Warped Passages - Lisa Randall [186]
This tells us that by exploring gravity at distances as small as (or smaller than) a proposed curled-up dimension’s size, and studying how gravity’s strength depends on the separation of masses at those distances, an experiment could study the behavior of gravity and look for evidence of extra dimensions. However, experiments that are sensitive to gravity at very short distances are formidably difficult to build. Gravity is so weak that it is readily overwhelmed by the other forces, such as electromagnetism. As mentioned earlier, at the time of the ADD proposal, experiments had searched for deviations from Newton’s gravitational force law and shown that the law applies at least down to distances of about a millimeter. If anyone could do better and study even shorter distances, they had a chance of discovering the large dimensions of the ADD proposal, which were just on the verge of experimental accessibility.
Experimenters rose to the new challenge. Motivated by the ADD idea, Eric Adelberger and Blayne Heckel, two professors at the University of Washington, designed a beautiful experiment whose purpose was to look for deviations from Newton’s law at very short distances. Others have also studied short-distance gravity, but this experiment was the most stringent test of the ADD proposal.
Their apparatus, located in the basement of the University of Washington physics department, is called the Eöt-Wash experiment. The name refers to a famous physicist who studied gravity, the Hungarian Baron Roland von Eötvös. The Eöt-Wash group’s experiment is illustrated in Figure 76. It consists of a ring suspended above two attractor disks, one slightly above the other. Holes are bored into the ring, and the upper and lower disks, and they are aligned in just such a way that if Newton’s law is correct, the ring won’t twist. However, if there were extra dimensions, the difference in gravitational attraction from the two disks would not agree with Newton’s law and the ring would twist.
Figure 76. The apparatus of the Eöt-Wash experiment. A ring is suspended over two disks. The holes in the ring and the disks guarantee that the ring will not twist if Newton’s inverse square law is obeyed. The three spheres near the top of the apparatus are used for calibration purposes.
Adelberger and Heckel found no twisting and concluded that no extra-dimensional (or other) effects modified the gravitational force at the distances they could study. Their experiment measured the gravitational force at distances smaller than ever before, establishing that Newton’s law applies all the way down to about a tenth of a millimeter. This meant that extra dimensions, even those for which Standard Model particles are confined on a brane, cannot be quite as big as the millimeter that ADD had suggested. They have to be at least ten times smaller.
Remarkably, millimeter-size dimensions are also prohibited by observations of outer space. The quantum mechanical uncertainty principle associates a millimeter with an energy of only about 10-3 eV, and a tenth of a millimeter with an energy of about 10-2 eV—either way, an extremely small energy, orders of magnitude less than that needed to produce an electron, for example.
Particles with such a low mass could be found in the surrounding universe and in celestial objects, such as supernovae or the Sun. These particles would be so light that if they existed, hot supernovae could produce them. Because we know how quickly supernovae cool and we understand the cooling mechanism (via neutrino emission), we know that there can’t be too many other low-mass objects emitted. The cooling rate would be too fast if energy leaked out in some other way. In particular, gravitons shouldn’t carry away too much energy. Using this reasoning, physicists showed (independently of terrestrial experiments) that extra dimensions should