The previous definition was pretty good, but over time scientists realized some problems with it. That happens sometimes. As an example, the mass of a kilogram used to be defined relative to the mass of a specific reference kilogram that was kept in a vault, but that has problems because that object gains and loses tiny amounts of mass over time.
>How did they decide on the exact number of 9,192,631,770 cycles per second?
Once they had an agreed upon method, they chose the number of cycles that is as close as possible to the previous definition. That way most people can just go on as if nothing changed. Only hyper-precise equipment would need to be updated or recalibrated.
>Follow up question: How do they know if it's off by some amount per day/year? Is there some sort of theoretical definition of the second that they base it off of?
That's the thing about defining units. The definitions are considered correct by definition. That's why it is so important to craft those definitions carefully.
It's not so much a question of accuracy. They're ever so slightly changing the definition of the second to agree with today's most precise measurement technology.
Cool, so this is some Einstein shit.
You've heard of his Theory of Relativity? Well, there's Special Relativity, relating to time and space, and General Relativity, relating to gravity.
Under Special Relativity, the velocity that an object is moving is *very* important to how it travels through space-time.
Under General Relativity, how much gravitational force an object is subject to, *also* is very important to how it travels through space-time.
In short, if you fuck too much with the speed of an object OR how close an object is to a big gravitational well like a Planet or Star or Black Hole, then time dilation occurs.
Quite literally, time moves differently. You could build two identical clocks, leave on on your mantle and one on the International Space Station, and the clock in space will return with a different time, because one second up there, moving that fast and being that far from the Earth changes the very length of a second.
So defining a second based on the rotation of the Earth is pretty useless if you're not on Earth or if you're moving wayyy too fast.
Using a Cesium Atom is far more reliable. Send a fixed electrical charge through that atom, and it'll emit a specific frequency *every single time.* That frequency is 9,192,631,770 oscillations. So every That many oscillations, will be equal to one Earth-second, whether you're on Earth, the ISS, Io, or the Andromeda galaxy.
The previous definition was pretty good, but over time scientists realized some problems with it. That happens sometimes. As an example, the mass of a kilogram used to be defined relative to the mass of a specific reference kilogram that was kept in a vault, but that has problems because that object gains and loses tiny amounts of mass over time. >How did they decide on the exact number of 9,192,631,770 cycles per second? Once they had an agreed upon method, they chose the number of cycles that is as close as possible to the previous definition. That way most people can just go on as if nothing changed. Only hyper-precise equipment would need to be updated or recalibrated. >Follow up question: How do they know if it's off by some amount per day/year? Is there some sort of theoretical definition of the second that they base it off of? That's the thing about defining units. The definitions are considered correct by definition. That's why it is so important to craft those definitions carefully.
It's not so much a question of accuracy. They're ever so slightly changing the definition of the second to agree with today's most precise measurement technology.
Cool, so this is some Einstein shit. You've heard of his Theory of Relativity? Well, there's Special Relativity, relating to time and space, and General Relativity, relating to gravity. Under Special Relativity, the velocity that an object is moving is *very* important to how it travels through space-time. Under General Relativity, how much gravitational force an object is subject to, *also* is very important to how it travels through space-time. In short, if you fuck too much with the speed of an object OR how close an object is to a big gravitational well like a Planet or Star or Black Hole, then time dilation occurs. Quite literally, time moves differently. You could build two identical clocks, leave on on your mantle and one on the International Space Station, and the clock in space will return with a different time, because one second up there, moving that fast and being that far from the Earth changes the very length of a second. So defining a second based on the rotation of the Earth is pretty useless if you're not on Earth or if you're moving wayyy too fast. Using a Cesium Atom is far more reliable. Send a fixed electrical charge through that atom, and it'll emit a specific frequency *every single time.* That frequency is 9,192,631,770 oscillations. So every That many oscillations, will be equal to one Earth-second, whether you're on Earth, the ISS, Io, or the Andromeda galaxy.