Defining a scholarly field that officially goes by the name of ‘decoherence’ is a daunting task for anyone, but Associate Professor David Craig of Le Moyne's Physics Department manages to succeed, illustrating some major issues in his field with a few well chosen examples.
This June, Craig traveled to Växjö, Sweden, to give a paper on “Decoherence in Quantum Cosmology.” He recently took the time to explain the fundamentals of his field to a novice who admitted her horrifying intention to simply use Google to learn more background for this piece.
In outlining decoherence, Craig shed light on his own work as well as on the spirit of international collaboration which has always distinguished physicists, who invented the Internet in order to better share their research results.
“One of the things about quantum mechanics is that it teaches you that the world is much more interesting than you ever imagined it could be,” Craig says. Among other peculiarities, quantum physics shows that it is possible to imagine perfectly sensible things that cannot actually happen, and that what can actually happen often does not seem sensible at all. Decoherence is how physicists sort out which is which.
The classic example is the “two-slit experiment.” In this experiment, particles must pass through a screen in which there are two slits cut, before being detected. Our imagination tells us that the particles that are detected on the other side of the screen must have passed through one slit or the other. Quantum physics shows that it is actually logically inconsistent to argue that this is the case unless there is a measuring device in place to determine which slit the particles pass through. And if you watch the particles, the results are completely different than if you do not. That’s decoherence. As crazy as this sounds, it’s confirmed by an experiment that Le Moyne physics students now perform every year.
Craig’s work applies the ideas of decoherence to a quantum description of the entire universe. Going from observations about a box of atoms on your desk to the universe is actually easier than it sounds, since “the universe has been very kind to us,” by being so symmetrical (both homogenous and isotropic), Craig explains.
Craig’s paper describes the first results of a collaboration with colleague Parampreet Singh of the Perimeter Institute for Theoretical Physics; Craig returned there for additional research in September 2009, helping to solidify a remarkable connection between Le Moyne College and this world-class research center, a dazzling facility which opened in Waterloo, Ontario, in 2004. As Le Moyne considers its own future role in the preparation of physics students thanks to the possibilities created by the McDevitt Endowment, we will all be learning a lot more about the fascinating field of physics. Learn more about the Perimeter Institute and Växjö, Sweden, the “greenest city in Europe.”