“Who ordered that?”

While you were reading this sentence, several hundred muons passed straight through your body. In fact, around ten thousand pass each square metre of Earth’s surface every minute. Slightly disconcerting, isn’t it?

A muon is essentially a bigger, fatter version of an electron. About 207 times bigger and fatter, in fact. Other than their size, they’re more or less exactly like electrons. Same charge, same spin. They’re both types of lepton. Unlike electrons though, they have extremely short lifespans. Muons take an average 2.2 microseconds to decay and break apart into an electron and a couple of neutrinos.

Despite their short lives, muons are everywhere. They’re being constantly created by cosmic rays. Confusingly, cosmic rays aren’t actually rays. They’re high energy particles (mostly protons) emitted from all kinds of celestial events like stellar fusion and supernovae. These high energy particles slam into the Earth’s upper atmosphere with the kind of gusto that the LHC could only dream of achieving. Striking molecules in the atmosphere, these collisions create a slew of subatomic particles, including pions. These pions then rapidly decay into muons, which proceed to rain down on us constantly.

With such a short lifetime, you wouldn’t think they’d travel very far. Left to their own devices, even with the speeds they attain, they’d barely be able to travel a kilometre. That’s where special relativity steps in again. Because the muons created in these collisions travel at relativistic speeds, they experience time dilation. As far as the muon is concerned, it still only lives for 2.2µs. To us, in our relatively static reference frame, they appear to live for much longer. Long enough to pass straight through anything in their way on the planet’s surface. Subsequently, they can penetrate surprisingly far into the Earth’s crust before they either decay or deflect off atoms that happen to be in their path.

In fact, so numerous are the cosmic rays that generate these muons that the moon actually casts a shadow (shown to the left here) in secondary muons. This image was taken from below the ground in the Soudan 2 detector in Minnesota. A mere 120 missing muons from a total 33 million mark out the Moon’s shadow in this image. The shadow is also not an exact match to the Moon’s actual location (shown by the cross in the centre of the image. That’s because despite their speed, cosmic rays are still charged. They’re deflected slightly by Earth’s magnetic field as they approach our planet — in much the same way the magnetic fields in a cathode ray tube deflect the electrons that make the picture on old TV screens.

Perhaps the most fascinating thing about muons though, is that in their fleeting existence, they can actually form into atoms. Muons can briefly orbit protons, forming muonic hydrogen, before they decay. Alternatively, if you can form an antimuon (the positively charged antimatter equivalent of a muon), then an electron can actually enter into an orbit around it. This “exotic atom” is known as muonium (with the chemical symbol Mu), and behaves a little like a lightweight equivalent of a hydrogen atom. It even has its own chemistry, being able to form short lived molecules like muonium chloride (MuCl).

Theoretically, if two muonium atoms live long enough to react together, you’d create a dimuonium molecule, with the formula Mu2. But I doubt it’s ever been observed in the lab. And most particle physicists aren’t really into Pokémon.

The title of this post, incidentally is actually a quotation. Due to the surprise discovery of such an odd particle, I. I. Rabi (the discoverer of nuclear magnetic resonance) famously quipped “Who ordered that?”

About Invader Xan

Molecular astrophysicist, usually found writing frenziedly, staring at the sky, or drinking mojitos.
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12 Responses to “Who ordered that?”

  1. Anonymous says:

    Re: Amino Acid Images
    Hello again!
    I am offering to pay to you produce images for the other amino acids!
    I sent you an email about this on 14 Feb.
    Did you get it?
    If you are not interested, no problem.
    If you might be interested, but not right now (or whatever), again, no problem!
    I am just concerned that you did not receive the email. As you did kindly reply to me earlier here on the comment thread, it seems odd that my email received no response, making me wonder if you got it in the first place.
    It’s from an aol.com address, and possibly it went to spam?
    (Possibly a reply went to my spam folder, and I missed it!)
    Even if you’re not certain of how to respond, I’d very much welcome an acknowledgement of receipt.
    I really don’t wish to bother you. You are probably very busy!
    Thanks again for your time.
    All the best!

  2. invaderxan says:

    No problem. I’m glad you enjoyed it!
    And thanks for that link…

  3. Anonymous says:

    I love the way you explained those terms (eletron-muon-pion) thanks.
    Now I only arrived at this tip because someone at http://www.planetparticles.com/root2.htm is onto something real big…

  4. invaderxan says:

    Re: Muons, amazing!
    Pretty fascinating, huh? :)
    Well, it’s mostly due to velocities, if I understand correctly. Electrons from beta decay typically have relatively low energies. As a result, they can be stopped by things like skin cells, causing ionisation damage. Secondary muons created by cosmic rays, travelling at relativistic speeds, are much more likely to pass straight through you without interacting much en route.
    On the other hand, statistically, a muon will occasionally be stopped by some unsuspecting organism. The majority of these muons won’t do a lot, but very infrequently they can cause spurious mutations and even tumours. Though it’s so infrequent that background radiation in most places is probably more dangerous.
    There’s no better reason to find something out, than purely wanting to know. ;)

  5. Anonymous says:

    Muons, amazing!
    That’s awesome! I thought muons were tiny little things like neutrinos. If they’re much bigger than electrons, why don’t our bodies get a bit annoyed by them passing through us? I mean, they get annoyed by beta radiation, don’t they?
    By the way, I’m not a scaredy-cat – I just want to know :D

  6. Anonymous says:

    Re: Making muons…
    Hi invaderXan
    There’s probably not enough deuterium for even single fusions via muons, since the odds of encountering another deuterium seem kind of low in the microseconds of a muon’s life. However there’s a non-zero amount of doubly deuterated water out there, so perhaps occasionally? In the clouds of Venus the deuterium ratio is much higher. I suspect muon fusion might be a neglected process…

  7. invaderxan says:

    Re: Making muons…
    Are there really sufficient concentrations of deuterium naturally for fusion to take place, though? Don’t forget, muons only act as catalysts…

  8. Anonymous says:

    Re: Making muons…
    As deuterium fuses it makes either helium-3 or tritium, which decays into helium-3 anyway. Thus if muons catalyse fusion of deuterium they make helium-3. But there’s probably not enough. On Venus…

  9. invaderxan says:

    Re: Making muons…
    Of course… I completely forgot about muon catalysed fusion. Indeed, if only they weren’t so troublesome to make! It’s kinda sad that if we can’t find a more convenient way to create muons, it may be one idea destined to be little more than a scientific curiosity. I wonder if anyone’s considered harvesting secondary muons from cosmic rays!
    And about helium-3? I do know that the lunar surface is enriched with helium-3, and it has something to do with the solar wind. Without looking it up, I forget the exact mechanism though…

  10. qraal says:

    Making muons…
    Hi InvaderXan
    What a shame there’s not an easy way of making muons because they catalyse deuterium fusion reactions quite willingly – they usually decay a bit too soon to ever produce more energy than what went in to making them though. With a continual rain of muons I wonder if helium-3 levels go up over time as deuterium is fused? Probably not since deuterium is massively outnumbered by protium.

  11. invaderxan says:

    Hehehe… Yeah, it’s bizarre, huh? Actually, a similar exotic atom is positronium — made from an electron and a positron orbiting each other. It only exists for a few nanoseconds before the two collide and annihilate, but in that time it can actually form dispositronium (Ps2) and positronium hydride (PsH)! :)

  12. Wow, that’s pretty cool! They can actually form short-lived atoms in their brief life?
    And most particle physicists aren’t really into Pokémon.

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