Physical constant

Some things in our world never change.

Scientists call these things constants. We cannot explain them with rules. We must measure them to know them.

One constant is how fast light moves. This speed is very important. It helps us define how long a meter is.

Other constants help us measure weight. There are many of these special numbers. Some scientists say there are 19 of them.

These rules help us understand our whole world. They stay the same everywhere in space.

Some things in our world never change. Scientists call these things physical constants. A physical constant is a value that we cannot explain with a theory. We can only find its value by measuring it in an experiment.

There are many different constants. The speed of light in a vacuum is one famous example. We use its speed to define the meter. Another is the gravitational constant, which is written as G. There is also the Planck constant, written as h. These numbers help us understand how the universe works.

Scientists use these constants to set our units of measure. Since 2019, all units in the International System of Units use them. For example, the kilogram is now linked to these fixed rules. This means we do not need a single metal object to define weight.

Some constants are just numbers without a unit. These are called dimensionless constants. One example is the fine-structure constant. It describes how strong an electromagnetic force is. Some people think these numbers are very special. They believe the universe is fine-tuned so that life can exist.

A physical constant is a special number in our universe. These numbers tell us how nature behaves. Scientists cannot explain these values using a theory alone. Instead, they must use experiments to measure them. A constant is different from a math constant. A math constant is just a fixed number. A physical constant involves a real measurement of the world.

How these constants work can be seen through their different forms. Some constants have units, like length or time. For example, the speed of light is a distance divided by time. Other constants are dimensionless. This means they are just pure numbers without any units. A good example is the proton-to-electron mass ratio. These values act like the rules for how everything moves and interacts. They stay steady so that the laws of physics work the same way everywhere.

History shows us how our understanding of these values grows. At first, the speed of light was seen as just a property of light. Later, Maxwell's equations showed it was part of electromagnetism. Eventually, the theory of special relativity changed how we saw it. Now, the speed of light is so important it defines the meter. This shows how a constant can move from a simple idea to a fundamental rule.

There are many specific constants used in science today. The speed of light is written as c. The gravitational constant is written as G. The Planck constant is known as h. The elementary charge is written as e. Scientists use these to define our measurement systems. Since 2019, the International System of Units uses these constants. Even the kilogram is now defined using the Planck constant and other fixed values.

These constants connect to the very existence of life. Some scientists study if these numbers ever change over time. They look at distant galaxies to check for tiny changes. There is even a idea called the fine-tuned universe. This idea suggests that if these numbers were different, life could not exist. This could mean our universe is special. It might even be part of a larger multiverse.

A physical constant is a fundamental quantity in nature that cannot be explained by any existing theory. Because theories cannot predict these values, scientists must determine them through experimental measurement. These constants are distinct from mathematical constants, which are fixed numerical values that do not involve physical measurement. While a mathematical constant is an abstract concept, a physical constant describes a real property of the universe. They act as the essential parameters that define how physical laws function in our world.

Physical constants can take different dimensional forms depending on what they measure. Some constants are dimensional, meaning they are expressed using units like length, mass, or time. For example, the speed of light, denoted as c, has the dimension of length divided by time. Other constants are dimensionless, meaning they are pure numbers without any units. An example of this is the proton-to-electron mass ratio. Physicists often use the term "fundamental physical constant" to describe universal constants that have dimensions. However, some scientists prefer to reserve that term specifically for dimensionless universal constants, such as the fine-structure constant, which measures the strength of electromagnetic interactions.

Our understanding of these constants often evolves as scientific theories become more advanced. The speed of light provides a perfect example of this shifting perspective. Originally, c was viewed as a simple property of light itself. When Maxwell's equations were discovered and verified, the quantity was linked to the entire system of electromagnetism. Later, the theory of special relativity emerged, and the speed of light became understood as the basis of causality. This progression shows how a constant can move from a specific property to a universal rule. Today, the speed of light is so fundamental that it defines the international unit of length.

In the modern International System of Units (SI), constants are used to define our very measurements. Since a major revision in 2019, all SI units are defined by fixed natural phenomena. This includes three specific fundamental constants: the speed of light (c), the Planck constant (h), and the elementary charge (e). This change means that units like the kilogram are now expressed in terms of these constants. For instance, the kilogram can be written using the Planck constant and an experimentally measured constant called ΔνCs. This shift also means the international prototype of the kilogram is being retired as a physical object.

Scientists can also use these constants to create "natural units" for specific fields of study. By combining different dimensional universal constants, they can build measurement systems that fit a particular area of science. For example, Planck units are constructed using c, the gravitational constant (G), the reduced Planck constant (ħ), and the Boltzmann constant (kB). These are very useful for studying quantum gravity. In atomic physics, researchers use atomic units, which are built from ħ, the electron mass (me), the elementary charge (e), and 4πε0. These systems allow scientists to work with scales that are most convenient for their specific research.

The exact number of fundamental constants depends on which physical theory is considered most fundamental. Currently, scientists rely on general relativity for gravitation and the Standard Model for other interactions. Together, these theories account for 19 independent fundamental constants. However, this number can change depending on how a scientist chooses to categorize them. For example, one list identifies 22 constants for the Standard Model, including the gravitational constant G and various Yukawa couplings. If new physics is discovered, such as the mass of the neutrino, the number of required constants could increase.

Researchers also conduct experiments to test if these constants change over time or space. Paul Dirac speculated in 1937 that constants like G might change as the universe ages. To avoid issues with arbitrary unit systems, scientists focus on testing dimensionless quantities. For the fine-structure constant, researchers have set an upper bound of change at roughly 10⁻¹⁷ per year. For the gravitational constant, observations of type Ia supernovae suggest an upper bound of less than 10⁻¹⁰ per year over the last nine billion years. Studying these values helps scientists determine if the laws of physics are truly universal and unchanging.

Finally, the specific values of these constants lead to profound questions about our existence. Some physicists explore the idea of a "fine-tuned" universe. This concept suggests that if dimensionless constants had different values, intelligent life might never have emerged. This observation is linked to the anthropic principle, which states that our existence as observers requires the constants to be exactly what they are. Some interpret this as evidence of a divine creator, while others suggest we live in a multiverse. In a multiverse, different universes might exist with different constant values, and we simply inhabit one that allows for life.