Recent content by SimonRoberts

Thanks for the excellent replies guys. I had realized that the Feynman diagrams were as you say a mneumonic for the complicated QM processes. The characteristic adjoint*operator*statevector structure of the fermion lines was the giveaway there. What I really mean is that I want to...

I have been studying how to apply the Feynman rules for QED to various simple first order process (as prescribed by Griffiths' 'Introduction to Elementary Particles'). So far, all is well. I can follow the rules, and develop the results, and it all looks very clever. My problem is that so...

As far as I am aware, there is currently no theoretical backing to the non-zero diagonal elements of the CKM matrix. They are entirely experimental. It is one of the most unsatisfying, niggling facts in particle physics today that we have no theoretical explanation of flavour mixing...

I would guess (and it is a guess, not being a golfer myself) - that a golf ball probably has the density it has to maximise this very problem. Maybe that would be a good starting point? Of course, golf balls have more complicated surface patterns with dimples and whatnot, but might give you a...

Don't forget that momentum is a vector quantity. So if it hits the wall with momentum magnitude P, the change in momentum is 2*P , to account for the reverse in direction. Hope this helps, Simon

Unbound neutrons are unstable, decaying to a proton, and electron, and an anti-electron neutrino in about 10 minutes. Neutrons bound with protons are just ordinary nuclei which arent dark. There is another thread running at the moment about the possiblilty of the presence of pure neutron...

As I understand it, one piece of evidence for dark matter is the rotation rate of a galaxy as a function of distance from the centre. After a certain point, whereas the rate would be expected to fall of with radius, it stays the same. Or at least, it doesn't fall off as much as it would. This...

Here's a +1 for them being the same, as far as I know.

If the other 78 are like me, they were hoping to open this one and explain to someone in less than a paragraph how to calculate torque about a pivot! Your question is rather more involved, and a little too on the engineering side of things for me to venture an answer. ,Simon

Sorry yes, multiplied is correct. I kept changing my mind as I tried to picture it, and looks like i settled on the wrong one!

Yes that was my point, hence my disagreeance with the concept of this 'Neutral Matter' that the OP referred to. Cheers, Simon

I think, just from drawing the 'triangle' that the speed of point C will be equal to the speed of A divided by the tangent of the angle at C. I'm fascinated as to the nature of this mechanical device though - do tell!

Both require something which most regard at first sight to be 'new and tricky'. For QM you will need to learn all about vector spaces and operator algebra, whereas GR needs a firm grounding in differential topology and tensor analysis. I suppose both are equally difficult at first look, since...

Surely 'neutral mass' by definition means massless. i.e, not interacting with the gravitational field. The same what that neutral charged particles do not interact with the EM field. We know Dark matter to interact strongly (I suppose 'positively' ?) with the gravitational field because of...

Same force, order of microteslas, compass needles are very light so are easily moved. The Earth and the magnet pull on each other. The Earth's magnetic field pulls on all magnets. I suppose its a bit like asking why a westerly wind doesn't blow all the cars into the ocean, since it does for...