The speed of light in a vacuum, at 299,792 kilometers per second (186,000 miles per second), is considered the ultimate speed limit of the universe, according to Einstein’s principles, since surpassing it would require infinite energy.
However, this doesn't imply that the speed of light can't be surpassed under specific conditions. When light travels through water, its speed decreases to 225,000 kilometers per second (139,800 miles per second). Under these circumstances, it can be overtaken by particles in environments like a nuclear reactor, leading to the production of Cherenkov radiation.
Moreover, 225,000 kilometers per second isn't the slowest light has ever been recorded to move. In 1998, scientists reduced its speed to an astonishingly slow 17 meters per second, which equates to just 61.2 kilometers (38 miles) per hour.
The goal of the experiment wasn't merely to slow down light. The researchers aimed to investigate Bose-Einstein Condensate (BEC), a state of matter theorized by Albert Einstein based on the work of physicist Satyendra Nath Bose. In this state, a gas of bosons, subatomic particles with integer spin, cool down near absolute zero and coalesce into a single quantum entity, effectively behaving like one giant atom.
"In this state, the wave function of a BEC reflects the ground state of a large-scale quantum object," a paper outlines. "Thus, a group of atoms in a BEC acts as a singular quantum unit."
First realized experimentally in 1995, this novel state of matter provides a macroscopic window into quantum phenomena.
The BEC exhibits peculiar properties, including zero viscosity. If placed in a container, it would climb the walls of the glass. It can sustain vortices that imitate black holes and erupt violently like a supernova, dubbed a bosenova. These unique characteristics are precisely why they are studied.
In 1998, scientists at the Rowland Institute for Science created a BEC by cooling sodium atoms in a vacuum chamber. They used laser beams (traveling at the conventional speed of light) to hit the sodium, slowing the atoms down as they absorbed photons. These atoms were then subjected to another series of lasers that pushed them back, further reducing their speed and cooling them, held steady by a strong magnetic field.
Once a condensate formed, a laser was shot across its width to induce quantum interference, followed by another laser across its length. Under these conditions, light was significantly slowed.
The scientists recorded, "We obtain a light speed of 17 meters per second for pulse propagation in an atom cloud initially prepared as almost completely Bose-Einstein condensate (with a condensate fraction of at least 90%)." They acknowledged that whether the cloud retained its condensate form during and after pulse propagation remained unaddressed in their initial report.
Recognizing the potential for further refinement in their method, the team soon advanced to halt light pulses altogether in an atomic cloud cooled to near the BEC transition temperature.
"Shortly after, we managed to stop a light pulse entirely in an atomic cloud just above the BEC transition temperature," the team detailed on the Hau Lab website. "While the pulse is slowed, compressed, and held inside the atomic model, we abruptly turn off the control laser and then reactivate it later on. This allows us to regenerate the light pulse: essentially, we can stop and deliberately restart the light pulse."
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