New observations have shown that the quantum structure of space, if there is any, is at much smaller scales than predicted.

In Einstein’s theory of relativity, space is a smooth, continuous fabric. In quantum theory, there is an inherent discreteness present in physics. In an attempt to reconcile the two theories, it is postulated that spacetime should be quantized at the smallest scales, that is grainy.

Now, ESA’s Integral gamma-ray observatory has placed new limits on the size of these quantum “grains” in space: if they exist, then they are much smaller than most quantum gravity theories predict.

Philippe Laurent of CEA Saclay and his colleagues used data from one of Integral’s instruments, IBIS, to study the difference in polarization between high and low-energy gamma rays emitted during one of the most powerful gamma-ray bursts (GRBs) ever seen.

GRBs are the most energetic explosions known in the Universe. Most of them are thought to occur during supernovae explosions (some are of unknown origin), producing a huge pulse of gamma rays that outshine entire galaxies during a few seconds, up to a few minutes.

If space has an underlying grainy structure, these tiny grains should twist light rays, changing the direction in which they vibrate, their polarization. In the case of gamma rays, high-energy ones should be more twisted than low-energy ones. By measuring the difference in polarization between high and low-energy gamma rays, astrophysicists can estimate the size of the grains.

That is precisely what Dr Laurent and his colleagues did with GRB 041219A, a gamma ray burst that took place on December 19, 2004.

According to some quantum gravity theories, the quantum nature of space should manifest itself at the Planck scale, around 10^{-35 }meter, which can be defined from three fundamental physical constants: the speed of light in a vacuum, Planck’s constant and the gravitational constant.

However, Integral’s observations, the most precise ever made, show that if there is any quantum “graininess”, it is at a level of 10^{-48 }meter or smaller!

An important detail is the distance at which GRB 041219A was observed: at least 300 million light years. Over large distances, the twisting effect should accumulate into a detectable signal. Here, absolutely nothing was detected, pushing the limits further.

What are the implications of this result? They are pretty important for many theories. Indeed, according to quantum theory, and more precisely string theory, any measurement below the Planck scale is meaningless to physics… Loop quantum gravity is also completely ruled out by this new result. This could also disprove the holographic universe hypothesis.

Finally, in the light of this new result, quantum theoreticians will have to rethink their theories, as most of quantum gravity theories have now been seriously challenged. Maybe Einstein was right once again, and spacetime is indeed perfectly smooth and continuous…

**Reference**

P. Laurent, D. Götz, P. Binétruy, S. Covino, A. Fernandez-Soto. **Constraints on Lorentz Invariance Violation using integral/IBIS observations of GRB041219A**. *Physical Review D*, 2011; 83 (12) DOI: 10.1103/PhysRevD.83.121301