Uniting gravity with its quantum nemesis might take a detector the size of the universe. So say two physicists who think they have found a way to resolve one of the biggest conflicts in modern physics using high-resolution maps of the infant cosmos.
The universe is currently described using two distinct frameworks: gravity for larger objects such as planets and black holes, and quantum mechanics for the tiny world of bosons and quarks. Even though almost everyone expects these realms to be linked, no one has been able to build a bridge between the two. Previous work focused on trying to finding the graviton – the quantum unit of gravity – the sheer existence of which would join the two theories.
"It's actually very hard to construct a consistent theory in which gravity is not quantised but the rest of the world is," says Steven Carlip at the University of California, Davis.
Force particles
Three of the four fundamental forces are proven to be carried by their own particle, such as the photon that carries electromagnetism or the gluon that carries the strong force. So it makes sense that gravity should also have its own particle – the graviton. But gravity is the weakest force of the four forces, so we'd need something very massive to have the sensitivity required to detect a lone graviton. Some researchers have even suggested that it is impossible to ever find them, because a sufficiently sensitive detector would have to be so massive it would collapse into a black hole.
Now, Nobel laureate Franck Wilczek of the Massachusetts Institute of Technology and Lawrence Krauss of Arizona State University in Tempe suggest that our best shot may be to find ancient ripples in space-time called gravitational waves, which are predicted by general relativity.
Instead of hunting the graviton directly, they say, look to maps of the cosmic microwave background (CMB), the first light that travelled across the universe after the big bang. Such maps give strong evidence that the cosmos rapidly expanded during its first fractions of a second. That burst, called inflation, should have triggered gravitational waves that would have scattered the photons of the CMB in preferred directions, creating patterns of orientation in the light that are an imprint of the relic waves.
Quantum-mechanical activity
Wilczek and Krauss say that if we can find those imprints, we would have our long-sought evidence for quantum gravity. Using a mathematical tool called dimensional analysis, they found a positive link between the primordial gravitational waves and Planck's constant, which is used in quantum mechanics. That means quantum-mechanical activity would have been needed to create the gravitational waves during inflation, Wilczek says.
The idea is plausible, although it relies on a few unstated assumptions, says Carlip, who was not part of the team. For instance, physicists will need to be able to show that the polarisation of the CMB is down to the proposed primordial gravitational waves and not the result of some other process.
Wilczek reckons we will not have the sensitivity to detect the influence of primordial gravitational waves in the CMB for at least 10 to 15 years, despite new high-resolution maps from the Planck satellite.
However, if the waves really are quantum in nature, it will be worth the wait. "Most theoretical groups are convinced the gravitational field should be quantised," says Wilczek. "But while no one was surprised by the existence of the Higgs particle, based on theoretical predictions, it's quite another thing to see the damn particle."
Journal reference: arxiv.org/abs/1309.5343
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