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ajb

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Everything posted by ajb

  1. Lee Smolin has written a good book on quantum gravity;"Three Roads to Quantum Gravity". It is not what I would call a book for the lay-person, but if you have a basic physics background then you will be ok. Although Smolin is best known for his work on loop quantum gravity, he also talks about strings. Reading it wont make you an expert in the subject, but it is the place to start.
  2. Atheist, your last point is exactly my question. Even if we assume that the Hilbert space is seprable and therefore has at least one basis. Can this (or any other) basis be written in terms of eigenvectors and eigenvalues of "some" operator? It is true that in standard quantum mechanics we can do this. However, my question really is "does this come for free"? Things like Zorns lemma and the axiom of choice are needed to set up a basis on any vector space. My question was not really aimed at that.
  3. I don't think there is any formal requirements that must be meet if you want to attend. Giving a talk would probably be a different matter. However, the talks will be of a fairly high technical standard, they are intended for researchers and postgraduate students. I expect you would get very little out of attending. You could contact the organisers and discuss it with them.
  4. Here is another paper on the archive that might be of interest http://arxiv.org/abs/hep-th/0606194
  5. I off to York on Monday for the North British Mathematical Physics Seminars, It should be good fun! http://www.maths.ed.ac.uk/NBMPS/
  6. It looks like a real mix of theory and phenomenology. did you get much out of the conference?
  7. He could be refering to the emf, the electromotive force, which is mesured in volts. It is defined as the work per unit charge that the device does in moving charge from the low potential to the high potential. When you do not have a connection between the two terminals the current is zero, and then the potential difference is equal to the emf (for an ideal emf device). When you connect it the emf is not the potential, but emf = IR where R is the resistance and I is the current. So I think the 30kV is the emf for the device when it is not supplying current. It maybe worth asking the sales person about this.
  8. In quantum mechanics it is assumed that any state can be expaned in terms of the eigenvectors of some operator. That is the eigenvectors form a basis for the Hilberts space. Does anyone know if this is automatic from the theory of Hilbert spaces and their operators or if it is a postulate of quantum mechanics?
  9. I am trying to write a notebook that will do some manipulations with expressions involving the NonCommutativeMultiply. I noticed that Times[a] gives a, where a is a single variable. However, NonCommutativeMultiply[a] does not give a. Does anyone know how I can tell Mathematica that I want NonCommutativeMultiply[a] = a? Any ideas welocme.
  10. I think you must distinguish between singularities in mathematical models and "real" singularities in nature. I think it is a general belief that there are no true singularites in nature. Singularities in a theory lead to new theories. This has been the case with classical electrodynamics for example. Lorentz knew that the description of an electon classically has a divergence in the self-energy. That is the electon produces a field which backreacts on itself and produces unphysical effects. Also, they realised it would take an infinite amount of energy to assemble an electron. Classical electrodynamics is full of such infinities. The resolution to these difficulties was a quantum theory of electrodyamics QED. There were originally infinities here also, much related to the infinites in the classical theory but they could be removed by rescaling of coupling conctants and masses; Remormalisation Theory. So the hope that the infinites that arrise in General Relativity could be removed is very natural and is one of the motavating ideas for quantum gravity. Exactly what is quantum gravity is another question. I think it was Hawking and Penrose that proved that within the framework of General Relativity the universe must have started from a singularity. So I think the overal point is that theories and models can have singularities. Does nature? Probabily not.
  11. This paper might be of interest. It is an introduction to the subject and does talk about experiments. http://arxiv.org/abs/hep-ph/0606153
  12. Thanks Kenshin, but I am no expert in the field of cosmology. I highly recommend the book by Andrew Liddle. His book "An Introduction to Modern Cosmology" is great. It does not assume any knowledge of general relativity, particle physics or anything like that. All you should have is a basic overview of classical physics. The book does give you an overview of dark matter, but not dark energy. Either way, it is the place to start. I have a copy of the first edition. Even my girlfriend who does not know much physics found it interesting! http://astronomy.susx.ac.uk/~andrewl/cosbook.html
  13. TurboRotary are you are talking about the interpretation of anti-particles via Feynman diagrams? It is true that anti-particles look like particles traveling back in time.
  14. woelen the argument you have stated is against accelerating a massive particle to the speed of light. This means that you cannot have "faster than light" time travel. It is however, not an argument against time travel in general.
  15. Glad to be of help. In fact I did a module on modern physics as part of my MPhys which included time-machines. I wonder how may university courses can boast that?
  16. What I would say is that it is not obvious that time travel is not permitted within the frame work of general relativity. Generally, when ever we have a rotating space-time the angular and temporal coordinates get mixed up we have a time-machine. Due to this mixing it is posible to find paths for which time becomes periodic. These are the so called Closed Timelike Curves, or CTCs. This is how Malletts time-machine works. The rotating cicular beam of light produces the phenomena of inertial frame dragging. This courses the space-time to rotate and we have a time-machine! Similar time-machines have been suggested involving rotating cylinders of dust, rotating black-holes, spinning cosmic strings, or even a rotating universe. The other kind of time-machine involves apparent faster than light motion. Things like worm-holes work like this. Essentially it is because there is more than one path that a light beam could be sent, one shortrer than the other. In this way you can make it look like you have sent a message faster than light and hence have a time machine. One conjecture you should know about is Hawking's "chronological protection conjecture". It simply states that the universe does not allow time-machines. As far as I know, just about every time-machine ever concidered can be shown to be flawed when investigated further. The question with Mallett's time-machine could be "what is stopping it working?"
  17. I think you mean Ronald Mallett http://www.physics.uconn.edu/~mallett/main/main.htm http://www.physics.uconn.edu/~mallett/main/time_travel.htm For more information on other time machines search SPIRES for time machines or time travel http://www-spires.dur.ac.uk This should convince you that time travel is a "real" area of research. However, I think that most people are not trying to create a real time machine, but rather push known theories to the limit. In this way we learn more about the theories (mostly general relativity) and point towards new physics. G.M. Shore (my MPhys supervisor) wrote a review of some time machines. http://arxiv.org/abs/gr-qc/0210048
  18. Does anyone know any references about fluid dynamics as a field theory? In particluar I wanted to know more about the symmetries of fluid dynamics and possible Lagrangians. The only references I know are HEPPH-0407101 and papers by the same people. Are there any others out there that I don't know? Cheers
  19. I was just wondering if there are any regular Mathematica users on this forum? I use Mathematica all the time, mostly for fun. I wanted to know about how to write packages. Any advice? Cheers.
  20. The main area of mathematics behind quantum mechanics if the theory of Hilbert spaces and their operators. Hilbert spaces are infinite dimensional complex vector spaces and the operator are infinite dimensional matrices. So, before looking at this I suggest you get familiar (if your not already) with linear algebra, things to do with vector spaces and matrices in finite dimension.
  21. You can also use a Monte Carlo method for calculating pi, but is it rather slow. I once wrote a programe in BASIC to do it several years ago now. I do not suggest that it is a "good" method of calculation but it does work. You can see how it work here http://www.eveandersson.com/pi/monte-carlo-circle
  22. The book by A Das & T Ferbel would give you an overview of nuclear and particle physics. It doesn't use any quantum field theory but assumes that you know some quantum mechanics. It might be the sort of book you are looking for. http://www.worldscibooks.com/physics/5460.html Halzen and Martin comes highly recommended, it was the bases of a course I did last year on further quantum mechanics. Like Severian said, it stops short of using quantum field theory, but does deal with relativistic quantum mechanics and Feynman diagrams. I never used the book myself, but I know people who have. If you want more advanced stuff there are plenty of book out there. I like Ryder, Kaku, Nash and Ramond. Hope that helps
  23. Hello all, I am new to this forum and would like to introduce myself. My name is Andrew and I am a PhD student at the university of Manchester UK. My research interests are in geometry and mathematical physics. Of interest lately has been the BRST transformations, the antibracket and (odd-)symplectic geometry. These things are all to do with how to treat general gauge theories correctly when quantising them. The underlying structures involved are all "super". That is they all involve anti-commuting objects. Before that I obtained a MSc in theoretical particle physics with a distinction at the university of Sussex. Mt final year project was on anomalies. And before that I did my undergraduate studies at the university of Swansea where I obtained a MPhys degree. To find out more, have a look at my website http://www.angelfire.com/geek/susy/index.html Thank you Andrew
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