Sometimes the most intriguing ideas you hear in science are offhand comments or anecdotes thrown out by a conference speaker. The American Physical Society meeting in Washington, DC, is chock full of fascinating talks. Not all of it is what you would traditionally call news, but there are plenty of great stories.
One session talked about searches for ways in which Newton's law of gravity might break down at short length scales. It's a topic we've covered in symmetry before. (See the sidebar to a feature on searches for extra dimensions.) Stephan Schlamminger of the University of Washington gave seven reasons to justify why we should look for deviations from Newton's law and a set of experimental results, but I want to focus on one of the less important stories because it's, well, a good story.
Some theoretical ideas would cause gravity to change on the scale of tens or hundreds of meters. When testing gravity, it usually helps to have a detector approximately the same size as the phenomenon you’re looking for. So where do you go to build an experiment on the scale of 100 meters where you can move large amounts of mass nearby and far away? Think on that for a moment before you read on…
One answer is the tunnel that hangs down inside most large dam walls. The walls typically have this tunnel so that the dam integrity can be monitored. But it’s also a long straight shaft of the right length, and the water level in the dam moves up and down, providing the movement of mass needed.
In one particular dam physicists hung a long wire with a mass on the end, attached the top to a very sensitive scale to weigh the mass and compared the weight with the water level over the course of hours and days. The weight of the hanging mass depends on the configuration of the mass near it. It won’t change much but enough to be detected and the weight is predictable from knowing the water level and various other details. If there is any variation in Newton’s Law over that distance, it should change the weight of the hanging mass just a little.
An older experiment did something similar but using gravimeters at different heights in a tower built for the experiment in a pumped storage reservoir in Germany’s Black Forest.
Over the course of the dam and reservoir measurements, the results perfectly matched predictions within the precision of the experiments, so there was no discovery of a deviation. Newton's Law is safe for now but it makes you think differently about the mass all around you, especially next time you stand on top of a large dam wall.
