Dork Scratchings

Lockdown reading My lockdown reading list consists of just one book. This might last a long time I thought, so it's my opportunity to make a 2nd stab at understanding Quantum Field Theory. Last time, I bought "Quantum Field Theory for the gifted Amateur" and I learned a lot from it. Mainly that I am not gifted! I got three chapters through it and then gave up on the book, and on quantum field theory. This time round I did my research better and found a much more gentle book: Student Friendly Quantum Field Theory, by Robert D. Klauber . It covers the same material, but takes pains not to lose the reader, by spelling out every ambiguity and subtlety. I'm half way through and feeling quite chuffed with myself. Here I am studying hard, on a sunny day in Lockdown Britain: No, the weights are not mine In this book, and every other in the QFT literature there is a concept of some particles being virtual. What's this about?  Why are some virtual and o...

Clockwise from top left: Occam, Deutsch, Everett, and Dirac Occam's razor The anthropic principle Forward reasoning Lack of any consistent alternative 1. Occam's razor Hugh Everett's 1956 thesis The Theory of the Universal Wavefunction opens with a mathematical summary of the then widely accepted Copenhagen Interpretation. "... there are two fundamentally different ways in which the state function can change:    Process 1:  The discontinuous change brought about by the observation of a quantity with eigenstates $\phi_1, \phi_2,...,$ in which the state $\psi$ will be changed to the state $\phi_j$ with probability $\lvert(\psi,\phi_j)\rvert^2$. Process 2: The continuous, deterministic change of state of the (isolated) system with time according to a wave equation $\frac{\partial \psi}{\partial t} = U\psi$, where $U$ is a linear operator." The 1st process is commonly known as the "collapse of the wavefunction" and ...

The End Is Always Nigh I suspect I am not the only person to feel this way, but for me the present moment seems to have a strong five minutes to midnight character to it.  Another way of putting it is the end is nigh , although I prefer five minutes to midnight because it carries fewer religious connotations.  (And also because it gives a nod to the famous Doomsday Clock started by the Bulletin of Atomic Scientists following the bombings of Hiroshima and Nagasaki.) From xkcd.com First and foremost in my mind is the climate crisis.  We're told by the IPCC the tipping point could be as low as 1.5 °C, which is only 0.4 °C away from where we are now.  And that (making optimistic assumptions) we have a CO 2 budget equivalent to 10 years of emissions at today's rates.  Beyond the tipping point the future looks so bleak that even if humans do survive it's hard to see there being very many of them, or their lives being very nice. But it isn't just th...

(Probably mad, but worth saving for posterity) Today's post follows on from this previous post  in which I argued that the entire universe is in a superposition of states. I showed how this can lead to the impression that only the system under study is in a superposition, whilst the rest of the world is in a single - classical - state.  To summarize that argument: Reality is a superposition of states for the entire universe, each of which can be thought of as a complete classical block universe. We don't directly experience the whole universe, so let's arbitrarily place a closed surface around us to demarcate what we "directly" experience.  Remember that this is a surface in 4 dimensional spacetime and so is itself 3 dimensional.  Also remember that it is completely arbitrary: if you like you can let it contain your entire body for your entire life; or it could just be your brain for some duration.  You could even let the surface enclose e...

Most Quantum Mechanics courses try to avoid imposing any kind of interpretation. This makes sense since the interpretation of QM is controversial but the mathematics is not. Unfortunately a little bit of interpretation always sneaks in through the back door. Whether you're being taught the Schroedinger Equation, Feynman path integrals, or QFT, the assumption is always that you can divide reality into That Which Is Under Study and the Rest Of The World... and that the nature of reality in the two parts is entirely different. If it's the Schroedinger Equation being taught That Which Is Under Study is represented by a state vector in a Hilbert Space that evolves with time; with Feynman Path integrals That Which Is Under Study is the set of all legal Feynman diagrams which complete the picture by joining neatly with the  diagram for The Rest Of The World; if it's QFT then the nature of reality inside That Which Is Under Study is a single state vector which can be converted in...

 Déjà vu Reality Checkpoint Cambridge ... the feeling of having been somewhere or experienced something before when you know you haven't.  Déjà vu demonstrates that knowing we've been somewhere before and feeling that we have are different things. What makes it so difficult to convince skeptics of the reality of Everett's parallel worlds is that it feels as if time flows through the present moment like water flowing through a hosepipe.  It doesn't feel like the present moment just exists as a droplet in a sea of moments, some of which happen to be a bit future- like , some a bit past- like (but most neither).  Past-like moments invoke a feeling of recognition in us, and we line them up like dominoes and call them the past.  But if you've ever experienced déjà vu you know that such feelings of recognition can't be trusted. So, if we could determine what neural activity or hormone , is associated with déjà vu, then maybe we could develop a drug th...

I would be glad to know your Lordship's opinion whether when my brain has lost its original structure, and when some hundred years after the same materials are fabricated so curiously as to become an intelligent being, whether, I say that being will be me; or, if, two or three such beings should be formed out of my brain; whether they will all be me, and consequently one and the same intelligent being. —  Thomas Reid letter to Lord Kames , 1775 If only nature could provide some way to distinguish between identical and the same  then one could answer Thomas Reid's question.  If a reconstructed Thomas Reid were the same being then that being would be him; if it were an identical being then it would merely be a person with the same memories, but without any continuity of experience linking it to the original Thomas Reid.  Incredibly, it turns out that nature does provide a way! A well known feature of modern physics is that if you perform the same experi...