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How frozen atoms could help us learn more from gravitational waves

We’ve seen ripples in spacetime only when the universe’s biggest events occur. Now there might be a way to spot them ahead of time.
October 21, 2019
The MAGIS-100 prototype
The MAGIS-100 prototypeUS Department of Energy

It’s been four years since the first detection of gravitational waves, those strange wobbles in spacetime caused when two massive objects in space collide. Finding that signal vindicated Einstein’s century-old theory of general relativity, which says accelerating objects produce curvatures in spacetime that propagate into waves. Since then scientists have observed these signals dozens of times, rippling out from many different parts of the universe and caused by very different types of cosmic collisions.

But ever since that historic first detection, scientists have been trying to unpick exactly what such observations can tell us about the universe. Unfortunately, all of them have one serious limitation: they’re a narrow snapshot of the moment when the two objects crash into one another, and little else. Worse, because we have no heads-up before these events will occur, we can’t even really use other instruments to study them. Without greater context, the gravitational waves we detect can tell us only so much before they exhaust their usefulness.

The key to getting more out of these signals might come from a new experiment taking shape deep in a 100-meter (320-foot) vertical shaft at Fermilab in Batavia, Illinois. This is MAGIS-100, a project designed to see whether shooting frozen atoms with lasers can be used to observe ultra-sensitive signals that might be stretching through spacetime. If successful, it could help usher in a new era of “atom interferometry” that could reveal some of the secrets of gravitational waves, dark matter, quantum mechanics, and other heady topics.

Here’s how MAGIS-100 should work: atoms are cooled to a fraction above absolute zero (to keep them stable) and then dropped down a vacuum chamber housed within the shaft. A laser is pulsed down this chamber between atoms in free fall, and the time it takes for light to travel from one to the other is measured. Because light in a vacuum travels at a constant speed, this time should be precisely predictable. Any delay would presumably be caused by sensitive external signals—gravitational waves, or potentially something else.

This is not altogether different from how conventional interferometers work. At its core, MAGIS-100 is sort of a shrunken-down version of the LIGO interferometers that made the first gravitational-wave detections in 2015. The difference is that LIGO uses mirrors stationed several of kilometers apart instead of atoms. These mirrors are susceptible to disturbances caused by perturbations in the ground, which makes it more difficult to discern actual signals from false “noise.” In theory, a free-falling atom won’t be affected in this way.

Stanford University physicist Jason Hogan, one of the leads for the project, likens the technology behind MAGIS-100 to a hybrid of an interferometer and an atomic clock. “These atoms basically act like extremely good stopwatches that keep time on the propagation of light and look for fluctuations caused by other signals,” he says.

The atomic-clock comparison makes sense. Whereas MAGIS-100’s 10-meter predecessor used rubidium atoms, the current instrument will use strontium atoms, which are currently used in the best atomic clocks in the world. They’re less sensitive to external magnetic fields than other atoms, which means “they lose one second over the lifetime of the universe,” says Hogan.

The hope is that a future, bigger version of MAGIS-100 will be able to pick up gravitational-wave events that fall outside the scope of the big projects such as LIGO or Virgo, which is based in Italy.

LIGO is limited to measuring signals at frequencies between 10 hertz and 1 kilohertz. That means it can only pick up on massive events like the mergers between two black holes or two neutron stars. At the start of those events, gravitational waves at frequencies lower than 10 Hz are emitted as the objects begin to orbit around one another. The closer they get, the faster they orbit (nearing 300 orbits per second), spinning so fast they eventually produce gravitational waves higher than 10 Hz. These “burst events” last about 100 seconds before the merger is complete and the gravitational waves taper off into much lower frequencies. What LIGO can “see” is actually just the finale of a long process that starts well in advance.

Atom interferometry, meanwhile, could measure frequencies from 10 Hz down to 100 mHz or less. It could pick up on smaller gravitational waves being emitted months or even a year in advance of a burst event. This would not only help reveal a more complete picture of how these bigger phenomena occur and why, but it could warn scientists where and when they will happen. That would buy the time to set up equipment that could observe them by other means, including radio waves, optical light, infrared, UV radiation, x-rays, and gamma rays.

“My dream scenario,” says Hogan, “is to make a detection of a source in the midband, like a neutron star or black-hole binary; figure out where it’s coming from in the sky; and give everyone a date and time and place to point their other instruments. We might be able to watch this merger happen in real time.”

Being able to pick up these lower frequencies might mean that gravitational waves emitted by quieter, less massive phenomena could be studied too. This might give us the chance to answer some cosmological questions about how the early universe was formed and evolved, says Hogan.

For example, atom interferometry might also make an impact in the search for dark matter. Some theories suggest dark matter is an ultra-low-mass material that behaves more like an electromagnetic wave. Its presence could lead to small interactions that would cause measurable energy effects on the order of about 1 Hz. Hogan and his colleagues are eager to test whether MAGIS-100, or a larger version, could detect these signals and possibly afford a direct glimpse of dark matter itself.

“You have two investigative goals that can be pursued at the same time with the same detector,” says Oliver Buchmueller, one of the leaders of the Atomic Interferometric Observatory and Network (AION) project in the UK, a proposal similar to MAGIS. “It’s an extremely intriguing way to kill two birds with one stone.”

All of this is speculative for now. MAGIS-100 is just an experimental prototype. An atomic interferometer would have to be over a kilometer long to be sensitive enough to make any discoveries related to gravitational waves. Hogan says he and his colleagues are already drafting ideas for a kilometer-long version and thinking about satellite versions of the technology, where atom interferometry could really shine.

Caltech physicist Rana Adhikari, who works on LIGO, cautions that even if you’re using atoms instead of mirrors, you’re still dealing with changes—extremely small, but still problematic—in Earth’s gravity field. A space-based atom interferometer, on the other hand, would effectively be the most sensitive instrument ever constructed, capable of observing gravitational waves at the lowest frequencies imaginable. “That would be the ultimate sensitivity achieved for this type of science,” says Buchmueller.

The Stanford researchers are not the only ones interested in this technology, although they are certainly leading the pack. Besides AION, groups in France and China are also developing atom interferometry systems, albeit with modifications (in France, for instance, the device runs horizontally). Just as LIGO uses three different detectors to confirm gravitational-wave signals, Buchmueller hopes these different atom interferometry projects can validate each other’s findings and prove the tech is the real deal.

So far, the Stanford team is putting the finishing touches on the MAGIS-100 prototype itself, and building up the sources of strontium atoms. On Fermilab’s side, installation is under way. Ideally, we’ll see the apparatus completely installed in summer 2021 and set for operations that fall. Testing will run over the following three years.

In the longer term, Buchmueller thinks there is also an opportunity for this work to influence applications beyond the hunt for gravitational waves. The ability to build such sensitive sensors and shrink them into compact devices could eventually be useful for ship navigation or military applications, he says.

We might see a future where a portable device that can fit in a car could be used to assist in oil prospecting, look for structural faults, or detect earthquakes well in advance, says Adhikari. “It might be that the atom interferometry and its spinoff technologies might prove much more beneficial to humankind in the long term,” he says.

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