Researchers Demonstrate that a Glowing Object Attracts Atoms

UC Berkeley physicists state that an individual’s physical attraction to hot bodies is real.

The blackbody attraction between a hot tungsten cylinder and a cesium atom is 20 times stronger than the gravitational attraction between them. (Holger Müller graphic)

To be clear, the physicists are not talking about sexual attraction towards a “hot” human body.

However, the researchers have demonstrated that a glowing object in fact attracts atoms, opposing to what most people – including physicists – would guess.

The small effect is much similar to the effect a laser has on an atom in a device known as optical tweezers, which are employed for trapping and studying atoms, a discovery that resulted in the 1997 Nobel Prize in Physics shared by former UC Berkeley professor Steven Chu, presently at Stanford, Claude Cohen-Tannoudji and William D. Phillips.

Three years ago, a team of Austrian physicists were the first to predict it, and before that no one believed that regular light, or also just the heat released by a warm object – the infrared glow seen when looking through night-vision goggles – could affect atoms in the same manner.

UC Berkeley physicists, who are skilled at measuring minute forces by employing atom interferometry, planned an experiment to analyze this. The prediction was confirmed when they measured the force applied by the blackbody radiation from a warm tungsten cylinder on a cesium atom.

The attraction is in fact 20 times the gravitational attraction between the two objects, however since gravity is considered to be the weakest of all the forces, the effect on cesium atoms – or any other atom, larger object or molecule – is usually too small to worry about.

It’s hard to find a scenario where this force would stand out, It is not clear it makes a significant effect anywhere. Yet.

Victoria Xu, Co-author and Graduate Student, Physics Department, UC Berkeley

Even though, gravity measurements become more accurate, they affect this small necessity to be taken into account. The next generation of experiments to sense gravitational waves from space may employ lab-bench atom interferometers instead of the kilometer-long interferometers presently in operation. Interferometers normally incorporate two light waves in order to detect small changes in the distance they have traveled; atom interferometers incorporate two matter waves in order to detect small changes in the gravitational field they have experienced.

This force would also have to be considered for extremely precise inertial navigation employing atom interferometers.

“This blackbody attraction has an impact wherever forces are measured precisely, including precision measurements of fundamental constants, tests of general relativity, measurements of gravity and so on,” said Senior Author Holger Müller, an associate professor of physics. Xu, Müller and their UC Berkeley colleagues published their study in the December edition of the Nature Physics.

Optical tweezers

Optical tweezers operate since light is a superposition of electric and magnetic fields – an electromagnetic wave. The electric field in a light beam allows charged particles to move. In a small sphere or an atom, this can indeed separate positive charges, such as the nucleus, from negative charges, such as the electrons. This produces a dipole, allowing the sphere or atom to act like a small bar magnet.

The electric field in the light wave will then be able to move this induced electric dipole around, just as one can use a bar magnet for shoving around a piece of iron.

Scientists can levitate a bead or an atom to conduct experiments by using more than one laser beam.

Müller’s team discovered that with incoherent, weak light, like blackbody radiation from a hot object, the effect is observed to be much weaker, but still there.

The team measured the effect by placing a dilute gas of cold cesium atoms – cooled to three-millionths of a degree more than absolute zero (300 nanoKelvin) – in a vacuum chamber and then launched them upward with a quick pulse of laser light.

Half are provided with an extra kick up towards an inch-long tungsten cylinder glowing at 185 oC (365 oF), while the other half continued to be unkicked. When the two groups of cesium atoms fall and then meet again, their matter waves inhibit, permitting the researchers to measure the phase shift brought about by the tungsten-cesium interaction, thus calculating the attractive force of the blackbody radiation.

People think blackbody radiation is a classic concept in physics – it was a catalyst for starting the quantum mechanical revolution 100 years ago – but there are still cool things to learn about it.

Victoria Xu

The research was financially supported by the David and Lucile Packard Foundation, National Science Foundation (037166), Defense Advanced Research Projects Agency (033504) and National Aeronautics and Space Administration (041060-002, 041542, 039088, 038706, and 036803). Other co-authors include Philipp Haslinger, Matt Jaffe and Osip Schwartz of UC Berkeley, Matthias Sonnleitner of the University of Glasgow, Monika Ritsch-Marte of the Medical University of Innsbruck in Austria and Helmut Ritsch of the University of Innsbruck.

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