Motus Mentis

There is a fascinating item making the rounds today. It seems that mysterious noise in an exquisitely sensitive gravity-wave detector may in fact be an indication that our instruments are now sensitive enough to detect the granularity of spacetime itself — and the argument is a very interesting one.

The last time physicists announced that an inexplicable noise in a sensitive detector was evidence that something funny was going on was when Arno Penzias and Robert Wilson were trying to build a quiet microwave antenna, back in 1964; it turned out they had stumbled on the remnant black-body radiation of the Big Bang. So probably we ought to pay attention.

The gravity-wave detector in question, near Hanover, Germany, uses laser interference between two beams pointed at right angles to each other — essentially the same idea as the Michelson-Morley experiment (arguably the most famous negative result in scientific history). The idea in this case is that any squeezing or stretching of spacetime in one direction will be visible as a change in the way the orthogonal beams interfere.

The marvelously accurate device, which can measure variations smaller than the diameter of a proton, has been troubled by random jitters. Nobody knows why — except, perhaps, a Fermilab physicist named Craig Hogan, who has a remarkable idea.

Hogan’s explanation has to do with the quantum-mechanical properties of the ‘event horizons’ that mark the boundaries of black holes. One of the objections made when Stephen Hawking announced that black holes can evaporate (by a mechanism that is a wonderful insight in itself, but we’ll leave that aside for now) was that when they are gone, they appear to have taken with them all the information they originally contained — everything about the quantum-mechanical state of the collapsed stars that formed them. But at the quantum level, information is supposed to be conserved.

It has since been shown, however, that all the information within the black hole is preserved in the patterns of quantum activity on the surface of the event horizon. In effect, there is a ‘lossless’ projection of the information inside the black hole onto its ‘skin’.

But if this is to be truly lossless, then we are saying that all the information inside the 3-D volume of the black hole is mappable onto its 2-D surface. If the ‘bits’ on the skin, then, are of the minimum possible quantum ‘grain size’ — a very small value known as the Planck length — then to make a one-to-one mapping to the contained volume, the 3-D ‘bits’ inside must be larger, because the volume of a sphere is larger than its surface. With me so far?

Hogan’s insight is that the observable Universe itself has an event horizon, namely the radius beyond which no light can have reached us since the Big Bang. If the same principle applies, then the quantum activity at the horizon, which happens in Planck-sized bits, must map the information of the entire visible Universe. But to make that possible, the bits down in here have to be a great deal larger than the Planck length. And random fluctuations at this much larger scale are exactly what is being observed in Germany.

The point? First, that our instruments may already be beginning to see things at the smallest scale possible: the inherent “grain size” of space and time. Second, Hogan’s idea, if correct, means that the entire Universe is a kind of colossal hologram.

Very heady stuff! Learn more here.