blog post

Slowing the Speed of Light to Zero

For centuries humans have been obsessed with speed. We are constantly pushing the limits for how fast we can make things travel. Currently in modern physics, light is regarded as the fastest thing in the universe and is the basis for Albert Einstein’s theory of special relativity. But what if the light you saw could be slowed down and even stopped? As hard as it is to believe, scientists at Harvard University have done just that!

The speed of light is constant when it is in a vacuum. It travels an astounding 186,000 miles per second. However, it does slow down when traveling through other common substances such as water (140,000 mi/sec), glass (124,000 mi/sec), and diamond (77,500 mi/sec). Despite the speed reduction when traveling through these substances, it is nowhere close to a speed that we can travel at.

Lene Hau is a world-renowned physicist at Harvard University, and she has figured out a way to stop light in its path. One of the tricks to slowing light to a halt is creating a cloud of nearly motionless atoms at near absolute zero (-460 degrees Fahrenheit). This is called a Bose-Einstein condensate.

This is a scientific apparatus that is used to make clouds of super-cooled atoms, or Bose Einstein condensates. This is not the one that Dr. Hau used, but is related. image source

This is a scientific apparatus that is used to make clouds of super-cooled atoms, or Bose Einstein condensates. This is not the one that Dr. Hau used, but is related. image source

To do this, Dr. Hau starts out with a clump of the element sodium at room temperature. While commonly found in table salt in its ionic form, elemental sodium is a nice shiny metal. She then heats up the metallic sodium in a container inside an oven. The heating process causes the atoms in the sodium to start to vibrate. Once the oven reaches 350 degrees Celsius the atoms leave the liquid phase and become a cloud of gaseous sodium atoms.

Dr. Hau then makes a tiny pin hole in the container which forces the atoms through the hole. As the streams of atoms are coming out of the container, her research team hits the atoms head-on with a laser beam. In other words, they simply shine the laser directly at the hole as the atoms fly out. This “kicks” the atom in a direction opposite of their current motion, which in turn causes them to slow down. Dr. Hau then bombards the atoms with laser beams from all directions in order to slow the atoms in the cloud even further. At this point the atoms are slowly vibrating around. She then uses an electromagnet to isolate the atoms with the lowest energy.

These isolated atoms are in a cloud that is approximately 0.01 mm in diameter. This tiny cloud contains around 5 to 10 million atoms. These specific atoms have been forced to remain extremely still. The stillness of the atoms results in a new state of matter that is colder than anything on Earth. The temperature is half-a-billionth of a degree above absolute zero.

Hau then shoots a beam of light into this cold atom cloud. By combining the coldest thing and the fastest thing on Earth, Dr.Hau was able to show that she could decrease the speed of light from approximately 186,000 miles per second down to 15 miles per hour. This is a speed that you could bike at! After a few more experiments, Dr. Hau figured out a way to tweak some properties of the atom cloud in such a way that she could completely stop a light beam in the cloud. It is as if the light wave is frozen inside this cloud, and stays there until it is warmed up again. This feat is something that Albert Einstein theorized was impossible.

The result of this finding may greatly further light-based research. Scientists believe that it could be possible to use light beams to store data and then send them over long distances. Some even theorize that there is a potential of accelerating light beyond the universal speed limit. There is still much to be learned and discovered in this field. It is truly amazing all that can be done using laser optics. I am currently doing laser research in Dr.Geiger’s laboratory, which led me to become interested in this topic. If you would like to learn more about the subject, you should take a look at some of the links listed below!

References & Further Reading
Light speed reduction to 17 metres per second in an ultracold atomic gas
Physics Central – Lene Hau

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