Pushing the Quantum Limit...

A zoom in on the Josephson junctions. Two layers of niobium are visible in the image, with the upper film colored blue and the lower film colored red. Josephson junctions are formed in the circular pits (they look a bit like an element of a muffin tin) where the two layers overlap (green). Credit: K. Lehnert/NIST/JILA

Topics: Black Holes, Dark Matter, General Relativity

Here’s a surprising fact: We don’t know what makes up 80 percent of the matter in the universe. I don’t mean that the matter is made of atoms, and we just don’t know which kind of atoms. What I mean is that four-fifths of the universe appears to be made of something that isn’t atoms at all, or more to the point, it’s not made from any of the fundamental particles that we know of.

Why do we think that this mystery matter exists? The short answer is that Albert Einstein’s theory of gravity, general relativity, has painted us into a corner. When we look through telescopes at stars and galaxies moving through the universe, something we can’t see is causing their motion to bend in a particular way. Einstein’s theory of gravity tells how much of this invisible mass—physicists call it “dark matter”—there must be to bend the trajectory of things we can see.

Faced with a situation like this, we make guesses (hypotheses) that we hope explain our strange observations. A good hypothesis should both be consistent with every known fact and have other detectable consequences. If we look for these other consequences and don’t find them, we discard or revise our hypothesis.

Somewhat to my surprise, I find myself working on an experiment designed to look for the consequences of a hypothetical dark matter particle known as the axion. This was surprising because physicists, like those in all professions, divide themselves up into distinct sub-fields. Predictably there are rivalries between, and stereotypes associated with, different cultures that build up around the subfields—the rough equivalent of engineering versus sales in the corporate world.

NIST: Pushing the Quantum Limit in the Search for Dark Matter, Konrad Lehnert