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The idea that spacetime is quantised at the Planck scale has been around for almost as long as physicists have attempted to reconcile general relativity and quantum mechanics. In the 1960s, John Wheeler coined the term quantum foam to describe the quantised structure of the universe at distances of around 10^-35 metres or so.

One question that has occupied physicists since then is how to detect this foamy structure. Today, Wayne Christiansen at the University of North Carolina at Chapel Hill and a few buddies say that various large telescopes are on the verge of being able to make measurements that could prove the existence of quantum foam or place important limits on its structure.

The thinking goes like this. One of the consequences of spacetime being quantised is that it places a fundamental limit on how accurately distances can be measured.

So imagine that you want to measure distance using a beam of light. This light will be influenced by the structure of spacetime and its wavefront will acquire a similarly foam-like structure. This limits the accuracy of the distance measurements that can be made with this light.

It also effects the way light from a point source should appear since the wavefront has a random foamy element to it. “In effect, spacetime foam creates a “seeing disk”,” says Christiansen and co, and this should be visible on images of certain distant point sources.

The trouble is that the effect is so tiny that it is only visible in images of objects over truly cosmological distances.

That’s OK though, say Christiansen and co, because the Hubble Space Telescope has photographed exactly the kinds of objects that are far enough away to demonstrate the effect. These objects known as high redshift quasars appear in an image known as the Ultra Deep Field.

Their paper today assesses these quasars in the Ultra Deep Field to determine to what extent they show evidence of quantum foam.

And here’s a thing: Christiansen and pals say that these quasars are blurred in exactly the way you’d expect from quantum foam in certain kinds of models of the universe. “The blurring is at a level consistent with a spacetime foam model,” they say.

But that’s not conclusive because the blurring may also have been caused by other effects such as some other scattering medium like dust in the intervening distance. Another possibility is that quasar may not be a true point source and the blurring reflects the structure of the quasar itself.

These are questions that the next round of observations may be able to resolve. Hubble’s recent refit has given it the ability to make better measurements. And the Very Large Telescope Interferometer now under construction could also make valuable contributions when it is completed.

Which means that we could be on the verge of finding the first evidence of the foam-like structure of spacetime. An area to watch.

Ref: arxiv.org/abs/0912.0535: A Cosmic Peek at Spacetime Foam


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