What is the fine structure constant? The equations of physics are peppered with "constants" - numbers which are the same everywhere and in everything. Some, like `π` - the ratio of the circumference of a circle to it's diameter, are simply mathematical constructs: they can never change by definition. Others, like the speed of light (`c`), are physical constants, but their values depend on the units that we use to measure them. A redefinition of the units means that the number changes. To be truly fundamental, a "constant" must be unitless (physicists call these numbers "dimensionless" - without dimension). One of the fundamental constants, uncalculable from any known theory, is known as the fine structure constant, denoted by the Greek letter alpha: `α = e2/ℏc.` In this equation `e` is the charge of an electron, `ℏ` is the (reduced) Planck's constant, which is fundamental to quantum physics, and `c` is the speed of light. Thus `α` ties together electromagnetism, quantum physics, and special relativity. `α` tells us how strong the electromagnetic force is. Its numerical value is measured to be approximately 1/137, which makes it much stronger than gravity, but weaker than the strong nuclear force. How do astronomers measure changes in it? So how can we tell whether `α` changes over space and time? One way is by looking at the quasar absorption spectra. Quasars are distant galaxies that emit light strongly. They are interesting in their own right (for example, they are powered by supermassive black holes) but for our purposes they are just a big candle. Sometimes the light that comes out of them pass through galaxy-sized gas clouds on the way to Earth. The gas clouds are themselves extremely distant, and because it takes so long for the light to reach the Earth, we are seeing things that happened a long time ago. The combination of quasar and gas cloud is known as a "quasar absorption system". When the light passes through the cloud the atoms and molecules in the gas cloud absorb some of the light. Light is made up of many different wavelengths. You can think of each wavelength as a colour: the visible region goes from blue at 400 nm to red at about 700 nm, but there are infra-red and ultraviolet regions too, among others. Each type of atom absorbs a particular set of frequencies (their "transition frequencies") which are unique. By seeing which frequencies were absorbed from the quasar light we can tell what atoms were present in the gas cloud. We can also compare the frequencies absorbed in the gas cloud with those absorbed by the same atoms on Earth. If the laws of physics were exactly the same in the cloud as on the Earth, the frequencies absorbed would be identical. If they are slightly different, however, it may be because `α` was different when the light passed through the cloud. Image copyright Dr. Julian Berengut, UNSW, 2010. May be used with appropriate attribution. A larger version is available here. The most recent results from our group, led by John Webb and Victor Flambaum at UNSW, suggest that `α` has a spatial gradient. That is, in one direction in the sky `α` seems to have been smaller in the past, while in the other direction `α` seems to have been larger (see image, above). More information about the project, and expecially about the astronomy side, is available at Michael Murphy's home page. What other evidence could prove the result correct? The simplest interpretation of the quasar results is that there is a spatial gradient in the values of `α`. This proposition can be tested using Earth-based measurements such as atomic clocks, the Oklo nuclear reactor, and meteorite data, since the galaxy and our solar system within it are moving along the gradient, from lower values of `α` to higher values of `α`. It may be visible in other cosmological data, too, such as supernovae, the cosmic microwave background, and the large-scale structure of the Universe. Most likely, if there is a spatial variation in `α`, then there will be a gradient in other constants as well. This should be visible in other measurements based on quasar absorption spectra, such as primordial abundances of deuterium and measurements of the electron-to-proton mass ratio. While these provide some hints of a spatial gradient in the same direction, there is not enough data to make a solid conclusion.