Are fundamental constants actually constant?

Physical constants are quantities which we believe to be universal and unchanging. But what if this isn’t true? The constancy of certain fundamental constants is a cornerstone of the Standard Model of physics, the best description of the universe we have today. Evidence that these quantities are not actually constant would be indicative of exotic and exciting new physics that we’ve never seen before. One of the scientific goals of SORCE is search for variation of fundamental physical constants.

Fundamental constants come in two types:

  1. Measured quantities – These are quantities that have units, such as the speed of light or the charge of an electron. The numerical value of these quantities depends on the system of units you use. For example, the speed of light is 300 million metres per second, but 670 million miles per hour.
  2. Dimensionless quantities – These quantities are unitless numbers, usually describing ratios of measured quantities, such as the ratio of the proton mass to the electron mass. These quantities are the same no matter what system of units you use.

How do we know if physical constants change with time, or are different in different locations? It’s very difficult to know if measured quantities are actually constant because we define our system of units based on them!

Plaque commemorating the measuring in October 1958 of Harvard Bridge using the height of MIT student Oliver Smoot as a unit of measurement, the smoot. (Credit: Wikimedia Commons)

Here’s an illustration of what I mean: Have you ever heard of the smoot? In 1958 a fraternity at MIT took freshman pledge Oliver R. Smoot, Jr. and rolled him head over heels the entire length of the Harvard Bridge in order to measure the length of the bridge in terms of his height, defining the unit of length the “smoot”. They measured the bridge to be be exactly 364.4 smoots (plus or minus an ear).  (As a fabulous endnote to this story, Oliver Smoot later became chairman of the American National Standards Institute and then president of the International Organization for Standardization.)

It’s 61 years later now. Let’s pretend you go out and remeasure the length of the bridge in smoots, carefully laying down the 80 year old Oliver Smoot on the bridge over and over. Let’s pretend you found the bridge to be 370 smoots. Here’s the question – did the bridge expand or did Mr. Smoot shrink? Maybe a little of both? This is the problem with measured quantities – it’s really hard to know whether the quantity you’re measuring is changing or your ruler is changing. So we try to make our rulers based on quantities that we think really shouldn’t change, like the speed of light, and hope for the best.

Dimensionless quantities don’t suffer from this problem. We can unambiguously determine if dimensionless physical constants actually change through experiments. SORCE is one such experiment that could detect the changing of fundamental constants. How? More on this in an upcoming post… stay tuned!


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