Top Eleven: Michelson and Morley

The next experiment in the Top Eleven is probably the most famous failed experiment of all time.

Who: Albert Michelson (1852-1931) and Edward Morley (1838-1923), American physicists.

When: Their first results were reported in 1887.

What: The famous Michelson-Morley experiment, which tried and failed to detect the motion of the Earth through the “luminiferous aether.” At the time, light waves were believed to be disturbances in some medium that permeated all of space, and was fixed in an absolute sense. In this picture, objects moving through space should also be moving relative to the aether, which should cause an apparent change in the speed of light along the direction of motion.

i-fe24eee45893230cac27bf097830695e-sm_1887_intf_color.gifThe speed of light is gigantic, so it was widely believed that measuring the change in speed would be impossible, but Michelson came up with a way to do it, using a device now known as a “Michelson Interferometer,” pictured at left. The idea is simple: you take a beam of light, and split it into two perpendicular paths of nominally equal length. Then you reflect the light back on itself, and re-combine the two beams. At the output of the apparatus, you will see a pattern of bright and dark lines (“fringes”) due to the interference of the two light waves.

The exact pattern of fringes is extremely sensitive to the difference between the times taken for each of the two beams to make the round trip. If the time is different by half of the oscillation period of the light (about 10-15 seconds), you’ll get bright fringes where you had dark ones, and vice versa. If you take this device and set it up with one arm pointed along the direction of the Earth’s motion through the aether (about 30 km/s as it orbits the Sun), you’ll see one pattern as the light on that path moves at a different speed than the light on the perpendicular path. If you then rotate the apparatus by 90 degrees, you switch the other arm into the direction of the Earth’s motion, and you should see a shift in the fringes. The change in the speed of light is a small one (about 1/10,000th of the speed of light), but the interferometer is so sensitive, you should see something.

Michelson and Morley set up their apparatus on a heavy granite slab (to eliminate vibrations), and floated the slab in a vat of mercury (to allow smooth rotation– OSHA didn’t exist in 1887), and made a long series of measurements as they rotated the interferometer, and saw absolutely nothing. The shift they reported was consistent with zero, that is, with no change in the speed of light due to the Earth’s motion through the aether.

Why It’s Important: There are conflicting stories about whether Einstein was aware of the Michelson-Morley result or not, and whether he cared about it if he was aware of it, but the Michelson-Morley experiment played an important role in paving the way for the acceptance of Relativity. The simplest explanation of their null result is that there is no aether, and without the aether, there’s no absolute rest frame.

Beyond that, it’s one of the earliest examples of the technique of interferometry, which is incredibly useful for measuring small changes in position, or small phase shifts. There are dozens of different schemes for interferometric measurements, which can be used to measure everything from obvious physical quantities like position and rotation to gradients in the gravitational field, and even gravity waves. The Laser Interferometer Gravity Wave Observatory (LIGO) is in the process of building what is basically the world’s largest Michelson interferometer in an effort to detect tiny expansions and contractions of space itself as a gravitational wave passes.

Reasons to Vote for Them:: Their null result brought about a dramatic shift in the way we view the world, and invented a crucial precision measurement technique. Also, Morley was a Williams grad (Class of 1860).

Reasons to Vote Against Them: Nostalgia for an absolute frame of reference, belief in aether dynamics.

(Michelson-Morley apparatus image stolen from this site, by way of the textbook companion site for my class this term.)

Comments

  1. #1 Roman Werpachowski
    February 1, 2006

    Michelson was born in Poland. Just bragging about ;-)

  2. #2 Chad Orzel
    February 1, 2006

    Most sources I’ve seen say that he was born in Prussia. Of course, given the way Poland has flickered in and out of existence and shuffled side to side, it may well be that the relevant part of Prussia then is part of Poland now.

    (My great-grandparents left eastern Poland in 1914 to avoid being drafted into the Russian army, which strikes me as an excellent tactical decision… We still have cousins over there, who my parents visited a couple of summers ago.)

  3. #3 Roman Werpachowski
    February 1, 2006

    Michelson was born in the so-called Prussian Poland, that is this part of Poland which during the partitions has been taken by Prussia.

    Saying that Michelson was born in Prussia is like saying that Marie Curie-Skg蒨瞘蒨采` 薫 采` 薫 采` 薫 采` 薫 采` 薫 采` 薫 采` 薫 采` 薫 采` 薫 采` 薫 采` 薫 采` 薫 采` 薫 采` 薫 采` 薫 采` 薫 采` 薫 采` 薫 7g 3=g 3糀0; < < 8
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  4. #4 Roman Werpachowski
    February 1, 2006

    … Saying that Michelson was born in Prussia is like saying that Marie Curie-Sklodowska was born in Russia.

    This blog does not like Polish characters ;-)

  5. #5 Vance Maverick
    February 1, 2006

    And that’s “Morley”, not “Morely”.

  6. #6 yellojkt
    February 1, 2006

    The US Naval Academy where this experiment was set up used to have a very nice display with the original instruments. I’m not sure where they moved it to since the building got flooded by a hurrican and completely gutted and renovated.

  7. #7 Eric Wallace
    February 1, 2006

    This experiment was performed at what is now Case Western Reserve University. They have a display in one of the campus buildings, but to be honest, I can’t remember if it claimed to be the original instrument or just a mock-up.

  8. #8 Chad Orzel
    February 1, 2006

    And that’s “Morley”, not “Morely”.

    Yes, yes it is.
    I’ve fixed that now, I hope.

  9. #9 Trent Goulding
    February 1, 2006

    Well, if we’re voting based on school affiliations, forget Morley and Williams; vote because Michelson taught at the Naval Academy, and I took freshman calculus in Michelson Hall there….

  10. #10 Rich Y
    February 2, 2006

    One of the more impressive things about the experiment that might not occur to modern readers is that they used an oil lamp for their interferometer light. This was before electric filament lamps, before steady electric arc lamps, even before luminescent mantles. It’s a shock for me, used to lasers and commercial optical tables and parts that you just screw together, to think about.

    Light from lamps has a much shorter coherence length than laser light, so setting up a white-light interferometer is much more difficult than the laser interferometer that you might put together in an undergraduate lab.

  11. #11 Thomas
    February 10, 2006

    Although being considered of historical importance, the Michelson – Morley experiment was completely unnecessary and pointless and its negative outcome could have been logically predicted on theoretical grounds alone: it is incorrect to set up an experiment that claims to examine the dependence of the speed of light on the velocity of the source/detector but can not properly define this velocity; the assumption of an ‘absolute’ reference frame on the other hand is a contradiction in terms as velocities are by definition relative (for the original experiment, the earth’s movement around the sun was used, but there is no physical reason why the sun should be a preferred (‘absolute’) reference frame for a completely self-contained earthbound experiment; the same argument holds for any other reference frame as well)
    The point is in fact that a light wave needs no carrying medium as it carries itself (to be more precise, according to Maxwell’s Equations, the electric wave carries the magnetic wave and vice versa; it is somewhat ironic that Maxwell himself did obviously not realize this as he believed in the ether theory and a positive outcome of the Michelson-Morley experiment).

    Thomas
    http://www.physicsmyths.org.uk

  12. #12 Jim Bourassa
    April 30, 2008

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