The 5% Rule: How Physics Keeps Your Blood Flowing

Imagine a world where your blood is either thick enough to stop your heart or thin enough to fail at transporting oxygen. This is not a medical condition, but a theoretical reality that could have been the fate of our universe had its fundamental constants been slightly different. A new paper published in Science Advances by physicists at Queen Mary University of London suggests that the very existence of life hangs on a razor thin margin of physical stability. The findings challenge our assumption that the laws of physics are arbitrary, pointing instead to a precise calibration that allows fluids to behave in ways compatible with biology.

The study, led by Dr Kostya Trachenko, does not look at the formation of stars or planets. Instead, it focuses on the microscopic dance of molecules in liquid. It proposes that the values governing how our universe operates must fall within a strikingly narrow band to allow fluids to flow in ways that sustain biological processes. This is a deceptively simple question with vast implications: why are nature's basic parameters so precisely calibrated to permit living organisms? The answer lies in the delicate balance of viscosity, the measure of a fluid's resistance to movement, which is governed by the deepest parameters of nature.

The Viscosity Floor

In 2020, Dr Trachenko and his collaborators demonstrated that fundamental physical constants impose a floor on how freely any liquid can flow. This means viscosity cannot drop below a threshold set by the electron's mass, its electric charge, and the Planck constant. These quantities dictate everything from atomic behaviour to the stellar reactions that forge elements like carbon and oxygen. While scientists regard these values as universal and fixed, nobody can explain why they take the precise values they do. The new research builds on this foundation, exploring what this boundary means for the complex fluids that make up living organisms.

Cellular life hinges on the constant movement of molecules. Nutrients must travel to where they are needed, waste must be cleared, and chemical reactions must unfold in liquid surroundings. Viscosity governs all of it. Dr Trachenko notes that understanding how water flows in a cup turns out to be closely related to the grand challenge of figuring out fundamental constants. Life processes in and between living cells require motion, and it is viscosity that sets the properties of this motion. If the constants shift, the motion changes, and life as we know it ceases to function.

The Blood Test

The investigation did not stop with simple liquids. In January 2025, the team turned their attention to blood, publishing their findings in The European Physical Journal E. Blood is far more complex than water, comprising plasma, red blood cells, and platelets whose interactions affect how it flows. Red blood cells clump together at low speeds, increasing resistance, which adds another layer of complexity to the physical constraints. The study demonstrates that the values governing how our universe operates must fall within a strikingly narrow band to allow these complex fluids to flow in ways that sustain biological processes.

Dr Trachenko warns that the margin for error is razor thin. He states that the viscosity of our blood would become too thick or too thin for body functioning with only a few per cent change of some fundamental constants such as the Planck constant or electron charge. This is a profound statement about the fragility of our physical existence. It suggests that the universe is not just fine tuned for the existence of atoms, but for the specific type of fluid dynamics required for complex life. The precision required is such that a slight deviation would render the blood of any potential organism useless for its biological role.

A Narrow Band

The research poses a deceptively simple question with vast implications: why are nature's basic parameters so precisely calibrated to permit living organisms? The answer, according to the study, is that they are not just calibrated for existence, but for the specific type of existence that involves complex fluid dynamics. The values governing how our universe operates must fall within a strikingly narrow band to allow fluids to flow in ways that sustain biological processes. This is a new dimension to the fine tuning argument, moving beyond the formation of stars and planets to the microscopic level of cellular function.

Scientists regard these values as universal and fixed, yet nobody can explain why they take the precise values they do. The new research adds to the growing body of evidence that suggests the universe is remarkably hospitable to life, not by chance, but by design. The fact that a few per cent change in fundamental constants would render blood non functional is a testament to the precision of our physical reality. It is a reminder that life is not just a biological phenomenon, but a physical one, dependent on the precise balance of forces that govern the universe.

Implications for Life

The findings have significant implications for our understanding of the universe and our place in it. They suggest that the conditions necessary for life are not just rare, but incredibly specific. The fact that our universe supports life is not a fluke, but a result of the precise calibration of its fundamental constants. This is a powerful argument for the fine tuning of the universe, and it provides a new perspective on the question of why we exist. It is not just that the universe is old enough for life to arise, but that it is physically structured in a way that allows complex fluid dynamics, a prerequisite for cellular life.

Dr Trachenko's work adds a new layer to the debate about the fine tuning of the universe. It moves the discussion from the macro scale of stars and galaxies to the micro scale of molecules and cells. This shift in perspective is significant, as it highlights the intricate connection between the fundamental laws of physics and the biological processes that sustain life. It is a reminder that the universe is not just a backdrop for life, but an active participant in its creation and sustenance. The precision required for life to exist is a testament to the beauty and complexity of the physical world we inhabit.

The Fragility of Existence

The study also highlights the fragility of our existence. A few per cent change in fundamental constants would have profound consequences for the functioning of our bodies. This is a sobering thought, and it underscores the importance of understanding the physical laws that govern our universe. It is a reminder that life is not guaranteed, but a result of a delicate balance of forces. The fact that we exist is a testament to the precision of our physical reality, and it is a source of wonder and awe.

The research by Dr Trachenko and his team is a significant contribution to our understanding of the universe. It provides a new perspective on the fine tuning argument, and it highlights the intricate connection between physics and biology. It is a reminder that the universe is not just a collection of atoms and molecules, but a complex system governed by precise laws. The fact that life exists is not a fluke, but a result of the precise calibration of these laws. It is a testament to the beauty and complexity of the physical world we inhabit, and it is a source of inspiration for future research.