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Revision as of 16:05, 20 August 2013

Welcome to the explain xkcd wiki!

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I'm glad to hear they're finally redefining the meter to be exactly three feet.
Title text: I'm glad to hear they're finally redefining the meter to be exactly three feet.


Ambox notice.png This explanation may be incomplete or incorrect: Created by a CONSTANT PLANCK. Links to resources would be good. Do NOT delete this tag too soon.

Standard units such as the kilogram, meter, and second are redefined from time to time as measurement technologies improve. These redefinitions are generally done to improve the precision to which the various units can be known or reproduced, without changing their actual value. The joke here is that redefining the kilogram to equal one pound would not only fail to improve on its precision, but would also significantly change the value of what a kilogram is.

On the day of this comic, the International Committee for Weights and Measures voted to redefine the kilogram by fixing it to the value of Planck's Constant. This is done by passing a measured current through an electromagnet to exert a force to balance 1 kg. The change will take effect on May 20, 2019, when the platinum cylinder International Prototype Kilogram that defines the unit will be retired. This means that the mass of a kilogram will no longer be tied to a physical object, but to the fundamental properties of the universe.. By fixing the value of Planck constant to 6.62607015×10-34 kg⋅m2⋅s−1, the kilogram will be defined in terms of the second and the speed of light via the meter.

The previous method of confirming that a kilogram is accurate is to use physical metal weights measuring exactly one kilogram, periodically transporting them around the world to an official weight lab to confirm they still weigh the same. Over time these physical objects have changed very slightly in their mass making them unreliable in the long run -- thus running into the issue that a kilogram did not stay a constant measure of mass. Note that these weights and comparisons are so precise that a fingerprint on one of the weights could throw them off.

In this comic, Black Hat announces that the kilogram has been redefined as equal to one pound. Ponytail and Cueball seem to think this makes things simpler, but Megan is rightfully alarmed. The metric system of measurement is the one used by most of the world and is the standard system used in science. It is considered superior to the United States customary system and the Imperial system (both of which the pound is part of). Therefore, redefining the kilogram to be based on the pound would make things much, much worse and outrage supporters of the metric system. More to the point, the pound is still often defined by metal weights, thus running right back into the very same problem they tried to escape from. Also, redefining the kilogram as being a completely different size from before will create a lot of confusion, since now when people read a mass in kilograms they need to work out whether it was written in old kilograms or new (pound-sized) kilograms.

In real life, the pound is officially defined as 0.45359237 kilograms, or less than half a kilogram. This makes defining a kilogram as one pound even more impossible as they are then stuck in a loop, as the pound must weigh less than half of a kilogram, meaning the value of each would be equal to zero.

In addition, the pound also exists as a unit of weight or force (lbf), whereas the kilogram is a unit of mass, thus fixing the kilogram to the pound would make even less practical sense.

The title text continues the joke by saying that the meter has been defined as exactly three feet. The yard, the closest US measurement to the meter, is three feet. However, a meter is about 9 centimeters longer than a yard. As with the pound, the metric system is used to define the yard as it is officially defined as 0.9144 meters.


[Black Hat talking to Ponytail, Cueball, and Megan while all stand in a row. Megan's hands are raised emphatically.]
Black Hat: To end many years of confusion, the International Committee for Weights and Measures has just voted to redefine the kilogram.
Black Hat: As of next May, it will equal exactly one pound.
Ponytail: Oh, cool.
Cueball: That does make things simpler.
Megan: No!!


To further expand on this, the classic definitions of all our various units of time, length, mass, and temperature are based on phenomena that are neither convenient to measure precisely nor in fact consistently reproducible. The duration of an Earth day and year vary unpredictably, the circumference of the Earth varies, the International Prototype Kilogram gains or loses mass any time it is handled (and in fact just sitting there it and its reference copies diverge from each other), and the value of baseline temperatures such as the freezing point of water depend on which isotopes of hydrogen are in the water molecules.

Nevertheless, there really are constants of nature. For example, one of them is ‘c’, the speed of light in a vacuum. The expressed value of c depends on your choice of the unit of distance and the unit of time, but it’s a constant in those units. Now just suppose we all had a reproducible way to define a specific unit of time, which just for fun we call a ‘second’. You might not know the length of a ‘meter’, but if I told you that measured in meters per second the universal constant value of c is exactly 299792458 meters per second, then I would have fixed the length of a meter to be exactly the distance light travels in a vacuum in 1/299792458 seconds. And in fact this is what the international body responsible for defining our SI units has done.

One second is defined to be a specific number of periods of the radiation emitted in a certain transition of a cesium 133 atom. The specific number was set in the year 1967, so as to match a previous astronomical standard called Ephemeris Time to the limit of human measuring ability at the time. The 1967 definition didn’t change the actual duration of a second, but it did make its measurement forever reproducible.

In 1983 the value of c was fixed to the value noted above. Prior to that it had been measured with respect to existing definitions of a meter, and had to be expressed with a measure of uncertainty. For example in 1973 a team at the US National Bureau of Standards refined c to 299,792,457.4 m/s ± 1 m/s. But from 1983 onwards, with an exact integer value for c that is quite close to that Bureau measurement, the length of a meter is now fixed with no plus/minus uncertainty. Furthermore, both the second and the meter match their predecessor definitions for all intents and purposes.

Similar redefinitions of units of mass and of temperature in terms of universal constants have been agreed to, mass with regard to the Planck constant h, and temperature with regard to the Boltzmann constant k. The constants h and k had previously been measured quantities, complete with uncertainties. The SI body fixed both of them to exact values, resulting in exact, no-uncertainty values for a kilogram of mass and a kelvin of thermodynamic temperature. As with the second and the meter, these new definitions match their predecessor definitions for all intents and purposes.

To expand on this even further, three additional universal constants that were previously measured and that had uncertainty values have been assigned fixed values, resulting in exact definitions of three corresponding units of measurement without affecting their applicability. Fixing the unit of elementary charge, e, serves to define the unit of electric current, the Ampere. Fixing the unit of luminous efficacy Kcd serves to define the unit of luminous intensity, the candela. And fixing the Avogadro constant NA serves to define the unit of amount of substance, the mole.

A very recent Wikipedia article about redefining the SI units of measure in terms of newly fixed values of things taken to be universal constants is Redefinition of SI base units.

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