timo

Actually, I always google for "tex" for that reason.

Actually, I didn´t. At least, I didn´t intend to. g, F1, F2 and 0 were supposed to be vectors. or |F1|=|F2| if they are vectors.

mg for the gravitational force acting on the light; F1 and F2 for the forces exerted by the cables. mg + F1 + F2 = m*a = 0.

Yes, your question did explain it. But I thought my answer also explained that I proposed you draw the forces into that diagram - because that´s the way to solve it unless you know how to work with vectors. EDIT: Actually, drawing the forces into the diagram is a good idea even if you know how to work with vectors. It is a great help for visualizing what you are doing.

Draw the forces in the diagram and see if that helps you. If not, post the diagram here; someone will tell you the next step (or what you did wrong, depending on what´s the case).

Can you perhaps draw a diagram of what you mean? The length of a curve? That´s usually calculated by integrating over the magnitude of the derivative with respect to a curve-parameter (integration is over the curve-parameter). Physics Example: A particle has a trajectory x(t). The length of its path between x(t=0) and x(t=1) is [math] L = \int_0^1 \| dx/dt \| dt = \int_0^1 \| v\| dt[/math]. not sure if that´s what you meant, though.

Ok, sounds like a good reason, especially since there´s a page that lists the most-required (by number of links) articles.

Can you quote the passage you mean, Jacques? I can´t find it.

Some suggestions from my side: 1) Do not import articles from Wikipedia (but feel free to get informations from there); Wikipedia is simply too good: 1a) You want your users to have the feeling of really doing something usefull. Writing some minor additions or corrections to WiSci articles might give them that feeling. Redisigning Wikipedia articles to get rid of unneeded links or "unnessecary information" doesn´t. 1b) Your self-written articles (the physics-ones, at least) look really crappy next to the imported Wikipedia articles. Why shouldn´t I use Wikipedia in the first place, now? 1c) I think the only chance for the project is to differ from Wikipedia, like "less history stuff", "more formulas" or perhaps being a source for learning, not only for looking up facts. By importing Wikipedia articles, you automatically take over the gross structure of their articles which you´ll never get out of the article unless you find someone who completely rewrites it all - and you´ll barely find any single person being able and willing to rewrite a Wikipedia article with a comparable quality. 2) When I open a page to edit on my Wiki (DokuWiki), there is a line above the edit window which links to a description of the text formatting syntax. I haven´t seen anything like that on WiSci, yet. You shouldn´t wonder why no one contributes if the user finds really through-thought guidelines on which unit system to use and which words to capitalize but no guide on how to actually write or edit an article. Here´s the page with the basic formatting syntax my Wiki shows, btw (it´s not the page my Wiki shows, of course, but mine is only slightly modified): http://wiki.splitbrain.org/wiki:syntax 3) Write stuff. Don´t sit there theorizing on style guidelines and unit conventions and wondering why nobody contributes. Just write articles/stubs and don´t be afraid if something might not be the ultimate experts´ formulation. It´s the idea of a Wiki that someone who comes by and thinks that he has a correction/improvement will improve the article later on. Speaking of myself, I wouldn´t want to start an article about ... say taus, but I´d gladly help improving/correcting an existing one. 4) Don´t fill your articles with links that go to "this page doesn´t exist, yet". Either don´t create that link or create the article it links to, too. Even of it´s just a one-liner like "Ernest Rutherford: <nationality> <profession> who lived <lifetime>. Best known for <what he´s known for, possibly the article that linked to it>."

Nevermind, according to the search button that user doesn´t even exist .

You´d tremendously increase your chances on getting an answer if you wouldn´t abbreviate the term "power producing lightsabers".

The only time I have seen a non-integer value being associated with the term "dimension" was with the Hausdorff Dimension (http://en.wikipedia.org/wiki/Hausdorff_dimension). However, it´s more maths than physics and I can´t think of any relevance for relativity, atm. Are you sure the website you saw wasn´t a math-page? EDIT: And I´d suggest that you don´t follow [Tycho]'s suggestion. I remember an incident not too long ago where someone in the same situation followed this advice only to be molested by someone else who complained why he started two threads with the same question ... .Just have the thread moved to the math section by a moderator if you feel it´s better suited there.

Here´s the post of Severian we (at least me) were talking about: http://www.scienceforums.net/forums/showpost.php?p=215519&postcount=58 It´s not direct scattering in the sense that there is a fundamental force for photon-photon interactions but an indirect one (at least to my understanding of the terms direct and indirect) because you need a second field (any charged field; it´s an electromagnetical interaction). However, this can happen in vacuum; you don´t actually need any electrons or other charged particles present for this process to happen.

Would it be (easily) possible to enhance the search function to allow searching for multiple usernames? The reason behind that question is that I wanted to post a reference one of Severians´ posts in which he had answered a question of mine. But since this forum doesn´t seem to have above option (I didn´t figure out how to do such a search, at least), I had to browse through quite a lot of threads I participated in to find it.

Seems like I didn´t know that 5 months ago when this thread was recent.

It´s mostly playing with the math of the spacetime of a Black Hole: You allow the coordiante r (which initially was supposed to be a radius and therefore >=0) to become negative. This effectively leads to a "parallel universe". The only path reaching this "parallel universe" is through r=0 which is the singularity. This is supposed not to be possible. There are, however, attempts to slightly modify the spacetime geometry close to the singularity to make a small region passable. If I recall this correctly, energy needed for this would need exotic properties (properties which no known matter has; can´t remember the details). Now you could also claim that this "parallel universe" you get by extending your coordiante range is simply the old one and that you´d simply come out at a different position. If the position is sufficiently far away, you might have found a shortcut through spacetime, which is the basic idea of Wormholes. It´s well possible that there are other ideas about Wormholes, but above is the one I know of. I didn´t read through it but from the pictures it seems like promising read: http://io.uwinnipeg.ca/~vincent/4500.6-001/Cosmology/Black_Holes.htm They wouldn´t prove Einstein wrong (unless he said "there are no Wormholes", which I doubt). Being faster than light is not that much of a deal when the path you take is shorter. Think about shooting a ray of light to a mirror on Jupiter and reflecting it back to your kitchen. I think you´ve got a good chance being there faster. Dark Matter: Matter with normal properties, we simply don´t know what it is (but there are some guesses). Needed to explain some astronomical observations (e.g. the observed rotational movement of spiral galaxies). Dark Energy = Something with (at least for matter) "exotic" properties; instead of working gravitational attractive, it works repulsive. There´s no explanation of what it is that relates it to anything known to standard physics (none I knew of, that is). Dark Energy is also called "Cosmological Constant". I use these terms as interchangable. If that´s not correct, I´d really like to hear an explanation on what the difference is. I think for such a general question, Wikipedia is also a good source and the people there are certainly better in explaining things than I am: http://en.wikipedia.org/wiki/Dark_matter ^^ An interesting note on above link: In the heading it sais that Dark Matter doesn´t emit enough EM-radiation to be detected. A few month ago, I attended a talk by an experimentalist who claimed to have found Dark Matter radiation in the WMAP (=a satelite experiment) data. Never heard of this again afterwards, but the talk was quite convincing (except that he didn´t show the raw data, of course).

I don´t really know the answers for sure' date=' but I can give it a try: None that I knew of. The relevant quantity for the fate of the universe is not Dark Matter but Dark Energy (those are two different things which easily leads to confsion). I don´t really know a definite answer, but I found a quote from a book (which is a pretty easy read in case you´re interested in the subject):

Bit of a hasty statement considering that Dave posted a message why he closed the thread and also also gave an explanation just three posts above yours.

1) To our current knowledge light in vacuum travels at the speed of light (big surprise!), not twice of it or any other value. 2) When you say "keep speeding up" it seems to imply that for some reason you think the light would acelerate. Why so? Objects which do not experience any force (such as friction) keep travelling at the same velocity.

It´s often helpful to draw a sketch for such questions. I assume you´re talking about a scenario I drew in the sketch atttached below. The key to calculating volumes is integration. In this case you can -losely speaking- add up all the circle slides lying in the water to get the answer. So [math] V(x) = \int_0^x \pi r(h)^2 dh [/math] r(h) here denotes the radius of the circle at height h (where h=0 is at the bottom and h=d is at the top). Now the trick you can use is that r(h)² = h*(d-h). Integrate.

Not nessecarily. The wiki I use (DokuWiki) has the option to set access restrictions for users.

No, you´re possibly mistaking Germany for the Netherlands.

None of those mentioned above are legal in germany and as a direct consequence, they´re also not taxed.

The energy light has is purely kinetic energy and connected to a momentum p=E/c. However, that´s only half of the answer. Having a state where your photon has a definite momentum will not produce anything you´d consider movement at all. I´ve written a small gnuplot file in which I try to visualize where movement of photons comes from. Copy the following lines to a file and load this file in gnuplot: f(x,t,k) = cos(k*x-k*t) g(x,t,k) = sin(k*x-k*t) r(x,t) = (f(x,t,1)+f(x,t,1.5)+f(x,t,2)+f(x,t,2.5)+f(x,t,3)+f(x,t,3.5)+f(x,t,4)+f(x,t,4.5))/(8**0.5) i(x,t) = (f(x,t,1)+g(x,t,1.5)+g(x,t,2)+g(x,t,2.5)+g(x,t,3)+g(x,t,3.5)+g(x,t,4)+g(x,t,4.5))/(8**0.5) w(x,t) = r(x,t)*r(x,t) + i(x,t)*i(x,t) set xlabel "position" set xrange [0:8] set ylabel "relative probability density" set yrange [0:10] plot w(x,1) title "time=1" replot w(x,4) title "time=4" replot f(x,1,1)*f(x,1,1)+g(x,1,1)*g(x,1,1) title "pure k=1, any time" f and g are the real and the imaginary part of a photon state with definite energy k at time t, respectively (note that when one puts c=1, energy and momentum are equal, which is the reason the same factor k appears before x and t). From these states with definite momentum, I now construct a linear combination of different momenta. The resulting state (whose real and imaginary part are denoted by the variables r and i) will not have any definite momentum -the momenta 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5 all have equal probability of being measured- but as you will see in the plot, it´s position can be determined better. To view the position of the photon, the probability density for its position is calculated in the next step. w(x,t) gives you the relative probability to find the photon at position x at time t (relative, because I was too lazy to normalize the wavefunctions). The next lines are only plotting commands, then. The probability density for the photons position are plotted for two different times and a reference curve for a photon with a definite momentum is plotted. The first interesting thing is that for the case of a definite momentum, there is no way to tell where the photon is. This is true for all times. This is the reason why I said that you cannot associate a state with definite momentum to motion. In contrast to that, the linear combination shows a clear peak region in which you´ll most probably find the photon. You can see that the photon moves to the right with velocity 1 (guess which phyiscal constant I set to one in above ...). I don´t know how much this helps you but I think it´s nice seeing some math and seeing it visualized. The output of above script is attached to this post, in case you don´t want to download gnuplot. EDIT: There is a typo the definition of i(x,t). The 1st summand has to be a g, of course. The plot below is made with this flawed version because the features I wanted to show remain the same, anyways.

I tend to visualize a 4D world the following way: When I have a date for lunch, I usually agree upon a place (3D) and a time (+1D). Theories with more than 4 dimensions are not generally accepted. They could be proven if someone found an effect which can be explained with those theories but which cannot be explained with 4D theories. Of course, all old results have to be reproduced, too.