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By Root 1935 0

—offering an explanation for why gravity appears weak (when you pick up a coffee cup, your muscles beat out the gravitational pull of the entire earth). If we could measure gravity’s strength over distances smaller than the size of the extra dimensions, we’d catch it before it’s fully spread and so we should find its strength to be stronger. To date, measurements on scales as short as a micron (10–6 meters) have found no deviation from expectations based on a world with three spatial dimensions. Should a deviation be found as physicists push these experiments to ever-shorter distances, that would provide convincing evidence for additional dimensions.

EXPERIMENT/OBSERVATION: Extra Dimensions and Missing Energy

EXPLANATION: If the extra dimensions exist but are far smaller than a micron, they will be inaccessible to experiments that directly measure gravity’s strength. But the Large Hadron Collider provides another means of revealing their existence. Debris created by head-on collisions between fast-moving protons can be ejected from our familiar large dimensions and squeezed into the others (where, for reasons we’ll get to later, the debris would likely be particles of gravity, or gravitons). Were this to happen, the debris would carry away energy, and as a result our detectors would register a little less energy after the collision than was present before. Such missing energy signals could provide strong evidence for the existence of extra dimensions.

EXPERIMENT/OBSERVATION: Extra Dimensions and Mini Black Holes

EXPLANATION: Black holes are usually described as the remains of massive stars that have exhausted their nuclear fuel and collapsed under their own weight, but this is an unduly limited description. Anything would become a black hole if compressed sufficiently. Moreover, if there are extra dimensions that result in gravity being stronger when acting over short distances, it would be easier to form black holes, since a stronger gravitational force implies that it takes less compression to generate the same gravitational pull. Even just two protons, if slammed together at the velocities mustered by the Large Hadron Collider, may be able to cram enough energy into a sufficiently small volume to trigger the formation of a black hole. It would be a wisp of a black hole, but it would yield an unmistakable signature. Mathematical analysis, going back to the work of Stephen Hawking, shows that tiny black holes would quickly disintegrate into a spray of lighter particles whose tracks would be picked up by the collider’s detectors.

EXPERIMENT/OBSERVATION: Gravitational Waves

EXPLANATION: Although strings are tiny, if you could somehow grab hold of one, you could stretch it large. You’d need to apply a force in excess of 1020 tons, but stretching a string is merely a matter of exerting enough energy. Theorists have found exotic situations in which the energy for such stretching might be provided by astrophysical processes, generating long strings wafting through space. Even if they were very distant, these strings might be detectable. Calculations show that as a long string vibrates, it creates ripples in spacetime—known as gravitational waves—of a highly distinctive shape, and hence they offer a clear observational signature. Within the next few decades, if not sooner, highly sensitive detectors based on earth and, funding permitting, in space, may be able to measure these ripples.

EXPERIMENT/OBSERVATION: Cosmic Microwave Background Radiation

EXPLANATION: The cosmic microwave background radiation has already proved itself capable of probing quantum physics: the measured temperature differences in the radiation arise from quantum jitters stretched large by spatial expansion. (Recall the analogy of a tiny message scribbled on a shriveled balloon becoming visible once the balloon is inflated.) In inflation, the stretching of space is so enormous that even tinier imprints, perhaps laid down by strings, might also be stretched sufficiently to be detectable—perhaps by the European Space Agency’s Planck satellite. Success