150
votes
Why was carbon-12 chosen for the atomic mass unit?
Why was Carbon-12 chosen for the atomic mass unit?
As is the case elsewhere in metrology, the answer is tied up in history, measurability, practicality, repeatability, past misconceptions, and ...
- 40.7k
112
votes
Accepted
How do you make more precise instruments while only using less precise instruments?
I work with an old toolmaker who also worked as a metrologist who goes on about this all day.
It seems to boil down to exploiting symmetries since the only way you can really check something is ...
- 8,400
110
votes
Accepted
Why is a second equal to the duration of exactly 9,192,631,770 periods of radiations?
That number, 9192631770, was chosen to make the new definition of the second as close as possible to the less precise old second definition. This means that--except for the most precise measurements--...
- 86.8k
104
votes
Accepted
What reference clock is an atomic clock measured against?
This is a good and somewhat tricky question for a number of reasons. I will try to simplify things down.
SI Second
First, let's look at the modern definition of the SI second.
The second, symbol s, ...
- 12.2k
103
votes
Why do atomic clocks only use caesium?
"Because that is how the second is defined" is nice - but that immediately leads us to the question "why did Cesium become the standard"?
To answer that we have to look at the ...
- 118k
100
votes
Has anyone charged an object with 1 coulomb? Why was such a ridiculously large charge chosen as the unit of charge?
Actually the ampere (SI unit for electric current) was defined first
(in 1881, see Wikipedia: Ampere - History).
They chose this size for $1$ ampere, probably because at this time
such a current could ...
- 32.4k
80
votes
Why is a second equal to the duration of exactly 9,192,631,770 periods of radiations?
It's a definition of a unit, which is an arbitrary choice. In the past we used to define a second as 1⁄86,400 of a solar day and later as "the fraction 1/31,556,925.9747 of the tropical year for 1900 ...
- 16.2k
76
votes
Why should a clock be "accurate"?
why it is necessary for a reference clock to worry about this, if the definition of the second itself is a function of the number of ticks the clock makes.
The concern is that somebody else (say a ...
- 25.4k
75
votes
Accepted
In nuclear physics, what length year in seconds is used?
A "year" without qualification may refer to a Julian year (of exactly $31\,557\,600~\rm s$), a mean Gregorian year (of exactly $31\,556\,952~\rm s$), an "ordinary" year (of exactly $31\,536\,000~\rm s$...
69
votes
Accepted
What are the proposed realizations in the New SI for the kilogram, ampere, kelvin and mole?
So the BIPM has now released drafts for the mises en pratique of the new SI units, and it's rather more clear what the deal is. The drafts are in the New SI page at the BIPM, under the draft documents ...
- 128k
65
votes
Accepted
How can a day be exactly 24 hours long?
You can't calculate the length of a mean solar day just by taking the mean of the shortest & longest apparent solar days. That would work if the apparent day lengths varied in a simple linear ...
- 10.7k
64
votes
Why are leap seconds needed so often?
It's not the rate of change of the rotation speed that's important, it's the current rotation speed (in the rotating reference frame that stays facing the sun) not matching a 24h day.
Thus leap ...
- 1,370
60
votes
How do you make more precise instruments while only using less precise instruments?
The more you measure things and add or multiply those measurements, the greater your errors will become.
Not necessarily. If the errors in a series of measurements are independent and there is no ...
- 42.1k
57
votes
Accepted
Why is the prospective new kilogram standard a sphere?
If you know the diameter of the sphere, you know everything you need to know about the dimensions. It all comes down to one single value.
Any other shape requires multiple dimensions and thus ...
- 1,510
53
votes
Has anyone charged an object with 1 coulomb? Why was such a ridiculously large charge chosen as the unit of charge?
Has anyone charged an object with 1 coulomb?
Not a problem nowadays with supercapacitors.
Why was such a ridiculously large charge chosen as the unit of charge?
Once the second and the ampere (both ...
- 86.8k
53
votes
What is a joule? I find the definition confusing
Pushing the ball bearing with 1 N for one meter and pushing a bowling ball with 1 N for 1 meter do exactly the same amount of work: 1 joule. As you say: it will take a much longer time for the ...
- 48.1k
51
votes
Will the volt, ampere, ohm or other electrical units change on May 20th, 2019?
Late last century electrical standards based on Josephson junctions became common. A Josephson junction together with an atomic clock can give an exquisitely precise voltage standard in terms of the ...
- 85.9k
44
votes
Accepted
Is the fact that 100 kPa equals about 1 atmosphere accidental?
This is a coincidence. There's nothing about the atmosphere that would make it have a nice relationship with the Earth's rotation or diameter, or the fact that water is plentiful on the surface.
On ...
- 128k
42
votes
Why are leap seconds needed so often?
It's a cumulative effect. Let's say that the mean solar day is about 1.5 milliseconds longer than the SI day of 86400 seconds. This difference accumulates every single day. After 1000 days, the total ...
- 14.3k
42
votes
Why should a clock be "accurate"?
For most of human history, we had a single mechanical clock: the spinning Earth.
Well, actually two mechanical clocks. The Earth’s spin rate is a good constant, but it’s tricky to measure directly. ...
rob♦
- 82.4k
42
votes
Accepted
Is the definition of a meter tautological?
Theoretically, we have not defined the speed of light in terms of the metre. We have defined it as a specific distance (that light can cover in one second).
Now take that distance and divide it with $...
- 49.5k
38
votes
Accepted
Has anyone charged an object with 1 coulomb? Why was such a ridiculously large charge chosen as the unit of charge?
The underlying reason that 1 coulomb seems like a large amount of charge is that most charged particles in ordinary settings are nonrelativistic -- moving at speeds $v$ much less than the speed of ...
- 3,660
37
votes
What reference clock is an atomic clock measured against?
BIPM and TAI
The International Bureau of Weights and Measures (BIPM) in France computes a weighted average of the master clocks from 50 countries. That weighted average then gives International Atomic ...
- 715
36
votes
Is there any truth to interpreting definition of a second as corresponding to oscillations?
The definition for the cesium clock is:
9192631770 cycles per second is frequency of the radio waves which cause maximum resonance, a physically measurable condition, in the cesium atoms.
This ...
- 7,099
36
votes
Accepted
The definition of 1 kelvin
To answer this question it may help to take an example from a more familiar area of physics, and then discuss temperature.
For a long time the kilogram (the SI unit of mass) was defined as the mass of ...
- 53.1k
36
votes
How long is a second?
A second is a second long by definition, but if you measure any time in seconds, the number of seconds you infer will be subject to an error of at least $\mathcal O(10^{-15})$ because of the ...
- 4,865
32
votes
Accepted
Why is the Planck constant an exact number with defined value?
Planck's constant relates two different types of quantities, namely energy and frequency. That means it is a conversion factor which converts the units of quantities from one form to another. If the ...
- 13.4k
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