Scientists have recently observed light working as a sort of weakly bound
molecule, acting as the "glue" connecting atoms.
According to physicist Matthias Sonnleitner of the University of Innsbruck,
"We have succeeded for the first time in polarizing multiple atoms together
in a controlled fashion, establishing a detectable attractive force between
them."
In a number of ways, atoms join together to create molecules, but in each
case, a trade-off of charges acts as a sort of "superglue."
Some of these molecules create rather strong connections by sharing their
negatively charged electrons, ranging from the simplest gases—the two linked
oxygen atoms we breathe in continuously—to the complex hydrocarbons seen
floating in space. Some atoms are attracted to one another due to variations
in their total charge.
Charge configurations surrounding an atom can change in the presence of
electromagnetic forces. Since light is an electromagnetic field that is
always changing, a suitable photon shower can move electrons into locations
where they might, in principle, bind.
This charge distribution varies a bit if you now turn on an external
electric field, according to physicist Philipp Haslinger of the Technical
University of Vienna (TU Wien).
The atom is polarized when the positive charge is slightly displaced in one
direction and the negative charge is slightly shifted in the other
way.
Using ultracold rubidium atoms, Haslinger, atomic physicist Mira Maiwöger,
and colleagues showed that light may polarize atoms in a manner similar to
that of magnetic fields, which causes normally neutral atoms to become
slightly sticky.
It takes a very thorough experiment to be able to quantify such a weak
attraction force, according to Maiwöger.
"The attractive force vanishes instantly when atoms are travelling swiftly
and with a lot of energy. The use of an ultracold atom cloud was due to
this."
The scientists used a magnetic field to confine a cloud of around 5,000
atoms beneath a gold-coated chip to a single plane.
In order to create a quasicondensate, the rubidium particles were cooled to
temperatures close to absolute zero (273 °C or 460 °F). As a result, the
rubidium particles started acting collectively and sharing properties
similar to those of the fifth state of matter, though not quite to the same
extent.
The atoms were hit by a laser and underwent a range of stresses. For
instance, the pressure from incoming photons' radiation might force them to
move along the light beam. As the atom moves away from the most intense
region of the beam, reactions in the electrons may cause it to return.
The researchers needed to perform some thorough calculations in order to
identify the slight attraction that is expected to form between atoms in
this flood of electromagnetic.
The atoms free-fell for around 44 milliseconds after the magnetic field was
turned off before arriving in the laser light field, where they were also
observed using light sheet fluorescence microscopy.
The cloud spontaneously extended throughout the fall, allowing the
researchers to collect data at various densities.
Maiwöger and colleagues discovered that at high concentrations, up to 18%
of the atoms were missing from the observational photographs they were
capturing. They postulate that these absences resulted from collisions that
were aided by light, which forced the rubidium atoms from their cloud.
This illustrated a portion of what was going on, showing that light
scattering off other atoms as well as the light coming in was having an
impact on the atoms. The atoms acquired polarity when the light made contact
with them.
Greater light intensity either attracted or repelled the atoms depending on
the type of light employed. As a result, they were either drawn to an area
of lower light or more light, and in both cases, they eventually gathered
together.
In their study, Maiwöger and colleagues state that one key distinction
between typical radiation forces and the light-triggered interaction is that
the latter is an effective particle-particle contact, mediated by scattered
light.
It attracts atoms toward areas of highest particle density rather than
trapping them in a fixed location (like the laser beam's focus).
Although the force pulling the atoms together is far smaller than the
molecular forces we are more familiar with, it may still accumulate on vast
sizes. Resonance lines and emission patterns, which astronomers use to help
us comprehend celestial objects, may change as a result.
Additionally, it could clarify how molecules arise in space.
Small forces may have a big impact in the immensity of space, claims
Haslinger.
Here, we were able to demonstrate for the first time that electromagnetic
radiation may produce a force between atoms, which may assist to clarify
hitherto unrecognized astrophysical circumstances.
This research was published in
Physical Review X.
