Physics Reference Studio
The essential handbook for every physics student. Access high-precision constants, browse key formulas, and decode scientific symbols.
Universal Constants
Speed of Light in Vacuum
Planck Constant
Gravitational Constant
Elementary Charge
Boltzmann Constant
Avogadro Constant
Gas Constant
Stefan-Boltzmann Constant
Permittivity of Free Space
Permeability of Free Space
Physico-Chemical Constants
Mass of Electron
Mass of Proton
Mass of Neutron
Atomic Mass Constant
Rydberg Constant
Bohr Radius
Standard Gravity
Standard Atmosphere
Speed of Sound (Air, 20°C)
Water Density (4°C)
Astronomical Constants
Astronomical Unit
Light Year
Parsec
Solar Mass
Solar Radius
Solar Luminosity
Earth Mass
Earth Radius
Moon Mass
Moon Distance
The Building Blocks of Reality
Physical constants are the "magic numbers" of our universe. They determine everything from the size of atoms to the color of the sun. Unlike variables (like speed or temperature) which change, constants are believed to be fixed primarily by the laws of nature.
Why Standards Matter
In 2019, the scientific community redefined the SI system. Instead of basing units on physical artifacts (like a metal cylinder for the kilogram), we now base them on fundamental constants like the Planck constant (h) and the speed of light (c). This means our measurement system is now truly universal—it would work the same way for an alien civilization as it does for us.
Understanding Uncertainty
Not all constants are known perfectly. For example, the Gravitational Constant (G) is notoriously difficult to measure because gravity is so weak.
That's why you'll see an "Uncertainty" value. A value of 6.67430(15) means we are sure of the first digits, but the last two might vary slightly.
The Language of Physics
Physics equations are sentences written in the language of mathematics. Symbols act as the vocabulary.
Knowing that μ (mu) usually means "micro" or "friction", while ρ (rho) usually means "density", helps you read equations like a pro.
Key Categories
- Universal Constants: Values named "Universal" apply everywhere in the cosmos (e.g., Speed of Light, Gravitational Constant).
- Atomic Constants: Detail the properties of subatomic particles (e.g., Electron Mass, Elementary Charge).
- Physico-Chemical Constants: Bridge physics and chemistry (e.g., Avogadro's Constant, Gas Constant).
Frequently Asked Questions
Why is the speed of light exactly 299,792,458 m/s?
Since 1983, the meter has been defined based on the speed of light. We defined the speed of light to be exactly that number, and then adjusted the length of a 'meter' to fit. So, there is zero uncertainty in this value.
What is the difference between h and ħ (h-bar)?
h is the Planck constant (6.626 × 10⁻³⁴ J·s). ħ (h-bar) is the reduced Planck constant, defined as h / 2π. It appears frequently in quantum mechanics equations like the Schrödinger equation to simplify calculations involving angles.
Why do we use Greek letters in physics?
Physics runs out of English letters quickly! Greek letters provide a standard way to represent specific concepts (like π for circle ratio, λ for wavelength, ρ for density) without confusing them with ordinary variables like x, y, or m (mass).
What is 'Standard Gravity'?
Standard Gravity (g = 9.80665 m/s²) is an average value of Earth's gravitational pull at sea level. In reality, gravity varies slightly depending on where you are—it's stronger at the poles and weaker at the equator.
What is the most precise constant?
The Rydberg constant (R∞) is one of the most precisely measured physical constants, with an uncertainty of less than 2 parts in 10¹². This precision makes it crucial for testing quantum electrodynamics (QED).
Are universal constants really constant?
As far as we know, yes. Measurements of light from distant quasars (billions of years old) suggest that the fine-structure constant has remained unchanged over the history of the universe. If it had changed even slightly, stars and atoms might not exist.
What is Avogadro's Number?
Avogadro's constant (6.022 × 10²³) is the number of atoms or molecules in one 'mole' of a substance. It bridges the gap between the microscopic world of atoms and the macroscopic world of grams and kilograms.
Why is 'c' used for the speed of light?
The symbol 'c' comes from the Latin word 'celeritas', meaning swiftness or speed. It was popularized by Einstein in his famous equation E=mc².
What is an Astronomical Unit (AU)?
An AU is the average distance from the Earth to the Sun (approx. 150 million km). It is used as a standard ruler for measuring distances within our solar system.
What is the Boltzmann Constant?
The Boltzmann constant (k) relates temperature to energy. It tells us how much energy corresponds to a specific temperature unit (Kelvin) at the particle level.