

Hot, Cold and Warm Ideas in Particle Physics 
Consider a unified model universe of particles where the only force acting between them is Gravity.
Newton's laws of motion apply at slow speeds, while Special Relativity enforces a global limit at the speedoflight.
Assuming low speeds, Newton's law of gravity states:
F =  G m_{g1} m_{g2} ⁄ r^{2}
For the sake of simplicity, we'll assume that G = 1 from here on.
Newton's Second Law of Motion states:
F = m_{i} a
In standard physics, all masses are constrained to be positive. This leads to a universe where gravity is always attractive, which was bascially fine before 1998.
We can further simplify things by assuming that all particles in our model have the same mass m = 1. This means that all particles are essentially the same and we can give them the same label "A".
If we consider a single particle (or "body") of type A, then it either sits still or moves in a straight line according to Newton's first law of motion.
From a philosophical point of view, if there's only one particle then the concept of relative motion is somewhat redundant because there is nothing else to measure against. We can however consider the Gravitational Field to give an idea of what the effect would be on other particles, if there were any.
In particular, we can make a plot of the Gravitational Potential versus distance. Using the convention that higher potentials are plotted in lighter shades and lower potentials in darker ones, we get:
This shows us what the environment would be like around a single planet or star, for example, where the attractive force of gravity increases as you get closer.
We can make things a little easier to understand by plotting lines of equal potential as follows:
We can think of the lines as a bit like a "marble run" to give us an idea of how other particles would behave if they were introduced into the system. This leads us neatly onto the next section...
We can analyse what happens when 2 particles of type A attract each other due to the force of gravity. This is known as the 2body problem. They behave in a familiar way, either colliding, orbiting around each other according to Kepler's laws of planetary motion, or flying past each other if their relative speed is high enough. The shape of each orbit is one of:
depending on their initial positions and velocity: 
Note that because the mass of each particle is the same, they each move in a similar way due to the gravitational attraction from the other. Another way of expressing this is via Newton's third law of motion which states that "for every action, there is an equal and opposite reaction".
Again we can make a plot of the gravitational potential to get an idea of what their combined gravitational effect would be on other particles:
This shows us what the environment would be like around a binary star, for example.
Note that although there is a theoretical point midway between the 2 particles where the forces balance and other particles could be located, in practice this arrangement is only semistable.
Things get more involved as soon as introduce a 3rd particle. It turns out that it is not possible to solve this problem analytically and so we must resort to numerical methods (i.e. computer simulation) to find out what happens in the general case.
Nevertheless, we can get a flavour of what goes on if we start with just 2 of the particles and consider the environment around them, as experienced by the 3rd particle.
With 2 particles we can assume that they will be rotating around each other in the general case. If we measure things relative to the rotating frame then there is is also a rotational potential as well as the gravitational potential from the other 2 particles:
Effectively, a 3rd particle experiences the rotating frame as a repulsion away from the centre point, at the same time as still being gravitationally attracted to the other 2 particles. This is commonly known as Centrifugal Force.
This tells us that if it's sufficiently far away from the first 2 then the 3rd particle might orbit around them as if they're a single combined A particle with twice the mass, located at the centre point.
On the other hand, if the 3rd particle approaches either of the first 2 or flies between them near the centre, then the resulting collision or a near miss can send them in all sorts of directions, disrupting whatever was going on beforehand.
There is a further bit of investigation we can do with the 3body problem, which is to consider the case when the masses of the first 2 particles are different and the mass of the 3rd particle is neglible by comparison. This is known as the Restricted 3Body Problem.
For example, if we use the relative masses of the Sun and Earth for the first 2 particles we get something like:
Interestingly, the repulsion due to the rotating frame dominates the attraction due to the Sun in all cases except a very close flyby, which explains why comet orbits are very elongated.
The attraction due to the Earth dominates close to the Earth (an effect known as the Hill Sphere).
We notice that there are 5 points on the plot (L1 to L5) where the gravitational and rotational potentials balance. These are known as the Lagrange Points.
It turns out that L1, L2 and L3 are unstable, but can be useful places to position spacecraft with a minor amount of stability correction.
On the other hand, L4 and L5 (known as the Trojan Points) are stable, which means that naturallyoccuring bodies such as asteroids may be found there.
Nearmisses can result in bodies orbiting around L4 or L5 in what are known as Tadpole Orbits.
Slightly further out we find that bodies can orbit around L3, L4 and L5 in what are known as Horseshoe Orbits.
We can introduce yet more particles of type A, all of which attract each other gravitationally.
Some restricted cases may be analysed such as the Solar System or Moons of Jupiter.
In the general case there is little that can be done to study nbody systems analytically, but they are amenable to computer simulation, using readilyavailable software such as Grav Sim.
There are many examples of star systems that are essentially "stable", giveortake the occasional closeencounter which results in the ejection of a star at high speed.
These systems can be found at a wide variety of scales:
System  Number of Bodies  Example  Image 

Open Clusters  101k stars  Pleides  
Globular Clusters  10k1m stars  Omega Centauri  
Dwarf Galaxies  10m100m stars  Small Magellanic Cloud  
Galaxies  1b100b stars  Andromeda  
Galaxy Clusters  1010k galaxies  Virgo Cluster  
Superclusters  101k clusters  Coma Supercluster 
At the Galaxy scale and above we find that there seemingly isn't enough matter there to hold everything together in purely gravitational terms. This is the problem known as Dark Matter.
Furthermore, at the scale of the entire universe, we find that gravity seemingly isn't attractive at all, but rather is repulsive. This is the problem known as Dark Energy.
Computer SimulationAll of the above can be simulated on a home computer using standard physics. A good place to start to understand the calculations can be found here, courtesy of Professors Piet Hut and Jun Makino. You can even download some PC software from GravSim, courtesy of the author. Included are some readymade models of globular clusters, which you can use as a starting point. For example, an artificiallygenerated cluster with 10,000 stars appears as follows: 
Although we can get a long way with Model A and alwaysattractive gravity, we find that there are cases where it breaks down. In particular, it cannot model any situation where the bodies are repelled by each other, as appears to be happening at the largest scales in the universe.
Conversely, if we attempt to use it as a unified model at the smallest scales then it's a nonstarter because Electrons  the very first subatomic particles discovered by J.J. Thomson in 1897  repel each other.
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