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Exactly, I could get a much better guess at the sun's surface temperature by using Wein's law and the spectral peak as you say. Or maybe by using the Maxwell-Boltzmann distrubution and linking that to the sun's luminosity. But that is astrophysics. All I did was take a very wild stab in the dark.

I assume that the outer layer of the star is made of ionised hydrogen, hence i pick 13 eV. This is of course a gross simplification as we have a high photon density which would lower this energy. What supprised me was how this very stupid guess gives a very good approximate upper limit on the stars temperature.

I have been "messing" Wien's law and the ionisation energies of Hydrogen and Helium to see of I can get a very naive guess at how hot stars are. Interestingly I get a temperature that is much higher than typical stars on the main sequence, but they are just within the range of the hottest stars observed. My approach was to assume that the temeperature of the star's surface is about the first ionisation energy of Hydrogen (13eV) and then Helium (25eV). This would be the "worse case scenario", atomic collisions would lower this temperature dramatically. I assume this as the photoshere of stars consists of plasma. This gives a surface temperature of 150,865 K and 290,125 K respectively. Stars at this temperature would radiate strongly in the high ultra-violet low x-rays. 150,865 K is just above the classification of O-type stars (30,000-100,000K). 290,125 K is just above record for a white dwarf, something like 250,000K. So it looks like my "stupid guess" gives an approximate upperbound on temperatures based on the composition of the star's surface. Anyone here know more about astrophysics like to comment?

It may be possible to look further back than the CMBR by looking for other types of radiation; gravitational waves and neutrinos. But I am sure there are plenty of techincal issues to address before this becomes possible.

You have to be careful trying to apply equations that govern classical physics to the (sub)atomic scale. So indeed, Maxwel's equations won't be able to explain what is going on inside a material at the quantum scale. The correct langauge to use is that of quantum mechanics. Light is slowed down in a medium, because it interacts with the electrons in the medium, so that the effects of this interaction shorten the mean free path and so the photon will on averege travel slower in the medium. I am sure you can find out more by using google.

Great stroy Severian. My girlfriend did not beleive I was involved in theoretical/mathematical physics. Not till I showed her my university card. But then it doesn't sound so cool as trainee astronaught! In the past saying your involved in physics or maths does not inpress the ladies. Maybe next time I will say that I am trying to make a "flux capasitor" or something!

An elementary but decisive observation.

Maybe we will soon be able to test the existence of extra dimensions, observe black hole evaporations, see supersymetric particles, detect (F or Cosmic?) strings in cosmology? String theory also makes some predictions on how GR could be Modifed. Maybe we could observe this one day. All of which could in principle support string theory to some degree. However, I would agree that string theory is not a theory in the strict sence, it is a mathematical frame work to construct theories. In the same way that quantum field theory (of point particles) is not a theory, but a mathematical frame work in which you can construct the standard model. The big problem right now is that string theory does not seem to be unique. This is the landscape problem. String theory has many many solutions all of which "could" be our universe. How to pick the correct one is the question. Before one could make very precise predictions this solution must be singled out. Maybe we are just asking too much of string theory? It provides the best way so far to combine gravity and quantum mechanics, but maybe it is not a TOE. Who knows.... But your point can be made of any quantum theory of gravity. Unless we can make observations on the scale of Planck energy we cannot test quantum gravity directly. How to do this, maybe we have to look for cosmological remnents of quantum gravity? See if there is a signal of a specific theory of quantum gravity in the CMBR. It is thought that strings (either cosmic or F-strings) may have contributed to the density fluctuations in the CMBR. Evidence of string theory? Also, as far as I know no other theory of quantum gravity makes predictions/speculations that string theory does/can not. For example, non-commutative space-time on small scales. Untill Planck scale physics is tested no one can rule out or in string theory or any other theory.

Photons are not gravitons. What is true is that they interact, this is becasue as you said photons carry energy. You can see they are not the same as although gravity and electromagneism are similar in many ways they are not the same. One main difference is that gravity appears to be only attractive as opposed to electromagnetism. This quantum mechanically manifests itself as the gravition being either spin-0 or spin-2. In fact in order to give the predictions of GR it must be spin-2. Other spins can be ruled out, fermionic gravitons won't give the 1/r^2 law, spin-1 can give a negative energy for static interactions and spins >2 are quantum mechanically inconsistent. They are thought to be massless as gravity is a long range force. However, it maybe possible to give them a tiny mass. But as GR is a gauge theory, I would expect the graviton to be massless.

Best thing you can do is get some rope/string and a slinky spring. Have a play with them and see what wave phenomena you can see. In particular look for longitudinal and transverse wave motions, reflected waves and how waves interfere with each other.

In physics ther are very few women. Im my undergraduate class of about 30 there was 3 or so. In my MSc there was again only a few women. In Mathematics it looks more like 50/50 or so, maybe a little less for PhD. But then very few go to do postdoc or become leturers in either maths or physics. How many maths or physics women professors can you name? Pesonally, only a few. In last months "Physics World" (the magazine of the IOP) the question of if we should recruit more women in physics or not was raised.

All I mean is that thsi thread seems to be going off track and becoming a bit off a personal attact on "beleivers" and "non-beleivers". I don't really wish to fight about the existance or not of gravitons, as gravitons at the moment are a pure theoretical objects, lets ask "sensiable" theoretical questions. Things you might like to think about is why gravitons are thought to be massless and why spin-2 and not spin-0 or higher spin?

So, back to gravitons... Any sensible questions about the subject that we can discuss or calculate?

Spoken as someone who has much experience of string theory?

Hi, I had a quick look at what you wrote, and to me it just looks like a list. I can see no structure in what you have written. What are you trying to say? Also, if your ideas are any good you should get them published in a reputable journal.

As you said, in atoms you have discrete energy levels, that is the energy of the orbiting electrons falls into discrete values. Now in a solid crystal we have a huge interaction between all these discrete energy levels of the individual atoms. This has the effect of "smearing" out the discrete energy levels into bands. That is electrons can now have continous energy, but between selective values. (unless the electron has enough energy to be free in which case it will be truely continous). There can be band gaps, bands can overlap etc... This is well understood using quantum mechanics, most books on the subject will say something about this, if not consult a book called something like introduction to condensed matter physics.

I think it was Dirac who first worked on the idea of a varying gravitational constant. I am not familiar with this work, but I think it would have been in the context of general relativity and cosmology as opposed to modified Newtonian gravity. All I can find is Consequences Of Varying G. (Talk) and The Variation Of G And The Quantum Theory. (Talk) on SPIRES.

People still work on fusion, (my dad for example). Have a look at JET.

I agree with Severian, all the equations I wrote down were classical. In fact, (classical) gravitation waves are usually considered as linear pertubations of a metric. Applying the standard methods of quantum field theory via path integrals to these pertubations we should arrive at the notion of a graviton as a "quantised gravitational wave". However, as I said this does not work as the theory is non-renormalisable. But if we ignore these complications for the mean while (i.e. treat gravity as an effective theory), then assuming that gravity is no different to the other forces then, as Atheist and Severian say we have no reason to question the existence of gravitons. The whole point of my posts was to give people an idea of what gravitons are.

So what is a graviton... In order to quantise general relativity one starts by power expanding around a classical around a classical solution to Einstein's field equations. Usually this solution is taken to be Minkowski space-time. [math]g_{\mu \nu}(x) = g_{\mu \nu}^{0}(x) + \kappa h_{\mu \nu}(x)[/math] where [math] h_{\mu \nu}(x)[/math] is the graviton field and [math]\kappa \approx \sqrt{G_{N}}[/math] (Newton's gravitational constant). Now given this one can now expand Christoffel symbols, (and hence the curvature tensors etc..) so that we can write the Hilbert-Einstein action as a power series in the graviton field [math] h_{\mu \nu}(x)[/math] . In doing so, each term in the expansion contains two derivatives and an increasing number of [math] h_{\mu \nu}(x)[/math] . Hence the action is nonpolynomial and non-renormalisable (in the standard sence). However, by treating it as an effective field theory you can still calculate the graviton propagator and extract Feynman diagrams. What you should note, is that we have an infinite number of interaction vertices. Usually, people only consider the three and four point interaction, but higher ones are present. So in principle we can calculate gravitational scattering processes to low order. The graviton is a spin 2 boson (gravity is a tensor theory) and is thought to be massless based on astonomical observations. So in words, the graviton is the quanta of a linear pertubation of the classical metric. They only make sence in a linearised approximation to gravity. Are they real will they be detected? If gravity follows similar rules to the other forces, then I would expect gravitons to be detected someday. If not directly, then maybe indirectly as loss of energy in some process. There are people on here who are much more awear of experimental issues, maybe they can help...

It is not a matter of blind belief like in religion. You work on a "theory" based on mathematical and (hopefully) experimental/observational evidence.

The circle is an exmple of what is known as a "manifold". As such you can always define a "point" on that manifold using (local) coordinates. You can just use your embedding of the circle in [math]\mathbb{R}^{2}[/math] to define local coordinates, i.e [math](x,y)[/math] such that [math] x^{2} + y^{2} = 1[/math].

Mathematics and computer science have a lot of cross over and computers are used in mathematics all the time. I don't think you should worry about it too much. But, if you asked me if it were easier to teach a computer scientist maths or a mathematician/physicist computer science, I know which one to chose.

22 papers on SPIRES talking about warp drive. I think that is the place to start looking. Generally, space-time engineering requires matter of some sort that is "unphysical". It will have some very odd equation of state or violate some energy conditions or something. trying to come up with "funny" space-times is more a test of our understanding of GR and a way to hint at new physics.

I found this on the Cern Document server, maybe it will help http://doc.cern.ch//archive/electronic/quant-ph/0404/0404042.pdf In fact, why don't you search the server yourself, Bekenstein has several papers on the topic. http://cdsweb.cern.ch/