After a long hiatus…

Hi folks, this is me again, some time ago since my last post. This is just a new entry to leave a comment about the future of this little blog of mine.

Time flies, things happen in live that make your time a scarce resource. I’m not quite sure if I will be posting again or this will last much, but I think that if I do, I’ll just use this blog to note down some resources, some activities that I’ll dedicate to in my spare time that have to do with Physics . Somehow, as a place to register (just for myself) my activity in subjects that interest me (Physics and Maths), but I’m pretty sure I won’t try to post long entries explaining complicate (or simple) concepts of Physics, as might have been my initial intention some time ago. That way lead me to abandon the blog as I didn’t have much time to review books, videos, etc. and prepair the blog entries. So I’ve decided to come back but just to do what I could be doing with a small notebook: include a link to a Physics related webpage that I’ve found, include a link to a video or set of videos that I’m interested in, etc.

Another objective that I want to keep on pursuing is writing in English oftenly. BUT, as English is not my mother tongue and I used to dedicate some time searching for the accurate word to say something and it consumed time…, from now on I will post without playing much attention to my incorrect use of the language (if it happens, which I have no doubt that will occur, sorry!). I want to use the blog for myself, in a fresher, quicker and more useful way for me.

Anyway, if you want to read my activity here or leave interesting comments, you’re welcome!

–Javier

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The Black Hole War: My Battle with Stephen Hawking to Make the World Safe for Quantum Mechanics

One of the latest books I’ve read: “The Black Hole War: My Battle with Stephen Hawking to Make the World Safe for Quantum Mechanics”, a book by Stanford well-known professor Leonard Susskind. I say “well-known” because anybody with some interest in Theoretical Physics may find his lectures on iTunes or YouTube (of course, for free). A great communicator Mr Susskind, one of my favourites, and also one of the fathers of String Theory.

In this book, a book that I think I will read again in some near future, Professor Susskind tells us the story about a theoretical dispute between him (“Is information lost when something falls into a black hole?“) and dutch physicist Gerard ‘t Hooft (1999 Nobel Prize in Physics), on one side, vs. the most famous physicist to the general public, Stephen Hawking, on the other.

The Black Hole War: My Battle with Stephen Hawking to Make the World Safe for Quantum Mechanics

The Black Hole War: My Battle with Stephen Hawking to Make the World Safe for Quantum Mechanics (image: amazon.fr)

 

This is the book description from the publisher:

A mind-bending book about modern physics, quantum mechanics, the fate of stars and the deep mysteries of black holes. What happens when something is sucked into a black hole? Does it disappear? Three decades ago, a young physicist named Stephen Hawking claimed it did–and in doing so put at risk everything we know about physics and the fundamental laws of the universe. Most scientists didn’t recognize the import of Hawking’s claims, but Leonard Susskind and Gerard t’Hooft realized the threat, and responded with a counterattack that changed the course of physics. This is the story of their united effort to reconcile Hawking’s revolutionary theories with their own sense of reality–effort that would eventually result in Hawking admitting he was wrong, paying up, and Susskind and t’Hooft realizing that our world is a hologram projected from the outer boundaries of space.

A great book I strongly recommend to anyone interesting in this challenging and profound topics. You can find it, for example, on Amazon.com, here.

You can get a grasp of what is explained in this book in this short YouTube video: Hawking vs Susskind.

The original 2005 publication by Professor Hawking entitled: “Information Loss in Black Holes” can be found here.

Carl Bender: Mathematical Physics

While I was searching the web for more lectures about QFT to complete the course by Cambridge Professor David Tong about the subject (great course, currently on lecture #6, next to review), I happened to visit the web of Perimeter Scholar International or PSI for short (by the Perimeter Institute for Theoretical Physics in Ontario, Canada), and its seminar archive at PIRSA (Perimeter Institute Recorded Seminar Archive). I was delighted to discover that lots of great lectures on Theoretical Physics were available online. Then I decided to start reviewing a course on Mathematical Physics by Professor Carl Bender (Physics Professor at Washington University in St. Louis). At the moment, I have only watched to the first lecture (I also found some of the lectures in YouTube here) about Perturbation Theory and I can say that Professor Bender is a great communicator and makes the subject comprehensible.

As I mentioned before, the videos, notes, mp3… of the course can be found here.

I am quite enthusiast upon the subject and, again, I am pretty sure that I will enjoy it very much.

Strings and Extra Dimensions

Chapter #21 of the course “Dark Matter, Dark Energy: The Dark Side of the Universe” by Prof. Sean Carroll. An interesting chapter to talk about these topics:

  • String theory: according to Wikipedia’s entry, “String theory is an active research framework in particle physics that attempts to reconcile quantum mechanics and general relativity. It is a contender for a theory of everything (TOE), a self-contained mathematical model that describes all fundamental forces and forms of matter”. I am pretty sure that if you are reading this blog from time to time, you must have heard of String theory (this could be another “coincidence scandal“, be reading this blog, I mean… or this expression is reserved for other stuff? 🙂
  • Quantized gravity: “Quantized Gravity or Quantum Gravity is the field of theoretical physics which attempts to develop scientific models that unify quantum mechanics with general relativity”. This model should be able to produce the same results that are currently known in the following two limits: a) when there is weak gravity (when we use QM and not GR), and b) when we use only GR at much larger distances than h bar (Planck’s reduced constant, i.e., when we do not take into consideration quantum phenomena). In addition to this, it should be able to explain physical situations in which both QM and GR are involved. The most famous quantum gravity theories are String Theory and Loop Quantum Gravity. I am pretty sure that I will be posting about them in future entries of this blog.
  • QED: relativistic QFT (quantum field theory) of electrodynamics. It describes how matter and light interact and involves both quantum mechanics and the special theory of relativity. The founder (one of the founders) of this theory was Richard FeynmanSurely You’re Joking, Mr. Feynman!
  • Planck length: length scale at which the structure of spacetime becomes dominated by quantum effects. In some theories, it is proposed that the structure of spacetime is discrete and its smallest distance is the Planck length.

    Planck length

    Planck length

  • M-theory: this refers to the theory proposed by Edward Witten who in 1995 said that the 5 viable string theories were nothing but different aspects of the same theory. Please, don’t ask me what the M stands for. We Physicists don’t know! (mother? magic? mistery? matrix? master?… the W in Witten’s surname but inverted? Choose the one that you like best).
  • QCD: quantum chromodynamics is the theory that explains the strong interaction that binds together quarks and gluons to form hadrons (i.e., protons and neutrons). One of the fathers of this theory was Murray Gell-Mann. If you want to see Gell-Mann in action, I strongly recommend you this TED Talk about “Beauty and Truth in Physics“. Gell-Mann, you are an incredible man, you know! 
  • Branes: mathematical concept that appears in string and related theories such as the M-theory. Enough for the moment…

Although I recommend purchasing the original videos from The Teaching Company, this chapter can be seen on YouTube here (part 1) and here (part 2) and here (part 3).

Inflation

Continuing with chapter #20 of the course “Dark Matter, Dark Energy: The Dark Side of the Universe” by Prof. Sean Carroll, now it is time to talk about “Inflation“. I remember when I was studying General Relativity at University that we didn’t spend much time on this concept. I thought by that time that it was a bit odd and also and old-fashioned idea but it has turned out to be a crucial theory (as it is seems to be quite stablished among many cosmologists) for the understanding of the very first moments of evolution of space-time, of our Universe.

As always, some of the concepts/ideas/people… whatever that Prof. Carroll has mentioned in this chapter are collected here for further reference (at least for myself):

  • Inflation: already mentioned in this previous post. It refers to a extremely short phase of the evolution of the Universe, at the beginning of the Big Bang, in which the Universe could have expanded exponentially fast, rapidly transforming curved space into flat one.
  • Alan Guth (1947 – ): american physicist that proposed the inflationary hypothesis in 1980.
Spectacular realization

Guth’s logbook showing the original idea of Inflation. December 7, 1979.

  • Inflaton: scalar field postulated to be the responsible of the rapid expansion of the Universe, known as inflation.
  • Reheating: this is a poorly understood process by which the temperature of the Universe prior to the inflationary phase gets back to its previous values. It is also known as thermalization. The reheating consists on a decay of the inflaton field into particles and radiation, starting the radiation dominant phase.
  • Multiverse: the multiverse is the hypothetical set of multiple possible universes or bubble universes that are popping into and out of existence and colliding all the time, with the space between them rapidly expanding.
  • Monopole problem:  Grand Unified Theories propose that at high temperatures, such as the ones taking place in the early universe, stable magnetic monopoles would be produced. Nevertheless, this heavy particles, which ought to be present today, haven’t been observed in nature so this is an open question in these theories. Here comes inflation to solve it: if a period of inflation occurred below the temperature where magnetic monopoles could separate from each other as the universe expands, the density of these particles would be highly lowered by many orders of magnitude and this could be the reason why there’s no track of them at the moment.
  • Horizon Problem: this referes to the problem of determining why the Universe appears to be homogeneous and isotropic. In a Big Bang model without inflation we couldn’t explain why two widely separated regions of the observable universe have the same temperature.
  • Flatness Problem: this referes to the problem of determining why the density of matter in the universe is comparable to the critical density necessary for a flat universe (Euclidean),  as recent observations of the cosmic microwave background have demonstrated. Inflationary theory solves this problem as it forces the universe to be very flat (to a very high degree, I mean).
  • Polarization of the CMB (Cosmic Microwave Background): one of the predictions of the inflationary universe is that primordial gravitational waves were created during the inflation era. These waves can be accessed by measuring the CMB polarization. Experiments to detect these perturbations are ongoing.

Although I recommend purchasing the original videos from The Teaching Company, this chapter can be seen on YouTube here (part 1) and here (part 2) and here (part 3).

David Tong: Lectures on Quantum Field Theory

I wish I was born in 2000 or later, I must confess. Why? Because when I was younger (let’s express it this way), there was no YouTube, no lectures on the Web, no nothing but plain text, etc. Nowadays you can follow, for example, a really interesting course on QFT (introductory level) from a Cambridge Professor, for free… This course is the one that I’ve started following. At the moment I’ve only seen the first class of it and hope it doesn’t get very complicated for me with respect to mathematics (not in its best shape of all time at the moment). Professor David Tong explains quite clearly the concepts, so I think I will enjoy the course.

The content of the course (please visit his webpage if you want to know it in detail) treats the following points:

  1. Preliminaries.
  2. Classical Field Theory:   
    Table of Contents; Introduction; Lagrangian Field Theory; Lorentz Invariance; Noether’s Theorem and Conserved Currents; Hamiltonian Field Theory.
  3. Canonical Quantization:   
    The Klein-Gordon Equation, The Simple Harmonic Oscillator; Free Quantum Fields; Vacuum Energy; Particles; Relativistic Normalization; Complex Scalar Fields; The Heisenberg Picture; Causality and Propagators; Applications; Non-Relativistic Field Theory
  4. Interacting Fields:   
    Types of Interaction; The Interaction Picture; Dyson’s Formula; Scattering; Wick’s Theorem; Feynman Diagrams; Feynman Rules; Amplitudes; Decays and Cross Sections; Green’s Functions; Connected Diagrams and Vacuum Bubbles; Reduction Formula
  5. The Dirac Equation:   
    The Lorentz Group; Clifford Algebras; The Spinor Representation; The Dirac Lagrangian; Chiral Spinors; The Weyl Equation; Parity; Majorana Spinors; Symmetries and Currents; Plane Wave Solutions.
  6. Quantizing the Dirac Field:   
    A Glimpse at the Spin-Statistics Theorem; Fermionic Quantization; Fermi-Dirac Statistics; Propagators; Particles and Anti-Particles; Dirac’s Hole Interpretation; Feynman Rules
  7. Quantum Electrodynamics:   
    Gauge Invariance; Quantization; Inclusion of Matter — QED; Lorentz Invariant Propagators; Feynman Rules; QED Processes.

Notes to the course can be found in this pdf (149 pages). The videos that I’m currently watching, in this link.

I want to thank Prof. Tong and the University of Cambridge for this wonderful initiative. By the way, I also hope they extend this (the publishing of the videos) to other courses by Mr Tong (String Theory, Solitons, etc.).

Was Einstein right?

This is the title of chapter #19 from the course “Dark Matter, Dark Energy: The Dark Side of the Universe”, already commented in previous posts. This time some of today’s notes have to do with to great physicists. Apart from equations or theories that try to explain our Universe, it is also important to know a little bit of History of Science and be aware of these names:

  • Urbain Le Verrier: Le Verrier is mentioned in this chapter as he was the guy that made a prediction about the existence of a planet, not known in the first half of the 19th century. That planet was Neptune. He was also the first to report that the slow precession of Mercury’s orbit around the Sun could not be completely explained by Newtonian mechanics and perturbations by the known planets. You know, the same precession that was explained later by Einstein`s General Theory of Relativity.
  • Mordehai Milgrom: Israelei physicist that proposed the MOND or Modified Newtonian Dynamics as an alternative explanation to Dark Matter. MOND is a hypothesis that proposes a modification of Newton’s law of gravity to explain the galaxy rotation problem.
  • Vera Rubinshe was the astronomer that pointed out the phenomenon known as the galaxy rotation problem (already mentioned): discrepancy between the predicted angular motion of galaxies and their observed motion.
  • Jacob Bekenstein: another Israeli physicist famous because he contributed to the foundation of black hole termodynamics. He was the first one to suggest that black holes should have an entropy.
  • The bullet cluster: two colliding clusters of galaxies into one, whose Gravitational lensing studies are supposed to provide the best evidence of Dark Matter.
  • The Friedmann Equation: it is a well-known relationship between the energy density of the Universe, the expansion rate (Hubble constant) and the curvature of space. It governs the expansion of space in homogeneous and isotropic models of the universe within the context of general relativity. That is:

(8πG/3 ) ρ = H2 + K

  • The Cassinni probe: spacecraft mission launched in 1997 to study Saturn and its moons.

Although I would recommend purchasing the original videos from The Teaching Company, this chapter can be seen on YouTube here (part 1) and here (part 2).

An additional comment/confession that I’m not sure if I’ve already done before: you may realize that some of these notes or definitions are in some parts copy-pasted directly from Wikipedia. This is because I’m just trying to have short notes to illustrate (mainly for myself) what I’ve been reading/watching/working on… And what a better definition (of course, not always) that the one that appears on the Wiki, validated many times before…