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Posted by on Sunday, March 20, 2011 at 10:20pm.

Muons are elementary particles that have a very short lifetime of about two millionth of a second (when at rest). They can be created in the upper atmosphere (at an altitude of say 10,000 meters) by cosmic rays, in which case they travel towards the surface of the Earth at very high speeds, say 99.9% of the speed of light. If we ignored special relativity, how far could they travel at this speed before they decay? You'll notice that even though they are very fast this distance is much less than 10,000 meters, and they would never reach the ground. This is not correct however, because we know that many of these particles do reach the ground, and experiments observe them all the time. How does special relativity resolve this apparent contradiction, and how far can they actually travel before the end of their life?

  • physics - , Monday, March 21, 2011 at 12:47am

    What they are calling the lifetime (2 microseconds) is actually about 1.28 half lives. There is no "end of life" number for a muon. Like all radioactivity, muons decay exponentially.

    I have some other objections to this problem. The "99.9% of the speed of light" number is too high. 10,000 meters is not the upper atmosphere. It well below where planes fly; most of the muons are created much higher in the atmosphere. They are bandying numbers around that are wrong, and implying that muons has some sort of a fixed life. If they are trying to teach you about special relativity, they should use real numbers, and explain the time dilation equation

    t' = t /sqrt[1 - (v/c)^2]

    Radioactive decay and all physical and life processes go more slowly on moving bodies, when observed Earth-frame coordinates. If t is the true half life of a stationary muon, t' will be measured half life when the muon is moving at velocity v.

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