There are three main areas of ongoing investigation in modern physics. One is an effort to better understand the world of the very small. This is the realm of quantum mechanics and the standard model of particle physics. Another is to understand gravity better, an understanding currently summed up in Einstein's theory of general relativity. The third is an effort to understand "emergent phenomena" -- things that happen in nature where the fundamental physical laws are well understood, but how they play out is not.
Quantum mechanics and general relativity are not consistent theories. General relativity is a "classical theory." It is deterministic and assumes an infinitely smooth universe. Quantum mechanics is a stochastic theory. It predicts in a highly accurate way the probability of individual or small groups of fundamental particles acting in particular ways. Quantum mechanics is normally considered in "Minkowski space" which means that the ordinary geometrical rules of Euclid modified by Einstein's theory of special relativity is used, but that Einstein's rule of general relativity is not used.
Both quantum mechanics and general relativity are four dimensional, with three dimensions in space and one in time.
The four forces
Modern physics is based upon the working assumption that there are four fundamental forces in the universe that govern everything. Three of those are described by quantum mechanics: electro-magentism, the nuclear strong force and the nuclear weak force. Part of quantum mechanics that explains electro-magnetism (i.e. electricity, magentic behavior, light, radio waves, X-rays, etc.), called QED, is about as close to perfect as a physical theory can get (although there remain some questions of how to interpret certain phenomena whose empirical behavior can be predicted). A generalization of QED called electroweak theory also does a good job of explaining the weak force (the weak force is the force that causes heavy radioactive elements to decay). A similar theory called QCD (for quantum chomodynamics) explains the nuclear strong force (which is what holds the nuclei of atoms together) although our ability to come up with experimental results is better than our ability to use the very difficult equations involved to actually make theoretical predictions.
The fourth force, of course, is gravity, explained by general relativity, which basically says that mass and energy curve space causing the phenomena we call gravity, which behaves in non-intuitive ways in very strong gravitational fields and when masses and energies are moving. In general relativity, gravity affects massless particles, like photons, as well as massive particles.
Why do we ordinarily not observe strong and weak forces? Because they only operate at very short distances, generally within atoms. Gravity and electromagnetism are the only long range forces known to modern physics.
The "fundamental" particles
In addition to the four basic forces, there is list of particles which have been discovered which appear to be "fundamental". These are broken into "force carrying" particles which have "spin zero", and the particles that make up more ordinary matter which have "spin one half".
The spin zero particles are: the photon (which carries the electro-magentic force, has no mass, and has the characteristics of both frequency and polarization), the W, Z+ and Z- particles (which carry the weak force, are massive, and have neutral, positive and negative electrical charges respectively), eight kinds of gluons (which carry the strong force) which are distinguished by their "color charge" which comes in types R, G and B (which have no connnection to real colors, they are just useful names).
The spin one-half particles all come in three generations, the first being the "electron generation", the next being the "mu" generation which is heavier and shorter lived that the previous generation but otherwise identical, and the thrid being the "tau" generation which is even heavier than the "mu" generation and even shorter lived than the previous generation. (What I have called generations are also sometimes called "families"). These are broken into two kinds of "leptons", the neutrino and the electron in each of three generations, and two kinds of quarks (the up and the down in each of three generations). All spin one-half particles are currently believed to have mass. Electrons (and their higher generation kin) have an electrical charge called -1. Up quarks have an electrical charge of +2/3 and down quarks have an electrical charge of -1/3. The strong force affects quarks but not leptons, and quarks have color charge (R, G or B or anti-R, G or B). All spin one-half particles come in matter and anti-matter versions. All spin one-half particles except neutrinos also come in "right handed" and "left handed" versions.
A proton is made of two up quarks and one down quark. A neutron is made of one up quark and two down quarks. Quarks only combine in combinations that are color neutral (i.e. either a color and its anti-color, or one of each color). There are also a great many of two and three quark combinations involving both up and down quarks, and higher generation quarks. Two quark particles are called mesons. Three quark particles are called bayrons. (Mesons and baryons collectively all called hadrons). In theory, five and six quark combinations (or more) are possible, but the experimental evidence for a pentaquark or higher is very new and has not been strongly and repeatedly confirmed.
Why is the world almost entirely composed of up quarks, down quarks, electrons and neutrinos? Because everything else is highly unstable and rapidly undergoes decay.
There are at least 165 different kinds of unstable particles whose lifetimes have been experimentally measured. Protons, electrons and first generation neutrinos appear not to decay. A neutron is the longest lived unstable particle and lasts on average about 886 seconds. The muon (a second generation electron) lasts on average for only 10^-6 seconds (a hundred million times shorter than a neutron). The next three longest lived particles that decay, kaons and pions (common two quark particles called mesons), last 1% of the life of a muon (one ten billionth as long as a neutron), which is 10^-8 seconds, and no other particle lasts more than about 1% of the life of a kaon or pion (a trillionth as long as a neutron), which is 10^-10 seconds. Only 36 possible particles last on average longer than 10^-22 seconds. The W and Z particles discussed above last on average 10^-25 seconds. The data is reviewed at length in this very long paper. In short, anything with a second generation particle in it is unstable, anything with a third generation particle in it is even more unstable, and the more unstable particles there are in something, the more unstable it will be.
There is no good theory to tell us why some "fundamental" particles are short lived and others are long lived, and why each has the mass that it does (although mass and lifetime appear to be related). We don't know for sure if there are really only three generations, or if this is all that our particle accellerators have been able to detect. We don't know whether or not there is a photon like particle to transmit the gravitational force (the hypothetical particle would be called a "graviton" and would have no mass and "spin two") or whether gravity is strictly a function of the curvature of time and space (which may be either infinitely continuous or come in small finite chunks), or both. In addition to the graviton, many theories propose (for mathematical reasons) that there exists a particle called the Higgs boson (which is massive and spin zero) which imparts mass to particles through intereactions with a "Higgs field" that fills the universe.
Almost nobody thinks we really understand all the fundamental laws of the universe, although many people think that we are tantilizingly close.
Two of the most important research programs to get to the bottom of it all are string theory and loop quantum gravity (and a number of similar quantum gravity theories).
String theory (also called M theory) argues that all particles are manifestations of one basic kind of particle, called a string, excited in different ways, and interacting with particular rules. It suggests that gravity is transmitted by a graviton, that there are probably 11 dimensions, that the universe as we know it may be confined to a slice of that multi-dimensional world called a brane, that gravitons cause gravity, that gravitons are not confined to the brane, and that there are a whole host of undiscovered particles out there (at least one for each one yet discovered) which are "supersymmetric". String theory seeks to be a "theory of everything" which describes all four forces and all the particles. It has produced very few results so far, despite about three decades of effort.
Loop quantum gravity is a more modest project. It seeks to develop a quantum mechanical theory of gravity. It does so by trying to view time-space as made of discrete chunks (its cousins also look at the structure of time-space), rather than gravitons. It is generally four dimensional, although often in an "emergent way" rather than fundamentally. It is also a work in progress, although it is currently moving along better than string theory.
A non-mainstream theory that tries to get to the bottom of the questions left by the standard model of particle physics is Preon Theory which argues that the "fundamental particles" may actually be composed of more basic units.
The big issues
Mainstream theories have issues. No supersymmetric particles have been discovered, nor have the Higgs boson or the graviton. Gravity waves predicted by general relativity have not been discovered yet either, although given our experimental limits this isn't necessarily too surprising.
Two big problems in astronomy are staring us in the face. One is the dark matter problem. Basically, the matter we can see cannot explain using general relativity, the way galaxies and galactic clusters and cosmic background radiation appear. The vast majority of matter required by general relativity to explain this behavior is invisible. Moreover, the evidence has shown that likely candidates like neutrinos and particles made of quarks and electrons don't fit the data. Thus, there must either be a new kind of matter (in the leading lambda cold dark matter theory, the main candidates are called WIMPs for weakly interacting massive particles, which don't exist in the standard model of particle physics), or the theory of general relativity must be wrong (the idea behind the minority view modified newtonian dynamics theories often called MOND, which holds that gravity is stronger than expected by general relativity at very large scales where gravitational fields are exceptionally weak).
The other problem is the dark energy problem. The universe is expanding at an increasing rate. There is no obvious reason for this, but it would match a uniformly spread weak field of energy throughout the universe or a cosmological constant in the equations of general relativity. This is called the dark energy problem and in the energy field version would account for 70% of the matter-energy density in the universe.
Quantum physics does predict that there is a "vacuum energy" associated with "empty space", but the amount of energy it predicts is vastly (hundreds of orders of magnitude) greater than that which would be necessary to account for "dark energy".
There are also potential problems at small scales. While standard gravitational equations work at scales down to 0.1 mm, there are rumors that unpublished work that could be published in the next year or two could show it to be weaker than expected at smaller scales.
Meanwhile, science is still struggling to understand a phenomena called CP violation (where charge and the left or right handedness of particles called partity don't comply with certain theories in certain situations) and to find deeper connections between the three forces of the standard model (which is called creating a GUT for grand unified theory).
We live in interesting times and while science has practical explanations for most phenomena, the ultimate basic laws are elusive. The physics world is full or more and less bizzare theories to explain it all, but none has one a consensus endorsement. Later posts on this blogs will examine some of the particular debates going on as physicists try to sort it all out.