The light, and winding roads

With a large enough sample, photons average the same distance regardless of obstacles. Andrew P Street reports.

Milk looks white because of scattering photons, but scatter enough and they take no longer to pass through than light through water.
Milk looks white because of scattering photons, but scatter enough and they take no longer to pass through than light through water.
Tony Robins/Getty Images

A beam of light, as we know, moves in a perfectly straight line in a vacuum (give or take the odd bit of gravitational lensing). When it’s moving through something that’s not a vacuum – air, for example, or water – you’d reasonably think it has a bit more troublesome a time of it because the photons keep bumping into things and moving off in complicated paths.

And, you’d figure, if you had a liquid that was filled with stuff – milk as opposed to water, let’s say – that beam of light would travel further within it because it would be constantly changing direction as atoms got in the way. After all, that’s why milk is white and vacuums are clear: the scattering of light waves in the former and the complete absence of scattering in the latter.

Except it turns out that if you test a bunch of them, the amount of distance covered by beams of light is the same, regardless of whether they zip through a clear glass of transparent water or a thick, turbid liquid. In fact, when all the data is added up, the distance that light beams cover, passing straight through or zig-zagging wildly from particle to particle, is exactly the same.

This unexpected idea was the theoretical prediction of physicist Stefan Rotter and his team at Technische Universitat Wien, Austria, back in 2014 – and now it’s been proved experimentally in a paper published in the journal Science.

Rotter and his colleagues fired beams of light through test tubes of water, to which nanoparticles were added. Measurement showed that no matter how much interference the light waves experienced, the average path length of the light was exactly the same.

And it’s that important word – average – that is the key to this counterintuitive result. Some photons of light will hit the surface and zip out while others will bounce around like pachinko balls, but the mean path length of the light through the liquid remains the same regardless of turbidity.

This experiment backs up the theoretical models of how waves behave in disordered media, and that has implications far beyond how light behaves in milk.

"The same rules that apply to light in an opaque liquid also hold for sound waves, scattered at tiny objects in air or even gravity waves, travelling through a galaxy,” Rotter explains. “The basic physics is always the same."

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Andrew P Street is a widely published journalist, non-fiction author and former columnist for the Sydney Morning Herald.
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