baryons are really just expected short lived combinations of quarks. The only stable free ground state of quarks are protons. The free neutron has already a slightly higher energy than the proton, which makes it unstable. Only the interaction with other protons and neutrons inside of nuclei can stabilize this quasi-stable particle. Given ...


14

The particle-antiparticle annihilation is on a per-particle basis. One electron annihilates on positron. One up quark annihilates one anti-up quark. One down quark annihilates one anti-down quark. Moving at relativistic speeds doesn't change the number of particles. For that matter, you could annihilate an electron with an anti-muon, since an electron and a ...


12

Each particle only annihilates its exact antiparticle. Electrons annihilate positrons. A blue up quark annihilates an anti-blue anti-up quark. A muon annihilates an anti-muon. The thing about anti-matter is that it postulates an exact opposite of every particular particle type (except for things like photons that are their own antiparticles). It's about ...


12

Charge is only the most familiar of the properties that are inverted between a particle and its antiparticle, but it's not the only one. So you should not consider "same mass and opposite electric charge" to be a definition of what an antiparticle is; it's merely a plain-English explanation. A list of properties in which particles and antiparticles differ ...


12

The Dirac equation implies negative energies as well as positive. This is due to energy-momentum relation $E=\pm \sqrt{m^2+p^2 }$. If we replace $E$ and $p$ by operators $E\to i\frac{\partial }{\partial t}$ and $p\to -i\nabla$ we get the Klein-Gordon equation $(\Box+m^2)\phi=0$ for scalar (spinless) fields $\phi$. The problem with this equation is that it ...


12

From the very basic understanding that they are created out of nothing mutually and collide to annihilate each other seems to indicate this happens due to an attraction. Why? this just means that if two of them are nearby, they can annihilate. Remember that particles are waves, and thus are quite spread out. They don't have to be directed to collide ...


12

See also: What is anti-matter? Currently there is no reason to believe/require antimatter has negative mass. It should therefore behave exactly the same in a gravitational field. The matter-antimatter distinction is pretty arbitrary. We found protons/neutrons/electrons first, so particles of the same families that exhibit similar behavior are "matter", and ...


12

Creating anti-protons is straightforward in principle because any high energy collision produces a shower of protons, antiprotons and various types of pions. The pions decay in a few nanoseconds, so you just have to wait for the pions to decay then separate the antiprotons from the protons. At Fermilab a 120GeV proton beam was collided with a nickel target ...


12

Dark energy is an unknown or unattributed form of energy that is separate and distinct from the other forms of energy. It is not anti-engery. It is dark energy. Anti-energy (were such a thing to exist) would annihilate any form of energy. Dark energy is called "dark" because we aren't exactly sure what it really is or what causes it. The most abundant forms ...


11

Although none of these questions is an exact duplicate, there is a lot of overlap, and I hope we can avoid stringing this kind of stuff out indefinitely. The good news is that you're apparently being very cautious about the safety hazards of your planned matter-antimatter spaceship -- hazards that science fiction authors typically blithely ignore. Please let ...



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