Transcript About Time

About Time
by Dr. Fred Raab
LIGO Hanford Observatory
December 17, 2005
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About Time
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Humans have a sense of time
» Do all people sense time in the same way?
» Do other animals sense time?
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How do we measure time?
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Astronomical clocks
Mechanical clocks
Atomic clocks
Coordinating time
Connections between space and time
» Navigation
» Relativity
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Different ways to answer, “What time is it?”
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A “Sense of Time”
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People perceive a “sense of time”
» Ice cream melts in a “short” amount of time on a summer day
» It will be a “long” time before summer vacation
People perceive an “arrow of time”
» If we see a movie of someone diving into a swimming pool and
splashing water about, it seems normal
» If we see a movie of water droplets jumping into the center of a
swimming pool and then a person rising feet first out of the water
and landing on a diving board, it seems funny and “weird”
Do all people experience time the same
» We share many impressions of time, but we also sometimes
disagree on whether something took a long time or not
Since animals cannot talk, we do not know if they
experience time like we do
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Astronomical measures of time are ancient
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Movement of the sun, moon and “stars” in the sky
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First way time was measured by humans
Sky only appears to move; actually we are seeing Earth’s rotation
Sundials measure Earth’s rotation in the daytime using shadows
At night we can use any bright star
Calculating Earth’s movements
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Earth makes one rotation each day
Earth makes a circular orbit around the sun each year
Earth’s rotation axis is tilted relative to its orbit about the sun
This tilt causes the seasons and also causes the positions of the
constellations to appear to rotate about the “North Star” each year
» This gave rise to dividing circles into 360 degrees (close to the 365
days in a year) and 24 hours (time to rotate 1/24th of circle)
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Motion of the Moon
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Earth rotation causes sun and stars to drift by 15 degrees per
hour ( = 360 degrees / 24 hours)
Earth orbit causes the constellation positions at midnight to drift
toward the West by 1 degree/day
But the Moon drifts from constellation to constellation and from
day to night making roughly 12 circuits each year
So a month is 1/12th of a year, or roughly 30 days
This effect is caused by the orbit of the Moon around Earth
These numbers 360, 30, 24 and 12, became the basis for
calendars and clocks because they could be obtained from each
other by multiplication and division
However they are not the exact numbers
For instance there are 365 days in a year and it takes 29 days
for a lunar orbit (bankers calculate interest on a 360-day year)
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Mechanical time
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Astronomical timekeeping is OK if you stay in the
same place, but if you move East or West on Earth,
then your astronomical time changes
So, a city that is 15 degrees East of us (Cheyenne,
Wyoming) would be 1 hour ahead of us in time
As long as it takes much longer than 1 hour to get a
message between Pasco and Cheyenne (before
1861) this is not a problem
But there was a problem measuring space that could
take advantage of a new way to measure time: the
“Longitude Problem”
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The “Longitude Problem”
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Columbus (1492) opened up exploration and trade routes that
ventured far to the West of Europe and by the late 1500’s, Earth
had been circumnavigated
World-wide trading companies were becoming the economic
basis of power for Spain, England and France
But world-wide shipping losses in money and lives were huge,
because ships could not determine their longitude and were
“lost” at sea
The British Parliament set up the “Longitude Prize” (worth more
than today’s Nobel Prize) to encourage the invention of a way to
measure longitude
This required that a “clock” be invented that could be carried on
a ship and kept “synchronized” to a clock in England; comparing
astronomical time to the “English” time (now Greenwich time) to
about 5-10 minutes accuracy, a captain could obtain his
longitude with sufficient accuracy to avoid wrecking his ship.
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Solving the Longitude Problem
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Galileo, Newton, Cassini, Halley and many other
scientists worked on “the Longitude Problem”
This work led to advances in astronomy (accurate
orbits for our Moon and the moons of Jupiter) and
physics (Roemer’s first measurement of the speed of
light)
Ultimately, John Harrison won the longitude prize for
the invention of a portable mechanical clock, which
Captain Cook demonstrated on a famous voyage to
the Pacific
Harrison developed both a ship-board clock (called a
“chronometer”) and the “pocket watch”
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Atomic clocks
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Ever since Harrison’s invention, timekeeping and navigation
have been intertwined
As we traveled farther and needed more navigational accuracy,
the demand for precision in timekeeping accuracy has become
more and more important
Eventually, quantum physics would lead to the discovery of
ultra-stable and unimaginably reproducible clocks: Atoms
Today, the second is defined as 9,192,631,770 cycles of
microwave radiation corresponding to a particular “hyperfine”
splitting of energy levels in the ground state of an atom of
Cesium with atomic number 133.
Devices that measure and count these “atomic seconds” are
called “atomic clocks”
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Atomic clocks are very good
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Today’s best atomic clocks will lose or gain less than
1 second of time in 1,000,000 years!
Atomic time is broadcast across the US from
Colorado and from the Global Positioning System
(GPS) satellites.
GPS navigation on Earth is accurate to within 20-ft
Differential-GPS surveying was used to position
LIGO foundations to a precision of 3/8 inch over 5
miles
You can buy “radio-controlled” wristwatches for under
$50, that receive broadcasts of atomic time and
synch to them.
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General Relativity: the laws of space and time
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Einstein developed Relativity from 1905-1916
Time and Space are stretched and shrunk by motion and matter
Theory first confirmed by detection of Sun’s space warp in 1919
Atomic clocks on GPS satellites “tick” more rapidly than the
same atomic clocks on the ground
The orbiting GPS clocks are becoming younger than us
because of the high speed of motion of the satellites
» This causes the GPS clocks to look slower (dilation)
But Earthbound clocks are slowed down because of the
stronger time warp due to Earth’s mass
» This causes the GPS clocks to look faster
Time warpage is stronger than time dilation at the height of the
GPS orbits, so GPS clocks appear to gain about a second per
century, relative to atomic clocks on the ground
General relativity is used to correct accurately for these time
dilations and time warps
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Coordinating Astronomical and Atomic Time:
What time is it?
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Universal time (UT) is astronomical time, counted from 0 hours
at midnight, with duration of a Mean Solar Day, defined to be as
uniform as possible despite variations in Earth’s rotation rate
International Atomic Time (TAI) is based on combining data from
a large number of atomic clocks around the world
Coordinated Universal Time (UTC) differs from TAI by
introduction of occasional “leap seconds” to bring it closer to UT,
so that over the millenia we will not have sunrise at “midnight”
» This is the time displayed in the LIGO control room, derived from GPS
satellites and compared to atomic clocks at Hanford & Livingston
Sidereal time is important to amateur astronomers; it measures
Earth rotation relative to distant stars as opposed to the Sun
Barycentric time has the duration of sidereal time, but is
calculated at the center of mass of the solar system, rather than
at Earth. It is important for measuring incoming signals from
space (like radio signals from pulsars), because it is more
immune to accelerations from orbiting planets and asteroids
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