Are the fundamental constants “constant”?

-Karthik Gurumurthy

I’ve always found the question of whether fundamental constants might change over time to be fascinating. When we consider physics, we often take these values as absolutely fixed, but there have been some intriguing theories suggesting otherwise.

Take the universal gravitational constant G, for example. The renowned physicist Paul Dirac proposed an interesting theory that G might actually be weakening over time, in proportion to the age of the universe since the Big Bang. According to his hypothesis, gravity would be only half as strong 10 billion years from now as it is today.

What I find particularly interesting is how scientists devised a clever test for this theory using the Moon. We know the Moon is slowly moving away from Earth due to tidal interactions – as tidal friction slows Earth’s rotation, the Moon’s orbit must expand to conserve the total angular momentum of the Earth-Moon system. If Dirac’s theory were correct, the Moon would be receding even faster than standard physics predicts.

The definitive test came from precise measurements using laser pulses bounced off reflectors that Apollo astronauts placed on the lunar surface. These measurements confirmed that the Moon is indeed moving away, but only at the rate predicted by conventional physics, not the accelerated rate Dirac’s theory would require.

The challenges of detecting changes in fundamental constants are considerable. Scientists continuously refine their measurements of physical constants, but they’re typically seeking more precise values rather than testing for temporal changes. If you measured the speed of light and got an unusual result, you’d likely assume experimental error rather than concluding that the speed of light had “hiccuped” that day.

Our measurement techniques are incredibly sophisticated but also complex, requiring numerous checks to ensure accuracy. If constants were actually changing, it would be extremely difficult to differentiate between measurement error and genuine variation without some theoretical framework to guide expectations.