An international research team has announced the first-ever successful observation of gravity’s effect on a falling quantum object, demonstrating a connection between gravity and the quantum world first predicted by Albert Einstein.
The research team said the result does not unify quantum mechanics and gravity, nor does it prove that gravity is a quantum phenomenon.
However, the researchers said their study offers experimental support that Einstein’s equivalence principle, a fundamental concept at the heart of the famed scientist’s theory of gravity, remains consistent with matter in the quantum world.
Unifying Einstein’s Theory of Gravity with Quantum Mechanics
When physicists want to describe nature, they rely on two distinct sets of rules. Einstein’s theory of gravity explains the mechanics of large-scale phenomena like stars and galaxies. Conversely, quantum mechanics helps scientists characterize the behavior of extremely small objects. Still, finding a scientific bridge uniting the two sets of rules has proven elusive.
“These two pillars of modern physics have so far eluded all attempts at a unified theoretical framework,” explained Professor Ron Folman from Ben-Gurion University of the Negev and the lead author of the new study.
Einstein did, however, identify one mechanism where the two worlds meet. Called the equivalence principle, the concept says that gravity should effectively disappear for an observer in free fall. For example, NASA simulates zero-gravity environments by flying astronauts in a plane, making successive rising and falling parabolic arcs. During the short periods of falling, the astronauts briefly experience ‘weightlessness” due to the equivalence principle.
Curious if the same phenomena would apply to the quantum world, the research team, which included Nobel Prize-winning physicist Professor Sir Roger Penrose, designed experiments. According to a statement announcing the research, the team hoped their setup would allow them to measure a “distinctive change in the quantum properties of atoms as they fell under gravity.”
Experiments Show Gravitational Pull Affecting Quantum Objects
To search for gravity’s effect in the quantum world, the researchers created clouds of rubidium atoms cooled to just above absolute zero. The researchers used a specially designed atom chip that allowed the team to manipulate the ultracold rubidium atoms.
Next, PhD student Or Dobkowski and colleagues used microwave pulses to place the atoms in quantum superposition. This unique quantum state lets each atom travel down two paths at once.
Using tiny electrical wires embedded on the specialized chip, the researchers generated tunable magnetic fields. The researchers said the field’s precision tunability allowed them to apply an upward force to one half of the atomic wave. For this experiment, the team created a magnetic field at the precise strength needed to counter gravity, effectively holding that half of the wave stationary relative to Earth.
After suspending the first half of the wave, the team used a magnetic pulse to lift the second part. Then, the team switched to a state almost unaffected by the magnetic field, allowing it to fall under gravity.
Once the second half completed its fall, the team used magnetic fields to reunite the two halves. When reunited, the two halves of the atomic wave interfered with each other. The researchers said that interaction allowed them to “measure the tiny difference in quantum phase accumulated while one was falling and the other was held still.”
“The phase measured in the new experiment is the same as the one predicted when Einstein’s principle is applied to such a quantum wave,” they explained.
“One of the Most Fundamental Questions in Physics”
When discussing the results, the research team said their result should not be construed as proof that gravity is quantum. The results also do not unite gravity and quantum mechanics. However, Folman noted, “this complex experiment gives more hints as to how such a unification may be achieved.”
Study co-author Professor Vlatko Vedral from the University of Oxford’s Department of Physics agreed, while also noting that their experimental results successfully demonstrated Einstein’s equivalence principle’s consistency with quantum mechanics, which alone provides an experimental connection between quantum physics and Einstein’s theory of gravity.
“This experiment pushes quantum mechanics into one of its most intriguing frontiers, gravity, and shows that, once again, its predictions hold,” Professor Vedral explained.
The professor also noted the experiment’s combination of hard experimental data with “a far-reaching theoretical interpretation, about one of the most fundamental questions in physics: How can gravity (described by Einstein’s theory of relativity) and quantum theory be unified into one understanding of the universe?”
The study “Observation of the quantum phase of free fall and the consistency with the equivalence principle” was published in Science Advances.
Christopher Plain has spent the last six years as Associate News Editor and Head Science Reporter at The Debrief. Follow and connect with him on X, learn about his novels at plainfiction.com, or email him at christopher@thedebrief.org.