7 ms·

Why is a second equal to the duration of 9,192,631,770 of radiations? (2016)

83 points by modinfo 3y ago

deleted 3y ago
[deleted]
tialaramex 3y ago
The choice to define SI such that the constants have known specific values, with the result that the exact meaning of the units is the thing our metrication discovers not the values of those constants - is a really clever trick. This completes the work of making SI truly universal. Once upon a time all of SI was defined in terms of prototypes, available only locally. In the case of objects like the kilogram and the metre, these were literally prototype physical objects, in some other cases what we measured was the planet we lived on, which is at least available in theory to all its occupants, although not much use if you're in Andromeda. While measuring the second according to its current definition is a bit trickier than estimating its first definition (in terms of a solar day) it's actually possible anywhere in principle.
- zmgsabst 3y ago
  I think it’s particularly clever in a relativistic universe to define length and time by light intervals: they’re a natural spacetime yardstick, for which all observers can agree.
  - johndunne 3y ago
    Indeed. I wondered how the definition would be affected by gravitational/relativistic time dilation. I learned that the calibrated clock would have been calibrated in the same local frame as the ceasium atom. It's already baked into the definition! Science is cool.
- Valgrim 3y ago
  The basis for the metric system was not defined by some arbitrary prototype at first, but by measurements everyone on the planet could reproduce using the sun and a bit of water, at least to a very acceptable approximation. The kilometer was based on 1/10000 the distance between the poles and the equator. The liter was based on the meter, the kilogram was based off the weight of water in a liter, the celcius was based off the freezing and boiling point of water at sea level, etc. The reference meter was put in place to fix the problem arising from the error margin in measurements, but for practical purposes, the definition was all that was needed for a country to join the metric system.
  - Cockbrand 3y ago
    I’ll never stop being fascinated by how neatly all of this fits together: take a hollow cube that has 10cm long edges. Fill it with water - now you have a liter of water. It weighs 1kg. Put it into the freezer, when it just starts freezing, it’s 0°C. Put it onto a stove, and when it starts boiling, it’s 100°C. Simple as that.
    - defrost 3y ago
      > Put it onto a stove, and when it starts boiling, it’s 100°C. Simple as that. Not quite - don't forget your elevation | air pressure. Of course not even elevation above mean sea level is straightforward .. what is the MSL of Australia, for example, and how does it relate to the Geoid, the long tidal run through the long narrow straits to the north of Australia, the WGS ellipsoid, etc.
      - jeffchien 3y ago
        Not to mention the dissolved solids as well, so it's not as simple as using tap water. IMHO Celsius is the SI unit least deserving of praise. It's just bolted onto the system through the Boltzmann constant and isn't intricately linked like other SI/CGS units. It's also a little too coarse for modern use. Sure, I could use decicelsius myself, but that doesn't stop every other weather station, AC/thermostat manufacturer, etc. from still using whole degrees. You can buy $2000 Italian espresso machines with elaborate PID controllers that can only be tuned to whole degrees, which ironically means that using Fahrenheit is more precise!
charlieyu1 3y ago
I thought kilogram was the only one that hasn't been redefined. Then I checked that and actually it is now defined by speed of light and Planck constant
- johndunne 3y ago
  The history behind the quest to define the kg is fascinating. When you consider how far science (and industry) has progressed where we can define with such ludicrous precision enabling experiments like LIGO and JWST, it leaves me thankful to be at this point in humanity's journey. Though, I still wonder what I'll be missing out on in another 100 years :D
  - scottapotamas 3y ago
    LISA (Laser Interferometer Space Antenna) is worth reading into if you don’t know about it already.
  - hanniabu 3y ago
    Does this mean the cherished perfect 1kg samples aren't so important anymore because we should be able to reproduce a kg from scratch?
    - cmehdy 3y ago
      The kg template had lost mass by the time this all happened: https://news.sky.com/story/scientists-redefine-kilogram-as-original-measure-loses-mass-11552690 https://news.sky.com/story/scientists-redefine-kilogram-as-o...
- jimmySixDOF 3y ago
  and ironically for this conversation, the new definition of kg ends with a ....."per second" /s
coin 3y ago
Why is the title edited? The “period of” got dropped.
- dfc 3y ago
  Humans are not perfect? Maybe the submitter tried to type the title from memory and copy+pasted the link. Why is your quote edited? It is "periods of" not "period of".
- curiousObject 3y ago
  It was too long. HN puts a limit on the number of characters in the title. So the submitter was forced to edit it.
  - wrycoder 3y ago
    And it should be, "periods of the radiation", anyway.
fanf2 3y ago
There are more historical details of where the number came from in my blog post https://dotat.at/@/2023-05-26-whence-time.html https://dotat.at/@/2023-05-26-whence-time.html https://news.ycombinator.com/item?id=37778496 https://news.ycombinator.com/item?id=37778496
Affric 3y ago
I always wonder with these sorts of questions if they are truly asked with naïveté. Sincerely, without judgement, do kids just ask the internet now? Do we not teach them the history of science or modern Europe? How is 9192631770 ever reached without any context?
- matteoraso 3y ago
  This is a fairly recent change, and not normally the sort of stuff that gets taught in school. FWIW, growing up I was taught that celsius was defined in terms of the freezing point and the boiling point of water, even though that hasn't been true since the 50s.
  - jenadine 3y ago
    It's not about how recent the change is, but how "useful" it is to people. The definition of second most people know is that it is 1/60 of a minute which is 1/60 of a hour which is 1/24 of a day. That's because people can relate to a day but not to the radiation of some atom. Edit: Unless you are a physicist, of course. Then it matters.

83 points by modinfo 3y ago

deleted 3y ago

[deleted]

The choice to define SI such that the constants have known specific values, with the result that the exact meaning of the units is the thing our metrication discovers not the values of those constants - is a really clever trick. This completes the work of making SI truly universal. Once upon a time all of SI was defined in terms of prototypes, available only locally. In the case of objects like the kilogram and the metre, these were literally prototype physical objects, in some other cases what we measured was the planet we lived on, which is at least available in theory to all its occupants, although not much use if you're in Andromeda. While measuring the second according to its current definition is a bit trickier than estimating its first definition (in terms of a solar day) it's actually possible anywhere in principle.

zmgsabst 3y ago
I think it’s particularly clever in a relativistic universe to define length and time by light intervals: they’re a natural spacetime yardstick, for which all observers can agree.
- johndunne 3y ago
  Indeed. I wondered how the definition would be affected by gravitational/relativistic time dilation. I learned that the calibrated clock would have been calibrated in the same local frame as the ceasium atom. It's already baked into the definition! Science is cool.
Valgrim 3y ago
The basis for the metric system was not defined by some arbitrary prototype at first, but by measurements everyone on the planet could reproduce using the sun and a bit of water, at least to a very acceptable approximation. The kilometer was based on 1/10000 the distance between the poles and the equator. The liter was based on the meter, the kilogram was based off the weight of water in a liter, the celcius was based off the freezing and boiling point of water at sea level, etc. The reference meter was put in place to fix the problem arising from the error margin in measurements, but for practical purposes, the definition was all that was needed for a country to join the metric system.
- Cockbrand 3y ago
  I’ll never stop being fascinated by how neatly all of this fits together: take a hollow cube that has 10cm long edges. Fill it with water - now you have a liter of water. It weighs 1kg. Put it into the freezer, when it just starts freezing, it’s 0°C. Put it onto a stove, and when it starts boiling, it’s 100°C. Simple as that.
  - defrost 3y ago
    > Put it onto a stove, and when it starts boiling, it’s 100°C. Simple as that. Not quite - don't forget your elevation | air pressure. Of course not even elevation above mean sea level is straightforward .. what is the MSL of Australia, for example, and how does it relate to the Geoid, the long tidal run through the long narrow straits to the north of Australia, the WGS ellipsoid, etc.
    - jeffchien 3y ago
      Not to mention the dissolved solids as well, so it's not as simple as using tap water. IMHO Celsius is the SI unit least deserving of praise. It's just bolted onto the system through the Boltzmann constant and isn't intricately linked like other SI/CGS units. It's also a little too coarse for modern use. Sure, I could use decicelsius myself, but that doesn't stop every other weather station, AC/thermostat manufacturer, etc. from still using whole degrees. You can buy $2000 Italian espresso machines with elaborate PID controllers that can only be tuned to whole degrees, which ironically means that using Fahrenheit is more precise!

charlieyu1 3y ago

I thought kilogram was the only one that hasn't been redefined. Then I checked that and actually it is now defined by speed of light and Planck constant

johndunne 3y ago
The history behind the quest to define the kg is fascinating. When you consider how far science (and industry) has progressed where we can define with such ludicrous precision enabling experiments like LIGO and JWST, it leaves me thankful to be at this point in humanity's journey. Though, I still wonder what I'll be missing out on in another 100 years :D
- scottapotamas 3y ago
  LISA (Laser Interferometer Space Antenna) is worth reading into if you don’t know about it already.
- hanniabu 3y ago
  Does this mean the cherished perfect 1kg samples aren't so important anymore because we should be able to reproduce a kg from scratch?
  - cmehdy 3y ago
    The kg template had lost mass by the time this all happened: https://news.sky.com/story/scientists-redefine-kilogram-as-original-measure-loses-mass-11552690 https://news.sky.com/story/scientists-redefine-kilogram-as-o...
jimmySixDOF 3y ago
and ironically for this conversation, the new definition of kg ends with a ....."per second" /s

coin 3y ago

Why is the title edited? The “period of” got dropped.

dfc 3y ago
Humans are not perfect? Maybe the submitter tried to type the title from memory and copy+pasted the link. Why is your quote edited? It is "periods of" not "period of".
curiousObject 3y ago
It was too long. HN puts a limit on the number of characters in the title. So the submitter was forced to edit it.
- wrycoder 3y ago
  And it should be, "periods of the radiation", anyway.

fanf2 3y ago

There are more historical details of where the number came from in my blog post https://dotat.at/@/2023-05-26-whence-time.html https://dotat.at/@/2023-05-26-whence-time.html https://news.ycombinator.com/item?id=37778496 https://news.ycombinator.com/item?id=37778496

Affric 3y ago

I always wonder with these sorts of questions if they are truly asked with naïveté. Sincerely, without judgement, do kids just ask the internet now? Do we not teach them the history of science or modern Europe? How is 9192631770 ever reached without any context?

matteoraso 3y ago
This is a fairly recent change, and not normally the sort of stuff that gets taught in school. FWIW, growing up I was taught that celsius was defined in terms of the freezing point and the boiling point of water, even though that hasn't been true since the 50s.
- jenadine 3y ago
  It's not about how recent the change is, but how "useful" it is to people. The definition of second most people know is that it is 1/60 of a minute which is 1/60 of a hour which is 1/24 of a day. That's because people can relate to a day but not to the radiation of some atom. Edit: Unless you are a physicist, of course. Then it matters.