Toward a Quantum Theory of Gravity?

[gwillybj]

Toward a Quantum Theory of Gravity? (Part 1)
Mario Livio, Astrophysicist, Space Telescope Science Institute
Posted: 04/16/2014 3:23 pm EDT Updated: 04/16/2014 3:59 pm EDT

The recent potential detection of ripples from the Big Bang by the BICEP2 telescope (Fig. 1) has justifiably generated huge excitement. If confirmed, the ripples represent an imprint on the cosmic microwave background by gravitational waves. Those gravitational waves are produced through a quantum process, providing, for the first time (again, if confirmed), evidence that gravity is governed by quantum mechanics. This point cannot be overemphasized. For decades the key goal of fundamental physics has been to unify the theory that works so well on the universe's largest scales -- Einstein's theory of general relativity -- with the theory of the subatomic world -- quantum mechanics. The BICEP2 results do not point the way to what that unified theory might be, but they provide evidence for the fact that general relativity and quantum mechanics can be made compatible.


Figure 1. The BICEP2 telescope (on the right) at the south pole. On the left is the South Pole Telescope. (Image in the public domain.)

In recent years physicists have taken a few encouraging steps toward a potential theory of quantum gravity. In particular, UCLA physicist Zvi Bern and his collaborators have revived from the dead a theory known as supergravity, which assumes that in addition to the gravitons -- the presumed quantum carriers of the gravitational force -- other mirror particles exist as well. While Bern does not suggest that supergravity is the ultimate theory, he hopes that it may at least provide the skeleton for a more complete theory.

One of the main problems in attempting to calculate gravitational interactions with gravitons has been that the calculations produced unphysical infinities at almost every step. The way particle physicists go about calculating the results of interactions is by summing up all the probabilities for all the possible processes. Originally, these labor-intensive efforts were believed to lead to values that simply blow up. Bern and colleagues, however, managed to enormously simplify the calculations by showing that, at least in some cases, gravitons can be replaced by two copies of gluons -- the carriers of the strong nuclear force. (Fig. 2 shows the "standard model" of particle physics and the place of gluons within it.) If this double-copy-of-gluons relationship holds in general, this clue could potentially lead to a dramatic breakthrough in the search for a quantum theory of gravity. The important point is that physicists already have at their disposal a fairly successful theory -- quantum chromodynamics -- to describe the interactions of gluons. The jury is still out on whether Bern's shortcuts and the idea of supergravity work in more complex calculations, and, more importantly, whether they truly apply to the real physical world, but one is beginning to sense a certain level of optimism.


Figure 2. The standard model of elementary particles. The gluons are the carriers of the strong nuclear force. (Figure in the public domain.)

One piece of the puzzle that, at the moment, appears difficult to crack is how to deal with black holes -- those singularities in classical general relativity that manifestly represent a breakdown of the theory on the quantum scales. In the next blog post I shall discuss an exciting new development in that realm.

Continued...

http://www.huffingtonpost.com/mario-livio/toward-a-quantum-theory-of-gravity_b_5153021.html

[gwillybj]

Toward a Quantum Theory of Gravity? (Part 2)
Mario Livio, Astrophysicist, Space Telescope Science Institute
Posted: 05/02/2014 8:25 pm EDT Updated: 05/02/2014 8:59 pm EDT

In classical general relativity, black holes represent those points at which the fabric of space-time becomes so steeply warped that nothing can escape from them. One of the key goals of a quantum theory of gravity is precisely to resolve such "pathological" situations and to describe black holes as complex but self-consistent quantum systems. The problem is that as physicists have attempted to do just that, they have encountered the so-called "information paradox." Quantum mechanics tells us that information about quantum states of a system falling into a black hole cannot be irretrievably lost. At the same time, black holes can evaporate through the emission of Bekenstein-Hawking radiation. What happens to the information, then? Put differently, can two observers, one outside the black hole's boundary of no return (the "event horizon") and one falling through it, compare or share information?

Opinions on these questions varied, with some physicists arguing that the information is lost, and other contending that it is not. In the language of quantum entanglement, when the Bekenstein-Hawking radiation is emitted, the particle that escapes and the one that is swallowed by the black hole are mutually entangled. However, research suggested that the escaping particle is also entangled with all the previously emitted Bekenstein-Hawking radiation, contrary to a principle of "monogamy" stating that no particle can be entangled with two systems at the same time. To resolve this paradox, it has recently been suggested that there exists at the event horizon a "firewall," high-energy quanta emitted as the entanglement between the infalling and outgoing particles is broken. However, this in itself violates an important general relativistic concept: that if the black hole is massive enough, nothing dramatic should happen at the event horizon, since any system there is at free fall.

Faced with the possibility of having to relinquish either a principle of quantum mechanics (loss of information, or "unitarity") or one of general relativity (no drama at the event horizon, or the "equivalence principle"), physicists are now considering the potential loss of another assumption: locality. Locality basically asserts that particle interactions occur when the particles find themselves at adjacent points in space-time. Until now, this is what allowed particle physicists to calculate probabilities of interactions using small diagrammatic representations known as Feynman diagrams (e.g., Fig. 1).


Figure 1. A Feynman diagram representing an electron-positron pair annihilating into a photon, which in turn produces a quark-antiquark pair, with the antiquark radiating a gluon. (Credit: Joel Holdsworth/Wikimedia Commons.) http://en.wikipedia.org/wiki/File:Feynmann_Diagram_Gluon_Radiation.svg

Now Princeton physicist Nima Arkani-Hamed and colleagues have introduced a new method for calculating such probabilities, by determining the volume of a geometrical figure known as the amplituhedron (see an image here). The calculations are currently done for gluons, and they don't assume locality at all. Since graviton interactions may perhaps be computed through copies of gluon interactions (see "Toward a Quantum Theory of Gravity? (Part 1)"), the amplituhedron may provide insights into a non-local description of quantum gravity.

One thing is clear: While the road ahead still seems to be long and winding, with the possibility that physicists will have to give up some cherished concepts, there is no shortage of new ideas and fascinating research.

http://www.huffingtonpost.com/mario-livio/toward-a-quantum-theory-of-gravity_b_5234837.html