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Does light travel at the "speed of light"?
In 1676{{#tag:ref|Some sources state 1675.<ref name="Moreux"/>|group=note}}, the Danish astronomer [[Ole Rømer]] became the first person who attempted to quantitatively estimate the speed of light. His method was based on observation of the orbit of the satellites of [[Jupiter]] such as [[Io]] and evaluating their [[eclipse]] data as seen from the Earth. The Dutch scientist [[Christiaan Huygens]] was first who applied the arithmetic onto Rømer’s estimate for the maximum time delay between periodical observations of Io's eclipse by Jupiter at the Earth's nearest and farthest position with respect to Jupiter.<ref>{{cite web |editor=Steven Soter and Neil deGrasse Tyson|title=Cosmic Horizons: Astronomy At The Cutting Edge|publisher=New Press, American Museum of Natural History|year=2000|url=http://www.amnh.org/education/resources/rfl/web/essaybooks/cosmic/p_roemer.html|accessdate=August 4, 2013}}</ref>
In 1676{{#tag:ref|Some sources state 1675.<ref name="Moreux"/>|group=note}}, the Danish astronomer [[Ole Rømer]] became the first person who attempted to quantitatively estimate the speed of light. His method was based on observation of the orbit of the satellites of [[Jupiter]] such as [[Io]] and evaluating their [[eclipse]] data as seen from the Earth. The Dutch scientist [[Christiaan Huygens]] was first who applied the arithmetic onto Rømer’s estimate for the maximum time delay between periodical observations of Io's eclipse by Jupiter at the Earth's nearest and farthest position with respect to Jupiter.<ref>{{cite web |editor=Steven Soter and Neil deGrasse Tyson|title=Cosmic Horizons: Astronomy At The Cutting Edge|publisher=New Press, American Museum of Natural History|year=2000|url=http://www.amnh.org/education/resources/rfl/web/essaybooks/cosmic/p_roemer.html|accessdate=August 4, 2013}}</ref>
In 1849, the French physicist Fizeau developed method which enabled to measure the speed of light between two points at the Earth by means of using the light source, rotating strobe disc (spinning toothed wheel), and the mirror. He shone a light ray between the teeth of a rapidly rotating toothed wheel. A mirror reflected the beam back through the same gap between the teeth of the wheel over the round-trip distance of 17 km. By varying the speed of the wheel, it was possible to determine at what speed the wheel was spinning too fast for the light to pass through the gap between the teeth, to the remote mirror, and then back through the same gap, and the rotational speed at which the light returned through the ''next'' gap. The experiment showed that the light traveled over the known distance in 1/18000 second. The resulting speed of light through air was obtained by dividing the known distance by time.
In 1849, the French physicist Fizeau developed method which enabled to measure the speed of light between two points at the Earth by means of using the light source, rotating strobe disc (spinning toothed wheel), and the mirror. He shone a light ray between the teeth of a rapidly rotating toothed wheel. A mirror reflected the beam back through the same gap between the teeth of the wheel over the round-trip distance of 17 km. By varying the speed of the wheel, it was possible to determine at what speed the wheel was spinning too fast for the light to pass through the gap between the teeth, to the remote mirror, and then back through the same gap, and the rotational speed at which the light returned through the ''next'' gap. The experiment showed that the light traveled over the known distance in 1/18000 second. The resulting speed of light through air was obtained by dividing the known distance by time.
The more precise methods, involving a rapidly rotating mirror, have been used by Americans Michelson and Newcomb.<ref name="Moreux">{{cite book |author=Théophile Moreux|title=Pour comprendre la physique moderne|year=1948|url=http://books.google.com/books?id=CetIPAAACAAJ&dq=Th.Moreux+Pour+comprendre+la+physique+moderne&hl=sk&sa=X&ei=lvL9UcmvKcfY4QTvm4DwBA&redir_esc=y|language=French}}</ref>
The more precise methods, involving a rapidly rotating mirror, have been used by Americans Michelson and Newcomb.<ref name="Moreux">{{cite book |author=Théophile Moreux|title=Pour comprendre la physique moderne|year=1948|url=http://books.google.com/books?id=CetIPAAACAAJ&dq=Th.Moreux+Pour+comprendre+la+physique+moderne&hl=sk&sa=X&ei=lvL9UcmvKcfY4QTvm4DwBA&redir_esc=y|language=French}}</ref>
==Does light travel at the "speed of light"?==
Surprisingly, the answer seems to be no. It appears to travel at about 1 part in 300 million slower than the speed that physicists denote '''c'''.
Much of modern theoretical physics, including all of relativity, involves a fundamental "speed of light", denoted '''c'''. Relativity has its origins in the behavior of light in various experiments, notably the [[Michelson-Morley experiment]]. So the behavior of light is used as the definition of '''c''' in various formulas, such as [[E=mc²]], [[E^2=(mc^2)^2+(pc)^2]], and [[Maxwell's Equations]]. This also fits in perfectly with the notion that a photon, being massless, must have speed '''c''' in all frames of reference. And it fits in with the notion that particles of exceedingly small rest mass, but reasonable energy, will travel at nearly the speed of light.
The electron neutrino is believed to have a mass of 0.25 Ev, or .44x10<sup>-36</sup> kg, and hence travel at very nearly the speed of light for reasonable energies. An announcement from the OPERA/Gran Sasso experiment in 2011<ref>http://www.nature.com/news/2011/111020/full/news.2011.605.html</ref> seemed to indicate that neutrinos were observed traveling faster than light. (This finding was later found to be flawed). Assuming the neutrinos had energy of about .27x10<sup>-8</sup> joules, their speed would have been slower than '''c''' by a factor of 1 in 10<sup>22</sup>, which would come out to .25x10<sup>-24</sup> seconds over the 730 kilometer test. That time difference would be many orders of magnitude too small to observe.
But 20<sup>th</sup> century theories of Quantum Electrodynamics indicate that photons interact with the "vacuum polarization" of the vacuum through which they are traveling.<ref>http://www.extremetech.com/extreme/184879-einsteinian-error-the-25-year-old-supernova-that-could-change-the-speed-of-light-forever</ref> Photons can spontaneously split into electron-positron pairs, which interact with each other gravitationally before recombining. This causes light to travel about 1 part in 300 million slower than '''c'''. Since neutrinos don't interact with the electromagnetic force, they are not subject to the vacuum polarization effect, and travel only 1 part in 10<sup>22</sup> slower than '''c''', which is faster than photons.
In order to observe this effect, and extremely long travel path for neutrinos and photons is needed. The supernova SN1987A provided such a path—it is 186,000 light years from Earth. (So the supernova actually occurred 186,000 years ago.) There are theoretical reasons to believe that the flash of light would have been delayed about 3 hours from the neutrino burst, since the photons had to pass through the exploding star, interacting with it, whereas the neutrinos had no such interaction. But the observation of the light flash was 7.7 hours later than the neutrino burst, indicating that it took 4.7 hours longer to travel to Earth. This is consistent with the predicted slowing of photons due to vacuum polarization.
==Notes==
==Notes==