by Wm. Robert Johnston
last updated 6 August 2002
Every object exerts a gravitational force on other objects. The force on you from a person down the hall is very very tiny. The force of gravity on you from the Earth you know is significant; the Earth is very big.
Suppose you're on a flat plain (and the Earth isn't spinning). If you hung a weight on a string, it would point towards the center of the Earth, because that's the direction of the force of gravity.
Suppose you move over next to a mountain. The hanging weight would no longer point towards the center of the Earth, but would be deflected slightly towards the mountain. The force of gravity on the weight is now the combination of the force of gravity from the Earth (big) and the force of gravity from the mountain (much smaller). (The effect is very small, but can be measured in a laboratory.)
When a tree falls in the forest near LIGO (even if no one is there to see it), it sends sound waves through the ground. These sound waves include ripples in the surface, ripples with a height a fraction of the width of a human hair, moving outward at about 750 km/hour (450 mi./hour).
Consider an instant when the ripples are moving past one of our LIGO buildings. Suppose there is a crest or high point to the left of the building and a trough or low point to the right of the building. Just like the weight hanging near the mountain above, this will cause a very slight deflection of gravity towards the left. This is called a gravity gradient.
For the sound waves I'm talking about, the gravity gradient is very small: about ten parts in a billion-billion of normal gravity. Most of you can safely ignore this effect. But here at LIGO, this is enough to swing the mirror back and forth slightly, by a fraction of the diameter of an atom, as the ripples move past.
Even worse, this effect is a gravitational pull, so it can't be stopped. The sound vibrations themselves transmitted through the foundation to the mount hanging the mirror can be reduced by improving the suspension, but this won't fix the gravity gradient "noise".
One possible solution is to have an array of seismometers, detect these tiny ripples, calculate their effect, and subtract the effect from the detector output. The seismometer experiments my group will be doing will be used to evaluate the problem.
The past few days for me have been spent working on a computer simulation of these ripples of underground sound and of the gravity gradient they produce.
In other news: We're still averaging one thundershower a day. I went to Sunday morning service at First Baptist Church in Hammond, the same church I went to last summer. There seems to be slightly more activity on the Southeastern Louisiana University campus (where we are staying) than last summer.
Image credits: Wm. Robert Johnston, © 2002
© 2002 by Wm. Robert Johnston.
Last modified 6 August 2002.
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