The effect of pandemic decayed away in 2023, so I attended many activities. This is the most dynamical year since the start of graduate school in 2019. I split the summary into two posts. In this first post, I will talk about two travels I had this year and my adventure in black-hole physics.

## Traveling aboard: APS Marching Meeting and Quantum Connection summer school in Sweden

The first onsite APS (stands for American Physical Society) March Meeting after the pandemic occurred in Las Vegas. There is no way to miss this opportunity to visit this city, which appears in many movies. What’s more, one of my friends that I met when learning physics course online, a professional poker, Mark Weitzman, is in Las Vegas! He really helped me a lot during my self-study project from 2016 to 2019. It’s a great opportunity to meet him. I also got to meet my first professor in physics, Jim Freericks, during the meeting.

The style of the building along the main road called Las Vegas Strip feels like ancient Rome and everything is related to Caesars. See the picture above. It made me felt like I was in a movie about ancient Rome. It is quite a new experience for me: the weather and landscape in a dessert, people with religious propaganda in hand walking in the street, inner layout of a house, and how strange people interact in the street.

In June, I had an opportunity to attend a summer school called Quantum Connection in Sweden. I talked about this in another post. This summer school covers a wide range of topics from high-energy physics to condensed matter physics. This completely rose my interesting in learning more about basics of elementary particle physics. I still cannot forget several interesting talks about quantum electrodynamics, especially David Gross’s talk. I guess I will grab an introductory textbook on elementary particle physics next year. Equally interesting is t’ Hooft’s talk about interpretation of quantum mechanics. It involves with an interpretation of quantum mechanics using cellular automaton. I’m very attracted to this topic and t’ Hooft recommended one of his textbook, *The Cellular Automaton Interpretation of Quantum Mechanics*, as a starting point. I plan to study it next year!

## Black hole physics: Newton, Einstein and quantum fluctuations

The notion of a black hole has fascinated me since high school time in 2010, when I just learned Newton’s gravity theory and read several popular science books by Steven Hawking. Newton’s gravity theory is so intuitive and simple that the physical picture of black hole in my mind has been the Newtonian one for almost ten years (as long as you understand the motion of a satellite studied thoroughly in high school physics, you will have no difficult to create a Newtonian black hole in your mind by treating light as particles). However, Hawking’s popular science book has convinced me about the gravitational red shift due to Einstein general relativity and something strange happens near the event horizon if we consider general relativity. However, the problem is that I don’t have Einstein’s gravity theory in hand to play with this toy model.

To change the situation, I started studying general relativity by following Prof. Scott Hughes’s MIT course since July 2022 and finished the course in March 2023. The course equips me with the basic mathematical tool to play with a black hole in detail. The examples in the course have established the physics outside the event horizon of a black hole, but what is most interesting is the physics near and inside the horizon, which seems like a more advance topic and is not covered in an introductory general relativity book. At that time, I wanted to understand a very specific thought experiment:

- Can I stay outside but very close to the event horizon and try putting my arm into the even horizon?
- When my arm is half inside and half outside the event horizon, do I have some strange feeling or not?

The standard textbook example can help understand my thought experiment partially, but not fully. Therefore, my next step naturally becomes to understand a black hole in general relativity and even with quantum effects. I find several popular science book and lecture videos that do a great job explaining the historical development and basic physics of a black hole:

- Leonard Susskind’s General Relativity lecture series (
*watched them**in April*). In later half of the course, Susskind focused on the black hole physics and discussed in great detail how to analyze the motion of falling into a black hole and formation of a black hole. Very advanced theoretical tools like Kruskal diagram and Penrose diagram is introduced. I believe I can use these tool to analyze my thought experiment in the future. - Kip Thorne’s popular science book,
*Black Hole & Time Warps: Einstein’s Outrageous Legacy*(*finished in October*). This is a great book! For a reader like me, it is an outstanding literature review, and it is a great joy to read. It goes into the details of why physicist believe a black hole should exist in our universe, instead of just an artifact solution of Einstein’s theory or the idealization that is unstable under perturbations. You will see how our current understanding of a black hole has been developed since Einstein, by great physicists like**Oppenheimer, Wheeler, Chandrasekhar, Zeldovich, Wheeler, Hawking**, etc. After reading this book, I have an intimate feel with the theory of black hole, instead of just a cold, inanimate, dead object floating in the space. The book has given me a panorama of this topic. *Truth and beauty: Aesthetics and Motivations in Science*by S. Chandrasekhar (*finished in June*). This collection of essays is more about philosophy. When I ask myself, what do I want to achieve as a scientist, I think I want to contribute to physics like Newton and Galileo did. But, what makes this two physicist so great? I try to find the answer in this book. Chandrasekhar examines and compares great scientists and artists like Newton, Beethoven and Shakespeare. In one of the essay, Chandrasekhar talks about his understanding of the beauty of general relativity and black holes:

- Susskind’s The Black Hole War (
*finished in November*) and Smolin’s Three Roads to Quantum Gravity (*finished in December*). These two popular science books talk about a much more advanced topic about quantum black hole, initiated by Hawking’s discovered of temperature of a black hole. They are more about understanding quantum gravity using black hole as a tool for thought experiments. Even without quantum effect, the physics near the horizon is pretty bizarre and hard to imagine. Quantum fluctuation adds tons of paradoxes and strangeness to this story. I guess I need to be familiar and comfortable with basic quantum field theory and more advanced mathematical tools for black holes in order to play with a quantum black hole. A long way to go, but it is exciting!