Light

Lene Vestergaard Hau: Bringing Light to a Stop by Laura Burn

Did you know the universe has a speed limit? It’s so fast, we’d need to make a giant sign to fit the number—300,000,000 meters per second (671 million miles per hour)! This is the speed of light. At this speed, light would be able to zip around the Earth seven and a half times in just one second!

Light moves at this speed when it is travelling through a vacuum—no, not the ones you use to clean the floor. In this case, we mean that light is travelling through nothing, not even air. Outer space is an example of a vacuum. So when light is travelling through outer space, it’s moving at this ultimate speed and can’t go any faster! It’s not that it will get a speeding ticket, the laws of physics just won’t let it go any faster! But what do you think happens when it moves through something like air or water? It slows down!

Why does the light slow down? It’s all about how much “stuff” is in the way, how many particles there are in one place. Think about how easy it is to run through air, but imagine how much harder it is to run through water. This is because water is denser than air, there are more particles in the same amount of space, like trying to walk through a crowded hallway. Light works in a similar way, it moves slower in these more dense materials, like water or glass, which have more particles for the light to bump into.

See this in action!

Grab a glass of water and something long, like a straw or pen, and place it in the glass of water. You should notice it seems to look bent or broken, this is the light slowing down and changing direction as it moves from air to water.

So, just how slow can light really go? Can we even bring it to a stop?

Meet Lene Hau: The scientist who stopped light in its tracks

Lene Hau is a Danish physicist who, in 1999, led a team at Harvard University to slow down light to the speed of a car, about 38 kilometers per hour. Just two years later, her team was able to bring light to a dead stop.

Hau’s journey into science started with a love of math in elementary school thanks to an inspiring teacher who made the subject exciting. When she got to university, she thought she’d become a mathematician, until she learned more about physics. She realised the math she had been doing could help explain how the world worked, and it amazed her to see that math could describe real things happening in nature.

That curiosity eventually led her to want to do experiments in the lab, where she could test the fascinating behavior of light. But getting there wasn’t easy.

Even though Hau had a brilliant idea, getting the equipment to do the experiments was a challenge. To access the equipment, she applied for funding but was "rejected on the grounds that she was a theorist (someone who only uses computers and mathematics to understand the behavior of tiny particles, not actual experiments) for whom such experiments would be too difficult to do." But Hau didn’t give up.

Instead, she got creative. She found other funding sources, built a team, and learned how to run the experiments herself, even down to handling the powerful lasers. It took about 15 months to carefully build and fine-tune the setup, working late into the night. Some of the experiments lasted for 27 hours straight! Hau and her team would even eat pizza while adjusting mirrors in complete darkness, having just 38 seconds to get it perfect each time. Finally, in 1999, Hau and her team became some of the first scientists to create a new form of matter called a Bose-Einstein condensate (BEC).

A BEC happens when a group of atoms is cooled down to extremely low temperatures. If you stepped outside on Pluto, it would feel warmer than a BEC! At this temperature, the atoms slow down so much that they start to act like one big "super atom”.

Remember how we saw that light slows down in water because it is denser than air, so there are lots of particles packed in? In water, light is like someone trying to run through a chaotic crowd where people are rushing and bumping into each other. There are lots of people, and they’re all moving differently, so it’s hard to get through quickly.

But when we cool things down, the crowd slows. And if we go super low, the crowd starts to move together as one. In a BEC, the crowd isn’t chaotic, it’s more like a marching band. Everyone is moving together, slowly and in perfect rhythm. Instead of pushing through, you have to move with them. This careful coordination slows light down even more than the rowdy crowd did!

This unique property of a BEC can make light travel, as Hau said, “so slow that you can beat it on a bicycle.” The first time her team saw signs of light slowing down was in the early hours of the morning, at 4 A.M. Just a few months later they were able to get the speed down to slower than the speed of an airplane, at which point Hau had to travel back to Denmark to teach a class. While on the plane she realised she was currently travelling faster than the light she had slowed down in her lab. She would be able to beat it to Denmark in a race across the Atlantic!

Two years later, after many late nights in the lab tweaking the set-up and likely drinking lots of coffee, Hau was successfully able to bring light to a complete stop inside the BEC for a fraction of a second.

Being able to control light in this way has many applications for technology; future computers could use light to carry information to make them faster and more secure. However, to stop light like this right now, we need a special room filled with equipment. But this is how the first computers were made too—and now we have computers in our pockets!

So now you can say you can run faster than the speed of light…but only with super cold atoms!

References

1. W. Cromie, Harvard Gazette: Hau Wins MacArthur, https://web.archive.org/web/20110928063328/http://www.news.harvard.edu/gazette/2001/10.25/01-macarthur.html.
2. Annenberg Learner, Interview with Featured Scientist Lene Hau, https://www.learner.org/series/physics-for-the-21st-century/manipulating-light/interview-with-featured-scientist-lene-hau/.
3. M. Browne, She Puts the Brakes on Light, The New York Times (1999).
4. W. Cromie , Researchers Now Able to Stop, Restart Light, https://news.harvard.edu/gazette/story/2001/01/researchers-now-able-to-stop-restart-light/.
5. L. V. Hau, Frozen Light, Scientific American (2003).
6. J. J. O’Connor and E. F. Robertson, Lene Vestergaard Hau - Biography, https://mathshistory.st-andrews.ac.uk/Biographies/Hau/.