ბგერის გავრცელების სიჩქარე დამოკიდებულია გარემოზე, რომელშიც ტალღა ვრცელდება და [[ტემპერატურა]]ზე.
Maxwell's equations]] lead to the prediction of electromagnetic waves in a vacuum, which are transverse (in that the electric fields and magnetic fields vary perpendicularly to the direction of propagation).<ref name="griffiths">David J. Griffiths, Introduction to Electrodynamics, ISBN 0-13-805326-X</ref> However, waves can exist in plasma or confined spaces. These are called [[ plasma wave]] s and can be longitudinal, transverse, or a mixture of both.<ref name="griffiths" /><ref>John D. Jackson, Classical Electrodynamics, ISBN 0-471-30932-X.</ref> Plasma waves can also occur in force-free magnetic fields.
In the early development of electromagnetism there was some suggesting that longitudinal electromagnetic waves existed in a vacuum. After Heaviside's attempts to generalize Maxwell's equations, Heaviside came to the conclusion that electromagnetic waves were not to be found as longitudinal waves in "''[[free space]]''" or homogeneous media.<ref>Heaviside, Oliver, "''Electromagnetic theory''". ''Appendices: D. On compressional electric or magnetic waves''. Chelsea Pub Co; 3rd edition (1971) 082840237X</ref> But it should be stated that
Maxwell's equations do lead to the appearance of longitudinal waves under some circumstances in either plasma waves or guided waves. Basically distinct from the "free-space" waves, such as those studied by Hertz in his UHF experiments, are [[Zenneck wave]]s.<ref>Corum, K. L., and J. F. Corum, "''The Zenneck surface wave''", ''Nikola Tesla, Lightning observations, and stationary waves, Appendix II''. 1994.</ref> The longitudinal mode of a [[resonant cavity]] is a particular standing wave pattern formed by waves confined in a cavity. The longitudinal modes correspond to the wavelengths of the wave which are reinforced by constructive interference after many reflections from the cavity's reflecting surfaces. Recently, Haifeng Wang et al. proposed a method that can generate longitudinal electromagnetic (light) wave in free space, and this wave can propagate without divergence for a few wavelengths.<ref>Haifeng Wang, Luping Shi, Boris Luk'yanchuk, Colin Sheppard and Chong Tow Chong, "Creation of a needle of longitudinally polarized light in vacuum using binary optics," Nature Photonics, Vol.2, pp 501-505, 2008</ref>