Severian

They might see light travelling less than c though (if it is ex-vacuo for example).

I was amused by this question I saw recently on an exam paper. Lets see who can answer it. A van is accelerating continuously at 12 ms^-2. In the back there are two baloons: one is filled with He (lighter than air) and attached to the floor by a piece of string; the oher is filled with C02 (heavier than air) and hangs on a peice of string from the ceiling. Describe the behaviour of the ballons as seen from inside the van.

I am against gay marriage. I think the state has no business asking, or condoning, or condeming, or even having an opinion on what 2 (or more!) consenting adults do in private. I think the state should abolish state condoned marriages of all kinds. There should be no tax breaks and no special rights for couples by default. Instead, if people want to get 'married' for religious reasons, they should go to their church and have a religious cerimony (with no associated legal binding). If they want to have the rights currently associated with 'marriage', they should go see their laywer and draw up a contract. I must admit though to being a little bit confused as to why the gay community is making such a stink about this. Surely there are only two reasons to get married - religous or legal (as outlined above). Since they can still do both of the things I suggest above (ie. have a religious cerimony, and visit their lawyer), why do they need the state to ratify their union?

:rofl: (This was a joke.... right? :scared: ) Edit: We need more smilies...

'Oscillion' is a dead givaway.

Sorry - that will teach me to read these posts a little more completely

I feel the urge to clear up a falacy here (I'm being pedantic). The electromagnetic force and the weak nuclear force have not been unified. They have only been decribed by a theory which inextricably links them (in fact the photon is a mixture of the U(1) gauge boson and one of the SU(2) gauge bosons). But they still have separate coupling constants and are still spearate forces (unlike the unification of electricity and magnetism into different manifestations of one force - elecrtomagnetism).

Traditionally 'matter' has been defined as the particles apon which forces act while the gauge bosons which mediate the forces are not considered matter (this is a bit of an arbitrary distinction though). Using that definition, a photon is not 'matter', but an electron is (with a positron being anti-matter). In fact, the theory of the big bang has all of matter being created in this way, eg [math]\gamma \to e^+ e^-[/math] Notice that in the above reaction there an equal amount of matter (e-) and anti-matter (e+) are created and this is the case in (almost) all reactions. So if it was all created in the big bang, this implies that there should be (almost) an equal amount of matter and antimatter in the universe. But looking around us, nearly everything is matter and there is very little antimatter to be found! One of the big questions in science right now is how do we reconcile the big bang with the absence of anti-matter. (The is the problem of the Baryon Asymmetry of the Universe.)

I hardly ever breath, I can hold my breath so long. I did take a long gulp once, when I was about 4 I think, but nothing since.

I can do about 10,000 or so a minute. I used to always do the one handed ones, but they were too easy, so now I always clasp both hands behind my back. Much more challenging!

The interference bands only have a one dimensional line where the destructive interference is entire. Around that line, the interference band will be dark but not completly absent of light. So unless the wires are infinitely thin (which I presume they are not) the experiment shows nothing at all, because the measurement is still capable of collapsing the wavefunction.

oh - sorry.

Why not use latex?

At 51 pages, I may be some time reading this.... As for other non-string QC approaches, as I mentioned above the foremost seems to be 'supergravity'. The other 3 forces in nature seem to be well described by the Standard Model: one imposes a symmetry on the universe (eg SU(3)) and then when one insists that the symmetry is 'local' (so causally disconnected observers can make different choices with respect to the symmetry) one is forced to introduce new paricles; these new particles are the guage bosons, the force carriers of the three forces. So that the gluon is a consequence of local SU(3) for example. This works amazingly well, so it makes sense to use it for gravity too. Now, the Standard Model has certain problems associated with it which are serious but generally aesthetic. The most famous one is the 'hierarchy problem'. When one calculates quantum corrections to the mass of one of the particles, the Higgs boson, one finds that it is naturally very very heavy (the Planck mass) unless one fine tunes the parameters very carefully. (This is actually a consequence of it being spinless.) The Higgs boson cannot be this heavy if it is to break elecroweak symmetry as in the Standard Model. But if one introduces a new symmetry - supersymmetry - one also introduces new contributions to the quantum corrections which exactly cancel the old ones, stabilising the Higgs mass. But this is not the most interesting thing about supersymmetry. In the 1970s Coleman and Mandula proved a theorem which says that the Poincare Algebra (the symmetry group of GR) is the largest possible symmetry group that space-time has - it cannot be extended. But in the 80s Sohnius et al showed that they had a flaw in one of their steps and you could extend space-time by adding 'fermionic' directions. So a field is then not just [math]\phi (x)[/math] but is [math]\phi (x,\theta, \bar \theta)[/math] where [math]\theta[/math] and [math]\bar \theta[/math] are new fermionic 'directions'. This of course is an alteration to space-time, so one might assume that it has something to do with gravity. In fact it is supersymmetry which provides the 'super' to superstrings. But even more amazingly, when one insists that supersymmetry transformations be local (just like we did for SU(3), SU(2) and U(1)) then we find that we generate a new force with a new force mediating particle. This is gravity and the mediator is a spin 2 particle known as the graviton. This is supergravity. Unfortunately it has a problem: while the leading order works fine, when one calculates quantum corrections to observables, infinities start appearing all over the place. My feeling is that supergravity is a low energy effective theory limit of a string theory at high energies. Just as the Fermi theory has infinities in it, once it is incorporated into a higher theory, the infinities will go away.

What do you mean by QG as such? String theory is an attempt to explain gravity quantum mechnically - is this what you mean? Although I don't know much string theory, I more or less understand supergravity, in which gravity is a consequence of local supersymmetry. Would you like to know something about this?

The Stefan-Boltzmann constant is [math]\sigma=\frac{\pi^2k^4}{60 \hbar^3 c^2}[/math] Natural units has [math]\hbar=c=k=1[/math] so then [math]\sigma=\frac{\pi^2}{60}[/math].

If you want to do a PhD at Cambridge you should do Part III of the maths tripos first. It will let you figure out what you want to work on for your PhD (if any). And badchad, there is nothing wrong with Buffalo - that is where Uli Baur is isn't it? (Or am I getting muddled up with SUNY at Buffalo?)

As I said before it is entirely inappropriate to use string theory to calculate low energy phenomena, not because string theory isn't up to it, but simply because it is too hard. It is much better to us the lower energy models to calculate theings like dark matter relic densities. (An anology would be that one does not use quantum mechanics to calculate the trajectory of a football - one uses Newtonian mechanics. The QM would give the correct answer but is far to complicated to be prectical.) I am aware of there being work done of what is euphamistically termed 'String phenomenology', which isn't phenomenology as an experimentalist would recognise, but is rather taking low energy limits of string theory to see which low energy effective models they reproduce. The most famous of these is the [math]E8 \times E8'[/math] heterotic string theory which breaks down into the Standard Model or other low energy theories. Even then though, this only tells you what your DM candidates are, and what their annihilation cross-sections are.. You still have to solve the Boltzman equation to find the relic density itself.

This is quite sufficient thanks!

OK - sorry, I was misunderstanding. I see what you mean. (I had thought that you were trying to predict the strength of electromagnetism from first principles, which if you could do it would be Nobel Prize time....)

Sorry - I was assuming 'admin' and 'mod' were interchangable. Thanks Sayonara3 - I got your PM and the email address you have is correct. I don't understand how I didn't get the confirmation email. The only thing I can think of is my spam filter, but since I have had 3 emails offering viagra this morning I don't think it can be that tight. Edit: great - I can edit now - thanks a lot!