Gravity: Strength in Weakness
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.Skip to next paragraph
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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.