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Blog

Chu’s right about bouncing atoms?

By Hamish Johnston

UPDATE: Chu and colleagues have uploaded a second preprint related to the gravitational redshift debate.

Steven Chu and Claude Cohen-Tannoudji shared the 1997 Nobel Prize for Physics (along with William Phillips) for their work on the laser cooling and trapping of atoms.

Now the two Nobel Laureates find themselves on opposing sides of a “preprint battle” over the re-interpretation of an experiment done in 1998. The experiment involved using vertical laser pulses to bounce atoms up and down in order to study the interference patterns that occur when different atomic trajectories meet.

In February 2010, Chu (who is now US energy secretary) along with Holger Mueller and Achim Peters published new calculations showing that the experiment confirms gravitational redshift to a few parts in a billion.

A result of Einstein’s general theory of relativity, gravitational redshift is the stretching of the wavelength of a particle as it moves away from a massive object such as the Earth. Other experiments have confirmed this aspect of general relativity to much greater precision, but these involved macroscopic objects. Any deviation in redshift for a quantum particle such as an atom could point towards a unified theory of gravity and quantum mechanics – the Holy Grail of physics.

But about 10 days ago, Cohen-Tannoudji and colleagues uploaded a paper to the arXiv preprint sever in which they argue that Chu and colleagues have not measured gravitational redshift after all.

As far as I can tell, Cohen-Tannoudji and colleagues argue that Chu and company made a mistake in their calculation of the expected interference caused by a deviation from gravitational redshift. When the correct calculation is done, they say, a deviation from gravitational redshift has zero effect on what was measured in 1998.

Now, Chu and colleagues have hit back with their own preprint. It argues that Cohen-Tannoudji and team made their calculations using mathematics that assumes gravitational redshift cannot be violated – essentially precluding its violation!

Stay tuned for round three.

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2 comments

  1. John Duffield

    Interesting problem, here’s my twopennyworth: the thing that people tend to miss about Einstein’s operational time and principal of equivalence is that the photon in the lift hasn’t actually changed frequency. Conservation of energy applies, it’s the observer and his measuring devices that have changed, and it’s the same for gravitational redshift. There’s a change in the coordinate speed of light, and a caesium atom can be simplified to a circular light-clock. This is subject to gravitational time dilation (see http://www.nist.gov/physlab/div847/grp50/primary-frequency-standards.cfm) wherein the light goes round slower where gμv is lower. Imagine you’re between two such clocks at different altitudes, and that their circular motion generates sinusoidal electromagnetic waves. One photon comes up towards you, the other comes down towards you, but their frequencies don’t actually change. You would thus expect to be able to detect a “beat”. Whilst the Mueller/Peters/Chu experiment involves superposition rather than photon emission, it seems to be emulating this scenario, so I’m leaning towards their version of events.

  2. Pascvaks

    Cohen-Tannoudji and colleagues are right, Chu et al are left.
    Time will tell.

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