What about the slowing and stopping of light?

by Dr Jonathan D. Sarfati

1 February 2001

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There have been a number of recent reports in scientific journals about variations on the speed of light. Last year, we posted a brief response by physicist Dr Russell Humphreys to experimental results allegedly showing information transmitted faster than the speed of light in a vacuum (called c, = 300,000 km/sec = 186,000 miles/sec). There have also been reports about the opposite effect—markedly slowing down light itself.

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Two years ago, a team led by Dr Lene Vestergaard Hau of Harvard University slowed light to just 17 m/s (38 mph) in a cloud of ultracold (only about a millionth of a degree above absolute zero) sodium atoms.^1,2 This cloud was a special phase of matter called a Bose-Einstein condensate, where all the atoms have collapsed to the same quantum state. Another ‘coupling’ laser beam, perpendicular to the main light and with a different polarization, split a single atomic absorption level into two, which cancelled each other out by destructive quantum interference. This cancellation of absorption produced the phenomenon of electromagnetically induced transparency.²

A number of enquirers asked if this experiment was significant for distant starlight travel time or c-decay theories. We pointed out that that light is always slowed down in matter, because it is constantly absorbed and re-emitted—the light is focused on the retina in your eye precisely because the lens and cornea slow down the light by about 30%. Many people need an outside lens which works on the same principle. However, this experiment has little to do with cosmology, because it’s the speed in a vacuum that counts for distant starlight travel time and for anything to do with c-decay theories. Einstein’s special theory of relativity states that light’s speed in a vacuum is constant. If anything, postulating that light had to travel slowly through a Bose-Einstein condensate would make the universe even older. [We do have answers, though—see How can we see distant stars in a young Universe?]

Now, Dr Hau^3,4 and another team led by Dr Ronald Walsworth and Dr Mikhail Lukin of the Harvard-Smithsonian Center for Astrophysics independently announced that they have slowed—and stopped—light.⁵ This time, Dr Hau’s team turned off the coupling beam before the probe pulse had emerged, so the pulse ground to a halt. When the coupling beam was turned back on, the probe pulse re-emerged. What happened was that the information in the probe pulse was stored in a ‘quantum coherence pattern’ imprinted on the atoms, and when the coupling beam was turned on, this information was ‘read out’ and converted into photons again. They could even chop up the original pulse into three, or ‘read out’ such that the re-emerging pulse was shorter and more intense.⁴ Dr Hau’s team claim that this system could be used in quantum information processing. [Note added 26 July 2001: this is even more plausible since researchers led by Alexey Turukhin of the Massachusetts Institute of Technology have demonstrated a similar effect in a solid. They slowed light to 45 m/s in a crystal of yttrium silicate doped with praseodymium, and the results were presented to the Quantum Electronics and Laser Science Conference in Baltimore in May.⁶]

Once again, this has nothing to do with creationist cosmology, but it’s still fascinating science!⁷ The nuclear physicist Dr Humphreys reinforced this point during a recent interview on AnswersLIVE: ( Click here to listen to the full broadcast)

“… the problem is how to do the reverse—how to speed it up faster than its speed in a vacuum, or its speed in air [which] is about the same as that in a vacuum. No one’s done that. I don’t think that’s necessarily impossible either—it’s just you would have to do something different than what people are doing now.”

References

1. Hau, L.V., Harris, S.E, Dutton, Z. and Behroozi, C.H., Light speed reduction to 17 metres per second in an ultracold atomic gas, Nature 397(6720):594–598, 18 February 1999. Return to text.

2. John Marangos, J., Slow light in cool atoms [comment on Ref. 1], Nature 397(6720):559–560, 18 February 1999. Return to text.

3. Chien Liu, Dutton, Z., Behroozi, C. and Hau, L.V., Observation of coherent optical information storage in an atomic medium using halted light pulses, Nature 409(6819):490–493, 25 January 2001. Return to text.

4. Cornell, E.A., Stopping light in its tracks [comment on Ref. 3], Nature 409(6819):461–462, 25 January 2001. Return to text.

5. Scientists Bring Light to Full Stop, Hold It, Then Send It on Its Way, New York Times, 18 January 2001. Return to text.

6. New Scientist 170(2291):25, 19 May 2001. Return to text.

7. [Note added 9 July 2001] See Dr Hau’s popular/semi-technical article, Frozen light, Scientific American 285(1):66–73, July 2001. Return to text.

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