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45

Whether the infalling material is matter or antimatter makes no difference. Fundamentally, the confusion probably comes from thinking of black holes as normal substances (and thus retaining the properties of whatever matter went into making them). Really, a black hole is a region of spacetime with certain properties, notably the one-way surface we call an ...


43

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 ...


38

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 ...


37

To maintain lepton number as a conserved quantity. Consider, in detail, what's going on in a beta decay (well, I'm going to ignore the nuclear context). The reaction is then $$ n \longrightarrow p^+ + e^- + \nu \,,$$ where you should take the symbol $\nu$ to mean some neutrino (without prejudice about matter-type or anti-matter-type for the moment). There ...


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 ...


28

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 ...


27

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 ...


25

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 ...


25

A particle isn't really a point particle; its position is best described by a wavefunction: the probability if finding it in any particular region in space. For annihilation to occur, the wavefunctions of the two particles has to overlap: to the extent that they overlap, there will be a probability that annihilation can occur. The greater the overlap, the ...


25

Mesons are not elementary, they are composed of quarks. So take a look at their quark content. The charmed eta meson consists of a charm and an anti-charm quark, denoted $c\overline{c}$. An anti charmed eta meson would therefore be an anti-charm and an anti-anti-charm (which is just a charm) quark, i.e. $\overline{c}c$, which is obviously the same as $c\...


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 ...


22

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 ...


22

Charged antimatter particles are stored using electric and magnetic fields in near vacuum conditions. (Near-vacuum conditions can be created on Earth) Anti-hydrogen is stored by exploiting its magnetic properties. (While neutral, it still has spin magnetic moment. The storage is done using strong superconducting magnets.) Antiparticles are easier to store ...


21

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 ...


21

At tree level, a matter-antimatter annihilation reaction doesn't just produce gamma rays, nor can you exclude neutrinos in the final state. Even the simplest such reaction can—given enough energy—produce a variety of particle-pairs. However, those pairs are subject to two subsequent processes: If the particles are not stable, they will decay towards ...


20

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

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 ...


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".


18

Interstellar space is an excellent vacuum, but it's not a perfect vacuum. For example Earth is constantly bombarded with protons from the solar wind, which stream outward uninterrupted until the heliopause when matter from other stars becomes more dominant. If there were, say, an antimatter star nearby, the place where its stellar wind of antiparticles met ...


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

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 ...


15

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 ...


15

You should not think a particle as point like. Classically, the probability of two point like particles colliding with random location and velocity is 0, that is why you said it never happens. However, at quantum mechanical level, these particle are described by wavefunction. It means that there is spreading in its spatial location, say 0.1nm (the minimum ...


15

Annihilation conserves everything What might possibly be unintuitive is that during matter-antimatter annihilation nothing disappears - the particles simply get converted to other particles and energy. From the point of gravity, however, energy is mass - so from the point of an outside observer, if no particles escape the system, then it doesn't matter if ...


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 ...



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