Underground Lab To Probe Ratio of Matter To Antimatter 82
Wired reports on the Enriched Xenon Observatory 200, a particle detector scientists hope will answer the question of why there is significantly more matter than antimatter in the universe. Quoting:
"The new detector will try to fill in the picture, determining basic features of [neutrinos], like their mass and whether or not they, unlike almost all other particles, are their own antiparticles. That quirk is why some scientists believe neutrinos could be the mechanism for the creation of our matter-filled universe. Almost all other particles have an antiparticle twin that, if it comes into contact with the particle, immediately annihilates it. But if neutrinos are their own antiparticles they could conceivably be knocked onto matter's 'team,' thereby causing the cascading win for matter over antimatter that we know occurred. As the Indian theoretical physicist G. Rajasekaran put it in a speech [PDF] earlier this year, neutrinos that are their own antiparticles would explain 'how, after [the] annihilation of most of the particles with antiparticles, a finite but small residue of particles was left to make up the present Universe.'"
Re:How so? (Score:5, Informative)
If there had been an exactly equal amount of matter and antimatter since the birth of the universe, it would have long ago annihilated into photons. Since this has manifestly not occurred, one is in excess of the other. Which one you label as "matter" and which one "antimatter" is purely down to personal preference.
Some differences can be observed in the behaviour of matter and antimatter; for example, in a magnetic field, electrons will curve in one direction and positrons in another. Another example is the asymmetries observed in the oscillations of mesons (the classic example being the K0) which reveal a clear, fundamental difference between matter and antimatter.
Re:How so? (Score:5, Informative)
Who says there is more matter than antimatter in the universe? Has anyone ever gone to the Andromeda galaxy? So how do we now it consists of normal matter?
Gas is observed between the galaxies, but not the hard radiation that would be produced when it reached a galaxy and annihilated.
Re:How so? (Score:5, Informative)
Even an easier explanation. The annihilation of antimatter with matter gives off very distinct spectral patterns. We know what these are, and we have calculated them for any observable red/blue-shift range. We've yet to observe any great quantity, or even a 'halo' around any other galaxies. In an equally distributed universe, you would see such things, as high-velocity matter from one universe collided with another, and give off pretty lights. We don't see such, hence, the universe is mostly matter.
Re:Questions for a physicist in this field (Score:5, Informative)
Does antimatter attract matter or repulse it
*Charged* antimatter will attract its matter counterpart electrostatically via the Coulomb force. For example, an electron will attract a positron. This does not occur for neutral particles.
The gravitational attraction between bodies must also be considered: it is negligible between individual particles, but comes into effect for macroscopic objects. The gravitational interaction is determined by the amount of mass present, and is always attractive, regardless of whether it is between matter or antimatter.
(could a double star, one of antimatter and one of matter, i.e. where the stars revolve around each other exist?).
A star will be (on average) neutrally charged, otherwise Coulomb repulsion between like charges will break it apart. Therefore it is fair to say that both the matter and antimatter star will be electrically neutral or at least not significantly charged.
Even if there were two charged objects, the Coulomb force, like the gravitational force, follows the inverse square law, so extra electrostatic attraction will be equivalent to more mass.
In either case, a stable orbit could form.
Would it be a prerequisite that a big bang produces as much matter as antimatter
I'm moving out of my depth here, but the answer is probably no. Even if it did, future interactions between particles may cause an imbalance (as is thought to have occurred in our universe). A prerequisite for what, anyway?
Basic answer (Score:4, Informative)
Does antimatter attract matter or repulse it
IANA particle physicist or cosmologist, but I can answer this one: it depends on which particles. For example, a position (anti-electron) has opposite charge to an electron and will thus attract electrons and repulse protons.
To pick a nit (Score:2, Informative)
It's been theorized, I think, that the former is true, that it really is it's own antiparticle, based on hypothesized neutrinoless double-beta decay--which, if true, insinuates the former. But this is clearly outside the standard model and is having difficulty gaining popularity, as far as I know, since it hasn't actually been seen yet.
The other I suppose "just so" method of defining a neutrino as its own antiparticle is by working nomenclature--there are four different kinds of neutrinos in this sense; muon, electron, and their two counterparts--anti-neutrinos if you will, which are still technically neutrinos.
I'm not a particle physicist, and I may very well be wrong in many places above. So if there is anyone about who can correct me I would very much appreciate it.
Re:Questions for a physicist in this field (Score:5, Informative)
Would it be a prerequisite that a big bang produces as much matter as antimatter?
I'm moving out of my depth here, but the answer is probably no. Even if it did, future interactions between particles may cause an imbalance (as is thought to have occurred in our universe). A prerequisite for what, anyway?
I'm afraid the answer is actually yes, at least if you replace 'a prerequisite' by 'excepted from the laws of physics'. Since there is nothing in the laws of physics to make us believe that matter is 'preferred' over anti-matter, we naturally assume that the amounts of both in the early universe are the same. The problem with this most natural assumption is that, if there was the same amount of matter and anti-matter in the universe, and they stayed in thermal equilibrium as the universe cooled (which is true for most of its history), they would almost completely annihilate and there would be too little matter left over today to make up what we presently see. So we need an event that favours matter over anti-matter to produce the required leftover matter we see. This is called the baryogenesis problem [wikipedia.org]. Of course, you CAN just demand that the required extra matter be put in as an initial condition, but most physicists shun that approach, especially since the matter excess is one part in 10 billion, an unnaturally small number which would have to be put in by hand. We prefer to find a way to generate that asymmetry dynamically.
Re:To pick a nit (Score:4, Informative)
See the graphic at http://en.wikipedia.org/wiki/Elementary_particle [wikipedia.org] and the article at http://en.wikipedia.org/wiki/Antiparticle [wikipedia.org] is also of interest.
Elementary particles with no charge cannot have an anti-particle, since the definition of anti-particle has to do with having the opposite charge, as I understand things.
Re:To pick a nit (Score:3, Informative)
Actually, in the standard model, both neutrinos and antineutrinos exist and are distinct. The key point is that it is the weak, not electromagnetic, force that is important.
The weak interaction 'connects' leptons and neutrinos. For example, an electron can turn into an electron neutrino by emitting a virtual W- boson. Conversely, a positron turns into an anti-electron neutrino by emitting a virtual W+.
Just like electrons have non-zero electromagnetic charge, neutrinos and leptons have non-zero weak charge, known as 'weak hypercharge'. This enables them to interact via the weak interaction. Neutrinos have opposite weak hypercharge to antineutrinos, so there is a definite distinguishing feature.
In contrast, bosons like photons and the Z0 do not have antiparticles because their charges (electromagnetic, weak hypercharge, etc) are all zero.
As a corollary, gluons have antiparticles because their colour charge is non-zero.
Re:How can you tell? (Score:4, Informative)
In a perfect vacuum, it isn't. However, even the intergalactic medium isn't a perfect vacuum, and somewhere there would have to be a border between a matter region and an antimatter region. Such a region would give off a very specific gamma ray spectrum, with a peak at 511 keV due to positron-antipositron annihilation and several peaks in the 70-400 MeV range due to proton-antiproton annihilation; the rate of interaction would be low, but the surface area of the frontier so large that we should be able to observe it from Earth. If it is a more localized phenomenon (like Niven's star system), then it would be travelling through the interstellar medium, inside a galaxy, which is far denser.