Tuesday, July 22, 2014

Book review: The Perfect Theory, A Century of Geniuses and the Battle over General Relativity

Amazon book review.

This is a wonderful book written for the general public whose topic is the history of general relativity, its evolution as a theory, and the modern evolution of gravitational theory to present attempts to obtain quantum gravity.  It is a very well written book by an astrophysicist at Oxford who works in this area. The author has his personal experiences with the theory mixed in throughout the book.

Its first chapter is entitled "If a Person falls Freely".  This is part of a thought experiment that Einstein used in order to come up with his ideas for general relativity. It discusses his ideas interspersed with Einstein's personal history as a Swiss patent clerk up to his becoming a physics professor at Berlin, then spending his later years at the Institute for Advanced Study at Princeton. There are also interesting tidbits throughout the book.  As an example, when Einstein skipped class as an undegrad which fellow student lent him his class notes so that he could see what he missed?

One of the earliest solution's of general relativity was Schwarzschild's showing the existence of black holes.  As expected black holes are a large topic with Oppenheimer, Synder, Wheeler. Penrose, Hawking, Bekenstein, Zel'dovich and other discussed.  Cosmological implications from general relativity are also a big topic with the expanding universe of Friedmann and Lemaitre, Einstein's introduction of the cosmological constant in order to have a static universe, up to present day modification to gravitational theory.  The history of general relativity is intermingled with experimental developments in astronomy which is explained very well.  This book shows how a theory and experiments grow off of one another and shows how science is advanced in this way.  One example of many in the book is shown very well with the work of Jim Peebles over many years.  The book discusses a variety of personalities involved in the development of general relativity and also shows how it was carried on in the secretive Soviet society.

The history to obtain a quantum theory of gravity as explained in this book and is very interesting and informative.  All the big names in physics that you can think of have tried their hand at this problem and none have come up with a solution.  The reasons why are discussed.   This book has others that you might not have heard of such as DeWitt and their tales.

If you are curious about Einstein's theory of relativity and its implications in astronomy,  cosmology along with recent attempts  to obtain a quantum theory of gravity this is a wonderful book to read and well worth your time.

Sunday, July 20, 2014

What Could Cold Dark Matter Be and Future US Searches

A fun thing to read about in physics is cold dark matter (CDM).  From our current understanding of the big bang creation of the universe, known as the lambda-CDM model. cold dark matter is what about 25% of our universe is made of.  But what is cold dark matter?  What is it made of?  From what is known at the present time it is not any known particle at all.  It is not part of the standard model of particle physics.  It must be part of physics beyond the standard model.  That is why there have been many dark matter searches in the past.  All of them unsuccessful in detecting CDM.  In fact the US recently announced the three projects that it will fund in our future attempts to detect cold dark matter.  A good article describing these is here 
 I've blogged about CDM in the past here, and here,
CDM's presence is only known from the gravitational attraction that it has to other matter.  It doesn't interact with the known particles and forces other than gravity.  All attempts to detect it in the lab have failed.  A few days ago I posted on physics.stackexchange.com the following question "Could dark matter particles that don't couple to quarks or leptons have been produced?".  Here's the statement of my question and one of the answers I got.
"With what we know about physics, is it possible that when the universe 'began', around when quarks and leptons were produced, another particle, which doesn't couple to either quarks, leptons or photons was also produced ? The only other way that we can observe its existence is via the effects of its gravitational field. In others words, some ''dark-matter-particle'' that doesn't interact with known forms of matter, except through gravity?"
Answer: Yes, there have been suggestions that such particles exist, and an example is the sterile neutrino.
But your question is a little more involved than you might think at first sight. For example if the sterile neutrino only interacts through gravity what interaction caused it to be created in the first place? There is nothing in the Standard Model that could create such particles. However we expect that the Standard Model is a low energy approximation and as we work backwards in time towards the Big Bang and the energies get higher we'll need a grand unified theory like SO(10) and ultimately a quantum theory of gravity (which may or may not be String Theory). These contain interactions that can create particles like sterile neutrinos. However this remains a speculative area of Physics and at the moment we can't say definitely whether such particles exist or if they exist how they were created.
So yes, such a particle the sterile neutrino might exist.  The problem is then:if this particle only interacts by gravity how are we going to measure it?  How are we every going to determine if this particle actually exists other than by its influence on other particles through gravity?  A very difficult experiment to carry out, if ever it can be carried out.  This is sort of like Freeman Dyson's question about detecting an individual graviton. Posted online in an IAS newsletter here, scroll down in the newsletter to Dyson's article (there are other interesting subjects in the newsletter).  Articles about  Dyson's question are here, an arxiv paper abstract here

Thursday, July 10, 2014

Where in a Quantum Computer is the Calculation Occurring?

     Reading about quantum computers is fascinating and fun but it brings up the question of where are the calculations happening in the qubit?  Where physically is the qubit and where does a state in superposition exist physically?

    From what I've read or seen on Youtube clips an example of a qubit is physically in the form of a single atom trapped.  The spin (up or down being 1 or 0) of the atom represents the 1 or 0 of a binary computer.  Now being a single atom its wavefunction can be in superposition of either up or down.  The spin of the atom can be simultaneously both up and down until it is measured.  Think the famous Schrodinger's Cat.  So when using a qubit to do a calculation what's happening in the computer?  In this strange superposition where is the calculation taking place?  In the single isolated trapped atom you would think.   But where in the atom?  This is a great example of quantum weirdness in how the quantum world is different than the classical world.  The classical world has no spin of the atom, spin is a quantum property of the atom.   The quantum world says that the atom can be simultaneously both spin up and spin down until it is measured.  A great video by PhD Comics that discusses what a quantum computer is given  here.  This cartoon, especially the part with the cube representing the qubit really got me wondering.  Wondering about where do quantum computers do their calculations?

  In a classical computer you have physical objects such as transistors which can be used to be on or off representing the 1 or 0 's  in a calculation.  In running a program in a classical computer you could in principle stop the program and look at the microprocessor down at the single elements in the chip and see where all the 1's and 0's are.  Kind of like taking a snapshot of the calculation occurring. You could advance the calculation by say one clock step and see how the program is evolving.   How all the 1's and 0's are moving around according to the program.  Apparently you can't do that with a quantum computer.  Once the calculation starts you can not observe what is happening.  An observation would ruin the calculation.  This isn't just an observation of say stopping it and taking a picture.  It is an observation of any kind and need not be by a conscious being.  A vibration of the qubit is an observation.  This is why the qubit is in an environment as cold as possible.  An observation leads to quantum decoherence of the state.  Anything that can effect the state of the object is an observation or a measurement.  This gets to the heart of one of the problems in quantum mechanics in what is meant by a measurement and what it does to a state of the system.   But how do you know what your program is doing in the quantum computer?  Where is the calculation taking place in time?  Or how is it evolving in time?  I have John Preskill's Quantum Computer lecture notes in  book form  loaded from his website here.  Maybe I just need to read more of his lectures. But not the whole book.

Thursday, July 3, 2014

Is Iraq Going to be Another Vietnam?

     First I certainly hope not and I don't think that it will.  My concern however is mission creep.  When I first heard a week or so ago that Obama was sending 300 or so soldiers to Iraq for embassy guarding and to advise the Iraqi military, I thought that the administration must be concerned enough about the fall of Baghdad they are beefing up security of the embassy even more.  Hopefully we aren't going to see a repeat of 1975 with people being evacuated from the rooftops[ of the embassy in Saigon as Saigon was being taken over.  Hopefully this won't happen again in Baghdad.  Also I was wondering why does the Iraqi Army need US military advisers?  We spent billions of dollars training the Iraqi Army and aren't they capable of defending Baghdad and regrouping to fight off ISIS after their initial disgrace of running away in their first encounters with ISIS ?  Running away in the ten's of thousands leaving behind their equipment.  Apparently we didn't train the Iraqis well enough or their soldiers felt that what they were fighting for was not worth their life or a combination of these or other reasons.

       Now we hear this week that we are sending in another 300 or so troops to advise the Iraqi army.  Didn't we train the Iraqi to deal with such a situation?  Why are they overwhelmed so much that they need our help?  Do they have in the back of their minds that don't worry the US will come in and save them?  I don't know.  What I do know is that we keep sending in more advisers just like what we did in the beginning of our involvement in Vietnam.  The President has said that we will not be putting in any boots on the ground.  I guess that means that we will not have soldiers with guns fighting in the streets.  But what do advisers do?  Sit outside the battles and observe and advise the Iraqis what to do?  Plan strategies for the Iraqis to defend Baghdad and maybe to start a counteroffensive?  Didn't we train them to do that?  Don't they have generals who know how to do that?  They are generals after all don't they know what to do?  Why do they need our help?  Do advisers depend on Iraqis to tell them what is going on and for intelligence.   I guess we have no confidence in the Iraqi or that they are doing.  What if a US adviser goes out into the field and gets killed. or a US pilot is shot down and captured?  What will we do?

    We were suppose to be done with Iraq when we pulled out in 2011.  The Iraqi government did not want any US combat troops stationed in Iraq so we left except for soldiers guarding the embassy or so I thought.  Are we going to keep sending in more advisers?  I hope not.  The middle east is important for oil coming to the US that is not reason enough to get involved in a religious civil war.  I know there are concerns about ISIS as a terrorist group but Afghanistan and Iraq should have given us lessons on appropriate ways of dealing with terrorist groups.  Starting large wars or restarting a war is not a solution to that problem.

Wednesday, July 2, 2014

Book Review: Not Even Wrong, by Peter Woit

This is a book I wish I would have read back in 2006 when it was published.  It gives a mathematical physicist's argument against string theory.  Woit has a blog started in 2004 under the same name as this book.   It discusses theoretical particle physics and mathematics and in particular is an ongoing criticism of string theory, multiverses and other theories that make no connection to observables and that can never be falsified.  The level of this book is the same as the blog, very readable.

The book gives a historical development of particle physics theory starting with the development of quantum mechanics in the 1920's and leading up to the culmination of the standard model of particle physics.  Then as the author states throughout the book "The discovery of the standard model is an intellectual achievement that will be remembered for the rest of human history.  One unexpected result of this progress has been that the field of theoretical particle physics has now been a victim of its own success for nearly a quarter century".  To this date (2014) there has not been any experimental result in particle physics that cannot be described by the standard model.   However, there are known problems with the Standard model.  Problems such as unifying the electroweak force with the strong force and explaining the values of twenty or so adjustable parameters in the standard model.  Many model and theories were developed along the way to the standard model one of them being string theory.

String Theory developed in the 70's was originally proposed to try and understand the strong force.   Over time hope began to grow that it could describe all the known particles and the four known forces.  It became of "Theory of Everything".  One big problem,  it couldn't calculate anything to compare to experiment.  The hope to relate it to experimental data was always over the next horizon waiting for the next big insight.  This situation has been going on now for thirty years or so with the author saying that string theory has been a drag on resources that otherwise could be used on working on other theories.  The author takes a similar view to what Lee Smolin discusses in his book "The Trouble with Physics".  In the world of folks opposed to string theory Woit's book and Smolin's book work together very well.  Both give compelling arguments against string theory and the negative effect it has had on physics in general.  The author discusses in a field with limited resources how string theory has negatively  effected the entire physics community.  However Woit being a mathematical physicist does say how string theory and certain areas of mathematics have grown together partly due to the string theorist Ed Witten.  This is a tread throughout the book in how over the past hundred years or so how physics and mathematics have helped one another in certain areas.  This to me made interesting reading when he discusses the collaborative efforts between the two disciplines.

Woit's account stresses the use of group theory in quantum mechanics especially the influence of Herman Weyl.  Various ideas in group theory are described very well such as describing what is a group what is a Lie group and why group theory is useful in physics.  He discusses what is an SU(3) group for example.  I wish he would have used equations and examples but that is not the purpose of this book. The level of discussion is that of what you would read in a Scientific American article.

I thoroughly enjoyed this book and would recommend it to anyone interested in particle physics, its relationship with mathematics and to understand why there is disagreement in the physics world about string theory and the negative effect it has had.on science.

Tuesday, July 1, 2014

Dark Matter and Cosmic Voids

     An interesting theoretical result has recently been published in Physics Review Letters about using computer simulations of dark matter along with dark energy showing that the distribution of the dark matter forms cosmic voids and walls.  A synopsis of this paper is given here

     What's really cool is that the simulations come up with the distribution of dark matter forming these bubble like voids with the walls being where the density is highest.  I can't access the paper but here's an image of the voids and walls from the synopsis.

      What's amazing is that these simulations show a distribution of dark matter that is similar to what is observed for the distribution of galaxies in our universe.  Years ago sky surveys showed that the distributions of galaxies also show these voids and walls.  Since dark matter constitutes about 25% of the mass-energy of the universe in the lamba-CDM models  (Cold Dark Matter) and visible matter about 5% it makes sense that the visible matter would cluster along with the dark matter.  The image below is from the Sloan Digital Sky Survey that shows the distribution of over 10,00 galaxies in a region of the sky.  More information about the photo is given here. Notice the walls and voids in the distribution of galaxies.  Pretty cool!!