Lecture 3: Experimental
measurements
(chpt 5, Feynman)
Importance of measurements
• Physics is an experimental science! Most physicists
are experimenters
• Measurability is a key part of the sciences
• First person who perform quantitative
measurement (scientist) is Galileo Galilei (15641642)
• Most of the Nobel prizes have awarded to invention
of instruments
• First US Nobel prize winner: Michelson, invention
of interferometer!
Michelson invented Michelson
Interferometer (Nobel prize 1907)
• Michelson-Morley
Experiment, measured the velocity
of earth relative to either.
What do you want to measure?
• Distance, Time, Speed
• Temperature (thermometer), pressure
• Weight, mass
• Energy, power, noise
• Brightness, telescope,
• Concentration, blood analysis,
• Electromagnetic fields, electric current, resistors,
capacitors, etc.
• Radioactivity
• ….
Galileo’s inclined-plane exp.
Discovery of laws of physics!!
Time measurement
• Something that happens again and again
• Counting pulse!
• Day, hour glass, noon to noon measurement is
nearly the same!
• Fraction of a day: other periodic motion!
• Galileo: pendulum: grand-father clock
• Make a pendulum swing 3600 times x24 in a day,
each swing is a second!
How do you make the time
measurement more accurate?
• Calibrate the longer period with shorter period
• Faster periodic motion: electric current oscillation
(electric oscillator)
• Accuracy of 10-12 achieved (quartz)
Atomic clock and light clock
• Atom moves periodically inside the atom.
• Atomic clock has been used as a standard for time
measurement since 1960’s and is the most accurate
devise now available.
• Usually radio-wave and has reached accuracy of
10-16
• More accurate measurement needs better
frequency measurement, light comb, (light clock)
• Jun Ye (JILA), the best guy in the world, 10-18
• Low-temp + relativity
Jun Ye, member of US NAS
Undergraduate
Student from SJTU (1984-1989)
Shortest time
• Lifetime of the elementary particles
• Strong interaction particle 10-24 sec, time it takes
from light travel through the proton, atomic nuclei.
• Higgs particles life time is very very short.
1.6 X 10-22 sec
Longer time
• Year, tree rings, river sediment
• Radioactivity dating!
• 14C dating (5000 yrs)
• 开始有人：600 万年
• 238U dating. Life time of the earth and solar system,
40 亿年，
• Age of the Universe: 130 亿年.
• Longer time, life time of some elementary particles
136Xe lifetime: 1021 yrs, I can measure it in my lab!
Proton life >1035 yrs
Distance measurement
• Finger span, foot, steps, height…, Sizes related to
human being
Harvard bridge: xyz Smoots + 1 ear
unit + count
• Height of a building: triangulization
• Height of a mountain
Astrophysical measurement
• Radius of earth, In the third century B.C., Eratosthenes was able
to mathematically calculate the earth’s diameter by comparing
differences in the angle of the sun’s rays at two separate
geographic points. He noticed that the difference in the angle of a
shadow in his location at Syene, which is present-day Aswan in
Egypt, and that of a shadow in Alexandria was about 7.2 degrees.
Since he knew the distance between the locations, he was able to
determine the circumference of the earth, and therefore the
diameter and radius as well. You can do this, too, by using his
method.
Read more : http://www.ehow.com/how_5895686_radiusearth.html
how many ways do you know to measure?
• Distance to the sun, how many ways?
• Distance to moon, laser measurement.
Distance to stars
• AU=distance to sun
• Parallax (par sec = 3 light
year)
• Color of a star, spheroids
• Distance to the center of
our Galaxy, 30000 light
years.
• Using triangulization
to measure the distance of
other galaxy.
Discovering dark energy (Nobel
prize, 2012)
• Using supernovae as a standard candle to meaure
distance.
Saul Perlmutter; Brian P. Schmidt; Adam G. Riess
"for the discovery of the accelerating expansion of
the Universe through observations of distant
supernovae"
Smaller distance
• Meter, 4x107 m around the equator
• Some bar in Paris, now as certain distance that light
travels in a certain time.
• cm, mm, …
• First major improvement: light as a wave!
Wavelength is on the order of 500nm! 1nm = 10-9m
• Optics experiment, Newton ring,
• Michelson interferometer
• X-ray determine the size of atom 0.1 nm
Even smaller distance
• Rutherford experiment found that atoms are quite
empty.
• He found the size of the nuclei around 10-15m =
1fm
• Since then some of the smaller distance
measurement done to 10-18m, LHC, large hadron
collider
• Smallest distance measurement: space oscillation
under gravitational wave, 10-22m. If found this is a
Nobel prize work!
More on measurements
• Measuring temperature,
• Very hot, very cold
• Speed measurement,
Light speed, speed of an atom
• Measuring magnetic field, electric field
SQID, Quantum hall effect
• Measuring fundamental constants
• Sensors, large data, data processing, information
science!
Equipment money
• A lot of measuring equipment were invented in
western world.
• Now China is supporting inventing new apparatus,
Ministry of science and technology, National
Science Foundation of China
• A lot of money
• One of these at SJTU, 4D electron imaging system.
原子尺度超高时空分辨兆伏特
电子衍射与成像系统
王西杰
2013年6月19日
原子尺度物质结构动力学
最高空间
分辨的电
子衍射与
成像技术
最高时间
分辨的飞
秒泵浦-探
测技术
这是目前科学界正在共同追求和探索的目标,
面临着巨大的技术挑战
原子尺度超
高时空分辨
电子衍射与
成像系统
21
（100fs+Å）
系统总体设计图
系统尺寸：6米x6米x3米
光
阴
极
电
子
枪
速
调
管
22
(三)飞秒高亮度电子源
系统各构成部分及功能
(一)飞秒激光系统（购置改造）
(三)核心设备：兆伏特飞秒高亮度电子源（自主研制）
（二）超快泵浦光源（研制集成）
（四）关键设备：高温超导电磁透镜
23
系统（联合研制）
(五)多功能样品室(购置改造）
（六）高灵敏探测系统
(与Gatan联合研制）