How Physics Got Precise

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How Physics Got Precise Or some events from the
history of physics that happen to intrigue the
speaker Daniel Kleppner Massachusetts Institute
of Technology
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How Physics Got Precise Or some events from the
history of physics that happen to intrigue the
speaker Daniel Kleppner Massachusetts Institute
of Technology
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The Length of the Year
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The year 1600 and the dawn of modern science On
January 1, 1600, Johannes Kepler set out for
Prague to work for Tycho Brahe. For me, this
event marks the birth of modern science. The
delivery was difficult.
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1609, July, Galileo learns of telescope 1609,
December, Galileo starts systematic
observations 1610, March, Galileo publishes The
Starry Messenger
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Refractive indices for different colors, to six
figures, five significant figures
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Page from Galileos notebook, Jan. 1610
Date of observation
Hours after sunset
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Ganymede
distance (Jupiter diameters)
time (hours)
Plotted by Alber Liau
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Europa
distance (Jupiter diameters)
time (hours)
Plotted by Alber Liau
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1682 more precise measurements are available
Europa period 84.223 hours from Kepler
85 hours
Ganymede period 171.71 hours from
Kepler 172 hours
Measurements taken from
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from Principia, Book III, The System of
the World, Motte translation translated by
Andrew Motte, 1729, revised by Florian Cajori, U.
of Cal. Press, Berkeley, 1947
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from Principia, Book III, The System of
the World, Motte translation translated by
Andrew Motte, 1729, revised by Florian Cajori, U.
of Cal. Press, Berkeley, 1947
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from Principia, Book III, The System of
the World, Motte translation translated by
Andrew Motte, 1729, revised by Florian Cajori, U.
of Cal. Press, Berkeley, 1947
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from Principia, Book III, The System of the World,
Law of Universal Gravitation And therefore (by
Rule 1 and 2) the force by which the moon is
retained in its orbit is the very same force
which we commonlycall gravity
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from Principia, Book III, The System of the World,
A Problem of Units
15 1/12 Paris feet, or, more accurately, 15
feet, 1 inch and 1 line 4/9
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Towards the end of the 18th century there
was total confusion in units and standards
Contemporaries estimated that under the cover
of some eight hundred names, the Ancien Regime of
France employed a staggering 250,000 different
units of weights and measures. Ken Alder,
the Measure of All Things, Free Press, 2002
The rod, 16 men of assorted height coming from
church. F.C. Cochrane, Measures for Progress,
NBS, 1996
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The Enlightment and a Triumph of Reason A
system of units based on Nature, not Mankind A
Metric System metric distance, metric mass,
metric time, metric calendar,. . . First triumph
of the Metric Revolution a new unit of
length--the meter Definition the meter is 1 ten
millionth the distance from the equator to
the pole
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Realization of the meter Survey, by
triangulation, the distance between two latitudes
along a convenient meridian, for instance the
meridian between Barcelona and Dunquerque.
Expeditions launched in 1792 Méchain
to Barcelona, Delambra to Dunquerque. Their work
concluded in 1798. A great story, see The
Measure of All Things, by Ken Alder, Free Press,
2002
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scale of 30,000 toises
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Paris!
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A digression-- the advance of precision
by Joseph von Fraunhofer, 1783-1829 -Genius of
optical instruments -Inventor of the diffraction
grating -A life of rags to riches
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The dispersive power of various glasses, Munich,
1814
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Absorption lines in the solar spectrum,the
Fraunhofer lines, 1814
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Table of refractive indices of various glasses
and materials.
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Thinking about diffraction, 1814. Fraunhofer
inventsdiffraction gratingin 1818.
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Fraunhofers shop
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NEEDED A BETTER METER 1873 Convention of the
Meter signed Bureau Internationale des
Poids et Mesures established New
definition of meter discussed 1889 Meter
redefined in terms of artifact Pt-Ir bar
1889 meter and kilogram
Meanwhile
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1879 letter from Maxwell to head of U.S. Naval
observatory
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Michelson-Morley experiment, December 1887
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1/8 Theoretical Shift
Experimental fringe shift
Michelson and Morley present their results for
the fringe shift due to motion through the ether,
December, 1887
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Directly following that paper, another.
On a Method of making the Wave-length of Sodium
Light the actual and practical standard of
Length.
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So, the 1889 definition of meter in terms of
artifact was obsolete when it was adopted.
Michelsons interferometric method was a)
more precise- could measure one meter to about
1/100 of wavelength of light, 2 parts in
108 b) more accurate- not susceptible to
aging, temperature, bumps and bruises
c) more practical- could be realized anywhere
d) based on a natural unit. In consequence,
the artifact definition was more or less
promptly set aside for a new definition.in 1960.
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Meanwhile- starting in 1882, Michelson published
a series of papers, pushing the limits of
interferometry. By studying the intensity of
interference fringes as he extended one arm of
his spectrometer over longdistances,
Michelson -Discovered the fine structure of
hydrogen -Learned how to measure the width of
spectral lines -Invented Fourier transform
spectroscopy -Discovered pressure
broadening -Confirmed Maxwells theory for the
speeds of atoms
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Hydrogen fine structure
visibility of fringes
Balmer-alpha
Balmer-beta
reconstructed spectrum
distance ----gt
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Comparison of Michelsons 1892 results with
state-of-the-art spectroscopy in 1939.
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Visibility curves and reconstructed spectra for
two Hg isotopes. top 3 line spectrum
bottom 2 line spectrum
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Measurements of the speeds of atoms from the
Doppler broadening of their spectral lines.
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Finally, in 1960, a new legal definition of the
meter was adopted, based on Michelsons
interferometric methods for counting wavelengths
of light. The meter is 1650 763.73 wavelengths
in vacuumof the orange-red line in the spectrum
of thekrypton-86 atom. BUT, in 1959 the laser
was invented, which revolutionized interferometry
and soon made the new definition obsolete.
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The story of TIME Historic definition of the
second There are 86,400 seconds in one
day Problem definition of day. Mean solar day
can be regarded as the average time between
successive sunrises. But, Earths rotation is
slowing due to tidal friction, and fluctuating
due to Chandler wobble and other small
effects. In 1952, the International
Astronomical Union proposed introducing
ephemeris time. Thesecond is 1/31 556 925.9747
of the tropical year 1900. Proposal was adopted
in 1958.
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Ephemeris time became legal in 1960. BUT first
atomic clock was demonstrated in 1954.
Louis Essen and Jack Perry, Cesium atomic beam
frequency standard NPL, 1954
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• The definition of the second in terms of
  ephemeris time was obsolete before it was
  adopted. So..
• in 1969 there was a new definition of the
  second
• The second is the duration of 9192 631 770
  periods of radiation corresponding to the
  transitionbetween the two hyperfine levels of
  the ground state of the cesium-133 atom.

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A word about time scales TAI average of
primary and secondary atomic clocks around
world UT1 based on Earth rotation UTC
Coordinated Universal Time Leap seconds
added to TA1 keeps UTC within 0.9s of
UT1 This is the commonly propagated
time scale.
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From Splitting the Second, Tony Jones, IOP, 1992.
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A problem in metrology Although time and
frequency can easily be measured to 1 part in
1013, Wavelengths (i.e. distances) cannot be
compared to better than 1 part in 1010. So, the
speed of light cannot be measured to better than
1 part in 1010. Result new definition of the
meter based on the distance light travels in a
given time.
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Realizing this new definition of the meter
requires measuring the frequency of light, i.e.
the frequency of a laser.
Frequency chain for measuring the frequency of a
visible laser. NIST, 1979.
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New definition of the meter, 1983 The meter is
the distance that light travels in 1/299 792 458
of a second. Unfortunately, for the next twenty
years there was virtually no way to implement
this definition. Fortunately, this did not
appear to cause any problems.
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Changing styles of precision. Three great
syntheses in physics -Newton Law of Universal
Gravitation -Maxwell proof that light consists
of electromagnetic waves, whose speed is given by
the electric and magnetic force
constants. -Bohr proof that the Rydberg
constant is given by a specific combination of
fundamental constants.
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Bohrs 1913 paper on his model of hydrogen. No
uncertainties are stated.
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The systematic treatment of experimental
uncertainty can be ascribed to R. T, Birge, who
carried out the first evaluation of the
fundamental constants. The Physical Review
Supplement later became the Review of Modern
Physics.
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The current state of high precision The most
accurately verified theory in physics is QED
theory and experiment have been found to agree to
about 1 part in 1011. This will soon be improved
significantly. A most unsatisfactory base unit in
physics is mass, which continues to be an
artifact at BIPM, Paris. This will not continue
for much longer. The most accurately measurable
quantity continues to be time. Current accuracy
is about 1 part in 1015. Major improvements are
expected.
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An unsatisfactory (though pretty) base unit
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A March of High Precision
Courtesy T.W. Haensch
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The revolution in precision optical frequency
metrology--counting cycles of light--invented by
T. W. Hänsch
Output from a modelocked (pulsed) laser. Here,
the pulse rate is not synchronous with the
carrier.
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An early frequency combgenerator
Synchronizing the carrier to form acomb of
coherent optical frequencies.
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Optical Atomic Clockbased on ultra-violet
transition in mercury ion
courtesy of Jim BergquistScience, 306, 1318, 2004
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Is Time about to bite the dust--i.e. involve an
artifact? The gravitational red shift of time at
the surface of the earth is about 1 part in 1018
per meter. Thus, to define time to 1 part in
1018, it will be necessary to fix where the
clock is located. Physics has progressed but
human nature has not. There will undoubtedly be
lively debate as to which laboratory gets the
new second. END