Around 350 years ago, Isaac Newton made a simple observation that would forever alter the field of physics: He realized that the force that pulls an apple down to Earth is the same force that keeps the Moon in orbit. The consequences of such a hypothesis are hard to overstate. The physical laws governing Earth were the same in the heavens. When we pointed our telescopes started looking at the most distant stars and galaxies in the visible universe, the laws of physics never changed. They are immutable and constant everywhere and for all time.

Well, probably. There's no way to be completely sure without observing the entire universe. That's why a group of researchers at the National Institute of Standards and Technology have spent much of the past two decades running an experiment to see if they can catch the laws of physics changing. Their method? To stare at clocks for fourteen years straight.

The scientists at NIST were looking at atomic clocks, which are some of the most precise measuring devices ever invented. Atomic clocks, rather than counting the time based on movements of gears or springs or crystals, operate based on oscillations in individual atoms. If an atomic clock were to run for a million years straight, by the end it would be off by less than a tenth of a second.

The atoms inside the atomic clock used for timekeeping all have one thing in common: when they’re hit with energy, they absorb that energy and then re-emit it at as light of a specific frequency. That frequency is so precise that it can be used to measure down to a billionth of a second. For instance, cesium, one of the more common atoms used in atomic clocks, emits light that oscillates exactly 9,192,631,770 times per second.

This number is dependent on a lot of things, like the strength of the electric forces inside the atoms and various aspects of quantum mechanics. In addition, over the fourteen years this experiment was run the Earth—and thus the atomic clock—moved closer and farther away from the Sun and its gravity, so this experiment is also affected by general relativity. If any of these laws of physics changed since the start of the experiment, the researchers should be able to tell.

And in an exhaustively-researched study, the NIST scientists revealed conclusively that they did not. Physics is the same everywhere and for all time. Well, again, probably. At least now we know the laws of physics certainly did not change over that fourteen-year period in the region of our solar system in a way detectable by an atomic clock experiment. This is better than nothing.

Really, what this experiment does is make us more confident in the laws we’ve spent centuries discovering. Although absolute proof of their immutability will always elude us, we can be reasonably sure that the laws of physics won’t change. If they haven’t changed noticeably over fourteen years and several trips around the Sun, there’s a very good chance they never will.

Source: Nature Physics via Wired

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