I have a few questions.Einsein came up with the energy formulae for energy, e=mcsquared where E=energy,M=mass and C=constant referring to speed of light squared.all very well but how could anyone work out the speed of light back then.There was no osiloscopes or radio to measure the frequency of light.even if you do know the wavelength which is only the distant between two peaks,that will not tell you the speed and unless one knew when the light left for example the sun how does one measure the speed especially as light is a continious stream.
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Feel free to reply to any comment by clicking the "Reply" button.The speed of light was definitely well known by then. It is fundamental to Maxwell's equations as it was recognized for that when it appeared naturally in those equations. Professor Maxwell published his equations in four parts from 1861 to 1862.
There are many ways that the speed of light can be measured. The first measurements were based on observations of the moons of Jupiter. Because of the finite speed of light, the time when the moons pass behind Jupiter can easily be predicted by Newtonian Gravity. The problem was they didn't go behind Jupiter at the right time because the time we observed it on Earth depended on the distance the Jovian system itself was from Earth at the time. The orbits of these two planets have been well understood since well before Maxwell so it was straightforward to notice and then calculate the finite speed of light. Here is a link with the relevant history.
More elegant methods of measuring the speed of light followed shortly. One physicist put a rotating mirror on a mountaintop and an observing device on another. As a result he was able to only see flashes at certain precise phases of the mirror which depend on the rotation rate of the mirror. My students do this experiment in my Calculus Physics class each year.
The measurement of the speed of light was quite accurate and well known well before Einstein's time. In fact MUCH more was known about it than just its speed. We didn't understand until Einstein why but we already had the surprising result that the speed of light is completely and always independent of the speed of the observer. If you seek a full explanation to my previous sentence, you will eventually stumble upon Einstein's Special Theory of Relativity from which E=mc^2 is derived.
The history of human knowledge is filled with such linkages and complexities. This is what makes history fascinating to me.
marmot84
Level 7
Jan 21, 2018
1
Uh. Wasn't Newtonian laws, as far as the mathematics, flawed? The mass of Jupiter could not have been known.
@Jack-of-scythes You don't need the mass. You don't really even need Newtonian physics. You just need a calendar and a telescope. Just time the orbits. If we didn't account for the finite speed of light they would not be regular. Then if you do account for it, they are regular.
The reason people were motivated to notice this is because of the Longitude problem. During Newton's time, the Brits worked very hard to solve what is known as the Longitude problem. Dava Soble gives an excellent account of this in her book Longitude.
I still do not see how they could measure the speed of light in practice,unless you have highly advanced modern scientific apparatus.considering speed travels at roughly 300,000 kilometres per second.and how does one know when the light was emitted from the sun to time it as it is a continious stream.
PeterJohn
Level 6
Jan 21, 2018
0
@astrochuck Yes, Michelson likely did this but it was first done by Fizeau.
The experiment which Michelson is now best known for is quite elegant and forms the bases for the current LIGO Gravity wave detectors based in Oregon and Louisiana. This experiment convincing shows that the speed of light is independent of the observer. I still marvel at Michelson's resourcefulness in conceiving of this experiment.
@astrochuck You sir, rock!
@astrochuck Hi Chuck, yes, sorry to appear to be distracting from your post. Your point is well take. Michealson was a first rate experimenalist. I did not know about his speed of light measurement. It is something I will look into. Thanks!
Firelands1973
Level 5
Jan 21, 2018
0
H i Firelands1973 thank you for your information,reading how the speed of light was first measured is the info that I wanted.One other question how in those days could they work out the diameter of the earths orbit around the sun,given that the earth revolves on its self as well as orbiting the sun.so a person standing in one position on planet earth which would be in a different posion every day to the sun .
@PeterJohn The Babylonians knew the orbit is elliptical
[en.wikipedia.org]
Johannes Kepler measured the orbit precisely because the Earth has a different position compared to the Sun. The Sun is more or less stationary as the Earth rotates. In reality, the Sun does experience a tug from every object in orbit around it, the effect is different based on the mass of the object in orbit. The size of the orbit can also be measured by comparing positions of far off stars at different times of the year.
Well [britannica.com]
The speed of light was first measured by Danish astronomer Olaus Roemer in 1676
Also light has a wave particle duality. Cool, huh?
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shockwaverider
Level 8
Jan 21, 2018
0
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