Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: waytogo on 12/09/2012 08:21:09
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Tell me something please...
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http://en.wikipedia.org/wiki/R%C3%B8mer%27s_determination_of_the_speed_of_light
This is the first measurement that bears close historical and scientific scrutiny
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Thanks, I know wiki and that link, but it does not explain the Q.
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I forgot this: I guess that it was used a math formula to calculate the stuff, so where is it?
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The results do not come from a precise formula but precise observation and prediction.
You must first realise that astronomers have been able to make quite precise measurements of times and positions of events in the sky for many centuries. The greeks well BCE were able to measure the size of the spherical earth and the approximate distance to the moon.
Romer's determination of the speed of light depended on reasonably accurate time measurements of precisely timeable events in the motions of the four large satellites of Jupiter. These events can be predicted quite accurately using standard orbit and geometrical theory.
However the relative motions of the Earth and Jupiter mean that jupiter is observable at a range of distances equal to the diameter of the earth's orbit. this results in the timing of events varying by around 16 minutes and gives a first approximate measurement of the speed of light
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That does not explain how.
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waytogo - you are gonna have to put a bit of effort in yourself. we can explain and help you get over sticking points - but the wikipage does explain Romer's idea
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Again, wikipedia is useless in that case (and many others as well).
Why that stuff is not explained at all? that's the point.
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This is so frustrating....I only read a few days ago the most wonderful article regarding the entire history of the measurement of the speed of light and I was in two minds about reposting it here......at the time...I decided no because I was pushed for time...........I wish i did now because I just can't locate it anywhere............arrrrghhhhhhhh !!
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You will find it Neil..
Or we will make you an offer you can't refuse.
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You will find it Neil..
Or we will make you an offer you can't refuse.
I've been searching high and low chum !!
It was on one of the many science communities on facebook and it's since scrolled out of view and I can't recall whcih community it was (I'm still looking now)...
....ewe can still make me the offer though ! [;)]
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FOUND IT :
SOURCE: https://www.facebook.com/fromquarkstoquasars
Light is something we often take for granted. In fact, the only time we ever really think about light is when there isn’t any, a thought that usually precedes running into something. But what about light itself?
As one of the most fundamental ways of expressing energy, light is extremely important in physics. One of the most elementary properties of light is it’s speed, usually expressed by using the letter “c”. Even though Einstein found what is perhaps the most useful property of the speed of light with his famous equation E = mc^2, the search for light’s speed started long before Einstein.
The first known proposed test was by Isaac Beeckman, who suggested in the early 1600s that a man with a stopwatch could sit atop a hill with a cannon, with a large mirror facing him approximately one mile away. Beeckman believed that when the cannon was fired, the man could start the stopwatch and stop it again after seeing the flash of light off of the mirror. Obviously this method is extremely flawed, and when a similar test was conducted by Galileo it yielded no results. Since then, we have calculated the time delay between the cannon firing and the mirror returning the flash to be approximately 11 microseconds, well below a human’s ability to distinguish a discrepancy in time.
Near the late 17th century, the astronomer Rømer conclusively proved that light had a finite speed, but he underestimated the speed of light by approximately twenty-six percent. Rømer started by assuming that the speed of light was so massive that it would take less than a second to span a distance equivalent to the Earth’s diameter. He was correct of course, as light can travel many times that distance in a single second. Rømer used his approximations for the direction and velocities of multiple bodies (though primarily Jupiter, Io, and the Earth) to conclude that it would take light approximately twenty-two minutes to span the distance equivalent to the diameter of Earth’s orbit around the sun. This leads to a value of about 220,000 kilometers per second, as opposed to lights actual speed of just under 300,000 kilometers per second.
A few decades later James Bradley devised another way to test the speed of light, and estimated it with a much higher degree of accuracy than had been done before. Bradley was the first to discover the aberration of light, a phenomenon where objects appear to be in a different location due to the time delay between seeing the light and when the light was reflected. By using this principal Bradley estimated the speed of light to be 301,000 kilometers per second by comparing the speed of light to the speed of Earth in its orbit. According to his research, light travels 10,066 times faster than the Earth does when orbiting the sun, and from there he calculated the speed.
In the early 19th century, Hippolyte Fizeau found a new way of attempting to measure the speed of light by using time-of-flight measurements of Earth. His study was substantially less accurate than Bradley’s, with an estimated speed of 315,000 kilometers per second, however his method lead to the more precise attempt of Léon Foucault. When Foucault tried his modification of Fizeau’s method he came up with an estimated 298,000 kilometers per second.
Just after the start of the 20th century, yet another attempt at measuring the speed of light was undertaken by Rosa and Dorsey. Their method allowed them to measure light’s speed without directly depending on a measurement of the propagation of electromagnetic waves (In other words, they weren’t directly observing light itself). Instead they used an electrical principal explained in one of Maxwell’s equations. They were able to estimate light’s speed at 299,710 kilometers per second, which is off by a mere 82 kilometers per second, and was by far the most accurate measurement of the day.
From 1926 to 1950, there were three additional attempts to measure light’s speed. Albert Michelson was the first to test light’s speed in that time, and he did so using a test remarkably similar to the very first proposed test by Isasac Beeckman. However Michelson’s test had a distance of approximately 22 miles and substantially more precise equipment. His test had an outcome of 299,796 kilometers per second. A few years later Louis Essen and A.C. Gordon-Smith calculated the speed of light in a cavity resonator. They had extremely accurate results and calculated light’s speed to be 299,792.5 kilometers per second. This was off by 0.042 kilometers per second.
K. M. Evenson is primarily credited with created the most accurate means of testing the speed of light, which he demonstrated in the 70s. Evenson’s method relies on the separate measurements of wavelength and frequency emitted by a stabilized laser. Evenson was able to get a value of 299,792.4562 meters per second, the most precise approximation of light’s speed to this day. That value is off by 0.0018 meters per second.
~Alan
Sources:
Huygens, C (1690) (in French). Traitée de la Lumière. Pierre van der Aa. pp. 8–9.
Bradley, J (1729). "Account of a new discoved Motion of the Fix'd Stars". Philosophical Transactions 35: 637–660.
Gibbs, P (1997). "How is the speed of light measured?". Usenet Physics FAQ. University of California, Riverside. Retrieved 2010-01-13.
Gibbs, P (1997). "How is the speed of light measured?". Usenet Physics FAQ. University of California, Riverside. Retrieved 2010-01-13.
Essen, L; Gordon-Smith, AC (1948). "The Velocity of Propagation of Electromagnetic Waves Derived from the Resonant Frequencies of a Cylindrical Cavity Resonator". Proceedings of the Royal Society of London A 194 (1038): 348–361. Bibcode 948RSPSA.194..348E.
Rosa, EB; Dorsey, NE (1907). "The Ratio of the Electromagnetic and Electrostatic Units". Bulletin of the Bureau of Standards 3 (6): 433. Bibcode 1906PhRvI..22..367R. DOI:10.1103/PhysRevSeriesI.22.367
Michelson, A. A. (1927). "Measurement of the Velocity of Light Between Mount Wilson and Mount San Antonio". The Astrophysical Journal 65: 1. Bibcode 927ApJ....65....1M. DOI:10.1086/143021.
Evenson, KM; et al. (1972). "Speed of Light from Direct Frequency and Wavelength Measurements of the Methane-Stabilized Laser". Physical Review Letters 29 (19): 1346–49. Bibcode 1972PhRvL..29.1346E.
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FOUND IT :
SOURCE: https://www.facebook.com/fromquarkstoquasars
The first known proposed test was by Isaac Beeckman, who suggested in the early 1600s that a man with a stopwatch could sit atop a hill with a cannon, with a large mirror facing him approximately one mile away. Beeckman believed that when the cannon was fired, the man could start the stopwatch and stop it again after seeing the flash of light off of the mirror. Obviously this method is extremely flawed, and when a similar test was conducted by Galileo it yielded no results. Since then, we have calculated the time delay between the cannon firing and the mirror returning the flash to be approximately 11 microseconds, well below a human’s ability to distinguish a discrepancy in time.
That's simply ridiculous and even do not explain nothing, the other one is the same story posted here but again, it is useless (the correct term would be disinfos).
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FOUND IT :
SOURCE: https://www.facebook.com/fromquarkstoquasars
The first known proposed test was by Isaac Beeckman, who suggested in the early 1600s that a man with a stopwatch could sit atop a hill with a cannon, with a large mirror facing him approximately one mile away. Beeckman believed that when the cannon was fired, the man could start the stopwatch and stop it again after seeing the flash of light off of the mirror. Obviously this method is extremely flawed, and when a similar test was conducted by Galileo it yielded no results. Since then, we have calculated the time delay between the cannon firing and the mirror returning the flash to be approximately 11 microseconds, well below a human’s ability to distinguish a discrepancy in time.
Hi there, thanks but that does not explain how, don't you? and with which tools they've misured 11 microseconds?
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Thnx Neil..
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Anyone?
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In 1612, scientists were still debating over whether the speed of light is finite or infinite. The cannon flash reflecting back to its source by a mirror was too short an interval to be measured with a stop watch. So the simple answer to the question is, "It wasn't."
With 20/20 hindsight, we can now devise an experiment which could have been done back then, using a spinning cylinder to send short pulses of sunlight which reflect back at the source by a corner reflector (http://en.wikipedia.org/wiki/Corner_reflector). (The corner reflector wasn't invented until 1938, but they could have easily made one in 1612. A big mirror would work well enough, though.) A very bright light source would shine thru a slit in the spinning cylinder and travel to the corner reflector and back. They didn't have carbon arc lamps, back then. Burning magnesium would have worked, except magnesium hadn't even been discovered, let alone refined. I'm not sure what light source they had that would have been bright enough. Sunlight might work if they could block out the ambient light. It's difficult enough to see a heliograph several kilometers away; try seeing a reflection of a heliograph thru a crude telescope.
Let's assume you would be able to see the reflected light by looking thru the same slit with Galileo's newly invented telescope, after the slit has rotated thru a certain angle. If the cylinder rotates 100 revolutions per second, and the reflected light is seen 1/100th revolution away from the light source, then the delay is 1/10,000 second. If the reflector is 15 km away, that would tell you that the speed of light is roughly 300,000 km/s.
Sounds easy with today's technology that we take for granted, but a lot of it hadn't been invented, yet. Today, we can use a laser light and count interference fringes to measure the delay in picoseconds.