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7 fundamentals of physics

What is the world made of and what basic building blocks is it made of? The ancient Greeks were already concerned with this question (e.g. Democritus with his first thoughts on the Atomos concept – everything consists of indivisible basic elements). It has not yet been conclusively clarified what our existence can really be traced back to in the end. Quantum mechanics and quantum field theories have already provided a good foundation here, but reconciling them with the theory of relativity has not yet been accomplished. Even theories like string theory or supersymmetry have not yet provided any experimental evidence.

If we are talking about basic quantities here, then the 7 SI units of physics (SI: French Système international d’unités) are meant. All other units can be derived from these units.

Let’s take length in meters [m] and time in seconds [s] as an example. 1 m is a length, 1 m² is an area, 1 m³ is a volume. m/s indicates a speed, m/s² an acceleration. So much for the simpler derivations. But even more complex units can all be traced back to the basic units, as in the examples below. Fortunately, other units are used here for the sake of simplicity.

Voltage in V (Volt): \( V = \frac{kg x m^2}{A x s^3} \)

Amount of heat in J (Joule): ​\( Q = \frac{kg}{m^2 x s^2} \)

The traceability of all measurements to the basic units is an important cornerstone for our calibration system and thus for the meaningfulness of measurements. There used to be descriptive original meters, original kilograms and the like. Since 2019, all basic parameters have been defined based on natural constants. Although this is a bit more difficult for beginners to digest, since natural constants are eternally constant according to nature, one has an everlasting same definition here. For example, the original kilogram has inexplicably lost some weight over the years (50 µg in 100 years). Such deviations have no place in our world of perfection.

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Temperature – symbol: T / SI unit: K for Kelvin

Temperature – Symbol: T / SI-E Temperature is a measure of the motion of particles (whatever: electrons, quarks, neutrinos, etc. and anything made of them). The higher the temperature, the faster these particles move. At absolute zero (0 K = -273.18 °C) there is no more movement and is therefore not achievable (= 3rd principle of thermodynamics). unit: K for Kelvin


The Kelvin unit is defined by the Boltzmann constant. One Kelvin is the change in thermodynamic temperature T, which corresponds to a change in thermal energy of exactly 1.380649 10-23 J.


Time – symbol: t / unit: s for seconds

Almost a philosophical question: what is the origin of time. Only through a reaction to an action does something happen and only here does time begin. The Big Bang was the second without yesterday…

… and probably there will be a second without a tomorrow. Today, the determination of the measurement of time is based on the principle of the atomic clock. Many atomic clocks use cesium. The inaccuracy is an unbelievable 1 s error in 20 million years.


One second is 9,192,631,770 times the period of the radiation that corresponds to the transition between the two hyperfine levels of the ground state cesium133.


Length: Symbol: l / SI unit: m for meter

The distance between two points. So at first glance it might be easier to imagine than time. However, if nothing moves over time, nothing cannot travel length either. In other words, if something is e.g. a meter away from me, but no particle or other information comes to me over a certain period of time, I will never know that something is there either. So it doesn’t exist for me either. For this reason, time cannot easily be viewed separately from space. However, this view is more for the philosophers and real physicists among us.


The speed of light in a vacuum is 299 792 458 m/s. A meter was therefore defined as the distance traveled by light in a vacuum within the time interval of 1/299792.


Mass: Symbol: m / SI unit: kg for kilogram

Not to be confused with weight (or actually weight). The weight results from the fall acceleration (gravity) and the mass 𝐹= 𝑚 × 𝑔. Mass itself results from the movement of matter (quarks, electrons, neutrons, etc.) through the ubiquitous Higgs field. It’s a bit like walking around a pool and wondering why you can’t go as fast as you can on land. We feel this resistance as a mass. Even when we stand still, the particles that make us move because we always have a temperature greater than 0 K. Interestingly, something also gets heavier as it gets hotter (though only very slightly).


A kilogram is defined by Planck’s quantum of action h (the smallest action or kick that can be performed in the real world). h is defined as 6.626 07015 × 10−34 ​\( \frac{kg m^2}{s} \)​. Since time [s] and length [m] are also defined with natural constants, the kilogram also results logically.


Amperage: Symbol: I / SI unit: A for amperes

Ampere is the flow of a charge (most often thought of as an electron) per second. One can think of 1 A as the movement of 6.2*1018 (6.2 trillion) electrons per second. Author’s opinion: one could actually have defined the coulomb (number of charges) as the basic unit.


An ampere is the flow of 1,602 10-19 elementary charges per second.


Amount of substance: Symbol: n / SI unit: mol

Absolute number of whatever particles considered (atoms, electrons, molecules, etc.). Since particles are very small and you would otherwise always have to calculate with very high numbers for everyday use, a handy conversion factor has been defined.


1 mole = 6,022 x 1023 particles = Avogadro’s constant. This is the number of carbon atoms that results from 12 g of the carbon isotope C12.


Luminous intensity: Symbol: Iv / SI unit: cd for candela

The choice of light intensity as the basic photometric variable initially seems incomprehensible. In everyday life, the use of luminous flux in lumens is also far more common than luminous intensity. But it is now defined as a base size. Candela is the amount of light emitted in a certain direction – expressed as a solid angle. It’s certainly the base size that’s a little harder to digest.

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A candela is the luminous intensity (luminous flux density) of a light source with a wavelength of 555 nm (green light, which we naturally see most intensively with the human eye), with an output of 1/683 watts per steradian (solid angle).

Article-Image: PTB

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