# DukeAtreides076

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1. ## So who is still around these days?

[quote name='Programmer One' timestamp='1318166927' post='4870785'] Sup. [/quote] Sup.
2. ## Antihydrogen Trapped For 1000 Seconds

[quote name='Alpha_ProgDes' timestamp='1304897271' post='4808287'] Back on topic. So what exactly are the hypothetical applications for anti-matter? Are there any? [/quote] Aside from blowing up the Vatican, antimatter, if it could be produced and stored in useful quantities, would be great for [url="http://en.wikipedia.org/wiki/Antimatter_rocket"]propulsion[/url]. There are also [url="http://en.wikipedia.org/wiki/Positron_emission_tomography"]medical applications[/url]. Also, I'd just like to point out that although the majority of physicists hypothesize that antimatter will fall "down", this has not yet been experimentally observed. There are theoretical arguments, yes, but these theories can still be falsified. This experiment is a direct test of these theories.
3. ## OpenGL vs Glide

I heard WinG is waayyy better.

5. ## [Common Lisp] What does the f stand for in setf, incf, etc?

It obviously stands for farctate.
6. ## magnetic moment of a moving charge

Quote:Original post by knighty You can always find a lorentz transformation (you have to apply it twice to a 2nd order tensor) for which the B's=0. You have to be a little bit careful here. It is true that the Lorentz transformation mixes E and B together, but there are certain restrictions that apply. Namely, the quantities E^2 - B^2 and E.B are invariant. (These invariants come from contracting the field tensor with itself and the field tensor with its dual). You can't always pick a frame where B' is zero: for example, a pure B field in one frame can't be transformed into a pure E field in another frame and vice versa since this would change the sign of E^2-B^2. You are correct though that you can always find a frame where the magnetic field from a point charge is zero (its own rest frame). ------------------------------ In response to Victor: I can point you to some references that explain things much clearer and in much more detail than I can. I already mentioned the book by Griffiths; it is very clear and very accessible. A more advanced text is Jackson's Electrodynamics. It is an extremely difficult text and everybody hates it, but almost all graduate level courses in electrodynamics use it because it is so comprehensive. Another nice (graduate level) book is the second volume of Landau and Lifshitz's Course of Theoretical Physics. Also try looking in some introductory relativity books. Quote:Original post by Victor-Victor http://en.wikipedia.org/wiki/Relative_velocity - "In kinematics, relative velocity is the vector DIFFERENCE between the velocities of two objects, as evaluated in terms of a single coordinate system, usually an inertial frame of reference unless specifically stated otherwise" The velocity that appears in the Lorentz force equation is relative to an observer sitting at rest at the origin of a reference frame moving at constant velocity. Similarly, E and B must also be measured by this observer in the same reference frame. The Lorentz force equation F = q(E + v x B) is equally valid in ANY inertial reference frame so long as E, v, and B are all measured in that particular frame (provided also that one uses the relativistic momentum when writing F = dp/dt). The "covariant" form referred to earlier is the exact same equation, just written with different symbols. I think you're missing a couple important things in your analysis. First off, you should also add in the E field when you write the force; as already pointed out the E and B fields are intimately related -- E in one frame becomes a mix of E and B in another. Secondly, (also pointed out earlier) the strengths of E and B are different according to different observers. The forces measured by two observers O and O' certainly won't be the same; however, this does not lead to a contradiction! An observer O can calculate the motion of a charge in his frame using his coordinates (x,y,z,t). Another observer O' moving at a different velocity can calculate the motion in her frame with her coordinates (x',y',z',t'). If the trajectory in O' is then transformed back to O with a Lorentz transform, the result is the very same trajectory that O calculated himself using his own coordinates, despite the fact that O and O' measure different fields and forces. Electrodynamics already has special relativity "built-in" in the sense that the Poincare group (Lorentz transforms + translations) is the set of transforms which leaves Maxwell's equations unchanged (when the proper field transform is used as well). Indeed, the Lorentz transform was known even before Einstein's famous 1905 paper.
7. ## magnetic moment of a moving charge

I can try to answer these knighty. Quote:Original post by knighty I must admit that I have some difficulties to understand this stuff. Is special relativity sufficient or should we use general relativity? after all, the particles are accelerating. Special relativity is quite capable of dealing with accelerating objects. Although there are no inertial reference frames where an accelerating object is at rest for all time, you can still choose a series of inertial frames where an accelerating object is instantaneously at rest (the fancy name for these frames is 'momentarily comoving reference frames'). GR is only needed for dealing with gravity. Quote:Original post by knighty I know that Coulomb law (like any oter 1/r^2 law) is conservative. is it the same with Biot-Savart law? Magnetic forces can't do any work anyway. Look at the Lorentz force equation: the magnetic force is always perpendicular to the velocity. Magnetic forces can change the direction a charge is moving in, but can't speed it up or slow it down. Quote:Original post by knighty Does Lienard-Wiechert potentials take into account the damping force due to the emission of radiations when the particles are accelerated? From the Lienard-Wiechert potentials, you can calculate the power radiated from an accelerating charge, and by conservation of energy the "reaction force" on the charge caused by radiation. See Abraham-Lorentz Force. Griffith's Introduction to Electrodynamics is a very clear reference for this kind of stuff.
8. ## magnetic moment of a moving charge

Quote:Original post by Victor-Victor My theory is the theory of General Relativity, only better. My Aether is a "metric of spacetime" as GR calls it. If your theory is so much better than GR, then why aren't you rich?
9. ## It is possible to write WoW in machine language?

Quote:Original post by Zahlman How long is a very long piece of string? Wolfram|Alpha isn't sure what to do with your input.
10. ## Lockerz - Too good to be true?

Quote: PTZ are Lockerz' proprietary loyalty "currency" that you earn whenever you buy, watch, play, share, invite friends, or even just show up on Lockerz. Forgot to mention this part eh?
11. ## Patch installation during gameplay

Erlang has support for code replacement while the system is running. Still, hot swapping code is a tricky business at best.
12. ## Tricky physics - how does friction affect velocity?

What sort of frictional force are you using? If it's a simple linear drag force like F = -b*v then (with no other forces acting) the velocity as a function of time is v(t) = v(0)*exp(-b*t/m). Then integrate to get the position as a function of time: x(t) - x(0) = v(0)*m/b * [1 - exp(-b*t/m)]. Take the limit as t goes to infinity and the total distance traveled is v(0)*m/b, so your initial speed should be v(0) = ||A-B||*b/m. Funny thing about the linear drag force is that it takes infinite time to reach the stopping point, but it doesn't really matter since it will very close in a reasonable amount of time depending on how large b/m is. However, it is possible to construct nonlinear drag forces that make the stopping time finite.
13. ## Brain Food - In Need of Book Suggestions.

You might be interested in Neal Stephenson's Snow Crash.
14. ## Is there any way that TortoiseSVN compress files when using an online repository?

I was under the impression that Subversion already uses zlib for compressing diffs.