|MadSci Network: Physics|
Hello Francisco, Your very question of "what is matter" is the very reason many curious people go to school for physics! It's something that most physicists (indeed, most curious people!) think about very often. There isn't exactly a short answer to everything that you've asked, but bear with me and I'll try to give you a good response. The definition that I remember from when I took my first course in physics was "matter is anything that has mass and takes up space." That's true enough in most senses, but to the scientists actually studying the more exotic kinds of matter the response is a little ill-posed (more on that later). So, first up matter, anti-matter, and light. There really is anti-matter. And indeed it's more common that a lot of people think. You can even find it (in small quantities and for very, very short periods of time) out in nature. So, what is anti-matter? Basically the difference between an electron and an anti-electron is that they are identical except for their respective charges. Anti-matter is extremely unstable in a sense. If a particle and its antiparticle interact, they can annihilate each other releasing energy. Typically you can think of this. An electron and anti-electron (positron) interact and produce two photons. The photons each possess both the kinetic energy and the rest mass energy of the electron-antielectron pair (1/2 to each photon). However there do not exist large quantities of anti-matter (that we know of) so you need not worry about bumping into any. It is also difficult to contain and isolate, making it hard to collect any reasonable amount of it. Now in terms of the different kinds of matter, there is a whole zoo of different particles. They are categorized based upon how they interact with other things. Or put in other words, they are classed by which forces of nature they respond to. The particles that can interact with all the forces (or force carriers) are known as Hadrons and are composed of quarks and gluons (or sometimes anti-quarks too). Strong, weak, and electromagnetic interactions are all allowed for these particles. Familiar things such as protons and neutrons fall into this category. In fact quarks themselves fall into this category. There are also particles called Leptons that are very familiar. They can only experience the weak and electromagnetic interactions, but not the strong interaction. They include electrons and neutrinos. Leptons also appear a bit different from hadrons in another way. They do not seem to have any physical dimension (at least to the limits of our best experiments, nor does it make sense for them theoretically). Or put a different way, they appear as points when we do experiments that try to resolve their size (and just to avoid confusion, I'm talking about unbound electrons in free space, not bound electrons). If that sounds strange, then good! Things are very unusual on the very small scale and our physical intuition of how we think they should look is often wrong. There is another kind of particle that is sometimes called "matter," but it depends upon who you ask, Bosons. Those are the force carriers for the different interactions; photons, W and Z bosons, and gluons for the electromagnetic, weak, and strong interactions respectively (the unseen graviton would also be here). Photons do not have a rest mass and hence do not fall under the classification of matter. Photons also only come in one variety, there are no photons and anti-photons, just photons (strictly speaking it's possible to say there are both, but we have no way of telling them apart. hence they appear the same). W bosons, Z bosons, and gluons all have a rest mass, but have a few other features that make them unusual. You may have noticed that I never really mentioned Gravity as a force of nature. That's because on the small particle scale, gravity interacts very, very, VERY weakly with everything. It is essentially neglected. However, we're here aren't we? The earth is exerting a gravitational attraction, therefore there must be more! So we suspect, strongly suspect, there are little particles called gravitons that mediate the gravitational interaction. They should not have any mass The last kinds of matter physicists sometimes talk about are those kinds that are speculative at this point. None of these particles have ever been observed in nature, but in some cases there exist very compelling arguments for them. The highest of these (on my list anyways) is the Higgs particle. It is predicted by the Standard Model of particle physics (1980) and is the crowning jewel of that theory. Because the theory has been so very, very successful at describing everything else within its framework, most physicists will be very surprised if this particle is never discovered. It is the mechanism through which all the other particles have mass(there may in fact be many different kinds of Higgs particle, not just one. but for now everyone would be very happy if any of them were seen). This is a current area of intense research. Fermilab has tried for a number of years to observe the Higgs, but has not succeeded. Now there is a new particle accelerator in Europe (the Large Hadron Collider) that should (we hope!) be capable of producing the energies needed to observe the Higgs particle. There is also a huge class of (to this point) never before seen matter known as super-symmetric particles. These are particles predicted to exist by some interesting theories. However, these are more speculative and people are a bit more uncertain about their properties (namely the mass) and whether or not they actually exist. In rough terms, they are used to account for the vast difference in the strength of the strong, weak, and electromagnetic forces from the very tiny gravitational force. Beyond these particles there are still more that are predicted on a more and more speculative basis. They are predicted in an effort to explain something and given properties which do not violate any laws of nature, but also make them thus far undetected. Tachyons, particles that have always been moving faster than the speed of light, are such an example. The existence of a tachyon does not (in principle) violate anything, but there is not a truly compelling reason for them to exist. Axions are another such particle. Scientists (very insightful, respected ones) are attempting to find direct evidence of such particles, but things are very uncertain and speculative at this point. Lastly I'll say this. My training and area of study is known as Condensed Matter Physics. I'm not a particle physicist and as such these things are not in my area of expertise. I may be missing a few of the finer points, but I've tried to make sure everything that I've said is correct. --------------------------- Michael S. Pierce Materials Science Division Argonne National Laboratory
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