timo

River_rat's post was slightly disambiguous considering that matter, so I covered this possibility in case (1). The case you meant is covered in (2), as you certainly saw. Of course you can define almost arbitrary non-invertible, non-closing operations on any sets - it's just not what I understand under "properly defined addition" and not what I expect when reading a plus sign. I assume that I'm not the only one thinking of at least a group operation when reading a plus sign, hence my comment that you shouldn't write such stuff to avoid confusion. @River_rat: Not only do not all elements have an inverse, but your "addition" additionally () also doesn't close. As soon as you define [math]\infty + (-\infty)[/math] to equal something, you run into exactly the problem with associativity I mentioned. To me, the "addition" you defined looks like little more than an implementation of "but I want this operation to exist" (don't get me wrong; that's an acceptable motivation) that quickly runs into its limits. With the limited range the operations might serve as some shortcut-operations like when checking the limits of composite functions where "undefined" then means "analyse with proper methods".

Thx for the offer. Like I said, I liked the opportunity to have a wiki in the backhand, here. But as long as I am the only one wanting it back (and potentially using it), you don't need to set it up atm.

Yes, I could think of quite a few of those (article about the twins paradoxon, a replacement for the "Lorentz Transformations" sticky in the relativity section, something clearing up the ambiguity of the term "mass", ...). In addition to that WiSci seems appropriate for replies to threads like http://www.scienceforums.net/forum/showthread.php?t=27343 which seem to call for a wider explanation than those usually given in a forum reply (we did have an article about redshift, although the cosmological redshift section wasn't written, yet). Any chance that WiSci will come back? EDIT: Sidenote: The table "site navigation" on the left side on the main page links to two non-existing sites: tfn and originsdebate.

When talking about addition, I talk about addition as a group operation. So depending on which way round I understand you statement, I get either: (1)[math] \infty + 1 = 1 +\infty = 1[/math]. That violates the uniqueness of the neutral element. (2) [math] \pm \infty + x = \pm \infty \, \forall \ x \in[/math] R. What's the inverse of infinity, then? It can only be [math] \pm \infty [/math] as far as I see. But then, the addition you just defined violates associativity: Assuming the inverse of [math] \infty [/math] was [math] -\infty [/math]: [math] (\infty + \, -\infty) + 1 = 1 \neq \infty + (-\infty + 1 ) = \infty + \, -\infty = 0 [/math].

Try skipping the carriage returns inbetween the latex tags and wrap each line with [ math] ... [ /math] seperately. The multiplication dot is "\cdot", btw.

I think Phi will like the new posters in the staff room better than the old ones (no offense to Carlo Rovelli or Enrico Fermi meant).

I think I've even seen some kind of graphical user interface for dosbox where you only need to set the commands for each game once. I cannot remember what the program was called (I'm not even sure what it did exactly), but perhaps it's worth to you to look around for it (or something similar). Perhaps this page helps: http://dosbox.sourceforge.net/wiki/index.php?page=DOSBoxFrontends

You could try using DOSBox.

But the 2 as your 3rd term is a strong indication that you did [math] \left( \sqrt{2} ^{\sqrt{2}} \right)^{\sqrt{2}} =2 [/math] instead of [math] {\sqrt{2}^{\sqrt{2}}}^{\sqrt{2}} = \sqrt{2}^{1.6\dots} < \sqrt{2}^2 = 2 [/math] (where I took the 1.6 from your numbers without checking it). @Blue_crystal: Very nice question. Two comments: (1) Perhaps don't give out the solution too early (I'm still not completely sure if it really works, but your explanation looks ok, so far). (2) Don't write stuff like "[math] \infty - 1 = \infty [/math]", even if it's just meant as a handwaving argument (unless you can properly define an addition on R [math]\cup \{ \infty , - \infty \}[/math], which I spontaneously doubt) - it confuses others into thinking this type of addition did exist.

It means "distance"; in the sense of distance from the point of creation. It's the radius.

Perhaps you should show the "process of deriving it" here. The term [math] 1 + m\, u^2 \, \gamma^2 [/math] looks quite suspicious by itself: You are adding a dimensionless number and an energy-like term there.

Approximation: Assume R to be the rate of annihilation (mass per time). Then the energy created per time-unit is Rc². Let this energy escaping at c, making the energy density (and thus the pressure) at distance d (assuming a spherical container) roughly [math] \frac{R c^2 \, dt}{4 \pi d^2 \ c \, dt} = \frac{Rc}{4 \pi d^2}[/math].

I do. Thx for the new option. I'd prefer that. For me, it's more important that the alignement is consistent within a single post (I tend to read them one-by-one, anyways) rather than that the alignement of the last lines is consistent over different posts.

I agree with ajb. I see no point at all to recommend QFT books to a beginner. I also agree with the "start with something easy"-proposal, I'd even go below 1st year university level for a start. You can read a general intro book and then start from the questions that were left open to go to more advanced books. Halliday, Resnick, Krane (and the 2nd part) seems like a good start for that, but it's the only english books I know, so I can't compare them to other books. EDIT: vv You're right, Rasori. But there's still no point in recommending the books

Like a big, fat blue star over the user avatar

To clarify: I was talking about a button, that adds "[ math] [ /math]" to the text, not a gui to compose TeX code.

A helium-filled balloon resting on the dashboard? It won't move relative to the car at all since it's squeezed too tightly between the dashboard and the windshield .

How about some button for TeX-code in the list of buttons on the "go advanced" page?

[ math] latex code [ /math], e.g. [math] \psi_\mu = J_\mu [/math].

I agree. There's a difference between passively reading texts and understanding the sentences in it and communicating a topic to others. Personally, I've always learned something new (or better) when preparing talks or tutorials. I also remember someone saying "if you really want to understand a topic, then give a lecture about it", so that statement seemingly extends up to the lecturer level (comments by lecturers about their attitude towards this statement welcome). Just do it for your own learning effect (which doesn't mean it shouldn't be potentially useful for others). Saves you a lot of possible disappointment about the fact that the others don't really care. If it's worth making it a sticky, then sure. Consider writing it on WiSci (in case it goes online again), so we can perhaps make an "interesting related articles on WiSci"-sticky someday.

Default is "new style". I've tried chosing "new(er) style" and switching to some other SFN pages afterwards: The choice "New(er) style" keeps selected.

I can't take the code from your page (I can, but then I have the nasty numbers when I copy/paste) and haven't looked into it, yet. One thing which you probably didn't pay attention to: Multiplication by factors of 10 is really easy in decimal base!

1) Hi! 2) The forum structure to some extent reflects the topics that are discussed. If you manage to interest people in geo-science topics or get some discussions about it running otherways, then that would be great (and also most likely result in an appropriate forum being created).

In other words: It's a spam message.

The height is important since the density of the two gases depends on the height via [math] \rho (h) =\alpha \exp (-mgh/kT) [/math], where [math] \rho (h) [/math] is the density, m the mass of the molecule, g the gravitational constant (if your container is soooo huge that g significantly changes, the term would probably become slightly more complicated), k the Boltzman constant, T the termperature, h height above container bottom and [math] \alpha [/math] some constant to make physical unit and total number of particles fit. As you might guess, the different masses of the two types of molecules make the function [math] \rho (h) [/math] behave differently. This results in different concentrations of the gases for different heights (the higher, the more share of the lighter gas). To find out to what extend this different concentration is measurable, you have to plug in values and check it out. EDIT AND SIDENOTE: Well, the question is about physics. I suppose the chemicists use this kind of physics (thermodynamics) at least as often as physicists, so the thread would have fit in there, too. Here, I'm not sure if "General Physics" or "Classical Physics" is more appropriate.