Longitudinal wave

A longitudinal wave is a specific way that energy moves through a medium. In these waves, the particles of the material oscillate in a direction parallel to the wave's travel. This means the movement of the particles is in the same or opposite direction as the wave itself.

Because these waves cause changes in pressure, they are often called pressure waves. They are also known as compressional or compression waves. This is because they create alternating regions of compression and rarefaction. A compression is a part where the medium is squeezed together. A rarefaction is a part where the medium is spread apart.

To understand the mechanism, we must look at how the medium responds to the energy. As the wave travels, it creates a cycle of density changes. First, the particles are pushed into a high-density area called a compression. This increases the local pressure. Next, the particles move apart, creating a low-density area called a rarefaction. This decreases the pressure. This back-and-forth process carries energy through the material without moving the material itself from one place to another. You can visualize this by imagining a stretched Slinky toy. If you push the end of the Slinky, a pulse of bunched-up coils travels down its length.

There are different types of waves to consider when studying physics. The main counterpart to a longitudinal wave is the transverse wave. In a transverse wave, the displacement of the medium occurs at right angles to the direction the wave travels. While longitudinal waves move through fluids like air and water, transverse waves can also move through solids. In solid materials, transverse waves are sometimes called shear waves to distinguish them from longitudinal pressure waves. Some authors use abbreviations like "L-waves" for longitudinal and "T-waves" for transverse. However, these abbreviations can be confusing because they have different specific meanings in fields like seismology or electrocardiography.

Scientists have spent a long time studying the math behind these movements. For longitudinal harmonic sound waves, researchers use specific formulas to describe frequency and wavelength. The frequency is how often the wave oscillates, while the wavelength is the distance between two points in the same phase. The speed of a sound wave depends on the temperature, the type, and the composition of the medium. In isotropic solids and liquids, the speed is determined by the elastic modulus and the mass density of the material. The elastic modulus involves the shear modulus and the bulk modulus, which describes how much a material resists compression.

One important concept in wave science is attenuation. Attenuation describes the loss of energy a wave carries as it travels through a medium. This energy loss happens for several reasons. It can be caused by the scattering of the wave at interfaces or by friction between molecules. Geometric divergence can also cause energy to spread out and weaken. In polycrystalline materials, which are made of many crystal grains, attenuation is common. When a wave crosses a grain boundary, a scattering event occurs. Scientists study this to evaluate the damage in engineering components without destroying them.

There are many notable examples of longitudinal waves in the universe. Sound waves are the most common example, as they are vibrations in pressure traveling through an elastic medium. In the Earth, earthquakes and explosions create seismic P waves. These are longitudinal waves that move through the ground. In 2022, NASA reported the sonification of a black hole in the Perseus galaxy cluster. This involved converting astronomical data from pressure waves into sound. Even in space, plasma waves exist as density waves of charged particles that can couple to electromagnetic fields.

While most electromagnetic waves in a vacuum are strictly transverse, there are complex exceptions. Oliver Heaviside concluded that electromagnetic waves would not be found as longitudinal waves in free space. However, they can show a longitudinal component when moving through certain materials. For example, they can appear in birefringent or inhomogeneous materials. Some theorists, such as Jean-Pierre Vigier and Bo Lehnert, have used the Proca equation to suggest that longitudinal electromagnetic waves might exist in a specific type of vacuum. However, most physicists doubt the existence of photon mass, which would be required for this to happen.