Ultra-Cold Atoms Harnessed to Hunt For Dark Energy
Just 0.0000000001 degrees above absolute zero. That is how deeply an atom cloud will be chilled, in a new ISS experiment. In a state of weightlessness, the atoms remain for 10 seconds, so physicists can study them and perhaps solve one of the major astronomy mysteries.
In 1995, when three US physicists cooled a gas of atoms to a few billionths of a degree above absolute zero, producing the first Bose-Einstein condensate, it was a sensation. Still, the physicists were not perfectly content. They dreamed of carrying out the experiment at the international Space Station, ISS, because in a state of weightlessness, an ultra-cold gas cloud will live much longer than on Earth, where scientists barely have time to study it. Back then, the vision was pure science fiction, as the cooling required large, heavy lasers, which could neither be launched nor fitted into the cramped space station.
Now, things are different. The large lasers of the past have been packed into a small chip, and the entire experiment set-up has been fitted into a box the size of a microwave oven. Named the Cold Atom Laboratory (CAL), the box will now be launched and mounted outside the space station by a team of NASA scientists, and subsequently, the experiments can be remote- controlled from Earth via radio signals.
Physicists throughout the world have already lined up to make experiments in the distant lab, as the ultra-cold atoms are almost completely unexplored. In a state of weightlessness, the gas will live for 10-20 seconds, so physicists have time to study and manipulate the cold atoms. According to scientists, the ultra- cold atoms can be converted into extremely sensitive sensors that can measure the strength of gravity with unprecedented accuracy, allowing the scientists to measure the extent of ice cap melting. Perhaps, the sensors can even find the unknown particles that produce the repellent black energy which makes the expansion of the universe accelerate.
Cold atoms converted into wave
Close to absolute zero, atoms behave very differently than they usually do. Under normal circumstances, according to the laws of quantum mechanics, atoms are both waves and particles at the same time, but close to absolute zero, the atoms lose their individual identities as particles, instead becoming a collective wave. The state, which is produced in gases at temperatures of a few billionths of a degrees above absolute zero, is known as a BoseEinstein condensate, as the phenomenon was predicted in 1924 by physicists Satyendra Nath Bose and Albert Einstein.
For years, scientists have dreamt of studying the ultra-cold atoms, but so far, Earth’s gravity has prevented it. Only 10 milliseconds after the production of a Bose-Einstein condensate, the atoms fall to the bottom of the experimental chamber, where the chamber wall heats the atoms, making the collective quantum state cease. However, the state of weightlessness of the space station will allow scientists time to make
measurements. Moreover, the state can be used to cool the atoms to even lower temperatures than on Earth. In the space lab, the Bose-Einstein condensate is produced by laser cooling, while the gas is captured in a magnetic field by ultra-cold atoms. When the magnetic field is "switched off", the gas spreads in the vacuum chamber.
The spread cools the atoms even more, as the longer the distance in between atoms, the more rarely they collide, and the colder they get. The principle is the same as when a spray bottle becomes ice-cold after the gas has been released and the pressure inside the container has been reduced. The longer the cloud develops, the colder the atoms.
Physicists hope to beat the cold record on Earth of 50 billionths of a degree above absolute zero, which was set by dumping a version of the experiment down a 146-m-high tower in Germany. The free fall produced five seconds of weightlessness, during which the gas expanded and was cooled, before the experimental chamber landed in plastic balls at the bottom of the tower.
In the space lab, the gas will expand and cool for 10-20 seconds, and the atoms will become even colder. If the cold record is beaten, the atom cloud at the space station will be the coolest spot in the universe – about 100 million times colder than empty space, in which the temperature is 2.725 degrees above absolute zero anywhere.
The space experiments also involve another major advantage: whereas scientists can only make three experiments a day in the tower, CAL will be available to physicists 24/7, and the remote-controlled experiments can be carried out over and over again from anywhere on Earth.
Highly accurate measurements
First, physicists aim to study the properties of Bose-Einstein condensates and manipulate the collective quantum wave, into which the cloud of ultra-cold atoms is converted, in order to see, if they can shake the wave or make it produce a circle.
When an improved version of the lab is launched no later than in 2021, it will be possible to carry out atomic interferometry, by which a laser beam splits the quantum wave in two, one half moving a little further away from Earth than the other, causing a tiny difference of the weak gravity acceleration from Earth, which the top and bottom waves are subjected to. When the two quantum waves are reunited, they produce a grooved pattern, that can be used to calculate the tiny difference and the strength of the gravity acceleration very accurately.
The ultra-cold atomic interferometer can make the world’s most accurate gravity measurements and is able to measure small changes in Earth’ field of gravity, as they are produced. Apart from observing melting ice caps, scientists will also be able to say to which extent the ground water deposits of an area are emptied of water for drinking and irrigation purposes.
Intense search for dark energy
An atomic interferometer will be sensitive to all types of fields and so, it could also reveal the physical mechanism behind dark energy, which, according to astronomers, makes the universe expand ever faster. The key is that the detector is located in empty space, where the particles’ rejection cannot be out-competed by the attraction of gravity.
The dominant theory involves that dark energy originates from empty space and has a constant strength – i.e. any given volume of empty space always contains the same quantity of dark energy. A competing theory says that dark energy is mediated by particles that change their masses and forces depending on their location. Near compact masses such as Earth, the particles have so much mass that not even CERN’s LHC accelerator can produce them. But at the ISS, a highly sensitive atomic interferometer might be able to detect the force fields produced by the particles. So, scientists hope that the ultra-cold atoms in space can soon solve the major mystery of astronomy once and for all, explaining dark energy.
An atom chip from the American company ColdQuanta is the heart of the space station experiment. The Bose-Einstein condensate is produced at the centre of the chip.