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Tuesday, January 26, 2021

UW-Madison Physics professor awarded fellowship to study atomic clocks, test Einstein’s theories

Atomic clocks, the most precise instruments known to man, are about to open a new door into our understanding of the fundamental nature of the universe. 

Shimon Kolkowitz, UW-Madison assistant professor of physics, has been selected to be one of 22 2019 Packard Fellows for Science and Engineering to continue his work with “ultra-precise atomic clocks” — hopefully answering some of the questions that follow Einstein’s theories.

The fellowship provides $875,000 in funding spread over five years. Kolkowitz will use this funding to actualize Einstein’s theories of relativity in experiments using atomic clocks, as well as somewhat freely explore what doors in physics these instruments can open. 

Kolkowitz was born in California and grew up in the Bay area. His grandfather was a professor in physics. His mother had a Ph.D. in Geophysics and met his father at Stanford while he was getting a master’s in computer science. 

Naturally, he was encouraged to get into science from a young age. Despite being told stories of all the research his family had done, it was a high school teacher in AP Physics that prompted Kolkowitz to pursue physics as an undergrad. 

“My AP physics teacher had what he called ‘Faith in Physics’ labs, where we’d have something like a ramp and we would have all these equations and were tasked to predict where the ball would land, and if you got it right the first time, you got an A,” Kolkowitz said. “It showed that these equations [and that physics] had real meaning that you could see and use.”

Kolkowitz got a B.S. in Physics from Stanford before going on to Harvard to receive a Ph.D.. It was at Harvard and the JILA institute, jointly run by UC Boulder and the National Institute of Standards and Technology, where he delved into quantum sensing and ultra-cold atoms — the mechanistic key of atomic clocks.

So what makes these clocks so special? 

It mostly has to do with gravity — which affects the rate at which time passes. This principle first alluded to by Albert Einstein is called relativity and it is illustrated strikingly well in the 2014 film starring Mathew McConaughey and Anne Hathaway, “Interstellar.” 

In the film, a team of astronauts on a mission to find a new home for humanity visit a planet close to a black hole. In accordance with relativity, the team that goes to the planet close to the black hole — which has an extremely high gravitational pull — experience time at a much slower rate than the people who remained on the spaceship. When they return, it’s been decades for the man who stayed — for them, it was hours. 

Although highly dramatized in the film, the principle it displays is very real. To put into even more tangible terms, due to Earth’s gravity, your head is older than your feet.

This difference in the passage of time due to differences in height with respect to gravity is called “the gravitational redshift,” and these clocks can measure it.

As humanity seeks to be more accurate, we measure things in relation to constants, constants like the speed of light. We now measure length with respect to time. A meter is how fast light travels in a fraction of a second. 

Now we are working to define weight to time, because currently, a vault in Paris holds the kilogram that we use as a reference. But that kilogram is subject to minute environmental changes. These changes over time create disagreements in what a “kilogram” means around the world.

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So how do these clocks work? How are they different from everyone’s clocks? 

Clocks use something that has a periodic cycle to it. Most rudimentarily, we used the Earth going around the sun. Then, humans started using things like a pendulum swinging to keep time. As we advanced, we used a quartz tuning fork, which has a defined frequency when it resonates and can be used to keep time.

All of these mechanisms of clocks all are subject to environmental conditions: temperature, dust, etc. all cause drift — time becomes less and less accurate. But Kolkowitz’s atomic clocks utilize a sort of feedback loop system. 

“[A quartz tuning fork] is vibrating at a [well-defined] microwave frequency which shines on an atom. And if it is on resonance with the atom, it will change the internal state of the atom. And if it isn’t, it won’t…when drift occurs, it is corrected by a signal that adds or subtracts whatever the frequency is off by,” Kolkowitz said.

To elucidate the extreme precision these mechanisms have in making these clocks accurate, Kolkowitz explained that if it were running since the Big Bang, it would agree on the age of the universe within a second.

Kolkowitz believes these clocks can have real-world applications in GPS by increasing the autonomy of satellites from the current hour or they can operate without human correction, to months. If a natural disaster were to occur or even a malicious actor intentionally severing communications to satellites, we could rely on the GPS network to still work for months. 

He also described that these clocks could be used in possibly predicting earthquakes and volcanoes because of their sensitivity to “masses moving inside the earth.”

First on Kolkowitz’s docket is to test relativity by putting two clocks at different heights on Earth, and sensing the relative difference, which would be a world record in measuring the gravitational redshift. 

Then he plans to test another aspect of relativity: The Einstein equivalence principle. It is best explained through an elevator thought experiment: Would being on elevator experiencing earth’s gravity be indistinguishable to an elevator in space experiencing acceleration?   

The Packard Fellowship gives Kolkowitz the freedom to explore what atomic clocks can do and lay the groundwork for future science. 

“It’s a complicated time in physics because we don’t know where to look,” Kolkowitz said. “But it’s also an exciting time in physics because we know that there are these mysteries and we know there must be answers to them — and these clocks are one way to access that.” 

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Alberto Kanost

State news and Science writer


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