Below is a short summary and detailed review of this video written by FutureFactual:
Cosmic Rays, Muons and Time Dilation: Experimental Verification of Special Relativity
Cosmic rays constantly strike Earth's atmosphere, producing a shower of particles including muons. Muons decay in about 2.2 microseconds in their own rest frame, yet many reach ground-based detectors. The video shows how this apparent paradox is resolved by special relativity: time dilation extends the muons' lifetimes in our frame, letting them travel kilometers before decaying. From the muons' frame, length contraction makes the atmosphere thin enough for the journey. This striking, accessible demonstration highlights a direct experimental verification of relativistic effects in everyday measurements.
Cosmic-rays collide to create muons
Every second, cosmic rays shower the Earth's upper atmosphere, producing cascades of particles when they strike air molecules. A significant fraction of these products are muons, charged particles that travel toward the surface at nearly the speed of light. Detectors on the ground observe the directions and energies of these muons, enabling scientists to study the original cosmic rays that created them.
Muon perspective and the decay paradox
Muons have an average lifetime of about 2.2 microseconds in their own rest frame. If created high in the atmosphere, one might expect most muons to decay before reaching the ground, yet detectors record many arriving muons. The video frames this not as a mystery of the muon itself, but as a consequence of relativistic motion.
"From the muon's perspective, it's the Earth and the atmosphere which are moving at 99.995% the speed of light towards the muon, and the lengths of moving objects are literally contracted by a factor dependent on their speed." - MinutePhysics
Time dilation: the key to muon survival
The video explains that time, as seen by a muon traveling close to the speed of light, runs slower compared with a stationary observer. This time dilation means the muon has more time to travel before decay, allowing it to reach detectors far from where it was created. In numbers, a muon traveling at 99.5% the speed of light would experience a lifetime that lengthens enough to cover several kilometers, explaining how ground-based detectors are able to observe them despite their brief lifetimes.
"This, in my mind, is one of the most awesome experimental verifications of special relativity." - MinutePhysics
From muon frame to atmosphere: length contraction
From the muon's frame, Earth and the atmosphere move toward it at nearly the speed of light. Because lengths in motion contract, the atmospheric path shortens dramatically, enabling a muon with a few microseconds to traverse the distance before decaying. The video notes that 50 kilometers of atmosphere can shrink to about 0.5 kilometers under these conditions.
"From the muon's perspective, it's the Earth and the atmosphere which are moving at 99.995% the speed of light towards the muon, and the lengths of moving objects are literally contracted by a factor dependent on their speed." - MinutePhysics
Detectors, experiments, and the verification of relativity
Detectors at Earth's surface observe muons, providing direct evidence of relativistic effects in nature. The video frames the muon journey as a dramatic demonstration of special relativity in action, emphasizing that the muons literally could not reach Earth without these relativistic effects. The observation is presented as a striking, accessible verification of time dilation and length contraction in the real world.
"They literally couldn't get here if it weren't for time dilation." - MinutePhysics
Conclusion: everyday evidence for SR
The discussion closes by highlighting how a natural, ubiquitous phenomenon—cosmic ray muons reaching the ground—serves as a tangible demonstration of Einstein's theory. The interplay of time dilation and length contraction underlines why particles can survive long enough to reach detectors and why SR remains a robust framework for understanding high-speed processes in the universe.