Does a charged particle accelerating in a gravitational field radiate?

Question: A charged particle undergoing an acceleration radiates photons.
Let's consider a charge in a freely falling frame of reference.
In such a frame, the local gravitational field is necessarily zero, and the particle does not accelerate or experience any force.
Thus, this charge is free in such a frame. But, a free charge does not emit any photons.
There seems to be a paradox. Does a freely falling charge in a gravitational field radiate?
Comment: I am sorry but "Let's consider a charge in a freely falling frame of reference." If it is falling it falls to some gravitational center, thus the statement " In such a frame, the local gravitational field is necessarily zero," is wrong . If the particle is in orbit, there is the angular acceleration and it will radiate. If it is free falling there is the acceleration of the gravitational field. The 1/r of a gravitational potential becomes 0 when r=infinity.
Comment: @annav this is true, but then the question could have been asked like this: "Imagine a static charge. It has an electric field but there is no magnetic field and hence no radiation. However, now let's transform into an accelerating reference frame. In this new reference frame we have an accelerating charge, so it should emit radiation. How can the charge emit photons in one reference frame but not in the other? And since an accelerating reference frame is the same as a gravitational field (Einstein's principle), does a gravitationally accelerated charge radiate or not?"
Comment: Duplicate of http://physics.stackexchange.com/questions/70915/does-a-constantly-accelerating-charged-particle-emit-em-radiation-or-not.  (While the linked question was asked later than this one, I believe that Ben Crowell's answer to that question is the authoritative one.)
Comment: possible duplicate of [For an accelerated charge to radiate, is an electromagnetic field as the source necessary?](http://physics.stackexchange.com/q/7014/)
Comment: No claims on correctness; there are some interesting points here: http://www.physicsforums.com/archive/index.php/t-72035.html
Comment: @annav That deserves to be expanded a bit and put in an answer :D
Comment: It's a very nice question, and I suspect the answer has to do with the fact that the electric and magnetic fields are components of a tensor in special relativity, which means that when you change reference frame the electric field can become magnetic and vice versa. I guess changing to an accelerating frame turns the particle's static electric field into an oscillating (radiating) electromagnetic one. But it would be nice to see an answer with the maths if someone knows it.
Comment: @Nathaniel this is a different question
Comment: The OP is asking about a particle accelerating in a gravitational field and then transforming to the frame where it's at rest; my version asks about a particle at rest and transforms to one where it's accelerating. While they are arguably different questions, Einstein's equivalence principle tells us they must have the same answer.
Comment: @Nathaniel I find a good article on this quastion http://xxx.lanl.gov/abs/gr-qc/0006037 thanks to  Manishearth
Comment: http://physics.stackexchange.com/q/89093/
Comment: There is a wikipedia entry for "paradox of a charge in a gravitational field."
Answer: The paradox is resolved as follows: the number of photons changes when you switch between non-inertial frames. This is actually a remarkable fact and it holds also for quantum particles, which can be created in pairs of particles and antiparticles, and whose number depends on the frame of reference.
Now, a step back. Forget about gravity for a moment, as it is irrelevant here (we are still in GR, though). Imagine a point charge, which is accelerating with respect to a flat empty space. If you switch to the rest frame of the charge, you observe a constant electric field. When you switch back to the inertial frame, you see the field changing with time at each point and carrying away radiation from the charge.
In the presence of gravity the case is absolutely similar. To conclude, switching between non-inertial frames makes a static electric field variable and corresponds to a radiation flow.
Another relevant point: When moving with charge, no energy is emitted, but when standing in the lab frame, there is a flux observed. However, there is no contradiction here as well, as the energy as a quantity is not defined for noninertial frames.
Comment: I am not at all convinced by this answer.  It feels heuristic at best.
Comment: To radiate is like to accelerate : it is an absolute. Therefore, if you radiate for someone, it must be possible to detect it in any other frame (inertial or not). A free falling charge does not radiate because it is inertial. A charge at rest on earth do radiate for a free falling observer. Yet, if you're also at rest with respect to the charge, even though you won't see it radiates, the electric field is curved because of strong equivalence principle ! Thus, the field of the charge acts on the charge itself in such a way that its all consistent, even from an energy point of view.
Comment: @sure, sorry, but it is not absolute. Consider a charge in vacuum - it certainly does not radiate for an observer, for whom the charge is at rest. However, an accelerated observer will see the energy flux. Therefore, the notion "radiate" is dependent on the observer. And therefore, some of your statements need to be restated with observers being specified.
Comment: @JerrySchirmer, by the way, even non-charged point masses may have self-force in GR, similarly to Abraham-Lorenz force.
Comment: " If you switch to the rest frame of the charge, you observe a constant electric field. " Could you please explain why? It is not clear what electric field will be, because it is no clear how the charge is accelerated and what is your definition of electric field in accelerating frame (ordinarily electric field is part of force due to other charges in inertial frame).
Comment: @JánLalinský: It is beacuase when you switch to an accelerated frame as an observer, metric will be constant with your proper time. If you write down Einstein-Maxwell equations, you will have equations for electric and magnetic fields, which do not depend on time. There may possibly be some flux components, though, I don't have a proof at hand.
Comment: Dear @JerrySchirmer, many thanks for pointing this out!
Comment: @JerrySchirmer, and yet, could you please kindly be specific in your statements? Is there any particular argument that you find wrong in my answer? Do you think some aspect of the question was not touched upon? Or in which way could in general the answer be better in your opinion?
Comment: @AlexeyBobrick: your answer doens't feel wrong to me, it just doesn't feel convincing -- in particular, I'd have trouble making this argument quantitative, to show how much radiation a charge $q$ will produce in a schwarzschild spacetime of mass $m$.  (I didn't downvote).  To me, the essential source of the problem is self-force, which I guess could be interpreted as a particle accelerating relative to its field.
Comment: @JerrySchirmer: I would say, the answer is also qualitative, it just asks why does the paradox appear. The paradox is resolved by two statements: 1) The charge in its own frame does not radiate only locally, 2) Additionally, static fields may turn into radiation when the frames are switched. So I just didn't go into calculations. And as you show in your answer, it takes an article size derivation to get the field estimates.
Comment: @AlexeyBobrick: fair enough :)
Comment: By absolute, I mean that if the charges radiate in an inertial frame, then there must exists a way to **know** it does. The same happens with acceleration. The non preservation of the syntaxic form of a quantity between frames does not mean that the quantity you're looking at isn't in some sense "absolute". That is, something non trivial happens somewhere, so something equivalent that is non trivial happens everywhere.
Comment: @sure: Well, I mean quite precisely, that if you take an observer and consider all the physical quantities (metric, fields, curvature) in his/her vicinity, you will get what observer 'sees'. This way, accelerated observer will 'see' that if it releases a test particle, the particle will get accelerated (cf to what you said). Similarly, accelerated observer can take a detector and measure the flux from a field of an inertial charge. Note, the charge didn't radiate for being inertial, but still can learn that the energy was extracted by a non-inertial observer (again, cf to what you stated).
Comment: An accelerated charge, seen from its proper frame, has curved field line and a non zero self-force. This is precisely what I mean by "you know you're accelerating even if you're in the accelerated frame". Or maybe I didn't understand something?
Comment: @sure: No, this is definitely fine, though I thought you meant acceleration in general. Yes, I do agree that an accelerated charge radiates non-locally to infinity, hence feels self-force. However, the radiation as a locally measured quantity is dependent on observer. I guess this settles the discussion.
Comment: Yes, we agree indeed.
Comment: How does this answer reconciles with the accepted answer of https://physics.stackexchange.com/questions/241522/is-the-number-of-photons-of-a-system-a-lorentz-invariant?
Comment: Lorentz boost is not the same as acceleration
Comment: > *"you see the field changing with time at each point. This naturally corresponds to appearing magnetic fields, and hence radiation"* -- Changing electric field is not sufficient to conclude there is radiation. Radiation means electric field disturbance spreads to infinity and decays with distance no faster than $C/r$.
Comment: Thanks a lot for your comment! You are correct, radiation does correspond to a C/r fall-off, and it requires more than just a changing electric field. However, my point was only that accelerated charges do radiate for inertial observers. I will edit the answer a bit to avoid confusion.
Answer: A recent answer by John Rennie linked this question as 'definitive' yet there are issues with the accepted answer by Alexey Bobrick.

The 'number of photons' mentioned makes one think that this is about a purely quantum effect. It is not. Motion and radiation of a point charge in curved space could be handled classically.
While it is generally true that in a curved background it is difficult to define invariantly what is radiated, there is a whole class of setups where it could be done globally and with ease (at least at conceptual level). Let us consider asymptotically flat spacetimes with time-like Killing vector field. Now consider the point charge that starts moving from infinity with constant velocity at $t=-\infty$ interacts with the nontrivial part of the metric around $t=0$ and flies away at $t=+\infty$. Killing v.f. gives us conservation of energy for the system 'charge ${}+{}$ electromagnetic field' and so the difference between initial and final kinetic energy would be well defined, and it had to be the energy radiated away. (Of course, if there is black hole horizon, 'away' may also mean into the black hole). Even for a charge in a bound motion the decay of bound orbits is such a case also would be observer independent. So if a charge's radius of orbit after, say, $10^{20}$ revolutions in a Schwartzschild metric becomes a half of its initial radius then no coordinate transform could erase the radiation at infinity.
One more note: this question is quite different from the question whether a uniformly accelerating charge radiates: there the motion is given,
the charge is being dragged by an external force, while here we are dealing with a charge and its field interacting with gravitational field without interference from additional forces, how the charge moves is unknown beforehand. And while for uniformly accelerated charge the main problem is how to extract radiation from the known EM field, in the current question we could sidestep this issue by restricting ourselves to asymptotically flat spacetimes. The main problem now would be to find the motion of the charge and by applying conservation of energy we would then know the energy of that was radiated away.

Jerry Schirmer's answer does link to a correct paper but does not provide an detailed explanation for the 'paradox'.
So here is the paradox resolution in terms of the question: in a freely falling frame gravitational field acting on a charge is indeed zero. However the charge would not be moving along a geodesic. Instead charge would be moving under the DeWitt-Brehme radiation reaction force:
$$
m{a^\mu} = f_{{\rm{ext}}}^\mu + {e^2}(\delta _{\;\nu}^\mu + {u^\mu}{u_\nu})\left({{2 \over {3m}}{{Df_{{\rm{ext}}}^\nu} \over {d\tau}} + {1 \over 3}R_{\;\lambda}^\nu {u^\lambda}} \right) + \\{} + 2{e^2}{u_\nu}\int\nolimits_{- \infty}^{{\tau ^ -}} {{\nabla ^{\left[ \mu \right.}}} G_{+ \,\lambda^{'}}^{\left. {\;\nu} \right]}(z(\tau),z(\tau^{'})){u^{\lambda^{'}}}\,d\tau^{'}, \tag{*}
$$
where $G_{+ \,\lambda^{'}}^{ {\;\nu}}(x,x^{'})$ is (retarded) Green's function of EM field (it is a bitensor: so indices $\lambda^{'}$ and $\nu$ correspond to (co)tangent bundles at different points), and integration is carried along the past motion of the charge. In the case of absent external force ($f_{\rm ext}=0$) and in a Ricci-flat metric this force is given only by a non-local integral.
Incidentally, original 1960 DeWitt & Brehme paper did not include the term with Ricci tensor. This was corrected in 1968 by Hobbs.
Green's functions in GR for massless fields have a richer structure than in flat space: it is generally nonzero inside the future lightcone of a point $x^{'}$, and so the integral would be nonzero. This  property  reflects  the  fact  that in  curved  spacetime,  electromagnetic  waves  propagate  not  just  at  the  speed  of  light,  but  at all speeds smaller than or equal to the speed of light, the delay is caused by an interaction between the radiation and the spacetime curvature. So the integral would generally be nonzero and the charge would radiate EM waves.
There is nothing mysterious in the non-local character of the force (*). It is the result of going from the system with infinite degrees of freedom 'charge ${}+{}$ elecromagnetic field' to a finite dimensional description in terms of charge motion alone. Locally point charge is acted upon by electromagnetic field. This electromagnetic field originated on the same charge in the past, been scattered by a gravitational field some distance away from it and produced a potentially non-zero force in the present.
Situation might be easier to understand if we consider the following flat space situation:  a point charge and a small dielectric ball at some distance $d$ from it. The ball gains dipole moment in the field of a charge and exerts a certain force on it. Now let us wiggle the ball a little around the moment $t=0$, then perturbations of EM field from this wiggle would be propagating and the original charge would feel additional force at time $t=d/c$. This is a usual Lorentz force, but since the only source of EM field is our charge we can write it as an integral of Green's function over the past worldline of the charge. This Green's function encodes all information about ball and its wiggle. And since there is diffraction on the ball, this function would be nonzero inside the future lightcone of its argument $x^{'}$. The force felt by the charge (both constant contribution and signal from wiggling) now would be written as an integral over the charge past, an expression similar to the DeWitt-Brehme force.
The actual calculations of Green's function in GR are quite complicated and for a sample I would recommend the review

Poisson, E., Pound, A., & Vega, I. (2011). The motion of point particles in curved spacetime. Living Reviews in Relativity, 14(1), 7, open access web.

Once that has been done the work done by the DeWitt-Brehme force allows us to calculate the energy of radiated EM wave. This has been demonstrated by Quinn & Wald:

Quinn, T. C., & Wald, R. M. (1999). Energy conservation for point particles undergoing radiation reaction. Physical Review D, 60(6), 064009, doi, arXiv.

who have proven that the net energy radiated to infinity equals to minus
the net work done on the particle by the DeWitt-Brehme radiation reaction force.
Comment: Very interesting discussion! Yes, your answer is more interesting and more to the point than mine. Indeed, there is self-force due to electromagnetic fields (thanks for the references!), and it makes electric charges deviate from geodesics. And it is an invariant statement even for spacetimes without Killing vectors.
Answer: The charge accelerates.  This is proven in a paper written by Bryce DeWitt and Robert Brehme in the '60s, cited in the paper at this link:
https://www.sciencedirect.com/science/article/pii/0003491660900300

Radiation Damping in a Gravitational Field, Bryce S. DeWitt, Robert W. Brehme, Annals of Physics: 9, 220-259 (1960)

The charged particle tries to do its best to satisfy the equivalence principle, and on a local basis, in fact, does so. In the absence of an externally applied electromagnetic field the motion of the particle deviates from geodetic motion only because of the unavoidable tail in the propagation function of the electromagnetic field, which enters into the picture nonlocally by appearing in an integral over the past history of the particles.

The article is out of print, and I had to look it up at a university library to read it.  The interesting part of the result is that the acceleration of the particle picks up a non-local term that depends on a path integral over the particle's path.
Comment: By the way, correct answer, but doesn't expicitly address the question. Indeed, the word 'locally' is the resolution of the paradox. The charge does not emit radiation locally in its comoving frame. And yet does so generally for accelerated or non-comoving observers.
Comment: There is a more recent article on this at https://link.springer.com/article/10.12942/lrr-2011-7
Answer: We have $F=m_1a$ where $m_1$ is the mass of the charged particle and $a$ the acceleration.
The gravitational force is $F=Gm_1m_2/r^2$.
Hence $a=m_2/r^2$ where $m_2$ is the mass of the large body (earth) towards which the charged particle is falling and $r$ is the distance from the center of gravity and $G$ the gravitational constant. There is always an acceleration, though when $r$ becomes very large the acceleration is very small and the photons emitted will be very low energy.
What is happening to the freely falling charged particle is that part of the potential energy it is giving up by falling, turns into radiated photon energy, rather than totally to velocity of fall towards the center of gravity, which will happen to an uncharged particle.
Here is a relevant theoretical  study of charge and acceleration.

The conditions in which electromagnetic radiation is formed are discussed. It is found that the main condition for the emission of radiation by an electric charge is the existence of a relative acceleration between the charge and its electric field. Such a situation exists both for a charge accelerated in a free space, and for a charge supported at rest in a gravitational field. Hence, in such situations, the charges radiate. It is also shown that relating radiation to the relative acceleration between a charge and its electric field, solves several difficulties that existed in earlier approaches, like the "energy balance paradox," and the "relativistic" nature of the observation of the emitted radiation

A more recent link is here. . It shows that a freely falling charge should not radiated after all. Only a  stationary one. See my answer to a newer relevant question here.
Comment: The quastion is in the plane of General Relativity. The free falling charge stay at rest in a local frame of reference. Does the observer, which is in the same frame, detect the radiation?
Comment: @anna v Dear Anna, thanks for your answer. My quastion was is in the frame of General Relativity. I find that the quastion is no agreement among physicist for today. It is clearly describe in this article http://xxx.lanl.gov/abs/gr-qc/0006037
Comment: You are using classical mechanics here. OP asks to interpret in terms of general relativity.
Comment: @Sergio you should be asking the question differently then, stating in the body of the question that you are talking of GR transformations. Still, your assumption that in the rest frame of the particle the gravitational potential is zero is wrong. It will be something complicated by the transformations to reach the rest frame of m1, but still there.  I expect that an observer at rest in the rest frame of m1 will not be seeing the radiation, in the same way as he/she would not know that the particle is falling and increasing its velocity in the total center of mass.
Comment: continued: the physical photons observed in the overall CMS would be cooled into the infrared by the same transformations to the point of being virtual, once one utilizes quantum field theory.
Comment: @Sergio I looked at the paper you linked. You have to realize that paradoxes arise in physics when mathematical frameworks are mixed. The mathematical framework of GR must not be mixed with the classical newtonian gravity, which happens if you imply GR and want to see gravitational fields. There are no gravitational fields in GR , just distortions of space-time. The statement " the local gravitational field is necessarily zero" is wrong because it mixes two such mathematical frames if you are implying that you are thinking in the GR framework. Solutions need  one consistent framework.
Comment: p.s The paper you linked is interesting and comes to the same conclusion as my newtonian argument in the comments.
Comment: Bit late, but doesn't this answer violate the weak equivalence principle? You suggest that a charged particle in a gravitational field accelerates slower than a neutral one. But gravity knows nothing of quantum numbers, charges etc because of the WEP.
Comment: @innisfree  Gravity at the moment knows nothing of quantum mechanics in a consistent framework. A charged particle necessarily is a quantum mechanical entity. We have to wait for the unified q uantum theory to be able to see what the weak equivalence principle will look in it. Till then semiclassical arguments have to hold, and it is an experimental fact that accelerating or decelerating charges emit radiation, and this energy will come from somewhere.
Comment: What experiment demonstrates that charge accelerated by gravity radiates? I agree that charges accelerated in EM fields radiate.
Comment: @innisfree our intuition that equivalence should hold from the quantum mechanical side too? here is a preprint that calculates this http://cds.cern.ch/record/403583/files/9910019.pdf , to be consistent with the equivalence principle. Maybe some day black hole observations might confirm this. It would be a small effect to the radiation emitted by by the hot gases , which is the standard explanation  I find for xray sources being black holes.
Comment: The abstract you quote is interesting, however; it says that charges supported at rest in a GF radiate! That means an electron sitting on the earth's surface. That is controversial. Without reading more, I can't follow how energy is conserved from the back reaction.
Comment: @innisfree I think it is a theoreticians idea of not bothering how it will be at rest. They want to stress that it is the distortion in the field that generates radiation  A free electron at rest in vacuum would take the energy from the gravitational potential. Sitting on the surface electrons are bound, Bound electrons do not radiate, which is the point of quantum mechanics.
Comment: @innisfree: Adding general relativity or quantum mechanics to the answer will not change the result.  There is no reason to complicate the answer with these things, which will cloud the physics. +1
Comment: The paper of Harpaz and Soker is beautiful, simple, and should be taught to any student in general relativity. It clearly close the question (I'm now totally convinced that there's no problem with electrodynamics and equivalence principle), and one can even conclude that the earth on which a charge lies on the ground Hawking's radiate as a result. Really beautiful piece of physics there.
Answer: A charge is surrounded by an electric field, which can be considered "attached" to the charge, moves with it, and stretches off to infinity.  It is just as much a "physical" object as the charge itself and has mass/energy and momentum density if moving.  Gravitationally accelerating a charge also gravitationally accelerates the local electric field around it, but it does not accelerate the parts of the electric field that are far away from the source of gravity.   These far way parts of the field will exert some drag on the charge and represents the energy lost to radiation.  When you realize the electric field stretches off to infinity you realize that a charge is a non-local object, and hence it is inappropriate to apply the Principle of Equivalence.
Comment: Principle of equivalence works everywhere you apply it. Check the article gave by anna v
Answer: I can't follow most of the answers and comments. I would like to keep
the question as simple as possible (which does not mean easy). First
of all, I find quite improper to involve QM, still worse QFT. It is
well known that relations between GR and QM are far from being settled... So I will insist on keeping the discourse strictly classical. I would like to see an answer to the following question
(not original, but not answered here, as far I can remember.)
We are on a lonely, non-rotating planet (or cold star it you like better). At some distance from its surface there is a stationary charged body. E.g. the body is kept fixed by a solid vertical bar raised from planet's surface. It seems to me that someone stated that in such situation, thanks to equivalence principle, the charged body is radiating. The obvious objection is: where does the radiated energy come from?
Let me explain better. The physical situation, as far planet and
charged body are concerned, is stationary. Moreover, there is no doubt
that spacetime is asymptotically flat. To me radiation means an energy flow whose total flux through a sphere of radius $r$ has a finite non-zero limit as $r\to\infty$. Since GR says that energy is globally conserved, I find it to forbid a continuous energy flux.
Please show where the error is in my reasoning.
Comment: Radiation does not necessarily mean there is non-zero Poynting flux, or non-zero EM energy, going out  through a sphere. It means, based on the known solutions of Maxwell's equations for accelerated charged points or oscillating currents, that from some distance on, magnitude of dominant component of EM field decays with distance $r$ as $C/r$ - no condition on the Poynting flux or energy flux is involved. For example, if there is equal intensity coming out as coming in, like in the half-advanced, half-retarded case, total Poynting flux may be zero.
Answer: This is an interesting topic, but I don't think the issue is fully resolved. Suppose there is a stationary charge in a gravitational field will it radiate ?
If we apply equivalence principle, it seems that it will radiate, since the gravitational field is equivalent to an accelerating frame. Hence the stationary charge in a gravitational field is equivalent to an accelerating charge in a zero-gravitational field. Hence it should emit radiation.Some physicists says, that won't happen. This is a true paradox which challenge the principle of equivalence.
Secondly consider an uncharged rocket which is accelerated by burning the fuel. Suppose, to get an acceleration of 1g we have used 1kg of fuel. How much fuel we have to use to accelerate a charged rocket ? less than 1kg of fuel or greater than 1kg of fuel ? If the charged particle emit radiation, then the second rocket need more fuel. But some physicist calculations shows that we need the same amount of fuel? Then there is a paradox !,What about the energy ? what is the difference between charged and uncharged rocket ?
I think this is the topic we physicists have to explore to uncover the mystery !
Actually there is a book titled $\textbf{Uniformly Accelerating Charged Particles Threat to the Equivalence Principle}$ by  $\textbf{Stephen N. Lyle}$ dedicated to explain this paradox.
Comment: There's nothing left to be resolved.  General relativity predicts that the particle will radiate.  This has been known since the '60s.
Comment: What about the equivalence principle. Is it valid in all the situation ? Can you apply the equivalence principle to an electron ?
Comment: Not globally.  And the electromagnetic self force breaks the premise of the equivalence princple.
Comment: OK, If this is true, how can you say that there is nothing left to be resolved ! If the equivalence principle cannot be applied in the quantum situation how can you surely say that we answered this question completely ?
Comment: What does it have to do with quantum mechanics?
Comment: Radiation is a pure quantum phenomena.
Comment: It has a classical explanation.  Single photon states aren't eigenstates of the hamiltonian, anyway.
Comment: Classical theories are the special cases of quantum theory when $\hbar\to0$ or $m\to\infty$ and photon itself is a quantum concept !
Comment: I understand the classical limit.  Show me a "virtual photon" state.  It's nonphysical nonsense that can clarify some things, but is also not observable and dependent on perturbation theory.  The field is what is real, the photon states are abstractions of the field.  Talking about virtual photons in answering this question is more misleading than just using the classical picture.


Galaxy S3 is unable to transfer files over USB: "USB device not recognized"

Question: I have Galaxy S3 with baseband version I9300XXLFB and I'm having problems when it is connected to the PC. I cannot transfer or retrieve any files or data over USB.
When I plug it in to my PC, it tells me USB device not recognized and the S3 does not appear anywhere for me to access it. I've tried different cables and PCs and they all seemed to exhibit the same issue. How can I get this working?
Comment: Is Settings > Developer options > USB debugging ticked?
Comment: Is the device unlocked at the moment you plug it? Have you checked the device manager (in case you're using Windows 7)?
Answer: You may need to use Samsung KIES (software for PC), or get your PC to recognize the 'new' "MTP" USB mode.
further background:
Ice Cream Sandwich supports USB mass storage after all, Galaxy Nexus does not
Wikipedia >> Media Transfer Protocol
Samsung >> SUPPORT >> Kies 2.0

Also see: Can I disable MTP mode and just have a regular USB connection?
Answer: For anyone having trouble connecting to Windows using USB in MTP mode, make sure you install the Samsung USB Driver for Mobile Phones from Samsung's website.  Here's a link for the Verizon model:
http://www.samsung.com/us/support/owners/product/SCH-I535RWBVZW
Navigate to the Manuals & Downloads tab, and then below the tabs, click Software.  There you will see the USB driver listed.
Answer: I had a similar problem connecting my Galaxy S3. The drivers were not installing properly. There were messages saying the USB device as malfunctioning. Despite being able to charge the phone, it was not being recognised as a mass storage device nor offering me an option on the phone as to what I would like it to connect to the PC as, until I changed my USB cable.
I had read some posts that had suggested this. After trying all the phone and device manager settings from debugging to reinstalling and updating drivers, this is what worked: trying a new USB cable. The drivers installed themselves. Then, I was able to select Use as Media Device from the phone notificaton menu. I double clicked My Computer, and opened Phone to display media. I was also able to select USB tethering from phone settings to allow connection to the internet, which is what kicked this all off in the first place.
Hope this helps some others.
What I also discovered during this process was how to set the phone up as a wi-fi hotspot. I just followed a neat youtube video detailing this, and was able to connect my laptop to the internet while I sorted out the rest of my problem.
Answer: I had a similar problem, but with no error message.  My computer just did not see my phone even though the computer recognized any other USB device i put in.  Note: on a previous session my computer DID recognize my phone and I was able to b/u some files.
My final solution was fairly simple, while the phone was still on I removed the battery from the phone and left it out for 5 minutes.  Then I reinstalled the battery and turned the phone on.  After plugging the phone into my computer via the USB cord I had a connection phone to computer.
No, I do not have a technical explanation for this.
Comment: This didn't work for me, but in opening up the case I did discover that it was desperate need of a cleaning, so it was helpful in that regard.
Answer: Have a look to the following Steps
Step 1 # Try cleanup and reinstall of drivers

http://www.droidforums.net/forum/galaxy-s3-general-discussion/216706-cant-connect-s3-pc-usb-3.html#post2340704
Pull the SIM card out and do a factory reset with format

Step 2 # You can to use Samsung KIES (software for PC), or get your PC to
   recognize the "MTP" USB mode.

Can I disable MTP mode and just have a regular USB connection?
Samsung Galaxy S3 not connecting via USB

Step 3 # Make sure you install the Samsung USB Driver for Mobile Phone

http://m.samsung.com/us/support/downloads
http://www.samsung.com/us/support/owners/product/SCH-I535MBBVZW
Driver issues when using Samsung Galaxy SIII via USB for development

Step 4 # For file transfers, you can use Airdroid, which connects through wifi.

http://www.airdroid.com/
Answer:
Install the latest driver
Ensure you are using the factory cable that came with the phone (some cables wont work with the S3, the computer will even lie to you and give you error saying the "USB device not recognized")
Answer: Just use KIES AIR (pre-installed on your S3). Works perfectly and is user friendly.
Answer: Before updating my Galaxy S III to Android 4.3, didn't have any problem with Windows 7. It's obvious that W7 should have recognized it even if Kies software were not installed. That was with main before updating to new android.
But saddlery W7 was not able to find the Galaxy SIII in Media device (MTP) mode!
Solution for me:
Installing: Device(Install),USB Drive (Software) - Nov 9, 2012
Answer: you might need to clean the micro usb port. Try using a toothbrush or a safety pin to get the lint out. This worked for me
Answer: You need to use USB 2.0 ports only.
Answer: If you are using Android 4.3 open the dial pad and dial *#0808#
Android 4.2 *#7284# this will solve your problem.
Comment: What, exactly, do those codes do? How will they help?


Can I invest in the housing market via the stock exchange?

Question: Buying houses is expensive these days, and interest on mortgages is extortionate.  Do investments via the stock market exist where you can invest in the housing market?  Maybe shares in real properties, or maybe some physically backed funds, or something along those lines?
Answer: You could look into an index fund or ETF that invests primarily in Real Estate Investment Trusts (REIT's). An REIT is

any corporation, trust or association that acts as an investment agent specializing in real estate and real estate mortgages

Many investment firms offer an index fund or ETF like this. For example, Vanguard and Fidelity have funds that invest primarily in real estate markets. You could also invest in a home construction ETF, like iShares' ITB, which invests in companies related to home construction. This ETF includes more companies than just REITs, so for example, Home Depot is included.
Comment: @Victor [Since 2008](http://web.archive.org/web/20080801195119/http://www.asx.com.au/products/real_estate_infrastructure/property_trusts.htm), LPT's are now known as REIT's as well (technically A-REIT's).
Comment: There is also what is called Listed Property Trusts (LPT), which are basically mutual funds invested in property trusts which are listed on a stock exchange.
Comment: I just checked in Australia and you are correct John. I don't really invest in them so I wasn't aware they were both the same thing. Thanks for the correction :-)
Comment: Post Properties (NYSE:PPS) is landlord to about 20,000 middle class renters/families across Atlanta, Dallas, Washington DC, and Tampa.    http://www.postproperties.com/post/investor/home.aspx
Answer: Have you considered a self-directed IRA to invest, rather than the stock market or publicly traded assets? Your IRA can actually own direct title to real estate, loan money via secured or unsecured promissory notes much like a hard money loan or invest into shares of an entity that invests in real estate. The only nuance is that the IRA holder is responsible for finding and deciding upon the investment vehicle.  Just an option outside of the normal parameters, if you have an existing IRA or old 401(k) or other qualified plan, this might be an option for you.


Reconciling the accounts of the calling of Simon Peter

Question: How do you reconcile the two accounts of Jesus calling Peter? (Matt 4:18-19 and John 1:35-42)
In the Matthew passage, Peter and Andrew are together fishing. In the John passage, Andrew was with John the Baptist and Peter was else where?
Answer: I don't see an inherent conflict between these passages.  Specifically, the John passage describes how both Andrew and Simon initially met Jesus; the Matthew passage describes how He called them as disciples.  Matthew never claims that the event by the seashore was Jesus' first encounter with the brothers.
Edit: In the account of John, Peter meets Jesus just after Jesus had been baptized, been tempted in the wilderness, and came back (John 1:32-34).  Peter's calling is in all of the gospels.  In Luke, Jesus was tempted in chapter 4.  Afterward, in chapter 5 (coinciding with the chronology in John 1), he calls Peter as a disciple (his calling is paralleled in Matt 4 and Mark 1). In Luke 4:38-39, just before Peter and Andrew were called, Jesus went from the synagogue to the house of - wait for it - Simon!  He went to heal His mother in law!  This is before Jesus called him as a disciple.  Also, in 5:3, he first refers to Peter simply by the name of Simon, just like in 4:38, so we can safely assume it is the same Simon whose house He had visited (otherwise Luke should have/would have given him a distinction, like Simon the fisher, in order to clarify it wasn't the Simon discussed only about ten verses ago in chapter 4, right?)  Jesus called him after healing his mother in law!  Of course He could have met Him as discussed in John 1 with his actual calling taking place soon after!  Does anybody see a flaw or hole in this line of logic?
Comment: Given that the fundamental circumstances are clearly different, and the John passage explicitly claims to be a first meeting between Jesus and the two men, that seems to be the clearest way to read this.
Comment: +1 While I agree there need not be a contradiction, Matthew sure makes it sound like the disciples had never met Jesus before they were called.  Bonhoeffer in _The Cost of Discipleship_ characterized the call as answered by faith and not reputation or former friendship or any prior contact.
Comment: @GalacticCowboy: So you are saying that the John passage takes place first so that by the time the Matthew passage takes place, Peter and Andrew would have already known Jesus, as Jon said in his comment?
Answer: Harmonizing John 1, Mark 1, Matthew 4:19, and Luke 5:1-11
All of these passages speak about a call of Peter.  But only Mark and Matthew recall the same event.  The order in chronology is John, Mark and Matthew, Luke.
JESUS MEETS PETER
In John 1, Jesus in introduced to Peter by Andrew.  Andrew was following Jesus since the previous day based on his former teacher's (John's) testimony that Jesus was the lamb of God.  Obviously, though Andrew did not know Jesus, he was taught by John about the Messiah who was to come.  So, when John said, "Look, there is the Lamb of God," Andrew knew that John was talking about the Messiah.  Hence, Andrew followed Jesus.
Andrew then brings his brother Peter to Jesus, saying "we have found the Messiah."  And Peter is named Cephas by Jesus.
JESUS CALLS PETER
NEXT, Matthew and Mark agree that Jesus was tempted before he called Peter to be a disciple, "a fisher of people."  In that call scene Jesus saw Peter casting a net into the water.  And Jesus called to Peter and he followed Jesus who was walking along the shore.
JESUS HEALS PETER'S MOTHER-IN-LAW
In Mark Jesus heals Peter's wife's mother after Peter is called.  Then it is mentioned how many are healed.  In Matthew there is no mention of the healing of Peter's mother-in-law but there is mention of many being healed after Peter's call.
In Luke Peter's mother-in-law is healed before the scene recorded in Luke 5:1-11
JESUS CONFIRMS PETER'S CALL
Luke 5 has Jesus standing by the sea of Galilee when the fishermen are washing their nets.  And Jesus has a crowd of people gathered around Him, listening to the Word of God.  This is not the same scene we see in Mark and Matthew because Jesus posture is different, and the boats are parked along the edge of the sea, not out in the water.  The fishermen are washing their nets, not casting them out.
Some have postured that this is the same day as the scene recorded in Matthew and Mark.  Hence, Matthew and Mark recorded the early part of the fishing day (which would have been the evening before because Peter says in Luke that they were "fishing all night") and Luke records the end of the fishing day.
Such a scenario would require that Jesus was there all night long watching the fishing, or that He was there in the evening, then returned the following morning.  This understanding is possible but there is no reason to demand such a scenario.  More troublesome is the fact that Luke lists the healing of Peter's mother-in-law before his call scene in 5:1-11.
Either Luke or Mark is out of order chronologically or Luke has recorded a different event from Mark & Matthew.  The latter is more likely.
Comment: Actually Peter's mother in law is healed in Matthew,but doesn't occur until chapter Matt 8:11.
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Answer: Before reconciling the synoptic account, generally, with John's account, it is first necessary to reconcile the different versions of the synoptic account.

In Mark 1:16, Jesus sees Simon and Andrew his brother casting a net into the sea and calls them to follow him, and that he will make them fishers of men. Later, in verses 1:29-30, he visits the house of Simon, where Simon's mother-in-law lies ill.

Matthew 4:18,8:14 follow Mark reasonably closely and do not present a contradiction against Mark.

In Luke 4:38, Jesus enters visits the house of Simon, where Simon's mother-in-law lies ill. Later, in 5:3 he finds Simon washing, not casting, his net and miraculously tells Simon how to catch a great haul of fish - Jesus says he will teach Simon to be a fisher of men. Andrew does not seem to be present and is only mentioned in chapter 6, where all the disciples are listed.

Wikipedia tells us, "The most widely accepted hypothesis today, however, is that Mark was the first gospel and was used as a source by both Matthew and Luke, together with considerable additional material." If Mark was the source used by the authors of Matthew and Luke, it ought to be the most accurate, at least of the synoptic gospels, and Luke should be ignored wherever it contradicts Mark, such as in the chronology of events. Comparisons should ideally be limited to Mark and John, although using Matthew presents no problem in this instance.
Issues to be reconciled
In both Mark and Matthew the meeting by the Sea of Galilee seems to be the first time Jesus has met the brothers, and we learn in Mark 3:16 that Jesus surnames Simon as Peter, with Mark indicating that Simon's name was changed on this occasion.
In John 1:35-42, Jesus most certainly first met Andrew with John the Baptist and first met Simon shortly afterwards, when Andrew called him. It was at this time (John 1:42) that Jesus called Simon by the name Cephas, or Peter.
In Matthew 3:13-14, John the Baptist immediately recognises Jesus, but John 1:31,33 has the Baptist twice saying that he had not known Jesus when he came to be baptised. However, Mark makes no mention of this, so the contradiction can be overlooked for present purposes.
Explanation
The scope of the problem with any attempt to harmonise the synoptic gospels and John's Gospel is summarised by Burton L. Mack, who says in Who Wrote the New Testament, page 177 that the Jesus, setting and storyline of the fourth gospel can not be aligned with the synoptics. It has not been possible to meld John's Jesus with that of the synoptics as if each had merely emphasised different features of the same historical Jesus.  He says John's Jesus is an altogether different kind of being.
Elaine Pagels says in Beyond Belief, pages 61-63, that she discerns in John's Gospel a distinct bias against Peter. John Dominic Crossan, in The Birth of Christianity, page 566, also points to typically oblique examples of the exaltation of the Beloved Disciple over Peter in John's gospel. So, a theme of John's Gospel seems to be to undermine what its author might have felt had become excessive veneration of the apostle Peter.
In keeping with the resolve to minimise the role of Peter in the gospel story, he is no longer the first disciple selected by Jesus, only following Jesus at the insistence of his brother, Andrew. At the same time, Andrew becomes the first to declare Jesus to be the Messiah, or Christ, at the very beginning of the gospel (John 1:41). In Mark's Gospel, Peter is the first disciple to venture that Jesus is the Christ, and only at verse 8:29.
John does not include a list of the disciples, corresponding to 3:14-19, so simply moves the renaming of Simon forward to this first meeting, where the difference passes unnoticed.
Answer: There are 2 stories here. The first is when Andrew and Peter first meet Jesus in John's account. The second is when Andrew, Peter, James and John are called as disciples, as described in the synoptic gospels.
In the first story, Andrew and another (often believed to be the apostle John, Andrew's business partner) is with John the Baptist when John sees Jesus and proclaims he's the lamb of God (Jn 1,36). Andrew starts to follow Jesus, and tells his brother Peter. Jesus therefore meets Andrew, Peter, and possibly John (Jn 1, 40-42).
Some time later, while John the Baptist was imprisoned (Mk 1, 14) we get the second story, with Mark and Matthew providing the shortened version, and Luke writing the more extensive account. Jesus is preaching and there are lots of people crowding around, so he gets into a boat and pushes out a little way so they can see and hear him (Luke 5, 1-3). After he's finished, he tells Peter to go fishing again - which he reluctantly does after explaining he had spent the entire night fishing without a catch, and they get a huge haul of fish. Peter (and implicitly Andrew) call out James and John in another boat to help get the catch ashore, and even with the additional boat the nets are breaking and the boats nearly sink (Luke 5, 6-10).
The accounts in Mark and Matthew are more brief. Here, there is no description of the crowds, the preaching from the boat, the previous night's fishing or the huge haul of fish. Instead, Mark tells us Andrew and Peter were fishing, and James and John were mending nets. Jesus calls them and they follow (Mark 1, 14-19) and (Matt 4, 18-22).
All these accounts fit together. Andrew, Peter and John met Jesus before he calls them to be disciples. Jesus is preaching and uses Peter's boat to be able to speak to more people. After finishing they fish, bringing in a huge haul that breaks nets and follow Jesus. In the shorter versions, they fish, mend nets and follow Jesus.
Comment: Welcome to Biblical Hermeneutics Stack Exchange Arranblonde, thanks for contributing! Be sure to take our [site tour](http://hermeneutics.stackexchange.com/tour) to learn more about us. [We're a little different from other sites.](http://meta.hermeneutics.stackexchange.com/a/803/423)
Answer: A basic mistake is often made when reading John's gospel as if it was an account parallel with the other three gospels. It is radically different, written at a different time, and for a different purpose.
John was the last apostle to write anything, and his writing was done when he was in his nineties. He was the only apostle left alive, and he was phenomenally old, for that era. He had received the visionary Revelation while on the penal island of Patmos, and obediently wrote down everything heaven had told him to write about events from his day to the return of Jesus Christ. That was what informed his gospel account. That is why it does not read like a biography and is not in chronological order. As one astute Christian author puts it, regarding John's gospel:

"Here is a man to whom God has drawn curtain after curtain, veil after
veil, till the the essence of things lies all revealed before the
conscious awareness of his inward sight. With his calm interior vision
he discerns the primeval genesis at the utmost reaches of time; he
sees the elemental meaning of things; he perceives in the visions of
the Almighty the purpose at the dawn of Creation...
His eyes beheld, he discerned, he saw the divine and heavenly light of
the glory of God in the face of Jesus Christ... He saw the hidden
mystery, of which Paul says, 'which from the beginning of the world
hath been hid in God'... That was how he saw the beginning. That was
why he wrote in the beginning. 'These are written that ye might
believe that Jesus is the Christ, the Son of God; and that believing
ye might have life through his name..' That 'believing' meant belief
in him as he had been revealed. And that revelation commences 'in the
beginning', John 1:1." John chapters 1 to 12 - The First Six Signs,
John Metclfe, pp.16-17
http://www.johnmetcalfepublishingtrust.co.uk/contact_us.htm

If John's Prologue is not appreciated, lots of strange things will appear to the reader who supposes his is but one of four (similar) accounts. No. John's gospel serves a unique purpose, only revealed to John as the end of the first century was about to break. That is why he writes what he does about Jesus having Andrew bring his brother, Simon, to him, and Jesus saying he would be called Simon Peter. On the morrow, Jesus commands them, and Philip and Nathaniel, to follow him. The next thing John records is the beginning of Jesus' signs, in Cana of Galilee, manifesting his glory, and his disciples believing on him (John 2:1-1).
But notice what John leaves out - he never mentions the baptism of Jesus (just what John the Baptist said about the Spirit's witness at that time.) He says not a word about the 40 days Jesus immediately spent thereafter, being tempted in the wilderness. Indeed, John leap-frogs from the logos creating everything "in the beginning" to the arrival of John the Baptist; as if nothing worth mentioning (for the purpose of his singular account) happened from creation till John the Baptist starting to prepare the way for Jesus. Think well on that. The Prologue is the key to grasping why John wrote what he did, and why he left out what he did.
Matthew's account of Jesus calling Andrew and Simon might well read very differently, for Matthew was long dead by the time John wrote his account, whereas John had just received heavenly visions of the risen Christ sharing the throne of God in heaven, with events leading up to his return. But please note that neither account contradicts the other. Different details are given. Put them together to get a fuller account. Yet Matthew's details served his purpose in writing mainly for a Jewish readership, while John was writing for Jews and Gentiles already suffering Roman persecution, with worse to come (as his visions had informed him.)
John wrote about Simon Peter before John the Baptist had been thrown into prison. Two of John's disciples believed John's testimony upon seeing Jesus, "Behold! The Lamb of God that takes away the sin of the world". They had been prepared for the coming Messiah by John's baptism. When Messiah appeared, they immediately followed him. Then three more men were called and started following, likewise so prepared by John. The first sign Jesus performed at Cana was before John was thrown into prison.
Matthew wrote, "Now when Jesus had heard that John was cast into prison, he departed into Galilee..." ch. 4 vs. 12, and in verse 18 records an incident by the sea of Galilee regarding Andrew and his brother, Simon, but calls him Simon Peter without mentioning the initial naming of Simon by Jesus prior to the miracle at Cana. Comparing the two accounts shows how the different authors had different details to record, according to the different purposes of their accounts. Not contradictory - different. Put them together, and there's no problem.
Answer: I am no scholar, but I am not concerned with the differing accounts.  They were written by Jesus' followers well after these events took place - likely as they aged.
I am 65 years old.  If I wrote my account of a childhood family experience, and shared recollections with my siblings, you can be sure our account and timelines would differ.  Also, the absence of collusion adds further credibility to the accounts.  They could have met together and said, " Okay guys, we need to get this right. Our accounts need to harmonize if we expect anyone to believe us.  That obviously did not happen.
What is important is the underlying message and harmony of the saving gospel message and divine identity of Jesus.  On this, they were without a doubt, on the same page.
Comment: Welcome to BHSX.  Thanks for your contribution.  Please remember to take the tour (link below) to better understand how this site works.  This answer could be improved with some references.
Answer: Here you get the reconciliation
I give you the timeline of incidents that occurred as below,

Jesus got baptized at BethaBara (John 1:28-34)

Next day John and Andrew met Jesus and stayed with him. Andrew testified Jesus to Peter (John 1:35-41)

Jesus was taken to the wilderness and tempted by the tempter (Matt 4:1-11, Luke 4: 1-13)

He went into Galilee and preached Gospel (Matt 4:12 -17, Luke 4:14,15)

He preached in Nazareth and people tried to kill him (Luke 4:16-30)

He preached in Capernaum (Luke 4:31-37)

He went to Bethsaida to heal Peter's mother in law (Luke 4:38-39)

Preaching gospel around Galilee (Luke 4:40–44)

Meeting Peter in Bethsaida (Matt 4:18-22, Luke 5:1-11, John 1:42)

Went to Capernaum (John 1:43,44, Matt 4:23)

I considered there is a time gap between John 1:41 and John 1:42.

40 Andrew was one of the two disciples who had followed Jesus. They had heard what John had said about Jesus. Andrew was Simon Peter's brother. 41 The first thing that Andrew did was to find his brother, Simon. Andrew said to Simon, 'We have found the Messiah.' ('Messiah' and 'Christ' mean the same.) 42 Then he brought Simon to Jesus. Jesus looked at Simon and he said, 'You are Simon, John's son. Your name will be Cephas.' This name is the same as Peter, which means 'rock'. (John 1 40-42)Easy English version)

Note the 'then' at the beginning of verse 42. The Greek word καὶ is generally translated as 'and' in English. But it has many more meanings in the root language.
If we consider this gap, then everything fits. I considered the basic understanding that Gospel writers choose what they record according to their purpose of writing it. So the apostle John came straight to the meeting of Jesus and Peter, ignoring so many other incidents in between as said at the end of his gospel:

25 But there are also many other things that Jesus did; if every one of them were written down, I suppose that the world itself could not contain the books that would be written
Comment: Hi Julian, welcome to BH-Stack Exchange, we are glad you are here. Please be sure to take the [site tour](https://hermeneutics.stackexchange.com/tour) and read our [code of conduct](https://hermeneutics.stackexchange.com/conduct). Thanks!
Answer: The event in Matthew at the sea and the one in Luke do not seem to describe different events. Matthew is a summary of the meeting described in Luke - he saw them fishing in Matthew and in Luke it details how they went out and fished together. I see no reason to take  Matthew's account to mean "as his eyes laid on them they were fishing". Initially they were washing nets but he told to go cast the nets out again. Hence his interaction with them involved fishing and it is accurate to say, "He saw them fishing," since Matthew felt that was the focal point of the story, especially considering Jesus' words echoed in Luke, "I will make you fishers of men."


How to Run A DB Sync Server On Daedalus?

Question: So I have been trying to do some fun things with Daedalus, and realised I should be able to query the node from it. This makes me wonder how to set up a limited database / querying system for my own needs out of Daedalus? Kinda like a smaller blockfrost.
Answer: The node by itself is stripped down of resources to keep active records of history, so the expectation that you could query node from Daedalus to have similar capability as cardano-db-sync (which maintains kinda everything from chain, as well as from ledger events, alongwith historical view). Thus, the resource requirements from dbsync are expected to be much more.
DB Sync v13 will allow you to stop querying ledger-state data which will shave off ~8-10GB (at the expense of not saving off-chain calcs coming from ledger-state). Instead if you're after creating small-scoped utility while not being able to spend much resources, you can look at projects below:

Ogmios / Kupo
Oura / Scrolls

Of course - the more you want to store and manage historically, higher the resources required will be
Answer: You can run cardano-db-sync, but be aware that is a rather heavy weight application that requires lots of RAM (ie 32 Gig) and will take about a week to sync.
Comment: @ErikdeCastroLopo If you run it long enough, you will see that one of them will eventually be killed with OOM error. It happened to me last week on a VM with 32 GM RAM. I upgraded it to 40 GB RAM and I also increased the swap (which was only 4 GB).
Comment: 32 GB will not be enough if you are also running cardano-node on the same machine. Or you will need to add a lot of swap...
Comment: Yeah looked at the repo this is not a viable option, for my computer and its over speced to train AI. That means I Have more Storage and RAM then most people.
Comment: @George-APEXStakePool I run a `node` and `db-sync` in a VM with 18 Gugabytes (16 was not enough).


Continuum mechanics: Finding the material derivative of plasma subjected to a decaying electric field

Question: I have been given the fact that a fluid is subjected to a decaying electric field of (scalar) magnitude: $$ e(\boldsymbol{x},t)=r^{-1}e^{-At},$$$$r^2=x_1^2+x_2^2+x_3^2$$ where $A$ is a positive constant, and the Eulerian velocity of the fluid is:
$$v(\boldsymbol{x},t)=(x_1x_3,tx_2^2,tx_2x_3)$$ and I have to work out the material rate of change of $e$ at time $t=1$ of the particle at the point $\boldsymbol{x}=(2,-2,1)$. I haven't really done much mechanics in maths so the 'decaying electric field' is kind of new to me.
I know that the material derivative is given by $$\nabla{e}{\cdot}v+e_t$$
So my questions is basically... how do I work out the gradient of $e$ when I have been given it in this form? Do I need to put in the given $x$ co-ordinates first, to obtain $r^2$ and then work out $\nabla{e}$ from there? Any help would be much appreciated!
Answer: First off MHD is a topic close to my heart!!
The material derivative is not as you have given (or a typo) and instead is given by
$$
\partial_t e + \mathbf{v}\cdot\nabla e
$$
To compute the $\mathbf{v}\cdot \nabla$ we go as follows
$$
\left(x_{1}x_{3},tx_{2}^{2},tx_{2}x_{3}\right)\cdot\left(\frac{\partial}{\partial x_{1}},\frac{\partial}{\partial x_{2}},\frac{\partial}{\partial x_{3}}\right)
$$
This equates to
$$
x_{1}x_{3}\frac{\partial}{\partial x_{1}} + tx_{2}^{2}\frac{\partial}{\partial x_{2}} + tx_{2}x_{3}\frac{\partial}{\partial x_{3}}
$$
for which you can sub back into the original "material derivative".
Then to complete your answer, you can compute the derivatives of r with respect to $x_{i}$ as normal. Alternatively you could of converted all the derivatives into cylindrical or spherical geometry, but I think the way i outlined above is more straight forward.
To explicitly show how to take the derivative, I will compute the first term.
$$
x_{1}x_{3}\frac{\partial e}{\partial x_{1}} = x_{1}x_{3}\frac{\partial }{\partial x_{1}}\left(-\frac{1}{r}\mathrm{e}^{-At}\right) = \mathrm{e}^{-At}\frac{-x_{1}x_{3}}{r^{2}}\frac{\partial r}{\partial x_{1}}
$$
You can finish off the differentiation.
So now since i decided to stay in these cartesian co-ordinates, it makes compute the derivatives more cumbersome, but not impossible. You will have to do the same for the derivatives with respect to $x_{2}$ and $x_3$.
Comment: haha you're completely right, sorry about that, haven't got this whole latex thing down... okay that makes a lot of sense, thanks!
Comment: I think it is counterintuitive to compute $\mathbf{v}\cdot\nabla$ first, wouldn't you get $\nabla e$ first and then compute the dot product?
Comment: @kleineg The material derivative is stated as $$\frac{D}{Dt} = \partial_t +\mathbf{v}\cdot \nabla $$ and then apply this operator to the quantity you want to look at. In physics, it is used to look for conserved quantities over that particular flow i.e. when it is set to zero. But having looked at your profile, you know this :).
Comment: so how would I apply this operator to e when it is in this form? I'm confused by the $r^2$
Comment: I have modified the answer to include how you would compute the derivatives.
Comment: for the first term that you have written, would I need to put in the partial derivative of r with respect to $x_1$?
Comment: yep. Similarly for the other derivative terms. Let me know if you need me to edit it into the answer. ps cheers for accepting :)


Macbook iSight not working

Question: I've tried to get my iSight camera working but for some reason don't know what's wrong. I've followed the instructions on both:
https://help.ubuntu.com/community/MactelSupportTeam/AppleiSight
and
http://turanct.wordpress.com/2010/06/11/use-your-macs-isight-on-ubuntu/#comments
The problem is after I type in terminal sudo apt-get install isight-firmware-tools I go through as the instructions say but when I put in the AppleUSBVideoSupport file path it says that there is no file located at that path? Not sure what to do now.
Any help is greatly appreciated thanks.
EDIT: here's the technical specs of the Macbook.
1.83 GHz MacBook (MA254LL/A)
Processor   1.83GHz Intel Core Duo
L2 Cache    2MB
System bus  667MHz
Memory  2GB of 667MHz DDR2 SDRAM (PC2-5300) on two SO-DIMMs
Hard drive  160 GB Intel Solid State Drive 320 Series(SSD SA2BW160G3H) 2.5", 3Gb/s SATA SSD160GB 5V 1A
Optical drive   Slot-loading Combo drive (DVD-ROM/CD-RW)
Graphics    Intel GMA 950 graphics processor with 64MB of DDR2 SDRAM shared with main memory1
Video camera    Built-in iSight
Display     13.3-inch (diagonal) glossy TFT widescreen display, 1280 x 800 resolution
Ports   One FireWire 400 port (8 watts); Two USB 2.0 ports (up to 480 Mbps); Mini-DVI port with support for DVI, VGA, S-video and composite video output (requires adapters, sold separately)4
Audio   Built-in stereo speakers, built-in omnidirectional microphone, combined optical digital audio input/audio line in, combined optical digital audio output/headphone out
Networking  Built-in 10/100/1000BASE-T (Gigabit)
Wireless3   Built-in 54-Mbps AirPort Extreme wireless networking (802.11g standard)
Comment: Possible duplicate of [this post](http://askubuntu.com/questions/103658/isight-on-ubuntu-11-10-macbook-4-1)?
Comment: Make sure to provide us with specific hardware information to minimize complications.
Answer: I'm actually just installing Tahr as we speak and after a quick Google it seems that Apple no longer use a USB camera for iSight anymore, rendering the USB firmware workaround you're trying useless. As far as I can tell there is no current workaround. I'm aware this is a very late response, but it may help others who come across this page.
Answer: Hi I just used the first link you provided to get mine working. I think it's all in the syntax of your command line statements. If you have the path to your files wrong than it won't work. It took me a while to work through the instructions but in the end I figured it out and it works. So any one reading this post just take note. And if you mess up you have to clean the mess up as mentioned in the link. I had to mount another on of my macbooks with a live cd to get the file needed. Moved it to a usb drive and then to my Ubuntu Studio. My paths where different than the instructions have and I had to figure that out before it worked. You have to make the script file and have it as part of the startup. They explain that but I didn't have the program installed so I had to install it to. I'm running 12.4 with latest updates FYI. It took me about a day of reading and careful testing before I had the cam working. but I'm slow lol.
Comment: Please consider adding some paragraphs to make your answer readable

Macbook iSight not working


Bad deck post crack

Question: This looks rather serious - should we replace or brace it ASAP?
Answer: I myself would prefer to replace it, as the crack/check goes down beyond the knot breakout or simply for aesthetics (compared to the treatment below). If that crack/check goes to the bottom or within 2-feet of it may split entirely, so replacement would be the only option.
However, if the crack/check goes a foot or less below the picture frame, then the installation of 1/2" nuts, washers and through-bolts in pairs every foot from the where it starts and ends will keep everything together, aligned and from growing.
This post is for up (wind lift) and down loads, it would only knee or elbow out if struck laterally. The goal is to just re-establish the trunk, leg or arm to full load capacity and avoid any shear-off or stepping-out when under load.
Comment: I do **NOT** like that first picture **at all** it appears to be broken all the way across both ways we can see - I'd sandwich some 2x material on each side and use longer bolts, at minimum, while arranging for a replacement post.
Comment: Hey Larry, wow you've been busy. Nice work! Nah, it ain't that bad, don't forget the rest of the lateral structure above spreading and sharing loads. You can see the big crack doesn't even go up beyond the picture and that's likely the same below...normal stress relief that found an edge. The front is normal checking too, they aren't quite connected, perfectly fine unless severely overloaded...timber frame buildings have these all over. Remember, each one of these are capable of supporting an entire car before they start to bow. Now, if a Hot Tub gets presented, then it must be replaced.

Bad deck post crack


Why would one player get the Fireside Friends achievement and not the other?

Question: I had a couple of new Hearthstone players over. The three of us (2 on Android, one on PC) signed in, and the PC player and I played some challenge matches. After a few games, the PC player got the Fireside Friends achievement. I did not. I checked to verify that I didn't already have the card back. He hadn't played at all prior to this event. Why might I not have gotten the achievement when he did?
Comment: If your friend played against you and your other friend. he would have unlocked it before you. You have to play 2 matches against 2 different people. Are you sure this did not happen?
Comment: @ChaseC Yes, I'm sure. We started playing as soon as he finished the tutorials. Player 3 was still working on the tutorials. From what I've read (and from the experience of the player that actually got the achievement), you can play all matches against the same player; you just need to have a third person also logged in on the same subnet.
Comment: There's some weird logic with it. I know in my case, I played several games, but never got it. Turns out, I had to select my friend from the "on your local network" tab instead of from my friends tab, play the games, and then I got it. You'll have to fiddle with some stupid stuff.
Comment: Is it possible you can only get it on a PC version?
Answer: Make sure you played at least 3 games against a player on the same subnet with at least 3 of you logged into Hearthstone on the same subnet.
You may have to sign out of Battle.net and sign back in. I know that some people said that swapping the device they play on worked out too. Try logging onto your friends PC and having 3 games on your account to see if it unlocks.


How can I add shine on an object

Question: Could someone please tell me how to add a small shine to an object, it should look like this:
Comment: You could also add it via a lens flare. Through the compositor. I feel it would give you more control as well. See this YouTube tutorial from Blender Diploma.
https://youtu.be/iMjcyL0QvIU
Answer: There are a number of key factors you must have to even get close to this effect:

You need to have a material with some amount of glossy mixed into it.
Then you need to bevel the edges of your geometry
Add a light to the scene (a strong one if possible).
You should be able to get some level of light reflection off the edge in the render.
Then in compositing you'll need to pump up the glint with nodes and then run it through a glare node to get something similar to the example.

Level Up Tip
If you can get that working, you might want to try Troy S's new LUT's for blender which allow your scenes to be rendered with high dynamic range. One of the main benefits of high dynamic range is that glints and glares are much more powerful in the compositing phase.
Screen Shots (using default sRGB LUT)

Material setup, geometry and light

Alternate angle of object in scene

Compositing Setup to accentuate glint and add glare effect
Comment: You can see in the screen shot that I'm using a spotlight from the back to get the glint on the front top edge bevel. I cranked the strength up to 5000 to add more reflection. The spotlight is good because it gives you more creative control.
Comment: Thanks Todd, what light would i use and how would i turn up the glint? Thanks again

How can I add shine on an object


Biosolvents for Life

Question: What biosolvents could, in theory, permit the emergence of life on other worlds? I have been recently looking through possible biosolvents and I have found a good number, but I am specifically looking for more esoteric or less common solvents or thalassogens. Something like Liquid Iron Pentacarbonyl or Molten Sodium Chloride. These two have been discussed here, but I am mostly looking for solvents that have not really been proposed in a published work or explored very well. I am just looking for a brief description of why the fluid might act as a good solvent for life and what element or compound could serve as the backbone, would it be carbon-based life? Tungsten-Chloride-based life? I just want it to be at least theoretically possible (it doesn't have to be plausible) at least so my story has scientific grounding. I am mostly looking for brand new solvent possibilities that even have not been proposed here, but I would be happy to learn more about solvents that are somewhat known like hydrogen cyanide. I am just asking for a simple list with brief explanations or overviews.
Comment: "I am just asking for a simple list with brief explanations or overviews." That isn't true. We have 35 questions that touch on your issue and you want something more, something new. That's anything but simple. Does "I want an answer not already listed on Stack Exchange" justify creating a new question?
Comment: When I said simple list, I was talking about the list being brief, I wasn't talking about the content. If this was easy I would have done it. Also, I didn't say I wanted an answer not already on stack exchange. It says "solvent possibilities that EVEN have not been proposed here." This place has a large variety of answers, I was enunciating the desire for a very new proposal. The word 'even' is the enuciation. You took my question out of context. I do feel the need to enunciate that point because I don't want to run into the problem of getting solvents I already know about.
Comment: I have explored every solvent question on this website. I am sure I have covered all 35.
Answer: The National Academy of Science has a report "The Limits of Organic Life in Planetary Systems" and they discuss water as a solvent, but then also go on to consider several other potential solvents and mediums for life.
They have a fairly extensive list that they divide into three groups solvents into three groups: polar solvents that are not water, nonpolar solvents, and cryosolvents. They have short discussions on
Polar solvents: Ammonia, Sulfuric Acid,  Formamide (HCONH2)
Nonpolar Solvents: Mainly hydrocarbons - methane, ethane, propane, butane, and so on
Cryosolvents: dihydrogen, dinitrogen, and supercritical CO2

Biosolvents for Life


Restoring a backup of 1 database to another puts both databases into Restoring mode

Question: We have a 2 databases-

Staging
Test

At times, we refresh out Test environment with a copy of Staging. We do this by taking a full backup of the Staging database and restoring it over the Test database.
When we do this, both databases go into Restoring... mode and only the Test databases comes back online after the restore.. the Staging database shows Restoring... until we issue a RESTORE DATABASE Staging WITH RECOVERY.
Why is this and what do we need to change about our process to prevent this happening?
The backup and restore event log on the Staging database doesn't show any evidence of a restore on the database.
I'm thinking it's something to do with Logical File Names. When we do the restore, we change the .dbf and .ldf files to point to the TEST files, however the logical files are still pointing to STAGING, something we can't change through SSMS during a restore.
The restore command that SSMS is running is:
USE [master]
BACKUP LOG [Chris21_STAGING] TO  DISK = N'F:\Microsoft SQL Server\MSSQL11.MSSQLSERVER\MSSQL\Backup\Chris21_STAGING_LogBackup_2016-06-24_11-12-03.bak' WITH NOFORMAT, NOINIT,  NAME = N'Chris21_STAGING_LogBackup_2016-06-24_11-12-03', NOSKIP, NOREWIND, NOUNLOAD,  NORECOVERY ,  STATS = 5
ALTER DATABASE [Chris21_TEST] SET SINGLE_USER WITH ROLLBACK IMMEDIATE
RESTORE DATABASE [Chris21_TEST] FROM  DISK = N'C:\Chris21_STAGING Backup 20160624.bak' WITH  FILE = 1,  MOVE N'Chris21_STAGING' TO N'F:\Microsoft SQL Server\MSSQL11.MSSQLSERVER\MSSQL\DATA\Chris21_TEST.mdf',  MOVE N'Chris21_STAGING_log' TO N'F:\Microsoft SQL Server\MSSQL11.MSSQLSERVER\MSSQL\DATA\Chris21_TEST_log.ldf',  NOUNLOAD,  REPLACE,  STATS = 5
ALTER DATABASE [Chris21_TEST] SET MULTI_USER

Answer: It seems to me that in UI you have 'Backup tail of the log, and leave database in restoring state' selected which is causing the issue. - https://dba.stackexchange.com/users/46744


Finding complex roots of integer polynomials

Question: How would one find approximates for complex root of polynomial with integer coefficients,I know for example the Newton's method $$x_n=x_{n-1}-\frac{f(x_{n-1})}{f'(x_{n-1})}$$
Anyway is it possible to do such when you're finding complex roots(for example $3z^3+z^2+z+1$)?Or are there any other methods for doing such?
Comment: Use a complex initial guess.
Answer: There are two improvements of Newton-Raphson iteration that work particularly well for polynomials since they find all roots simultaneously.  (Newton-Raphson finds only one at a time.)  The first one is the Durand-Kerner method that was actually already known to Weierstrass (and re-invented again by me when I was a student).  The second one is the Aberth-Ehrlich method.  It's been quite a while since I used those methods but if I remember correctly the latter is superior in terms of convergence rate.
Oh, and as stated in the comments, do not pick all initial candidate roots real since iteration will never "come off" the real line in that case.  Just some random complex initial values will do for both methods.
Comment: and Aberth-Ehrlich recommends not using starting points that are symmetrical about the real line, either.


Uploading image to Cloudinary (Mean)

Question: This question includes particulars to a framework, but I just have the general question on how to upload the photo file to an api. What is the actual part of the image that I submitting? What is the content of the photo?
I am able to successfully upload an image upon clicking a button (using angular-file-uploader. However, I don't know what part of this object I should be sending to my api.
The code below will complain about a type error: it can't convert circular structure to JSON.
So I can also try to grab the file out of the queue, but then I when it makes it to cloudinary, I receive an error: TypeError: file.match is not a function.
html

    <input type="file" nv-file-select uploader="user.uploader" />
    <button ng-click="user.upload()">Upload</button>

controller
vm.upload = function() {
    console.log(vm.uploader);
    var photo = {file: vm.uploader.queue[0]}
    UserDisplay.uploadPhoto($stateParams.user_id, vm.uploader.queue[0])
}

api
cloudinary.uploader.upload(req.body.file, function(result) {
    console.log("made it here");
    console.log(result)
});

Answer: Cloudinary's Angular library supports ng-file-upload, not angular-file-uploader.
There's some code that isn't displayed here. For example what is UserDisplay?
You get this error because queue holds FileItem objects and not strings.
You should initialize FileUploader with the upload url, and formData that includes the upload_preset. That way there is no need to override the upload function.
You can see the sample project on Cloudinary's AngularJS repo: https://github.com/cloudinary/cloudinary_angular/tree/master/samples/photo_album

Uploading image to Cloudinary (Mean)


What is the longest working cubesat?

Question: For example VZLUSAT-1 is 5 years old and it is still working.
Comment: BugSat-1 was on the same payload launcher as UNISAT-6 and is also still active.
Comment: Not specifically a cubesat SAUDISAT 1C, was launched in 2002 and is still active. The list is really quite long for cubesats (the first launched in 2003) - CANX-2 for example is still listed as operational (when over its ground station) and that was launched in 2008. (the ones I listed earlier are from 2014)
Answer: Under the specific CubeSat design specification, one of the earliest CubeSat launches was on 30 June 2003 from Plesetsk, Russia, with Eurockot Launch Services's Multiple Orbit Mission. The CubeSats were injected into a Sun-synchronous orbit and included the Danish AAU CubeSat and DTUSat, the Japanese XI-IV and CUTE-1, the Canadian Can X-1, and the US Quakesat.
According to this: https://opensea.io/assets/ethereum/0xb4612c2f7b7dac79e176a1d8513c9a5b3a33e868/14

The first mission was a multiple launch of 6 CubeSats using a Rokot-KM rocket, a remake of the old SS-19 Stiletto soviet intercontinental ballistic missile. The CubeSats were: AAU-Cubesat Aalborg University Cubesat (DK), CanX 1 Canadian Advanced Nanospace eXperiments (CA), CUTE 1 CUbical Tokyo institute of technology Engineering satellite (JP), DTUSat Danmarks Tekniske Universitet Satellite (DK), QuakeSat (USA) and XI-IV (JP).
CubeSat XI-IV (Sai Four), the OSCAR 57, is still active.

CUTE-1 is also considered still active.
As of 2022, makes these 2 the oldest CubeSats still active.

What is the longest working cubesat?


How to format disk without installing system?

Question: I want to move system to another machine. I have full backup in .tar.gz file as described here. Now I need to partition hard drive, install grub and unpack archive. How to do that? When I tried to use Live CD it started installing system immediately after partitioning and I need clean partition to unpack my archive to.
Comment: Did you use gparted to partion the disk? I never experienced what you described, when using gparted.
Answer: You can do this, for instance, with the GParted Live CD.

GParted Live is a small bootable GNU/Linux distribution for x86 based computers.
  It enables you to use all the features of the latest versions of the GParted application.
GParted Live can be installed on CD, USB, PXE server, and Hard Disk then run on an x86 machine.
Installation Instructions
To install GParted Live on CD, download the .iso file and burn it as an image to a CD.
Accounts in GParted live
GParted live is based on Debian live, and the default account is "user", with password "live". There is no root password, so if you need root privileges, login as "user", then run "sudo" to get root priviliges.

But  if you already have a Ubuntu Live CD you can run GParted or Disc Utility and from there GParted after choosing the option Try Ubuntu instead of Install Ubuntu.
Answer: Don't run the installer then.  Use the Disk Utility or gparted to partition and format the drive.
Answer: Use live and GParted or fdisk  Not the installer auto-config partitioner if you want to do this.  You can let the auto-partitioner do it's thing and kill the installer, but then you have to install grub from rescue/live.
All the parts of this question have been asked before:
How do I partition my drive from live
How to restore is covered on the page you cited.  You need to do the restore BEFORE you do the grub reinstall.
How do I reinstall grub


Do Australian universities pay expenses to attend a faculty interview?

Question: I have been invited for an interview for a lecturer position at a university in Australia. I am based in Europe at the moment. It is not expicitly clear, at least to me, in the invitation email that they will pay for the travelling and accommodation expenses, whether in full or in part. I thought I should ask them for clarifications, but I am not sure how to ask it, and perhaps it is better to find out more about the custom before asking.
Comment: Just ask them who makes the travel arrangements and who pays.
Answer: The short answer is 'yes'. I know some colleagues who were invited for an interview for a lecturer position at an Australian university and they were indeed reimbursed for all their travel+accommodation+food+visa procedure expenses for the trip. The reimbursement was not conditional on if they would accept the job if offered or not. It was also written clearly on their invitation letters (needed for their Australian visa application) that they would be reimbursed all these expenses.
However, you must NOT assume anything. It is going to be a good amount of money travelling from Europe to Australia, and you must clarify about your reimbursement.
Comment: Yes, always get these things "in writing". I once moved 3000 miles with a (written!) promise of moving expenses paid, but at the far end they didn't want to, by default, until I scrounged-up that piece of paper.
Comment: I have a different experience.  A colleague of mine was interviewed over Skype.  Given the current state of Australian universities, I doubt they will pay for your 'lovely' trip from Europe!
Comment: @user35582, if an Australian university goes interviewing candidates, they must have made sure to secure funding to invite candidates over! I would just get it specified (in writing, as paul garrett emphasised) to avoid any confusion.
Comment: @John, you must work for a rich university!  Usually, the university will pay for relocation if they make the offer.
Comment: hI don't work in Australia. But I have worked in a university there in the past and I know the system at least generally. They do want to hire overseas candidates. And no overseas candidate, especially an early stage academic, would go all the way to Australia on his/her own expense to finally find out they didn't get the job! So the universities do have to pay the candidates' travel expenses. As I said, the OP must confirm this in writing as a general common sense.


Swipe left to Delete or Swipe right to Delete?

Question: For actions like "Edit" and "Delete", which should go with Swipe left and Swipe right?
I myself found Swiping left (right handed) is less natural than Swiping right, so "Delete" can be assigned to Swipe Left. Any use case on this gesture that you recommend?
Comment: It totally depends on the reading direction LTR or RTL. Either of the direction you should show Edit and Delete button at the end of the list. You should read this for more details: https://www.nngroup.com/articles/contextual-swipe/
Answer: Generally swipe right is a forward or positive action, left is a back or negative action.
But unless you are careful and particularly when you are choosing between quite unrelated options using swipes at all can be very unintuitive.
Comment: I suspect that this very much depends on culture.
Comment: You might think so but in countries where the writing reads the other way the browser forward and back arrows are not reversed!
Comment: But eye tracking heat maps are, even when viewing non-text data. I'm not saying one way or another - I'm just saying it's probably worth a bit of research on the part of the implementer.
Answer: I'd say that guidelines for actions when swiping left contra right on list elements aren't really established. Looking at the Android UI patterns you usually dismiss/exit elements by swiping them either right or left. Meaning, swiping right or left invokes the same action.
If you want some reference I can suggest the Sony email client which has different actions for swipe direction. Swiping Right will trigger a remove of the list item (then also providing an "Undo" action in the same element which the user can tap or swipe back in the opposite direction to trigger). Whereas swiping Left will provide two different actions on tap, either "Move to folder" or "Mark as read").
These patterns are formed by Sony themselves, they didn't really have any reference. It's a delightful feature however and it does feel natural, but users will most likely not recognize it as something familiar at first.

One disclosure though is to not rely solely on this pattern to trigger these actions (which you may already be fully aware of). Hiding fundamental actions behind interactive patterns such as these are nifty for the ones that discover them, but detrimental for novice users who won't. So keep the actions also on a details page or similar of the items.
Comment: Piggy backing on my comment on @JamesRyan's answer, I suspect that Sony's implementation was influenced by their right-to-left read-direction. Just to add a bit more to think about.
Answer: I would say, it depends,... ;)
Have a look at the platform you are developing for. For ios for example: swipe left shows "delete/mark/more" and swipe to right opens a option to mark the row (email for example as: mark as unread). So there is no clear "guideline" which direction to use.
Use the direction which the target platform offers, so you can make sure, user does not have to learn a new gesture or even worse, are confused by a misleading (180°) solution.
Answer: Mailbox app does this with visual feedback such as "Red" arrow or 'X' to delete and "Green" arrow or check mark to archive. You can see this as you swipe and know what you are going to to do if you continue. This is better than having to remember the swipe direction/actions.


Use of CC0 licensed code which used CPOL licensed code

Question: I want to use for the creation of an application that I plan to add to Google play (free to use) some code that is licensed under a CC0 license (https://github.com/tedbarnett/read-exif-in-unity). However this code use the work from an other licensed code (CPOL license https://www.codeproject.com/info/cpol10.aspx). I was wondering how I was supposed to deal with this situation: Am I allowed to use the code and if I am how exactly should I proceed to publish my application on Google play.
Comment: Could you please give some details how the code under CC0 is using the code under CPOL? Is it a direct copy, a dependency, ...?
Comment: It is a direct copy of the whole CPOL code which is used like a tool for his own code.
Answer: I drilled down into the project you linked and saw that there is sample code, which is licensed under CPOL 1.0. In your comments to your question you mentioned that this CPOL-licensed code is directly copied into the CC0-licensed project.
I think this should not be. CPOL-licensed code must not be re-licensed as CC0. When you (fork and) re-license you can only reduce freedoms and add restrictions, not the other way around; but CC0 would be removing restrictions compared to CPOL. I believe we are witnessing a license-incompatibility.
If I wanted to use this code (and there is no alternative) I would directly address the re-licensing question with the developers of the 2 projects and ask for their thoughts.
Alternatively you could fork the CC0 project, remove the CPOL code from it, and implement that same CPOL code as a dependency directly in your own project. You can then license your own project with any license that is compatible to CPOL and any other dependencies used in your project.
Comment: Thank you for the thoughtful response but I am still a bit confuse about the CPOL license... It says I should make sure any user agree with the license so doesn't that mean that I have to provide a link to the original code or just to a copy of the license ? Should I also cite the names of the 2 works I used or can I use them without saying anything ?
Comment: Most of the terms in the CPOL License only apply to the 'Work' (as defined in the license) but not to 'Derivative Works' (as defined). It will be proper to include a link to the Work and its license in the 'About' page of your App, in the same way that you will need to do that with the other dependencies (I see a lot of Unity dependencies).


When I back out of an app is it still using up my data?

Question: When I am coming out of an app such as facebook, do I just back out of it and is it still using up my data?
Answer: Yes, such internet-based applications normally use background services to stay connected with their servers and use mobile data when Wi-Fi is not available. But data usage is often very low for such background apps.
If you want to stop any data usage of any app when you are not using internet apps, you can disable mobile data temporarily. To do so, Go to Settings -> WiFi and networks -> Mobile data and uncheck the option to disable mobile data.
Also, if you are using Android 4 ICS or newer, you can disable background mobile data usage for some apps. Just go to Settings and under Wireless and Networks, click on Data Usage. Then you will see a list of apps that has used your data. Select the app you want to restrict and in newer window, scroll to bottom and check option "Restrict background data". Then, no background mobile data for that app is allowed.
Comment: Odd, I don't see an option to restrict background data. Nexus 7, android 4.1.1. I looked at the entry for Chrome.
Comment: @offby1 If you see that option for other apps, so it's most probably Google has disabled it for Chrome. However, I don't have 4.1 JB and maybe the option location is different in, or removed from your Android version.
Comment: It's also worth looking at the app's own settings, they often have an option to limit or stop their background checking and syncing. For Facebook in the Facebook app you can go to **Menu** -> **Settings** and in the **General Settings** section there's a "Refresh interval" settings that you can change to set how often (or if) Facebook will use your data connection while it's running in the background.