Physicists Slow Down the Speed of Light in Groundbreaking Experiment
In the last two years, physicists have proposed that the "speed of light" constant is either slower than previously thought or not a constant at all. Now, researchers from Glasgow and Heriot-Watt universities have provided the first experimental evidence that the speed of light is not constant, even in a vacuum, and that it should be seen as a "limit" rather than a "constant."
In the real world, the speed of light can be slowed by many factors, even something as simple as a glass of water. The "constant" refers to the speed of light in a vacuum, which was thought to be an invariable 299,792,458 m/s. But in this experiment, the researchers showed that even light inside a vacuum can be slowed down by changing its shape. In order to demonstrate this, they shot two photons side-by-side in a vacuum, but sent one of the photons through a device that warped its structure. The altered particle consistently traveled at a slower speed than the unaltered particle.
As a result, they concluded that the speed of light is only constant for light that is structured in a specific way. Plane waves represent the "ideal" structure for light, in which the fronts of the waves move in perfect parallel. While physics textbooks tend to characterize all light as plane waves, this experiment and others indicate that the structure of light is more variable. So when generalizing about the speed of light, it should be thought of as an upper limit rather than a mathematical constant, as any variance in structure can slow it down even in a vacuum.
From the paper: "That the speed of light in free space is constant is a cornerstone of modern physics. However, light beams have finite transverse size, which leads to a modification of their wavevectors resulting in a change to their phase and group velocities... In both cases, the delay is several microns over a propagation distance of the order of 1 m. Our work highlights that, even in free space, the invariance of the speed of light only applies to plane waves. Introducing spatial structure to an optical beam, even for a single photon, reduces the group velocity of the light by a readily measurable amount."
If this experiment can be replicated, and it proves to be true that the speed of light is a limit rather than a constant, then it would have reverberating effects in the scientific community. In the field of cosmology, in particular, the speed of light as a mathematical constant has been taken for granted in the development of a multitude of theories. Virtually every cosmological theory would need to be reevaluated, including the length of time it takes for the Sun's light to reach Earth, measurements of Earth's distance from celestial bodies, etc. All of cosmology would essentially need to be started from scratch.