You may remember the hubbub that a Fermilab physicist caused last year when he started to investigate some strange results coming from the GEO600 gravitational wave experiment.
In a nutshell, GEO600 — a mindbogglingly sensitive piece of kit — started to detect what particle physicist Craig Hogan interpreted as quantum “fuzziness." This fuzziness, or blurriness on the smallest possible scales, could be interpreted as evidence for the “holographic universe" hypothesis.
This hypothesis describes the 3-dimensional universe we live in as a projection from a 2-dimensional “shell" at the very edge of the universe. As with any projection, the projected “pixels" will become fuzzy the closer you zoom in on them. The quantum fuzziness GEO600 seemed to detect could be evidence for this projection effect. The Universe is therefore a hologram, so the idea goes.
Spurred on by the GEO600 results, Hogan is currently working on a project to build a “Holometer" at Fermilab to probe these quantum scales, hopefully shedding some light on what this fuzziness could be.
However, as announced this week, a space-borne European satellite that should be able to measure these small scales too, doesn't appear to be registering any quantum fuzziness. In fact, it has yet to detect anything quantum, indicating that spacetime's “graininess" is composed of quanta that a lot smaller than predicted — and in my view, puts a question mark over the interpretation of the GEO600 results.
Gamma-Ray Bursts and Grains of Quanta
The European Space Agency's Integral gamma-ray observatory can make very precise measurements of the gamma-rays emitted by energetic (and often mysterious) gamma-ray bursts (GRBs).
GRBs are thought to be caused by the collapse of massive stars as they reach the end of their lives, explode and form neutron stars or black holes. As they explode, they blast a high-energy pulse of gamma-ray radiation from their poles, outshining entire galaxies. If correctly aligned with Earth, we can detect GRBs as a bright, transient flash.
As the gamma-rays — high-energy photons that exist at the extreme end of the electromagnetic spectrum — travel through space, their polarization (or “twist") is affected by the spacetime they travel through.
If spacetime is composed of tiny quantum “grains," the gamma-ray photons' polarization should change from random polarization (at the GRB source) to biased toward a certain polarization when received by the Integral spacecraft.
Also, high-energy gamma-rays should be more twisted than lower energy gamma-rays; the difference in the polarization can therefore be used to estimate the size of the quantum grains.
What's the Polarization?
If spacetime was smooth and continuous (as Einstein viewed the Universe), the polarization will remain random, and there will be no difference between high- and low energy photons no matter how far the gamma-rays travel. But if spacetime is composed of grains (as quantum mechanics predicts), the further the gamma-rays travel, the greater the polarization difference.
So, Philippe Laurent of CEA Saclay and his collaborators analyzed the polarization of gamma-rays from a very energetic gamma-ray burst. GRB 041219A occurred on Dec. 19, 2004, and it was immediately recognized as being in the top one percent of GRBs for brightness.
Also, due to its distance — 300 million light-years away — data from this explosion should have also revealed a measurable difference in the polarization between low- and high-energy gamma-ray photons.
Alas, no polarization difference was detected.
Some theories predict the quantum graininess should manifest itself at scales of around 10-35 meters — a scale known as the Planck length, the fundamental scale for quantum dynamics. Through the precise nature of its polarization measurements, Integral hasn't found any quantum graininess down to a scale of 10-48 meters; that's 10,000,000,000,000 times smaller than the “fundamental" Planck length.
So, if quantum predictions are correct, the spacetime quanta must be made from grains that are 10-48 meters in scale or less.
What does this mean?
Holographic Universe… or Not?
For Hogan's interpretation of the GEO600 results to be correct, this graininess should be measurable over larger scales. In fact, GEO600 started to detect quantum fuzziness at scales of around 10-16 meters — that's 10,000,000,000,000,000,000 times larger than the Planck length.
At first glance, the Integral results appear to contradict the GEO600 interpretation, therefore disputing the holographic universe hypothesis all together. If these “fuzzy" 10-16 meter scales aren't detected through Integral's polarization measurements of gamma-rays, perhaps the GEO600 quantum fuzziness is an effect of overlooked instrumental error.
However, all may not be lost.
The Integral polarization results depend on spacetime being constructed from discrete quanta that behave in a way that fits with quantum theory. The holographic universe hypothesis goes one step further, constructing 3-dimensional spacetime from projections of a 2-dimensional “shell" — perhaps gamma-ray photons behave differently in this fuzzy, projected, quantum world, and this could be why no polarization difference between gamma-ray photons are detected.
Proving or disproving a holographic universe, of course, isn't the focus of this Integral study; it is an attempt at revealing the very fabric of spacetime, helping physicists understand what our Universe is made of.
“This is a very important result in fundamental physics and will rule out some string theories and quantum loop gravity theories," said Laurent in the ESA press release.
“Fundamental physics is a less obvious application for the gamma-ray observatory, Integral," added Christoph Winkler, ESA's Integral Project Scientist. “Nevertheless, it has allowed us to take a big step forward in investigating the nature of space itself."