One big problem with theories of everything is that their predictions of how space-time behaves at the smallest scales are way beyond the reach of our experiments. At least, that is the common wisdom. Now it seems that certain properties of space-time predicted by these theories may have already influenced experiments without anyone noticing.
The challenge for a theory of everything is to unify Einstein鈥檚 description of gravity with quantum mechanics into a successful theory of quantum gravity. There are many candidate theories that predict that space-time fluctuates rapidly on so-called Planck scales of about 10-35 metres, but that鈥檚 too small to be probed directly. 鈥淭o smash our way down to these scales in a particle accelerator would take inconceivable amounts of energy,鈥� says Charles Wang, of the University of Aberdeen, UK.
Despite this, there may yet be a way to measure these space-time fluctuations indirectly, say Wang and teammates Robert Bingham and J. Tito Mendon莽a of the Rutherford Appleton Laboratory near Oxford, UK. They were inspired by Einstein鈥檚 work on Brownian motion 鈥� the phenomenon in which pollen grains suspended in water can be seen jiggling about when looked at through a microscope. Einstein realised that the relatively large grains were being jostled by the far smaller, and invisible, molecules of water. 鈥淓instein was able to deduce the properties of the water molecules he couldn鈥檛 see by looking at the pollen,鈥� says Wang. 鈥淲e just needed to find an equivalent to pollen that will be jostled by fluctuations in space-time.鈥�
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鈥淲e just needed to find an equivalent of the pollen in Brownian motion that would be jostled by space-time鈥�
Wang believes that a technique called matter wave interferometry could be the answer. According to quantum mechanics, matter has wave-like properties, and a beam of atoms can also be thought of as a beam of matter waves. In matter wave interferometry, a single coherent beam of atoms is split into two. The beams are sent along different paths and then recombined to produce a pattern of light and dark interference fringes. Wang says that kinks in space-time would knock the matter waves about and force them to 鈥渄ecohere鈥�, or return to classical, particle-like behaviour, causing the interference fringes to blur slightly (see Graphic).
Such blurring has already been seen in experiments carried out by Nobel laureate Steven Chu at Stanford University in California, and also by a German group led by Klaus Hornberger at Ludwig Maximilian University in Munich. Chu used a beam of caesium atoms, while Hornberger used a beam of much larger fullerene particles. Both teams attributed the blurring to the decoherence of beams caused by heat and other environmental factors, says Wang.
Now Wang鈥檚 team has re-analysed those results and seen a curious coincidence. They have shown that even though the two experiments used atoms with very different masses, the decoherence in both experiments can be explained by space-time kinks at scales of about 10-31 metres (Classical and Quantum Gravity, vol 23, p L59). 鈥淵ou wouldn鈥檛 expect both atom beams to be disturbed in the same way unless the disturbance was caused by a fundamental property,鈥� says Wang. 鈥淭he experimental results may have been much more significant than first realised.鈥�
鈥淵ou wouldn鈥檛 expect both atom beams to be disturbed in the same way unless it was caused by some fundamental property鈥�
Using these preliminary results, Wang鈥檚 team estimated the scale at which quantum gravity transitions to classical gravity as described by general relativity, and found it to be about 10-31 metres, much smaller than the 10-19 metres predicted by certain string theories.
To ensure accuracy, Wang wants similar experiments to be carried out in space, where vibrations and the Earth鈥檚 rotation won鈥檛 distort the results. The European Space Agency plans to send an atom interferometer into space on its HYPER mission by 2020. 鈥淚f we carry out the experiment again in space and our results remain unchanged, that will have big implications for string theory,鈥� Wang says.
Hornberger says that more work needs to be done before conventional explanations for the blurring can be ruled out, but he adds that Wang鈥檚 tantalising work is worth following up: 鈥淭his idea could allow us to investigate regions [of space-time] that would otherwise lie far out of reach.鈥�
