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UD physicist Marianna Safronova (right) and postdoctoral
researcher Charles Cheung are part of an international team that
recently resolved one 40-year astrophysics puzzle where theory and data
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Scientists are constantly searching for ways to understand how
galaxies, black holes and stars form, grow and reorganize in the
universe. Astrophysics is the field tasked with helping to explain some
of these things.
A lot of what astronomers and physicists know comes from analyzing
the light that we receive from these cosmic phenomena using theoretical
calculations, only some of which can be validated experimentally. This
causes tension between what theoretical calculations suggest and what
experimental data can prove.
One current example of this involves a 40-year-old puzzle in
astrophysics where experimental observations and theoretical predictions
of two specific radiation lines of iron ions found in space plasmas
disagreed by up to 15%,
a difference several times larger than expected. The controversy cast
doubt on how well scientists understand and describe very hot gases that
originate from cosmic X-rays.
X-rays are a form of energetic radiation emitted by very hot gases in
space that exist in the vicinity of objects, such as black holes, huge
clusters of galaxies, in-stellar flares and even the corona of the sun.
Data about the intensity of these X-rays are important for understanding
physical conditions in space, such as temperature and density.
University of Delaware theoretical physicist Marianna Safronova and
postdoctoral researcher Charles Cheung are part of an international team
led by the Max Planck Institute for Nuclear Physics that recently
resolved this longstanding disagreement using an extremely precise
experiment. Improved theoretical calculations contributed by Safronova
and Cheung played an important role in the work.
Plasmas are very hot gases that make up about 99% of all visible
matter in space. Because they are so hot, the atoms inside these gases
break into their components, leaving positively charged ions and
electrons that move independently.
Safronova is an expert in computing the properties of atoms. Atoms
have distinct levels of energy, and when an electron jumps from one
energy state to another, it emits a particle of light called a photon.
How fast this jump takes place is called the transition rate, which
changes depending on the energy levels that are involved in the process.
Analyzing this light is the main way astronomers learn about stars
and galaxies in the cosmos. But these experimentalists need help from
theoretical physicists like Safronova, who can calculate where to look
for these transitions. To do this, theorists use atomic-scale models and
then point experimentalists in the right direction.
Safronova explained that in the problem they were working on, a
theoretical calculation of a key property of two very bright, visible
transitions of iron called 3C and 3D (which occur in most hot
astrophysical plasmas) strongly disagreed with experimental results. The
property in question showed how strong one transition was relative to
The discrepancy had previously been attributed to theory not being
accurate enough. Yet, no matter how theoretical physicists like
Safronova modified the advanced calculations, the predicted transition
rate didn’t change in any meaningful way. This led the theorists to
remain certain their calculations were correct, but it still bothered
So, Safronova and her collaborators at other institutions worked to
improve their methods for calculating this transition rate using
supercomputers, including UD’s Caviness and DARWIN high-performance
parallel computing systems. Key to this process was Charles Cheung, a
postdoctoral researcher in Safronova’s group who earned both bachelor’s
and doctoral degrees in physics at UD in 2016 and 2021. Cheung is
credited with writing the new parallel version of the atomic code that
helped solve this long-standing discrepancy.
“Charles was instrumental in terms of theory for making this work happen,” said Safronova.
Cheung recalled the laborious process of running code on a single
computer for an earlier paper and waiting weeks for the computer to
return a single number, then repeating the calculation over and over
with different parameters. The power of high-performance parallel
computing, harnessed by Cheung’s new parallel code, greatly speeded up
this process on the current work.
“For that first paper it took me months of runtime just to figure out
the numbers I needed. Now, I can rerun that original two-week
calculation and have an answer in 15 minutes,” said Cheung, the paper’s
second author. “I can run problems that are over 100 times larger, too,
to explore exciting new systems in atomic physics that are of extreme
interest to experimental groups for building quantum sensors.”
The new version of the code allowed the theoretical team to make much
larger calculations than previously possible while reducing numerical
errors. Cheung’s first version of parallel code scaled with about 50%
efficiency, but with additional refinements, today main parts of the
code perform with 99 to 100% efficiency.
“With our new code, we were able to put uncertainty numbers on our
predictions,” he continued. According to Safronova, this ability to put
precision on theoretical predictions in such complicated systems is new,
and it provided even greater confidence that the team’s atomic-scale
models were correct.
Meanwhile, Safronova’s experimentalist collaborators decided to redo
the experiment at PETRA III, a German synchrotron light facility at the
DESY laboratory outside Hamburg. The improved experimental precision
allowed the research team to obtain data on the tails of the spectral
lines they observed. The new measurements confirmed that the theoretical predictions were correct.
The researchers recently published their findings in Physical Review Letters.
Now that the theoretical and experimental data agree, it means that
researchers using X-ray data from space telescopes can have greater
confidence in the theoretical atomic models behind them.
Safronova said this new agreement between theory and experiment is
particularly interesting as it makes it possible to study quantum
electrodynamics (QED) — the study of how light and matter interact —
when even more precise experimental data for 3C/3D transitions become
available. This is because both theory and experiment must be very
precise to see the subtle, but fundamental effects of QED in atoms and
ions containing more than a few electrons.
The ability to perform very large-scale atomic computations also
opens the door to testing other complicated fundamental physics
questions that previously weren’t possible.
“Now that we know the computation is right, we can use it to guide
experiments with other systems where we can now reliably predict atomic
properties,” she said. “It also is a milestone that theory is trusted
and our ability to compute things with quantum mechanics in complicated
systems has reached a new level.”
Quantum mechanics is a fundamental physics theory that describes the
behavior of matter and light at atomic and subatomic scales. It gives
scientists the mathematical framework for computing properties of atoms
to make precise theoretical predictions without an experiment.
Following postdoctoral studies, Cheung hopes to pursue a faculty position and inspire future theoretical physicists.
“Many students want to get their hands on things like lasers in the
lab, but only a small percentage of people are interested in doing the
theoretical calculations,” said Cheung. “I want to recruit more people
interested in doing these types of calculations which will be monumental
over the next decade for the new technologies and quantum sensors that
are being developed now.”
Article by Karen B. Roberts, photo illustration by Jeffrey C. Chase
Published March 02, 2023