The Speed of Light

1
The Speed of Light

• Santa Rosa Junior College
• Physics 4D
• May 11, 2006
• Dr. Younes Ataiiyan
• Jon BeVier, Michelle Fong, Travis Yaeger

2
The Speed of Light299,792,458 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

3
What exactly is the speed of light?

• 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 Its
  Lights 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.

4
Calculation of the speed of light

• People credited with developing the speed of
  light concepts
• In 1629, Isaac Beeckman, proposed an experiment
  of observing light reflecting off a mirror
  attached to a cannon placed one mile away.
• 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.
• 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.
• 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.
• 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/F
izeau-Foucault_apparatus
5
Is it possible to go faster than the speed of
light?

• NO
• 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

6
It is not possible to move at the speed of light

• 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.

7
Is it possible to go faster than the speed of
light?

• YES
• 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.

8
It is possible to go faster than light

• 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.

9
Just because nothing has gone FTL doesnt mean
nothing can

• 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.

10
What would you see if you moved at the speed of
light?

• 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/I
mageRedshift.png
11
What can go faster than the speed of light?

• 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 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.

• 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.

Photo of TRIGA reactor http//en.wikipedia.org/wi
ki/ImageTrigaReactorCore.jpeg
12
References

• http//van.hep.uiuc.edu/van/qa/section/New_and_Exc
  iting_Physics/Relativity/20020211082231.htm
• http//www.enchantedlearning.com/subjects/astronom
  y/glossary/indexs.shtml
• http//www.physorg.com/news12084.html
• http//math.ucr.edu/home/baez/physics/Relativity/S
  R/scissors.html
• http//en.wikipedia.org/wiki/Speed_of_light.22Fas
  ter-than-light.22_observations_and_experiments
• http//www.thekeyboard.org.uk/Faster20than20ligh
  t20speed.htm
• http//math.ucr.edu/home/baez/physics/Relativity/S
  peedOfLight/FTL.html16
• http//www.irishastronomy.org/user_resources
• http//lilly.barkbark.net/albums
• http//lofi.forum.physorg.com/What-can-go-faster-t
  han-the-speed-of-light_4767.html