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Stellar Aberration Experiment |
One of the proposed explanations for the null result of the Michelson-Morley
experiment is the hypothesis that the ether is drug along with the earth. In
this case the earth would always be measured to be at rest with respect to the
ether. But if the ether were drug along with the earth, the light rays in the
vicinity of the earth would also be drug along with the earth.
Without ether drag, a light ray coming from a star on the ecliptic pole would travel straight down, perpendicular to the motion of the earth around the sun. In order to see the star in the center of the field of view, the telescope would have to be pointed forward in the direction of the motion of the earth. The concept is the same as walking with an umbrella in the vertically falling rain. In order to keep your feet from getting wet, you have to tilt the umbrella forward. Similarly, in order for the light ray to traverse the central axis of the forward moving telescope, the telescope will have to be tilted forward. The amount of tilt is not very much, but it is enough to be measured and has been observed.
As the earth orbits the sun, its direction of motion changes, and the direction of tilt changes accordingly. This phenomenon is known as stellar aberration and its occurrence is proof positive that the earth really does orbit the sun.
But if ether drag were true, this phenomenon would not be observed. The result would be the same as if you were walking in the rain at the same speed and in the same direction as the wind is blowing. Since you are moving along with the air and the rain drops are falling vertically through the air, the rain drops would fall vertically downward from your perspective and you would not need to tilt the umbrella at all to keep your feet dry. This is true regardless of what the wind is doing high above you. Indeed, the wind could be blowing in one direction high above you and in another direction where you are. (You can actually see this phenomenon taking place from time to time in distant rain storms. The rain leaves the cloud falling in one direction and hits the ground falling in another.)
In a similar fashion, ether drag would pull the incoming light rays along with the earth and cause them to pass through the telescope the same as if the earth were at rest. There would be no need to tilt the telescope in the direction of the earth's motion and the stellar aberration phenomenon would not be observed.
Since stellar aberration is observed, the ether drag theory must be invalid.
P1. Stellar Aberration Calculations.
The earth has an orbital radius of about 150 million kilometers and an orbital
period of one year. Calculate (a) the earth's orbital speed (assuming the orbit
is circular), (b) the tilt angle for stellar aberration (the angle in degrees,
minutes, and/or seconds that the telescope must be tilted forward to see a star
on the ecliptic pole centered in the field of view). (c) If the resolution of
the telescope system is 0.05 arc seconds, could the stellar aberration
phenomenon be observed with this equipment?
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