Scientists have long discovered that in quantum physics, particles can feel the effects of magnetic fields that they have never been directly exposed to. Now, a new study published in the journal Science suggests that this bizarre quantum phenomenon, known as the Aharonov-Bohm effect, applies not only to magnetic fields but also to gravity.
The Aharonov-Bohm effect
In classical electromagnetism, electric and magnetic fields are the fundamental entities responsible for all physical effects. For example, particles are only affected by an electric or magnetic field (e.g., acceleration, deceleration, turning) only when they are in direct contact with an electric or magnetic field.
The electromagnetic field can be expressed as a quantity called the electromagnetic potential, which has a value anywhere in space. From the electromagnetic potential can be easily derived electromagnetic field. But the concept of electromagnetic potential has always been considered to be a purely mathematical concept and has no physical significance.
In quantum physics, however, things get even more interesting. In 1959, physicists Yakir Aharonov and David Bohm proposed a "thought experiment" that linked the electromagnetic potential to measurable results. In this thought experiment, a beam of electrons is divided into two paths, each moving around the sides of a cylindrical spiral, and the magnetic field is confined to the inside of the coil. Thus these two electron paths can pass through an area without a field, but the electromagnetic potential of this fieldless region is not zero.
The Aharonov-Bohm effect is a quantum mechanical effect. In this effect, when a particle moves around a region containing a magnetic field, its phase changes, even though the magnetic field is zero everywhere the particle passes. | Photo credit: E.Cohen et al., Nature Rev. Phys.,2019
Aharonov and Bohm theoretically demonstrated that electrons on these two different paths undergo different phase changes, and that when electrons on these two paths are recombined, they can produce detectable interference effects. Since the change in phase can be calculated from the strength of the magnetic field, interference can be interpreted as an effect that the electrons never actually pass through the magnetic field. Today, the Aharonov-Bohm effect has long been verified by many experiments.
Weak gravitational effect
In the new study, physicists have experimentally confirmed that the same incredible physical phenomena can occur in gravity.
One of the biggest challenges scientists has always had when scientists want to use gravity to perform similar experiments is that the gravitational effect is too small and too difficult to capture compared to electromagnetic forces. Physicists have been trying to design experiments that can detect this effect for decades, but it wasn't until 2012 that a team led by Michael Hohensee came up with a protocol that could be implemented with current technology.
The idea of Hohensee et al. is to first make ultracold atoms and then use pulsed laser beams to control their motion, including putting them into an area where the gravitational potential (rather than the field) differs from other locations. If you can split an atom into two waves of matter, move them to regions with different gravitational potentials, and then pull them back together, you can observe the interference patterns they produce, measure their phases, and quantify the gravitational Aharonov-Bohm effect.
Making "atomic fountains" in the laboratory
Ten years later, physicist Chris Overstreetda and his colleagues used an atomic interferometer to detect the Aharonov-Bohm effect for the first time in gravity.
In the new experiment, they created an "atomic fountain" that emits ultracold rubidium atoms into a 10-meter-high vacuum tube, allowing the atoms to do free-fall motion in the tube. The researchers controlled the atom fountain by emitting a series of laser pulses at different times, splitting, orienting, and recombining these atomic wave packets so that each atom could be in a quantum superposition of two paths at the same time.
In quantum mechanics, microscopic particles behave like waves, so each particle can be represented as a "wave packet." In the "Fountain of Atoms" experiment, atoms are emitted vertically from the bottom of a vacuum tube and follow a free-fall trajectory. At three different times, laser pulses can separate, orient, and recombine the wave packets of atoms. | Reference: A. Roura
At the top of the vacuum tube, there is a tungsten ring with a mass of 1.25 kg. Atoms in one path fly high, close to the tungsten ring; atoms in the other path fly lower and farther away from the tungsten ring. The two paths can be spaced up to 25 cm apart. This is done to detect tiny phase shifts caused by time expansion, because in a gravitational potential, general relativity predicts that clocks at two different heights will pass at slightly different speeds. When the atoms regroup to produce an interference pattern, the researchers can read the difference in the phase changes they experience from the interference signals of the two paths.
It is important to note that these atoms are not flying in an area without a gravitational field. Instead, the experiment was designed to allow researchers to filter out the effects of gravity, showing only the bizarre Aharonov-Bohm effect.
To observe this effect, physicists need to consider the phase shift produced by the gravitational (gravitational field) drag of the tungsten ring. To achieve this in the experiment, the researchers continually changed the minimum distance between the atoms that flew higher along the path and the tungsten ring, plotting a curve for the change in the phase difference between the two paths and the tungsten ring.
When the two paths are closer together, the spacing of the wave packets is small compared to the distance from the tungsten ring, so it should not be sensitive to time expansion. As it turns out, it is. They found that when the distance between the two paths in the atomic interferometer was small, the measured phase curve coincided with the expected phase shift caused only by the gravitational field.
But the situation is different when the distance between the two paths is greater, and the results show that there is something other than a gravitational field that causes phase shifts. The researchers interpret this "other thing" as relativistic time dilation, which they believe suggests that gravity produces an Aharonov-Bohm phase shift similar to that triggered by electromagnetic interactions.
Tiny phenomena, great achievements
It's a very tiny, hard-to-catch phenomenon, but with the help of sufficiently sensitive atomic interference, physicists have detected the change. In this way, the new experiment not only recreates the bizarre Aharonov-Bohm effect in a new setting, but also demonstrates the potential of many subtle effects that may be contained in gravitational systems.
In addition, through this experiment, the researchers also noticed that the observed phase shift was proportional to the mass of the atom, and these phase changes depended on Planck's constant (h) and Newton's gravitational constant (G). The gravitational constant G is a natural constant that reveals the intensity of gravitational force, but so far we know it far less precisely than other fundamental natural constants. The researchers believe that the atomic interferometer device used in the new study will be used to better measure the value of the gravitational constant.
General relativity and quantum mechanics are the two basic theories of this experiment. Physicists have long wanted to combine them to describe reality. The new result is a remarkable achievement that shows a great triumph of quantum mechanics under the action of gravity, and while this is not enough to prove the quantum nature of gravity itself, perhaps one day physicists will achieve this goal.
#创作团队:
Author: Light rain
Typography: Wenwen
#参考来源:
https://physicsworld.com/a/physicists-detect-an-aharonov-bohm-effect-for-gravity/
https://www.sciencenews.org/article/quantum-particles-gravity-spacetime-aharonov-bohm-effect
https://www.space.com/space-time-curvature-measured-atomic-fountain
https://www.science.org/doi/10.1126/science.abl7152
https://www.science.org/doi/10.1126/science.abm6854
https://physics.aps.org/story/v28/st4
https://physics.aps.org/articles/v5/s87
#图片来源:
Cover Art & Top Image: Principles