It took a few months of work, but my youtube spacelab entry is ready.
This experiment tests the interior solution to gravity. Unlike other gravity experiments, this experiment will test what happens inside a mass. An experiment directly testing what would happen if a test mass was dropped into a source mass has never been done. Science is about collecting empirical evidence, and until this or a similar experiment is done, our understanding of gravity will be incomplete.
Gravity is a very weak force. But scientists have tested gravity since the 18th century, with Cavendish and his torsion balance. Space-based gravity experiments that oscillate through the center of a mass have been proposed before, but rejected because they were not accurate enough to improve on the value for G, the gravitational constant. However, in this experiment, we are looking to experimentally prove that a test mass will oscillate through a source mass.
This experiment is similar to the ‘gravity train’ problem found in many physics text books, where gravitational oscillation through the earth is used as a hypothetical means of transportation.
***Note: The test mass and source mass will be in two separate orbits. Any two orbits at the same altitude cross paths twice, so the test mass will also oscillate through the source mass once every orbit around the earth (about every 90 minutes). However, this ‘orbital oscillation’ can be subtracted from the actual motion of the test mass to find the gravitational oscillation. The magnitude of the orbital oscillation can either be found by using math or by using a control test mass without a source mass. Alternatively, the tunnel of the source mass can be placed on the orbital plane so that there is no orbital oscillation, but this setup might be more inaccurate or difficult to pull off.
The ‘orbital plane’ is the sheet that contains the path of the satellite.
A ‘gravity gradient’ is what happens when one mass is slightly closer to the earth, and thus slightly more attracted to the earth.
Experiment Question: What would happen gravitationally if a test mass was dropped through a hole in a larger, source mass? Would the test mass oscillate through the center of the hole?
Hypothesis: Yes, the test mass will oscillate thought the center of the source mass.
Method: A test mass and a source mass (the source mass made out of a dense material such as lead or tungsten) will be released in the microgravity environment of the ISS. Care will be taken to prevent noise due to gravity gradients, inertia, and air resistance.
Expected Results: The test mass will oscillate at about the rate predicted by the equation shown in the video. As gravity experiments are very sensitive, this experiment may be somewhat inaccurate. But since we are only looking to confirm radial gravity induced oscillation, qualitative data is all that’s needed.
David Levi did the research, filming, and experiment outline
Sara Ingram did the animation
Sources and Links of Interest:
[Henry Cavendish’s 18th century gravity experiments] http://tiny.cc/f2l4a
[An old NASA proposal that has good examples of gravity experiments in space] http://tiny.cc/dd0ws
[Good description of a ‘gravity train’] http://tiny.cc/m6j6u
[Some math behind a ‘gravity train’] http://tiny.cc/4zgzf
The equation for gravity induced oscillation was found in Sears and Zemansky’s University Physics:
Young, Hugh D., Roger A. Freedman, A. Lewis Ford, and Francis Weston Sears. “Chapter 12 Gravity.” Sears and Zemansky’s University Physics. San Francisco: Pearson Addison Wesley, 2008. 402-03. Print.
Unfortunately, I cannot find a good derivation of the formula online. Look into the idea of a gravity train if you want to understand where it comes from. If needed, I can supply a derivation.