The first time I saw living cells emit light, I realized I was watching biology express itself in an entirely different way. Not through molecules, genes, or structures, but through a faint physical signal carrying unknown information about the state of life.

As a young scientist entering the field of biophysics, I had an ambitious goal: to detect a hidden pattern of communication in living systems that could expand our understanding of biology and potentially enable new medical technologies.

I was in graduate school when I first walked into a room surrounded by six inches of copper shielding, what scientists call a Faraday chamber. It was built to be as dark as possible. Once the door closed, the outside world disappeared. I could not see what was in front of me, not even my own hands.

Somewhere in that darkness was a dish of living skin cancer cells. Beside it was a photomultiplier tube, an instrument sensitive enough to detect light far too faint for the human eye to see. These instruments can pick up extremely weak light signals, the kind astronomers study from distant stars millions of light-years away.

My goal sounded deceptively simple to me, but I eventually found out it was technically challenging: measure the patterns of light emitted by living cells, and ask whether those patterns change when cells are healthy or cancerous. Essentially, can we create an optical signature of cancer?

But in this kind of experiment, every stray photon mattered. A tiny gap around a cable, a crack in the ceiling leaking light from the floor above, a warm electronic component, or even the detector itself can produce signals that overwhelm the light emitted by living cells. Much of the work was not finding the photons, but proving that they truly came from the life in the dish and nowhere else.

I started my measurements. Then the signal appeared.

The cells were flashing, releasing tiny bursts of light! Not a glow like a firefly, and not anything visible to me in the room, but measurable spikes of light activity that my instrument detected. When I finally analyzed the data, the light patterns from cancerous and noncancerous cells were different enough to suggest something amazing to me: living tissues may carry optical information we are only beginning to learn how to read.

I haven’t been able to stop thinking about what I saw more than 11 years ago.

In preparation for my PhD studies, I had dug deep into the early work of Alexander Gurwitsch, whose experiments nearly a century ago suggested that weak light from one onion root could stimulate cell division in a completely unconnected neighboring root. Gurwitsch called this “mitogenetic radiation.” Over the decades, a small but determined group of scientists continued to investigate and validate the idea that living cells do indeed emit light. Today, these faint light emissions are often discussed as biophotons or ultraweak photon emissions, and they remain one of the most fascinating and challenging open questions in biology.

The work of these scientists helped move the field from asking whether living cells emit light to the much harder questions we are asking today: Why does life emit light at all?

Are these photons simply byproducts of metabolism, like heat from an engine? Or are they useful signals that reveal stress, aging, disease, or repair? Or, in some cases, do cells use these faint light signals as part of the way they communicate?

The honest answer is that we do not yet know, and it is exactly the place where I am most excited to help move it forward.

Biophoton research has never had a smooth trajectory. Since Gurwitsch’s early observations, the field has started and stopped more times than most scientists care to count. It has moved through cycles of excitement and skepticism, slowed by difficult measurements, controversial findings, and instruments that were often not sensitive enough to settle the biggest questions. But that is changing. The mystery of how these photons are produced, and what they might do, is now gathering renewed interest with better photon-sensing tools, more sophisticated signal-analysis methods, and new interdisciplinary perspectives from biophysics, quantum biology, and neuroscience. Together, they are giving the field new life.

As extremely faint particles of light naturally emitted by living cells, biophotons are physical products of normal chemical reactions, especially those involving oxygen, metabolism, and mitochondria, the tiny structures in our cells that help transform energy.

But not all faint biological light comes from the same source. One important challenge in the field is distinguishing true endogenous biophoton production—where the biophotons originate within an organism—from delayed luminescence, where biological material absorbs external light and re-emits some of that energy later—like the light skin releases after it is exposed to sunlight. This distinction is important because the main question is: what kind of light does life emit, and under what conditions? And just as importantly: do those photons carry any biological meaning?

Unlike the relatively intense glow of a firefly, biophotons are too weak to be reliably perceived by the human eye. They are often measured at roughly 10 to 1,000 photons per second per square centimeter from biological tissue. Their wavelengths commonly fall in the near-ultraviolet to visible range, about 200 to 800 nanometers, and researchers are interested in them because their intensity and wavelength can change with stress, aging, oxidation, growth, cancer state, and cellular damage.

Crucially, biophoton emission is not a quirk of human biology. It is a property of life itself, not a feature unique to any one organism. Across the tree of life and at every scale, living systems emit light: Bacteria produce measurable photon emissions tied to their metabolic activity. Plants not only absorb sunlight for photosynthesis; they emit light of their own, with emission patterns that shift depending on their physiological state.

Perhaps most strikingly, when an animal dies, biophoton emission decays alongside it. The light fades as metabolism starts to shut down, which means these emissions are genuinely coupled to the energetic processes that define being alive. Biophotons are not an artifact or a coincidence. They are a signature of life in motion.

But a signature is not necessarily a message.

With the development of better light-sensing and signal-analyzing technologies, we now have a few answers about what these signals are. But we still don’t fully understand their functional role in the body.

As my career progressed, my own lab has focused on whether these faint emissions can help us read biological states that are otherwise difficult to detect. We have used biophotons to screen human cells, tissues, and whole animals for signatures of cancer. We have also measured ultraweak light around the human head and found that it can correlate with electrical brain activity and cognitive tasks, such as attending to sounds.

This does not mean the brain is “thinking with light” in any simple way. It indicates a complex behavior we need to probe carefully. To me, this is a more exciting implication: living tissues may have an optical layer of information that reflects their energetic and physiological state.

If we can learn to read that layer, medicine could gain a new kind of sensing technology for diagnostic or health tracking.

Imagine tracking early shifts in metabolism before tissue damage becomes obvious, detecting cancer-promoting changes in the cellular environment before conventional imaging can reveal a tumor, or reading subtle brain states not only through electricity, but also through faint light linked to neural activity and energy use.

This future is not here yet. The signals are too weak and are still difficult to measure through the wet, noisy, complex system that is our body. These signals are also vulnerable to contamination from room light, heat, instrument noise, and biological variability. Progress will require better sensors, stronger controls, careful replication, and experiments that can separate correlation from causation—but we’re getting there.

The hardest question—but one of the most exciting—is whether these endogenous biophotons do or mean anything.

Some researchers have put forward the idea that biophotons are not the result of active signaling in cells, but just simple byproducts of metabolism. In other words, while these signals might predict physiological states, they don’t cause them. We know, for example, that light from the brains of aging mice shifts toward the blue end of the visible light spectrum, characterized by shorter, more energetic wavelengths. Is this solely a marker of an aging brain? Or do those light emissions cause the brain to function differently, in the same way that chemicals might?

That’s still an open question.

But even if biophotons are byproducts of chemical reactions, they still have the potential to serve as meaningful signals. We see a parallel in chemical signaling. When cells make energy, they break down molecules such as glucose through a cascade of reactions that produces ATP, heat, and a range of smaller molecules. Some of these products are simply cleared or recycled. Others become signals that tell the cell about its metabolic state. Biophotons may follow a similar logic in a biophysical model of life, where they may begin as byproducts of metabolic energy, but still become useful physical signals that nearby cells can detect and respond to.

That possibility becomes easier to take seriously when we remember that light already interacts with the body in ways that go far beyond vision. In photobiomodulation, researchers—and increasingly clinicians—use light to change physiology. Researchers have found that UV, infrared, and visible wavelengths of light can affect human tissues ranging from skin to brain, and even bone, in distinct ways. Red and near-infrared light, for example, can influence cellular metabolism and has been studied for its effects on inflammation, wound repair, and brain function.

If externally applied light can alter biology, it becomes reasonable to ask whether the light produced naturally inside tissues might also have biological consequences. Still, it’s unclear whether cellular biophotons, which are comparatively much weaker than the light used in photobiomodulation, could have similar effects on neighboring cells. Even less certain is the idea that these signals are regulated or organized in a goal-oriented or purposefully instructive way at the cellular scale.

This is the ultimate question. If cells are communicating by way of hidden optical networks within our tissues, then decoding those messages will unlock a rich tapestry of biological information and inspire a new wave of technologies for reading and supporting human health.

For example, we might be able to monitor the optical environment of tissues and map early signs of trouble, including cancer-promoting changes in the microenvironment. We could catch the disease before it becomes visible by conventional methods—at which point, it may be too late to prevent suffering.

This possibility becomes even more compelling when we remember what light can do. It can vary in intensity, wavelength, timing, polarization, and direction, giving it multiple physical dimensions through which information could, in principle, be encoded and decoded by our cells. If living systems produce and respond to these patterns in a regulated way, then the optical microenvironment of cells matters.

This would also change how we think about the light we’re exposed to every day. If cells are sensitive to light as a biological signal, then sunlight, artificial lighting, and the screens we carry in our pockets may not be simply the background of our lives, but a part of the physical environment our tissues are constantly interpreting as meaningful information.

Even if biophotons are only metabolic exhaust, they may still serve as powerful biomarkers. Just as a car’s exhaust can tell a mechanic whether the engine is burning fuel properly, overheating, or under stress, faint cellular light may one day help us read the state of the body, from metabolism to subtle brain-state changes.

But if biophotons are also signals—even some of the time—then biology contains a communication channel we have barely learned to detect and understand. That is the question that keeps my team going back into the lab: Are these photons only the glow of metabolism, or are cells using them to carry information?

The faint flashes coming from our cells may not answer biology’s deepest questions on their own. But they may help us ask better ones. And sometimes, in science, a new field begins not with something blinding, but with a signal so faint that almost everyone missed it.