Muons are created when high-energy particles smash into atomic nuclei, producing short-lived intermediary particles called pions that quickly decay into muons. This happens naturally in Earth’s upper atmosphere billions of times per second, and it can be replicated artificially using particle accelerators. About 10,000 muons rain down on every square meter of Earth’s surface each minute, making them one of the most common subatomic particles you’ve never heard of.
Cosmic Rays and the Upper Atmosphere
The vast majority of muons on Earth are born roughly 15 kilometers above your head. The process starts with cosmic rays, high-energy particles (mostly protons) that travel through the near-vacuum of interstellar space. When one of these particles slams into the atmosphere, it encounters a dramatic change: the density of matter jumps by a factor of 100 quintillion compared to open space. A collision with a nitrogen or oxygen nucleus is almost inevitable.
That collision doesn’t just bounce the particles apart. It shatters the nucleus and releases a spray of new particles, most importantly pions and kaons. These are unstable and exist only briefly. Charged pions survive about 26 nanoseconds on average, while kaons last roughly 12 nanoseconds. In that flicker of time, they decay into muons and neutrinos. Neutral pions take a different path entirely, decaying almost instantly into gamma rays instead.
The result is a constant shower of muons streaming toward the ground at nearly the speed of light. At sea level, about one muon passes through every square centimeter every minute. They’re passing through your body right now.
Why Muons Survive the Trip to the Ground
Here’s where things get strange. A muon’s average lifetime is just 2.2 microseconds. Even traveling at 99.99% the speed of light, a simple calculation says it should only cover about 0.6 kilometers before decaying. The atmosphere is roughly 10 kilometers thick. By all rights, muons created in the upper atmosphere should never reach the surface.
But they do, in enormous numbers. The explanation is one of the most elegant real-world demonstrations of Einstein’s theory of special relativity. Because muons move so fast, time passes differently for them than it does for an observer on the ground. From Earth’s perspective, the muon’s internal clock runs about 70 times slower than normal, a phenomenon called time dilation. That stretches the muon’s effective travel distance to around 42 kilometers, more than enough to reach the surface.
You can also look at it from the muon’s own perspective. The muon “sees” the atmosphere compressed by that same factor of 70, shrinking it from 10 kilometers down to just 140 meters. Either way, the math works out: the muon reaches the ground. This was one of the first experimental confirmations that relativistic effects aren’t just theoretical abstractions. They’re measurable, and they happen constantly in the sky above you.
What Exactly Is a Muon?
A muon is essentially a heavier version of the electron, carrying the same negative charge but with about 207 times the mass. Unlike the electron, which is stable and can exist indefinitely, the muon is unstable. After its brief 2.2-microsecond life, it decays into an electron and two neutrinos.
Despite being heavier than electrons, muons are still classified as leptons, meaning they don’t interact via the strong nuclear force. This gives them a remarkable ability to pass through dense matter. While protons and neutrons get stopped quickly, muons can penetrate meters of rock or metal, losing energy gradually along the way.
How Accelerators Produce Muons
Scientists don’t have to wait for cosmic rays. They can manufacture muons by recreating the same basic physics in a laboratory. The recipe is straightforward in principle: fire a beam of high-energy protons at a dense target material. The protons smash into atomic nuclei in the target, producing pions, which then decay into muons and neutrinos, just as they do in the atmosphere.
The target materials vary by facility. Fermilab’s NuMI beamline uses a 1.2-meter-long graphite target struck by protons at 120 billion electron volts. Oak Ridge National Laboratory takes a different approach, using flowing liquid mercury as the target. For proposed next-generation muon colliders, the favored design calls for a high-speed jet of liquid mercury inside a powerful magnetic field that captures the pions as they fly outward.
The challenge isn’t just making muons. It’s making enough of them and corralling them into a usable beam before they decay. Muons emerge from the target scattered in all directions with wildly varying energies. Powerful superconducting magnets, starting at fields as strong as 20 tesla, funnel the pions into a decay channel where they convert into muons. A proposed muon collider would need a 4-megawatt proton beam just to produce an adequate number of muons, making it one of the most demanding accelerator designs ever conceived.
Once you have a muon beam, the clock is ticking. The beam must be cooled (its spread in energy and direction reduced) and accelerated to higher energies before the muons decay. A technique called ionization cooling passes the beam through liquid hydrogen absorbers sandwiched between powerful magnets and radio-frequency cavities. The entire process, from creation to acceleration, must happen in microseconds.
Why Scientists Go to the Trouble
Muons are uniquely useful precisely because they sit between electrons and heavier particles in mass. They’re heavy enough to produce clean, high-energy collisions without the messy radiation losses that plague electrons, but they’re fundamental particles rather than composite ones like protons, so their collisions are simpler to analyze.
One of the most celebrated uses of muons is measuring their magnetic properties with extreme precision. Fermilab’s Muon g-2 experiment, which published its final result in 2025, measured the muon’s magnetic anomaly to 127 parts per billion, surpassing its own design goal. This measurement tests the predictions of the Standard Model of particle physics at an extraordinary level of detail. Any discrepancy between the measured and predicted values could point to undiscovered forces or particles.
Muon Imaging of Large Structures
The same cosmic-ray muons that rain down naturally have found a surprising practical application: scanning the insides of massive objects. Because muons penetrate deeply into solid matter but are partially absorbed along the way, placing detectors beneath or beside a large structure lets you map its internal density. More muons arriving from a given direction means less material in the way; fewer muons means denser rock or stone.
This technique, called muon tomography, made headlines when a team used it to discover a previously unknown large chamber hidden inside Egypt’s Great Pyramid of Giza, a void above the Grand Gallery that had gone undetected for 4,500 years. The same approach has been used to image the interiors of volcanoes, inspect nuclear reactor containment vessels, and scan shipping containers for hidden dense materials. No artificial muon source is needed. The atmosphere provides a free, constant, and deeply penetrating beam.