The answer to the enduring question of the smallest thing in the universe has evolved along with humanity.
People once thought grains of sand were the building blocks of what we see around us. Then the atom was discovered, and it was thought indivisible, until it was split to reveal protons, neutrons and electrons inside. These too, seemed like fundamental particles, before scientists discovered that protons and neutrons are made of three quarks each.
“This time we haven’t been able to see any evidence at all that there’s anything inside quarks,” said physicist Andy Parker. “Have we reached the most fundamental layer of matter?”
And even if quarks and electrons are indivisible, Parker said, scientists don’t know if they are the smallest bits of matter in existence, or if the universe contains objects that are even more minute.
Parker, a professor of high-energy physics at England’s Cambridge University, recently hosted a television special on the U.K.’s BBC Two channel called “Horizon: How Small is the Universe?”
Strings or points?
In experiments, teensy, tiny particles like quarks and electrons seem to act like single points of matter with no spatial distribution. But point-like objects complicate the laws of physics. Because you can get infinitely close to a point, the forces acting on it can become infinitely large, and scientists hate infinities.
An idea called superstring theory could solve this issue. The theory posits that all particles, instead of being point-like, are actually little loops of string. Nothing can get infinitely close to a loop of string, because it will always be slightly closer to one part than another. That “loophole” appears to solve some of these problems of infinities, making the idea appealing to physicists. Yet scientists still have no experimental evidence that string theory is correct.
Another way of solving the point problem is to say that space itself isn’t continuous and smooth, but is actually made of discrete pixels, or grains, sometimes referred to as space-time foam. In that case, two particles wouldn’t be able to come infinitely close to each other because they would always have to be separated by the minimum size of a grain of space.