BLOOMINGTON, Ill.— An international team of physicists including Illinois Wesleyan
University Professor of Physics Gabe Spalding has shown waves of light can seem to
travel back in time.
It may seem like science fiction, but the experiment did not violate the laws of physics.
Spalding, his physics student Joseph Richards ’16 and a team of scientists tackled a century-old intuition
from Lord Rayleigh regarding the speed of sound. Rayleigh theorized that music being
played on an object traveling faster than the speed of sound, a supersonic jet for
example, would result in a listener hearing the music playing in reverse. The Spalding
team simulated what an observer standing still would see when looking at a superluminal
(faster than the speed of light) occurrence. The results of the scientists’ experiment,
conducted last summer at Heriot-Watt University in Edinburgh, Scotland, have been
published in Science Advances.
“The existence of an absolute limit, the speed of light, is the natural source of
the question: what would happen if we cross this limit?” lead author Mattero Clerici
told a writer for a post on IFLScience. “Light sources, however, may move faster than the speed of light when their speed
is not associated with the physical motion of matter. Following this line of thought,
we devised a way to experimentally investigate the [effects] of superluminal motion.”
The B. Charles and Joyce Eichhorn Ames Professor of Physics at Illinois Wesleyan,
Spalding has long worked with advanced imaging, and came up with the idea for studying
superluminal spot motion and mathematically determined the conditions for observing
time reveals and spot pair creation and annihilation. Using a new sort of camera based
upon an array of single-photon avalanche diodes (SPADs), researchers captured scattered
laser light by shining ultra-short laser pulses at different points on the screen.
Researchers were able to create a live video of the light traversing the surface in
picoseconds (a picosecond is a unit of time equal to one trillionth of a second).
“This is incredible in its own right,” said Richards, who was tasked with verifying
that the light sources had indeed moved at superluminal speed.
The experiment showed pairs of images forming and annihilating each other. In a video
of the experimental results, virtual spot pair annihilation corresponds to a spot
source changing its speed and crossing the boundary between sub- and superluminal
speed. “A local observer would report that the wavefront is moving from left to right
at every point, but once the spot becomes superluminal, there is time reversal,” said
He described a further phenomenon in the experiment where a local observer would report
a wavefront that is always moving from left to right, but the time reversal associated
with the transition to superluminal velocities gives rise to detection of a pair of spots – one moving forward in time and one moving backward in time.
The new findings might have applications far beyond the ultra-high-speed cameras.
Spalding said faster-than-light imaging will open many areas of scientific inquiry.
“For example, it may play an important role in new, advanced forms of microscopy based
upon very rapid signals from fluorescently tagged biomolecules,” said Spalding.
For Richards, however, the impact has been immediate.
“To have a publication of this magnitude to my name before the end of my undergraduate
career is quite rare,” said Richards. “My experience with Dr. Spalding both here at
Illinois Wesleyan and in Scotland has most certainly catapulted me into the next level
of graduate study and I am forever grateful to both him and the University.”
For more than a decade Spalding has traveled to Scotland in the summer to work with
cross-functional teams of scientists and engineers, often accompanied by Illinois
Wesleyan students such as Richards. A native of Glenview, Ill., Richards said he was
both “absolutely terrified but also excited” to work in a lab with Spalding’s longtime
collaborators in Scotland. “However, Dr. Spalding was very helpful in settling me
into my new position, and we quickly hit the ground running.”
In Scotland Richards said he learned a great deal pertaining to the process of academic
research. “I feel I have a strong grasp on the process from start to finish and feel
confident that I can be a benefit to research groups in the future,” said Richards,
who will enter the master’s program in electrical and computer engineering at the
University of Illinois Urbana-Champaign this fall.
Spalding is currently on sabbatical in Scotland, where he has been working on yet
another version of array-based versions of SPADs. He has been promoting educational
versions of such detectors as a cornerstone to a new generation of instructional labs
in modern physics. To date, he has shipped 420 such SPADS to colleges and universities
across the country. Spalding’s leadership in making this equipment available across
the country, and his research in the field of optical micromanipulation was detailed
in an IWU Magazine article.