Journey to the 1Oth Dimension!

Unfortunately, to go any further—to describe exactly how modern particle physics treats gravity, and henceforth the difficulty of coming up with a reason for why it should be so much different from the other forces, requires a little refresher course on the state of particle physics today. The current model, which has become so well tested and generally accepted that everyone just refers to it as the standard model, was the major accomplishment of physics in the second half of the 20th century. And everyone believes it is accurate, though no one believes it is True, and the person to replace it will probably be the 21st century’s Einstein.

Unfortunate but essential aside summarizing the present state of particle physics:

The standard model describes how everything in the subatomic world works. It is the ultimate (for now) and most general (again, for now) extension of quantum mechanics. It is basically a listing of all the fundamental particles and a set of rules governing how those particles interact. And how do they interact?
Particles interact by exchanging particles with other particles. For example, an electron exerts a force on another electron by shooting a little photon (a particle of light) out to the other electron, which the second electron catches and responds to. The preferred anthropomorphism is that particles “communicate” forces using “mediating” particles, like photons. This is what the process looks like in my head.

As you can clearly see, the two electrons “communicate” by tossing the basketball-like photon back and forth to each other. This tossing pushes the electrons apart, which agrees with what we see in the world—negatively charged electrons repel one other. With particles other than electrons, the net effect can be attraction, not repulsion, but the principle remains the same. This is the essential point necessary to understand the rest of this stuff: A force—any force—is caused by things throwing particles at other things. The more particles that are thrown (and caught), the stronger the force will be.


OK, so where does gravity fit into all this? Just treat it like any other force—gravity is caused by massive particles throwing “gravitons,” attractive particles, at each other. These gravitons work to pull massive particles closer together. Simple as that. (Aside within the aside: You may have caught wind of another theory of gravity called general relativity. A fellow named Einstein came up with it almost 100 years ago. Conceptually, it could not be any more different from the standard model. General relativity explains gravity by invoking the warping of space-time; the standard model explains it and everything else by invoking the exchange of subatomic particles. Problems happen when we try to put the two theories together, when we try to describe things that are both very massive and
very small, like black holes. Problems like mathematical inconsistencies, zeroes in denominators, nonsensical results. String theory has been developed at least in part to avoid these problems and combine quantum mechanics with general relativity, using a new structure of space-time and all that stuff I talked about a few pages ago.)

End of aside.

Now it is finally possible to understand why gravity’s weakness is such a pressing problem. Within the standard model, there is a symmetry between the graviton and the other force-carrying particles. They share a common conceptual description. This common description implies that the forces the particles produce should also be similar, in both character (for example, the theory correctly predicts that the strength of both the electromagnetic and gravitational force diminishes with distance) and in magnitude. Yet, as we have seen, gravity is much weaker than every other force. And so we are left with the question: What makes gravity so special?


Enter brane theory. Recall that brane theory postulated that we are trapped in our three-dimensional world, which is itself floating in a higher-dimensional space. We cannot travel into this higher-dimensional space. In fact, nothing we know of can travel into it—not electrons, or quarks, or exotic muons—except for the graviton. It alone can journey into the higher dimension. And as gravitons spread out into that extra dimension, there are fewer here to do the work of pulling heavy things together. As we learned earlier, a force—any force—is the result of particles throwing particles at other particles. When there are fewer particles being caught, that force gets weaker. According to brane theory, we lose gravitons out into the fourth dimension. The result: Gravity is weak.