The second only became important when society became interconnected through fast-moving railways where cities needed to agree on exact timekeeping. By the 1950s, several global systems required every second to be accounted for with as much precision as possible.  Today, devices are synchronised to help us maintain a globally consistent time, from radio signal transmitters to satellites for global positioning systems.

The growing importance of the second

The second as we know it was one introduced in the late 1500s when the Gregorian calendar began to spread across the globe alongside British colonialism. The Gregorian calendar defined a day as a single revolution of the Earth about its axis. A day could be divided into 24 hours, each hour into 60 minutes, and each minute into 60 seconds. Yet, the second was more a mathematical idea than a useful unit of time.

The history of the second

Before timekeeping could go fully atomic, countries had to decide which atom would work best.  At the Thirteenth General Conference of the International Committee for Weights and Measures in 1967, researchers decided on Cesium-133. The element conformed to the following specifications:<ul><li>It had a long-lived and high-frequency electron oscillation for precise, long-term timekeeping.</li><li>It maintained a reliably measurable quantum spin </li><li>It could easily vaporise.</li></ul>

Finding the right element

Determining how many ticks of a caesium atom were in a second, the conference used the most precise astronomical measurement of a second available at the time.  They started with the number of days in a year and divided it down. Compared to the atom’s ticking rate, one second was defined as exactly 9,192,631,770 ticks of a caesium-133 atom.

The cesium standard

As early as 1955, researchers started to develop atomic clocks, which relied on the laws of physics to establish a new foundation for timekeeping. An atom consists of negatively charged electrons orbiting a positively charged nucleus at a consistent frequency. You can lightly disturb an electron's orientation if you expose an atom to an electromagnetic field such as light or radio waves. If you briefly tweak an electron at the right frequency, the vibration will resemble a ticking pendulum and can tick for centuries.

Atomic clocks

Discover how scientists developed atomic clocks, which use the vibrations of atoms to measure and maintain a globally consistent time. -- In 1967, researchers gathered to answer a long-running scientific question: just how long is a second? It might seem obvious at first. A second is the tick of a clock, the swing of a pendulum, the time it takes to count to one. But how precise are those measurements? And what is that length based on? John Kitching digs into how we scientifically define this fundamental unit of time. Lesson by John Kitching, directed by Tjoff Koong. Support Our Non-Profit Mission ---------------------------------------------- Support us on Patreon: http://bit.ly/TEDEdPatreon Check out our merch: http://bit.ly/TEDEDShop ---------------------------------------------- Connect With Us ---------------------------------------------- Sign up for our newsletter: http://bit.ly/TEDEdNewsletter Follow us on Facebook: http://bit.ly/TEDEdFacebook Find us on Twitter: http://bit.ly/TEDEdTwitter Peep us on Instagram: http://bit.ly/TEDEdInstagram ---------------------------------------------- Keep Learning ---------------------------------------------- View full lesson: https://ed.ted.com/lessons/who-decides-how-long-a-second-is-john-kitching Dig deeper with additional resources: https://ed.ted.com/lessons/who-decides-how-long-a-second-is-john-kitching#digdeeper Animator's website: http://greenhouseanimation.com/directors/tjoff-koong/ ---------------------------------------------- Thank you so much to our patrons for your support! Without you this video would not be possible! Po Foon Kwong, NinjaBoffin, Jesse Jurman, Josue Perez Miranda, Scott Markley, Elija Peterson, Ovidiu Mrd, Lawrence Wu, Xavier Dupont, Aravind Battaje, Nathan Giusti, Helen Lee, Anthony Benedict, Karthik Balsubramanian, John Hong, Annastasshia Ames, Sebastiaan Hols, Aries SW, Amy Lopez, Liz Candee, Kathryn Vacha, Anthony Arcis, Jeffrey Segrest, Karmi Nguyen, Yelena Baykova, Harshita Jagdish Sahijwani, Nick Johnson, Carlos H. Costa, Mohamed Elsayed, Les Howard, Jennifer Kurkoski, Ryan B Harvey, Abhishek Bansal, Jose Arcadio Valdes Franco, Karl Laius, JY Kang, Charles A Hershberger, Robert Seik, Heidi Stolt, Alexis Hevia, Brady Jones, Christina Salvatore, Karlee Finch, Michael Goldberg, Mario Mejia, Nicolas Silva, Kurt Almendras, Denise A Pitts, Doug Henry and Kristiyan Bonev.

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Discover how scientists developed atomic clocks, which use the vibrations of atoms to measure and maintain a globally consistent time. 

--

In 1967, researchers gathered to answer a long-running scientific question: just how long is a second? It might seem obvious at first. A second is the tick of a clock, the swing of a pendulum, the time it takes to count to one. But how precise are those measurements? And what is that length based on? John Kitching digs into how we scientifically define this fundamental unit of time.

Lesson by John Kitching, directed by Tjoff Koong.

Support Our Non-Profit Mission
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Support us on Patreon: http://bit.ly/TEDEdPatreon
Check out our merch: http://bit.ly/TEDEDShop
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Connect With Us
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Sign up for our newsletter: http://bit.ly/TEDEdNewsletter
Follow us on Facebook: http://bit.ly/TEDEdFacebook
Find us on Twitter: http://bit.ly/TEDEdTwitter
Peep us on Instagram: http://bit.ly/TEDEdInstagram
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Keep Learning
----------------------------------------------
View full lesson: https://ed.ted.com/lessons/who-decides-how-long-a-second-is-john-kitching
Dig deeper with additional resources: https://ed.ted.com/lessons/who-decides-how-long-a-second-is-john-kitching#digdeeper

Animator's website: http://greenhouseanimation.com/directors/tjoff-koong/
----------------------------------------------

Thank you so much to our patrons for your support! Without you this video would not be possible! Po Foon Kwong, NinjaBoffin, Jesse Jurman, Josue Perez Miranda, Scott Markley, Elija Peterson, Ovidiu Mrd, Lawrence Wu, Xavier Dupont, Aravind Battaje, Nathan Giusti, Helen Lee, Anthony Benedict, Karthik Balsubramanian, John Hong, Annastasshia Ames, Sebastiaan Hols, Aries SW, Amy Lopez, Liz Candee, Kathryn Vacha, Anthony Arcis, Jeffrey Segrest, Karmi Nguyen, Yelena  Baykova, Harshita Jagdish Sahijwani, Nick Johnson, Carlos H. Costa, Mohamed Elsayed, Les Howard, Jennifer Kurkoski, Ryan B Harvey, Abhishek Bansal, Jose Arcadio Valdes Franco, Karl Laius, JY Kang, Charles A Hershberger, Robert Seik, Heidi Stolt, Alexis Hevia, Brady Jones, Christina Salvatore, Karlee Finch, Michael Goldberg, Mario Mejia, Nicolas Silva, Kurt Almendras, Denise A Pitts, Doug Henry and Kristiyan Bonev.

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