Graduate

Applied Physics/Physics Colloquium: Recent results on Gravitational Waves from LIGO and Virgo

Topic: 
Recent results on Gravitational Waves from LIGO and Virgo
Abstract / Description: 

Over the last two years, the Advanced LIGO and Advanced Virgo detectors have observed a handful of gravitational-wave events from the inspiral and merger of binary black holes in distant galaxies. These events have resulted in the first measurements of the fundamental properties of gravitational waves, tests of General Relativity in the strong-field, highly-dynamical regime, and the population, masses and spins of black holes in the universe. Most recently, signals were detected from the inspiral of a binary neutron star system, GW170817. That event is thus far the loudest (highest signal-to-noise ratio) and closest gravitational-wave event observed. A gamma-ray burst detected 1.7 seconds after merger confirms the long-held hypothesis that BNS mergers are associated with short gamma-ray bursts. The LIGO and Virgo data produced a three-dimensional sky localization of the source, enabling a successful electromagnetic follow-up campaign that identified an associated electromagnetic transient in a galaxy ~40 Mpc from Earth. A multi-messenger view of GW170817 from ~100 seconds before merger through weeks afterward provides evidence of a "kilonova", and of the production of heavy elements. For the first time, using gravitational waves we are able to constrain the equation of state of dense neutron stars and infer the rate of local binary neutron star mergers. When we include EM observations, we are able to directly measure the speed of gravitational waves, constrain its polarization content, independently measure the Hubble constant, probe the validity of the equivalence principle, and gain new insight into the astrophysical engine driving these events.

Date and Time: 
Tuesday, November 14, 2017 - 4:30pm
Venue: 
Hewlett 201

SCIEN Talk: Next Generation Wearable AR Display Technologies

Topic: 
Next Generation Wearable AR Display Technologies
Abstract / Description: 

Wearable AR/VR displays have a long history and earlier efforts failed due to various limitations. Advances in sensors, optical technologies, and computing technologies renewed the interest in this area. Most people are convinced AR will be very big. A key question is whether AR glasses can be the new computing platform and replace smart phones? I'll discuss some of the challenges ahead. We have been working on various wearable display architectures and I'll discuss our efforts related to MEMS scanned beam displays, head-mounted projectors and smart telepresence screens, and holographic near-eye displays.

Date and Time: 
Wednesday, November 29, 2017 - 4:30pm
Venue: 
Packard 101

SCIEN Talk: Near-Eye Varifocal Augmented Reality Displays

Topic: 
Near-Eye Varifocal Augmented Reality Displays
Abstract / Description: 

With the goal of registering dynamic synthetic imagery onto the real world, Ivan Sutherland envisioned a fundamental idea to combine digital displays with conventional optical components in a wearable fashion. Since then, various new advancements in the display engineering domain, and a broader understanding in the vision science domain have led us to computational displays for virtual reality and augmented reality applications. Today, such displays promise a more realistic and comfortable experience through techniques such as lightfield displays, holographic displays, always-in-focus displays, multiplane displays, and varifocal displays. In this talk, as an Nvidian, I will be presenting our new optical layouts for see-through computational near-eye displays that is simple, compact, varifocal, and provides a wide field of view with clear peripheral vision and large eyebox. Key to our efforts so far contain novel see-through rear-projection holographic screens, and deformable mirror membranes. We establish fundamental trade-offs between the quantitative parameters of resolution, field of view, and the form-factor of our designs; opening an intriguing avenue for future work on accommodation-supporting augmented reality display.

Date and Time: 
Wednesday, November 15, 2017 - 4:30pm
Venue: 
Packard 101

SystemX Seminar: Efficient battery usage for wireless IoT nodes

Topic: 
Efficient battery usage for wireless IoT nodes: Battery lifetime prediction
Abstract / Description: 

The prediction of the life-time of battery-powered IoT devices and wireless sensor networks is almost exclusively based on the assumption that the total charge in a battery (i.e. the mA-h) can be linearly consumed in time. This is not the case in reality. Batteries are complex electro-chemical systems and their discharge behavior depends heavily on the timing and intensity of the applied load. There is very little empirical data or reliable models available for these kinds of batteries and loads that are typically used in IoT sensor nodes for very long operational time, 5 -10 years.

We characterize the inexpensive CR2032 Li-coin cells using carefully controlled synthetic loads and a wide range of IoT-typical load parameters. We observe that actual lifetimes can differ from predicted linear ones by almost a factor of three. Furthermore, loads with similar average currents can vary significantly in the amount of capacity of the battery they can utilize. We conclude that short duration loads generally are faring better than sustained loads which was not anticipated. We suggest a better prediction model, that captures the non-linear short duration behavior, which can be implemented in constrained IoT devices.

Date and Time: 
Thursday, November 16, 2017 - 4:30pm
Venue: 
Huang 018

IT Forum: Tight regret bounds for a latent variable model of recommendation systems

Topic: 
Tight regret bounds for a latent variable model of recommendation systems
Abstract / Description: 

We consider an online model for recommendation systems, with each user being recommended an item at each time-step and providing 'like' or 'dislike' feedback. A latent variable model specifies the user preferences: both users and items are clustered into types. The model captures structure in both the item and user spaces, and our focus is on simultaneous use of both structures. We analyze the situation in which the type preference matrix has i.i.d. entries. Our analysis elucidates the system operating regimes in which existing algorithms are nearly optimal, as well as highlighting the sub-optimality of using only one of item or user structure (as is done in commonly used item-item and user-user collaborative filtering). This prompts a new algorithm that is nearly optimal in essentially all parameter regimes.

Joint work with Prof. Guy Bresler.

Date and Time: 
Friday, November 10, 2017 - 1:15pm
Venue: 
Packard 202

ISL Colloquium: Delay, memory, and messaging tradeoffs in a distributed service system

Topic: 
Delay, memory, and messaging tradeoffs in a distributed service system
Abstract / Description: 

We consider the classical supermarket model: jobs arrive as a Poisson process of rate of lambda N, with 0 < lambda < 1, and are to be routed to one of N identical servers with unit mean, exponentially distributed processing times. We review a variety of policies and architectures that have been considered in the literature, and which differ in terms of the direction and number of messages that are exchanged, and the memory that they employ; for example, the "power-of-d-choices" or pull-based policies. In order to compare policies of this kind, we focus on the resources (memory and messaging) that they use, and on whether the expected delay of a typical vanishes as N increases.
We show that if (i) the message rate increases superlinearly, or (ii) the memory size increases superlogarithmically, as a function of N, then there exists a policy that drives the delay to zero, and we outline an analysis using fluid models. On the other hand, if neither condition (i) or (ii) holds, then no policy within a broad class of symmetric policies can yield vanishing delay.

Date and Time: 
Thursday, November 9, 2017 - 4:15pm
Venue: 
Packard 101

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