"Rarest Event Ever" Had A Half-Life 1 Trillion Times Longer Than The Age Of The Universe - How Did We See It?

In 2019, researchers with the XENON Collaboration saw something unexpected. The device is designed to find evidence of the elusive dark matter, a hypothetical substance that is believed (with good reason) to exist everywhere. Instead, it saw something weird happening to the xenon in the device. One of the atoms decayed. This was a surprise as the half-life for that particular xenon was a half-life of 18 billion trillion years. That’s more than 1 trillion times longer than the current age of the universe.

This has been described as the rarest event ever recorded, and it is not hyperbole. Still, it is important to understand the meaning and context of a half-life of 18 billion trillion years, and how in the end we can see such an event, even if they are extremely rare. The term "half-life" refers to the amount of time it takes for half of a given quantity of a specific atom to decay into another form.

When we think of radioactive decay, we tend to think of things happening very fast. There’s a good reason for that. With the advent of the nuclear age, discussions of half-life have all been about unstable elements that disappear in seconds and can trigger explosive chain reactions. In medicine, we use radioactive elements that might decay in hours or days, but their half-lives might be a lot longer than that.

Take Uranium, for example. Its most common form has a half-life of almost 4.5 billion years. So when the Earth formed, it had twice as much Uranium. Still, you wouldn’t want to be near Uranium for long, because the atoms do decay constantly, albeit slowly. Uranium is not super dangerous naturally, but it is in our uses that can pose a more serious health risk.

Still, the half-life of xenon-124 is about 4 trillion times longer than that of uranium-238. How did we even measure that? The detector has 2 metric tons of xenon in it, which is almost 10,000 trillion trillion atoms. So if you put enough of these atoms together, you should see a single atom decaying every few minutes.

Should is the operative word here. Because looking at atoms is not like looking at a handful of red marbles waiting for one to turn blue. It is like looking at an overwhelming number of marbles, where one might get slightly more massive and create a flash of x-rays or throw away an electron. In 177 days of data collection, the team saw around 9 events.

A problem with a lot of these rare events with an enormous half-life is actually catching them in the act. And without seeing the event, we do not even know if it happens. 

Take the proton, for example, the tiny, positively charged particle at the heart of every atom. Some theories in physics predict that protons might eventually decay. But so far, in all of our experiments, we've never seen it happen. That means if proton decay does occur, it must take an incredibly long time, so long, in fact, that scientists estimate its half-life to be at least 1.67 billion trillion trillion years. 100 billion times longer than Xenon-124.

It is not easy looking for events that make the lifetime of stars look like seconds.  

The observations were reported in detail in the journal Nature.

An earlier version of this story was published in 2019.