A constant in science is a quantity whose value does not change under specified conditions, serving as a fixed reference point that allows researchers to describe, predict, and compare natural phenomena. Some constants, like the speed of light in a vacuum, are treated as universal truths baked into the fabric of reality. Others, like the gravitational constant, are surprisingly hard to pin down with precision. The idea sounds simple, but the deeper you look, the more you realize that “constant” covers a surprisingly varied family of numbers, and the question of whether any of them truly never change is still an active area of research.
Not All Constants Are the Same Kind of Thing
When people hear “constant in science,” they usually picture a single type: a number that nature insists upon, like the speed of light. But physicists actually recognize several distinct categories, and lumping them together can create confusion. One useful distinction is between dimensional constants and dimensionless constants. A dimensional constant has units attached to it. The speed of light, for instance, is about 300,000 kilometers per second. Change your unit system and the number changes, even though the underlying physical reality hasn’t. The Planck constant and the gravitational constant are dimensional too. They depend on the measurement system you happen to be using.
Dimensionless constants are different. These are pure numbers with no units at all, which makes them arguably more fundamental. The most famous is the fine-structure constant, usually written as alpha, which characterizes the strength of the electromagnetic force. Its value is roughly 1/137, and that number doesn’t depend on whether you measure in meters or miles, kilograms or pounds. Because dimensionless constants are independent of any human-chosen unit system, many physicists consider them the truest window into the underlying structure of nature.1International Journal of Informational Integrative Medicine. On the Principle of Universality of physical laws: Does God use constants?
Beyond that split, there are also what you might call conversion factors. The speed of light, for example, now functions partly as a conversion factor between units of distance and units of time. And then there are constants that describe specific particles or forces, like the mass of an electron or the charge of a proton. A paper in The European Physical Journal Special Topics frames fundamental constants as “a rich blend” that includes conversion factors, properties of specific particles, universal features of empty space, and genuine dimensionless constants like the fine-structure constant.2SpringerLink / The European Physical Journal Special Topics. On the nature of fundamental physical constants The upshot is that calling something “a constant” tells you it doesn’t change, but it doesn’t tell you why it doesn’t change or what role it plays in the architecture of physics.
How Constants Anchor Our Measurement Systems
For most of modern history, the basic units of measurement were tied to physical objects. The kilogram, for example, was defined by a platinum-iridium cylinder stored in a vault near Paris. That arrangement had an obvious weakness: the artifact could gain or lose atoms over time, subtly shifting the definition of the kilogram for the entire world. In 2019, the International System of Units (the SI) was overhauled so that all seven base units are now defined by fixing the numerical values of seven fundamental constants.3PubMed Central. How to Define the Units of the Revised SI Starting from Seven Constants with Fixed Numerical Values
The kilogram is a good example of how this works in practice. Instead of relying on a metal cylinder, the kilogram is now defined through a fixed value of the Planck constant. A device called a Kibble balance uses electromagnetic force to compare mechanical power to electrical power, allowing scientists to realize the kilogram from the Planck constant itself.4Comptes Rendus Physique. The Kibble balance and the kilogram The Bureau International des Poids et Mesures (BIPM) has carried out mass measurements of a one-kilogram artifact in terms of the Planck constant using this approach.5Metrologia. The BIPM Kibble balance for realizing the kilogram definition
The meter follows a similar logic: it’s defined by fixing the speed of light. The second is defined through a specific frequency of radiation emitted by cesium atoms. By anchoring units to constants rather than objects, the system becomes reproducible anywhere. A lab in Tokyo and a lab in São Paulo can independently realize the kilogram without ever consulting the same artifact. The constants haven’t changed, but they’ve been promoted from things we measure to things we measure with.
Who Decides the “Official” Values?
The task of cataloging the best-known values of fundamental constants falls to the Committee on Data of the International Science Council, known as CODATA. Every few years, the CODATA Task Group on Fundamental Constants collects all available experimental and theoretical data, runs a massive statistical adjustment, and publishes a self-consistent set of recommended values. The 2018 adjustment, for instance, incorporated all data available through the end of that year.6PubMed Central. CODATA recommended values of the fundamental physical constants: 2018 These recommended values serve as the international reference that scientists, engineers, and standards bodies rely on.
The process has been going on for decades. The 1998 CODATA adjustment replaced a set of values that had stood since 1986, incorporating over a decade of new measurements and theoretical refinements.7Journal of Physical and Chemical Reference Data. CODATA Recommended Values of the Fundamental Physical Constants: 1998 Each new edition typically narrows the uncertainty around most constants, though occasionally a new measurement technique reveals that a previous value was slightly off. The constants themselves aren’t changing; our knowledge of them is getting sharper.
The Gravitational Constant and the Limits of Precision
Not every constant is known to the same level of precision, and the gravitational constant, usually written as G, is the most stubborn holdout. Despite being one of the first constants ever identified, G still carries the largest relative uncertainty of any fundamental constant. Different laboratory experiments, using different methods, have produced values that don’t quite agree with each other.8Classical and Quantum Gravity. Measurement of the Newtonian constant of gravitation by precision displacement sensors
The reason is that gravity is extraordinarily weak compared to the other fundamental forces. To measure G in a lab, you need to detect the tiny gravitational pull between human-scale masses, and that signal is easily swamped by vibrations, temperature fluctuations, and other noise. Improving G requires advances in force-calibration technology. By contrast, electromagnetic constants can be measured with exquisite precision using quantum phenomena like the Josephson effect and the quantum Hall effect. The result is a strange situation: we can predict the behavior of electrons to twelve decimal places, but we know the strength of gravity to only about five.
Are the Constants Actually Constant?
This is the question that keeps physicists up at night. We call them constants, but we’ve only been measuring them for a few centuries at most. The universe is about 13.8 billion years old. Could the constants have been different in the distant past, or could they vary from place to place?
The best-tested candidate for variation is the fine-structure constant, alpha. Because alpha governs how atoms absorb and emit light, astronomers can look at light from extremely distant quasars and check whether the atomic absorption patterns match what we see in laboratories on Earth. If alpha were different billions of years ago, the patterns would shift in characteristic ways. A study using the Very Large Telescope in Chile examined a quasar whose light left when the universe was only about 800 million years old, making it the most distant direct measurement of alpha to date. The result was consistent with no change over time, with the deviation from the modern terrestrial value being statistically indistinguishable from zero.9PubMed Central. Four direct measurements of the fine-structure constant 13 billion years ago
That said, when the same researchers combined their measurements with existing data from other directions in the sky, they found a hint of spatial variation, preferred over a no-variation model at a statistically suggestive level.9PubMed Central. Four direct measurements of the fine-structure constant 13 billion years ago This doesn’t mean alpha definitely varies across the cosmos. The analysis depends on atomic calculations that are themselves complex and uncertain.10Canadian Journal of Physics. Atomic calculations and search for variation of the fine-structure constant in quasar absorption spectra But it does mean the question isn’t settled. “Constant” may be our best current description, not an eternal guarantee.
The Hubble Tension and Constants That Disagree with Themselves
Sometimes the problem isn’t that a constant might change over time but that different ways of measuring it give different answers right now. The Hubble constant, which describes how fast the universe is expanding, is the most famous example. Measurements based on the cosmic microwave background (the afterglow of the Big Bang) give one value, while measurements using nearby supernovae and other distance markers give a somewhat higher one. This disagreement, known as the Hubble tension, has persisted for years.
One proposed explanation was that our local patch of the universe happens to be unusually dense or sparse, which could skew the nearby measurements. But research using the distance-ladder method has found that local density contrasts are too small to account for the discrepancy. The distance-ladder measurement doesn’t appear to be affected by local structure, and the Hubble constant can in principle be measured to better than one percent precision using this approach.11The Astrophysical Journal. The Local Perspective on the Hubble Tension: Local Structure Does Not Impact Measurement of the Hubble Constant Whether the tension points to new physics or to subtle measurement errors is still debated, but it illustrates a broader point: a value can be called a constant and still be genuinely difficult to nail down.
When “Constant” Is a Misnomer
Some quantities carry the word “constant” in their name for historical reasons, even though they aren’t truly fixed. The solar constant is a classic example. It refers to the total solar irradiance, the amount of energy the Sun delivers per unit area at Earth’s distance. For a long time, scientists assumed this value was essentially fixed. But satellite measurements beginning in 1978 showed that it actually fluctuates. Over minutes, solar convection and oscillations cause tiny variations at about the 0.01% level. Over days to weeks, sunspots and bright regions called faculae push the irradiance up and down by roughly 0.1%. And over the roughly 11-year solar cycle, the Sun is about 0.1% brighter at solar maximum than at solar minimum.12Journal of Space Weather and Space Climate. Magnitudes and timescales of total solar irradiance variability
These variations are tiny in percentage terms, but they matter for climate science. Early recognition that the “solar constant” varies on timescales from minutes to decades came from instruments like the ACRIM experiment aboard the Solar Maximum Mission satellite.13Atmospheric Research. Solar irradiance variability Today, many scientists prefer the term “total solar irradiance” precisely to avoid the misleading implication that it never changes. The name “solar constant” persists mostly out of habit.
Constants Outside Physics
The concept of a constant extends well beyond fundamental physics. In biology, one of the most striking examples is Kleiber’s Law, which describes the relationship between an organism’s body mass and its metabolic rate. In the 1930s, the Swiss biologist Max Kleiber showed that the metabolic rate of animals scales as roughly the three-quarter power of body mass. This relationship holds across an astonishing range of organisms, from bacteria to elephants, and has even been observed in some algae and plants.14PubMed Central. Kleiber’s Law: How the Fire of Life ignited debate, fueled theory, and neglected plants as model organisms
The three-quarter exponent has been confirmed even at the level of individual organisms. In studies of planarians (flatworms that can grow and shrink dramatically), the scaling exponent was found to be 0.75, exactly matching Kleiber’s interspecies value. The fact that the same exponent appears within a single species and across species suggests a deep underlying principle at work.15eLife. Body size-dependent energy storage causes Kleiber’s law scaling of the metabolic rate in planarians That said, the exponent isn’t perfectly universal. In fish, for example, the intraspecific scaling exponent varies between species and appears to depend on factors like red blood cell size, which affects how efficiently cells exchange gases.16PubMed. Intraspecific metabolic scaling exponent depends on red blood cell size in fishes
Kleiber’s Law is a good illustration of how biological “constants” differ from physical ones. The three-quarter exponent is an empirical regularity, not a value derived from first principles. It holds remarkably well on average but allows for variation in individual cases. In quantitative linguistics, similar power-law regularities appear: Zipf’s Law describes how word frequencies in any large body of text follow a predictable distribution, with the most common word appearing roughly twice as often as the second most common, three times as often as the third, and so on. These statistical constants are real and useful, but they’re patterns that emerge from complex systems rather than bedrock rules of the universe.
Constants Set Fundamental Limits on Technology
Some constants don’t just describe nature passively; they impose hard limits on what technology can achieve. A striking example comes from information theory. In the early 1960s, the physicist Rolf Landauer argued that erasing a single bit of information must release a minimum amount of heat, set by the temperature of the environment and the Boltzmann constant. This is now known as the Landauer limit, and it sets a floor on the energy consumption of any irreversible computation.17PubMed Central. Landauer Bound in the Context of Minimal Physical Principles: Meaning, Experimental Verification, Controversies and Perspectives
For decades, this was a theoretical curiosity since real computers dissipate vastly more energy per operation than the Landauer limit would require. But as transistors have shrunk and energy efficiency has improved, the limit has become increasingly relevant. Simulations of nanomagnetic systems have shown that the energy dissipated during bit erasure does approach the Landauer limit with high precision when operations are performed slowly enough.18PubMed. Exploring the thermodynamic limits of computation in integrated systems: magnetic memory, nanomagnetic logic, and the Landauer limit The Boltzmann constant, in this context, isn’t just a number in a textbook. It’s the reason why computers generate heat, and it places an absolute boundary on how efficient they can ever become.
Further theoretical work has refined the picture. The fundamental lower bounds on energy cost apply not just to erasure but to measurement as well, though the details depend on the symmetry of the memory being erased.19PubMed. Minimal energy cost for thermodynamic information processing: measurement and information erasure For engineers designing next-generation processors, these aren’t abstract concerns. They define the ultimate wall that miniaturization and optimization will eventually hit.
Why the Specific Values Matter So Much
One of the most debated topics in modern physics is why the constants have the specific values they do. It turns out that many of them appear to be finely tuned for the existence of complex chemistry, long-lived stars, and ultimately life. If the cosmological constant were much larger, the universe would have expanded too rapidly for galaxies to form. If the strong nuclear force were slightly weaker, atomic nuclei heavier than hydrogen wouldn’t hold together. The fact that several constants, including the cosmological constant, the Higgs mass, the electromagnetic coupling strength, and the proton-to-electron mass ratio, all fall within the narrow ranges that permit a universe like ours has generated one of the most contested debates in the foundations of physics since the 1970s.20Cosmological and Astrobiological Review: Journal for the Study of the Universe, Life and the Natural Sciences. FINE-TUNING OF FUNDAMENTAL CONSTANTS AND THE ANTHROPIC PRINCIPLE: METAPHYSICAL IMPLICATIONS AND A CRITIQUE OF “CAUSAL EXPLANATION THROUGH SELECTION”
Some physicists invoke the multiverse hypothesis: if an enormous number of universes exist, each with different constant values, we naturally find ourselves in one of the rare configurations that permits observers. Others argue that a deeper, yet-undiscovered theory will eventually explain why the constants must take the values they do. Statistical analyses have found that the degree of fine-tuning varies depending on which constant you examine. The ratio of the gravitational constant to the square of the Hubble constant, for instance, appears finely tuned in some models, while the amplitude of primordial density fluctuations does not.21Journal of Cosmology and Astroparticle Physics. Is cosmological tuning fine or coarse? The fine-tuning question sits at the intersection of physics, philosophy, and cosmology, and nobody has a fully satisfying answer yet.
The Push for Even Sharper Definitions
Even with the 2019 SI redefinition anchoring units to constants, the quest for better precision continues. The second, currently defined by a microwave transition in cesium atoms, is the weakest link. Optical atomic clocks, which use transitions at much higher frequencies, have already surpassed the cesium standard in both accuracy and stability, reaching estimated uncertainties in the range of one part in a billion billion. Work is underway toward a future redefinition of the second based on one of these optical transitions, supported by advances in ultra-stable lasers and new methods for comparing clocks at distant locations using optical fibers and transportable standards.22Comptes Rendus Physique. Towards a redefinition of the second based on optical atomic clocks
A more precise second would ripple through the entire system of units, since many other measurements ultimately depend on accurate timekeeping. It would also improve tests of whether constants truly are constant: if your clock is precise enough, you can detect smaller drifts in the quantities it depends on. In that sense, the science of constants is self-reinforcing. Better measurements of constants lead to better instruments, which lead to even better measurements, which occasionally reveal that what we thought we knew was slightly off. The constants themselves may not change, but our relationship with them keeps evolving.