By Christopher Cocuzza
SC Staff Writer
The Society of Physics Students has recreated one of physics’ most famous experimental devices. The “muon detector” is a device used to detect particles called muons and was used to prove Einstein’s theory of special relativity.
A “muon” is a subatomic particle with the same charge as an electron but with a much greater mass. Similar to electrons, they are not composed of any other particles and are therefore called “elementary.”
Most muons found on Earth are created by cosmic rays interacting with the Earth’s atmosphere.
Muons are capable of penetrating solid objects, and this feature is exploited to detect them when they reach the surface of the Earth.
The muon detector’s significance begins with Albert Einstein.
In 1905, he published his first paper on his theory of Special Relativity. From Einstein’s new theory resulted two new physical effects known as “length contraction” and “time dilation.”
Length contraction and time dilation occur whenever an object is moving.
From the perspective of the moving object, distances will appear shorter. This effect is known as length contraction.
From the perspective of someone observing the moving object, the time as kept on the object will be slower than the time that the observer keeps. If the moving object happened to be a clock, it would seem to be ticking slower than a clock the observer is holding. This effect is known as time dilation.
These effects are only significant when objects are traveling near the speed of light. Everyday objects usually do not travel anywhere near this speed. Length contraction and time dilation are typically effects that are observed on the subatomic scale where tiny particles can travel extremely quickly.
Einstein’s new theory was controversial when it was first introduced. It aimed to supplant previous theories that had been failing to conform to empirical evidence but had become entrenched in the scientific community. Since Einstein’s theory had little empirical evidence itself, many were reluctant to accept it.
Over the following decades, most physicists accepted special relativity solely due to its great explanatory power. But it was not until 1940 that length contraction and time dilation were actually observed in an experiment by Bruno Rossi and D. B. Hall.
In the Rossi-Hall Experiment, a muon detector was used to measure muons hitting the Earth’s surface. Scientists already knew that muons decay quickly and travel at over 99 percent the speed of light. At these speeds, the effects of special relativity are prominent.
Ignoring the effects of special relativity, the muons do not have enough time to reach the Earth before they decay. Therefore, scientists would expect to measure almost no muons with their muon detector.
However, Rossi and Hall measured thousands of muons reaching the Earth every minute. This result can be explained using length contraction and time dilation.
From the perspective of the muons, the distance they need to travel to reach the surface of the Earth is contracted. This shorter distance allows them to reach the Earth before decaying.
From the perspective of an observer on Earth, time as kept on the muon is slowed. Effectively, the muon’s decay time is extended, and this longer time period allows them to reach the Earth before decaying.
This experiment conclusively proved Einstein’s speculations from 35 years prior. The muon detector had cemented its place in scientific history as the device that proved special relativity.
The muon detector built by the Society of Physics Students currently resides within the Gessner Science Hall. A second muon detector is currently under construction.
When the second detector is a finished, the Society of Physics Students plans to run its own experiment. One detector will be placed near the top of Gessner Science Hall and the other at the bottom.
The difference between the amounts of muons detected will shed light on the effects of special relativity and the properties of muons.
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