Researchers have found a theoretical possibility that the speed of light may not be constant and it may change while interacting with the ephemeral particles in the vacuum.
European Physical Journal D
Researchers based their report on the findings related to vacuum. Vacuum is a space that was thought to be fully empty of matter but at the smallest and the most basic level of quantum physics, it is not empty as it is occupied with particle pairs such as electron-positron or quark-antiquark pairs that appeaar and disappear continuously with extremely short lifetimes.
In one study, researchers including Marcel Urban and his collagues from the University of Paris-Sud, located in Orsay, France, found “quantum level mechanism” to understand the vacuum and identified that the vacuum is not completely empty but it is filled with the virtual or ephemeral particles having various levels of energy.
Based on this finding of changing energy levels and vacuum fluctuation, researchers proposed that the speed of light may also change.
In another study, researchers including Gerd Leuchs and Luis L. Sánchez-Soto, from the Max Planck Institute for the Physics of Light in Erlangen, Germany, proposed that some of the physical constants, such as the so-called impedance of free space, (which shows the different levels of the electric and magnetic fields of electromagnetic radiation moving through the free space,) depends only on sum of the square of the electric charges of particles but not on their masses.
The value of vacuum impedance, which is important for the determination of the speed of light, combined with the value of the speed of light, shows the total number of charged elementary particles found in nature.
Both of the studies presented the concept of the interaction of the light with the virtual particle-antiparticle pairs thereby affecting the speed of light.
Leuchs, G., & Sánchez-Soto, L. (2013). A sum rule for charged elementary particles The European Physical Journal D, 67 (3) DOI: 10.1140/epjd/e2013-30577-8
Urban, M., Couchot, F., Sarazin, X., & Djannati-Atai, A. (2013). The quantum vacuum as the origin of the speed of light The European Physical Journal D, 67 (3) DOI: 10.1140/epjd/e2013-30578-7