The world we see around us seems to be rooted in scientific laws. Theories and equations that are absolute and universal. Central to these are fundamental physical constants. The speed of light, the mass of a proton, the constant of gravitational attraction. But are these constants really constant? What would happen to our theories if they changed?
Although our physical theories give us a powerful understanding of the universe, they don’t explain physical constants. We don’t know why the speed of light is 299,792,458 meters per second. That is just the result we get when we measure the speed of light. The same is true with every universal constant. They lay at the heart of physical science, yet all we can do is measure their value.
Since these constants are rooted in physical properties, it is generally thought they cannot change over space and time. Every electron, for example, has the same charge. They should have the same charge whether they are here on Earth or in a galaxy billions of light-years away. The charge they have now should be the same charge they had at the dawn of time.
While that makes sense, it isn’t necessarily true. Lots of our “obvious” assumptions have been proven wrong, from the idea that the Earth is the center of the cosmos to the idea that space is Euclidian. So there have been many scientific experiments trying to prove whether this assumption is true.
Most of these experiments are astronomical. Because light takes time to travel, when we look deep into the cosmos we also look deep into the past. A galaxy a billion light-years away appears to us as it was a billion years ago. So if the physical constants are the same in distant galaxies as they are here, this means they are constant not only in space but also in time. And this is what most experiments have shown. But a recent study suggests at least some of them might not be.
This latest study looked at what is known as the fine structure constant, alpha. This constant is a ratio of the charge of an electron with the speed of light and Planck’s constant of quantum theory. It’s known as a unitless constant because the units cancel out, so it has the same value regardless of which units of measure you use. It is also central to the energy levels of an atom. If it were to have a different value, then the line spectra of atoms and molecules would change in a measurable way.
The team looked at light from a quasar known as J1120+0641. The light left the quasar when the universe was only 800 million years old, and it passed through several interstellar gas clouds before reaching us. They measured the line spectra of the light as it passed through four regions at different distances, and found no evidence of a change in alpha, meaning it doesn’t appear to change over time. But the value of alpha they got is slightly different from the value found from similar studies. This would suggest that the fine structure constant could have a different value depending on where you are in space.
This has led some popular news articles to declare that physical constants do change after all, but that conclusion is unwarranted. To begin with, the deviation found by the team is very small, and well below a level considered conclusive. The team also only looked at light from one quasar. That’s understandable given that this kind of study can be difficult to do, but it means there is nowhere near enough data to draw radical conclusions. This is only one study of several, and the others have all support the idea that physical constants don’t change.
The best evidence remains on the side of unchanging physical constants.
Reference: Wilczynska, Michael R., et al. “Four direct measurements of the fine-structure constant 13 billion years ago.” Science Advances 6.17 (2020): eaay9672.