Black holes and quantum mechanics are two of the most intriguing physics topics. Their strange and exotic features certainty capture the imagination. Now, new research in the American Physical Society’s journal Physical Review Letters brings aspects of the two together in an experiment that shows, for the first time, that gravity stretches and squeezes quantum objects through tidal forces.
|A macroscopic quantum state explores curved spacetime.
Image Credit: Peter Asenbaum.
As the name implies, tidal forces are responsible for the tides. Like any two objects, the moon and the Earth are attracted to one another by gravity. The strength of a gravitational attraction depends on the distance between the two objects—the closer they are, the stronger the attraction. This means that the moon always tugs a little harder on the side of the Earth closest to the moon, causing the Earth to stretch and deform just a little. The moon pulls on water in the same way, and this uneven attraction is largely responsible for the tides.
An object will experience tidal forces whenever the strength of a gravity changes over the space the object occupies. This is dramatically illustrated by black holes. The strength of gravity skyrockets once you cross the event horizon of a black hole, the point of no return. If you were to cross an event horizon, your head (or whichever part of your body crossed the event horizon first) would experience a gravitational pull so much stronger than your feet that you would be stretched out like a spaghetti noodle.
In his general theory of relativity, Einstein characterized gravity as the curving of spacetime by objects with mass (or energy). His predictions are a great match to what we see on the scale of stars and planets, but what about in the weird world governed by quantum mechanics? A thorough understanding of the universe has to include an explanation of how gravity affects quantum objects.
In this new research, scientists from Stanford University and the University of Birmingham used an extremely sensitive tool called an interferometer to observe the tidal forces of gravity acting on a quantum system. According to the authors, this is the first time the curvature of spacetime has been seen at a quantum scale.
An interferometer is like a high-tech, extremely precise ruler. While interferometers can be designed to measure things of all sizes, their value is in the ability to measure tiny differences in length. If the term rings a bell, it may be thanks to LIGO’s recent detection of gravitational waves.
LIGO is a gravitational wave observatory made of large optical interferometers. Each interferometer consists of two arms of the same length that form an “L” shape. A laser beam is split in two and each beam travels along a separate arm. When a beam reaches the end of its arm, a mirror reflects the light back. The two beams meet and recombine at their intersection. If the beams have traveled exactly the same length, they interfere in a signature way. If the beams travel slightly different lengths (perhaps due to a gravitational wave warping spacetime), their interference pattern will tell you by how much.
In this new experiment, the team used an atom interferometer. The details and technology behind atom interferometers are different than those of optical interferometers, but the principle is the same. Early in the 20th century, physicists realized that matter behaves both as waves and particles. Atom interferometers observe the interference patterns produced by matter waves that have traveled two different paths. As our understanding and technology have improved, these interferometers have become important tools for measuring fundamental physical constants as well as accelerations, rotations, and the strength of gravity.
In an atomic interferometer like the one used by the Stanford team, a cloud of atoms is cooled down to close to absolute zero inside of a vacuum chamber. The atoms are then launched upward and pulses of light send them into a combination of two quantum states whose waves travel along two different paths. When the atom waves are recombined, the interference pattern highlights any change in path length.
This particular experiment involved two interferometers, each utilizing a subset of the same cloud of atoms. In order to see whether tidal forces would act on a quantum sample, the team optimized their design to be super-sensitive to changes in the curvature of spacetime, a process that took several years and pushed the boundaries of existing interferometers. Then, the researchers created spacetime curvature by placing 200 pounds of lead bricks near the top of one of the interferometers.
The interferometers were close enough together that they experienced the same background noise, which could then be cancelled out in the results. However, the interferometers were far enough apart that only the waves traveling along one of the paths experienced significant spacetime curvature from by the lead brick. As the results showed, tidal forces due to this curvature did stretch and squeeze the quantum samples.
According to Peter Asenbaum, the lead author of this journal article, “Intuitively, one might expect that placing some lead bricks close to the experiment could not possibly influence the experimental outcome. However, our apparatus is so sensitive to gravitational interactions that the presence of the bricks influenced the outcome substantially!”
Along the path to observing how gravity interacts with quantum objects, the team made many important improvements to their atom interferometers. These advances aren’t limited to this one experiment—they will enable scientists to make more precise measurements of many other interactions and quantities too. As we learn more about the universe through ever-improving tools, I suspect there will never be a shortage of discoveries that engage the imagination.