Through a variety of tests on Earth and throughout the universe, physicists have not measured any changes in time or space for any of the fundamental constants of nature.
All modern physics rests on two basic pillars. One of them is Einstein’s theory of general relativity, which 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. Using these theories, physicists can explain the vast swathes of interactions throughout the universe.
But these theories do not fully explain themselves. Within the equations appear fundamental constants, which are numbers that we must measure independently and plug in manually. Only with these numbers in place can we use theories to make new predictions. General relativity is based on only two constants: the gravitational force (commonly called G) and the cosmological constant (commonly called Λ, which measures the amount of energy in the space-time vacuum).
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The Standard Model requires 19 constants to be interfered with in the equations. These include parameters such as the masses of nine fermions (such as the electron and the top quark), the strength of the nuclear forces, and constants that control how the Higgs boson interacts with other particles. Since the Standard Model does not automatically predict the masses of neutrinos, to include all of their dynamics we have to add seven more constants.
These are 28 numbers that define absolutely all the physics of the known universe.
Many physicists argue that having all these constants seems a bit artificial. Our task as scientists is to explain as many diverse phenomena as possible with as few initial assumptions as we can throw away. Physicists believe that general relativity and the Standard Model aren’t the end of the story, especially since these two theories are incompatible with each other. They believe that there is a deeper, more fundamental theory that unites these two branches.
That fundamental theorem can have any number of fundamental constants associated with it. It could have the same set of 28 that we see today. They can have independent constants of their own, with the 28 constants appearing as dynamic expressions of some fundamental physics. It can’t even have constants at all, with the underlying theory being able to fully explain itself without having to add anything manually.
No matter what, if our fundamental constants aren’t really static — if they vary across time or space — that would be a sign of physics beyond what we currently know. And by measuring these differences, we can get some evidence for a more fundamental theory.
Physicists have devised a number of experiments to test the stability of these constants.
One of the tests involves ultra-accurate 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 observing the same clock for long periods of time can reveal if any of these constants are changing.
Another ingenious test involves the Oklo uranium mine in Gabon. Two billion years ago, the site was a natural nuclear reactor that operated for a few million years. If any of the fundamental constants were different at the time, the products of that radiative process, which have survived to this day, would be different than expected.
Looking at the largest scales, astronomers have studied the light emitted by quasars, which are super-bright objects powered by black holes billions of light-years away. The light from those quasars had to travel such huge distances to reach us, and it passed through countless clouds of gas that absorbed some of that light. If the fundamental constants were different throughout the universe, this absorption would change and quasars in one direction would look very different from quasars in other directions.
On larger scales, physicists can use the Big Bang itself as a laboratory. They can use our knowledge of nuclear physics to predict the abundance of hydrogen and helium produced in the first 10 minutes of the Big Bang. 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 from a long time ago, it would appear as a mismatch between theory and observation.
In these experiments and more, no one noticed any difference in the fundamental constants. We can’t completely rule it out, but we can place very strict limits on their potential changes. For example, we know that the fine structure constant, which measures the strength of the electromagnetic interaction, is the same throughout the universe to one part per billion.
As physicists continue to search for a new theory to replace the Standard Model and general relativity, it seems the constants we know and love are here to stay.
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