Why and How to Use Encrypted Emails

The Problem

Privacy (often misunderstood as secrecy) is recognised as a fundamental human right in almost all modern civil societies, and is a cornerstone of democracy. However, it goes without saying that all major free email service providers, such as Gmail, Yahoo, Outlook.com etc., do not adhere to the highest standards of privacy protection. Although they have made their systems top notch in terms of convenience and security (from external attackers), your email communications are not private from them. Since you do not pay them for their service, they get their revenue elsewhere such as from advertisement companies to whom your data is the product. Anyway, even if they decide to stop using your email conversations for targeted advertisements, they still have the ability to read your personal correspondences at their convenience. They do not even need a warrant. A rogue top-level employee could read your emails for malicious purposes, for instance. Also, what about the idea of mass surveillance of whole populations by governments (for example, to suppress dissent or activism)? Are you comfortable with that kind of snooping (which is very different from a targeted search and seizure upon warranted suspicion of a crime)? Of course, all of this happens with your permission because you agreed to their privacy policy when you signed up for the free account.

What is Learning? — An Example of Linear Classification

Problem. Suppose that we are a bank trying to learn the creditworthiness of our current customers. Our resources include a database of the history of previous customers who have been already classified as defaulters or not. Based on this history, we are to learn the function $f$ which takes a customer as input and spits out the (binary) value of their creditworthiness ("yes" or "no").

[Assume that the customers are linearly separable.]

What is Learning?

To put it in simple words, learning is the process by which an entity gains the ability to predict outcomes in an unknown domain by experiencing or observing patterns in a known domain of data. Learning means to study the features of a given sample of data and come to an inductive generalization or extrapolation of such characteristics for the entire population of which the sample is merely a part. For example, you observe that the sun rises in the east and sets in the west today and tomorrow and the days after that every day for a year. By the end of the year, if you predict that the sun will rise again in the east and set in the west—correctly so—then you have learnt.

Fermions in the Standard Model

This post is more like a note-to-self for future reference.

Wick Rotation — What happens to the integration limits?

🚧🚩 IMPORTANT: Mathjax typesetting does not load in mobile version. Please use desktop page. Apologies for the inconvenience.

This is a genuine question that arises in the minds of students in a class where Wick rotation is discussed carelessly. It is a very simple observation.