Recent content by muppet

If you have a classically motivated differential equation of motion for some degree(s) of freedom, you can usually hack your way towards a Lagrangian that yields that equation. A reasonable starting point is to multiply the equation on the left by your DOF (here, \psi) and start integrating by...

I'm not entirely sure I follow (7.10.13) (why p is identified with the expression containing grad S), but is it not simply the case that by acting on the product \psi = N e^{ i S} with two derivatives it's inevitable that you'll pull down a term that looks like the second derivative of S, just...

Can I ask why you say they are "like a graviton with spin zero"? To me a graviton is necessarily a spin -2 particle -the spin, the gauge symmetry corresponding to classical diffeomorphism invariance, and the fact that it couples to the energy-momentum tensor are all closely interrelated.

They would both be considered BSM. Axions are a species of particle simply not present in the standard model. Neutrinos, on the other hand, are present- or at least "left handed" neutrinos are. The thing is that in the SM, from the viewpoint of the fundamental definition of the theory, "left...

The standard answer to this, which is treated in many standard textbooks or widely available lecture notes, is that yes: there is a conceptual leap, which lies in the fact that relativistic quantum mechanics for fixed number of particles described by a conserved probability density is not...

Been looking for a nice introduction to this topic once I get my damn PhD corrections out of the way- thanks for sharing :)

OK, thanks. Just checking- I've never actually studied a real-life example of projectile motion, rather than one with made-up numbers before!

In this clip the mythbusters are investigating projectile motion at a firing range: Using the fact that the bullet fell approximately 1 inch as it traveled 100 feet, I predicted it should travel 600 feet down the firing range before hitting the ground from a height of 36 inches. Instead, it...

I just found out about this via Twitter: http://phys.org/news/2014-10-function-electron.html I'm too tired to have got my head around all the details, but it looks as if there's a fascinating new experimental perspective on what a "measurement" in QM actually is. DOI for the original journal...

Everyone learns the picture associated with e.g. the Balmer series in Hydrogen: a photon with a precise energy flies in and is absorbed by an electron which is excited into a higher energy state, which then decays to the ground state, re-radiating a photon of that precise frequency. If we...

I'm not 100% sure on this... but your post clearly assumes you're talking about spin-1/2 particles, which don't have a spin-zero component anyway, so think the addition of angular momentum should proceed in exactly the same way. However, if I think about a collision occurring in the...

I suspect that here the M^{\mu\nu} are operators that act on Hilbert space, which is different to matrices that act in a vector or spinor space with which the indices on e.g. a Dirac field are associated. (Think the angular momentum operator in some abstract representation, and not a matrix!)...

Belatedly, thanks to everyone for your replies.

Thanks for all the replies! It looks to me as if the deceleration parameter q=-\frac{\ddot{a}a}{\dot{a}^2}=-\frac{\ddot{a}}{a H^2}=-\frac{1}{2}(1+3w)\left(1+\frac{k}{\dot{a}^2}\right) is only constant in a flat universe?

George: on p.5 Kinney claims that the Hubble length "sets the scale of the observable universe"- is he not talking about the horizon there? In the concrete case of a flat universe with vacuum energy, it seems to be an exact equality: proper horizon size =1/H :confused: