timo

You write "as I read in 'The Fabric of Cosmos' by Brian Greene, the branes on which the strings end start to vibrate when they collide which causes this effect" and everyone is happy. On topic: Wormeholeman, it´s rather interesting that you think things should be explained in easier terms. My perception of the treatment of physics in popular scientific debates is quite the contrary: Most problems and misunderstandings arise due to loose, incorrect or incomplete formulations. Take the infamous "E=mc²" for example. Is it really such a hard mindtwist to simply stick to "E² = (mc²)² + (pc)²" and consequently use the restmass? You miss absolutely nothing doing so since relativistic mass is just another name for energy, anyways. But you gain a whole lot like not being confused how a photon can have zero mass while it has energy. There´s another catch in the consideration of how much to water down a scientifically valid statement in order to make it more easily understandable: If you don´t understand an explanation because the formulation was too difficult (or simply badly worded) you can always ask for clearification. If it´s oversimplified, you say "thanks for posting the Lorentz Transformations, now I understand General Relativity" and happily head for the trapdoor at the end of the alley (ok, it´s a bit of an unrealistic example as noone in here gets as technical as to post the Lorentz Transformations ...). To sum it up in a single sentence: I can agree to saying "everything should be explained as simple as possible but not simpler" but imho a lot of discussion is done below the "not simpler" line. Also, I´d like to wholeheartedly agree with Blueniose saying "not everyone is a good teacher". In fact, I am repeatedly impressed by how the only member here of whom I know he actually teaches physics is able to give seemingly easily understandable, yet accurate, explanations.

I could agree with this statement. But I still wanted to comment on it: 1) One might take GR (and especially the equivalence principle) to a point where one sais that gravity doesn´t exist in GR. To some extend, the whole stuff with curved spaces is about getting rid of gravity and making it a fictional force appearing due to the use of "improper" coordiante systems. 2) That manifold is called "spacetime". I was assuming that the term was commonly known so I didn´t explain anything on it. That manifold is described by its metric (-tensor). That´s the point where my previous post started. 3) Minor technical correction: It´s a Lorentz Manifold. A Riemann Manifold does (at least to my sources) have a positive definite metric. Of course I don´t know which equation you are talking about but the equation describing the relation between spacetime structure and energy content is the "Einstein Equations". All of the appearing objects are tensors which ultimately brings us back to the difficulties about defining "differ" raised in my previous post. Perhaps you should also post the link or at least specify "the article".

I didn´t really want to complain about the current or a future arrangement of the physics subforums. I was simply asking what "Modern/Theoretical Physics" is supposed to mean (well, and I gave it a few comments so that my post wouldn´t look so tiny). If I had been asked for a suggestion what to change at this point (before Dave posted a suggestion for a new arrangement) I had simply said "let's replace Modern/Theoretical Physics with Eletrodynamics". My question what "Modern/Theoretical Physics" means hasn't been answered yet, btw. For attempting to give a reasonable suggestion for the subdivision of the physics forum I´d have to know why to subdivide it at all. There´s several reasons I could think of like giving an additional information on what the poster is talking about at all, making it easier for people to give an adequate reply, giving the visitors the possibility to directly browse to the threads they are interested in, being easier to administrate or simply easing the forum navigation. The list posted by Dave seems pretty ok. It´s more application-oriented than what I´d have had in mind but perhaps thats quite a good idea. Only thing I´d worry is whether the "Classical Physics and Quantum Basics" isn´t a bit crowded with topics (Classical Mechanics, basic QM, Electrodynamics and Thermodynamics).

Gluons do. http://en.wikipedia.org/wiki/Gluons In contrast to QCD where you get confinement due to the gluon-gluon interaction I could imagine that you wouldn´t get such a scenario in quantum gravity. Reason: While the number of color-anticolor pairs is not limited and each gluon-coupling has the same strength, your energy is limited and therefore the coupling between the gravitons might decrease with an increaing number of them.

I´d think they´d do: Reason 1 (from GR): Gravitons contribute to the energy-density tensor and couple to it. Reason 2 (from gauge theory): The Lorentz Group is non-abelian.

There is a question I was wondering about since I joined here and perhaps this might be the right point to ask it: What is "Modern/Theoretical Physics" ? Is it supposed to be "modern theoretical physics" or "modern physics and theoretical physics"? If it´s the former I´d wonder why there´s such a specialized subforum. If it´s the latter then it seems like a curious mixture to me. I cannot see a real reason why theoretical physics should have a seperate forum when experimental physics and applied physics don´t. In fact, I could rather understand why there´d be a subforum for experimental physics and none for theory. After all, there are some things in experimental physics that theoreticans don´t really bother about (feasability of measurements, applicability of different devices, ...) while I can think of little theoretical physics that isn´t used (verified, for example) by experimentalists.

If I remember correctly, the Bethe-Weizsäcker Formula is pretty good at describing the energies of different nuclei. Most of the appearing terms should be rather easy to understand as it's mostly a classical model. EDIT: I already had a strange feeling writing above because I remembered there was some trapdoor with Bethe. So I looked it up ... it´s the Bethe-Weizsäcker formula I meant, not the Bethe-Bloch formula. The first Wikipedia entry describing a bit about it was http://en.wikipedia.org/wiki/Semi-empirical_mass_formula And in case the connection from a formula for the energy of a system to decays of nuclei isn´t obvious: You can use it to verify whether a nucleus would lose or gain energy when it was changing to another isotope by either absobing an electron or by releasing one (neutrinos left unconsidered).

I don´t even know what Swansont commented on, but in case your "get a real answer using the mathematics" refered to the original question: It seems you didn´t notice, my answer was a mathematical one. In Relativity, the structure of spacetime is entirely described by the metric tensor. Being a tensor, it´s an invariant object where "invariant" means it remains the same under arbitrary coordinate transformations e.g. the frame where the object is moving. This invariance, which physicists usually call "covariance", has a slight catch, however. While the objects remain unchanged under coordinate transformations, their representations can change. That´s what my comparison with the EM field was about. The EM field can be described by the EM field strength tensor. The electromagnetic field strength tensor is ... right: A tensor, and therefore invariant under coordiante transformations (charge at rest <-> moving charge). If one writes down this tensor in a suitable representation (it´s a 2nd order tensor so a matrix is suitable) one can identify 6 independent entries of this matrix. Three of them can be combined to form the electric field, the three others form the magnetic field. And as we all know, the magnetic field of a moving charge and a charge at rest differ. So to sum it up and to repeat what I already said in my first post: The question whether the gravitational field of a moving object differs from that of the same object at rest depends on what your preferred understanding of "differ" is (tensor level or representation level). In the case of this thread however, there is an additional problem: I simply don´t know what a "gravitational field" is supposed to be in GR. Gravitiy is an effect completely determined by the spacetime structure. So I´d see little point in introducing a gravtiational field at all.

What do you do with the energy your solar panels collect, then?

Temperature is not energy; it´s a parameter describing the energy distribution of a system. 0 K doesn´t mean "no energy" (unless you adjust your E=0 level to this point). The inobtainability of T=0 K is a statistical argument not a "the world will end if we reach T=0"-one or a "that´s forbidden by uncertainty"-two.

Would you consider the electromagnetic field of a moving charge and a charge at rest (only electric field, then) as different? If "yes", then the gravitational field of a moving body probably differs from that of the same body at rest. If "no, it´s the same field seen from different frames of reference" then a moving body and the body at rest should have the same gravitational field. Pick the answer you like. Although I´m aware that I´m not being very popular with these kind of questions, I can´t resist asking the following question: What´s a gravitational field? And to give a little explanation why I´m asking this: I think it´s something you should ask and try to answer for yourself (mathematically, ideally), not for me.

There is a very wide range of random generators. Quality of the random numbers usually isn´t the only criterion for chosing a certain solution. Often, the random generator deliviered with your programming language is sufficient. For some programs like computer games your prime criterion is performance, not quality of the numbers. Therefore, you´ll chose a quick solution like reading out the processor time and perhaps XORing it with something to hide the continuitivity. For scientifical programs like physics simulations, the standard random generator sometimes isn´t good enough quality-wise. So you switch to algorithm which gives sufficiently good random numbers and still is fast enough.

I think you are correct by saying that no deterministic algorithm can produce random numbers - it would be somewhat self-contradicting. Within the scope of computer sciences, one usually speaks of pseudo-random numbers. Those are numbers generated by an algorithm which are good at faking being random. There are several tests that one can perform on pseudo random numbers to quantify their quality. The more primitive ones are checks on the distribution (you take a large amount of those numbers and see if their distribution over the interval you wanted to take randoms over is roughly the one you wanted) and their periodicity (the computer can only handle a finite number of different numbers therefore each infinite series has to have a periodicity - the higher the periodicity the better). Then, there are also more sophisticated tests on the correlation between the numbers (ideal random numbers are totally uncorrelated). But I have to admit that I wouldn´t have a single definition for "correlation" in mind, atm. An interesting test is trying to compress a large number of randoms with some compressing program. Ideal randoms cannot be comressed. There are methods that are thought to create "true random numbers". But those are hardware solutions. An example would be connecting the computer to a geiger counter and measuring the decay of some radioactive substance. However, while one reads about those methods from time to time, I hadn´t heard of or even seen an actual random generator working that way.

What´s the difference between annihilating and canceling out? You mean because they don´t simply vanish to nothing? Particles and anti-particles both have positive energy. Since energy is a conserved quantity an encounter of both cannot result in nothingness.

Actually, it might be that I was wrong about the WIMP part. I read one or two papers today in which WIMP refered to particles with little interaction - not nessecarily particles with weak interaction only.

And for those who don´know what the "Intelligent Design case" is: Could you perhaps edit in what this is all about? I´ve read several pages of dialogues and reports now but still I haven´t figured out more than that it´s about ID and that some court is involved.

I wouldn´t want to stop anyone from trying to fiddle around with equations but in this case I must tell you that I don´t really know what you´re up to. Furthermore, I don´t have the feeling that you´re knowing what you´re doing either. That´s not nessecarily bad. Sometimes it´s interesting just to do some stuff and critically review what you actually did afterwards - it sometimes yields a very personal learning experience. Are you aware that "curvature" (whatever you think it is) is usually connected to some second derivative, for example? Well, one point where I can help you out a bit (at least that´s what I hope): >> Would dr/dt be the same as v=at? If r is a position, then the total derivative dr/dt (t being time) is called velocity, in other words: dr/dt=v. In your case r was supposed to be a radius of a sphere, so it´s more or less up to you to decide to what extend you could still talk about a velocity (one might think about things like "expansion velocity of the sphere").

What´s your point in debating here Fiend? Write a PM to one of the moderators why you were banned and wait for an answer (note: Due to the mods not being paid for being online 24h hours a day it can take a few moments to get the answer). What´s the point in arguing that this and that cannot/shouldn´t have been the reason when you don´t even know (at least if one can trust your words in this case) what the ban was for?

The interaction with other matter (or itself) would depend on what dark matter is. For the two classes of candidates you mentioned, the interactions are pretty much described by their names already. If it´s WIMPs, then they´d interact via the weak force. If it´s SUSY particles then they´d mostly have the same interactions as the particle they are the SUSY partner of. EDIT: And to elaborate on the 2nd part of your original question a bit more: I do not think dark matter and dark energy are related. They enter theory for different reasons and -which would be the more important point- unless I´m mistaken here they also do have different properties. Dark energy is the attempt to map a constant (called "Cosmological Constant") that may appear in the Einstein Equations on a physical object. Dark matter is needed if the Einstein Equations are correct - regardless of the quantity of the Cosmological Constant.

Like in the case of Bose. That´s why it´s called the Bose-the-nonwhite-Einstein condensate. And from looking at the Chandraseka mass we finally know why those things above it collapse to black holes.

Temperature is a statistical variable, namely a parameter in the energy distribution. The reason why it´s often called a measure of the average kinetic energy of the particles in a substance is that Thermodynamics is often (especially in introductionary courses) done on systems whose "only energy" is kinetic one. Due to being a statistical approach, the whole concept of thermodynamics can usually only be applied to a large number of particles and not to individual particles or small numbers of them. Therefore, one usually cannot consistently speak about the temperature of an individual atom. Normally, temperature will decrease if the average energy of the system´s particles dereases and increase if the average energy increases. Therefore, absorbing energy which increases the total energy and thus also the average energy of the system will usually increase temperature. Same goes for giving away energy like by emitting a photon (where it´s loweing the temp, of course).

Most basic computer programs would probably run into the problem at m= -0.5 where the value runs towards infinity. Is it just this one term you want to have the extremal values for? In this case you´re better off doing the calc by hand and perhaps learning the math required for it if you don´t know it already. Or if the term above was just an example for terms you want to calculate, please try to specify more precisely what kind of terms you need to be worked on.

I think I´ve heard once that an octave is an increase in frequency by a factor of two. Mathematically, there is no problem increasing the frequency to whatever value you want. From the physics side, I think you´ll arive at a point where you can´t treat air (or wherever your sound waves travel in) as a continuous medium anymore but must account for its structure (the individual molecules and their interaction). From the "human" side, you´ll simply not be able to hear the tone anymore at a certain frequency (around 14 kHz ?).

Well' date=' it probably wasn´t the CIA .... I can only speak for physics, here but I´d guess it´s more or less the same for all natural sciences: It´s a common misconception that science was about explaining why something works. It´s about creating models of how it works. And a big point in about these models is that they should allow for predictions (not sure to what extend this applies to biology). You can explain everything with "it´s gods will" but this "theory" won´t allow for verificable predictions. For science, it does absolutely not matter whether a model works because it´s the "real explanation" of a phenomenon or because nature works like the model does due to some other reason (because god made it appear like that, for example). I can´t. Who cares? I had a pretty long answer to this but I deleted it because it would bring the thread way too off-topic. So I´ll stick to my message without giving much explanations on it: There is a difference between a teenager letting his/her imagination running wild and an adult who has an agenda of getting his/her idea slip into education of future generations through the back-door. I have yet to meet a single scientist who claims that there is no such thing as god (but admittedly god is usually not a big topic in science, anyways). See my comment above why the question isn´t relevant. (partly) on topic: I am in absolutely no position to judge the annoyance the ID threads cause to this forum or the administration - I simply don´t read them. I am aware that the discussion about ID is more than just "a few crackpots made up their 'darwin is a plagiatist'-argument because they didn´t understand relativ.... aehm .... evolution"-thing but that it does have (in the US - ID seems like a US-only thing to me) political component. However, I always feel a bit sad when a thread is closed "because it´s crap". There are good reasons why ID shouldn´t be considered science, aren´t there? Well, I´m pretty sure I´ve read some convincing ones some time ago. Why not make a sticky "discussion about intelligent design", post an initial statement why you (whoever is the driving force on the SFN crusade against ID) post an initial statement why you don´t consider ID a scientific theory, then simply dump all ID threads there (okok, I can´t judge on the amount of moderation this dumping needs). Then, either ignore or sometimes visit the thread from time to time if your mood drives you there. Are you afraid that the SFN members/visitors are too stupid to tell a good statement from a bad one? I´d also like to have a "proving relativity wrong" sticky, btw. Well, that´s just my thoughts about the "ID-problem" that seems to be quite an issue, lately. Like I initially said, I´m aware that it has a component beyond "it´s crap, scientifically" - the policital one. But I also do think that empty-headed closing of respective threads only fuels the uninformed (to say the least) "scientists are all anti-christs"-arguments.

Well, guess the test is over now so my answer probably comes a bit late. Nevertheless: You don´t need to know velocity or acceleration at all in this problem. You have some kinetic energy. This kinetic energy will be completely lost due to friction. The force due to friction is the force of gravity times the friction coefficient. And finally the point most likely to be the one you missed: Energy is force integrated over distance (force times distance for the spimples problems like this one). That´s enough to solve the problem.