How cosmic rays live longer lives

Every few years there is some discovery or other in the physics lab or the astronomical observatory that is invariably headlined by the media as “confirming that Albert Einstein was right in his theory of relativity.” This week it was the turn of Gravity Waves.

Whenever I read these headlines at the breakfast table, I get so frustrated I tend to spill marmalade on the pages of my copy of Quantum Electrodynamics Today. The reason for my frustration is that Einstein’s theory of relativity is confirmed experimentally every minute of every day in the most elegant possible way and I’m astonished that neither the media nor ordinary physics lectures do not make this demonstration the centre piece of their explanations.

The experimental evidence that confirms Einstein so regularly and so beautifully concerns cosmic rays. The earth’s atmosphere is continuously bombarded by cosmic rays from deep space – mainly protons from Quasars and Supernovas. These particles are travelling at almost the speed of light. When one hits the nucleus of an atom in the upper atmosphere the collision gives rise to particles called muons. The muons from these collisions continue in about the same direction as the original proton, towards the earth’s surface, and also at almost the speed of light.

However, a muon has a very short lifespan – about 2 microseconds – before it decays away. Moving at close to the speed of light, a muon will travel only about 450 metres in this short time.  But the upper atmosphere is more than 300 miles above the surface – so no muons should ever reach the earth’s surface. In fact, around 10,000 muons reach every square metre of the surface every minute.  How do they do it?

They do it because in the muon’s time frame, they only exist for 2 microseconds, but because they are travelling so fast in relation to the earth, they experience Einstein’s time dilation effect from our point of view. Although only 2 microseconds elapses for the muon, a much longer time elapses on earth – long enough for the particle to reach the surface and penetrate deep into the crust where they can be detected even down coal mines.

In the earth’s frame of reference, muons live longer than 2 microseconds because of their very high velocity – long enough to survive the journey to the surface.

This not only confirms Einstein’s theoretical predictions, it also shows that the time dilation effect is not merely a theoretical outcome but a phenomenon that can have concrete effects on our lives.

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