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by
Ciro Santilli
(@cirosantilli,
32
)
Time
Table of contents
Frequency
Period
Frequency
Hertz
Frequency
Megahertz
Hertz
Clock
Quartz clock
Clock
Atomic clock
Clock
Caesium standard
Atomic clock
Unit of time
Decimal time
Unit of time
Second
Unit of time
Day
Unit of time
Calendar
Day
Year
Unit of time
Frequency
Time
Period (physics)
Frequency
Hertz (1857-1894, Hz)
Frequency
Named after
radio
pioneer
Heinrich Hertz
.
Megahertz (MHz)
Hertz
Mega-
Hertz
, i.e. a million
Hertz
.
Clock
Time
Quartz clock
Clock
Video 1
. How a quartz watch works by
Steve Mould
(2017)
Source
. Mentions
feedback loop
loop with the
quartz
tuning fork
for the
piezoelectricity
and an
amplifier
. Also mentions the choice of 32768
Hertz
(
2
1
5
) as the first power of 2 that is outside of the
human hearing range
, and then how a
frequency divider
is used to reduce the frequency to get the
second
counter.
Atomic clock
Clock
Video 1
. How an atomic clock works, and its use in the global positioning system (GPS) by
EngineerGuy
(2012)
Source
. Shows how conceptually an atomic clock is based on a
feedback loop
of two
hyperfine structure
states of
caesium
atoms (non-
radioactive
caesium-133
as clarified by the
Wikipedia
page). Like a
quartz clock
, it also relies on the
piezoelectricity
of quartz, but unlike the
quartz clock
, the
quartz
is not shaped like a
tuning fork
, and has a much larger resonating frequency of about 7
MHz
. The feedback is completed by producing
photons
that
resonate
at the right frequency to excite the
caesium
.
Video 2
. Inside the
HP
5061A Cesium Clock by
CuriousMarc
(2020)
Source
.
A similar model was used in the
Hafele-Keating experiment
to test
special relativity
on two planes flying in opposite directions. Miniaturization was key.
Contains a disposable tube with 6g of
Caesium
. You boil it, so when it runs out, you change the tube, 40k USD. Their tube is made by
Agilent Technologies
, so a replacement since that opened in 1999, and the original machine is from the 60s.
Detection is done with an
electron multiplier
.
youtu.be/eOti3kKWX-c?t=1166
They compare it with their 100 dollar
GPS
disciplined oscillator, since
GPS
satellites
have
atomic clocks
in them.
Video 3
. Quick presentation of the
atomic clock
at the
National Physical Laboratory
(2010)
Source
. Their super accurate setup first does
laser cooling
on the
caesium
atoms.
Caesium standard (1967, 3.26 cm)
Atomic clock
tags:
Caesium
Uses the
frequency
of the
hyperfine structure
of
caesium-133
ground state, i.e spin up vs spin down of its valence electron
6
s
1
, to define the
second
.
International System of Units
definition of the second since 1967, because this is what
atomic clocks
use.
TODO why does this have more energy than the hyperfine split of the
hydrogen line
given that it is further from the nucleus?
Why
caesium
hyperfine structure
is used:
physics.stackexchange.com/questions/191871/why-do-atomic-clocks-only-use-caesium
Unit of time
Time
Decimal time
Unit of time
Second (s)
Unit of time
Day (d)
Unit of time
Calendar
Day
Year (y)
Unit of time
Ancestors
International System of Units
List of systems of units
System of units
Physics
Natural science
Science
Index
Incoming links
Spacetime interval
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