After illustrating last time how a force can be created by the exchange of particle, it is about time to make a list of which forces there are in nature, and which of them are included in the standard model.
Actually, there is only one force in nature which we currently know and which is not included in the standard model of particle physics. This is gravity. That is the force which pulls one inevitably to the ground, as long as one is not actively working against it. And the one which makes it so hard to get up in the morning. Or so.
It is actually not only the ground, and thus the earth, that is pulling at you, but actually also the earth is pulled by you. However, since the earth is much heavier than you are, it is rather ignorant of your presence. However, it cannot ignore the pull of the moon, to which it reacts with the tides. Nor can it ignore the sun, and this makes earth orbiting around it. On a larger scale, the solar system feels the pull of the milky way, making the solar system orbiting the center of it. And our galaxy the center of the local cluster of galaxies.
In fact, any object which has mass pulls any other object towards it, which has also mass. Actually, this is not entirely correct: Mass is not necessary, it suffices if there is energy in the game. This will lead a bit too far astray now, as it is necessary to delve into the theory of relativity for why this is the case, and I will leave this to later.
However, the generic concept that some objects act a force on each other because they both have a certain property is far more general. It is the simplest example of a charge. Gravity is simple in that everything pulls everything else to itself. In other cases, which will be encountered next time, this is not always the case: Some charges pushes away other charges.
So, why is gravity not included in the standard model (yet)? The simple answer is that we do not yet know how to really do it. There are quite a number of ideas, going by the fancy names of string theory, quantum loop gravity, and many others. However, none of these ideas could have been yet made so precise that it would actually explain how gravity quantitatively fits into the standard model.
The major problem encountered is that it is very hard to make gravity a quantum theory. That has rather technical reasons, and there are some hot leads how we can possibly circumvent this in the future. But not yet. The basic problem is essentially that we do not yet know how to cope with a pileup of gravitons, the (hypothetical) particles carrying the gravitational force, which inevitable always occurs in a quantum theory. That is actually an involved technical problem. For that reason gravity is not yet part of the standard model of particle physics, but instead described by a classical theory, general relativity.
The question is whether this matters when we want to talk about particle physics. The fortunate answer is that it does not, in most cases. The reason is that gravity is a very weak forces. Compared to those described by the standard model, it is about 10000000000000000000000000000000000000 times weaker than the weakest other force of the standard model. Therefore, only if there is a large charge - thus mass or energy - gravity becomes important. That happens only at energy scales which are more than 10000000000000 times larger than accessible in any experiment so far. In nature, it only occurs very close to a black hole or very, very early in the history of the universe. So, for most purposes, and in particular the ones of this blog, gravity can be neglected.
However, there are a number of open questions related to our limited understanding of gravity which have to do with large scales rather than particles: E.g., why is the universe expanding today? Also these questions will not be discussed for the moment in this blog.