Introduction to quantum physics - simply explained
In the 20th century, some astonishing experiments led to the development of quantum physics, which is not so easy to explain for laypeople. It applies above all to physical objects and sizes of microphysics, i.e. the smallest particles and their characteristics. Quantum physics contradicts the classic ideas of physics that nature is always built up continuously and is 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 described that an electrically negatively charged zinc plate was discharged when irradiated with UV light. The removal 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, even if the light intensity is increased.
- Even a positively charged plate cannot be unloaded by radiation. A mercury vapor lamp, on the other hand, provides ultraviolet light: this UV radiation has enough energy to remove the electrons from the negative zinc plate.
- If the intensity of the ultraviolet radiation is increased, the discharge of the zinc plate takes place more quickly. (UV light has a shorter wavelength and a higher frequency than visible light.)
Interpretation of the photoelectric effect
The test result 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 become faster even with more intense light, although more energy is provided overall first explain Albert Einstein in 1905.
- He postulated that the energy of light is not continuously distributed in the room, but must be present in certain portions of energy. These are called energy quanta or photons. Their energy results in: E = h ⋅ f and is indivisible.
- With the photoelectric effect, a photon with a sufficient amount of energy is swallowed 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 they are not more energetic. The higher the frequency of the incident light, the faster the free electrons are after they are released. This linear relationship is described by the constant factor h - Planck's constant: 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 a momentum p = m ⋅ c = h ⋅ f / c. Nevertheless, photons are not particles because they show the typical interference phenomena of waves.
- However, electrons that fly through a double slit also do not show a clearly determined result as expected: rather, the points of impact on a detector screen vary strongly and randomly and are unpredictable. Electrons are like quantum objects like photons: it turns out that in addition to protons and neutrons, atoms and molecules are also 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 (see Photon).
Wave-particle duality
The juxtaposition of wave and particle models is called dualism. From the experiments by the Englishman Taylor, who worked with the smallest intensities of photons or electrons at the double slit, the unpredictability could finally be interpreted as a probability wave of the quantum objects (1926 Max Born).
- If many particles (no matter whether 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 gap of width Δx and this gap width is reduced, the diffraction figure on the screen increases (contrary to what would have been expected in classical physics).
- The particles received a transverse impulse in the x-direction ("px") when they passed through the gap Δx.
- Werner Heisenberg used the equation Δx ⋅ Δpx = h to relate the inaccuracy of the location and the momentum to each other. This Heisenberg uncertainty principle sets a limit for the simultaneous determination of location and momentum.
- It is impossible to precisely determine the location and momentum of a quantum object at the same time. It results from the wave-particle dualism (and not because of the measuring equipment and possible measuring errors).
Special features of quantum physics summarized
According to the Heisenberg uncertainty principle, not all sizes can be determined and measured: causality and determinism are eliminated in the micro range - predictions for individual objects are hardly possible anymore. Quantum objects follow statistical laws.
In the next practical tip, we will explain the theory of relativity to you.