Through a variety of tests on Earth and throughout the universe, physicists have not measured changes over time or space for any of nature’s fundamental constants.
All modern physics rests on two main pillars. One is Einstein’s theory of general relativityWhat do we use to explain the force of gravity. the other is the standard model, which we use to describe the other three forces of nature: electromagnetism, the strong nuclear force, and the weak nuclear force. With these theories, physicists can explain vast swaths of interactions throughout the universe.
But those theories are not completely self-explanatory. Within the equations appear fundamental constants, which are numbers that we must measure independently and connect by hand. Only with these numbers in place can we use the theories to make new predictions. General relativity depends on only two constants: the force of gravity (commonly called G) and the cosmological constant (usually denoted by Λ, which measures the amount of energy in the vacuum of space-time).
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The standard model requires 19 constants to plug into the equations. These include parameters such as the masses of nine fermions (such as the electron and the up quark), the strengths of the nuclear forces, and the constants that control how the Higgs’ Boson interacts with other particles. Because the Standard Model does not automatically predict the masses of neutrinos, to include all their dynamics we have to add seven more constants.
There are 28 numbers that completely determine all the physics of the known universe.
not so constant
Many physicists argue that having all these constants seems a bit contrived. Our job as scientists is to explain as many varied phenomena as possible with as few initial assumptions as we can. However, physicists believe that general relativity and the standard model are not the end of the story, especially since these two theories are not mutually compatible. They suspect that there is a deeper and more fundamental theory that unites these two branches.
That most fundamental theory could have any number of fundamental constants associated with it. It could have the same set of 28 that we see today. It could have its own independent constants, with the 28 appearing as dynamical expressions of some underlying physics. It might even have no constants at all, with the fundamental theory able to fully explain itself without having to add anything by hand.
No matter what, if our fundamental constants aren’t really constant, if they vary over time or space, then that would be a sign of physics beyond what we currently know. And by measuring those variations, we might get some clues to a more fundamental theory.
And physicists have devised a series of experiments to test the constancy of those constants.
Constants to test
A test implies ultra-precise atomic clocks. The operation of an atomic clock depends on the strength of the electromagnetic interaction, the mass of the electron, and the spin of the proton. Comparing clocks in different locations or looking at the same clock for long periods of time can reveal whether any of those constants change.
Another nifty test involves the Oklo uranium mine in Gabon. Two billion years ago, the site acted as a natural nuclear reactor that operated for a few million years. If any of the fundamental constants were different then, the products of that radioactive process, which survive to this day, would be different than expected.
Looking at larger scales, astronomers have studied the light emitted by quasarswhich are ultraluminous objects powered by black holes sitting billions of light years away from us. The light from those quasars had to travel such enormous distances to reach us, and they passed through countless clouds of gas that absorbed some of that light. If the fundamental constants were different throughout the universe, then that absorption would be disturbed, and quasars in one direction would look subtly different from quasars in other directions.
On the largest scales, physicists can use the big Bang Same as a lab. They can use our knowledge of nuclear physics to predict the abundance of hydrogen and helium produced in the first twelve minutes of the Big Bang. And they can use plasma physics to predict the properties of the light emitted when our universe cooled from a plasma to a neutral gas when it was 380,000 years old. If the fundamental constants were different a long time ago, then it would show up as a mismatch between theory and observation.
In these experiments and more, no one has observed any variation in the fundamental constants. We can’t rule it out entirely, but we can place incredibly strict limits on your possible changes. For example, we know that the fine structure constant, which measures the strength of electromagnetic interaction, is the same throughout the universe to 1 part per trillion.
As physicists continue to search for a new theory to replace the Standard Model and general relativity, it seems that the constants we know and love are here to stay.