Exciting Physics - open discussion to inspire!

Well, the thing I do like about sonoluminescence is that tiger pistol shrimp can do it with their claws

Cheers for having a go I look forward to the day when fusion really is reality because problems regarding materials are finally rounded up

Although was it not he who pretty much invented/ developed QED. I presume thereofre that before it they didn't get very far with it.

Just to clarify, where does the Dirac equation fit in in all of this?

I assume you are a 4th year? I'm hoping to go further into quantum as it is by far my biggest area of interest.

You integrate over all internal momenta and the series sum (which you truncate) is summing over all configurations, so essentially yes, you average every single possible possibility.
Even for scalar $\phi^{4}$ theory there are Feynman rules. Dirac did the ground work 20~30 years before Feynman, so he must have had some way of doing it. QED was pretty much Feynmans work though (though I think he shared the Nobel Prize with someone else).
The Lagrangian for QED involves a fermionic field which is the electron. The field function $\psi(x)$ MUST obey the Dirac equation because it's a fermion.
Yep, I'm a 4th year Outside Oxbridge QFT is PhD work I think. I'm hoping to do a PhD at Southampton next year and they take peope with the assumption you've never done any of the stuff I'm doing in my 4th year now. Quantum field theory, symmetries, string theory, standard model, advanced general relativity, it's all 1st year PhD stuff, even in Durham too!

I'm not sure it is entirely intentional. I would assume they create sound waves which just so happen to cause nearby bubbles to undergo sonoluminescence. Though it may well have benefits for them in terms of being able to see etc. Thats evolution for you...

If you do have any specific questions I can try and clear them up with some explanation, but I can't pretend to compete with the IOP or the Scientific American

Although I assume all that would come under the banner of theoretical physics, which Im not sure if I want to take, sounds very tough

The symmetries section comes under 'Lie Algebras and Representation Theory' which a bunch of my pure maths friends did. Lie groups and algebras are integral to particle physics and general relativity is essentially 'slightly applied differential geometry'.

It's tough (Part III is meant to be ) but it's very interesting at the same time. I've read a couple of text books and papers recently on string/M theory and they are all very interesting. It's slightly sad to say perhaps, but seeing the inherent symmetry and beauty in theories you don't understand make you want to learn more.

Really? Thats really interesting. What sort of diffraction experiments have been carried out? Any more details you can give us maybe?

I'm an X-ray photoemission spectroscopist so I don't know that much about diffraction, however, the synchrotron is used widely for protein crystallography and many materials such as super and semi conductors. The technique I use is called Normal Incidence X-ray Standing Wave. We set up x-ray standing waves from bragg planes in single crystals and calculate the position of adsorbate atoms on the surface of these crystals by observing the change in x-ray absorption (measured as the height of specific photoemssion or auger transition peaks) as the bragg condition is scanned.

www.esrf.fr
www.srs.ac.uk

I've been fascinated by String Theory ever since i saw the programme aired on TV all that time ago by Brian Greene which you can watch on the PBS website:

http://www.pbs.org/wgbh/nova/elegant/program.html

I've had the book of the programme for a long time now and i've decided to try read it all again now that i feel more confident being at the end of my A2 course.

Thursday just gone i was at the Royal Holloway University at an open day in Particle Physics which was very interesting, practical activities about the Cloud Chamber, and the BaBar Experiment at SLAC in which we used data from that experiment to estimate the life of a particle which i forget now =P. But the best part of the day was listening to a lecture from Glen Cowan who studied at Berkley and has one of the few genuine copies of the photo that proved the existence of the positron particle in the Cloud Chamber. He was given it by Carl D. Anderson who discovered the particle, in 1987 when Glen graduated.

Ahh thats cool , made any interesting discoveries or breakthroughs as yet?

I take string theory with somewhat a pinch of salt.

In my mind a physical theory is only really viable if it makes predictions that can actually be tested. As yet I have not seen any such predictions. How is one meant to see a string of such ridiculously small size. Beyond discovering other dimensions which seems unlikely and can't see how these strings can be detected.

The other problem is that the theory is inconsistent. There are to my knowledge at least 5 different forms of string theory depending on the types of string you have... normal string, tube like string, closed string... different sizes of string. I believe they all have names like heteurotic type I, heteurotic type II and so on. The then leads onto M-theory and all the rest of it. Though, alpha is I am sure, more knowledgabe about this than me.

Those experiments at Royal Hollaway sound really good. I've never actually played with a cloud chamber myself, though I have seen one. Out of interest, what was the lecture actually about?

The first lecture was from Dr Fabrizio Salvatore (sounds like a mobster lol) which was about matter in general, designed for people who knew nothing about particle physics so it was more of a revision session which definitely helped me. The second one from Glen Cowan was about antimatter and it's basic history. I asked him after lecture what he thought about string theory and he said the same as you, he'll pay attention to it when it can make accurate predictions .

Healthy skeptacism is always good
It's not so much that you want to see the strings, but to see an effect they have which a point particle theory doesn't say should be there. As yes the theory is a bit too immature to have got to the point of making it's own predictions. If does develop general relativity spontaneously within itself though, which is a good sign (that was one of the reasons Michael Green gave for why he thought it was worth researching).

There are beginning to be ideas on how to look for extra dimensions too. Things like how gravity behaves.
Inconsistent is the wrong word I think. It gives multiple perspectives which on the surface don't seem to share enough commonalities to be linked. Your worry is well founded though and until 1995 it was a thorn in the side of many a researcher till Witten, then people like Hull, Townsend, Schwarz, Green etc, realised that if you hypothesised the existence of 11 dimensions (the theories you name are 10 dimensional) and then use various ways to remove that dimension you get the 5 different theories.

It's vaguely like some old Hindu/Buddist tale about a village of blind people trying to work out what an elephant looks like (I hope I'm remember this right rather than inventing a crazy story). One villager 'fumbles' the trunk and says an elephant is like a huge snake, long and thin and thrashing about. Another feels a leg and says an elephant is like a tree, thick, round and tall. A third feels the ears and says an elephant is like a bat's wing, thin, flat and flapping about. None of them get the complete image and without actually seeing the whole elephant they never get the right description by trying to mesh their versions together. M theory is like that. We've realised we're seeing snippets of something greater and only just beginning to find out about that.

It's hard to get across the elegance of it, particularly if you don't show the maths, but it's not clumbsy or mish-mashed together. I think it's Weinberg who says something like 'It's hard to think that something so rich and elegant in structure is totally wrong.'. Even if the current theories end up being swept away by time, I think that the hypothesis of particles being described by objects with internal structure (ie strings or membranes) is so powerful a notion something of it will remain in physics for a long time to come.

I'm assuming you people know about these Richard Feynman lectures?

http://www.vega.org.uk/video/subseries/8

I haven't watched them all yet but i thought i could stick them here incase you guys haven't, and perhaps for other peoples reference .

Feynman is the greatest physicist who ever lived...