Can a wave be two dimensional?

Question: I am having a hard time picturing waves, the image that comes to mind is a bobbing device submerged in still water which generates pulses in all directions (similarly in air). Then how can a wave be two dimensional? The classic image of an electromagnetic wave is a 3D wave function, x-y represent the electrical component, z-x represents the magnetic component. But this is a wave function, so its just a plot, and that is not how I should picture it (or is it?). So in real life, if I have a source of EMR in space, I can imagine concentric spheres of Electrical fields, but what about the magnetic fields? How will they be perpendicular to a sphere of electric fields? Can someone help me picture EMR? Thanks! (apologies in advance for the noobish language, I am an absolute beginner to physics)
Comment: The strength of an electric field is a vector, so it has a magnitude and direction, but the magnitude is not spacial.
Answer: The picture below shows 3-D wave with wave fronts.

Imagine holding the wave fronts and turning into half a circle. When you look into the same 3-D wave from the upside now, you will just see half circle. This is your 2-D wave. The same you see in books with half circles (wave fronts) emanating from a point.

In the following picture, take off the components of the electric and magnetic field below the line of propagation from your mind. See the upper half, if you increase the length of wave front you will get the first picture. Similarly it applies to the other component. I hope it helped.

Answer: Here's my visual approach to answering your question: the "spherical electrical waves" that you're referring to are just a way to represent the wave crests of light. When considering the oscillating electric field component of the electromagnetic wave, the individual waves propagate through space symmetrically in all directions when being emitted from a single source, one wave for each direction in 3D space, and when you connect the crests of the propagating waves at a specific instant of time, you obtain the spherical surfaces you're referring to. And when you visualize the crests and troughs of the oscillating magnetic field component of the wave, the same thing will apply, i.e: when you connect the crests of the magnetic waves, you will also obtain spherical surfaces identical to those you obtain when connecting the electric wave crests. So at a certain instant of time, those spherical surfaces rather connect the crests of each individual unidirectional electromagnetic wave emitted from the source, and that over time, span a sphere whose radius increases at a rate equivalent to the speed of light in the respective medium.
Answer: Ok I think I understand your question.
So you're talking about how the electromagnetic waves are always perpendicular to each other, but now, what if we have a spherical electromagnetic wave?
Ok there's a few problem to sort out. First off, electromagnetic waves always travel in a straight line (ignoring effects like general relativity, diffraction (and other scattering processes),    etc.). You might already know that, but when bringing up a spherical wave you are probably getting confused. What is this spherical wave and how is it formed? Well, this EM source sounds a lot like the bobbing device in water...at least from what you're describing...causing an outward flux of EM waves in a spherical manner. This actually is just a bunch of EM waves going in a single direction. The photons within the EM waves are traveling in straight lines still. The "spherical" wave is just a bunch of regular EM waves combined together. That explains why the intensity diminishes the farther the radius from your source is.
So instead of picturing waves as 3D, picture them as a bunch of 2D waves strung together. Physics' current understanding is that photons are point particles, so if a point moves in a line (well a curved line I suppose), it is making a 2D shape.  The magnetic wave is orthogonal to the electric wave, so the EM wave actually spans 3 dimensions.
So to now answer your question, each part of the EM wave moves in 2 dimensions and you can think of it as a sine wave or a cosine wave plot. Both of them together spans 3 dimensions, but each individual part just spans 2 dimensions.
Comment: "electromagnetic waves always travel in a straight line" - how about diffraction http://en.wikipedia.org/wiki/Diffraction and other wave optics phenomena?
Comment: Ok I know I know lol, you get my point though. They don't run off in random directions by themselves, there's always something acting on them, I'll edit the answer to be more general
Answer: Waves from all particles of the universe cascading downward combine from a distance radius forming the sinusoidal wave medium density(space) compressing and expanding two opposing vortices at each and every point of space, or contracting and expanding virtual pair's popping out of existence.. .  .The total amplitude formed by a rings series of inward spherical waves colliding at maximum compression points at wave crests and wave troughs always seeks a minimum.  . ..are equally balanced by opposite expansion at interchanging compression points as trillions of waves cancel the sum of opposite vectors mathematically is always zero.  . ..leaving space with an average temperature, or pressure stillness of the wave amplitude squared.. .  .This polarized superposition of the wave amplitude squared is the reason for entanglement and the symmetry or conservation laws in human physics or why, at the speed of C, time and space are zero due to length contraction and time dilation.  . ..  A light wave always transverses one unit of space per one unit of time.  . ..In a sense they are on the edge between space and time like the zeros and ones of a computer screen always forming a blank canvas that we can interact with turning the possible into the actual.. .  .

Can a wave be two dimensional?


What are the differences between sparse coding and autoencoder?

Question: Sparse coding is defined as learning an over-complete set of basis vectors to represent input vectors (<-- why do we want this) . What are the differences between sparse coding and autoencoder? When will we use sparse coding and autoencoder?
Comment: Sparse coding actually constitutes a specific type of autoencoder known as [sparse autoencoder](https://en.wikipedia.org/wiki/Autoencoder#Sparse_autoencoder). So you can consider sparse coding as a subset of autoencoders if you will.
Answer: Finding the differences can be done by looking at the models. Let's look at sparse coding first.
Sparse coding
Sparse coding minimizes the objective
$$
\mathcal{L}_{\text{sc}} = \underbrace{||WH - X||_2^2}_{\text{reconstruction term}} + \underbrace{\lambda  ||H||_1}_{\text{sparsity term}}
$$
where $W$ is a matrix of bases, H is a matrix of codes and $X$ is a matrix of the data we wish to represent. $\lambda$ implements a trade of between sparsity and reconstruction. Note that if we are given $H$, estimation of $W$ is easy via least squares.
In the beginning, we do not have $H$ however. Yet, many algorithms exist that can solve the objective above with respect to $H$. Actually, this is how we do inference: we need to solve an optimisation problem if we want to know the $h$ belonging to an unseen $x$.
Auto encoders
Auto encoders are a family of unsupervised neural networks. There are quite a lot of them, e.g. deep auto encoders or those having different regularisation tricks attached--e.g. denoising, contractive, sparse. There even exist probabilistic ones, such as generative stochastic networks or the variational auto encoder. Their most abstract form is
$$
D(d(e(x;\theta^r); \theta^d), x)
$$
but we will go along with a much simpler one for now:
$$
\mathcal{L}_{\text{ae}} = ||W\sigma(W^TX) - X||^2
$$
where $\sigma$ is a nonlinear function such as the logistic sigmoid $\sigma(x) = {1 \over 1 + \exp(-x)}$.
Similarities
Note that $\mathcal{L}_{sc}$ looks almost like $\mathcal{L}_{ae}$ once we set $H = \sigma(W^TX)$. The difference of both is that i) auto encoders do not encourage sparsity in their general form ii) an autoencoder uses a model for finding the codes, while sparse coding does so by means of optimisation.
For natural image data, regularized auto encoders and sparse coding tend to yield very similar $W$. However, auto encoders are much more efficient and are easily generalized to much more complicated models. E.g. the decoder can be highly nonlinear, e.g. a deep neural network. Furthermore, one is not tied to the squared loss (on which the estimation of $W$ for $\mathcal{L}_{sc}$ depends.)
Also, the different methods of regularisation yield representations with different characteristica. Denoising auto encoders have also been shown to be equivalent to a certain form of RBMs etc.
But why?
If you want to solve a prediction problem, you will not need auto encoders unless you have only little labeled data and a lot of unlabeled data. Then you will generally be better of to train a deep auto encoder and put a linear SVM on top instead of training a deep neural net.
However, they are very powerful models for capturing characteristica of distributions. This is vague, but research turning this into hard statistical facts is currently conducted. Deep latent Gaussian models aka Variational Auto encoders or generative stochastic networks are pretty interesting ways of obtaining auto encoders which provably estimate the underlying data distribution.
Comment: Sorry to bring this question up again. I think Autoencoder can encourage sparsity as well, i.e. sparse autoencoder.
Comment: Thanks for your answer! So is that mean sparse coding should not be used in any case, but autoencoder? Also, there should be an extra term in sparse coding which regularize W?
Comment: There is no general rule like that. SC has one benefit over AEs: encoding via optimisation can be very powerful.
Comment: Sorry, can you elaborate on this?
Comment: Having a fixed map that has been estimated in order to follow some constraint (here: sparse result) is less powerful than having an optimiser that tries to find a solution like that over possibly many iterations.
Answer: In neuroscience the term Neural Coding is used to refer to the patterns of electrical activity of neurons induced by a stimulus. Sparse Coding by its turn is one kind of pattern. A code is said to be sparse when a stimulus (like an image) provokes the activation of just a relatively small number of neurons, that combined represent it in a sparse way. In machine learning the same optimization constraint used to create a sparse code model can be used to implement Sparse Autoencoders, which are regular autoencoders trained with a sparsity constraint. Bellow more detailed explanations for each of your questions are given.
Sparse coding is defined as learning an over-complete set of basis vectors to represent input vectors (<-- why do we want this)
First, at least since (Hubel & Wiesel, 1968) it's known that in the V1 region there are specific cells which respond maximally to edge-like stimulus (besides having others "useful" properties). Sparse Coding is a model which explains well many of the observed characteristics of this system. See (Olshausen & Field, 1996) for more details.
Second, it's being shown that the model which describes sparse coding is a useful technique for feature extraction in Machine Learning and yields good results in transfer learning tasks. Raina et al. (2007) showed that a set of "basis vectors" (features, as pen-strokes and edges) learned with a training set composed of hand-written characters improves classification in a hand-written digits recognition task. Later Sparse Coding based models has been used to train "deep" networks, stacking layers of sparse feature detectors to create a "sparse deep belief net" (Lee et al., 2007). More recently astonishing results in image recognition was achieved using sparse coding based models to construct a network with several layers (the famous "Google Brain"), which was capable of distinguish a image of a cat in a purely unsupervised manner (Le et al., 2013).
Third, it's probably possible to use the learned basis to perform compression. A haven't seen anyone really doing it though.
What are the difference between sparse coding and autoencoder?
An autoencoder is a model which tries to reconstruct its input, usually using some sort of constraint. Accordingly to Wikipedia it "is an artificial neural network used for learning efficient codings". There's nothing in autoencoder's definition requiring sparsity. Sparse coding based contraints is one of the available techniques, but there are others, for example Denoising Autoencoders, Contractive Autoencoders and RBMs. All makes the network learn good representations of the input (that are also commonly "sparse").
When will we use sparse coding and autoencoder?
You're probably interested in using an auto-encoder for feature extraction and/or pre-training of deep networks. If you implement an autoencoder with the sparsity constraint, you'll be using both.
Comment: This answer has many interesting points and important references. However, the first paragraph is *wrong*. Sparse coding and sparse auto encoders are different beasts.
Comment: Where is it stated that they are the same thing? Please, tell me and I'll correct the answer.
Comment: In the first sentence.
Comment: "Sparse Coding is just one of the available techniques for training autoencoders". This sentence doesn't define "Sparse Coding" as the same "beast" as autoencoder. It says that, between all available techniques for training autoencoders, one of them is "Sparse Coding". I agree that the sentence has indeed some ambiguity, which I believe is clarified by the rest of the answer.
Comment: You say that sparse coding is a method to train auto encoders. That is clearly not the case, as auto encoders have an explicit decoder that is not implemented with an optimisation algorithm. Given an arbitrary auto encoder you cannot train it with sparse coding.
Comment: I edited the answer to eliminate ambiguities. Take a look.
Comment: The term "sparse coding" precisely is not used to describe a method for training autoencoders, but there is indeed a kind of autoencoder which uses a constraint inspired by sparse coding: the Sparse Autoencoder. I edited the answer and changed the terms "sparse coding" for "sparse coding based". As the OP cited the UFLDL sparse coding definition, I presume he was interested in understanding the relationship between the concepts in the context of the tutorial. I can't say I agreed with every word you said, but your complains about the precision of the terms used were helpful.
Comment: Thanks for your answer! "If you implement an autoencoder with the sparsity constraint, you'll be using both." What do you mean by this?
Comment: Autoencoder and "Sparse Coding" are not mutually exclusive options (so you don't have to choose "when" to use one over the other). If you choose to implement your autoencoder adding to it a sparsity constraint (i.e. a Sparse Autoencoder), you'll be at the same time using an autoencoder and a sparse coding based model.
Comment: For autoencoder use case, I would say data de-noising, compression, and dimensionality reduction for the purpose of visualization. according to this [post](https://www.quora.com/Why-are-autoencoders-considered-a-failure-What-are-their-alternatives)
Answer: A sparse coder is kind of like half an auto-encoder.  An auto-encoder works like:
<code>input  =>  neural net layer  =>  hidden outputs => neural net layer => output
</code>
For back-propagation, the error signal, the loss, is: input - output
If we apply a sparsity constraint on the hidden outputs, then most will be zeros, and a few will be 1s.  Then the second layer is essentially a set of linear basis functions, that are added together, according to which of the hidden outputs are 1s.
In sparse coding, we only have the second half of this:
<code>                                codes => neural net layer => output
</code>
The 'codes' is a bunch of real numbers, selecting for the basis functions represented by the weights in the neural net layer.  Since in Olshausen's paper, they are applying a sparsity constraint to the codes, the codes are, just as in the sparse auto-encoder, sparse: mostly zeros with a few ones.
The difference we can see now clearly: for the sparse coding, there is no first half of the neural network: the codes are not provided for us automatically by a neural net.
How do we get the codes in sparse coding?  We have to optimize ourselves, which we do by using gradient descent or similar, to find the set of codes that best provides output matching the input image.  We have to do this for every image, including for every test image, each time.
Answer: You might want to read this recent paper, https://arxiv.org/abs/1708.03735v2 on precisely this same topic. In this paper the authors show that indeed one can set up an autoencoder such that the ground truth dictionary is a critical point of that autoencoder's squared loss function.


How to get deskbar-applet in Xubuntu?

Question: How to get deskbar-applet in Xubuntu? Or is there any alternative box where I can type for programs/files and they appear?
I've installed deskbar-applet but I don't think I can pin it to the Xubuntu panel.
Answer: Deskbar-applet appears to be very specific to gnome and gnome-panel.
As an alternative try synapse

As you can see, it is invoked similarly via the XFCE panel or via a hotkey (defaults to CTRL+SPACE).  There are a number of default plugins which are similar in functionality as deskbar as well.
It uses the Zeitgeist backend - so it is similar to Unity - when you use a file or application it is automatically indexed for you to find again.
Comment: synapse is amazing and instant! Thanks! It's better than any other I've used!
Answer: Catfish is built in, it's just not obvious it's a file system search application. Put a shortcut to it on your panel.
Answer: I'd recommend trying Gnome Do, it's not as snappy as synapse, but the difference is marginal and it supports more apps and is much better at finding files. Also XFCE has Application Finder, very small app that allow you to find applications :)

How to get deskbar-applet in Xubuntu?


Block cipher block and key sizes

Question: Is there any point in making block size less than key size in block cipher?
For example, Russian GOST symmetric cipher uses 256-bit keys to encrypt 64-bit blocks. That means there are many keys which give the same result, so why making keys so large?
Comment: I've answered, but I could also live with closing the question as a dupe for the AES / Rijndael specific questions. I'll leave that up to the rest of the crypto crowd.
Answer:
GOST symmetric cipher uses 256-bit keys to encrypt 64-bit blocks. That means there are many keys which give the same result

This is actually incorrect. A block cipher, when given a key, maps $n$-bit plaintext blocks to $n$-bit ciphertext blocks (and vice versa for decryption). Here's a toy example for $n=2$:

$E_k(00) = 01 \\
E_k(01) = 10 \\
E_k(10) = 00 \\
E_k(11) = 11$

Although there are $2^n$ possible plaintext blocks and $2^n$ possible ciphertext blocks, there are actually $2^n!$ different possible ways to map plaintext blocks to ciphertext blocks. In this toy example, there are actually $2^2! = 4! = 1\times2\times3\times4 = 24$ different ways of mapping each two-bit input to a unique two-bit output.
Back to your question, GOST uses 256-bit keys, meaning that there are $2^{256}$ possible keys. However, with a 64-bit block size, there are $2^{64}!$ different possible permutations, and $2^{64}!$ is much larger than $2^{256}$.
Comment: Shouldn't some of those 2^64! be same??
Answer: This is because the set of possible permutations of 64 bit blocks of plaintext ($2^{64}$ possibilities) to 64 blocks of ciphertext is very high. A key selections just one of these permutations. Even a 256 bit key space is smaller by far than the number of possible permutations.
Some plaintext blocks will likely map to the same ciphertext block for a few of these permutations. But the permutation itself that gets selected by the key is still very likely to be unique, and it is very improbable that you find the matching pairs in the first place.
So your statement "...that means there are many keys which give the same result" is not correct. The result of the key is a specific permutation, and there are plenty of permutations to divide over the keys.
Also see this question as well as this question which explain this for AES/Rijndael specifically.

Block cipher block and key sizes


Simple Number Theory Task

Question: I am doing exercises with middle grade students and looking at their capacities to create arguments for simple number theory conjectures. I want three tasks, and I have two so far: 1. The sum of three consecutive integers is divisible by three. 2. The sum of two odd numbers is an even number. I need one more task that is similar in difficulty, but not too similar that the same warrants would arise. Any ideas?
Answer: A nice third problem arises by continuing from the second problem to show the power of parity arithmetic for solving problems in integer arithmetic. For example, we can use it to show that large classes of polynomials have no integer roots. Let's consider a simple example.
$f(x) = x(x\!+\!1)+2n\!+\!1\,$ has no integer roots since $\,x(x\!+\!1)\,$ is even, so adding $2n\!+\!1$ yields an odd so  $\ne 0;\,$ equivalently $\,f(0)\equiv 1\equiv f(1),\,$ i.e. $f\,$ has no roots mod $\,2,\,$ so no integer roots.
Remark $ $ This idea generalizes widely as follows
Parity Root Test $\ $ A polynomial $\,f(x)\,$ with integer coefficients
has no integer roots when its $\rm\,\color{#0a0}{constant\,\ coefficient}\,$ and $\,\rm\color{#c00}{coefficient\,\ sum}\,$ are both odd.
Proof $\ $ If so then $\  \color{#0a0}{f(0)} \equiv  1\equiv \color{#c00}{f(1)}\,\pmod 2,\ $ i.e. $\:f\:$ has no roots in $\,\Bbb Z/2 = $ integers mod $\,2,\,$ therefore $\,f\,$ has no integer roots. $\ $ QED
In the same way, we can often reduce problems to "smaller", simpler problems in modular images. Because (ring) homs preserve the ambient algebraic structure, as above, we can often deduce information about the original problems from information gleaned in the simpler modular images. Such problem solving by modular reduction is an algebraic way of "dividing and conquering".
As above, one can often introduce the germ of such ideas in very early grades through simple instances such as parity arithmetic.  When teaching at such early levels I strive to present examples that will provide good intuition for the generalizations that students will encounter in later studies. The Parity Root Test is a model example of such. You can find much further discussion of such in my many MSE posts on the Parity Root Test.
Answer: The NRICH website has several posters that I like to pick interesting problems like this from. Right now, these three are my favorite:
1. Big Powers
2. Make 37
3. Mystic Rose
They have more poster problems for secondary and primary ages. The problems are generally simple to state and allow students to play around with the math a bit before making conjectures and proving them.
Answer: I like your question a lot :) and got some new ideas from other answers. So I like to contribute some of mine in the hope they provide useful to others as well. I usually distinguish if problems are  relating to place value system or not.
1. Palindromic numbers ($ANNA$-numbers). Using two different digits, you can build two different
$ANNA$-numbers. For example using $7$ and $3$, you can build $7447$
and $4774$. What can you say (and prove) about their difference?
2. Mirrors. Sometimes, the product of a two digit numbers and their mirrors are equal. Like $23\times 64= 32\times 46$. Or $34\times 86 = 43\times 68.$
Can you figure (and prove) a special property of numbers where this is true?
3. Sum of consecutive numbers. Actually, your first problem generalizes a lot and allows to equal or more challenging problems, like : What numbers can not be written as a sum of consecutive numbers.
Remark-1: Actually, this problem is also on nrich.
4. Gaps in Prime numbers. For each pos. integer $n$ there is a sequence of $n$ numbers, none of which is prime.
Remark-1: this is nice to show that the general solution (i.e. following a rule like take $(n+1)!$, $(n+1)!+1$, $(n+1)!+2$, $\ldots$, $(n+1)!+(n+1)$ does not always produce the easiest example possible, as you can see e.g. setting $n=8$.
Remark-1: If your students know Euclids proof that there are infinitely many primes, you can also do the following problem:
Prove that for $n\geq 3$, there is at least one prime number between $n$ and $n!$.
Answer: Maybe discuss the twin primes conjecture briefly and then ask them to investigate how many triplet primes of the form $p,p+2,p+4$ there are.  They should be able to prove (using the kinds of arguments you'd use in tasks 1. and 2.) that there is only one such triplet.  I think this both shows them that number theory is pretty deep and that conjecture is part of the natural cycle of doing mathematics.
Comment: Hm... I really like this one actually. I'm wondering though how middle grade students will do with this. I think their arguments will be mostly inductive which is fine.
Comment: I used this problem with 9th graders.  It's been a while, but I remember it going pretty well.  Students made the right conjecture (at least one of the three will be divisible by 3) and many of the were able to figure out why.
Answer: The product of an even number and an odd number is an even number.
or
The product of two odd numbers is again an odd number.
Answer: The divisible by 3 thing (digits addition)?

Simple Number Theory Task


Why does the multimeter need to be set to AC voltage to measure the voltage on secondary winding when driven by a square wave on the primary?

Question: I have a transformer being driven by a square wave at about 9 kHz with a 10/11 duty cycle. In order to measure the voltage on the secondary I have to set my meter to AC. Why is it so? The primary voltage is not AC so why is the secondary AC? and is it a sinusoidal wave? or alternating square wave? My theory is that it has to do with the way to magnetic field on the primary is constantly increasing and decreasing which would induce an alternating current on the secondary.
Comment: Well it sure isn't DC. A square wave is certainly AC, just not a sine wave. Neither is the mains voltage, actually.
Comment: Assuming a non-true-rms-ac mm, neither DC nor AC is the correct setting for a square wave, but one of the two is a better approximation.
Comment: Ahh, your primary voltage is AC.  (A square wave still counts as AC... just not a single frequency.)  How did you measure your primary voltage?
Comment: Well the primary is being driven by the output of a 555 and the time constant is .1 ms
Comment: AC implies that polarity is reversed. The square wave I am creating alternates between 0 V and some positive value
Answer: The output of a transformer is always AC.  In the long run, the average current thru a transformer secondary with resistive load is always 0.
For example, consider driving the primary with a 0-10 V pulse train that is low for 9 µs and high for 1 µs.  The DC level is 1 V going into the primary.  However, that 1 V will be lost coming out of the secondary.  The open-circuit voltage you get out is -1 V for 9 µs and +9 V for 1 µs, multiplied by the turns ratio.  If the primary has 100 turns and the secondary 300, then you will get -3 V and +27 V, for example.
A good meter set to DC will always show this as 0 V.
Different AC voltmeters will show you different voltages for the -1 to +9 V example pulse waveform.  That is because most meters don't read true RMS.  Most will measure the average of the absolute value, then apply the correction factor from that to RMS for a sine wave.  That correction factor will be incorrect for something like the pulse train in this example.
To get the voltage of non-sinusoid waveforms, you either have to get a true RMS meter, and make sure your signal is within its frequency range, or use a scope and do the math yourself.  Some scopes have RMS measurement built-in as a math function, which makes them in effect true RMS meters.  Again, keep the limitations of the RMS meter in mind.
Comment: Disregarding the setting of the multimeter, why is the voltage on the secondary always alternating?
Comment: @Alan: It's fundamental to how transformers work.
Answer: Depending on the transformer, it might be a good or poor bandpass filter by itself.  The better the bandpass, the more sine-y the output becomes, but it only does anything when fed that one frequency.
Most likely though, is that you still have a fairly square-ish waveform coming out of it, which like Plasma said, will not play well with either setting.  Also, the transformer will remove any DC offset (which you shouldn't give it to start with; they don't like that) so that the DC setting will likely read close to zero on average.
You really need an oscilloscope to see what's going on, which you can sometimes approximate using a PC sound card.  At 9kHz, your signal might be on the edge of what a good one can do, so try to find a card that can record at 96kHz or higher, and even then the internal filtering might round off some corners above 20kHz.  Also make good and sure that the signal into the card is low enough to not blow it up (done that before), so do some math and aim for about 2V max.


Why does Picasa open RAW images correctly then desaturate them?

Question: I have an Olympus E-PL2 and am taking photos in raw.  I have the codec downloaded from Olympus, so I can view the photo with Picasa in Windows, and it looks fine but a few seconds after loading the image, it goes and does something in which it looks like it desaturates the whole photo.  What is going on?  I know there are many posts asking 'what is a good raw viewer' but I'm looking more for a solution for Picasa if possible since I use it for all my photo management.
Comment: another similar [question](http://photo.stackexchange.com/questions/9497/why-is-aperture-changing-the-color-of-my-raw-photos)
Comment: I agree that this is effectively a duplicate (because the same thing happens in other software), but that one is specific to Lightroom and this one to Picassa.
Comment: indeed, i found it difficult to describe the 'problem' and didn't see a suggested match as I was posting my ?
Answer: Sorry but you are confused. Picasa first shows you the JPEG preview that is embedded in RAW files.
It then loads the RAW data and lets it be converted by the codec. What you see then is the RAW file with default convertion.
In order to get something good from a RAW file, you have to work at it. IIRC there are questions here regarding matching the JPEG conversion and about why RAW files do not look so good by default.
Comment: Like this one: [How can I reproduce the camera-internal postprocessing?](http://photo.stackexchange.com/questions/13091/how-can-i-reproduce-the-camera-internal-postprocessing)
Comment: Yes, the image shown is the embedded JPEG which is not ambiguous to render. The desaturation happens because colors and tones of RAW can be interpreted differently.
Comment: P.S.: +1 to this answer, but I don't think being confused is anything anyone has to be sorry about. :)
Comment: I'll accept your answer, but it's odd that you say it shows me the jpg and then the raw.  I took the advice of another questioner and installed faststone image viewer, and the raw preview looks great, nothing at all like the picasa preview.  Is it actually showing me a jpg preview and not the raw?
Answer: Most RAW photos include a JPEG thumbnail (which is what you'll see initially); Then Picasa runs the RAW decoding (with different parameters to your camera), giving you your desaturated image.
Answer: We have this same question for Apple Aperture, Adobe Lightroom, Adobe Camera RAW, and possibly others I haven't found right now. In each case, the answer is the same: there is no such thing as displaying an "unedited RAW file". By definition, a RAW file is sensor data not in a useful form for display. The closest you might get is an RGBG matrix with linear color, which might look kind of like an image if you stand back and squint. Instead, what you're seeing initially is the embedded JPEG preview, and then when the preview changes, you're seeing the default conversion.
If you don't like the defaults, you can change them in most programs. In Picasa, I'm not sure you can very much.
Answer: Picasa does not use the camera's settings for conversion of the RAW but instead its own algorithm.
It is one of the worst converters out there, but you get what you pay for.
I started using Irfanview and have been quite happy with it. Picasa is where I do my organization.
Comment: The RAW data from different cameras, even from the same manufacturer, varies wildly in terms of colour points and more, and it's quite a task for any RAW editing software to support them all equally well.  Picasa may have poor results in some cameras and great results in others.  Other editors may have poor results in other cameras and great results with your camera.  Big players like Adobe usually support all cameras (made up to the date the RAW editor was released) very well which is great if you have something like a recent version of Lightroom.
Answer: One of the possible issues here is colour management. The raw rendition which you see after the initial delay may have no embedded profile, or you may need to set Picasa Color Management to "on" in View menu. It may be both issues, too.
Answer: It will be showing the jpeg preview (a jpeg preview is embedded in each raw file) until the RAw file fully loads up. All raw files need a little PP in order to look anything like the jpegs you get out of camera.
I remember it used to annoy me!


Can a Jew be polygamous today?

Question: Polygamy is allowed by the Torah but outlawed in the countries where Jews live. So, because of Dina dmalchuta dina, Jews cannot have more than one wife. Rabbenu Gershom imposed a ban on polygamy for Ashkenazim, but it is now expired. So my question is: Would a rabbi perform a second wedding for a married man, knowing the marriage would not be recognized by civil authorities, but would by Jewish law?  If not, why not?
Comment: What do you mean would a rabbi perform a wedding? In Judaism you don't NEED a rabbi. You could do everything on your own as long as you have witnesses and everything is done correctly.
Comment: No, because a "marriage" that is not civilly recorded and recognized does not count for civil authorities.
Comment: "Rabbenu Gershom imposed a ban on polygamy for Ashkenazim, but it is now expired" - that is debatable, but the question would be better if it just ignored that part and posited a Sefardi Rabbi from a community that traditionally did not accept the ban. You could also expand the question to inquire about locations that recognize polygamous [Cohabitation contracts](https://en.wikipedia.org/wiki/Samenlevingscontract) but don't allow equivalent "marriage."
Comment: Don't you already provide the reason why he wouldn't in your question?
Comment: In a number of countries, marriages that are not recorded are illegal and rabbis do not perform them. They will ask for a proof of civil marriage before doing the religious one
Comment: In the US, some Mormons, Muslims, and maybe others, practice "de facto" polygamy, where one wife is official and registered with civilian authorities and the others are unofficial.  I am not aware of this being illegal in civil law.  My question was: Is there anything in Jewish law that prevents a Jew from doing the same?  Reference please.
Comment: What about *dina d'malchusa dina*? In most civilized countries polygamy is illegal.
Comment: DD"M only applies to monetary laws when they don't contradict the Torah - it's irrelevant in weddings.
Comment: If I'm not mistaken R' Gershom didn't invalidate the second marriage, only banned the person so it didn't change the Halachic status of the second marriage.
Comment: I think you should rephrase your question as it is unclear what you asking. Can someone marry twice? Can a rabbi marry someone else? Is R"G ban over?
Comment: Where do pilagshim/concubines fit within your question (like the Yaavetz who I believe permits them)? They're not a wife even halachically, yet could look like multiple wives from a secular perspective
Comment: (on the issue of dina d'malchuta in Israel)
 https://judaism.stackexchange.com/questions/57065/dina-dmalchuta-with-regards-to-the-state-of-israel
Comment: @MauriceMizrahi "I am not aware of this being illegal in civil law". It is, it is just ignored a lot in a time where cohabitation without marriage, adultery and the "consenting adults" standard are social norms, it is more of an anachronism than anything else. It tends to be prosecuted when there is an actual second civil marriage done under fraudulent claims of not being married, there was deception of one of the spouses involved, or someone under 18 years old was involved.
Answer: No. See Shulchan Aruch Even Haezer 1:9, and the Ba'er Heitev 19(ibid).
The Ba'er Heitev explains that since the custom among Jews is to marry only one woman, even without the cherem of R' Gershom, marriage nowadays is based on the premise/condition of monogamy.
Even in a place where polygamy is permitted by law(not that dina d'malchuta dina would matter, in my opinion) and the cherem of R' Gershom didn't apply(for theoretical purposes), it would still be forbidden to take a second wife because she may claim that she only agreed to be married on the condition of being his only wife. Even if she agreed to have a tzara, Rabanim will not and should not perform the marriage ceremony without special circumstances permitting(see below).
Only in certain situations where a woman is not willing to live with her husband, and won't accept a get; is he allowed to take a second wife after deliberation and permission from 100 Rabanim.
Comment: Indeed all Jews should move to Israel!
Comment: What about communities that don't have the eastern European custom of the baer heitev
Comment: @chachamNisan, Shulchan Aruch Even Haezer 1:10 says (after the long Ramo) לא פשטה תקנתו בכל הארצות. Jews in Muslim countries tended to not follow it, including in Israel.
Comment: @DoubleAA Then I guess Jewish polygamists should move there.
Comment: The Meiri on Ketubot 62b (couldn't find a free source online) infers from the gemara there that polygamy was frowned on by the Rabbis even in the Talmudic era. AFAIK none of the sages had more than one wife.
Answer: Some Jews have taken it upon themselves to be polygamous today. There's apparently an underground movement based on this idea in Israel. I've heard personal accounts and found this article from a Google search that claims these ideas could go all the way up to the Sephardic chief Rabbinate.
Source: https://www.timesofisrael.com/in-defiance-of-israeli-law-polygamy-sanctioned-by-top-rabbis/
But even this article is shrouded in claims and mystery with little hard evidence. Which means it would be hard to be openly polygamous since the social stigmas are very large and real (according to modern social norms adultery is more acceptable than polygamy). So one would probably not be able to legally "register" their second/third/etc wife/marriage in any modern country outside of the Middle East.
Comment: I don't see where in that article you get "There's actually a seemingly underground movement based on it in Israel from some top (mostly Sephardic) Rabbis" from that article. Besides for bing a click-baity article, it quotes one unnamed Rabbi "who serves a community in [...] Hod Hasharon" who allegedly has a signed letter from R' Shlomo Amar allowing him personally to marry a second wife (without detailing what the specifics of that letter are). While I don't doubt that there are crazy extremist cults promoting polygamy, to claim it is supported by 'top Rabbis' needs a bit more proof.
Comment: Eh, I'm not the downvoter, but I still don't think my point is addressed. Your answer as to whether a Jew can be polygamous sounds like it's saying 'Yes, and here's an article speaking about a secret Illuminati club that does it!' Which admittedly, doesn't make an answer invalid (just like saying a sourceless 'I heard people who do it' doesn't invalidate it either), but it doesn't make for a strong answer either. (And even if everything in that article was 100% true, it wouldn't prove the validity of the group, just that the group exists)
Comment: @Heshy By that logic by law countries should make adultery illegal. But they don't, just polygamy. It's Christian politics, nothing else. If a woman doesn't want a sister wife then she has the right to request that in the ketubbah.
Comment: You should mention the bein adam lachaveiro aspect.  It's never going to be permitted if it causes pain to (one of) the wives - which is more likely nowadays than in the past because of what society accepts.  This is deoraita and probably the biggest issue even for Ashkenazim, unless everyone involved is ok with it.
Comment: Wouldn't the group mentioned in the article be transgressing the principle of dina d'malchusa dina?
Comment: Of course countries should make adultery illegal if they make polygamy illegal!  Adultery is worse in every possible way!  But I'm addressing halacha, not secular law.  If you hurt someone else that is forbidden mideoraita.  If you have a Sefardi couple in America who weren't so religious but had an Orthodox wedding and choose an Ashkenazi rabbi+ketubah because they didn't know any better and are now becoming more religious, are you seriously going to say she implicitly agreed to him marrying other wives?  (Again, if she's ok with it of course this doesn't apply)
Answer: Polygyny is permitted the problem is that most people think its not permitted so it is socially not common.  Dina Malkuth Dina is mostly irrelevant as Jewish Torah Marriage and Civil Marriage is usually handled as seperate transactions one can easily do just Jewish Torah Keddushin and in fact many marriages are done without civil REGISTRATION.  Further since cohabitation and having several woman one has a sexual aspect to their relationship is not illegal in most Western Countries with exception of a few US States it can easily be done without any concern of Dina Malkuth Dina from another interesting point of view is Jurisdiction their is a concept of LAW regarding four corners and if you apply that every Huppa is a seperate private Jurisdiction and therefore it can be claimed in even a WESTERN COURT that it is not even in the GOVERNMENTS appointed LEGAL JURISDICTION.
Further people including Rabbi's have not studied and so don't even understand Rebenu Gershon's decree as he implemented it and as it was exempted during his lifetime and in the period afterwards.
The views of not practicing has more to do with adopting the morals forced upong the Jewish Communities by the surrounding Roman Catholic and other Christian Clergy since the last 1000 years and continues today.
The biggest challenge is not if its Legal by Torah Law but by the fact that very few people are open to the practice which is sad as HaShem who is the master and overseer of all Shiddichim can seen and decide that a women would be a good match for such a man who has a wife but then nobody accepts it because of ignorance and wanting to behave as Monogmist Christians.
I have studied this subject extensively and discussed various aspects of this hundreds of times over the last 1+ years with learned Rabbi's  all Sephardim.
Finally I will quote what my Rabbi said " We dont forbid what the Torah Permits."
I welcome any questions or queries from those interested in genuine discussion.
And yes if I found the right sephardi woman or women I would ask them to marry me according to the Torah of Mosheh and Yisrael.
Shalom Itzik Michael
Comment: Welcome to MiYodeya and thanks for this first answer. Since MY is different from other sites you might be used to, see [here](https://judaism.meta.stackexchange.com/a/3887/11501) for a guide which might help understand the site. Can I also recommend you take the [tour](https://judaism.stackexchange.com/tour) to get a sense of how the site works?Great to have you learn with us!
Comment: Note that in many European countries it is forbidden to have a Jewish wedding without first having a civil wedding. Since polygamy is also forbidden, it would be illegal to have a second Jewish wedding. Last, are you aware of any Jewish community in which polygamy was commonly accepted?
Comment: Pre State of Israel it occurred from Morocco to Afghanistan so both Iberian and Syrian and Edut HaMizrach countries such as Iraq and Persia and even I have read of cases in Bukhara.  Of course we know it was very common in Yemen.  I was not aware of the stance in Europe but is that really the case? Is it only applicable if a Rabbi is licenced by the state for Marriages what of a private ceremony by someone who is not licenced by the State?
Comment: Yes in at least some of Europe (e.g., Switzerland, France) it is strictly forbidden and no respectable rabbi would marry someone who didn't go through a formal civil ceremony before. Rabbis are not licensed there, it is simply the law.
Answer: In those countries where polygamy is illegal, it would be forbidden for a Jewish man to take on multiple wives, based on the principle of dina d'malchusa dina.
Comment: This is incorrect. *Dina d'malchuta dina* only applies to monetary law (דיני ממונות).
Comment: Is the secular legal definition of polygamy met if there is only a halachic marriage?
Comment: @ezra it could be in the case of Yibbum
Comment: I believe when the Yemenites entered Eretz Yisrael they were allowed to keep their extra wives but not accrue anymore
Comment: @Aaron Because the Israeli government made a special exception for them.
Comment: What makes you say Dina D'Malchusa Dina applies to this case (marriages)? Your own link seems to imply that it doesn't.
Comment: @Salmononius2 It's not like it's a mitzvah to have more than one wife

Can a Jew be polygamous today?


Why is leapfrog integration symplectic and RK4 not, if the latter is more accurate?

Question: In a system where energy theoretically should be conserved, the most accurate simulation would conserve energy (as well as giving accurate positions, velocities and etc). RK4 is more accurate than leapfrog, yet leapfrog conserves energy and RK4 doesn't. Why is this?
Answer: TL;DR: It depends on what kind of accuracy you need.
Energy conservation does not automatically equal accuracy. Suppose, you want to simulate the solar system, and you are using a solver that – to use an extreme example – just rotates the entire system by some angle every second. These solutions obviously conserve energy, but they are blatantly incorrect.
On the other hand, if you want to predict celestial motions for a sufficiently short time span, then the effects of a Runge–Kutta method not preserving energy are negligible. Rather, this takes its toll on long-term simulations. On short time scales, a Runge–Kutta method will give you more accurate results than leapfrog – at least for a comparable computational effort.
Now, on long time scales, neither method yields very accurate results in the sense of predicting the precise future of some initial condition (which may also get difficult due to the butterfly effect). However, the leapfrog method at least yields some plausible solution, as energy is preserved. This is sufficient for many simulations where the qualitative behaviour of the investigated systems is of interest.
Comment: This went beyond what I asked to exactly what I needed to know, in particular the strengths of each on differing timescales. Also, that example aided my understanding greatly. Thank you very much.
Comment: Note that symplectic methods conserve an energy that is CLOSE to the correct value, but slightly in error, according to their order.


Arcade games in Apple IIc's ROM

Question: Is there a website where I can find the old games that where pre-installed in the Apple IIc ROM? When I could not get an OS disk, I could still have some fun with my Apple IIc by playing those arcade games from the ROM (space-invaders, …) and there was a shortcut that gave me access to the source code, too. Also, is there any corresponding emulator for modern PC/Mac computers?
Comment: There are no games in the Apple //c ROM. Memory is a funny thing - but not that memory! Possibly the machine you used had a hard drive.
Comment: Are you *sure* there were pre-installed arcade games in the original ROM? I just checked both the official ROM listing and a dump of v4, and couldn't find anything (and no source code, either). Maybe you had some special hardware?
Comment: If you actually find a ROM image with these, please post a link, I'd be very curious to see that.
Comment: Well, yes I am sure, things like the old space invaders, and many others. I highly doubt that it was some customized hardware....
Comment: @NickWestgate: I vaguely recollection the //c ROM containing something game-ish.  ROM chips come in power-of-two sizes, but the total amount of ROM required in the //c is not a power of two.  By the time it came out, the prices of different size ROM chips were such that manufacturing a machine with more ROM (though possibly not using it all) would likely have been cheaper than manufacturing one with exactly the required amount.
Comment: @supercat Then you need to provide proof. The ROMs are readily available on the internet. The four types (5 if you include the IIc+) are described here on [Wikipedia](https://en.wikipedia.org/wiki/Apple_IIc#Revisions).
Comment: Questions like this do not bother me.  However, how is this not like my question here?  http://retrocomputing.stackexchange.com/questions/1964/what-are-the-apple-ii-specific-books-written-since-2000  A question that could never be answered because it would be a list of websites vs. a list of books?
Answer: I feel like you're remembering the Apple At Play disk that came with the Apple //c.  It had Lemonade Stand, Quick Quiz, and Space Quarks (more of a Galaxian clone than Space Invaders, but I didn't know Galaxian when I was young, so I compared it to Space Invaders, too).

On Windows 10, I use AppleWin, which is really a //e emulator, but it has all the nostalgia of the //c I loved as a child. I used it to capture the screenshots above. I've never used a Mac, so I can't help you there.
And there used to be an online repository of all things Apple // called asimov.net.  There are still mirrors floating around the web.  I don't know how well they can be trusted, so I can't specifically recommend any site, but I did use an asimov mirror in order to obtain the pic of Space Quarks.
Also, archive.org lets you play ancient emulated games directly in the browser.
Comment: Right: that's the one!!! (+1). Any links, resources, emulators?
Comment: @alainlompo I added some links to resources I use, and one piece of information I can't recommend, but it's there for you to explore.
Answer: I don't remember any embedded games, but I would highly recommend Virtual II (http://www.virtualii.com/) for Apple II emulation.  Its free if used in a limited fashion, otherwise you need to buy a license, but it does just about everything you could possibly think of.
As far as a short-cut for access to the 'source code', the only thing I can think of is that you are asking for how to access the ROM monitor.  From the command prompt:
<code>]CALL -151
</code>
This will drop you into the monitor, where you can query RAM address and disassemble whatever is in memory (RAM/ROM).  For example, after entering the ROM monitor you will get a * prompt.  You can type a memory address followed by the letter L (with no spaces) to list (disassemble) memory, like this:
<code>*FF69L
</code>
More information regarding using the monitor can be found here: http://www.classiccmp.org/cini/pdf/Apple/Apple%20II%20(Redbook)%20Reference%20Manual%2030th%20Anniversary.pdf starting around page 68 or so.
Comment: What about [AppleWin](//en.wikipedia.org/wiki/AppleWin)? It's free and open source ([Github](//GitHub.com/AppleWin/)), and emulates an Apple ][ e.
Comment: and MAME, while we're at it.

Arcade games in Apple IIc's ROM


How can I use an on-camera TTL flash with a Yongnuo 605N to trigger my off-camera manual speedlights?

Question: I am using a Nikon D7000. I want to use an SB700 on camera in TTL as well as have my Yongnuo RF-605N trigger my off-camera speedlights in manual. The RF-605N does NOT perform TTL pass-through. I see that there are some hotshoe cords with a hotshoe on both ends, but have heard reviews that claim that they do not fire both hotshoes simultaneously. I don't understand what sense that would make?? I am not opposed to using a flash bracket or having the trigger connected via cable if possible.
Comment: possible duplicate of [Firing a TTL on-camera flash and manual off-camera flashes via radio triggers](http://photo.stackexchange.com/questions/61040/firing-a-ttl-on-camera-flash-and-manual-off-camera-flashes-via-radio-triggers). My answer there was get a set of YN-622Ns, and stack triggers.
Answer: The SB700s have built-in wireless triggers for multiple flash-unit photography. I am assuming all your flashes are SB700/800/900 or compatible. You will not need the Yongnuo RF.
The master flash unit on your camera commands the remote units. The flash mode can be set on each flash unit.
http://downloadcenter.nikonimglib.com/en/products/245/SB-700.html
If you have additional flashes that are not Nikon, you could use optical triggering for them.
Comment: I am using Yongnuo 560IV & 510EX flashes. I am not using the CLS system or optical slave because I may need to fire flashes that are concealed behind barriers. I am also using the RF605N because it very easy to control each flash on/off simpler (one button) than through the Nikon menu.
Comment: I was wondering if that might be the situation. You may want an iTTL pass-through hotshoe with a pc connection for the RF. The reliability issue with multiple flashes is due to voltage needed to fire multiple flashes. With an RF trigger, this may not be a problem.
Comment: Btw, I've gotten a slave on a cheap dice-sized cube optical trigger to work behind obsticles (backlight a decorative window) simply by using a well-placed mirror.  A sensor cube on a cord should work too.
Comment: Would this cord work for what I am looking for? http://www.bhphotovideo.com/c/product/1005408-REG/energizer_ene_ttlu_multi_fit_ttl_flash_cord.html  The switch on it looks as if you can only use one hotshoe at a time though.  I would mount the flash on one of the hotshoes, and the 605N on the other. I want this setup so that I can control the off camera lights on/off. Optical trigger does not help because I may need to turn off some flashes from the camera.


Why do combustion chamber combustion gasses turn the engine when it only burns?

Question: I asked this because if I have a small portion of fossil fuel and light it, the fuel will only burn. But inside a combustion chamber of a vehicle this same combustion will propel the piston downward keeping the vehicle moving; why?
Comment: LostPecti:   please edit if my changes do not capture your intent.
Comment: No you captured my question perfectly. Thanks for your help.
Comment: The main thing is that you're turning the fuel into a fine mist. If you light a puddle, it will burn slowly, if you atomize it and light the cloud, it will explode. Pretty much anything is dangerous when its small enough & in high enough quantities mixed with the air. Just google about flour mill explosions, even flour is explosive when mixed well with air making flour mills super dangerous. We compress it so there is even _more_ oxygen around the substance we want to burn. Add these together and you get quite the explosive force.
Answer: During combustion, the pressure in the combustion chamber is increased, and this pressure pushes down the piston. There are two reasons for this:
Increase of the amount of gas molecules
Let's say we use hexane as fuel. To burn one hexane molecule consisting of 6 carbon and 14 hydrogen atoms, we need 13 oxygen atoms (6.5 oxygen molecules) and get 7 water and 6 carbondioxide molecules:
<code> 1* Hexan           + 6.5* oxygen -> 7* water  + 6* carbondioxide

     H H H H H H
     | | | | | |
1* H-C-C-C-C-C-C-H  + 6.5* O-O    ->  7* H-O-H + 6* O-C-O
     | | | | | |
     H H H H H H
</code>
Because air consists of just 20% oxygen and 80% nitrogen, there are four nitrogen molecules for each oxygen molecule in the chamber. They do should not react during combustion, so you simply add 26 molecules nitrogen on both sides.
So, before combustion, there are 1+6.5+26=33.5 molecules and after, there are 7+6+26=39 molecules.
One interesting fact about (ideal) gases is that a certain volume at certain temperature and pressure always contains the same amount of molecules, regardless which kind or mixture of molecules.
Let's say we still have the same volume in the combustion chamber and neglect the increase in temperature, the increase of number of molecules by a factor of 39/33.5=1.16 results in an increase of pressure by a factor of 1.16, too.
Heat expansion
If you increase the temperature of a gas, it would expand. If it can't because it's enclosed in the combustion chamber, the pressure rises instead. For example any (constant) volume of ideal gas at room temperature (20°C) increases its pressure by a factor fo 4.3 when heated to  1000°C.
All together
During combustion, the pressure increases by a factor of 1.16 as the number of molecules rises, and another factor of 4.3 due to the temperature, leading to a total factor of 5 in pressure increase.
Let's say the combustion chamber has a piston of 8cm diameter (typical bore), which corresponds to a surface of 50cm². A pressure of 5000hPa (difference to environmental pressure of 1013 hPa) will apply a force of 2500N (or 560lbf) onto the piston and push it down.
What I didn't say here is that a real motor first compresses the air/fuel mixture by a factor of about 14, which increases temperature and pressure in the cylinder. (It invests energy here, but it gets it back after combustion) Also, I don't know which temperature is reached during combustion.
Also, this is a very basic calculation neglecting some effects, but I think it clearly shows how the force on the piston is created.
Oh, and if you ignite a little fuel in an enclosed bin, you will also notice an increase in pressure. But since the process is quite slow, most of the heat leaves the bin, it doesn't become that hot, and the pressure isn't that high. (But be careful: Fuel vapors can explode, and then you have the high pressure...)
Comment: Great answer. I can add a few comments here. Exhaust manifold temps can reach 1200-1600 F. The ratio of pressures before and after combustion is usually modeled by automotive engineers as "explosion ratio". In this way they don't have to worry too much about the chemistry behind the scenes
Comment: First off thanks you for your detailed answer. Let me see if I understand. The idea of a fuel is to make enough pressure to push the piston down. It does this using two methods. The first is using molecules, the more molecules added the more pressure in the cylinder. The other method is using heat, since we know gas have atoms and can expand better then solid atoms or liquid atoms. When the atoms in the gas are heated they expand causing even more pressure. So in all combustion chambers rely on high amount of pressure. I have a  question why do air/few have to be compressed first?
Comment: @LostPecti  I think your question is great.  Your other question in your comment, "Why does air/fuel need to be compressed?" is another question.  You should ask it.   :-)  Cheers!
Comment: @LostPecti: Well, atoms/molecules don't expand, they are more like small boxers keeping their neighbors at distance by punching them. Heat is their dope (->more strength, more distance, more space needed by all boxers) and of course more boxers want more space. The piston also takes lots of punches, which force it down. (Hmm, should have put that into the answer...). But you are right, the idea of a piston engine using fuel is to generate pressure inside the cylinder to push the piston down. Steam engines generate pressure outside and lead it into the cylinder.
Answer: AFAIK the way this works when you burn the fuel,  you have it in liquid state at atmospheric pressure. In actuality you're not burning the liquid, you're burning the vapors that form over the liquid.
When in the engine, fuel is disbursed into droplets which creates a lot of surface area for the fuel to expel vapor. What the ECU does is create a very specific mixture of this fuel, vapor, and air that is needed to create the high energy burn that makes the engine turn.
Without that very specific ratio you don't get the more energetic burn.  The chemistry behind why that is I don't know.
I hope that helps.


Why was the previous Literature SE closed?

Question: What's the history behind the first Literature StackExchange? Why was it closed? How can we avoid the same thing from happening to this one?
Comment: [Why did Literature fail?](http://meta.stackexchange.com/q/131397/278659)
Comment: [How will Literature be run different this time?](http://discuss.area51.stackexchange.com/q/25915/124276)
Comment: [How can we make this literature proposal succeed when other literature proposals have failed?](http://discuss.area51.stackexchange.com/q/22392/124276)
Comment: @Benjamin I'm not going to fight you on that! However, "off-topic" seems like the wrong close reason, and marking it as a duplicate won't work since the dupe is on another SE. Hmm.
Comment: The answer to the _practical_ question is "for reasons the current iteration of the site isn't suffering from."
Comment: IMHO, even with previous discussions on the topic, this is is still a valuable discussion to have _now_, specifically in the context of the new Beta.
Comment: @Randal'Thor Thanks. I actually found the first one myself right after asking. The others are interesting, thanks!
Comment: I'm voting to close this question as off-topic because it is a duplicate of http://meta.stackexchange.com/questions/131397/why-did-literature-fail.
Comment: @tobiasvl I was going to mark as a duplicate, but because it was off-site off-topic was the closest.
Comment: If someone like @Randal'Thor would want to answer this question with just the links to the other relevant questions, I'd be happy to accept that answer so this metastack actually does supply the background and history to other curious people.
Answer: I'm going to answer this question in the form of quotes from several previous meta discussions on the same or related issues, along with some of my own commentary based on how this incarnation of the site has actually been coming along so far. Brace yourself: this may be long.

Why did Literature fail?

The highest-voted answer, from the user known here as Standback, says:

Since I found this site, I've never been able to square with its site definition. That's a lot of what kept me away.
On the one hand, the most popular topics - recommendations, story identification, and reading order - seemed to me inconsequential and uninteresting. Their utility is obvious, but generally speaking, these are the opposite of expert questions. These are (usually) trivialities and everybody-can-answer questions. On a more active site with wider scope (e.g. SciFi), these questions might be a mere nuisance in my own opinion, but when they seem like the primary focus of the site, they drown it out. It becomes a site that discourages visitors who were hoping for more than that.
On the other hand, the SE format seems an extremely poor fit for getting into deeper literary discussions. Analysis of a work can be structured very naturally as a dialogue; I honestly don't see what can be gained by shoehorning it into a question/answer, problem/solution format. I never have. And while I wasn't very active on the site, I did peek in from time to time, even gave a shot or two myself, and never saw anybody manage to get a serious analytical discussion off the ground at a level that interested me - and I think the SE format is the primary reason for that.
Regarding both these categories of questions, my feeling is that these questions work just as well or better on a regular forum than they do on a strictly-formatted SE platform. This being the case, SE hampers rather than helps.

How does this assessment compare with the new Literature site?
Well, recommendation questions are firmly off-topic this time round, and while there have been both story identification and reading order questions here (mostly good-quality ones, IMHO), neither has been occurring in dangerously large volumes or threatening to swamp the site.
As for literary analysis questions: while they have inspired several meta debates about their workability given the SE model and the community we have here, they have on the whole been successful and provoked well-thought-out, carefully written answers. I can provide example links to support my claim that literary analysis Q&A have been working well here so far, but I don't think I need to - if anyone disagrees, please tell me why.
The second answer, from DForck42, says:

I think the very first thing was letting recommendations be on-topic. With 85 out of 434 questions they were about 1/5th of the site, but drew little traffic. People get tired of looking at stuff that doesn't interest them.
I think another issue is that we lost a lot of traffic to sci-fi because it shares such a huge portion of content with literature.

Again, recommendations are off-topic this time round. As for overlap with Sci-Fi & Fantasy, that's been discussed here, and (as a very active user on both sites) I honestly don't think it's going to be too much of a problem. Sure, there are a few SFF people who can't be bothered to participate here because the only fiction they read is sci-fi and fantasy, but there are also plenty who've happily joined up here and are helping to build this site too. SFF and Movies & TV have coped for years with having scope overlap without either site collapsing. Furthermore, so far it looks as though the 'tone' of this site - the style of questions and answers encouraged here - is shaping up to be very different from SFF's (again, I can elaborate on this if anyone wants me to), which means that although many literary works could be asked about on either site, the types of question asked may differ between here and SFF, which reduces the 'competition' for content.

How can we make this literature proposal succeed when other literature proposals have failed
The top answer, from Hamlet, says:

Instead, I think the problem is a lack of effort. While the last literature proposal had 89 followers, it also only had 45 example questions (many of which were downvoted and/or closed). In order for a proposal to succeed, it needs 40 questions that have 10 or more upvotes. Therefore, I would encourage everyone to ask five example questions. If you don't have time to create an original example question, you can copy questions from the old literature proposal or from the old literature site (which was closed back when Stack Exchange had different standards about site activity).
And of course, I would also encourage everyone to promote this site on other Stack Exchange sites, on other online communities, and among people whom you personally know.
Let's make this proposal a success!
EDIT: In response to questions I have received about the closure of the formal literature site, I'm going to briefly discuss why that site was closed. In my opinion, the site was closed for three reasons:

As I stated, earlier, the old literature site was closed back when Stack Exchange had different standards for about site activity for beta sites. Obviously, those standards no longer apply to beta sites, and if this proposal eventually reached the private beta phase it wouldn't be in danger of being shut down for that reason.
The second reason why the old literature site wasn't successful was that it had turned into a site dedicated to book recommendations. People weren't asking questions about literature; they were asking questions along the line of "could you recommend a book that I would enjoy to me." This problem could easily be avoided in the new site: we would just have to be proactive about closing book recommendation questions.
Looking at the data dump of the old literature site's meta section, I got the impression that people were confused about the scope of the site. It wasn't clear whether the site was supposed to be a book recommendation site, a site for literary analysis, or a site to answer basic plot points in works of literature. I don't think that this proposal should be a book recommendation site -- there are other proposals for that -- but I do think it could be successful as a combination of the second and the third topics.
I am worried that there don't seem to be very many literary analysis questions on the area51 page, and since Stack Exchange originated as a programming site, I'm not sure if this proposal will attract many teachers/scholars of literature who have the expertise necessary to answer those questions. However, it certainly is worth a try.
I've asked and answered many literary analysis type questions on Mythology.SE, and I would be willing to ask and answer such questions on a literature site if it was created. The question that remains is whether others would be willing (and have the expertise necessary) to do the same. I think it's worth a shot.
As I mentioned earlier, I disagree with the assertion, made by some people, that questions about literature ill suited to the Stack Exchange format. Previously in this answer, I gave examples of questions asked and answered on Stack Exchange's mythology website about relatively complex literary elements in mythology. I think these examples show that even complex literary analysis questions can be successful on Stack Exchange.

How does this assessment compare with what's actually been happening in the first week and a half of our lovely shiny new site?
Well, the issue of followers and example questions has already been resolved, in order for us to get this far. (It wouldn't be boasting to say that a significant proportion of this was thanks to me. I'm active in a lot of different chatrooms, and I bugged a lot of people to follow the proposal and use up all their upvotes on example questions. Without that effort, who knows what would have happened.)
I agree with most of the rest of Hamlet's answer, the important points being that SE's standards are different now (making it much less likely that we'll fail in beta), recommendation questions are banned now (again!), we can mix literary analysis with trivia/plot questions (further reading), and yes, literary analysis really can work (see above).

How will Literature be run different this time?
The top answer to this one is more or less the same as the answer just quoted above, so I'll skip straight to the second answer, written by myself:

A clearer and better site scope.
Judging from what I've read about the failure of the previous Literature site - e.g. Why did Literature fail? - it seems that many of the problems there were due to either an unclear understanding of site scope, or an overabundance of questions which were on-topic and really shouldn't have been (e.g. recommendation questions). This time, let's be more proactive both about deciding on a clear scope and about shutting out questions that will be bad for the site. I plan to start several different scope-related meta threads as soon as private beta starts.
More experience.
The previous incarnation of Literature was, what, 4, 5, years ago? In that time, sites such as Science Fiction and Fantasy, Movies & TV, and Anime & Manga have grown to maturity. We can now see what a successful site based on analysis of fictional works (rather than programming, mathematics, etc.) actually looks like. Many of the followers and committers of the Literature proposal are already active on those sites. I'm a mod at SFF; you're a former mod at M&TV let's bring our experience to the table and help to make Literature great!

The first of these issues isn't fully resolved, as there's still some confusion about whether various borderline questions should really be on-topic or not. Maybe it'll never be fully resolved, but many healthy sites have Scope Wars to one extent or another. Most of the questions being posted here have never faced a single close vote, and that's heartening. Our scope debates may rage long and hard, but I'm not too worried about them destroying the site.
The second issue is definitely an advantage. We've had active users and moderators from many related sites - SFF, M&TV, RPG, Writers, even Arqade - sharing their knowledge and experience here and helping this site to become its own newly-fledged beast, taking wisdom from many sources and yet unique and distinct from all of them. Exactly as it should be.
Comment: My takeaways are these: (1) What we're trying to do is hard. (2) We have the dedication and people to make it work. (3) We're doing the work we need to, to make the scope clearer; we've already answered many central questions to this effect. (4) A userbase with broader literary interests helps - which this one is. (5) We've found a way to make analysis and longer-form thought work. (6) Maybe (just maybe), we're more patient with harder questions and longer answers. I think we're on the right track.
Comment: But it's certainly worth the attempt; I think previous experience has showed us some areas and tones to steer clear of, and I'm very happy to put in some effort participating to see whether we can make it work :)
Comment: I do still consider the workability of Literature Q&A to be an open question. I have some serious reservations - I'm concerned that a lot of content doesn't fit well into Q&A (I'm fascinated by the issue I raised [here](https://literature.stackexchange.com/questions/304), but unconvinced Q&A is a helpful format to discuss it) ; I'm concerned about the site being flooded by "cheap" questions of the "What is the significance of trivial detail X in book Y" mold; I'm concerned about the existence of "expert-level" questions, i.e. whether Lit "experts" will truly have questions they want answered.
Answer: I disagree with @Standback and @DForck42 on the reason the previous site failed - but, then, maybe that's part of why it failed - because we couldn't even agree on a definition of success.
The biggest struggle I faced, from a moderator standpoint, was meeting the aggressive numbers targets set by StackExchange with no idea how to promote the site - they offered to help me promote it, but I was often left feeling like it was up to me to come up with the ideas, and that just isn't a skillset I have.
Comment: It's fortunate, then, that SE has since [changed their policy](https://meta.stackexchange.com/q/257614/278659) on what's required for a site not to be shut down. Even if we don't graduate for a long time, we don't seem to be in danger of going the same way as Lit.1 this time round.


Adding a Image Upload Button in CKEditor

Question: I have installed CKEditor in Wysiwyg profiles. While publishing any node if i click on the image upload button it shows options to add images by a image link, but what is the way to add a dynamic image upload button so that users will be able to Browse and choose any image from their computer in content?
Answer: Check out IMCE with IMCE WYSIWYG bridge.
Once you install and configure those two modules, select "IMCE" as a button in CKEditor and then you can upload and insert images in WYSIWYG.
Comment: two issues , first one is it shows a server upload and in that frame i got a upload button, so is there any way to use the only Browse button, and second issue is though the photo uploaded successfully in wysiwyg body but after save the content uploaded photos get disappear ,, ?
Comment: 1) IMCE allows you to *browse* the files on the server. That is why you get the second upload button. This is good if your files are already on the server or if you want to use the same file in several pages for example. Otherwise you would have to upload a file each time you want to use it - which is not good from management point of view. 2) Check your text input filter and if it supports the img tag.
Answer: I know I'm somewhat late to this question, but I just found out about One Click Upload which does exactly what you want and may help others that have this same problem and arrive to this question (the only caveat is that it needs flash to be enabled to work).
Answer: The easiest way is to use according plugins: http://doksoft.com/soft/ckutils/index.html
It allows user to upload images, preview and re-size uploaded images when needed.
Comment: I am afraid the plugins are not free.
Answer: There is also the media module but it is much more than a simple image uploader and you should use it only if you want to be able to manage files through Drupal.
Answer: in drupal 7 you can work with ckeditor and imce modules.
after you've installed these two modules:

in imce config, under "Configuration profiles" you can add new profile and edit it's setting and under "Directories" add directories that you want the user access and add images.
go back to imce config page and under "Role-profile assignments" attach a role that the user have to the profile that you've created.
then go to ckeditor config page. profile advance -> edit. under file browsing setting, set "File the browser type" to imce.

that's it . now the user can click on picture icon on ckeditor and upload a new image in directories that you've set.
hope it helps.


What happens if a dragon lacrima of another element is implanted?

Question: In fairy tail, we know that the third generation dragon slayers have been taught dragon slayer magic by dragons and also that they have dragon lacrimas implanted.
If a dragon lacrima of an element of different type is implanted what will happen?  Will they gain two types of dragon slayer magic?
( For example if Sting is implanted with a fire element type dragon lacrima will he gain two types of dragon slayer magic? i.e. holy light type magic taught by Vicelogia and fire type magic gained from the lacrima)
Comment: Probably. You can be taught two different kinds of magic, so why wouldn't two dragons be able to teach you two different kinds of dragon slayer magic? The same would probably go for the lacrimas, but I don't think there is any proof of a dragon slayer having two elements other than Natsu who is somehow magically able to keep using Laxus' magic, but it was never explained why this is the case.
Answer: Spoilers to recent Manga chapters, But to be put simply they gain the ability to use and presumably eat that element. This is shown by

 One of the Elite Members of the Alvarez Empire,God Serena, who used to be a wizard saint, rank 1, who had 8 Dragon Lacrima implanted into his body.

This is shown when (Even Bigger Spoiler):

 he uses the Cavern, Purgatory, and Sea King Dragon slayer magics in rapid succession to beat down four wizard saints ranked 2,3,4,and 5. Draculos Hyberion, Wolfheim, Warrod Sequen, and Jura Neekis were all incapacitated by his 3 attacks of those 3 dragon slayer magics. He then prepared to finish them off with Gale Dragon Slayer Magic, but was interrupted before he used that one.
Answer: I believe it is dependent on the Dragon Slayer. Natsu has done this multiple times (Laxus, Zancrow) and Gajeel has accomplished it once. It could be a similar process like when people take small amounts of poison to build a tolerance to it. As was seen, Natsu became sick after using lighting, but over time it seems he has been accustomed to it as it doesn't have such an adverse effect on him when he uses it.
Answer: The lacramas shouldnt hold restrictions since there is a character with 8 dragon slayer elements from them. But for dragon slayers to gain dual elemental modes like Natsus Lightning Flame Dragon mode or Gajeels Iron Shadow Dragon mode, i personally believe it can only be taken from 2nd or 3rd generation dragon slayers. Laxus gave Natsu his lightning in a similar way to how Natsu had consumed the ethernano and other flames, but only the lightning has stayed with him without being a fire based attack. I like to believe that since Laxus has a lightning Lacrama, his lightning was shared by having enough energy consumed to create a new lacrama in Natsu. Gajeel then did the same with Rouges shadow by absorbing the power from the lacrama half of Rouge instead of the shadow magic taught by his dragon. The final example i could think of was future Rouge saying he "took" Stings magic after killing him. The only possible way for that is through his lacrama, turning Rouges future self into a White Shadow Dragon


.NET XBox Live Account API

Question: Is there a .NET API available to get data from your XBox Live account?
All I'm really interested in is who's online, but messages would be cool too.  And some sort of event driven notifications of user sign-on would be great, but I'll poll if need be.
Answer: Check out the Xbox Community Developer Program.
Answer: You can also try and take a look at https://xboxapi.com/
They provide a API system to get all sorts of info.
Example dev: https://xboxapi.com/dev/profile/Major+Nelson
their documentation is here, https://xboxapi.com/documentation
it provides info to get data in PHP json, xml, or dev.
Sadly I dont think anything like Messages and some notifications, only publicly visible info. But if you track profile info one can generate a history and if you poll the online status, you can make a popup or something

.NET XBox Live Account API


Can 35mm film be developed if it's been ripped out of canister?

Question: I am a total beginner and my Pentax K 1000 has jammed a few times. Being the noob I am I ripped the film from the canister. I have the film rolls but the film itself is not in the canister. Can I still develop the film?
Answer: Film works by changing when exposed to light.  If you have exposed the film to light, then any information it captured will be lost as the film will have been completely over-exposed and washed out.  If you did it in complete darkness or, if it's orthochromatic, limited spectrum of light to which the film isn't sensitive (like the red lights in a dark room) then you'd be ok, but it doesn't sound like that's the case here.
Comment: Most modern film emulsions are __panchromatic__, i.e. sensitive to the whole spectrum of visible light, therefore they should be handled in complete darkness! __Orthochromatic__ films, on the other hand, are sensitive only to green and blue parts of the light spectrum, so they can be handled under a red darkroom safelight.
Comment: @lightproof - good to know.  I know the basics of film, but I got serious after film had already passed it's prime so I never personally got a chance to learn darkroom work.
Answer: If you've done this in absolute darkness and kept the film in absolute darkness: Yes, absolutely.
If it received some light: no.


What books to read to learn about geography and how it affects economic development?

Question: I saw youtube videos like how geography doomed Africa and heard how Japan's prosperity is an anomaly due to its isolation and they got me interested.
Comment: @MikeJ Krugmans work does not really focuses on economic development though. Rather he tries to explain why so many industries tend to cluster together like Silicon Valley and some puzzling results from international trade that were not already explained by classical trade models. I don't think that's what the OP is looking for although of course everything in economics is interconnected so these are part of the big picture.
Comment: Paul Krugman has done a lot of work on economics and geography.

https://pr.princeton.edu/pictures/g-k/krugman/krugman-increasing_returns_1991.pdf                           I will make a post with more information/ resources shortly.
Answer: A good starting point which should be accessible to people at all levels is the book "Guns, Germs, and Steel" by Jared Diamond.
I believe this is one of the first and main seminal works in this area.
That being said, it is not settled science that geography (alone) can doom a country's economy.
Answer: As mentioned by @BBKing, "Guns, Germs, and Steel" by Jared Diamond is a great book. Another interesting one is "Why Nations Fails" by Daron Acemoglu and James A. Robinson.
They argue that geography is insufficient in explaining the political and economic success or failure of states. They support their thesis by comparing country case studies.

They identify countries that are similar in many of the
  above-mentioned factors, but because of different political and
  institutional choices become more or less prosperous. The most
  incisive example is Korea, which was divided into North Korea and
  South Korea in 1953. Both countries' economies have diverged
  completely, with South Korea becoming one of the richest countries in
  Asia while North Korea remains among the poorest. Further examples
  include the border cities Nogales (Sonora, Mexico) and Nogales
  (Arizona, USA). By referencing border cities, the authors analyze the
  impact of the institutional environment on the prosperity of people
  from the same geographical area and same culture. (source Wikipedia)

Regarding @PulseReborn's comment

I was expecting more of textbook recommendations

I can recommend "Modern Economic Growth" by Acemoglu, a graduate textbook. In particular, Chapter 4 has a section called "Geography". He presents some arguments for and against this explanation.
Comment: @PulseReborn could you tell us a bit about your level of economics training? Many textbooks on this can be fairly technical and mathematical if you are a beginner so that information would be important.
Comment: I already read Why Nations fails. GGS is one of the first books I come across when searching the topic, the other being Prisoners/Revenge of Geography. I didn't finish Revenge and GGS. I feel lost at the introduction, of the former and GGS's size makes it seem like it's full of fluff or repetition. I didn't mind size and density but after reading Why Nations failed and others I became sceptical of popular books. I was expecting more of textbook recommendations; books that relatively to the point, makes it clear, has summaries, talks  of relevant topics and with actually usefu table of contents
Comment: I updated my answer!


How to specify the quickest movement a human can do based on MTM standards?

Question: I am looking for a benchmark as a reference regarding the human Kinesiology that specifies the quickest movement a human can do on basic everyday movements like computer mouse clicking. As far as I know, there is an standard called MTM (Measurement Time Methods) in the field of industrial engineering and ergonomics which specifies the required time for each basic movement to be done by human. This MTM standard time is provided on the basis of not putting the human (worker) in an overload or underload working situation. So I assume MTM cannot provide me with the shortest possible duration (time) a basic movement can be performed.
I am not sure whether we have such a benchmark based on human physiology, Do you have any suggestion in this regard? I recorded response times and have a distribution of these response times for many subjects, however, I would like to estimate the minimum possible response time a human can do in my experiment
Comment: I'm no expert but i would look to the group of bodily relexes, actions that are automatically taken in response to a stimuli e.g. ducking when a pigeon suddenly flies near your head. As to the fastest of these i would suggest eyeball action when sitting on an underground train whilst looking at adverts or fixtures on the tunnel as the train hurtles by. These eye movements are super-human fast.
Answer: I don't know about any MTM standards or benchmarks, but motor response latencies have been assessed in many studies. Importantly, auditory response latencies are smallest, then tactile followed by visual response times. This, because the auditory system is swift, while the visual system is sluggish by comparison (source: Packman & Packman).
For example, Jain et al. (2015) report a mean visual response latency of 247 ms and an auditory response latency of 228 ms. Another study reports approximately 144 and 133 ms, respectively. The lower values are probably because the authors chose the lowest reading as their final measurement Gavkare et al, 2013). Other technical differences may also have yielded deviations. Importantly, stimulus intensity affects reaction time (e.g., louder auditory stimuli decrease reaction time) and other stimulus parameters do so as well (source: Packman & Packman). Importantly, age is positively correlated with reaction time (Ng & Chan, 2012).
A large data set where the authors stratified their subject pool in age groups as well as other parameters can be found in Ng & Chan, 2012.
References
- Gavkare et al,, Indian Medical Gazette, June 2013
- Jain et al., Int J Appl Basic Med Res (2015); 5(2)
- Ng & Chan, IMECS (2012)
Comment: Although the OP mentions 'response times' I do not believe the response times you mention in this post are the ones he was referring to: _"the shortest possible duration (time) a basic movement can be performed"_.
Comment: Yes then the question becomes whether a neural component is included yes or no. In my answer it is and afaik pretty much all studies take that into account.
Answer: Theoretically visual stimuli would be faster than auditory stimuli because of the given speed of light vs sound.  Therefore I personally believe the time from stimuli to recognition would depend on the individual and furthermore the time from recognition to the time of response (ex. Clicking a mouse immediately upon recognition of a sound or sight, followed by [roughly] identifying said object)
With all those variables, in MY OPINION there are  absolute's: 1) light travels faster than sound; 2)visual contains more data than audio
Comment: Previous answers have citations that refute your belief. Do you have any citations/research to support your answer in the setting proposed?
Comment: Most studies of reaction time are interested in neural processing and not propagation delays. If you wanted, in a typical lab you could introduce a non-negligible propagation delay of maybe 10 ms, but with headphones the propagation delay is on the order of 100s of micro seconds and lost in the measurement noise.


What's the best way to level after you get into the Outlands?

Question: So, recently I got my Undead warrior to the Outlands. I thought it was great to be in the other planet of WoW. I was ready to power level up to around 70 to get back to Northrend. But it seems like leveling in the Outlands (especially Zangarmash) takes a LONG time. I cant use Refer a Friend because none of my friends play WoW. If all other hope is lost I'll resort to dungeoning. Any hints out there?
Comment: "Resort to dungeoning"?  If you're a warrior, pick up some tank gear, and LFG.  No queue time, it levels you _crazy_ fast, and you get awesome gear.  (When WotLK came out, I leveled my tauren warrior from 16 to 70 entirely in dungeons)
Answer: Questing is always the best way to level, especially without Refer a Friend's bonus giving you more xp for kills.  Just follow the quests progressions through Burning Crusade content and you'll be of the proper level to head to Northrend after like 3 zones.
Also if you already have the means to do so, getting heirloom gear is a great boost to leveling as well.
Answer: With heirlooms I generally do all the quests in Hellfire and then dungeon my way the rest of the way to level 68 - at level 68 you can begin queuing for WotLK dungeons which grant significantly more experience.
With a full set of heirlooms you should be most of the way to level 68 by the time you finish the quests in Hellfire.
Answer: I know this post is over a year old, so this comment is just some quick bullet points for leveling in Outlands as of patch 6.2.2. This comment is directed towards players who are only interested in leveling as QUICKLY as possible.
•Secure flight training as SOON as you hit level 60. Same for Northrend with Cold Weather Flying, and Pandaria with Wisdom of the Four Winds. This is going to more than double (I'd almost go as far to say triple) your questing rate. Plus you don't have to worry about killing a bunch of useless mobs that you are bound to aggro on a ground mount.
•Heirlooms are still your best friend. With the XP stacking with the more pieces you have, it will give you an unparalleled boost in XP gains per quest, dungeon, or even just a regular mob kill.
•If you haven't done so already, get yourself into a guild. Doesn't even have to be a good one, as long as the guild is level 25. You will get a slew of helpful abilities (mass resurrection, shorter cooldown on hearth stone, portable bank, and even allows access to pieces of heirloom gear otherwise unobtainable without being in a guild). If you aren't like me (I took the first G-invite I got from a level 25 guild), you most likely can find a fun "leveling guild" that will help you with quests, dungeons, and even have done people to share your victories with!
•Professions. If you're looking to power level to max level in a hurry, leveling your professions will slow you down more than you think (some may disagree but meh, I digress). For example, if you have good momentum going thru quests and you stop to mine or gather every ore or flower you see, it will kill your momentum instantaneously. If you have a main that is decently wealthy, you can always just buy mats at the auction house and power level your professions later on.
•As entertaining and delightful world events can be (ie. Brewfest, Winter's Veil, Midsummer Fire Festival), try to refrain from getting involved with the enticing activities. You won't get much from them, aside from a few pieces of gold and a couple achievements. Not really worth your time while trying to speed level.
•If you are questing as a DPS, then queue for random dungeons as a DPS. If you want to waste time switching specs every time you want to run an instance, then be my guest. Personally, I'd rather sacrifice the shorter queue times in exchange for being able to continue blasting thru quests while waiting for a group invite.
•Remember it's just a game and it's meant for you to have FUN! I lost sight of that notion back in WotLK while power leveling 8 characters to max level, and all I ended up with was just that. 8 characters at max level, and no really great memories of leveling any of them. If you can find friends to quest with, or run dungeons with, it'll make the leveling experience (even the power leveling experience) a lot more memorable and noteworthy.
•Finally, I would recommend having a reason in mind as to WHY you are leveling your toon. If you're leveling it just to level it, you're most likely going to lose interest in it. Set some long term goals like eventually queuing up for raids, or gearing up for some pvp action, or like me, to get your toon at max level in preparation for the next expansion. Having a real reason to level, along with some solid goals, will help keep you focused on continuing the grind to max level. The next time you feel like you're staying to get bored, think slot those things for a moment, and see if it doesn't give you a bit of encouragement!
That's all for me guys and gals. I know this was a bit lengthy but I wanted to cover a lot of ground since this post hadn't been updated in a while. Hope you all are having a wonderful day, and I wish each of you the best of luck on all your future leveling adventures!
-S
Answer: You should follow the Questline of Burning Crusade, and additionally do some dungeons once, because the quests are a big boost in EXP.
Additionally you should consider to stay in lower level areas until the quests are green, because the quests will be significantly easier to do if they are not on the same level as you are, which results in a lot faster questing time.
Answer: The TourGuide addon http://www.curse.com/addons/wow/tourguide hasn't been updated in years, but all its data for Ouland are still completely up to date. It will point you in a nearly optimal path to completing the quests in a minimal amount of time. If you have the TomTom addon as well it will put up a big arrow on your screen pointing exactly where to go to acquire or complete the quest quest in the list.
Also make sure you learn flying as soon as you hit 60 to minimize travel times.
Finally, you can get a huge amount of experience from doing a single ramparts and a single blood furnace instance. They each have multiple quests that turn in for a nice chunk of XP. I typically go from 58-65 in Hellfire alone between all the quests and those two instance runs.
Answer: When I recently ran Outlands on my DK I breezed through Hellfire Peninsula quests, following that line all over the place until I could move onto Terrokar Forest. I did Terrokar Forest and moved left into Nagrand until I was able to fly north to the Netherstorm. Bombed through that for a few levels until I could hit Northrend.
I know you didn't ask, but, once in Northrend I slid from Left-to-Right, Borean Tundra to Dragonblight to Zul'Drak, then north into Stormpeaks on back left to Icecrown.
If you're not a tank, then while you're running your quests be sure to always queue for random dungeons so you get that bonus XP. If you're tank, you usually get an instant-queue hit and it can take longer to level via dungeons (since you can only do the dungeon quests once per dungeon per toon) and you'll spend more time in dungeons than getting quests done.