(Note: this is part of a series of posts, in which I explain a basic physics concept and put it in a wider context. The first two dealt with pendulums; I do have one more post about pendulums to come as well.)
“Identical” is one of those words that is most often used imprecisely. That’s for good reason: very little is actually identical in the everyday world. An example is “identical twins”, when a single fertilized egg splits into two embryos — both of which carry the same genetic information. Despite this, genetic changes can still occur between the twinning event and adulthood, so twins are never completely identical, though the differences are small enough that even DNA tests can be stymied. (Dogs evidently can do better in some instances!) And of course even if genetics are the same (as in clones of various sorts), environmental factors serve to shape the development of an organism.
When going into the microscopic regime, identical becomes a more and more precise description. If we are hard-headed science types, we can say two things are identical if no experiment can tell them apart. Imagine a game of pool where all the balls are exactly the same, as opposed to the regular game with its cue ball and 15 numbered balls (that are of course the Devil’s tool) with their various colors and patterns. In a strange game of identipool, you wouldn’t be able to tell which ball is which. Sure, supposedly you could watch all 16 balls as they move around the table, but if someone distracted you momentarily and switched the positions of two of them, you wouldn’t notice. The more balls, the harder it is to keep track. Imagine the Calvin & Hobbes story line in which he duplicates himself 5 times, and the chaos that ensued.
In quantum mechanics, particles are precisely identical. Every electron is identical to every other; photons of the same color (as in a laser) are also indistinguishable. Richard Feynman, pictured above, imagined a way of thinking in which there is only one electron: positrons (the antimatter versions of electrons) are electrons moving backwards in time. This means that all the collisions, annihilations, and other interactions are simply the result of one electron bouncing back and forth in time. The idea is less far-fetched than it sounds, even though it ultimately doesn’t seem to match the real world: the amount of matter in the universe appears to be greater than the amount of antimatter. Nevertheless, the difference between electrons and positrons is less than it might seem at first; either way, every electron is identical to every other electron, so they might as well be the same particle as far as experiments are concerned. Even though each type of particle may be distinguishable from others in nature, the particles within a type cannot be told apart, no matter how sophisticated you are!
A truly dramatic example of the identical nature of quantum mechanical particles occurs with a type of particle called a boson, named for Indian physicist Satyendra Nath Bose (the handsome gentleman shown at right). Bose’s work in collaboration with Albert Einstein showed that if you assume all particles in a boson gas are truly identical, they can merge their identities together into a single entity at low temperatures. Called a Bose-Einstein condensate, this new super-object behaves like a single particle of (relatively) enormous size — a quantum Voltron, if you will.
Because all the particles were originally identical, when their identities merge, it is no longer possible to even say there are individual particles within the condensate. They are identical, and they have identical quantum states, so together they make the perfect collective. The first Bose-Einstein condensate in a laboratory was made in 1995, at the University of Colorado at Boulder. The bosons were rubidium atoms cooled to within a whisker of absolute zero (170 nanokelvins, to be more precise). Photons, which are also bosons, were made to form a Bose-Einstein condensate in 2010.
The very identical nature of particles on a quantum level has many implications and results. I’ll return to some of these ideas in later posts, but here’s a teaser: the identical character of electrons and neutrons plays a role in the structure of neutron stars, bizarre corpses of stars that act like giant atomic nuclei. Nothing in our daily experience is truly identical, but on the smallest scales, the indistinguishability of particles is not only precise, but necessary for understanding their behavior.
(I intended to include a picture of Paul and Pauline, the identical aliens from Planet Purple on “Mr. Rogers’ Neighborhood”. However, they seem to be sufficiently obscure characters that I can’t find a good picture of them on the entire internet! Internet, you have failed me. We are no longer on speaking terms.)