Physicists embark on a hunt for a long-sought quantum glow
A new approach could make it possible detect the elusive Unruh effect in hours, rather than billions of years.
Date:
April 26, 2022
Source:
Massachusetts Institute of Technology
Summary:
Researchers say they've found a way to significantly increase
the probability of observing the Unruh effect, a 'quantum glow'
phenomenon that was first proposed in the 1970s.
FULL STORY ==========================================================================
For "Star Wars" fans, the streaking stars seen from the cockpit of the Millennium Falcon as it jumps to hyperspace is a canonical image. But
what would a pilot actually see if she could accelerate in an instant
through the vacuum of space? According to a prediction known as the
Unruh effect, she would more likely see a warm glow.
========================================================================== Since the 1970s when it was first proposed, the Unruh effect has eluded detection, mainly because the probability of seeing the effect is infinitesimally small, requiring either enormous accelerations or vast
amounts of observation time. But researchers at MIT and the University
of Waterloo believe they have found a way to significantly increase the probability of observing the Unruh effect, which they detail in a study appearing in Physical Review Letters.
Rather than observe the effect spontaneously as others have attempted
in the past, the team proposes stimulating the phenomenon, in a very
particular way that enhances the Unruh effect while suppressing other
competing effects. The researchers liken their idea to throwing an
invisibility cloak over other conventional phenomena, which should then
reveal the much less obvious Unruh effect.
If it can be realized in a practical experiment, this new stimulated
approach, with an added layer of invisibility (or "acceleration-induced transparency," as described in the paper) could vastly increase the
probability of observing the Unruh effect. Instead of waiting longer
than the age of the universe for an accelerating particle to produce a
warm glow as the Unruh effect predicts, the team's approach would shave
that wait time down to a few hours.
"Now at least we know there is a chance in our lifetimes where we
might actually see this effect," says study co-author Vivishek Sudhir, assistant professor of mechanical engineering at MIT, who is designing
an experiment to catch the effect based on the group's theory. "It's a
hard experiment, and there's no guarantee that we'd be able to do it,
but this idea is our nearest hope." The study's co-authors also include Barbara Soda and Achim Kempf of the University of Waterloo.
========================================================================== Close connection The Unruh effect is also known as the
Fulling-Davies-Unruh effect, after the three physicists who initially
proposed it. The prediction states that a body that is accelerating
through a vacuum should in fact feel the presence of warm radiation
purely as an effect of the body's acceleration. This effect has to do with quantum interactions between accelerated matter and quantum fluctuations
within the vacuum of empty space.
To produce a glow warm enough for detectors to measure, a body such as
an atom would have to accelerate to the speed of light in less than
a millionth of a second. Such an acceleration would be equivalent to
a g-force of a quadrillion meters per second squared (a fighter pilot
typically experiences a g-force of 10 meters per second squared).
"To see this effect in a short amount of time, you'd have to have
some incredible acceleration," Sudhir says. "If you instead had some
reasonable acceleration, you'd have to wait a ginormous amount of time
-- longer than the age of the universe -- to see a measurable effect."
What, then, would be the point? For one, he says that observing the
Unruh effect would be a validation of fundamental quantum interactions
between matter and light. And for another, the detection could represent
a mirror of the Hawking effect -- a proposal by the physicist Stephen
Hawking that predicts a similar thermal glow, or "Hawking radiation,"
from light and matter interactions in an extreme gravitational field,
such as around a black hole.
========================================================================== "There's a close connection between the Hawking effect and the Unruh
effect - - they're exactly the complementary effect of each other," says Sudhir, who adds that if one were to observe the Unruh effect, "one would
have observed a mechanism that is common to both effects." A transparent trajectory The Unruh effect is predicted to occur spontaneously in
a vacuum. According to quantum field theory, a vacuum is not simply
empty space, but rather a field of restless quantum fluctuations, with
each frequency band measuring about the size of half a photon. Unruh
predicted that a body accelerating through a vacuum should amplify these fluctuations, in a way that produces a warm, thermal glow of particles.
In their study, the researchers introduced a new approach to increase the probability of the Unruh effect, by adding light to the entire scenario --
an approach known as stimulation.
"When you add photons into the field, you're adding 'n' times more
of those fluctuations than this half a photon that's in the vacuum,"
Sudhir explains.
"So, if you accelerate through this new state of the field, you'd expect
to see effects that also scale 'n' times what you would see from just
the vacuum alone." However, in addition to the quantum Unruh effect,
the additional photons would also amplify other effects in the vacuum --
a major drawback that has kept other hunters of the Unruh effect from
taking the stimulation approach.
Soda, Sudhir, and Kempf, however, found a work-around, through
"acceleration- induced transparency," a concept they introduce in the
paper. They showed theoretically that if a body such as an atom could
be made to accelerate with a very specific trajectory through a field of photons, the atom would interact with the field in such a way that photons
of a certain frequency would essentially appear invisible to the atom.
"When we stimulate the Unruh effect, at the same time we also stimulate
the conventional, or resonant, effects, but we show that by engineering
the trajectory of the particle, we can essentially turn off those
effects," Soda says.
By making all other effects transparent, the researchers could then have
a better chance of measuring the photons, or the thermal radiation coming
from only the Unruh effect, as the physicists predicted.
The researchers already have some ideas for how to design an experiment
based on their hypothesis. They plan to build a laboratory-sized particle accelerator capable of accelerating an electron to close to the speed of
light, which they would then stimulate using a laser beam at microwave wavelengths. They are looking for ways to engineer the electron's path
to suppress classical effects, while amplifying the elusive Unruh effect.
"Now we have this mechanism that seems to statistically amplify this
effect via stimulation," Sudhir says. "Given the 40-year history of this problem, we've now in theory fixed the biggest bottleneck." This research
was supported, in part, by the National Science and Engineering Research Council of Canada, the Australian Research Council, and a Google Faculty Research Award.
========================================================================== Story Source: Materials provided by
Massachusetts_Institute_of_Technology. Original written by Jennifer
Chu. Note: Content may be edited for style and length.
========================================================================== Journal Reference:
1. Barbara Soda, Vivishek Sudhir, Achim Kempf. Acceleration-Induced
Effects
in Stimulated Light-Matter Interactions. Physical Review Letters,
2022; 128 (16) DOI: 10.1103/PhysRevLett.128.163603 ==========================================================================
Link to news story:
https://www.sciencedaily.com/releases/2022/04/220426162601.htm
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