New Atomic Clocks Can Measure Quantum Gravity

Chuckles Freely

Scientists can now measure gravity’s effect on quantum particles, helping to reconcile Einstein’s Relativity and Quantum Mechanics.

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Since the beginning of the 20th century, physics has had a major problem: it’s two theories of explaining the universe, General Relativity and Quantum Mechanics, refuse to get along. These two monoliths have achieved unbelievable accuracy explaining everything from planets, stars, and galaxies to quarks, atoms, and molecules, yet they’re fundamentally incompatible. The main problem is that measuring the effects of Relativity, which governs our understanding of macro-scale gravity effects, on quantum particles is seemingly impossible, and that measuring the quantum effects on large objects seems equally out of reach.

While a lot of theoretical work has gone into trying to reconcile these theories, progress has faltered due to a lack of physical measurements. However, quite astonishingly, researchers from the University of Wisconsin have advanced the science of quantum clocks far enough that another team from University of Colorado has begun to take such measurements.

Time-dilation and the Wavefunction

When Einstein unveiled Special Relativity in 1905 and General Relativity in 1915, he turned the scientific community upside-down. Newton’s laws of motion, which stood for more than 200 years, were dethroned. Time and space were no longer constant, and the universe was far more bizarre than we realized. Among the many consequences of this theory, and the one that is relevant to this discussion, is time-dilation. Einstein showed mathematically that the measurement of time is unique to an observer’s frame of reference. That is, two people with synchronized, identical clocks will both measure time accurately within their own frame of reference, yet their clocks won’t agree. In other words, the flow of time is relative, as it won’t be the same for one observer as it is for another. While this is not apparent in our everyday lives, it becomes obvious under relativistic conditions, such as velocities approaching the speed of light. The faster two frames of reference are traveling relative to each other, the more pronounced the effects of time-dilation.

And yes, we’ve measured the effects of time-dilation countless times. For example, in the Hafele-Keating experiment, physicists Hafele and Keating flew 4 cesium-based atomic clocks on 2 airplanes headed in opposite directions. When the planes returned they compared the clocks to one at the United States Naval Observatory. Despite all the clocks being synchronized beforehand, they all showed different times, as they were moving at different speeds relative to each other. Another classic example is the fact that we can detect solar-created muons on Earth’s surface. Muons created in the Sun have a half-life of 1.56 microseconds, not enough time to reach detectors on Earth. The solution to this paradox is that muons are traveling near the speed of light, and so time in their frame of reference moves much slower to time in our frame of reference. For us, it seems they don’t have enough time to reach Earth before decaying, but for them, they can survive long enough to travel the 93 million miles from the Sun and hit our detectors. Time-dilation continues to be verified in particle accelerators, with GPS, among many other examples.
Objects orbiting Earth are traveling at high speeds relative to the Earth’s surface, and the further away they are faster they travel.(DVdm, CC BY-SA 3.0, via Wikimedia Commons).

Around the same time Einstein was revolutionizing our understanding of the universe, scientific giants like Plank, Bohr, and Heisenberg were developing Quantum Mechanics. They helped show that, among many other counter-intuitive revelations, light is both a particle and a wave, energy only comes in discreet values, and particles exist as a wave of probability, the latter of which is the focus of this discussion. The probabilistic nature of matter is described by the Wave Function, governed by the Schrödinger equation. According to this, a particle in a box, for example, has certain probabilities of existing at different locations, and, in fact, it does exist at all of these probabilities until it’s forced to choose. Matter, then, is not a solid object but a blur of probabilities.
The Wave Function shows that matter exists as a spectrum of probabilities.(Maschen, Public Domain)

And we’ve proven this countless times as well. Perhaps the most well-known experiment is the Davisson-Germer experiment, conducted from 1923 to 1927. Based on Young’s 1801 double-slit experiment, Davisson and Germer fired electrons through a screen with 2 vertical slits with another screen used a detector on the other side. The detector originally displayed an interference pattern, consisting of light and dark bands, suggesting the electrons were behaving like waves. However, when a detector was placed near the slits, the electrons produced a pattern consistent with particles. This demonstrates the wave-particular nature of particles, in that they exist as a wave until detected, at which point the wave function collapses. In other words, they exist as a wave of probabilities until they are forced to exist at only one location.

While Relativity and Quantum Mechanics have proven themselves to be extraordinarily accurate, they don’t agree. Scientists have struggled to make them work, to develop a Theory of Everything, yet have faltered due to an inability to adequately measure the effects of one on the other. However, in a step forward to reconcile the two, scientists from the University of Colorado published new research in the journal Nature in which they explained that new atomic clocks from the University of Wisconsin are accurate enough to measure the effects of time-dilation on a quantum particle’s wave function.

A Quantum Step Forward

First developed in the 1950s, atomic clocks are still used today to synchronize the modern world due to their extreme accuracy. The atomic clocks at the US Naval Observatory and the National Institute of Standards and Technology (NIST) are so precise that they produce an error of only +/- .03 nanoseconds everyday, meaning at worst they’re off by 1 second every 100 million years. They work by firing cesium atoms through a tube where they interact with radio waves, causing them to resonate. The frequency of these radio waves needs to be just right to cause the cesium atoms outermost electrons to oscillate between energy levels at exactly 9,192,631,770 per second, which is now the official definition of a second. Despite having such amazing precision, they’re not accurate enough to measure the effects of gravity on the quantum world.
This is NIST-F1, the time keeper of the United States and reference point for other official clocks(National Institute of Standards and Technology, Public Domain)

Researchers from the University of Wisconsin developed a new type of atomic clock that is orders of magnitude better than NIST-F1, the clock that sets the international standard. The problem with typical atomic clocks is that they’re limited by the “local oscillator.” This means that the vibrations of the cesium atoms depend on the radio waves, which can only achieve limited accuracy. The team claims that their results “demonstrate that applications involving optical clock comparisons need not be limited by the instability of the local oscillator.” They accomplished this by trapping strontium atoms in a one-dimensional lattice and exciting them with a shared laser. The strontium atoms interact with one another, in what is known as atomic coherence, allowing to them synchronize and washout individual errors. They described their idea as a “one-dimensional optical lattice clock, in which spatially resolved strontium atom ensembles are trapped in the same optical lattice, interrogated simultaneously by a shared clock laser and read-out in parallel.” The team also hinted at how these new atomic clocks could be used in cutting-edge physics like the study of gravitational waves, dark matter, and of course quantum gravity.

Building on these new ideas, researchers from the University of Colorado claim to have measured quantum gravity. They took a lattice of strontium atoms and measured the time-dilation between the top and the bottom. Because the top of the lattice is further away from the surface of the Earth, it’s moving faster than the bottom, meaning each experiences the flow of time slightly differently relative to the other. They claimed that they measured “a linear frequency gradient consistent with the gravitational redshift within a single millimetre-scale sample of ultracold strontium” because these new clocks are so accurate that they’re “sensitive to the finite wavefunction of quantum objects oscillating in curved space-time.” Their results showed that the time difference across only a millimeter sized lattice was about 10^-19, far outside our ability to see with the naked eye.

The work of these teams of scientists haven’t reconciled Relativity and Quantum Mechanics, although their work is an important step in doing so. Now that we can measure the relativistic effects of gravity on the quantum world, we are on the cusp of a radical shift in modern physics. Both teams are confident the precision of these new atomic clocks will only get better, meaning we might be closer than we realize to achieving what the best scientists in over 100 years have struggled to do: find the The Theory of Everything, a final theoretical framework that encompasses every physical aspect of the known universe.

Read more about how the universe works in Neil deGrasse Tysons' (Amazon Affiliate Link) Astrophysics for Young People in a Hurry.

Originally published at on February 19, 2022.

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