Finding Awe in the Balance

In those wonderful moments when I run into friends and we have the opportunity to catch up, we often engage in the time-honored tradition of sharing photos on our phones. When it’s my turn to scroll, chances are you’ll see shamelessly touristy photos of my wife and I kayaking through the canals of Gdansk or shots of our rescue dog Iggy. Recently, I’ve added a new series into this highlight reel of my life: two photos I took on a tour of the National Institute of Standards and Technology (NIST).

I’ve long heard tales of the incredible work that takes place at NIST in Gaithersburg, Maryland, but it was only recently that I saw this work for myself. The entire tour was fascinating, but my encounter with the NIST-4 Kibble Balance is the thing I can’t stop thinking about. So let’s get to the photos.

In photo 1, Darine Haddad, a researcher in NIST’s Fundamental Electrical Measurements Group, invites us to observe the elaborate balance for measuring mass. The hulking assortment of shining gears, tubes, and rivets sits atop tons of concrete in a room-size Faraday cage. The driving concept behind the design is that to measure mass using unchanging fundamental constants, you ultimately want to do so through mass–energy equivalence. It turns out that the most precise way to measure mass requires using fundamental lessons about the quantum properties of individual photons.

In photo 2, Haddad shows us a green foam wedge that slopes from one side to the other and has a bit of a bulge in the middle. The wedge is a three-dimensional model of the warping of space-time in that room.

A gravimeter on the other side of the room continuously measures the acceleration of gravity within that space. Most people tend to think of gravity as effectively constant, or at least consistent in a specific location. But, of course, at the scales and in the contexts in which physicists and astronomers concern themselves, that is often not the case. When you build a device that can measure a 1-kg mass to within two-millionths of 1 percent, variances in gravity at a given moment in time—for example, from shifts in the earth’s tides—need to be considered. The scales of this balance shift and tip with the slightest movement of the massive things around us, including the moon.

Standing there, I acutely felt myself on the sphere of the world in relation to the moon. Everything felt a little smaller and closer together as several fundamental physics concepts converged in this space, part of what it takes to make very precise measurements.

But the gravimeter is not in the exact location where the balance registers its measurements. This is where the green wedge comes in. To translate a gravity measurement on one side of the room into what is happening at that moment a few feet away, inside the balance, you have to model how space-time is warped in that room.

The room is mostly underground. As I understand it, the low end of that wedge is low because the wall that Haddad is facing has soil on the other side. In contrast, the wall behind her backs up to other work spaces. The different materials affect gravity in ways visualized using the green wedge model. The visible bulge illustrates the effect of the powerful 1-ton magnet that operates inside the balance, which is also warping space-time.

When I heard this, the hairs on the back of my neck stood on end, not dissimilar from when I stared down the epochs etched into the walls of the Grand Canyon. Who knew something like this was waiting for me in an office park in Gaithersburg, Maryland? This massive mechanism right out of a steampunk hallucination can weigh a gram at levels so precise that it requires, and makes visceral, an understanding of the warping of space and time by gravity and electromagnetism. This device is only possible because of advances in our understanding of quantum mechanics and Planck’s constant. The balance is itself the result of a dialog between breakthroughs in the physical sciences going back centuries.

This specific device played a role in that history. Proposed by British physicist Bryan Kibble in 1975, the Kibble balance made possible the 2019 transition to defining the kilogram with universal constants instead of a physical artifact.

None of this was fated. If scientists like Kibble had not forcefully made the case for this approach, things could have gone in a different direction. Indeed, there was active debate in the scientific literature in the 1990s about how best to replace the artifact-based standard for mass with a fundamental constant. The other leading proposal involved counting every individual atom in a 1-kg sphere of silicon-28. Experiments were run. Papers were presented. Careers were made. Devices were constructed.

This is where I found awe in the balance and why I pull these photos out at parties and family events. What I saw was every bit as incredible as the towers at Torres del Paine, or the perfect, regal fur on our little Pomeranian, Wendy.

Haddad, that little green wedge, and the NIST-4 Kibble Balance represent all that is so awe-inspiring about the nature of NIST’s quest in metrology. As they work toward increasingly precise methods for measurement, all manner of counterintuitive fundamental breakthroughs in how we understand the universe come into play. The contents of this room gifted me my first direct embodied experience understanding aspects of relativity and quantum mechanics that I had previously understood in more of an abstract, thought-experiment sort of way.

Because not everyone can visit the balance, I wanted to share my experience—and these photos—here with you, along with gifting them to AIP’s Emilio Segrè Visual Archives, where they will join some 30,000 other photographs documenting the lives, work, and accomplishments of physical scientists in the 20th and 21st centuries.

This collection of openly available photos, based on an original donation of Nobel laureate and amateur photographer Emilio Segrè’s personal photo collection, has grown through donations from a wide range of scientists and science professionals. A core strength of this collection is its inclusion of both formal and candid photos of physical scientists. If you have photos that speak to your own personal experiences with people, places, and communities in the physical sciences that you would like to consider gifting to this collection, the team at AIP’s Niels Bohr Library & Archives would love to hear from you.

Add Your Photos to the Collection!

To view AIP’s Emilio Segrè Visual Archives, which now includes these photos, visit repository.aip.org/emilio-segre-visual-archives-general-collection.

Learn how you can gift photos to the archives at aip.org/library/ex-libris-universum/adding-your-photos-to-the-story-of-science-is-now-easier-than-ever.