The connection between relativity and quantum mechanics has been a black box for the world of physics for decades. That partially stems from the difficulty in collecting data on systems that interface between the two of them. Relativity is the realm of the supermassive, while quantum mechanics can best be described as the realm of the minuscule. But, there is, in fact, one particular realm where they overlap. One of the results of relativity is that gravity can affect the flow of time. Commonly known as “time dilation,” this effect has now been studied by researchers at the National Institute of Standards and Technology (NIST) in the US using an extraordinarily accurate atomic clock.
Time dilation itself is a well-established concept. This isn’t even the first time NIST engineers have proven it using atomic clocks. Conceptually time dilation means gravity itself slows down time. So an object that is experiencing high gravity will thereby experience less time.
One of the most famous examples of the concept was probably captured in the movie Interstellar. The heroes end up on a desolate world, searching for an explorer who had only experienced a few hours on the planet’s surface while years went by on Earth. In the movie’s case, that time dilation is due to the high gravity caused by a black hole near the planet, but the same effects can be seen at a minute scale even here on Earth.
In the past, NIST scientists have proved this by measuring the time dilation of two atomic clocks placed on top of one another only 33 cm (1 ft) apart. Even with that slight distance, they could detect perceptible changes in gravity. Now for their next trick, they narrowed that distance to only a millimeter.
Having two separate atomic clocks that close together is physically impossible, so Dr. Jun Ye and his team devised a novel one for use specifically in this experiment. Typically these devices use the vibration of a certain type of atom to count time. The definition of a second itself is based on the vibrations of a cesium atom.
The researchers used a structure known as an “optical lattice” that holds about 100,000 individual strontium atoms in a defined structure. Importantly, they also developed an imaging system that could closely monitor the top and bottom of the lattice, which could be thought of as looking similar to a stack of pancakes, just at an atomic scale. The distance between the top and bottom of the optical lattice measured only a millimeter, making it the smallest distance ever seen in this type of experiment.
Still, there was a perceivable difference in the time experienced by the upper part of the lattice vs. the lower one. It was only 0.0000000000000000001 seconds, but the scientists were definitely counting. That was in line with expectations of what general relativity says it would have been.
Proving a theory that has already been proven dozens of times before wasn’t the only outcome of the experiment, though. The technique the researchers used points to the potential construction of a clock that is 50 times more accurate than any existing today. Again, that might seem like overkill, as most atomic clocks are more than adequate to measure anything in our macro world. But on the order of quantum mechanics, time itself gets funny as the slight differences in gravity over minimal distances is a confounding factor in our understanding of that realm. More accurate clocks could potentially explore those small distances in a way that has never been possible before, and this new “atom cloud” based atomic clock may be one way to do it.
There’s still an order of magnitude improvement in accuracy needed before any such experiments could be run. But with luck, determination, and continued funding, better atomic clocks could pave the way to unlocking one of physics’ biggest mysteries.
NIST – JILA Atomic Clocks Measure Einstein’s General Relativity at Millimeter Scale
Nature – Atomic clocks measure Einstein’s general relativity at millimeter scale
Science News – An atomic clock measured how general relativity warps time across a millimeter
Chowdera – Nature cover paper verifies general relativity on millimeter scale
Experimental setup of an atomic clock utilizing a cloud of strontium atoms
Credit – R. Jacobson / NIST