“Many physicists believe that there must a beautiful set of laws in Nature and that one way to quantify the beauty is by symmetry. Some of the symmetries may be hidden in our world, but they should manifest themselves if we look at Nature at a more fundamental level. We showed that this expectation is wrong once we take into account the gravity,” said Caltech’s Hirosi Ooguri, Director of the Kavli Institute for the Physics and Mathematics of the Universe, about a new study that has found that, when gravity is combined with quantum mechanics, symmetry is not possible.
There are four kinds of fundamental forces in Nature: electromagnetism, strong force, weak force, and gravity. Of the four, the gravity is the only one still unexplainable at the quantum level. Neither the Newtonian model nor relativity is a fundamental explanation for gravity. We still don’t know how the fundamental, quantum properties of mass, energy and space-time combine to create the phenomenon.
There ought to be particles that deliver the gravitational force in the same way that the electromagnetic force, for example, is delivered by photons. This picture is known as quantum field theory. But we have yet to find any proof of the existence of these hypothetical particles, which have been dubbed “gravitons”.
Researchers believe the holographic principle is an important hint to combine the gravity and quantum mechanics successfully.
A hologram makes three-dimensional images pop out from a two-dimensional screen. Similarly, the holographic principle allows physicists to study gravitational systems by projecting them on a boundary that surrounds the entire Universe. The AdS/CFT (anti-de Sitter/conformal field theory) correspondence, developed in the late 1990s by Juan Maldacena, has been particularly useful because it gives a precise mathematical definition of the holographic principle.
In the paper published on May 17, Ooguri and Daniel Harlow, Assistant Professor at Massachusetts Institute of Technology, proved that symmetry is not possible in a gravitational theory if it obeys the holographic principle.
A diagram above used to prove that quantum gravity cannot have any global symmetry. Symmetry, if existed, could act only on the shaded regions in the diagram and causes no change around the black spot in the middle. The shaded regions can be made as small as we like by dividing the boundary circle more and more. Thus, the alleged symmetry would not act anywhere inside of the circle.
Previous work by Harlow and others had found a precise mathematical analogy between the holographic principle and quantum error correcting codes, which protects information in a quantum computer. In the new paper, Ooguri and Harlow showed such quantum error correcting codes are not compatible with any symmetry, meaning that symmetry would not be possible in quantum gravity.
Their result has several important consequences. In particular, it predicts that the protons are stable against decaying into other elementary particles, and that magnetic monopoles exist.
Details of their study were published in Physical Review Letters on May 17 and selected for Editor’s Suggestion “due to its particular importance, innovation, and broad appeal.”
The Daily Galaxy, Sam Cabot via Kavli Institute for the Physics and Mathematics of the Universe
The Hubble Space Telescope image at the top of the page shows NGC 4388 a spiral galaxy in the Virgo Cluster. Gravity is changing the outer edges of the galaxy and triggering bursts of new star formation