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Scientists have spotted the first hints that Albert Einstein's formulation of gravity operates in the quantum realm.
The hints have to do with observations of a falling quantum object, and though this may not seem like a huge deal (literally, we are talking about atoms here), it could be an important step on the road to a theory of ... everything.
To start, we have to go back to the early years of the 20th century. Consider Einstein's iconic theory of gravity, general relativity, and the elusive realm of quantum mechanics. While the former describes the universe on scales of planets, stars and black holes, even demonstrating how galaxies and galaxy clusters formed, the latter describes the counterintuitive physics found on scales smaller than atoms. But there is a problem: these two important theories don't play well together.
There is no quantum theory of gravity. However, scientists think we're getting closer to one.
A team of researchers has performed an experiment to find the point where quantum mechanics and general relativity meet. Namely, how the quantum properties of atoms change as those atoms fall under the influence of gravity.
"This is a unique paper, in the sense that it combines a hard experiment 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?" team leader Ron Folman of the Ben-Gurion University of the Negev said in a statement. "These two pillars of modern physics have so far eluded all attempts at a unified theoretical framework, but this complex experiment gives more hints as to how such a unification may be achieved."
At the cornerstone of the team's experiment is an important element of general relativity called the equivalence principle.
The equivalence principle states that, for an observer in free fall, gravity should disappear. If that sounds confusing, consider this popular analogy: You are inside a closed elevator with no view of your surroundings. You feel heavy. You might drop a ball, and it would fall to the floor of the elevator. Why would these things happen? Well, there are two options.
You might be sitting stationary on the ground floor, or the elevator could be moving upwards at an acceleration equal to the gravitational field strength of Earth, 9.8 meters per second squared. You can't tell the difference between gravity and acceleration. That's the equivalence principle.
Now, let's shake that up. Suddenly, you can't feel weight anymore. You drop the ball, and it floats. Gravity has disappeared, or the upward acceleration has ended. Inside the elevator, two things may have happened, and you can't distinguish between these two either. The elevator cable may have snapped, and you may now be in free fall, or the upward acceleration may have carried you to space and then ceased.
The big question now is: How do you test this on a quantum scale, especially for particles that behave as waves as they travel?
To measure the equivalence principle at a quantum scale, the researchers turned to a piece of equipment called the Quantum Galileo Interferometer. This allowed them to split the quantum wave associated with a traveling atom into two paths. One was held in place while the other was allowed to fall freely.
The team then reunited the wave to observe the changes gravity had introduced to the falling wave.
For the experimental subjects, the team chose a cloud of rubidium atoms, which they cooled to just above absolute zero. These were placed close to the surface of a specially designed atom chip. Using microwave pulses, the scientists placed the atoms in what is known as a quantum superposition.
A superposition is one of those really weird things unique to quantum physics, effectively allowing a quantum system to simultaneously exist in two or more contradictory states. In this case, the superposition allowed each atom to travel along two different paths at the same time.
Magnetic fields generated by the atomic chip applied an upward push along one path, counteracting the downward pull of gravity and holding it stationary. The other path was subjected to magnetic pulses pushing the atoms upwards. The pulses were then shut off, allowing the atoms to fall freely under gravity like a ball thrown into the air.
When the waves were reunited, the team used an effect called interference, which occurs when the peaks and troughs of two misaligned waves meet, to measure changes in the free-falling wave.
This represented the first time that the effects of gravity and free fall on the phase of a quantum wave had been measured.
However, while this is an effective demonstration of Einstein's equivalence principle in the quantum realm, it isn't a demonstration of quantum gravity. Yet it remains a step along a frustratingly long road physicists have been traveling down for over a century with no end in sight.
"We have no consistent theory telling us why quantum physics should fail," team member Vlatko Vedral said in the statement. "This experiment pushes quantum mechanics into one of its most intriguing frontiers, gravity, and shows that, once again, its predictions hold."
The team's research was published on Wednesday (Sept. 2) in the journal Science Advances.