Hank: Thanks to Linode for supporting this SciShow video. You can get a $100 dollar 60-day credit on a new Linode account at linode.com/scishow.

What is the best way to measure the passage of time? For me, I got the ol' "one-mississippi, two-mississippi" trick that might help you out in a pinch, but there's a lot of science and technology out there that needs way more accurate time keeping. And these days, the best atomic clocks can tick so precisely, you would have to wait longer than the age of the universe for them to be off by 1 second.

But some scientists are on the hunt for an even more accurate clock, and they have proposed a literal nuclear option: a clock the size - not just of an atom - but of an atom's nucleus.

[intro]

To make any clock, you need something that goes back and forth repeatedly at a fixed rate. That's called an "oscillator," and it has a frequency: how many times it oscillates over some specified length of time. The most accurate and reliable  oscillators out there are light waves, which are made of electric and magnetic fields that wobble at fixed frequencies. But how to light waves and atoms come together to make an atomic clock? Well, we're gonna have to get a bit quantum mechanical.

See, atoms are made up of a nucleus surrounded by a swarm of electrons, and those electrons can only exists where they have specific fixed energies. Metaphorically, you can picture these energy levels as rungs on a subatomic-sized ladder. And if you want an electron to climb up or down that ladder, it has to either gain or lose the exact amount of energy that will get it to another rung.

Now electrons are fond of getting that energy by either absorbing or emitting light with the appropriate frequency, because according to quantum mechanics, the energy and frequency of light are directly related: the lower the energy, the lower the frequency, and vice versa.

So to make an atomic clock, scientists take a bunch of identical atoms, like cesium-133, and hit them with a laser. That laser light has a frequency and therefore, an energy. And that's as close as possible to the energy needed to bump just one of each atom's electrons up to another rung. Any electrons that do get a boost will eventually shed their excess energy and drop to their original rung, and that means that they will emit light on the way down. Again, that light oscillates, or metaphorically, it ticks at a precise frequency. And scientists count those incredibly consistent ticks to make the passage of time.

With decades of laser science research behind them, atomic clocks are simple enough that we can shoot them into space, and they'll still be accurate down to the nanosecond. But we can do even better, because atomic energy levels aren't the only game in town. An atom's nucleus has its own, even smaller, subatomic ladder. And just like electrons, a nucleus can jump from one of its proverbial rungs to another if it absorbs or releases just the right amount of energy.

For keeping track of time, a nuclear clock can offer some advantages over the traditional atomic ones. While an electron's ladder has rungs at very specific energy levels, those levels aren't always constant; the position of the rungs can shift by teeny tiny amounts if, say, there's a slight shift in some external electric or magnetic field. And if your clock is the size of a single atom, teeny tiny shifts are a big deal and can throw off your time-keeping ability.

But nuclear energy levels are less affected by changes going on around them, because the protons and neutrons inside an atom's nucleus are so tightly bound together. These super strong bonds can help a nuclear clock tick more steadily than an atomic clock, but there's a catch; it also means the gaps between the energy levels - the distance between the rungs on the ladder - are on a completely different scale.

Typically, we're looking at energies that are millions of times greater than what electrons are dealing with, and remember, the amount of energy needed to jump between rungs relates to the frequency of the light we need to shine on them. For the average nucleus, you'd need to blast it, not with the microwaves of traditional cesium atomic clocks or the optical lasers that we use with more state-of-the-art clocks that use other elements, no - you would need a laser that shoots gamma rays: the most energetic and highest frequency light out there. And that's just not feasible with current technology.

But as luck would have it, there is one nuclear energy gap we know of that would work; it's found in the radioactive element thorium-229. The problem is that while scientists know the gap corresponds to some kind of ultraviolet light, no one's been able to pin down the exact energy. And without knowing that, you can't excite any thorium nuclei inside your fancy new clock.

But a big breakthrough came in 2023, when a team at CERN used a somewhat unconventional approach for their experiments; instead of taking thorium nuclei and trying on a bunch of different laser frequencies to see which one excites them, they started with another radioactive element - actinium-229 (protons: 89, neutrons: 140). These actinium not only decay into thorium-299* (protons: 90, neutrons:139, *excited), they decay in such a way that the new thorium starts our in the excited state scientists were looking for. The team just waited for this excited thorium to de-excite so they could measure the frequency of the light that came out in the process. In theory, this frequency would tell us exactly what a nuclear clock's laser needs to be in order to get some regular thorium in that clock a-tickin'.

While these CERN experiments weren't accurate enough, to spit out one single, perfect frequency to power the world's first nuclear clock, they drastically narrow down the options for follow-up research. Which is exciting, because it means we can start to imagine cool things to do with nuclear clocks, including more precise GPS and other kinds of extreme precision monitoring. Like, maybe people could track tiny movements in tectonic plates that may signal upcoming volcanic eruptions, or do state-of-the-art searches for astronomy's mysterious dark matter.

but the most interesting application could be to see if the very laws of physics themselves change over time. Because while things like the speed of light seem constant, some physicists think they might actually vary by tiny amounts over cosmic time scales. So if they see an ultra precise time signal from a nuclear clock start to drift over time, that could indicate the very forces of nature themselves are changing. There's still work to be done in building a nuclear clock, but with the right innovations, the future of timekeeping could arrive any second.

This episode of SciShow is supported by Linode, a cloud computing company from Akamai. Linode values communication a lot, just like we do here on this science-communication channel. They stick to that value so well that Linode has user-friendly interfaces and award-winning customer support staff that you can chat with at any time, any day. And I mean real people will walk you through how to set up the stuff you want to do with Linode's toolbox. And that's an extensive toolbox, including dedicated CPUs, storage, memory, and firewall protection, to name a few uses. To get started with Linode, you can click the link in the description below or go to linode.com/scishow for a $100 60-day credit on a new Linode account.

Thanks for watching this episode of SciShow, and keep that two-way communication going in the comments.

[outro]