0:01

Welcome to brain Stuff, a production of iHeartRadio.

0:06

Hey brain Stuff, I'm lorn vogel bomb, and

0:09

this is a classic episode of the podcast.

0:12

In this one, we dig into the weighty history

0:14

of the kilogram. It turns

0:16

out this standard measure has been redefined

0:19

over time. Hey

0:22

brain stuff, I'm lorn vogel bomb. And for

0:24

nearly one hundred and fifty years, the official

0:27

weight of a kilogram was determined by

0:29

a shiny cylinder of platinum locked

0:31

away in a French vault. The

0:33

kilogram, like the meter and the second, is

0:35

one of the seven fundamental units of measurement,

0:38

also known as the International System of Units

0:40

or the metric system the SI for short.

0:43

These were first formalized in the eighteen

0:45

seventy five Treaty of the Meter. Back

0:48

then, the best way to agree on the weight of a kilogram

0:51

was to forge a single hunk of metal and

0:53

dubit legrand k, and

0:56

for more than a century all scientific scales

0:58

were calibrated back to that one physical

1:00

reference point, with copies stored in

1:02

a dozen countries. But even

1:04

solid objects can change over time.

1:07

When La grand k was weighed in the nineteen eighties,

1:09

it was a couple of micrograms lighter, meaning

1:11

that all highly accurate scientific scales,

1:14

not like the one in your bathroom, had to be

1:16

recalibrated, and that's what nerds

1:18

call a real pain in the mass. Luckily,

1:21

a team of metrologists was already on

1:23

the case. Metrology being the science

1:25

of weights and measures. They were searching

1:28

for a universal constant that would generate

1:30

a fixed value for the kilogram that's true

1:32

now and will still be true a million years

1:34

from now. They had already found

1:36

such a physics fix for the second, which

1:39

was redefined in nineteen sixty seven from

1:41

its previous value of a fraction of a day

1:43

one eighty six thousand, four hundredth of a

1:45

day to be precise, to something much

1:47

more confusing, but much more constant.

1:51

It takes nine billion, one hundred and

1:53

ninety two million, six hundred and thirty one

1:55

thousand, seven hundred and seventy oscillations

1:58

of a special microwave beam to excite

2:00

atoms of the isotope caesium

2:02

one thirty three to a higher energy

2:04

level. Since that number will

2:06

never change unlike the exact length of a day,

2:09

that's your new second. Same

2:11

for the meter, instead of being defined as the

2:13

length of a single meter long metal pole

2:16

forged back in eighteen eighty nine. It

2:18

was redefined in nineteen eighty three as the distance

2:20

light travels in a vacuum in a particular fraction

2:22

of a second one two hundred and ninety nine

2:24

million, seven hundred and ninety two thousand, four

2:27

hundred and fifty eighth of a second. It

2:29

wasn't until twenty seventeen that scientists

2:32

working at the US National Institute of Standards

2:34

and Technology and similar bodies worldwide

2:37

finally agreed on a universal constant for

2:39

the kilogram. The achievement required

2:41

solving one of the thorniest physics problems

2:43

of the last century, coming up with a numerical

2:46

value for planks constant without

2:48

getting too technical. A physicist, Max

2:51

Planck proved in nineteen hundred that matter

2:53

releases energy in discrete chunks

2:55

called quanta. His equation

2:57

for measuring these packets of energy is

3:00

included a constant called H hitherto

3:02

known as planx constant. Thanks

3:05

to Einstein, we know that energy and mass

3:07

are mathematically related, that whole e

3:09

equals mc squared thing, so physicists

3:11

figured out the planks constant, being

3:13

a fixed unit of energy, could yield the world's

3:16

most accurate measurement of mass. Calculating

3:19

the exact value of planks constant took

3:21

decades and some serious technological innovation,

3:24

specifically a nifty device called a kibble

3:27

balance. But they did that work, and

3:29

we now know that planks constant is six

3:31

point six two six zero seven one

3:33

five zero times ten to the power of negative

3:36

thirty four jeweles per second I

3:38

mean duh. In

3:41

mid November twenty eighteen, at the annual

3:43

meeting of the International Bureau of Weights and Measures

3:45

in Versailles, France, representatives

3:47

from more than sixty countries voted to approve

3:49

a new and everlasting definition of

3:51

the kilogram as calculated by the plank

3:54

constant. No more hunk of metal.

3:56

The kilogram's mass is now tied to planks

3:58

constant. Definitions were also announced

4:01

for SI units, the ampeer electrical

4:03

current, the kelvin for temperature, and

4:05

the mole the number of molecules or atoms

4:07

in an element. These new definitions

4:10

will take effect on May twentieth, twenty nineteen.

4:13

The original platinum kilogram prototype

4:15

will remain in that underground French vault,

4:17

while countless generations of scientists will

4:19

make world changing discoveries using the Kilogram

4:22

two point zero. Today's

4:29

episode is based on the article the Kilogram

4:31

is Dead Meet the Kilogram two point zero on

4:33

how stuffworks dot Com, written by Dave Ruse.

4:36

Brain Stuff is production of iHeartRadio in partnership

4:38

with HowStuffWorks dot Com and is produced by Tyler

4:40

Klang. Four more podcasts from iHeartRadio,

4:43

visit the iHeartRadio app, Apple Podcasts,

4:45

or wherever you listen to your favorite shows.