Quantum gravity corrects QED
Nov 3, 2010 10 comments
This week's issue of Nature includes a paper that's remarkable for two reasons: it is about quantum gravity – a topic usually not covered in the journal – and it is written by just one person. Now, after a little digging, physicsworld.com can answer all of the important questions about this paper.
So, whose citation index ranking is about to go into the stratosphere?
The paper was written by David Toms, a Canadian mathematical physicist and lecturer at Newcastle University in the UK.
What has Toms done?
He has shown that interactions between quantum gravity and quantum electrodynamics (QED) cause electric charge to vanish at very high energies (above about 1015 GeV). He told physicsworld.com that his technique can be generalized to apply to the two other "gauge couplings", which define the strong and weak forces.
Why should electric charge vanish at high energies?
A major problem with QED, which describes the interaction between charged particles and photons, is that electric charge increases at higher interaction energies. This is a result of vacuum polarization, whereby the spontaneous creation of electron–positron pairs tends to screen the electric charge of a particle at low energies. At higher energies, however, the screening is much reduced and the effective charge increases – and this cannot be correct.
Can you explain?
Physicists already know that the strong force – which binds together quarks within hadrons – goes to zero at extremely high energies. This property is called asymptotic freedom and its discovery earned Frank Wilczek, David Gross and David Politzer the 2004 Nobel Prize for Physics. If it can be proved that quantum gravity makes QED asymptotically free then it could stand as a viable theory on its own.
Can you elaborate slightly?
The main reason why QED was viewed as incomplete, prior to Gross et al, was that without asymptotic freedom the electric charge becomes infinitely large at some energy scale and the theory is no longer reliable. For their calculations to be reliable at high energies, physicists expect the strong, weak and electromagnetic forces to become unified and become asymptotically free.
Hold on, didn't Frank Wilczek and Sean Robinson establish gravity-induced asymptotic freedom of charge in 2006?
Yes, sort of. Robinson and Wilczek came up with the idea of gravity-driven asymptotic freedom and worked out that it applied to all three gauge couplings (Phys. Rev. Lett. 96 231601). It was later pointed out, however, that there were errors in their calculations. This caused a flurry of activity as other physicists tried and failed to do the calculation using different approaches.
Now, Toms has worked out a way of avoiding these errors by performing a set of careful checks to guarantee that the calculation meets certain mathematical and physical criteria. In doing so, he has shown that Robinson and Wilczek's idea was correct all along.
So what do they have to say?
"Toms' work is important equally as much because of the way in which he did the calculation as the result itself," said Robinson who is a lecturer at Massachusetts Institute of Technology. He said that an important feature of the technique is that it is "demonstrably flawless". He also pointed out that while Toms' paper was under review at Nature, an independent group of physicists at Tsinghua University in China posted a preprint (arXiv: 1008.1839) using a similar "flawless" technique but a different set of cross-checks. The Tsinghua team obtained essentially the same result as Toms, illustrating the power of the technique.
That must be good news for physicists working on unification?
Sort of. Toms has shown that quantum gravity causes asymptotic freedom in all the gauge couplings. This is handy if you want to show that all forces unify in a single (very weak) force at very high energies. However, he treated quantum gravity by simply quantizing Einstein's general theory of relativity. This approach breaks down at the very energies that unification is expected to occur. To take things further, physicists would need to integrate more exotic aspects of quantum gravity such as additional dimensions and supersymmetry.
Where can I find out more?
About the author
Hamish Johnston is editor of physicsworld.com