Colloquium

Almost fifteen years after LIGO started listening to the cosmos, and 100 years after Einstein discovered general relativity, gravitational waves have been detected by ground-based interferometers, opening a new window on the universe. In this talk I will address some of the most exciting areas of research advanced LIGO will allow us to explore in the coming years. Detection and characterization of gravitational wave transients will be discussed, as well as their impact on astrophysics.

In recent decades probing for the subtle indications of new physics in
experimental data has become increasingly difficult. The datasets have gotten
much bigger, the experiments more complex, and the signals ever smaller. Success
stories, like LIGO and Kepler, require a sophisticated combination of statistics
and computation, coupled with an appreciation of both the experimental realities
and the theoretical framework governing the data.

In this talk I will look broadly at data science in physics, and how and why it
has taken an increasingly central role. I'll highlight specifically my current
area of research, radio cosmology: discussing why it is one of the most
challenging areas for data science, and describing my work developing optimal
and efficient statistical methods for turning terabytes of timestreams into
cosmology.

I will begin by giving an overview of the current state of exoplanet
science, a field that has advanced tremendously in just the last few
years.  While specialized instrumentation and observational facilities
have provided the data driving this advance, the development and
application of statistical techniques to interpret this data have been
of critical importance.  These same tools are also at the core of all
data-driven science, and are thus applicable to many other fields of
astrophysics that will be acquiring increasingly large and rich
datasets in the coming years.  I will highlight a few examples of
particular interest to fundamental physics, including gravitational
wave observations, fast radio bursts, and cosmological surveys.

Gravitational waves, as predicted by Einstein one hundred years ago, have been detected by the advanced Laser Interferometer Gravitational-Wave Observatory (LIGO) in September last year. This finding marks the beginning of gravitational-wave astronomy. From now on, we are able to probe our universe using both long-range forces in nature. 

In the foreseeable future, the gravitational wave astronomy will be exponentially growing, with more detectors targeting gravitational waves at different frequencies and more events detected every week. This will provide us great opportunities but also impose challenges as well. In the first part of my talk, I will discuss the physics of gravitational wave detectors, emphasizing its basic principles and dominant noise sources, as well as ongoing efforts to improve detector sensitivity to fulfill the growing need. In the second half of my talk, I will discuss the astrophysics of gravitational-wave sources - especially compact sources such as black holes and neutron stars. I shall explain the spacetime dynamics, gravitational-wave emission during compact binary mergers and possible electromagnetic radiation from these compact objects. In the next few years with new gravitational-wave observations and possible electromagnetic counterpart measurements, we may dramatically improve our understandings on the astrophysics of these sources.

In any quantum field theory, the energy flux at a point of spacetime can be negative.  This would produce a repulsive gravitational field causing nearby light rays to defocus.  This in turn threatens to produce a variety of exotic phenomena including traversable wormholes, warp drives, time machines, and evasion of singularity theorems.  I will describe a new "quantum focusing conjecture" that prevents such pathologies.  In the flat spacetime limit it reduces to a novel lower bound on the energy density, which can be proven for several classes of field theories.

Precision atom interferometry is poised to become a powerful tool for discovery in fundamental physics.  Towards this end, I will describe recent, record-breaking atom interferometry experiments performed in a 10-meter drop tower that demonstrate long-lived quantum superposition states with macroscopic spatial separations.  The potential of this type of sensor is only beginning to be realized, and the ongoing march toward higher sensitivity can enable a diverse science impact, including laboratory tests of general relativity and the equivalence principle, searches for dark matter, probes of quantum mechanics, and detection of gravitational waves.  With the recent LIGO results, gravitational wave astronomy is particularly compelling since it provides a new window into the universe, collecting information about astrophysical systems and cosmology that is difficult or impossible to acquire by other methods.  Atom interferometric gravitational wave detection offers a number of advantages over traditional approaches, including simplified detector geometries, access to low frequencies that are complementary to LIGO, and substantially reduced antenna baselines.

Radio pulsars are Nature's most perfect clock and hence are useful for precision work on a wide variety of physical and astrophysical topics, ranging from sensitive tests of relativistic gravity to constraining the equation of state of ultradense matter. I will describe current ongoing surveys for radio pulsars using the two largest radio telescopes in the world, and how these surveys are also valuable for searching for Fast Radio Bursts, a newly recognized astrophysical phenomenon of unknown origin. 

In the summer of 2015, the speaker led a team at Microsoft Research, comprised primarily of research interns from seven universities, to demonstrate a social, ambulatory, Mixed Reality system for the first time.  Each intern developed a preliminary, domain-specific exploration of how such a system could be used.  One of the interns, Andrzej Banburski of the Perimeter Institute, demonstrated an interface to Mathematica.  The very concept and terminology of Virtual Reality had been first articulated by the speaker almost four decades earlier precisely in the hopes of experiencing this type of interface.  Mathematical expressions floating in the air is not just more vivid that writing them on a blackboard; a different workflow and mental framework becomes possible.  For instance, functions appear as lenses that can be compounded as they are lined up.  One can peer through a stack of such lenses at different data or starting conditions.  Instead of erasing and replacing portions of expressions on a blackboard, a stack of lenses can fork and merge, so that a range of variations can be explored at once as aspects of a sculptural form.   The purpose is not just to make math more accessible to those who might find this interface more inviting, but to free math from pre-computational, notation-bound conventions.  The speaker will argue that this type of interface, while still only barely explored, could eventually have a significant impact both on computer science and other disciplines, including theoretical physics. 

A quantum entanglement is a special kind of correlation; it may yield a strong correlation that is not possible in a classical ensemble, or hide the correlation from all local observables. Especially important is the entanglement that arises from local interactions for its implications in many-body physics and future’s quantum technologies.
I will review a few characteristics of entangled qubits in connection to fault-tolerant quantum information processing, and present a class of long-range entangled many-body states that are ground states of gapped local Hamiltonians on lattices. The class is qualitatively unconventional in many ways, and substantially boosts the richness of many-body entanglement. Implications in mechanisms of localization, renormalization group flow, quantum information storage, and topological order will be discussed.

Topological phases of matter are phases of matter which are not characterized
by classical local order parameters of some sort. Instead, it is the global properties
of quantum many-body ground states which distinguish one topological phase from
another. One way to detect such global properties is to put the system on a topologically
non-trivial space (spacetime). For example, topologically ordered phases in (2+1)
dimensions exhibit ground state degeneracy which depends on the topology of the spatial manifold.
In this talk, I will discuss how one can use a {\it unoriented} space (spacetime)
to detect non-trivial properties of topological phases of matter in the presence
of discrete spacetime symmetry, such as time-reversal or reflection symmetry.
In particular, I will show how interaction effects on topological insulators and
superconductors can be understood using quantum anomalies on unoriented spacetime.

Topological quantum computation is based on the possibility of the realization of some TQFTs in Nature as topological phases of quantum matter.  Theoretically, we would like to classify topological phases of matter, and experimentally, find non-abelian objects in Nature.  We will discussion some progress for a general audience.