Gravity is a big deal in our universe. We, the land around us, and even the air we breathe are locked down to the Earth as we hurtle through space at thousands of miles per hour. Gravity molds the stars and planets into their spherical shapes, and sets them in orbits around each other. In the violent last seconds of a giant star's life, gravity can distort the fabric of space and time to such an extent that light itself is sucked into the dying star's corpse, creating a bottomless pit called a black hole. Given all that celestial drama, it may seem surprising that one of the great mysteries of modern physics is why gravity is so incredibly weak compared to the other forces of nature.
When you actually stop to think about it, gravity (despite dropped plates smashing to the floor, or certain bad-tempered 17th century physicists getting clocked by an apple) is ridiculously easy to overcome. Take our attraction to the Earth, for example. The reason we don't fly into space as our world whips around the Sun is that the Earth's gravity attracts everything to the center of our planet. But, unless I'm much mistaken, you're probably not free-falling toward the Earth's hot inner core right at this moment.
If gravity had its way, all the matter that comprises the Earth would be accelerated down at 9.8 meters per second (per second of falling), and we'd all end up packed inside a tiny black hole where the Earth's core used to be. But that's silly; the reason we don't fall toward the center of the Earth is that the floor is in the way, as is the ground, bedrock, and several thousand miles of molten rock and metal. The structure of the Earth keeps us on solid ground, so to speak.
But in the realm of physics, this is far from obvious. In reality, there's nothing solid about matter. Everything we touch is really made up of interacting electromagnetic fields. If you press down on the tabletop in front of you, your hand is not actually coming into direct contact with the table. Both your hand and the table are made up of atoms, the outermost particles of which are electrons. Electrons all have a negative charge, so before the atoms can physically touch in any way, the electric fields of the electrons repel each other. That's why the floor holds us up; the electrical repulsion of your atoms against the floor is way stronger than the force of gravity, which is pulling you down.
Moving a bit farther into the realm of atoms, there are two other natural forces that only come into play at very small scales. Named the strong and weak forces, these forces act to hold tiny particles called quarks together to form the protons and neutrons in atomic nuclei. Appropriately named, the strong force is in fact the strongest natural force we know of, and is responsible for protons (all of which have positive charges and should repel each other) sticking together in the nucleus.
The weak force is really only weak compared to the strong force (it's the second strongest natural force), and creates an interaction that allows neutrons to turn into protons under some conditions. At the scale of atoms, gravity is almost negligible. The strong force attracting two protons together is 10^40 (that means a 1 followed by 40 zeroes) times stronger than the force of gravity between them.
Now, in my mind, one of the most important parts of being a scientist is asking annoyingly obvious questions. Take, for example, the question "Why is gravity so much weaker than all the other forces?" This seems to be a natural candidate for the answer "It just is." That's the way the universe works.
We measured the strength of all the forces, and gravity came in last. End of discussion. But that's not enough of an answer for scientists, and the real reason gravity is so weak may break open the next major advance in our perception of the universe. There are some tantalizing suggestions that gravity is, in fact, not weak at all, it's just diluted by having to act over more than our familiar four dimensions (three of space and one of time).
By now you may have heard that scientists strongly suspect that there are more than four dimensions in our universe. The big question is why we can't experience those other dimensions directly; why do they seem to be hidden from us? An increasingly popular theory of fundamental physics, call Brane Theory, is being used to explain that, and a whole lot more about how our universe really works.
The word "brane" is meant to be short for "membrane." We usually think of a membrane as a two-dimensional surface that separates two three-dimensional volumes. Brane theory tells us that our four-dimensional universe may actually be a kind of membrane existing between volumes that contain higher dimensions in space. These mysterious higher-dimensional regions are inaccessible to us, as we and all the forces of nature lie embedded in our limited brane.
Talking about higher dimensions is always difficult, because being three-dimensional beings; we can't easily visualize what higher dimensions might be like. Sometimes an analogy helps, even though all analogies are limited.
Take, for example, one of those "water-skeeter" insects that skim along the surface of ponds. The insect can't swim or fly, it just scoots around on the surface tension of the water looking for food. Effectively, the bug can only move in two dimensions in space (the surface of the pond), plus one of time. Above and below the skeeter are vast three-dimensional volumes (the sky and the water), but the bug can't move in them, and doesn't experience them in any direct way. In the same way, our universe may be surrounded by higher-dimensional volumes (which scientists are currently labeling the "Bulk") that we haven't been able to peer into, at least not yet.
The problem with the skeeter analogy is that to make it accurate here, all the natural forces would have to act only on the two dimensions of the pond surface. All the electromagnetic forces and the strong and weak forces could only exert themselves on the two dimensions of the surface. That's why we could never expect to see any of the Bulk outside our universe using telescopes, which see light. Light is an electromagnetic wave, and when we say that electromagnetic forces are limited to our brane, that means they can't travel off our three dimensional space.
But here's the kicker: gravity may not be constrained to our brane. Using the skeeter analogy, while all the other forces are locked onto the two-dimensional pond surface, the bug might still be able to feel the gravitational attraction of objects in the water below or the sky above. Moving up a level to our three-dimensional universe, although we'll never be able to physically see anything outside our brane, we may be able to feel the gravitational attraction of other regions in the higher-dimensional Bulk.
These higher dimensions explain why gravity seems so weak to us. As it turns out, gravity may not be that weak at all. It's just acting over several higher dimensions, making it seem weaker when we only view it in our familiar three.
In some way, gravity has a lot more directions to pull in, so it gets spread out. How many dimensions are we talking about here? That's still open to some debate, but many particle physicists think there may be at least seven more dimensions, for a total of eleven. And be assured, there is no one on the planet who really understands what eleven-dimensional space means in any kind of intuitive way. It's going to take us quite a while to figure out how to even begin to think about that.
But the thing that gives me goose-bumps is thinking what else lies out there in the Bulk.
There are some theoretical reasons to believe that there are other branes out there besides our own, separated from us by a dimension we can't travel in. Cosmologists are getting pretty excited about a new model of how the universe began, with one or more branes interacting with each other. There may even be observational evidence of this in the microwave background radiation, leftover heat from the very beginning of our universe. The implications of this theory are staggering. Not only is the door left wide open to the possibility of entire parallel universes existing out there in the Bulk, but now we have the real possibility that gravity may allow us to explore them, to a very limited degree.
For decades astronomers have been perplexed by so-called "dark matter" in our universe. It might be slightly more accurate to call dark matter "excess gravity," as what astronomer actually observe is gravitational force pulling stuff together where no matter appears to be present. This dark matter is currently measured to make up more than 90% of the mass of our universe, and has such intimate affects as holding our Milky Way galaxy together.
Is this extra gravity in fact coming from another brane, an entirely different universe? When we map the distribution of dark matter around our galaxy, are we in fact creating the first map of a parallel universe? Did the pull of gravity from another brane influence the way the structure of our universe formed and evolved? In the end it will all come down to gravity, and what its weakness tells us about the structure of reality.