I understand that it is ridiculously difficult and expensive to produce antimatter by artificial means. So I am asking here only theoretically.
If an object were to be made out of antimatter, and it was large enough to be seen by the human eye, would it reflect light in the same fashion as ordinary matter?
I did read the Wiki page on antimatter, but there was no mention of this.
Short answer: yes.
Longer answer - a photon is its own antiparticle, and the laws of QED work exactly the same with antimatter as matter, so photons are reflected in exactly the same manner from interaction with positrons and antimatter as they are with interaction with electrons and ordinary matter.
Wow, four minutes! Astonishing! I watched a show last night which said that it is speculated that light goes right through dark matter instead of being reflacted; and that got me thinking about why exactly it is that ordinary matter reflects light, and in what way dark matter would have to be different in order to not be reflective and … well, you understand. 
I realize that antimatter and dark matter are two different things; it was just all part of my general rumination.
Thanks, **Francis Vaughan. **
Basically, any matter that has electric charge (or contains charged particles) can interact with light, or EM waves of some frequency. “Dark matter” is hypothesized to have no electric charge because otherwise we would observe its effect on light traveling through clouds of it.
All this implies that light would not be reflected from dark antimatter, though. It wouldn’t interact with it any more than it does with dark matter.
Yup. And dark matter, whatever it is, will also have antiparticles: Either separate particles, or they’ll be their own antiparticle (or some mixture of both).
Also note that, even if a dark matter particle and its antiparticle are distinct, if they annihilate each other, it won’t be by converting into photons (in fact, converting directly into photons is pretty rare for matter-antimatter annihilation, and is mostly limited to just the electron, muon, tauon, and W). They might convert into some other unfamiliar particle, or into neutrinos, or maybe even into some sort of mesons.
Annihilation to photons is allowed. It just has to be at loop level.
What happens when protons and anti-protons collide? Or even better, if a cloud of hydrogen and anti-hydrogen collide? The electrons/positron collisions release gamma rays, and the proton/antiprotons release ???
Do they collide? Why not just “orbit” each other?
When they collide at low energies (i.e., nearly at rest), they annihilate to a handful of mesons, most commonly pions and kaons but also heavier mesons (eta, rho, omega, phi, …). These mesons are all unstable particles (each being made of a quark and an antiquark), and so they in turn quickly decay into lighter mesons and/or muons, electrons, neutrinos, and photons. Some of those particles are also unstable, and in the end you’ll be left with electrons, neutrinos, and photons.
If your proton and antiproton are colliding at higher energy, then heavier products can spew forth from the collision. This is what many high-energy particle colliders do, with the now-shuttered Tevatron being the highest energy proton/antiproton collider ever. Those collisions can produce anything in the particle zoo with some probability, subject to certain conservation laws (including energy conservation).
The answer is pretty much the same as the above “at rest” case, except now there are electron/positron pairs in the mix which produce, as you correctly note, photons.
They can “orbit” each other, but it’s tricky to get them to do so. You either bring them together violently and hope some bind up, or you bring them together delicately and hope some bind up. Either way, the bound state is short lived, with the proton and antiproton annihilating after a microsecond or so.
Other particle/antiparticle pairs form bound states. Many mesons are themselves this sort of state. Electron/positron pairs can form “positronium”, which has a lifetime around 100 nanoseconds.
To be more specific, the most likely outcome of low-energy proton-antiproton is three pions, either three pi0, or one each pi-, pi0, and pi+ (with the latter being about twice as likely as the former). Any pi0 will decay into a pair of photons. A pi+ will most likely decay into a mu+ and a mu neutrino, and likewise a pi- will decay into a mu- and a mu antineutrino. A mu+ will in turn decay into an e+, a mu antineutrino, and an e neutrino, and reverse that for a mu-. And eventually, all of the e+ and e- you produced (there will be equal numbers of them) will find each other and annihilate to photons (or, more likely, the e+ will find and annihilate ambient e- from the environment, which will then be replaced by the e- you produced, but that amounts to the same thing).
In terms of numbers, then, proton-antiproton will eventually result in an average of about 3 1/3 photons and 4 neutrinos per original particle pair. In all cases, the bulk of the extra energy in a decay will be carried off in the kinetic energy of the lighter particle to be released, and the biggest jump in mass is from the muon to the electron (where the lighter particles are neutrinos), so bulk of the energy release will be in neutrinos.
Now, I hadn’t yet got far enough in my ruminations to arrive at the postulation of dark antimatter… this is all fascinating!
Are there any theories about how matter could be formed from uncharged particles?
Also, if a sufficient quantity of particulate antimatter happened to exist in close proximity, would it tend to aggregate into clumps?
Hey, wait a minute … if I understand you guys correctly, you are saying that it is supposed that there is both anti-dark-matter, and dark antimatter. That is sort of mind blowing to me. I went to the wiki pages on “antiparticle” and “photon” to get an explanation of how photons can be their own antiparticle, and now I believe that I get it.
Overall, it seems to me that the people who figure these things out must have an especially good grip on slippery concepts.
Every particle has an antiparticle. Sometimes the antiparticle is the same as the particle itself, but there’s always something.
And as a general rule, everything that matter does, antimatter does, too. If you take any situation that happens in the real world, replace all the particles with their antiparticles, you’ll get (almost) a situation that also happens in the real world. This is called C symmetry.
(stop reading here if you want)
That “almost” deserves an explanation. It’s absolute as far as electromagnetism and the strong force are concerned, and we’re guessing also for gravity. But it turns out it doesn’t work for the weak force: If you’ve got the weak force involved, then you need to also replace everything by its mirror image, and then you’re back to a real physical situation. This is called CP symmetry.
Well, again, almost. It turns out that if you replace every particle by its antiparticle, and mirror-image everything, all the same processes can occur, but some of them (again, all involving the weak force) occur at slightly different rates, a difference of about one part in a thousand. If you really want to get exactly back to symmetry, then you need to swap particles and antiparticles, take the mirror image, and run time in reverse. This is called CPT symmetry.
How can particles bind with their anti-particles? Surely they should just annihilate?
Briefly.
This question warrants some terminology clarification. “Matter”, “antimatter”, and “charged” all get used in multiple ways depending on context, so your question could be answered in multiple ways.
There are a handful of fundamental particles of nature that we know about (electrons, quarks, photons, … everything in this table). Those entities are usually called “particles”, and they have “anti” counterparts that are usually called “antiparticles”, although in some cases the particle and antiparticle are actually the same entity. To confuse things, though, sometimes these two sets are called “matter” and “antimatter”. To confuse things more, sometimes only the quarks and leptons are called “matter” (and the antiquarks and antileptons called “antimatter”), with the bosons on the right of the table being left out of the definition.
These fundamental particles can join up to make composite particles. The most familiar are the proton (made of two up quarks and a down quark) and the neutron (made of two down quarks and an up quark). Neutrons and protons can bind up via the strong force to form nuclei, and these nuclei can capture electrons to form atoms. In certain contexts, one would use “matter” to mean these nuclei or atoms and would use “antimatter” to mean analogous nuclei or atoms made from antiprotons (made of antiquarks), antineutrons (made of antiquarks), and antielectrons (a.k.a. positrons).
So, can uncharged particles combine to form matter? Depends what you are interested in. A lone neutron could be called matter, and it is neutral. A hydrogen atom is neutral, and two of these can combine to form H[sub]2[/sub] which is undeniably matter. On the other extreme, gluons are electrically neutral yet we expect they can form bound states with each other. Whether you want to call the resulting “glueball” matter or not is just a choice of definition. (Probably it’s not what you have in mind.)
The gluon example gets us to how “charged” can mean different things. In everyday language, “charged” means “electrically charged”, but it’s not the only type of charge there is. An electron binds to a proton to form hydrogen because both have electric charge. A proton and a neutron bind to form a deuterium nucleus because each is made of things that have “strong” charge. (Protons and neutrons have no strong charge themselves, but the strong charge of their constituents quarks bleeds out, analogous to electrically neutral atoms forming molecules because the components of the atoms are electrically charged.)
Dark matter is electrically neutral, but it may have other charges. There are many viable theories in which dark matter particles interact with each other through their own dark-sector forces(*), forming dark atoms. These dark atoms could then reasonably be called “dark matter”, except that the term “dark matter” is already taken to mean generically anything that does what dark matter needs to do cosmologically and astrophysically.
(*) To make the connection explicit: forces and charges are related. Things that have strong charge interact via the strong force. Things that are electrically charged interact via the electromagnetic force. Some new “dark charge” on dark matter would lead to a dark force.
Given the above, I’ll reword your question to what I think you are asking: “If a sufficient quantity of antiatoms (made of antineutrons, antiprotons, and antielectrons) were found together, would they clump together to form antiplanets, antistars, antigalaxies…?” Yes, as far as we know. We haven’t seen any large anti-clumps out in the universe (which is a story all its own), but they should behave basically the same as if they were made of matter.
Not exactly. Particles have antiparticles, so dark matter particles could reasonably have “anti” versions of themselves. But whether you call these entities anti-dark-matter or dark antimatter is just a terminology choice.
It should also be noted that “matter” and “antimatter” aren’t really well-defined categories. It’s straightforward to say that two particles are antiparticles of each other, but it’s not always straightforward to say which is which. Up quarks, down quarks, and electrons are familiar to us, so we call those matter, and anti-up, anti-down, and positrons antimatter. We can extend that to mu-, tau-, charm, strange, truth, and beauty being matter, because they’re similar to electrons, ups, and downs, with their antiparticles being antimatter. But the pi+ and pi- are each others’ antiparticles, and neither is familiar, nor directly analogous to anything familiar. Which one is matter and which is antimatter? It doesn’t, pardon the expression, matter. Likewise with the W+ and W-, or many other pairs.
Let’s look first at an antimatter-free example. Consider the isotope potassium-40. It has a nucleus with 19 protons and 21 neutrons. 19 electrons are bound to that nucleus. Each electron has a position around the nucleus that can only be discussed probabilistically. The lower-level electrons have a more compact probability distribution around the nucleus, but the main detail is that the probability distribution for the electrons’ positions can overlap the nucleus (which has its own very compact distribution).
Potassium-40 decays with a half-life of a billion years. 10% of the time, the decay mechanism is electron capture. In electron capture, one of the electrons and one of the protons combine to form a neutron and a neutrino. This new state (argon-40 plus a free neutrino) is favorable from an energy point of view, but the transformation doesn’t happen as soon as you make potassium-40. All those electrons can be happily bound around all those protons for a billion years. The timescale for the transformation (i.e., the radioactive decay) is governed by the degree of overlap in the positions (spatial probability densities) of the various particles, by the difference in the kinetic energy of the various particles before and after, by the number of particles involved, and in general a few other things.
The situation is the same for an electrically charged particle binding up with an electrically charged antiparticle. You’ve got two entities that would happily convert into something else if given the chance, but the process is random and happens over some characteristic time scale (i.e., with some half-life), and in the meantime, they will happily form an exotic atom.
Pi+ and Pi- are ambiguous because they are composed of a quark and an anti-quark.
For dark matter, is it possible that even the fundamental particles may not be unambiguously matter or unambiguously antimatter? I’m guessing that it is, but maybe that depends on which is your favorite dark matter particle candidate.
Definitely. Note that Chronos’s W boson example is just such a case within the Standard Model. That is, the W is a fundamental particle with a distinct antiparticle, but there is no particular reason to call one the particle and the other the antiparticle. They are simply each others antiparticle.
Note that I’ve already trademarked the term “Darkon” for the dark particle that mediates the dark force.
