Can quantum physics explain everything

Introduction to Quantum Physics - Simply Explained

In the 20th century, some amazing experiments led to the development of quantum physics, which is not that easy to explain for laypeople. It applies above all to physical objects and sizes in microphysics, i.e. the smallest particles and their characteristics. Quantum physics contradicts the classical notions of physics that nature is always built up continuously and always measurable. In this article you will learn the most important findings and statements of modern quantum physics.

Explanation of quantum physics: the external photoelectric effect

In 1888, the German physicist Hallwachs describes that an electrically negatively charged zinc plate is discharged when exposed to UV light. The leaching of electrons by light is generally referred to as the external photoelectric effect.
• When exposed to visible or infrared light, however, there is no effect, not even when the light intensity is increased.
• Even a positively charged plate cannot be discharged by irradiation. A mercury vapor lamp, on the other hand, supplies ultraviolet light: this UV radiation has enough energy to release the electrons from the negative zinc plate.
• If the intensity of the ultraviolet radiation is increased, the zinc plate will discharge more quickly. (UV light has a shorter wavelength and higher frequency than visible light.)

Interpretation of the photoelectric effect

The result of the experiment that only light with a sufficiently high frequency provides sufficient energy for the "release work" of the electrons, as well as the fact that the released electrons do not get faster even with more intense light, although more energy is provided overall, could be achieved first explain to Albert Einstein in 1905.
• He postulated that the energy of the light is not distributed continuously in the room, but has to be present in certain energy portions. These are called energy quanta or photons. Their energy is given by: E = h ⋅ f and is indivisible.
• In the photoelectric effect, a photon with a sufficiently large amount of energy is swallowed up by the electron in the metal plate: With this energy (h ⋅ f) the electron can leave the metal plate (work function) and may have additional energy for its movement: h ⋅ f = work function + Kinetic energy against an electric field.
• A higher light intensity means more photons, but these are not more energetic. The higher the frequency of the incident light, the faster the free electrons are after they have been triggered. This linear relationship is always described by the same factor h - Planck's quantum of action: h = 6.6260 ⋅ 10¯³⁴ Js.

Particle properties for light, wave properties for particles

With the special theory of relativity and the relationship E = m ⋅ c², the photons receive both a mass m = h ⋅ f / c² and an impulse p = m ⋅ c = h ⋅ f / c. Nevertheless, photons are not particles, because they show the typical interference phenomena of waves.
• But even electrons that fly through a double slit do not show a clearly determined result as expected: rather, the points of impact on a detector screen vary greatly and randomly and are not predictable. Electrons are quantum objects like photons: it turns out that besides protons and neutrons even atoms and molecules are quantum objects.
• The particle model cannot describe their interferences either. In 1924 Louis de Broglie therefore introduced the concept of the matter wave, which also assigned a wavelength to the particles: λ = h / p (cf. photon).

Wave-particle dualism

The juxtaposition of wave and particle models is called dualism. From the experiments of the Englishman Taylor, who worked with the smallest intensities of photons or electrons on the double slit, the unpredictability could finally be interpreted as a probability wave of the quantum objects (1926 Max Born).
• If many particles (regardless of whether they are photons or electrons) hit the detector screen, the well-known interference pattern of a wave appears: however, no prediction is possible for individual objects.
• Photons can therefore be interpreted both as electromagnetic waves and as probability waves - matter waves only as probability waves.

Uncertainty principle in quantum physics

If a stream of quantum objects (electrons or photons) of the momentum p = h / λ hits a slit of width Δx and this slit width is reduced, the diffraction figure on the screen increases (contrary to what would have been expected in classical physics).
• When passing through the gap Δx, the particles received a transverse impulse in the x direction ("px").
• Werner Heisenberg used the equation Δx ⋅ Δpx = h to relate the inaccuracy of the location and the momentum to each other. This Heisenberg uncertainty relation sets a limit for the simultaneous determination of position and momentum.
• It is impossible to determine the exact position and momentum of a quantum object at the same time. It arises from the wave-particle dualism (and not due to the measuring equipment and possible measuring errors).

Special features of quantum physics summarized

According to Heisenberg's uncertainty principle, not all quantities are determinable and measurable: causality and determinism are canceled in the micro range - predictions for individual objects are hardly possible any more. Quantum objects follow statistical laws.
In the next practical tip, we will explain the theory of relativity to you.