The Speed of Light
Santa Rosa Junior College
Physics 4D
May 11, 2006
Dr. Younes Ataiiyan
Jon BeVier, Michelle Fong, & Travis Yaeger
The Speed of Light:
299,792,458 m/s
“Nothing travels faster than the speed of light with
the possible exception of bad news, which obeys its
own special laws”
-Douglas Adams
What exactly is the speed of light?
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The speed of light is the speed at which electromagnetic waves
moves in a vacuum. The speed of light has been defined as
exactly 299,792,458 meters/sec (186,000 miles/second).
Mnemonic phrase using the letters on telephone keypad:
Constant Which We Remember Well Because It’s Light’s Velocity
The speed of light was used replace the SI unit definition of the
meter
The distance light can travel in one year is called a light year.
The speed of light is represented by the letter c, which is
celeritas in Latin, "speed", and also known as Einstein's constant.
Calculation of the speed of light
People credited with developing the speed of light concepts:
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In 1629, Isaac Beeckman, proposed an experiment of observing light reflecting off a mirror attached to a
cannon placed one mile away.
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In 1638, Galileo proposed an experiment to observe the speed of light by measuring the time delay of
uncovering a lantern. His experiment involved two people each with a covered lantern some distance away
from each other. When one uncovered the lantern, the other person who saw the light would than uncover
his/her lantern and the original person who uncovered the lantern would record the time. Because the speed
of light is much faster than could be recorded using this method, this experiment would be an inaccurate
measurement of the speed of light and would actually be measuring the human reaction time.
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Descartes studied eclipses and determined that the speed of light was infinite. He believed that the Sun, Earth,
and Moon would not be aligned during the lunar eclipse if the speed of light were finite.
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The first quantitative estimate of the speed of light was made in 1676 by Danish astronomer, Olaus Roemer.
He studied the eclipses of the moons of Jupiter. He observed that Io revolved around Jupiter once every 42.5
hours when Earth was closest to Jupiter, and that when Earth and Jupiter moved further apart, it took extra
time for light to cross the extra distance between the planets. Roemer estimated that it would take 22 minutes
to cross the diameter of the orbit of the Earth. The modern estimate is 16 minutes and 40 seconds.
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The first measurement of the speed of light by using an apparatus on Earth, was conducted by Hippolyte
Fizeau in 1849. He used a beam of light directed at a mirror several thousand meters away. At a certain rate
of rotation, as the beam passed through a rotating cog wheel, it would pass through one gap of the teeth on
the way in and another gap on the way out. The speed of light was calculated by the distance to the mirror, the
number of teeth on the wheel, and the rate of rotation.
Diagram of the apparatus Fizeau used to measure the speed of light.
http://en.wikipedia.org/wiki/Fizeau-Foucault_apparatus
Is it possible to go faster than the
speed of light?
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NO:
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According to Einstein's theory of special relativity, published in 1905, nothing can exceed
the speed of light
Einstein says that speed is a fundamental constant of nature: It appears the same to all
observers anywhere in space
Reason:
 The theory says that objects gain mass as they speed up, and that speeding up requires
energy. The more mass, the more energy is required. By the time an object reached the speed
of light, Einstein calculated, its mass would be infinite, and so would the amount of energy
required to increase its speed. To go beyond the infinite is impossible
 So far, no experiment has contradicted special relativity. Sub-atomic particles have been
accelerated to within fractions of a percent of the speed of light, but not equal to or
exceeding that speed
It is not possible to move at the
speed of light
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Henri Poincare said “Perhaps we must construct a new mechanics, . . . in which the
speed of light would become an impassable limit” in an address to the International
Congress of Arts and Science in 1904 before Einstein announced special relativity in
1905.
It is a consequence of relativity that the energy of a particle of rest mass m moving
with speed v is given by:
E = mc2/sqrt(1 - v2/c2)
As the speed approaches the speed of light the energy approaches infinity. Hence is
should be impossible to accelerate an object with rest mass to the speed of light and
particles with zero rest mass must always go at exactly the speed of light otherwise
they would have no energy. This is sometimes called the “light speed barrier.”
Is it possible to go faster than the
speed of light?
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YES:
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According to the theory of relativity, nothing can go faster through space
than the speed of light in a vacuum
However, particles can move faster than the speed of light in a material,
which is less than the speed of light in a vacuum
Reason:
 There is the group velocity principle that states that it is possible to move
faster than the speed of light, however the transmission of information at
this speed is impossible.
It is possible to go faster than light
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The group velocity principle is often thought of as the velocity
at which energy or data is conveyed along a wave. Most of the
time this is correct and the wave can be conveyed as a single
velocity or a waveform. However, if the wave is travelling
through an absorptive material, this may not remain true. For
example, it is possible to design experiments in which the group
velocity of laser light pulses sent through prepared materials
significantly exceeds the speed of light in a vacuum. Data
transfer at this speed is not possible, since the signal velocity is
still less than the speed of light. It is also possible to reduce the
group velocity to zero, stopping the pulse.
Just because nothing has gone
FTL doesn’t mean nothing can
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The theory of relativity explains why it is not possible to physically go faster than the
speed of light, however, it does not rule out FTL travel. Particles have been known to
decay instantly into other particles which fly off at high speed. These particles could
be accelerated and the resulting decay and release of other particles might be faster
than the speed of light. Even if such particles exist the circuits reading the
information are too slow and the FTL transmission of data is negligible.
 When particles move faster than the speed of light, in a material, we get
something called Cherenkov radiation. Cherenkov radiation is similar to a sonic
boom. When a plane moves faster than the speed of sound, it creates a big
"boom" that is a shockwave of energy. When a particle goes faster than the speed
of light through a material, it gives off a shockwave of energy, and that is
Cherenkov radiation.
 Light can be slowed down in materials because photons interact with particles of
the material. It is then possible for a particle to move faster than the slowed down
light, and Cherenkov radiation occurs.
 We can measure this radiation and identify particles that travel faster than the
speed of light in a material.
What would you see if you moved at
the speed of light?
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Observers traveling at large velocities will find that distances and times are
distorted ("dilated") in accordance with the Lorentz transforms. However,
the transforms distort times and distances in such a way that the speed of
light remains constant. A person traveling near the speed of light would find
that colors of lights ahead of them were blue shifted and that those behind
them were red shifted.
Image of red shift:
http://en.wikipedia.org/wiki/Image:Redshift.png
What can go faster than the speed of
light?
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The picture seen below is TRIGA (Training,
Research, Isotopes, General Atomics) reactor
submerged in water, you can see the blue shift
caused by the accelerated particles that are
moving faster that the speed of light in water,
approx 0.75c. This is an example of
Cherenkov radiation. Cherenkov observed the
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emission of blue light from a bottle of water
subjected to radioactive bombardment.
Cherenkov also made a detector to detect particles
moving at faster than light speeds. The Cherenkov
detector was used in Sputnik III for scientific data
collection.
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Photo of TRIGA reactor:
http://en.wikipedia.org/wiki/Image:TrigaReactorCore.jpeg
A team of scientists announced that they
had successfully sent a pulse of light
through a special chamber at a velocity
faster than the speed of light. The Scientists
that succeeded were from the NEC
Research Institute in Princeton, New Jersey.
They sent a pulse of light through a six
centimeter chamber that contained an
unnatural form of Cesium at the even more
unnatural temperature of nearly absolute
zero. The pulse of light travelled so fast that
its peak actually exited the Cesium chamber
slightly before it entered. They calculated
the speed of light in the chamber to be
300c. This is so fast that the main part of
the pulse exits the chambers far side before
it enters the near side.
One beam of light is shone on the chamber
and excites the Cesium atoms, then a second
beam passes through the chamber soaks up
some of that “excited” energy and gets
amplified when it passes through the
Cesium atoms.
References
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http://van.hep.uiuc.edu/van/qa/section/New_and_Exciting_Physics/Relativity/20020211082231.htm
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http://www.enchantedlearning.com/subjects/astronomy/glossary/indexs.shtml
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http://www.physorg.com/news12084.html
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http://math.ucr.edu/home/baez/physics/Relativity/SR/scissors.html
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http://en.wikipedia.org/wiki/Speed_of_light#.22Faster-than-light.22_observations_and_experiments
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http://www.thekeyboard.org.uk/Faster%20than%20light%20speed.htm
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http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/FTL.html#16
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http://www.irishastronomy.org/user_resources
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http://lilly.barkbark.net/albums
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http://lofi.forum.physorg.com/What-can-go-faster-than-the-speed-of-light_4767.html
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Thornton, Stephen T., and Andrew Rex. Modern Physics for Scientists and Engineers, 3rd ed. Belmont: Thomson Brooks/Cole,
2006.