- 4235 -
LIGHTSPEED - measured to the nearest second? Scientists are discussing whether it's time
to redefine the second again according to optical clock oscillations, using UV
and visible light sources in place of microwaves. But while several important questions still
need to be answered before this happens, it's clear that the precise definition
of a second is subject to change.
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--------------------- 4235 - LIGHTSPEED - measured to the nearest second?
- Wait just a second
and I will start this review.
Ready? The length of a second
depends on how you're measuring it. The
measurement of a second is not as constant as you might think.
-
- There are 24
hours in a day, 60 minutes in an hour, and 60 seconds in a minute. So a second is just 1/(24 x 60 x 60), or
1/86400, of a day, right? Well, it turns out that defining time isn't that
simple.
-
- We are used to
thinking of a second as a fixed increment of time, but this small unit has
changed several times over the centuries.
The second was originally based on the length of the day. People observed the sun passing overhead and
started measuring its movement using sundials. Devices like that give a time
based directly on the position of the sun in the sky, which is called “apparent
solar time”.
-
- Sundials have a
few drawbacks. Aside from the obvious problem of not being able to read a
sundial when the sun isn't visible, relying on Earth's daily rotation, known as
“astronomical time”, is surprisingly inaccurate.
-
- The Earth rotation
is not precisely constant. The Earth
speeds up and slows down over time. There's a seasonal variation, big
unpredictable variations from decade to decade due to changes in the molten
core, and a longer-term slowing caused by the tides moving backwards and
forwards.
-
- How can we
precisely measure time if using the length of a day is so unreliable?
In the 16th century, people turned to technological solutions
to this problem, and the first recognizable mechanical clocks began to emerge.
-
- The heart of
making a clock basically moved from keeping time by following the position of
the sun, to making an oscillator and defining a fixed number of oscillations to
be equivalent to one second.
-
- The earliest
mechanical examples were pendulum clocks, which were designed to tick at a
specific frequency, equivalent to an astronomical second, averaged over the
course of a year. Over the next several hundred years, scientists worked on
building better, more precise oscillators and developed a myriad of other
timekeeping systems, including springs and gears.
-
- By around 1940,
quartz crystal clocks had become the new gold standard. If you apply a voltage to a carefully shaped
piece of quartz, it starts vibrating and you can tune the frequency of that
oscillation very precisely. But while
this precision is fine for general use, it's just not good enough for really
technical applications, like the internet, GPS systems or studying fundamental
research.
-
- Problems arise
because every piece of quartz is unique and resonates slightly differently
depending on physical conditions such as temperature and pressure. To be truly
accurate, clocks need to be set against some independent, unchanging reference.
This is where “atomic clocks” come in.
-
- Atoms have natural
fixed resonances. They exist only in particular energy states and can only
change from one state to another by absorbing or emitting a fixed amount of
energy. That energy corresponds to a
precise frequency, so you can use that frequency as a reference for time
keeping.
-
- The first
practical atomic clock, unveiled in 1955, measured the number of these
microwave-induced energy transitions in cesium atoms during a single
astronomical second.
-
- In 1967, the
global scientific community agreed to redefine the second according to this
number, and the International System of Units and Measurements now defines a
second as the duration of 9,192,631,770 energy oscillations in a cesium atom.
-
- Since then, the
astronomical second has continued to vary, while the atomic second has remained
at precisely 9,192,631,770 oscillations. These variations in astronomical time
actually mean that, every few years, scientists must add a leap second to allow
Earth's slowing rotation to keep up with atomic time. This leap second is being
abolished in 2035, but scientists and government agencies haven't yet figured
out how to handle this tiny discrepancy.
-
- But scientists are
not content to rest with this definition, which is accurate to
10^-15s or
one-quadrillionth of a second.
-
- Research teams are
working on even more precise optical atomic clocks, which use higher-energy
visible light-induced atomic transitions in elements such as strontium and
ytterbium to improve this accuracy more than 100-fold.
-
-
-
- We can not even
figure out how to get rid of daylight savings time. And, that changes a full hour. Science has its work cut out for it!
-
-
November 22, 2023 LIGHTSPEED - to
the nearest second? 4235
------------------------------------------------------------------------------------------
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------ “Jim Detrick” -----------
--------------------- ---
Thursday, November 23, 2023 ---------------------------------
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