The Speed of Light Santa Rosa Junior College

The Speed of Light Santa Rosa Junior College Physics 4 D May 11, 2006 Dr. Younes Ataiiyan Jon Be. Vier , Michelle Fong, & Travis Yaeger Jon Be. Vier, Michelle Fong, & Travis

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? n n n 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: n In 1629, Isaac Beeckman, proposed an experiment of observing light reflecting off a mirror attached to a cannon placed one mile away. n 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. n 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. n 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. n 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? n NO: n n n 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: n 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 n 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 n n 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 = mc 2/sqrt(1 - v 2/c 2) 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? n YES: n n n According to 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: n 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 n 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 § 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 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. n 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. n 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? n 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? n 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. 75 c. This is an example of Cherenkov radiation. Cherenkov observed the n 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. n Photo of TRIGA reactor: http: //en. wikipedia. org/wiki/Image: Triga. Reactor. Core. 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 300 c. 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.

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