Michelson-Morley Experiment 2012년도 1학기 𝐸=𝑚𝑐2

Light is a wave 2012년도 1학기 𝐸=𝑚𝑐2

Why light is a wave Diffraction - high-school physics class - Diffraction gratings Interference - simple explanation on Young’s double-slit experiment - Young’s experiment using a laser - Advanced experiments - Thin-film interference 2012년도 1학기 𝐸=𝑚𝑐2

What is the medium of the light wave? 2012년도 1학기 𝐸=𝑚𝑐2

Speed of Light is very large! Recall that v(air)<v(water)<v(diamond)=12km/s c=299,792,458 km/s >v(diamond) If the medium of light wave exists, then it must be harder than diamond and much lighter than that. What is it? They called the medium of the light wave aether. 2012년도 1학기 𝐸=𝑚𝑐2

Questions on the aether Light from a far galaxy travels through vacuum. If aether exists, then it must fill the space even in the vacuum. Unlike the sound, the light wave is an electromagnetic wave that is transverse. Therefore, if the light wave is similar to the sound, then the aether, which is very light, but must have properties of solid whose bondage is very strong. 2012년도 1학기 𝐸=𝑚𝑐2

Questions on the aether Aether is transparent It must be completely uncompressible It must not be viscous because the motion of planets does not decelerate 2012년도 1학기 𝐸=𝑚𝑐2

The experiment 2012년도 1학기 𝐸=𝑚𝑐2

Michelson and Morley Albert Abraham Michelson (1852~1931) Nobel Prize Winner (1907) Edward Williams Morley (1838~1923) 2012년도 1학기 𝐸=𝑚𝑐2 그림출처(좌):http://en.wikipedia.org/wiki/File:Albert_Abraham_Michelson2.jpg (으): http://ko.wikipedia.org/wiki/%ED%8C%8C%EC%9D%BC:Edward_Williams_Morley.jpg

Must Watch Michelson-Morley Experiment (1/3) 1887, Cleveland, Ohio - When Einstein was 8 - 23 years after the Maxwell’s equations - Michelson and Morley wanted to prove the existence of aether by direct measurement - Michelson measured the speed of light most accurately 2012년도 1학기 𝐸=𝑚𝑐2

Michelson’s first Experiment Michelson-Morley Experiment (2/3) 1880 in Berlin, Germany - c= 3× 10 8 m/s - v= 3× 10 4 m/s - necessary to measure the travel time difference of 2 light beams of order 𝑣 𝑐 2 = 10 −8 - The instrument was not dependable enough - No conclusion - Stopped working on the project 2012년도 1학기 𝐸=𝑚𝑐2

Michelson-Morley Experiment 1887 Clevland, Ohio, USA - Greatly improved his old interferometer with Morley - Tried to detect the aether, again! - The result was in complete disagreement with the existence of aether 2012년도 1학기 𝐸=𝑚𝑐2

Michelson interferometer A half-silvered act as a beam splitter. 50 % -> the mirror M1 50 % -> the mirror M2 2012년도 1학기 𝐸=𝑚𝑐2 그림출처: http://en.wikipedia.org/wiki/File:Michaelson_with_letters.jpg

Michelson interferometer M1을 향하는 광선과 M2를 향하는 광선은 은도금 거울에서 다시 만났을 때 경로차가 생긴다. 𝑝𝑎𝑡ℎ 𝑑𝑖𝑓𝑓𝑒𝑟𝑒𝑛𝑐𝑒=2 | 𝑑 1 − 𝑑 2 | 이러한 경로 차이 때문에 두 광선은 서로 간섭하여 Detector에서 간섭 무늬가 나타나게 된다. 만약에 경로 차이가 다음과 같다면, 2 𝑑 1 − 𝑑 2 =𝑛 𝜆 𝑛=0,1,2,3… , 서로 보강 간섭을 하여서 간섭 무늬 중앙이 밝고, 그렇지 않고 다음과 같다면, 2 𝑑 1 − 𝑑 2 = 𝜆 2 (2𝑛+1) 𝑛=0,1,2,3… , 서로 상쇄간섭을 하여서 간섭 무늬의 중앙이 어두워진다. Path 1= 𝑑 1 Path 2= 𝑑 2 그림출처: http://en.wikipedia.org/wiki/File:Michaelson_with_letters.jpg

Michelson-Morley Experiment Ether wind 그림출처: http://en.wikipedia.org/wiki/File:Michaelson_with_letters.jpg Interferometer를 돌려가면서 간섭 무늬를 확인한다.

Efforts to resolve theoretical dilemma 2012년도 1학기 𝐸=𝑚𝑐2

FitzGerald Contraction (1892) Suggested modified experiment by shortening an arm of the Michelson-Morley experiment Predicted Length contraction depending on the v(earth) Experimentally verified later George Francis FitzGerald (1851~1901) 2012년도 1학기 𝐸=𝑚𝑐2 그림출처: http://en.wikipedia.org/wiki/File:George_Francis_FitzGerald.jpg

Lorentz (1895) Michelson-Morley Experiment (3/3) To most physicists FitzGerald’s idea was absurd. Developed the Lorentz transformation and explained the FitzGerald contraction Hendrik Lorentz (1853~1928) 2012년도 1학기 𝐸=𝑚𝑐2 그림출처: http://en.wikipedia.org/wiki/File:Hendrik_Antoon_Lorentz.jpg

Poincaré (1899) Offered a general explanation of the experiment Calling it the principle of relativity Absolute motion will never be detected Predicted entirely new dynamics is involved that is distinguished from Galilean relativity Henri Poincaré (1854~1912) 2012년도 1학기 𝐸=𝑚𝑐2 그림출처: http://en.wikipedia.org/wiki/File:JH_Poincare.jpg

Implications of the results Galilei’s inertia: No difference between “at rest” and motion with “𝑣≠0” If Michelson’s experiment proved the existence of aether, then it disproves the Galilei’s idea that any inertial reference systems are equivalent. 2012년도 1학기 𝐸=𝑚𝑐2

In Summary, Michelson-Morley Experiment was the most brilliant failure in science history In Summary, Michelson has done the following: 1. Measured c most accurately 2. First measurement of the diameter of a star 3. Proved that the speed of light is invariant in any inertial reference frame. However, he never accepted this. 2012년도 1학기 𝐸=𝑚𝑐2

References http://galileo.phys.virginia.edu/classes/109N/lectures/michelson.html 2012년도 1학기 𝐸=𝑚𝑐2