So I am, among other things, a bored AP Physics student. Fortunately for me, my AP Physics book has cool stuff in it, like a list of hadrons and leptons, plus cool pictures. However, in the very back of the book, there’s a section called “The Standard Model for the Evolution of the Universe.”
This section is only a few paragraphs, but it’s really interesting, so I’ve read it a few times. It describes how until approximately 10[sup]-43[/sup] seconds after the big bang, all the forces were unified. Then, they each split off one at a time, until we end up with four fundamental forces, Electromagnetic, Weak Nuclear, Strong Nuclear, and Gravitational.
Now, I find this all to be really cool. However, the book never explains why. Why did the forces separate? Was it the falling temperatures? Expansion of space? Passage of time? Magical gremlins?
I know there are a few dopers knowledgable on this subject. Care to explain?
A terrific place to start is Coming of Age in the Milky Way and The Whole Shebang by Timothy Ferris.
The basic idea behind the splitting-off of the forces is similar to the freezing of a liquid. When the temperature is high enough for molecules to bounce around and spin every which way, the substance is liquid. When the temperature drops, the molecules settle in next to each other and crystals form with a definite direction of alignment.
The forces arise from particle interactions. (See, for example, James Trefil’s From Atoms to Quarks .) In the early universe, the temperature was high, and the particles went every which way, and all forces looked alike. When the universe cooled, the particles settled into a groove, and particles moving across the groove acted differently from particles moving along the groove. You have to understand that this groove doesn’t have a direction in actual space; it lives in the “internal space” of the mathematical description of the particles. I know that’s not very clear, but to explain it would take a book. (I have actually written that book, but it won’t be published for about two years, and you want an answer NOW.) When the temperature drops even more, then the particles settle into a groove within the groove, and the forces split off even further.
Hope this helps.
Groovy! 
Thanks, FriendRob. I didn’t think anyone would be able to put it into terms a high school student could understand, but it makes a lot more sense after your explanation.
What’s the book going to be called, btw?
I forgot to say that the freezing-out phenomenon is called “spontaneous symmetry breaking”. It’s the basis of the so-called “standard model of elementary particles”, and is also crucial to “Grand Unified Theories” (GUTs). If you google any of those terms, you’ll come up with lots of sites.
The title is still under negotiation. Two possibilities:
Smashing Symmetry - because of the symmetry breaking mentioned above, or
The Theory of Almost Everything - because all forces except gravity are included in the Standard Model
Which do you like better?