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Light travels at many speeds, new studies suggest

Since the days of Einstein, physicists have held the speed by which light travels through a vacuum to be a constant and have constructed many theories of the universe accordingly. There is only one problem, however, as two new studies point out: Space is not a vacuum. Consequently, the speed of light is not constant, after all.

The studies, published in European Physical Journal D, include one study whose lead author was Marcel Urban of the University of Paris-Sud in Orsay, France, and another one co-authored by the Max Planck Institute for the Physics of Light researchers Gerd Leuchs and Luis Sánchez-Soto. Urban’s study suggests that deep space teems with tiny subatomic “virtual” particles that possess fluctuating energy levels. Leuchs and Sánchez-Soto suggest in their study that these particles’ electrical and magnetic fields can impact light as it travels through them.

The slowing effect could be greater or lesser, depending on the concentration of particles within the given area of space. On average, the studies posit, the matter within a square meter of space can slow light by about 50 “attoseconds”—an attosecond is one quintilllionth (10^-18) of a second.

It is too miniscule a measure of time for human observers to notice. One attosecond is to a second what one second is to 31.71 billion years. Nonetheless, given the great gulfs of distance that space comprises, those attoseconds could add up to slow light enough that refined scientific instruments, high-resolution lasers in particular, could register it.

The authors’ findings thus hold some implications for study of the universe. Space researchers have been using the benchmark of light-year—e.g., the distance that light travels in a calendar year, equivalent to just under six trillion miles—to calculate the distance of the galaxies, star clusters, and other objects throughout the universe.

The slowing effect won’t actually change any of the prevalent theories concerning physics, but it may require some modifications of the current understandings of said theories. It is also possible that researchers could calculate the existence and concentrations of these virtual particles within space by measuring the slowing effect. Additionally, physics textbooks that have always stated the speed of light as the conventionally accepted 186,000 miles per second will have to be revised to include a caveat that the speed varies.

Before any of this occurs, though, the two studies will first need to survive further vetting. Both have yet to finish the peer-review process.