If you think cosmos is freezing, try a plasma that makes the vacuum of space feel like a tropical vacation.
The unimaginably hot plasma that exists in the cores of dead stars billions of light-years from Earth has been simulated in a lab by physicists at Rice University. Lasers cooled the plasma until it reached -460 degrees Fahrenheit, about 50 times the temperature of deep space. A cryo-substance mirroring ultra-hot star guts? If that sounds paradoxical, read on.
Plasma is a cloud of extremely dense gas made up of ions and free electrons. This fourth state of matter is able to conduct electricity and usually occurs in extreme environments. Think ridiculously hot celestial bodies, like the sun, and extreme gravity zones like the core of Jupiter or an ultra-dense white dwarf star.
So how do you simulate plasma that hot by making it the polar (pun sort of intended) opposite? “Laser cooling of a neutral plasma is a challenging task because of the high temperatures typically associated with the plasma state,” said team lead Tom Killian and colleagues in a study recently published by Science. “By using an ultra-cold neutral plasma created by photoionization of an ultra-cold atomic gas, we avoid this obstacle and demonstrate laser cooling of ions in a neutral plasma.”
Vaporizing strontium allowed the scientists to suspend that vapor in the beams of a powerful ten-laser array to cool it off. Enter another laser used a short pulse to ionize that strontium vapor once it had cooled. Plasma was created when each strontium atom was tripped of one electron by the pulsing laser’s energy, which also caused that plasma to rapidly expand.
Then came the deep freeze. Blasted with another laser array, the expanding plasma plunged to temperatures more glacial than deep space — 50 times over.
The reason plasma can be created this way is a phenomenon known as strong coupling. Losing an electron gives every one of those laser-blasted strontium atoms a positive charge. Positively charged ions will repel each other in the plasma, but in this case, that gets swallowed up by the immense amount of kinetic energy they produce. That kinetic energy literally brings the heat.
When positively charged ions are forced uncomfortably close together in extreme gravity environments like white dwarfs and Jupiter, even though the plasma is still hot like you wouldn’t believe, repulsive forces end up overtaking the kinetic forces. Desperately seeking equilibrium, the ions start repelling each other, but there is a method to this madness. All ions are repelled by their neighbors equally. Strong coupling occurs when they have found that balance.
By producing repulsive forces that are much stronger than the kinetic forces, it has become possible to simulate the ultra-hot and ultra-dense plasmas in space by creating ultra-cold ones here on Earth without an assist from Jupiter-level gravity that cannot be replicated here on Earth.
For now, just imagine Doctor Evil saying “plasma” instead of “magma”.