Light: A Quantum Conundrum

Look!

Up in the sky!    

It’s a wave! It’s a particle!

Actually, it’s a quantum of electromagnetic energy that exhibits wave-particle duality that was emitted by an electron that had been disturbed by the hot and high-pressure fusion of a hydrogen atoms into helium atoms inside our sun.

Is light a wave that varies in frequency from long radio waves to extremely short-wave gamma rays? Or is light a beam of particles we call photons that enter the stomata of green leaves and help power the production of carbohydrates that help the plant grow and stay healthy.

The controversy has been debated for centuries, and is not fully settled today. The current general consensus is that light acts as both waves and particles depending how you detect it. The idea of light being two different things at the same time is counterintuitive, but much of quantum mechanics is that way. Activity at the atomic and subatomic levels is often confusing and hard to understand.  As physicist Richard Feynman put it, “If you think you understand quantum mechanics, you don’t understand quantum mechanics.”

Although we don’t know how light can be in two quantum states at once, we do have some knowledge of where it comes from and how it gets to us.

Our sun has been producing heat and electromagnetic energy in its core for around 4.6 billion years, and everything on Earth, which is a bit younger at 4.5 billion years, has evolved with this energy and learned to make use of it. It is believed that even the inorganic atoms that combined to make the first organic molecules used sunlight to help them produce the amino acids and proteins. The energy generated by our sun is our lifeline. Without it, Earth would be a cold, lifeless and desolate place.

Like most stars, our sun is made of mostly hydrogen atoms. The sun’s gravity is so intense inside the sun’s core that when two hydrogen atoms, each with one proton and one electron circling it, collide they fuse together and create a new heavier atom of helium. This atomic collision of two hydrogen atoms disturbs the electrons in them which causes them to leap from a low energy orbit to a higher level and absorb a speck of energy. Then when the electrons calm down and return to their original low energy level in less than a split second, that extra absorbed energy is emitted from the electrons as packets of energy that are at the same time both high energy electromagnetic waves and gamma ray photons. The fused hydrogen atoms then become helium atoms with two protons, two electrons, and at that point the new atom is also joined by two neutrons.

Trillions of hydrogen atoms fuse every second sending clouds of high-energy, high-frequency, massless gamma ray bursts bouncing around in the middle of the sun. The packets of energy are tiny, somewhere between the size of an electron and a neutrino, and the sun is huge, so sometimes the energy packet will bounce around in the crowded sun for years haphazardly running into hydrogen protons, helium neutrons, and electrons before it reaches the surface of the sun. Each one of these atomic obstacles pulls out a small amount of the packet’s energy so that by the time it reaches the sun’s surface, it may have been reduced in frequency and electromagnetic power. On the other hand, if it has not had many encounters with other particles to reduce its strength and frequency, it could still be a strong, high frequency gamma ray or X-ray.

Then 8 minutes and 19 seconds later, a small percentage of these packets of energy come into Earth’s atmosphere. Those with high frequency such as gamma and X-rays collide with atoms and molecules in the atmosphere and are further reduced in radiative power, which is good since too much gamma and X-ray exposure is dangerous to many forms of life, including humans. Lower frequency packets, from ultraviolet to radio, make their way to Earth’s surface.

Fortunately, the energy packets in the middle of the electromagnetic spectrum with frequencies from 32 to 15 millionths of an inch long were just right for the evolution of animal eyes. Today you and I see this band of frequencies as our old buddy Roy G Biv: red, orange, yellow, green, blue, indigo, and violet.

Among the first living things to make use of the sun’s packets of energy on our planet were cyanobacteria, a type of bacteria that about 3 billion years ago evolved to contain enough chlorophyll to be able to combine carbon dioxide from Earth’s atmosphere, light from the sun, and water in a process called photosynthesis to make its own food. These photosynthetic bacteria started an evolutionary revolution.

A few million years later, green plants, following the lead of cyanobacteria, learned to use photosynthesis to make the glucose they needed to grow and produce seeds to carry on their species. Today photosynthetic plants are the backbone of our food chain. All humans survive on plants or animals that eat plants.

With photosynthesis plants could take care of themselves. Animals, however, lacked the ability to make their own food and had to hunt for it. In primitive animals, hunting consisted of little more than blindly roaming around until they happened to bounce into something edible. It was not a very efficient system.

It seems that natural selection does not tolerate inefficient systems for long. So, around a billion years ago, many animals developed a light sensing protein that enabled them to tell dark from light. This system of light detection was not great at finding food, but no doubt played a part in helping the animal from becoming food for another creature. If a small animal sensed something near it that was big enough to blot out the light coming into its photoreceptor protein, it could react by scurrying away to a place with more light. In this way, photoreceptor proteins were important in perpetuating a number of species that might have gone extinct due to predation.

Thus, in the relentless march of evolution and natural selection, animals with functioning photoreceptor cells survived when others did not. After many generations, photoreception improved until around 541 million years ago, an ocean dwelling creature called a trilobite developed a compound eye that enabled it to actually see what was out there in front of it.

Since then, many different types of eyes have evolved. Whether, compound – as in insects, or camera type eyes – as in reptiles and mammals, or mere eyespots as in jellyfish, most of them work by taking in light reflected off objects in a creature’s environment and then sending the light to the creature’s brain where, through a complex electro-chemical process, it is turned into an image.

By the time humans began evolving, about 6 million years ago, the camera eye in primates was fully developed. The human eye lets in light through a lens then sends an electro-chemical signal into the brain. The signal first goes through the hippocampus and then to the occipital lobe where an image is formed. The hippocampus helps us remember the images we had seen. We could see predators, prey, and each other and remember what each one looked like.

Early in human history, light was taken for granted. It came from the sun in the day and the moon at night. As the years passed and we evolved to become curious about how things worked in our environment, some people began to suspect that light might be a moving entity that had a speed as it moved from the sun to our eyes, but it was too complex a subject to undertake.

Some progress was made, however in understanding light and how it influenced eyesight. For example, Arab mathematician Ibn Al-Haytham (also called Alhazen) 965 – 1040, was one of the first to determine that vision occurs when light reflects from an object into one’s eyes and that the image is actually made in the brain. For his ground-breaking work, he is known today as the ‘father of modern optics”.

Some 600 years after Alhazen, Galileo Galilei, in the early 1600s, attempted to determine that light actually traveled and had a speed, and was one of the first to do experiments with it. In one of these experiments, he and a helper stood on hilltops about a mile apart holding lanterns. The idea was that Galileo was to quickly uncover his lantern, then measure the time it took for his helper to see the light and then uncover his lantern. The hilltops were too close together, however, and Galileo came to no definite conclusion about the time lapse. But at least he had set a precedent that got other scientists interested in the problem.

A few decades later in 1676, Ole Romer, using precise observations of the movement of Jupiter’s moon Io, determined the speed of light to be 124,000 miles per second. This was a good estimate considering the level of technology of his day.

In 1728, in an experiment that involved observing the changing positions of the stars as the Earth traveled around the sun, astronomer James Bradley put the speed of light at 185,000 miles per second, very close to the light speed we recognize today. Over the years, several scientists, such as Leon Foucault, Albert Michaelson, and others worked on figuring the exact speed of light, each one getting a little closer to the actual speed that was determined by an international commission in 1983. Today the speed of light has been determined to be 299,792,458 meters per second or 186,282.4 miles per second

For years most people considered light a wave of energy emitted by the sun. In the early 1600s, Italian priest and mathematician Francesco Grimaldi had done experiments with glass prisms and noticed that sunlight was made of many colors. From his experiments Grimaldi decided that light was a wave, and coined the word diffraction to describe the bending effect the prism had on sunlight.

A few years later, around 1670, Isaac Newton became interested in light and did experiments with prisms similar to what Grimaldi had done. In his experiment he darkened his room except for a small hole which let in sunlight on to a prism. When the light went through the prism, it was broken up into the seven colors: red, orange, yellow, green, blue, indigo, and violet. He then set up another prism, the second one inverted, and shone light through both at the same time. The colored rays of light from the first prism became white again when they went through the second prism proving to him that sunlight is made up of many colors.

His results led him to believe that light was made of particles instead of being a continuous wave. He named the array of colors from his prisms, spectrum, the Latin word for spectre or apparition. The colors he saw we today call the visible spectrum. Each color has its own wavelength from red with a wavelength 620 nanometers wide to violet that is only 380 nanometers wide. These are extremely small distances, but the rods, cones, and optic nerves in our eyes respond to them and send the red, blue, and green  frequencies to the brain which pulls out from those three colors, all of the other hues we see.

Newton’s work further perpetuated the wave vs. particle controversy that had been going on since the days of Democritus who in the 5^th century BC was one of the first people to theorize that everything was made of atoms, including light. For years the pendulum swung back and forth between those who believed that light was a wave and those who saw it as a beam of particles.

A notable experiment concerning light was done by Thomas Young around 1802. He passed light through a narrow slit and got a wave pattern on the barrier on the other side. He then passed light through two slits at the same time and got waves interfering with each other as if you throw two rocks into a pool of water and the flowing waves increase or decrease each other. He decided that the wave patterns confirmed that light is a wave. Young’s ‘double slit experiment’ is still being used in light research today.

Then in 1900, Max Planck came up with the idea that light is made of tiny packets of energy he called quanta — plural, or quantum — singular. At first the theory was not taken seriously. But in 1905 Albert Einstein worked on what came to be known as the photoelectric effect and showed that a beam of light, which he called Lichtquant, would cause a metal plate to emit electrons if the frequency of the light was high enough. His experiments showed that light acts not only as a wave, but as a beam of particles as well. Einstein’s work confirmed Planck’s quantum theory and prompted Einstein to conclude that, “there are therefore now two theories of light, both indispensable…without any logical connection”.

After Einstein’s experiments showing that light consists of packets or quanta of energy, the idea of light being made of particles caught on. In 1916, physicist Leonard Troland gave the particles the name photons, which he described as ‘units of illumination stimulating the eye’. The name stuck, and that is what we call particles of light today.

Max Planck went on the receive the Nobel Prize in physics in 1918 for his quantum theory and Albert Einstein won the award in 1921 for discovering the photoelectric effect. To express the duel nature of light being both waves and particles, in 1928 physicist Arthur Eddington coined the term ‘wavicles’.

Planck’s and Einstein’s work have led to numerous discoveries. For example, work done by Arthur H. Compton, in an experiment similar to Einstein’s photoelectric effect, concerns the way energy is absorbed by matter. He demonstrated that X-ray quanta lost some energy and frequency when they hit electrons, leading to the concept that light acts as both waves and particles when it interacts with matter. For this discovery Compton was awarded the Nobel Prize in physics in 1927. This phenomenon, called the Compton Effect may explain what happens inside a star as packets of energy emitted by electrons are reduced in energy and frequency as they encounter protons, neutrons, and electrons before they reach the surface of the star.

These and other experiments with light have led to the invention of the maser –microwave amplification by stimulated emission of radiation in 1953 by Charles Townes, James Gordon, and Herbert Zeiger. This was followed by the development of the laser –light amplification by stimulated emission by Theodore Maiman in 1960. Both masers and lasers are today used extensively in industry and science.

Perhaps one of the most sought-after uses of light at this time is in the development of quantum computers in which photons or electrons called qubits store and transfer information. Scientists in laboratories all over the world are making progress on this endeavor and working prototypes are already being developed. Quantum computers are much faster than digital computers and the data is more difficult to hack or alter.

So, after 3,000 years of speculation, the wave-particle duality of light conundrum has been solved: it is both. Coming out from the surface of the sun are packets of energy that at the same time are both tiny particles as well as waves of electromagnetic energy. By the time they get to us, these wavicles cover the full spectrum of electromagnetic radiation from low frequency radio waves with wavelengths that could be several meters wide to gamma rays with wavelengths of less than one nanometer. Radio waves, microwaves, infrared, the visible spectrum, ultraviolet, X-rays, and gamma rays are all forms of electromagnetic radiation that life on Earth from bacteria, to plants, to animals and Homo sapiens have evolved with and learned to use.

Photons are the superheroes of the universe. Although tiny, massless, and with no electrical charge these quanta of light pack enough pure energy to leave all our Earthly heroes literally and figuratively in the cosmic dust. In fact, if it were not for light waves and photons, there would be no heroes at all… no comic book heroes, military heroes, nor even those special unsung heroes — the caring people who try to be kind and helpful to everyone every day.

For if no light, no photosynthesis; if no photosynthesis, no plants; if no plants, no food; if no food, no life. It all ties together very nicely, and we humans have taken advantage of the process to become Earth’s top species. In a marvelous way our planet has put together the things available to it – light, water, atmosphere, and soil – and created a beautiful place to live.

Thus, if our fear of the unknown still compels us to revere entities beyond ourselves, it is light and the myriad other phenomena of nature that make up stars, galaxies, and our universe, and all of the elements we are made of that deserve our revering. Those forces of nature created us and sustain us.

Ted McCormack