This is one in a series on careers in science, technology, engineering and mathematics made possible with generous support from Arconic Foundation.
Out in the ocean, a see-through squid swims inconspicuously. Sunlight from above shines into the water and through the squid. Well, right through most of it anyway. Only the squid’s eyes block the sunlight, casting a shadow below. That shadow poses a danger for the squid. Deep-sea predators below are looking upward, searching for such shadows. That’s how they find food.
Many species of glass squid, however, have evolved a solution to this problem. They emit light from under their eyes. Now, from below, the animal seemingly disappears. As nonsensical as it might seem, they use light to hide.
And these squid are not alone in harnessing light as a survival skill. Deep below the ocean’s surface is a world of light. Almost every creature down there shines in some way. Consider the sea worm. It shoots out yellow sparks to scare off predators.
Alison Sweeney is a physicist at the University of Pennsylvania in Philadelphia. “In the deep ocean, bioluminescence is the rule, rather than the exception,” she notes. By bioluminescence (BY-oh-loom-in-ESS-ens), she’s referring to the ability of many organisms to give off light.
Hiding from predators is far from the only reason animals generate light. The viperfish, for instance, emits a blinking beacon that lures prey — to become its dinner. Even that emblem of summer, the firefly, emits light. It's goal is to find a mate. The pulsing yellowish green flashes not only tell potential mates “I’m here,” but also warn predators that this insect is too toxic to make a healthy lunch.
Long ago, people figured out how illuminate their world with electric lighting. But people are far from the only creatures that can light the darkness around them. Many animals do too. And as strange as it sounds, even bubbles may sometimes emit light.
Here we meet three scientists who study such all-natural sources of light. What they’ve been turning up may lead to better devices for illuminating the human world. Their findings might even yield better sources of energy — and perhaps a better understanding of geriatric stars lurking in deep space.
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The chemistry of light
Many animals that make light have special cells where chemical reactions create a glow. Glass squid, for example, have patches of these cells around their eyes. The light they issue will now fill in the shadow their eyes would have otherwise cast. A firefly, in contrast, has a clump of these cells in the last segment of its abdomen. They all glow at once to make a bright flash.
Where those cells are and how closely they are spaced will depend on what the animals do with their light. The variety is a bit “like all the different types of lights you find at Home Depot,” Sweeney says. “You have huge flood lights, Christmas lights and flashlights.” Each has its purpose.
Sweeney became interested in the ability of animals to generate light when she was in college. How sunlight affects plants and animals fascinated her. Eventually, she decided to probe how this connection evolved.
In graduate school, she began to study how animals emit and use light. That’s what led her to study a genus of glass squid, called Galiteuthis (Gal-ih-TU-thiss). Like fireflies, the glass squid relies on a trio of chemicals to make its light. One is the element oxygen. Another is a pigment called luciferin (Loo-SIFF-ur-in). The last is an enzyme (a molecule that speeds up a chemical reaction) called luciferase (Loo-SIF-ur-ace). When luciferin and oxygen interact, they produce light. Luciferase speeds up that reaction. (Both luciferin and luciferase take their names from the Latin word lucifer, for “bearing light.”)
The glass squid cells that emit light look like thin cables, each about 50 micrometers long. That’s about the width of a human hair. Sweeney thought the cells also acted like cables. They seemed to direct their light downward, toward the ocean floor.
But while studying these cells carefully, she and her colleague Amanda Holt turned up a surprise. Those cells were “leaky,” Sweeney explains. They lost light all along their length. That meant light was even exiting the cell sideways.
At first, this struck the women as inefficient. But then Sweeney considered the animals' world. The squid would need to hide from predators lurking not only below but also to both sides. So they would have to avoid casting eye shadows to either side. And the leaky light did just that. It effectively hid the squid’s eyes from all angles.
Sweeney and Holt reported their finding in the June 2016 Journal of the Royal Society Interface.
“These animals are masters of manipulating the tools to make light,” Sweeney concludes. They seem to have tricks for working with light in extremely small spaces (a single cell) and in bizarre environments.
Researchers hope to make new devices that do the same thing using nanotechnology. That term refers to things that are made from structures with sizes on the scale of 100 billionths of a meter (some 25 billionths of an inch) or less.
One idea is to use the luciferin-based chemical reaction to make tiny structures glow. These nanorods might offer an alternative to LEDs, or light emitting diodes. As their name suggests, LEDs are electronic devices that emit light. They do so by allowing electrons to pass through a semiconductor. Such a material can transmit electrons, but won’t heat up the way the metal filament does in traditional (incandescent) light bulbs. That heat is just wasted energy. So LEDs are much more efficient than traditional light bulbs at providing a glow.
But LEDs have a problem. They need electricity. Nanorods that glow via a chemical reaction wouldn’t. So this might be one way to make LEDs even more efficient. Still, inefficiencies in generating light aren’t all bad, as Sweeney’s work shows. Glass squid use inefficient ways of generating light to live better lives. “We want to understand how squid do it, so we can do it too,” she says.
Not all animals rely on chemical reactions to make their glow. Some instead fluoresce (floor-ESS). This process converts one form of light to another.
Certain molecules in their cells absorb light of one wavelength, or color. This bonus energy excites the molecules. But they don’t want to stay excited. So they release this energy — but at another wavelength, usually a longer one.
For example, an object that absorbs ultraviolet radiation may re-emit that energy as visible light. And that will make it glow.
Matt Liptak got interested in studying molecules that fluoresce after one of his colleagues hit a snag in his research. Liptak is a chemist at the University of Vermont in Burlington. His colleague Ivan Aprahamian is a chemist at Dartmouth University in Hanover, N.H.
Aprahamian was studying molecules made from three materials: the elements boron and fluorine, and a compound called hydrazone. He was interested in these molecules because they are molecular rotors — tiny machines. In them, two parts of the molecule can rotate with respect to one another.
The molecules that interested him would fluoresce after violet light shined on them. This had been expected. What was not expected: In a thin, runny liquid, such as water, the molecules glowed weakly and red. In a thicker liquid, such as maple syrup, they instead glowed bright green.
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That wasn’t supposed to happen. In fact, in the thicker materials the molecules shouldn’t glow at all!
Fluorescent molecules typically absorb light, which then makes them vibrate, stretch and give off light. All of these processes release energy, which brings the molecules to a more “relaxed” state. In that relaxed state, the molecule will hold less energy. So as the molecule relaxes, it can shed extra energy as light.
Yet the molecules Aprahamian studied glowed more strongly when they were put into thicker and thicker liquids. This was unexpected since most molecules that fluoresce emit less light in thicker liquids. Clearly, something odd was happening.
Aprahamian asked Liptak for help in figuring out why his molecules didn’t relax in the thick liquids. Liptak's team made computer models of the fluorescent molecules. (These are analyses done with a computer to mimic how a physical system works.) This model by Liptak’s group led to predictions about the molecules. The researchers could then test those predictions in experiments. And when they did, they found that these molecules behaved very differently from most fluorescent compounds.
“The thickness of the fluid, what we call its viscosity, was shutting down the molecule’s relaxation,” Liptak realized. This didn’t let the molecules vibrate well. So they could not twist at the bottom. That kept them from shedding as much heat as might have been expected. The “excited” molecules responded by emitting light, which led to a brighter glow.
In essence, the research team had discovered a new way fluorescence can work. They described it in the September 2016 Nature Chemistry.
“It’s really exciting,” Liptak says, “because we realize now there’s a whole world of these molecules to explore.”
If the team can find molecules that emit light like these molecules, but even more brightly, then these chemicals might also find use in helping improve LEDs. No one knows, yet, the potential that such molecules might hold for the next generation of LEDs. But they “open up a new research pathway,” Liptak says.
Bubbles don’t seem like they should be able to emit light. But under the right conditions, they do through a process known as sonoluminescence (SO-no-loom-in-ESS-ens).
Bubbles are little pockets of gas. Sound waves can crush them. And doing so, the bubbles will release energy in a fantastic burst of heat and light. This happens in nature, when snapping shrimp clamp their claws shut.
As the two parts of the claw snap into each other, the change in pressure makes bubbles shoot out. It also triggers the sound of a snap. That sound travels as a wave. As it does, it causes changes in the pressure and density of the water.
Such large pressure changes can squeeze bubbles, making them collapse. When that happens, the gas inside heats. In fact, that gas can get hotter than the surface of the sun. In may get so hot that electrons are pulled off of their atoms. This creates plasma, a soup of electrons and charged atoms.
Eventually the electrons and atoms recombine. Some scientists think that’s when the burst of light emerges. All of this happens in less than a billionth of a second.
It’s pretty incredible, notes Alex Bataller. He is a physicist at North Carolina State University in Raleigh. And there’s no reason this way of making light should be restricted to snapping shrimp. He says that people could use it, in theory, to make energy.
We use a lot of energy to power our gadgets, heat our homes, drive places, keep the lights on, cook food and do all of our other daily tasks. Generating that energy causes pollution. And common sources of energy, such as coal and oil, won’t last forever. So scientists are looking for cleaner, sustainable ways to make energy.
One way would be to fuse atoms together. Fusion releases a lot of energy. It’s the process that fuels the sun’s production of heat and light. The sun’s energy will last for another 5 billion years or so. Scientists would like to figure out how to safely fuse atoms to generate energy on Earth.
Sonoluminescence might be the ticket. But scientists need more control over the process than comes from the bubbles and light that snapping shrimp make. Scientists also need to know more about what’s happening inside the bubbles. What’s the temperature? What’s the pressure? Can you make the gas inside the bubbles get hotter than it normally would? Can you get it hot enough to fuse atoms?
These are questions Bataller has tried to answer. Until recently, he was a graduate student at the University of California, Los Angeles. There, he spent a lot of time shooting laser beams into tiny bubbles. These bubbles were micrometers in size. That’s just a few ten-thousandths of an inch — about the size of a single red blood cell.
He focused the laser to about the same size. The goal was to get the bubble to absorb the laser light. That should boost the temperature of the gas inside.
Ultimately, Bataller wanted to get the bubble’s internal temperature to reach some 10 million degrees Celsius (18 million degrees Fahrenheit). Then, when the bubble collapsed, maybe — just maybe — the gas inside would get hot enough to fuse its atoms.
So far, no one has gotten this bubble fusion to work. It’s unclear if there’s any way to get the inside of a bubble that hot. But researchers are continuing to try.
Bataller, however, has moved on. He is now using what he learned blasting bubbles with lasers to study other materials. For instance, he’s probing semiconductors and the plasmas that can form inside them. This work, still in its early stages, also has to do with creating cleaner energy.
And even if bubble fusion doesn’t work, Bataller says, scientists can still use sonoluminescence to study a type of dying stars known as white dwarfs. They are about as hefty as the sun but only the size of Earth. Their cores have collapsed. In the process, they’ve blown off a lot of gas. But they still contain plasma, like light-emitting bubbles do. So studying those bubbles may offer insights into white dwarf stars.
As the work of these researchers shows, science and engineering have much to learn from how nature can set bits of the world aglow.
angle The space (usually measured in degrees) between two intersecting lines or surfaces at or close to the point where they meet.
atom The basic unit of a chemical element. Atoms are made up of a dense nucleus that contains positively charged protons and uncharged neutrons. The nucleus is orbited by a cloud of negatively charged electrons.
bioluminescence The light emitted by certain animals — such as fireflies, squid and deep-sea fishes — and by some shallow-water algae.
boron The chemical element having the atomic number 5. Its scientific symbol is B.
cell The smallest structural and functional unit of an organism. Typically too small to see with the unaided eye, it consists of a watery fluid surrounded by a membrane or wall. Depending on their size, animals are made of anywhere from thousands to trillions of cells. Some organisms, such as yeasts, molds, bacteria and some algae, are composed of only one cell.
chemical A substance formed from two or more atoms that unite (become bonded together) in a fixed proportion and structure. For example, water is a chemical made when two hydrogen atoms bond to one oxygen atom. Its chemical formula is H2O. Chemical can also be an adjective used to describe properties of materials that are the result of various reactions between different compounds.
chemical reaction A process that involves the rearrangement of the molecules or structure of a substance, as opposed to a change in physical form (as from a solid to a gas).
chemistry The field of science that deals with the composition, structure and properties of substances and how they interact with one another. Chemists use this knowledge to study unfamiliar substances, to reproduce large quantities of useful substances or to design and create new and useful substances. (about compounds) The term is used to refer to the recipe of a compound, the way it’s produced or some of its properties. People who work in this field are known as chemists.
colleague Someone who works with another; a co-worker or team member.
compound (often used as a synonym for chemical) A compound is a substance formed from two or more chemical elements united in fixed proportions. For example, water is a compound made of two hydrogen atoms bonded to one oxygen atom. Its chemical formula is H2O.
computer model A program that runs on a computer that creates a model, or simulation, of a real-world feature, phenomenon or event.
core Something — usually round-shaped — in the center of an object. (In geology) Earth’s innermost layer.
density The measure of how condensed some object is, found by dividing its mass by its volume.
electricity A flow of charge, usually from the movement of negatively charged particles, called electrons.
electron A negatively charged particle, usually found orbiting the outer regions of an atom; also, the carrier of electricity within solids.
element (in chemistry) Each of more than one hundred substances for which the smallest unit of each is a single atom. Examples include hydrogen, oxygen, carbon, lithium and uranium.
engineering The field of research that uses math and science to solve practical problems.
environment The sum of all of the things that exist around some organism or some device and the condition those things create for that organism or device. Environment may refer to the weather and ecosystem in which some animal lives, or, perhaps, the temperature, humidity and placement of components in some electronics system or product.
excite (in chemistry and physics) To transfer energy to one or more outer electrons in an atom. They remain in this higher energy state until they shed the extra energy through the emission of some type of radiation, such as light.
filament Something with a thin, thread-like shape. For instance, the fragile metal wire that heats up to emit light inside an incandescent light bulb is known as its filament.
firefly An insect in the family Lampyridae. Also called lightning bugs, fireflies are not flies at all, but beetles. They are known for their beautiful nighttime flashes of light, created through a process known as bioluminescence. There are over 2,000 species of firefly around the world.
fluorescent Capable of absorbing and reemitting light. That reemitted light is known as a fluorescence.
fuel Any material that will release energy during a controlled chemical or nuclear reaction. Fossil fuels (coal, natural gas and petroleum) are a common type that liberate their energy through chemical reactions that take place when heated (usually to the point of burning).
fusion (v. to fuse) The merging of two things to form a new combined entity. (in physics) The process of forcing together the nuclei of atoms. This nuclear fusion is the phenomenon that powers the sun and other stars, producing heat and forging the creation of new, larger elements.
generation A group of individuals born about the same time or that are regarded as a single group. The term also is sometimes extended to year classes or types of inanimate objects, such as electronics or automobiles.
geriatric Relating to old age. Doctors who specialize in conditions affecting the elderly are known as geriatricians.
graduate school A university program that offers advanced degrees, such as a Master’s or PhD degree. It’s called graduate school because it is started only after someone has already graduated from college (usually with a four-year degree).
graduate student Someone working toward an advanced degree by taking classes and performing research. This work is done after the student has already graduated from college (usually with a four-year degree).
incandescent light The old-style lighting technology that relied on a glass bulb. Electricity passing through the bulb heats a thread-like tungsten filament, making it glow white hot. Thomas Edison commercialized this technology in 1879. By that time, the technology was already about 50 years old. Incandescent lights have been used to illuminate everything from tiny flashlights to whole rooms. Many governments have moved to ban these bulbs because they waste so much of their energy as heat.
insight The ability to gain an accurate and deep understanding of a situation just by thinking about it, instead of working out a solution through experimentation.
journal (in science) A publication in which scientists share their research findings with the public. Some journals publish papers from all fields of science, technology, engineering and math, while others are specific to a single subject. The best journals are peer-reviewed: They send out all submitted articles to outside experts to be read and critiqued. The goal, there, is to prevent the publication of mistakes, fraud or sloppy work.
laser A device that generates an intense beam of coherent light of a single color. Lasers are used in drilling and cutting, alignment and guidance, in data storage and in surgery.
LEDs (light emitting diodes) Electronic components that, as their name suggests, emit light when electricity flows through them. LEDs are very energy-efficient and often can be very bright. They have lately been replacing conventional lights for home and commercial lamps.
micrometer (sometimes called a micron) One thousandth of a millimeter, or one millionth of a meter. It’s also equivalent to a few one-hundred-thousandths of an inch.
model A simulation of a real-world event (usually using a computer) that has been developed to predict one or more likely outcomes.
molecule An electrically neutral group of atoms that represents the smallest possible amount of a chemical compound. Molecules can be made of single types of atoms or of different types. For example, the oxygen in the air is made of two oxygen atoms (O2), but water is made of two hydrogen atoms and one oxygen atom (H2O).
nanotechnology Science, technology and engineering that deals with things and phenomena at the scale of a few billionths of a meter or less.
organism Any living thing, from elephants and plants to bacteria and other types of single-celled life.
oxygen A gas that makes up about 21 percent of Earth's atmosphere. All animals and many microorganisms need oxygen to fuel their metabolism.
physical (adj.) A term for things that exist in the real world, as opposed to in memories or the imagination. It can also refer to properties of materials that are due to their size and non-chemical interactions (such as when one block slams with force into another).
physicist A scientist who studies the nature and properties of matter and energy.
pigment A material, like the natural colorings in skin, that alter the light reflected off of an object or transmitted through it. The overall color of a pigment typically depends on which wavelengths of visible light it absorbs and which ones it reflects. For example, a red pigment tends to reflect red wavelengths of light very well and typically absorbs other colors. Pigment also is the term for chemicals that manufacturers use to tint paint.
plasma (in chemistry and physics) A gaseous state of matter in which electrons separate from the atom. A plasma includes both positively and negatively charged particles. (in medicine) The colorless fluid part of blood.
predator (adjective: predatory) A creature that preys on other animals for most or all of its food.
pressure Force applied uniformly over a surface, measured as force per unit of area.
prey (n.) Animal species eaten by others. (v.) To attack and eat another species.
radiation (in physics) One of the three major ways that energy is transferred. (The other two are conduction and convection.) In radiation, electromagnetic waves carry energy from one place to another. Unlike conduction and convection, which need material to help transfer the energy, radiation can transfer energy across empty space.
red blood cell Colored red by hemoglobin, these cells move oxygen from the lungs to all tissues of the body. Red blood cells are too small to be seen by the unaided eye.
electrical resistance The tendency of an electricity-conducting material to oppose the passage of a current through it. That resistance (usually measured in units known as ohms) will convert some of the electric energy into heat.
sea An ocean (or region that is part of an ocean). Unlike lakes and streams, seawater — or ocean water — is salty.
semiconductor A material that sometimes conducts electricity. Semiconductors are important parts of computer chips and certain new electronic technologies, such as light-emitting diodes.
society An integrated group of people or animals that generally cooperate and support one another for the greater good of them all.
sound wave A wave that transmits sound. Sound waves have alternating swaths of high and low pressure.
species A group of similar organisms capable of producing offspring that can survive and reproduce.
squid A member of the cephalopod family (which also contains octopuses and cuttlefish). These predatory animals, which are not fish, contain eight arms, no bones, two tentacles that catch food and a defined head. The animal breathes through gills. It swims by expelling jets of water from beneath its head and then waving finlike tissue that is part of its mantle, a muscular organ. Like an octopus, it may mask its presence by releasing a cloud of “ink.”
star The basic building block from which galaxies are made. Stars develop when gravity compacts clouds of gas. When they become dense enough to sustain nuclear-fusion reactions, stars will emit light and sometimes other forms of electromagnetic radiation. The sun is our closest star.
sun The star at the center of Earth’s solar system. It’s an average size star about 26,000 light-years from the center of the Milky Way galaxy. This term can also refer to some other sunlike star.
toxic Poisonous or able to harm or kill cells, tissues or whole organisms. The measure of risk posed by such a poison is its toxicity.
transmit (n. transmission) To send or pass along.
ultraviolet A portion of the light spectrum that is close to violet but invisible to the human eye.
variety (in agriculture) The term that plant scientists give to a distinct breed (subspecies) of plant with desirable traits. If the plants were bred intentionally, they are referred to as cultivated varieties, or cultivars.
vibrate To rhythmically shake or to move continuously and rapidly back and forth.
viscosity The measure of a fluid’s resistance to stress. Viscosity corresponds to the idea of how “thick” a liquid is. Honey is very viscous, for instance, while water has relatively low viscosity.
wave A disturbance or variation that travels through space and matter in a regular, oscillating fashion.
wavelength The distance between one peak and the next in a series of waves, or the distance between one trough and the next. Visible light — which, like all electromagnetic radiation, travels in waves — includes wavelengths between about 380 nanometers (violet) and about 740 nanometers (red). Radiation with wavelengths shorter than visible light includes gamma rays, X-rays and ultraviolet light. Longer-wavelength radiation includes infrared light, microwaves and radio waves.
white dwarf A small, very dense star that is typically the size of a planet. It is what is left when a star with a mass about the same as our sun’s has exhausted its nuclear fuel of hydrogen, and collapsed.
Journal: H. Qian et al. Suppression of Kasha's rule as a mechanism for fluorescent molecular rotors and aggregation-induced emission. Nature Chemistry. Vol. 9, January 2017, p. 83. doi:10.1038/nchem.2612.
Journal: A. Holt and A. Sweeney. Open water camouflage via ‘leaky’ light guides in the midwater squid Galiteuthis. Journal of the Royal Society Interface. Vol. 13, June 8, 2016. doi: 10.1098/rsif.2016.0230.