Posted on 10/08/2015 6:26:20 PM PDT by LibWhacker
Sometimes you have to think outside the box. Faced with some of the universe’s most stubborn mysteries, such as the identity of dark matter, physicists are turning to a technique that employs the weird laws of quantum mechanics: atom interferometry.
Atom interferometers allow the study of various physical phenomena by splitting atom waves using a nanograting, such as this one. Composed of silicon nitride, this grating, imaged with a scanning electron microscope, has a period of 100 nm. Image courtesy of Alex Cronin (University of Arizona, Tucson, AZ).
This method, which takes advantage of the fact that quantum particles behave like waves, crashes these waves together to study gravity and atoms, and to pinpoint the value of fundamental physical constants. The hope is that the relatively new technique can be developed into a measurement tool of unprecedented sensitivity to test these phenomena, as well as to reveal potentially unseen particles and forces.
“It has a lot of untapped potential,” says Holger Müller of the University of California, Berkeley. But he admits, “Honestly, what drew me mostly was that it’s extremely challenging experimentally ... and I love an experimental challenge.”
The idea that particles can behave as waves that can be sloshed together dates back to the birth of quantum mechanics about a century ago. But it took decades to test because atoms have much shorter inherent wavelengths—about a billionth of a centimeter at room temperature—than subatomic particles or photons of light. Optical elements capable of manipulating such tiny waves were not developed until about 25 years ago, ushering in the first atom interferometers (1, 2). These elements include fabricated gratings with nanometer-sized slits and pulses of finely-tuned laser light.
Many atom interferometers use three such optical elements: one to separate a group of atoms into two divergent streams of atom waves, a second to bring the two streams back toward each other, and a third to make them collide and come into focus.
If one stream travels farther—or encounters different physical conditions—than the other, the peaks and troughs in each stream will not line up perfectly when recombined, creating a pattern of bright and dark spots called “interference fringes.” Changing the length or conditions in one stream shifts the fringes, allowing researchers to interpret what each stream encountered and how its atom waves responded. “It’s almost like having a ruler with calibration marks on it,” says atom interferometry pioneer David Pritchard of the Massachusetts Institute of Technology.
Light waves have long been collided in such designs, but atom waves are able to test more varied phenomena, and to higher precision. Atoms interact very strongly with electric and magnetic fields, other atoms, and gravity, whereas photons of light remain more aloof, says Pritchard. And because the wavelength of room-temperature atoms is 10,000-times shorter than that of light waves, he explains, atom interferometers have 10,000 times the potential resolution of those using photons.
Gravity Tests
That sensitivity could aid the search for dark matter, whose presence has been deduced based on its pull on ordinary matter, such as stars within a galaxy, but whose identity is unknown. Dark matter may form clumps, and so may vary in density as the Earth moves through its orbit. This variation may be detectable as fluctuations in how fast atoms fall toward Earth in atom interferometers placed at different locations in a laboratory or measured at different times of year. The reigning theory of particle physics, known as the standard model, does not account for dark matter, so measuring it this way would put researchers in direct contact with an entirely new realm of physics. “If we were to find direct evidence of dark matter, it would be a huge revolution,” says Alex Cronin of the University of Arizona in Tucson. Varying the type of atom used in such a direct measurement could help test competing models of the mysterious stuff.
Different atoms could also test a fundamental tenet of Einstein’s general theory of relativity: the equivalence principle, which says that objects of different mass should fall at the same rate because of gravity. This principle has been tested to a sensitivity of one part in 10 trillion by comparing how fast the Earth and Moon fall toward the Sun, for example. But Mark Kasevich of Stanford University and his colleagues hope to do 100-times better by dropping two different isotopes of rubidium atoms down a 10-meter-tall interferometer in the basement of the university’s physics building.
Kasevich is also studying how atom interferometry could be used to detect ripples in spacetime. Predicted by general relativity, these gravitational waves should be triggered by the movement of astronomical bodies, such as merging black holes, but have yet to be detected directly. If such a wave were to roll through spacetime in the presence of one or more atom interferometers, however, it could squeeze spacetime there, changing the length traversed by falling atoms and affecting their interference pattern. An atom interferometer placed in space would escape the gravitational vibrations of tides and earthquakes on Earth and might just be able to outperform proposed gravitational wave probes that use light interferometry, Kasevich says.
Atomic Physics
Interferometers can also be used to study the behavior of atoms themselves, says Cronin. By subjecting two atomic streams in an interferometer to electric fields of differing strengths, Cronin measured how much the positive and negative charges within the atoms shifted to align themselves with the fields. Using measurements of this “polarizability,” which the team has made for a handful of atoms (3), they can do a better job of predicting “how atoms collide, or the long-range interactions between atoms that enable them to form molecules,” he says.
The technique also presents a new way to determine the value of the fundamental constants of nature. Recently, for example, a group of researchers, led by Guglielmo Tino of the University of Florence in Italy, used free-falling rubidium atoms and a ton of weights to measure Newton’s constant, known as G, which describes the strength of the gravitational pull between bodies (4). So far, the technique has proven no more sensitive than previous methods, which have returned a confounding wide range of values for G, but researchers believe atom interferometry can do much better in the future.
Other teams are using the technique to measure the fine-structure constant, which determines the strength of interactions between charged elementary particles and light. The standard model predicts this strength, so a precise measurement of the constant will test the standard model “at a very deep level,” says Müller. Another application seeks to test how large an object can be made to obey the laws of quantum mechanics and behave like a wave.
But the technique is still young, says Cronin, and “the pleasure of discovering new applications for atom interferometry is very much alive.”
Never cross the streams.
We’ll cross the stream when we come to it.
Dark matter lives.
Starting to sound like “ether”.
ether or?
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