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June 22, 2015

During the first full day of talks at Convergence, attendees were challenged to be more ambitious and to think about the big questions that theoretical physics can address.

**By David Harris**

“These are amazing times for physics…. Many of us believe physics is poised for a new revolution.” With those words Perimeter Institute Director **Neil Turok** officially opened the first Convergence conference.

A day of talks outlined some of the latest and greatest achievements in physics and posited some potentially fruitful directions to explore. The Convergence conference is happening at a time many physicists are calling a time of “crisis”, referring mainly to the remarkable simplicity of the laws of physics that seem to govern the universe at small and large scales, and the fact that is has been incredibly difficult to find any cracks in those laws to make progress to the next iteration of knowledge.

“The great surprise is we knocked on the door of GeV scale physics expecting a party but when the door opened there was just one little person — the Higgs boson,” said Turok. He also pointed out that physicists require just six numbers to determine the large-scale structure of the universe, making it simpler than the structure of an atom.

Of course, not all the universe is so simple, just on the extreme ends, with a lot of interesting complexity in the middle. Turok suggested there are three main frontiers to be explored in physics at the moment: the tiny, the huge, and the quantum. It is the combination of these three frontiers through the interplay of information, gravity, and quantum fields that is driving a lot of current work.

Turok encouraged the conference attendees to be more ambitious and to think about the big questions that theoretical physics can address.

The theme of simplicity recurred through a panel discussion, with cosmologist **Paul Steinhardt **commenting, “The universe is extraordinarily simple and at the same time very puzzling… Something that is simple demands a simple explanation – and that’s what theoretical physicists are good at.” Condensed matter theorist **Matthew Fisher** said, “In a complicated dance, the underlying choreography could be quite simple.”

Particle physicist **Natalia Toro **said that physicists must ask more questions and do more exploratory experiments. When questioned on what sorts of experiments those should be, she responded that small invisible-axion experiments, dark matter particle searches in the LHC, and small accelerator-based experiments searching for dark matter could be fruitful. However, not ready to give up the role of theory to experimental dominance, she was quick to point out: “Theory and experiment working together form a formidable partnership.”

Mathematical physicist **Freddy Cachazo** leapt straight into the abstract world of quantum field theory and foundational concepts, suggesting that, in the way forward, “Locality is something that we’ll have to give up at some point,” indicating that progress beyond the physics “crisis” could require some uncomfortable choices in the way we think about the universe.

Theorist **Savas Dimopoulos **outlined some of those other approaches, praising the potential of experiments in atom interferometry, among others, and asserting that we should be searching hard for axions as a solution to the dark matter problem. He proposed the creation of a “super-lab” that would operate as a user facility in the style of high-energy physics in which scientists come in and do small-scale experiments at the frontier of physics. He believes that such a lab could define a new field of physics, which (being far less costly than huge accelerators) could be more politically palatable to fund, both by governments and private industry.

After the morning’s concentration on the progress of current and potential of future experiments, the early afternoon turned to quantum information theory. **Rob Spekkens** discussed cause and effect in the quantum world. He pointed out that “There is currently no consensus on the interpretation of quantum physics,” but that the “instrumentalist” interpretation, sometimes called the ‘shut up and calculate’ interpretation, is inadequate: a good scientific theory should not only predict the results of an experiment but it should also guide you as to what experiments you should do next. The work he discussed revolved around the paradoxes that can occur when you conflate causal influence with inference and outlined a program for trying to disentangle the statistics of experiments from the causal structure of physical phenomena. He pushed for an approach involving quantum generalizations of causal models, showing that such a direction would allow for a new application of quantum information theory to tasks other than information processing.

After the abstract but exciting direction outlined by Spekkens, **John Preskill** discussed the possibilities of combining quantum theory, computer science, and information theory into quantum information science. He said that a good use of quantum computers is problems that are classically hard but “quantumly” easy, such as simulating real quantum systems like chemical reactions, quantum field theory, and even quantum gravity.

Preskill then went on to pose two of the biggest questions that quantum information science might tackle: “What is inside a black hole?” and “What’s the quantum structure of spacetime?” He showed how the combination of results from information theory and quantum gravity could lead to new insights such as understanding the nature of a black hole and how the geometry of spacetime could be built up by considering entanglement between particles as a spacetime wormhole between them.

He also discussed how entanglement of particles on the boundary of a volume is related to the geometry of the volume itself. Finally, Preskill touched on the intriguing topic of how quantum error correction, developed for quantum computers, could be related to the holographic principle, which says that all the information contained in a 3D volume of space can be represented as information on the 2D surface of that volume.

Wrapping up the formal conference talks of the first day, **Nergis Mavalvala **discussed the dawn of the gravitational wave astrophysics. By detecting gravitational waves, physicists will be able to test many hypotheses of general relativity and newer attempts to form a theory of quantum gravity. Gravitational wave interferometers like LIGO have reached such a level of technical sophistication that they are now limited in their measurements by the fundamental quantum limits on measurement and the quantum structure of the vacuum rather than any specific engineering problem.

Even though LIGO hasn’t positively detected a gravitational wave yet, the fact that it didn’t detect such waves during an observed gamma-ray burst has allowed scientists to determine that the burst wasn’t coming from the rapid inspiraling of binary stars, but from a different astrophysical mechanism. Completing the day’s theme of bringing together subfields of physics, Mavalvala showed that using techniques of quantum engineering, learned from quantum optics, could allow a 25 percent improvement in the noise levels, below the current quantum noise limit.

The day brought together many varied subfields of physics to approach difficult problems in new ways and potentially make progress beyond the “crisis” that faces fundamental physics at this time. Whether this is successful will depend very much on whether theoretical physics can rise to the challenge posed by Turok with the Convergence conference.

**David Harris is a theoretical physicist turned science journalist and communicator. He is the on-site rapporteur for the Convergence conference at Perimeter Institute. Follow him on Twitter. **

“Theory and experiment working together form a formidable partnership."

- Natalia Toro

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