[♩INTRO ]
  In 1958, an MIT student named Oliver Smoot  measured a bridge as part of a fraternity
  pledge.
  But he wasn't allowed to use a ruler or  a tape measure.
  He had to use himself.
  He laid down at the start of the bridge, a  couple of students marked where his head and
  his feet were, and then he moved over and  laid down again.
  After an hour and a half of this, the results  were in: The bridge was 364.4 smoots long
  -- plus or minus an ear,
  because measurements are meaningless without  error bars.
  That's about 620 meters, by the way, for  those of you in backwards countries that haven't
  switched to smoots yet.
  … Of course, smoots aren't an official  unit, even though Smoot later became chairman
  of the American National Standards Institute,  where part of his job was to define and standardize
  the units Americans use.
  Yet, there are plenty of units used every  day that can seem just as weird.
  And even if you've never heard of them,  you're benefiting from obscure units every
  time you put on your shoes or read about dark  matter.
  People were measuring things long before the  French Revolution brought the metric system
  thundering across Europe.
  It was easiest to use something common and  familiar, so unsurprisingly, a lot of older
  units came from farming.
  Like the furlong.
  It started as the length of land that an ox  could plough without resting, which apparently
  tended to be about 200 meters.
  But there were no universal standards — I  mean, for one thing, every ox is different
  — so different places ended up with different-length  furlongs.
  Today, you don't really hear much about  furlongs.
  That is, unless you're into horse racing,  where the track stretches for one furlong.
  Horse heights are also still measured in units  of hands instead of centimeters or inches.
  A hand is exactly a third of a foot, in case  you were wondering, or about 10 centimeters.
  You'd think measuring in feet would work  just as well, at least in the US, but I guess
  equestrians find it handy.
  If you look closely, though, you'll find  furlongs hidden all over the place.
  City blocks in Salt Lake City, Utah, stretch  one furlong.
  And in the United States, it's pretty common  to hear about land measured in acres, which
  are just furlongs in disguise.
  An acre was once the land area that a team  of oxen could plough in a day, so acres were
  originally thin rectangles a furlong long  and about a tenth of a furlong wide
  — although the different furlongs in different  places lead to very slightly different acres
  on either side of the Atlantic.
  Both are a little more than 4000 square meters.
  Even though acres started as thin rectangles,  today you'll hear something described in
  acres no matter what shape it is.
  And whenever you do, you're hearing one  of the last hurrahs of the ancient furlong.
  Have you ever wondered about the logic behind  shoe sizes?
  I mean, a size 9 isn't nine centimeters  or nine inches or nine meters -- and it's
  not like a size 9 is a inch or an centimeter  longer than a size 8.
  Well, if you're in an English-speaking country  like the US or Australia, the barleycorn is
  your answer.
  When animal-based measurements were a little  too big, people looked for smaller things
  to measure with.
  And in England, they settled on the barleycorn:  a single grain of barley.
  A barleycorn is about eight and a half millimeters  long—roughly a third of an inch.
  But originally, it was exactly a third of  an inch -- because an inch, by definition,
  was three barleycorns long.
  Today's inches, though, are ultimately defined  through the speed of light, and measuring
  with barleycorns is mostly a thing of the  past.
  Except that shoemakers didn't get the message.
  Because even if they don't use actual barleycorns  any more, shoe sizes are still based on that
  ancient unit.
  There's a lot of variation between countries  when it comes to details like how much wiggle
  room your feet get and where size numbers  start, but anyone with the English system
  uses the barleycorn.
  Kids' sizes generally start at size 0, which  fit adorable tiny feet that are eleven barleycorns
  long.
  Then, each size is one barleycorn larger,  up to 24 barleycorns in length, or size 13.
  Somewhere around there, the adult sizes start,  and again each one is one barleycorn bigger
  than the last.
  There are other systems in other parts of  the world; European shoe sizes, for instance,
  get about two-thirds of a centimeter longer  with each size.
  But if you're using the English system,  you have the barleycorn to thank if your shoes
  fit -- or to blame, I guess, if they don't.
  The micromort was first detailed in a 1989  paper by Ron Howard.
  No, not that Ron Howard.
  A Stanford professor named Ronald A. Howard  was looking for a concrete way of discussing
  the risks of certain actions.
  Howard proposed that if something has a one-in-a-million  chance of killing you, it carries one micromort
  of danger.
  Which means that instead of being defined  by convention or universal constants like
  the rest of this list, the micromort is defined  by statistics.
  And that means that something's micromort  count will change over time.
  Back in 2000, going skydiving in the US would  add about 11.9 micromorts to your day -- because
  there were 32 skydiving fatalities that year,  out of 2.7 million jumps.
  But by 2016, that number dropped to about  6.5 micromorts per jump -- because there were
  more jumps and fewer deaths.
  The count also changes based on where you  are.
  Generally, running a marathon exposes you  to about 7 micromorts.
  But if you run in a smoggy city where it's  hard to breathe, that number can go up.
  Which, yes, makes running a marathon more  dangerous than skydiving these days.
  If you're micro-morbidly curious, you can  find all sorts of lists and tables online
  with micromort counts of different activities.
  And even though micromorts only measure the  risk of death instead of the risks of any
  sort of injury, they can still help inform  your decisions.
  Lots of people won't swim in the ocean because  they're afraid of sharks, for example.
  And in Australia, one of the countries with  the most recorded shark attacks, swimming
  in the ocean has about 12.125 micromorts of  risk.
  But 12 of those 12.125 micromorts have nothing  at all to do with sharks;
  they're from the risk of drowning -- which  is something people tend not to worry about
  even though its risk is almost a hundred times  higher.
  And the 0.125 that is from sharks is about  the same as what you get from kangaroos.
  And both of them combined are less risky than  sitting on a chair in Australia, due to the
  likelihood of dying when you fall off of it—which  kind of puts things in perspective.
  Maybe you've heard that the jiffy started  as a real unit of time, just like the second.
  But what really happened is actually more  complicated than that.
  "Jiffy" used to just mean any short amount  of time.
  Like, "be back in a jiffy!"
  But since science and engineering have lots  of fast things, people in different fields
  all reached for this fun-sounding word to  describe something fast.
  Physicists love the speed of light, so one  common definition you'll hear is that a
  jiffy is the time it takes light to go a centimeter  in a vacuum.
  That's about 33 picoseconds, or 33 trillionths  of a second.
  Which definitely qualifies as "fast."
  But 33 picoseconds isn't the only jiffy  in physics.
  Other physicists might say a jiffy is a hundredth  of a second, since that's a convenient amount
  of time for measurements.
  And others would say it's closer to three  trillionths of a trillionth of a second -- roughly
  the time it takes light to go the length of  an atomic nucleus.
  It all depends on what they're talking about,  since no one ever bothered to standardize
  things.
  And the trouble doesn't stop with physicists.
  Electrical engineers tend to care way more  about cycles of the power from an outlet than
  they do about the speed of light, so they  started saying a jiffy is one full cycle of
  that power.
  Different countries have different cycles,  making an electrical jiffy 1/60 of a second
  in the US and 1/50 of a second in Europe.
  And in computer science, processor cycles  are important -- so a jiffy can be the time
  it takes a computer to complete one computation.
  Jiffies aren't really used for anything  official, so the confusion doesn't cause
  too much trouble.
  But the next time someone tells you they'll  be back in a jiff, you might want to ask if
  they mean a physics, engineering, or computer  science jiffy.
  Just to be safe.
  In pictures, it's good to include a banana  for scale.
  But bananas aren't just good for measuring  size.
  They're also pretty good for comparing exposures  to radiation.
  The banana equivalent dose, or BED, is how  much radiation you'll get from radioactive
  atoms in your average banana.
  Yup.
  That radiation happens because a small fraction  of Earth's potassium is an isotope called
  potassium-40.
  Potassium-40 radioactively decays, so eating  a banana means exposing yourself to a tiny
  bit more ionizing radiation than not eating  one.
  Roughly 1 BED of ionizing radiation comes  to about a ten-millionth of a sievert -- the
  usual unit for radiation.
  One BED is a really tiny amount.
  Each day, you're naturally exposed to about  a hundred BEDs just from rocks and bricks
  and being out in the world.
  And a hundred BEDs a day still isn't worth  worrying about -- so there's absolutely
  no reason to stop eating bananas just to avoid  radiation.
  They're delicious.
  Even though one BED is so little radiation  that it basically doesn't matter, it can
  still help us understand the risks of different  behaviors.
  An international flight, for example, exposes  you to about four hundred BEDs.
  And medical tests like X-rays and mammograms  can range anywhere from hundreds to tens of
  thousands of BEDs, depending on the test.
  That's relatively safe as long as you aren't  having them too often, but the medical professionals
  in the room do sometimes use protective lead  walls, since they're conducting these tests
  all the time.
  Radiation therapy, on the other hand, is around  twenty million BEDs.
  Now that's a lot -- but then again, the  whole point of radiation therapy is to kill
  cells.
  That's why most types of radiation treatment  involve keeping the radiation to as small
  a part of the body as possible.
  A fatal dose of radiation, meanwhile, is up  around a hundred million BEDs.
  Now, even though they're convenient — and  kind of hilarious — BEDs aren't a perfect
  unit.
  Potassium-40 doesn't give off all types  of ionizing radiation, and different types
  can have different effects on your body.
  Plus, radiation has different effects inside  and outside the body.
  But next time someone talks about radiation,  converting into bananas can help you know
  what they're talking about.
  Atomic bombs and nuclear power both rely on  chain reactions of neutrons running into atoms
  like uranium, which lets off more neutrons.
  So as you might imagine, the scientists working  on the first atomic bomb were pretty interested
  in just how often neutrons would run into  atoms.
  And since they were always talking about how  big uranium atoms were and whether uranium
  was easy to hit or not, they invented a new  unit that was about the size of a uranium
  nucleus: The barn.
  The name came from the fact that trying to  hit a big, bulky uranium atom was a bit like
  trying to hit the broadside of a barn … or  something like that.
  But weird name or not, it stuck as a unit  of cross-sectional area.
  One barn is equal to about a tenth of a trillionth  of a trillionth of the size of the "Play"
  button on this video.
  Assuming you are watching on the computer.
  Nuclear and atomic physicists are always talking  about particles hitting or missing or interacting
  with atoms, so barns still naturally come  up all the time.
  And barns are even out at the frontiers of  knowledge.
  We still don't know what dark matter is,  but a lot of people think it might occasionally
  ram into atoms of regular matter -- the stuff  you and I are made of.
  And when they're talking about the chance  of an atom getting hit by dark matter, real
  physicists write real papers with units like  "millibarns" in them.
  We live in a very strange world.
  But as strange as these units are, they do  make it easier to talk about things.
  That's why units exist.
  And while here on SciShow, we almost always  use metric units plus common oddballs like
  light-years because they're what most people  worldwide understand, you never know when
  you'll need to measure something in furlongs,  or in barleycorns.
  Or, even, in smoots.
  Thanks for watching this episode of SciShow,  brought to you by our patrons on Patreon.
  If you want to help support us while getting  awesome perks, you can head on over to Patreon.com/scishow.
  [♩ OUTRO ]
     
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