The Speed of Light Can Vary

By manipulating the structure of light pulses, scientists managed to slow down some photons and challenge textbook wisdom

light beams
John Rensten/Corbis

Much of how we understand the way our universe is built hangs on constants—physical measurements that describe, for example, the charge and mass of the electron. These quantities are the foundation for basic theoretical equations, and they don’t change. Except, scientists just found out that the most famous constant, the speed of light, isn’t quite as constant as we thought.

When light travels through glass or water, it’s slowed down. But scientists thought that, when it flashes through a vacuum, it's kept to the unvarying speed of 299,792,458 meters per second. New work, published at, shows that this isn’t always the case. Depending on the structure of the light, the researchers observed, photons, the basic unit of light, will travel slower. 

The researchers essentially pitted differently structured photons against each other. Light is often represented as a a wave (though the whole particle/wave thing can get twisty), but that's just an approximation, writes Andrew Grant for Science News. Think about a laser, he suggests—that structures light as a concentrated or bull's eye–shaped beam. Or, think about how light behaves when passed through a lens—it can convenge on a point.

So, the structure of light can vary, and that's what this experiment hinged on. Grant writes:

The researchers produced pairs of photons and sent them on different paths toward a detector. One photon zipped straight through a fiber. The other photon went through a pair of devices that manipulated the structure of the light and then switched it back. Had structure not mattered, the two photons would have arrived at the same time. But that didn’t happen. Measurements revealed that the structured light consistently arrived several micrometers late per meter of distance traveled.

“It’s very impressive work,” Robert Boyd, an optical physicist at the University of Rochester, told Science News. “It’s the sort of thing that’s so obvious, you wonder why you didn’t think of it first.”

Previously, two different research groups had come up with the idea the speed of light might change, primarily because what we think of as a vacuum isn’t really empty. They discussed the possibility that space is actually "a great big soup of virtual particles that wink in and out of existence in tiny fractions of a second," reports Livescience. Those winking particles would impede light and cause fluctuations. But those two papers were theoretical. The new paper actually includes observations that show some photons were slowed.

This doesn’t mean that our whole understanding of the universe is thrown out of whack. Or at least, it’s not something that the typical person will notice in daily life. Only those physicists working on specific problems, such as studying very short light pulses, would need to change what they do. The rest of us can still use the value written in our textbooks. And don’t worry, we’ll still be able to get surprisingly accurate measurements of the speed of light at home using a microwave and a bar of chocolate.

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