We know about gravity, but we still don't understand it. Our best theory of gravity describes bent space-time as the result of the activity of quantum fields. Physicists have tried to use quantum field theories to describe gravity, but they are incomplete.
One of the most promising of those efforts is that gravity can be seen as a hologram, a three-dimensional effect that pops out of a flat, two-dimensional surface. The only concrete example of a theory like this is the AdS/CFT correspondence, in which a particular type of quantum field theory, called a conformal field theory, gives rise to gravity. A finite boundary can be found in the AdS space. Juan Maldacena, the theory's discoverer, called it a "universe in a bottle."
Our universe is not a bottle. The universe is flat. Our universe is far away in space and time. Physicists call this capsule thecelestial sphere.
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Physicists want to determine the rules for a CFT that can give rise to gravity in a world without the curves of AdS space. They are looking for a CFT for flat space.
The CFT is more ambitious than the one in AdS/CFT. Concepts of space and time are broken down by the fact that it lives on a sphere. It could explain how space and time come to be if the CFT depended on space and time.
Physicists are hopeful that they are on the right track after recent research results. The results use fundamental symmetries to constrain what this CFT might look like. Some wonder if the connection is more than coincidence after researchers discovered a set of mathematical relationships between these symmetries.
Nima Arkani-Hamed is a theoretical physicist at the Institute for Advanced Study in New Jersey. The thing we are going to find is going to be mind-blowing.
Symmetries on the Sphere.
Physicists use particles to see what happens in nature. This is calledscattering. Particles fly in from distant points, interact with each other, and then fly out to the detectors in a transformed state dictated by quantum forces.
Physicists can use quantum field theory to calculate the results of scattering problems if the interaction is governed by more than gravity. Physicists want to learn about gravity.
There is an amazing animal out here.
Nima Arkani-Hamed is a woman.
Steven Weinberg showed in the 1960s that certain quantum scattering problems can be calculated. Monica Pate of Harvard University said that they have nailed the behavior in this low-energy limit. The predictions of general relativity are reproduced by quantum gravity. The low-energy scattering problems are being used by holographers like Pate and Pasterski to determine some of the rules the hypothetical CFT must obey.
They look for symmetries. Physicists calculate the products of scattering and what they should look like when they hit the detectors in a scattering problem. Researchers look for patterns on the detector that correspond to rules or symmetries, which are what the scattering process must obey. The outcome of a scattering event should not change if you apply certain transformations to the detector.
Researchers working on quantum gravity hope to translate scattering problems into symmetries in the sky, so that they can fill out the CFT rulebook.
Pasterski said that they were trying to start from the basics of the dictionary.
A group led by Andrew Strominger of Harvard University published a paper in November that describes the CFT's "symmetry algebra". Different symmetry transformations can form new ones. Strominger and his colleagues, including Pate, have been able to further constrain the potential CFT by studying the structure of the transformations. They discovered that the group of symmetries obeyed a well-established and thoroughly studied algebra, which is related to the description of well-known quantum systems such as the quantum Hall effect.
David Skinner, a theoretical physicist at the University of Cambridge, said that the structure you landed on is something that people have explored and played with before.
Infinite issues.
Problems arise when you have a theory that applies to an infinitely distant sphere. Consider the particles that come together. They will be far apart if they scatter apart at any nonzero angle. The idea of distance is no longer valid. Our theories rely on locality, in which the strength of interactions between objects depends on their distance from one another. If everything is far away from everything else, the CFT has to go somewhere.
The concept of time on the sphere is far away in the past and in the future. There is no meaning here.
The concept of space and time is a feature, not a bug, according to Arkani-Hamed. It has the potential to explain space-time as a fundamental theory.
Some people temper their enthusiasm. Skinner thinks there is a long way to go. There are some major challenges that need to be overcome.
Arkani-Hamed does not disagree. The whole thing is trying to figure out what the question is. The stakes are also high.
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