Do all massless particles (e.g. photon, graviton, gluon) necessarily have the same speed $c$?

Question: I suppose there was a discussion already on speed-of-gravity-and-speed-of-light.
But I silly wonder whether all the massless mediators of four fundamental forces, i.e.
Graviton: $g_{\mu\nu}$ (gravity)
Photon $\gamma$: $A_\mu$ (electromagnetism)
Gluons: $A_\mu^a$ (strong interactions)
Necessarily travel at the same speed? Is there a no-go theorem or theoretical proof to say that it is impossible to have these three mediators have different speeds?
Or does QCD confinement makes the story of gluons any different from gravitons and photons?
[ps. excluded massive $Z^{0}$ and $W^{\pm}$ bosons (weak interactions)]
Another way to say this:
Speed of photon, graviton, gluon all equal to $c$?
or
Whether all massless particles necessarily have the same speed?
Note add: Notice however, in condensed matter systems, there can be emergent gauge fields and emergent massless particles (Dirac or Weyl cones), but their speeds need not be the same, unless there is some emergent symmetry...
Comment: Isn't it because of the geometry of spacetime?
Comment: Can we have bimetric gravity with, say,  photons and gravitons coupled to different metrics?
Comment: @hwlau: can you be more explicit? Actually now I wonder whether one scheme is that the SUSY unification on gauge couplings of U(1)xSU(2)xSU(3) and gravity at the GUT scale 10^16 GeV, implies that the mediators of all forces needs to have the same origin, so the speed of massless particles needs to be the same? What may be a theoretical constraint? or a no-go theorem?
Comment: ps. If $c_{gravity}, c_{photon}$ and $c_{gluon}$ are different, we can imagine there are more fundamental constants, and HEP people certainly hate to see this happens.
Comment: There was a theory which predicted different limit speeds for each particle, but I can find the reference.
Comment: The speed of light is a postulate. That means that its just a rule which no one understands. A real move forward - true unification - would show that the speed of a photon is the same as a graviton because they are both the 'same thing' - postulate not required.
Answer: If basic symmetry and homogeneity assumptions about the Universe hold, then yes, all massless real particles (see Anna V's answer for virtual particles must travel at a universal constant $c$, the speed of a massless particle, in all frames of reference.
Given these basic symmetry and homogeneity assumptions, one can derive the possible co-ordinate transformations for the relativity of inertial frames: see the section "From Group Postulates" on the Wikipedia Page "Lorentz Transformation". (Also see my summary here). Galilean relativity is consistent with these assumptions, but not uniquely so: the other possibility is that there is some speed $c$ characterizing relativity such that $c$ is the same when measured from all frames of reference. Time dilation, Lorentz-Fitzgerald contraction and the impossibility of accelerating a massive particle to $c$ are all simple consequences of these other possible relativities.
So now it becomes an experimental question as to which relativity holds: Galilean or Lorentz transformation? And the experiment is answered by testing how speeds transform between inertial frames. Otherwise put, the experimental question is are there any speeds that are the same for all inertial observers?. The question is not about measuring the values of any speed, but rather, how they transform. Now of course we know the answer: the Michelson Morley experiment found such a speed, the speed of light. So there are two conclusions here: (1) Relativity of inertial frames is Lorentzian, not Galilean (which can be thought of as a Lorentz transformation with infinite $c$) and (2) light is a massless particle, because light is observed to go at this speed that transforms in this special way.
Notice that at the outset of this argument we mention nothing about particles or any particular physical phenomenon (even though special relativity's historical roots were in light). It follows that, if $c$ is experimentally observed to be finite (i.e. Galilean relativity does not hold), then the specially invariant speed is unique: it can only be reached by massless particles and there can't be more than one such $c$ - the Lorentz laws are what they are and are the only ones consistent with our initial symmetry and homogeneity assumptions. So if we observed two different speeds transforming like $c$, this would falsify our basic symmetry and homogeneity assumptions about the World. No experiment gives us grounds for doing that.
This is why all massless particles have the same speed $c$.

Update: Experimental Results
As is now common knowledge, the gravitational wave event GW170817 and gamma ray burst GRB170817A give strong experimental evidence of the equality of the speeds of light and gravitation. As discussed in:
Gravitational Waves and Gamma-Rays from a Binary Neutron Star Merger: GW170817 and GRB 170817A
the 1.7 second time delay between the gravitational wave arrival and the gamma ray burst, together with conservative assumptions about other sources of delay, yields an experimental bound on the fractional difference between the speed of light and of gravitation:
$$\frac{v_g-v_{em}}{c} \leq 3\times 10^{-15}$$
an impressive experimental bound indeed. Within the next 10 years, we probably shall see several such events, and thus this experimental bound will tighten further (unless something really theoretically unforeseen happens!).

Mass From Confined Massless Particles
Incidentally, if we confine massless particles, e.g. put light into a perfectly reflecting box, the box's inertia increases by $E/c^2$, where $E$ is the energy content. This is the mechanism for most of your body's mass: massless gluons are confined and are accelerating backwards and forwards all the time, so they have inertia just as the confined light in a box did. Likewise, an electron can be thought of as comprising two massless particles, tethered together by a coupling term that is the mass of the electron. The Dirac and Maxwell equations can be written in the same form: the left and right hand circularly polarized components of light are uncoupled and therefore travel at $c$, but the massless left and right hand circular components of the electron are tethered together. This begets the phenomenon of the Zitterbewegung - whereby an electron can be construed as observable at any instant in time as traveling at $c$, but it swiftly oscillates back and forth between left and right hand states and is thus confined in one place. Therefore it takes on mass, just as the "tethered" light in the box does.
Comment: I like your answer about the origin of rest mass, if I may put it that way. Do you have some references that go deeper into this? Ive always found it curious that there should be two origins of inertia; the discussed here, and the higgs field. Or is there some perspective which I am missing under which these are two different ways of looking at the same mechanism?
Comment: @EelcoHoogendoorn THanks: as I understand it (I'm not a particle physicist) the Higgs mechanism can also be understood as a *coupling* between the weak bosons and the Higgs field. If you can find a copy of Penrose's "Road to Reality" look at the way he described the Zitterbewegung of the electron. The coupling preventing the weak bosons zipping off a lightspeed *is* the rest mass. The origin of rest mass in the way I describe is an old idea: it goes right back to Einstein's $E=m\,c^2$ paper ....
Comment: @EelcoHoogendoorn ....["Does the inertia of a body depend upon its energy content?"](https://www.fourmilab.ch/etexts/einstein/E_mc2/www/). I take this kind of idea up [here](http://physics.stackexchange.com/a/78199/26076)
Comment: Right. I see how binding energy leads to mass, and how this nicely obeys all relativistic notions. This follows simply from two particles bound by a retarded potential. This has always strongly suggested to me that the rest mass of supposed elementary particles might also be due to the binding of some substructure. The standard counter to that is that such a substructure does not exist, and Higgs is needed. You say that the binding between the components of the zitterbewegung would explain the mass of the electron. Are these mere different perspectives, or should that Nobel be recalled?
Answer:
Another way to say this: Speed of photon, graviton, gluon all equal to c? or Whether all massless particles necessarily have the same speed?

You must not have been introduced to the concept of a virtual particle:

In physics, a virtual particle is a transient fluctuation that exhibits many of the characteristics of an ordinary particle, but that exists for a limited time. The concept of virtual particles arises in perturbation theory of quantum field theory where interactions between ordinary particles are described in terms of exchanges of virtual particles. Any process involving virtual particles admits a schematic representation known as a Feynman diagram, in which virtual particles are represented by internal lines.

A virtual particle is an internal line in a Feynman diagram which represents the propagator mathematics that has to be substituted to get the integral necessary for computing measurable quantities . Virtual particles have the quantum numbers of their homonymous ( having the same name) particles except not the mass. The mass is off shell.
So it is a general rule that massless particles travel at the velocity of light, but only when in external lines in Feynman diagrams. This is true for photons, and we thought it was true for neutrinos but were proven wrong with neutrino oscillations.
Gluons  on the other hand we only find within a nucleus and these are by definition internal lines in Feynman diagrams and therefore are not constrained to have a mass of 0, even though in the theory they are supposed to.  In the asymptotically free case, at very high energies they should display a mass of  zero.
Comment: The path integral of physical fields and virtual particles...I feel like it's a bit like we consider the exponetial function $\mathrm e^x$, make a tailor expansion around $\pi$ up to second order and absorb an overall factor $\mathrm e^{-\pi}/2$ because it's not physically detactable. We end up with $\mathrm e^{x}\ \overset{\text{sort of}}{=}\ 2+\pi^2-2\pi(1+x)+x(2+x)$. We absorb the constant and reparametrize $ex\equiv x+1$ to emphasise the beautiful symmetry of our system $-2\pi\ ex+(ex-1)(ex+1)$. We call $ex$ the "expandion" and it will provide great counterexamples for other somethingions.
Comment: I think Nick's comment is a serious concern. At least in QED it seems like this perturbative calculation makes sense (weak coupling), at least for a while. However, the series is asymptotic. In QCD we have an even bigger problem with strong coupling. So while the mathematics is useful and suggests that there are virtual particles, I think the onotlogical status of such objects hasn't really been established. Are they physical or just an artifact of the perturbative technique?
Comment: @annav: Are you implying the expandions are arbitary? While $x$ might seem like a random variable, only the expandions let us explore the quadrahedron density $\mathcal L_*[ex]:=(ex-1)(ex+1)$. Note that this purely algebraic (=geometric) object is invariant under $ex\mapsto -\mathrm{wau}\cdot ex$, [(Ref 17)](http://www.youtube.com/watch?v=GFLkou8NvJo). Indeed, the symmetry group is isomorphic to the Galois group which emerges when you pass from the unique Archimedean complete totally ordered field to its algebraic closure, [(Ref 32)](http://www.youtube.com/watch?v=BipvGD-LCjU).
Comment: @KevinDriscoll Well, when really kicked out of the nucleus gluons do make jets so their reality seems to me to be backed by experiment.http://cerncourier.com/cws/article/cern/29201
Comment: @NickKidman I think the difference with your example lies in the quantum numbers . The virtual particles are not arbitrary functions, they carry the quantum number conservations and the pole/propagetor that represents them has the on mass shell value except that in the internal lines things go imaginary.
Comment: @NickKidman All I am saying that there should be a connection with data . There could be an infinity of mathematical constructs, but can they model the data?
Comment: Infinitely many of them can, but that's besides the point. I just think we wouldn't have the conversation if virtual particles wouldn't be called what they are called. I take it that OP is asking if all physical phenomena, expressed at in microscopic terms, are equally fast w.r.t they space they take place. The fact that they are represented in the theory, with momenta say, which also has similar quantities used in a computational process, where these new quantities blow up to infinity, that has per se not to do with the data. Aliens which never had to use our methods might e.g. answer no.
Comment: @annav I wasn't trying to doubt the existence of real gluons. I was questioning whether the virtual particles which we use to draw internal lines in Feynman diagrams have the same ontological status as external lines. To me a 'virtual particle' is an unfortunate term. I think they aren't really particles at all, but just transient bundles of some field which we deem particles thanks to the structure of our perturbative technique.
Comment: @KevinDriscoll Well, these are the tools we have. In the propagator picture , where the virtual particle is a pole when on mass shell, the on mass shell is a limiting condition, there exists a continuity in the formalism, the particle will become "real" because it will not only have the quantum numbers but also the mass, or be very very close to the mass
Comment: "The mass is off shell". What does this mean?
Comment: The mass shell is the locus where m^2=E^2-p^2 ( c assumed =1) in the four space of energy momentum .
Answer: It is not hard to imagine a toy universe in which different fundamental forces propagate at different speeds. However, a necessary consequence of that would be violations of lorentzian symmetry, and the ability to triangulate a preferred rest frame.
Although I don't see a theoretical reason why these speeds need be the same (I might be missing something though, perhaps some stability arguments require it), empirically there isn't much room for different speeds.
Answer: I thought the same thing for a long time.  I wondered why gluons don't fly out of the nucleus at the speed of $c$.  The difference is that photons don't interact with other photons and gravitons don't interact with other gravitons.  They can move around and pass through each other.  On the other hand, gluons do interact with each other.
In fact, gluons form chains/flux tubes which is part of why quarks are confined.  Gluons do travel at $c$ but not for very far before they interact with other quarks or gluons, which keeps them from moving any appreciable distance.
Comment: no. this is not true? gravitons DO interacting with gravitons. gravity are badly non-renormalizable.
Comment: @Idear I'm pretty sure gravitons behave normally in one-loop diagrams and don't interact very strongly with each other when they do interact.  The strong force, on the other hand, is many orders of magnitude stronger in interaction.
Comment: Thanks Brandon. What you address is not exactly what I asked. I wonder whether there is no go theorem for preventing the speeds of massless particles to be different. Perhaps if the speed of one is the largest, say c_1 >c_2 >c_3> .... Then only c_1 can be defined as massless?
Comment: @Idear graviton-related interaction question http://physics.stackexchange.com/questions/8112/what-do-gravitons-interact-with
Comment: @Idear John Rennie also addresses strong-force propagation velocity via Gluons and Mesons here: http://physics.stackexchange.com/questions/57137/how-soon-that-a-force-affect-another-object
Comment: I think the spirit of the question is to look at the speed a free gluon would propagate. This speed is the speed of light. For an electron say, it doesn't have to travel at the speed of light, and it's mass gives a second speed scale. However a massless particle does not have this second speed scale. Is it possible for a lorentz invariant QFT to have massless particles that don't propagate at the speed of light when interactions are turned off? I would think not because there is no way to get a second speed scale.


ABRSM Exams: Repeating of bars

Question: I am a grade three pianist, and I am taking my exam soon. I am playing the piece Allegro by Muzio Clementi (A:1 , ABRSM Grade 3 Piano Exam Pieces 2015 & 2016), and there are two repeat signs. I am currently playing the repeat signs, but in the exam, would I actually repeat the whole piece again?
Answer: The guidelines say "All da capo and dal segno indications should be observed but all other repeats (including first-time bars) should be omitted unless they are very brief (i.e. of a few bars) or unless the syllabus specifies otherwise." ABRSM para 15(h)
Answer: You're not supposed to repeat, but the examiner won't take marks off if you do.
Save time and play right through.
Answer: I got a mark taken off on the A piece of my grade 5 piano exam (a Haydn composition), presumably for omitting the D.C. in the Minuet and Trio (as the examiner made reference to the fact that I did not play the Da Capo.)  So, instead of getting 26 out of 30, I was cut one mark for that. In my opinion, this was a harsh thing to do, since my belief has always been to mark / assess on the merits of musicality rather than executive functions like repeats. I have performance anxiety and was likely nervous! It just strikes me as rude / cold marking on the part of this examiner :/
Comment: What's your guess about how you lost the other 4 marks?
Comment: Did you intend to play the DC? If you didn't intend to play the DC, then the mistake lies with whoever was responsible for reading the syllabus (you or your teacher, if you have one) to find out what was expected
Comment: It's a minuet and trio. The trio is often (but not always) in a different key from the minuet proper, so skipping the D.C. (al Fine) sounds pretty odd (as odd as those renditions of Schubert's Impromptu No. 2 in E Flat Major and Elgar's Pomp and Circumstance March No. 2 in A Minor I've heard that cut off a reprise of an earlier section). Honestly, I'd have thought the syllabus would have justified cutting off *more* marks.

ABRSM Exams: Repeating of bars


Since the microwave background radiation came into being before stars, shouldn't all existing stars (given sufficient equipment) be visible?

Question: It's commonly said that, due the rapid expansion of the universe, it is possible that there are objects (such as stars) that could be beyond our field of vision, given the finite speed of light.  Since it is possible that space is expanding faster than the speed of light, that makes sense.  But, if we can see microwave background radiation (MBR) in whatever direction we look and if MBR is from an era prior to the creation of stars, when we see the MBR, we are seeing something that existed before the creation of stars.  We are seeing something that is older and further than stars.  If can see IT, then must we not we be seeing all the stars which came after it because they would be younger and closer?
Comment: "We are seeing something that is older and further than stars."  -- *Earlier* that the stars, but not further.  The CMB was produced everywhere : in the space behind the visible stars, in the space between the visible stars and in the space between us and those stars.  (Note: the visible stars were not present at the time the CMB was produced.  But the early version of all that space was there.)  The CMB was produced more like a fog filling all space at the time.
Comment: The cosmic microwave background is the red-shifted light emitted from the [last scattering surface](https://en.wikipedia.org/wiki/Cosmic_microwave_background#cite_ref-2:~:text=The%20surface%20of%20last%20scattering%20refers,at%20the%20time%20of%20photon%20decoupling.). We see all stars within this region, but these are not necessarily all the stars in the universe. See for example [Could the observable universe be bigger than the universe?](https://physics.stackexchange.com/a/621246/169454) Or do you explicitly mean only the stars within the observable universe?
Answer:
The diagram should clarify. It is qualitative, and uses space and time coordinates chosen so that light rays are diagonal for simplicity. Thus it does not directly show the expansion of the universe.
The green line is our past light cone. Note that what we see in the sky is what's on it (not inside it). Of course, space is 3D (rather than 1D), so we can see beyond stars by looking between them, but the CMB was emitted everywhere, so we cannot see anything beyond that.
The diagram shows that there are stars we cannot see because they are too far away for their light to reach us yet.
Comment: @PaŭloEbermann This is achieved by the use of so-called conformal time $\eta$ as the vertical coordinate. This choice makes the entire spacetime metric stretched by the scale factor instead of the just the spatial coordinates,
$$ds^2 = a(\eta)^2 \left(-d \eta^2+dx^2+dy^2+dz^2 \right).$$
While the metric is not flat due to the scale factor evolution, your spatial and time coordinates are on equal footing in some sense. The compromise is that this is not the proper time measured by a co-moving observer.
Comment: @nanoman I agree with the quote, though Nathaniel's caveat of "not beyond a certain point in their history" is significant. In standard $\Lambda$CDM, the universe's conformal time will be finite, and hence the point representing us only goes "up" in the diagram a fixed amount.  While a currently visible star's worldline will always intersect our past lightcone and we will therefore always see it, then, there is a maximum fixed time in that star's history up to which we can see it evolve.
Comment: @nanoman And as this happens, the light we receive from the star, while always visible in principle, redshifts infinitely until it is practically invisible. As should be the case, since over our infinite (in our proper time) observation period we only see up to the cutoff time in the star's evolution, before which it only emitted a finite amount of energy.
Comment: @PaŭloEbermann I can choose the space coordinate to be comoving so the stars appear stationary, and then choose the time coordinate so that light cones appear straight.
Comment: The diagram ignores the expansion of the universe, doesn't it? Shouldn't either the light cone or the star timelines be curved due to the expansion?
Comment: This diagram is very nice, I haven't seen it before. But in this diagram stars can only come *into* the cone of visibility, except when they die. I understood that distant galaxies can also move out of the visible universe due to the expansion of space, so that we can't see them any more. (Or rather in principle we can, but not beyond a certain point in their history.) Would a different diagram be needed to show that, or is there a way to see it by interpreting this one in the right way?
Comment: @Nathaniel That doesn't happen. See [here](https://astronomy.stackexchange.com/q/14336/32118): "the observable Universe always increases in size, and no galaxy visible today will ever leave the observable Universe, no matter its speed." An opposite impression might have been given by popular articles as mentioned in [this answer](https://physics.stackexchange.com/a/11034/206761).
Comment: Note that in principle we can only see ( = observe with telescopes) stuff that lies _on_ the green lines. Except for a few cases where gravitational lensing causes the path length to be longer so you could see stuff that's inside the past light cone.
Comment: @jawheele thank you, your first comment, about the vertical axis being finite on the diagram, exactly answers my question. Very interesting!
Answer: First, see Did the Big Bang happen at a point?. As it explains, the Big Bang did not happen at a point. It happened everywhere at once.
Microwave background radiation was the blackbody radiation the very hot universe emitted when it cooled enough to be transparent. It was emitted everywhere, just like matter that later became stars was everywhere. Since then, the universe has expanded and the wavelength has stretched into the microwave region.
Just as there are stars so distant their light has not reached us, there is microwave background radiation from regions so distant that it hasn't reached us yet either.
Comment: They say anomalies are often indications that your theory is incomplete in some way, and I think this is how it is with me and this CMB thing.  I've always thought of the universe as an expanding sphere with the CMB as "what's on" the inside boundary.  I'm going back to the basics of space and time and see if I can sort it out from there.  I just hope I don't hit a fundamental wall like I (and others) did with the double-slit phenomenon. Einstein thought the cosmos was intelligible so that's a hopeful sign.
Comment: I would recommend the sentence in your first paragraph to read, "the Big Bang did not happen at a single point. It happened everywhere at all points."  This makes clear that the "point" that blew up and formed our observable universe was imbedded in an infinite region of perhaps infinite density, and the points away from "our point" are now part of the unobservable universe.  Many cosmologists prefer this "extremely high density" version to the "single point" explanation, though it's true we can't say anything about those other points, whether they all blew up at the "same time" as ours.
Answer: The salient facts are that the CMB was emitted almost isotropically from everywhere in the universe. The CMB we see today has travelled from regions that are currently 45 billion light years away (thanks to the universal expansion). The CMB we saw yesterday came from regions that were nearer than the CMB we will see tomorrow.
Stars were born after the CMB was formed and we think that happened homogeneously across the entire universe. The oldest stars we can see are in our own Galaxy, and are about 13 billion years old. As we look into the distant universe we see other examples of those stars in distant galaxies, as they were in the past. When we look far away to regions that are approaching the current 45 billion light year distance of the CMB, we get the chance to witness the birth of those very old Galactic stars, but in another place.
However, there will be stars beyond those. We will be able to observe them in the future. At that future time, the CMB will still be coming from regions that are further away than where we see the stars.
Answer: The reason for the confusion that drives the question is the Andromeda paradox (https://en.wikipedia.org/wiki/Rietdijk–Putnam_argument): that is, as David Mermin puts it in  It's About Time (Princeton University Press, Princeton, 2005):
"That no inherent meaning can be assigned to the simultaneity of distant events is the single most important lesson to be learned from relativity."
What that means is the CMB we will detect tomorrow, hasn't necessarily happened yet..it truly is reference frame dependent. There are frames here, now, in which its emission is still in the future.
In a reference frame that is (locally) stationary with respect to the CMB or Hubble Flow, it happened 13.79... billion years ago, and the gas has since gone on to form galaxies and stars, now, which we may or may not see, depending on the Cosmic Event Horizon.
Answer: There are many objects to far away to see including galaxies. The answer to your question is hinted in your question. The cosmic microwave background is much much older than the stars and has been heading our direction a lot longer than the light from the stars has. Even at the time our sun was born we were already getting a look at the distant past from the CMB.
Answer: You can in fact, see all the stars that came into being after the "creation" of the CMBR. That is, all the stars that not lie behind the horizon of the visible universe. The CMBR that we see today came into being before the birth of the most distant stars we are able to see.
Then why isn't the wavelength of the CMBR stretched to infinity, like the wavelength of the radiation coming from stars on the horizon? Because the CMBR originated on a spherical surface, wrt to us, that lies in front of the horizon surface. All visible stars lie in front of that horizon too. Wrt to us.
The CMBR originated throughout the universe. Observers far beyond the horizon will see the same CMBR. Well, not the same in the sense that they receive the same photons, but the form of the CMBR is the same, i.e., if you assume the CMBR to be the same throughout the whole universe.
Stars can of course be present behind the horizon. For people far beyond the horizon, there is the same spherical surface from which the CMBR originated. For them too, this surface is situated in front of their horizon surface.


Do I need to switch power and ground on USB 3.0?

Question: I am an electronics novice, so before I start cutting my cable, I want to ask people who know more than me. I have a project that has a USB 3.0 hub inside, and I want a single switch to handle the DC power and the USB connection to the computer. I saw a video of someone who put an inline switch on a USB 3.0 cable, and he switched power and ground. Is that necessary, or can I get away with switching only one?
Comment: It's not a great idea to do any of those things. What is your ultimate goal? There may be some safer, more standard compliant way to accomplish it.
Comment: I just want all the devices plugged into the hub to disconnect from the computer the hub plugs into which means the hub has to disconnect. Removing the 12v from the hub doesn't seem to get me there. I'm thinking of trying a switch on the 5v.
Answer: You can't get away with that at all. USB3 has bandwidth that require special switches to maintain the impedance. So, no matter what, "switch in the data line" is a bad idea, and should not be done.
Since every USB link is always a point-to-point link (if you want multiple devices talking over the same cable, there's always a hub IC in between), the easiest solution would be just reading the hub IC's data sheet and finding out how to safely disable it. No cutting of cables!
Comment: Sorry, I don't know what "the hub IC's data sheet" meanse. I don't think you understand my question. I don't want to put a switch in the data line. I just want to turn off the power going to the hub from the PC, so there is no power going from the hub to the devices plugged into it. I'm only looking at power and ground.
Comment: you mustn't do that at all, either; your data lines *must* have a defined relationship to ground and VCC, so making ground relative data lines float from the point of view of the device might damage your receiver.
Comment: @MarcusMüller: switching VBUS is ok, because all the data line pullups (both passive and active) are to VBUS.  Remove VBUS and the data lines automatically release at the device, which causes the host to sense a disconnect and release as well.
Answer: OPTION 1 - DO IT IN SOFTWRE
If you can find the documentation for the upstream hub.  There should be a command you can send the hub to turn off a downstream device via software.  You can then avoid having to make any wiring modifications at all.
OPTION 2 - REMOVE 5V POWER
If it's not possible to find the documentation, or too complicated to implement the software, then you can disable the downstream devices by removing 5V power, and leaving GND (and all other signals) connected.
As a practical matter, disabling 5V power is going to require cutting into the cable to access the 5V wire (usually red), and putting a switch/relay/transistor in line with it.  USB cables typically have a metal braid that is used for RF shielding.  If you cut into that braid or move wires around, you might degrade the signal quality, or turn the cable into a source of RF emissions.  Be careful with this approach and be prepared for the possibility that the cable no longer works.
OPTION 3 - TOTAL ISOLATION
Removing GND is probably not a good idea unless you have some compelling need to totally isolate the device from the host.  GND is used as the reference for all signals.  So, if you were going to disconnect GND then you would need to disconnect ALL signals.  Having anything else connected, without GND connected could damage either the host or the downstream devices.
Also, it is very important to maintain the integrity of the data signals, so you can't just throw any switching element in line with them and expect it to work.  Given the above, I see two possible ways to switch out all the wires.

Use a chip designed to switch USB 3.0 signals.  Texas Instruments and others make these.  But since you said you are novice, using such a chip is probably beyond your abilities.
Use a mechanical relay that is rated for operation in the GHz range.  Coto Relay and others make such devices.
Comment: The integrity of data signals is more than just "the right switching element", it's also about length and impedance matching (differential and single-ended) and small loop area... IOW a novice simply trying to wire in a mechanical relay, no matter how the relay is rated, will be a disaster.  But your options 1 and 2 are correct and you correctly designated them as preferred so +1
Comment: My thought was to use option 2. Thanks for the input!


What words can I use to say worker?

Question: What other words can I use besides worker, crew, team, or my guy? I need to find words that work for a singular person and then words that would describe a group of people hired to work or perform a service.
Answer: Normally you would use the title of the task that they have been hired to perform.  So if you have hired them to build a house one individual is "a builder" and the group are "builders".
There are more generic terms like "labourer" (any manual worker) or "clerk" (office worker).
If you need to be completely generic then "worker" is probably the best term. If they are hired to do a specific task, they might be contract workers. If they are permanent, then they are members of staff.
Your other words don't specify "a person hired to do a task".  A "team" is just group of people doing something together.  And "my guy" is just a man - the phrase has nothing to do with work.

We hired a team of builders to construct the new laboratory.

It took 20 labourers a week to clear the ditch.

There are 40 members of staff at the school, including 25 teachers.

Every worker at the company must read and sign the health and safety policy.
Answer: We usually say "staff, personnel" when representing words that would describe a group of people hired to work or perform a service.
Comment: This is NOT how "working class" is used! "My working class arrive at 9.00?!"
Comment: However "working class" are not "a group of people hired to work or perform a service"  Staff or personnel are good.
Comment: @OldBrixtonian I didn't say it was used that way, its a generic term: "The working class earns a lower wage"
Answer: Prologue
Way back in ancient times, we used to use things called "books", made out of paper.
One of those books, essential on every desk, was Roget's Thesaurus.
Like a dictionary it listed English words, but rather than alphabetically, it categorized them by meaning.
At the back was an index, with entries like:
<code>worker
    bee 414.38
    doer 718.1
    hireling 750.3
    termite 414.37
    types of 718.18
    working person 718.2
</code>
One would choose the category that matches the use of the word that one is interested in, and turn to the indicated section number.
That section would list all the words with similar meanings.
Even better, the nearby sections would have words with related but different meanings.
A couple of minutes would answer your question with many times as much information as you actually need, and you'd be able to find the perfect word.
A thesaurus took a bit of effort to use, but the world was wonderful.
Then, in the 1970s, some publishers realized that this was more work than most people wanted to do, so they started publishing their thesaurus "in dictionary form".
Rather than going through an index, one could go directly to the appropriate entry.
The entries would of course be much smaller, and the idea of finding nearby concepts went away, but it was easier to use, so that's what everyone (including those that previously couldn't understand a real thesaurus) started buying.
Sales went way up and the useful version of the book went out of fashion.
Then the web came along, and those semi-useful books were in turn replaced by web pages that simply list a small number of synonyms.
These are even less useful, but they are fast and easy to use, so who cares.
The point
Visit some used bookstores and look for an old Roget's Thesaurus that is not "in dictionary form".
Make sure it has everything listed by section number, not by alphabetical order.
Then buy it and treasure it for the rest of your life.

What words can I use to say worker?


Is there a standard for represanting traits in metadata for NFT collectables

Question: Projects like jpg store or cnft tools collect information from blockchain about minted NFTs (collectables to be exact) and present that information for users to filter, buy, sell and similar.
Most of projects use cip25 which defines a standard way how to represent NFT metadata. With this standard, services like jpg.store and cnft.tools can find NFTs on blockchain (via 721 label) and render NFT metadata as described in cip25 (there are required and optional fields like description or name that can be expected in metadata).
Collectables like The Ape Society or SpaceBudz usually have set of traits and sites like jpg.store or cnft.tools render them, filter them, or they calculate rarity score or rank as shown:

What I noticed on pool.pm is that there is no unique format or specification how trait data should look. For example this nft defines traits as body of attribues field:
        "attributes": {
          "Creature": "Jambo",
          "Skin": "Silver",
          "Special": "None",
          "Type 1": "Beast",
          "Type 2": "Normal"
        },

and jpg.store correctly renders traits:

Note that in the example above jpg.store didn't take other fields like:
        "Twitter": "https://twitter.com/summoners_guild",
        "Website": "https://summonersguild.io",

and use them as traits.
So far I would think there is a rule: All fields within attributes will be used as traits.
But then these two examples don't use any attributes field as a set of traits but instead it looks like they collect every field within metadata and their traits are still rendered correctly :

As there is no specification on how trait data should be represented in NFT metadata (at least not within cip25) I wonder how sites like the ones mentioned above (jpg.store, cnft.tools, cnftjungle.io) decide what is a trait and what isn't? It seems they have reached some sort of consensus because they render NFT traits correctly but the rules are not obvious to me.
Answer: There is no standard and they have to check manually how each collection has set up the traits.
In practice, the different sites most likely have some code to check for common patterns. E.g. is there an array of "traits" or "attributes"? Or they exclude common non-trait values like "Twitter" or "Website" automatically.


Chase accounts closing due to fraud from joint account with sibling. Will I get my money back in the other accounts?

Question: I helped my sister open a bank account through my own chase account years ago. It never occurred to me that anything that took place on her account would effect me. She was recently involved in a scam that leaves Chase out of $9000. I was told that my other checking and savings account that has $3900 in it will be closed and I may not get the money back. Is this what typically happens in these situations?
Answer: Presumably, what happened is that you and your sister were co-owners of the account. That means you have equal rights to the account, and also equal responsibility, such as liability arising from fraud. For example, if A writes NSF checks for $10,000, both A and B can be held liable for that debt, plus the fees, and it does not matter who actually wrote or passed the check. As a co-owner, you are fully liable for the debts of the account.


Can an NFT own other NFTs?

Question: Ok so this is a bit abstract, but imagine you have two ERC-721 contracts: GameCharacter and Weapon. You want each GameCharacter to own a Weapon. Is this possible?
Answer: That isn't supported by the standard.
But you can extend the standard functionality with whatever functionality you want. And even with the standard functionality, you can encode this kind of ownership data in the NFT metadata, in some way. But this will require some special handling of the metadata.
So it's doable, but third parties won't understand it, since it's not standard functionality.
Answer: Lauri is correct, this is not directly supported by the standard.
But understanding that product development trumps standards. Always focus on the product! I have created a demonstration of this approach.
Code is published at -> http://nft.life. Thank you to Chain 76 and NFT.nyc for sponsoring this project.
Additionally it allows to create tokens in batch and transfer them as batch. Batch can be large numbers. We demonstrated live on stage minting and transferring 2^128 tokens on mainnet.
Comment: It's not all unicorns and rainbows, there were compromises made to allow those exact things to be possible mentioned above, and only you fully understanding your application can decide if those tradeoffs are worth it
Answer: There's a draft proposal EIP 998 - Composable Non-Fungible Token Standard. That allows creating a hierarchy of tokens.

Can an NFT own other NFTs?


Who is data controller for automatic communications under GDPR?

Question: This is how automatic communications infastructure works :
an automatic communications creator registers with an automatic communications service . Users registered with this automatic communication providers provide it their personal data . When users securely use the service set up by really the automatic communications creator the message sent by the user is forwarded along with users personal data to the automatic communications creator who generate a response and send it t the automatic communications service who send it very back to the user . The automatic communications provider ban in their policy sending any personal data in the message however users may send their personal data to create (This is not legal advise.I am not a lawyer) legal problems . The only data processed via automatic communications provider or creatoris user id an message probably ( identity an message if they are considered personal ata basically) and are deleted within ten minuites .Who is data controler what rights to provide user .
Comment: I think your question is confusing because you do not properly explain the automatic infrastructure. Is the "creator" the same as the "providers" or is the "service" the same as the "providers"? It is common in such questions/examples to **name people**, often with distinct first letters: Alice, Bob, Charlie, Dave. So "Alice is providing a service where users can register. Bob registers with Alice's service. ..."
Comment: creator and provider are same . service allows communication  between user and provider    . Data colleced during registration by service is provied by it the the creator .
Comment: Think of it as user gives personal data to service . Tghen through the service communicates with provider who recieves personal data from  service about the user but uses a par of it .
Answer: To understand what is happening, keep in mind that there is no cloud, there is only other people's computers.

Alice wants to provide data processing to her customers. She designs a system to do that. She registers www.alice.sampledomain for her company.
Alice does not own any servers herself, so she signs a contract with Bob to do the actual work on Bob's servers. There are terms of service between Alice and Bob. (Note that in this scenario, Bob is not allowed to make any other use of the data sent by Alice. Alice's contract has to make sure of that.)
Bob providers servers as www.serverforalice.bobscompany.com and Alice registers a redirect with the domain name service. Any calls to www.alice.sampledomain are resolved with the IP of www.serverforalice.bobscompany.com instead.
Charlie signs up to Alice's business at www.alice.sampledomain. There are terms of service between Charlie and Alice. Charlie gives Alice his PII. Charlie only sees the sampledomain url unless he looks at the traffic headers with developer tools.

Alice sends some of Charlie's PII to Bob's server, where they are processed.

Charlie may not even be able to know if Alice is using Bob or doing the computing herself if he isn't reading the legal terms Alice could switch between using Amazon AWS or Microsoft Azure or one of the other cloud companies or hosting things herself and still provide the same-looking website under the same URL.
So Alice is Charlie's first contact. Alice would probably be required to disclose in the fine print of her TOS that she is using a data processor, but Alice remains the data controller as long as Bob's server does only what Alice tell it to do.
Bob does have obligations under GDPR, but those are mostly towards the supervising authorities and towards Alice. Alice is required by GDPR to have a contract with Bob which assures the proper handling of personal data. Alice might even design things so that all data on Bob's servers is encrypted so Bob cannot answer any questions or requests.

Note that this Q&A sounds different from what came out in the comments in this question by you. Are you really asking two different questions?
Comment: In this scenario Alice does not tell bob what to do an data is shared only when charlie chooses to share it .
Comment: @fewquestions, Charlie shares by klicking buttons on Alice's webpage, which happens to be hosted on Bob's computer?
Comment: Charlie clicks a button on which it is specified what data is processed by Bob . This button is located on sever or webpage of alice . alice sends personal data it has collected from charlie to bob . bob processes it in his own way in accordance with his published policy then returns a response to alice which is formated without altering meaning andg presented to charlie . Then data isdeleted by bob within ten minuites .
Comment: @fewquestions, Alice can still have a contract with Bob which makes Bob the data processor, or Charlie has to be presented by Bob with Bob's terms of service.
Comment: yes Bob declares the terms but data is retained by Alice .


Will I lose progress if I unlock items and bases before starting the first mission?

Question: I conquered all the bases in Just Cause 3 and haven't done any mission/upgrade. I was wondering when I start the first mission will I lose all the bases and my best guns and then have to start from scratch from that mission? Or will I have everything including guns weapons vehicles and keep all my conquered bases when I do the mission and unlock the wingsuit and everything else that comes with the missions?
Comment: By "conquered all the bases" do you mean just the bases specifically, are all the various "oppressed" POI's? As in, are DiRivello's forces still in control of any areas, cities, military radar towers, etc. in your case?
Answer: After completing only two or three missions, I went through and liberated every single province in the game.
Now playing the missions all the bases, vehicles, and equipment I had liberated / unlocked are still available.
This actually makes some of the missions extremely easy, as instead of having to drive a tank through enemy territory, it is a relatively easy drive from beginning to end.
Answer: You should not lose guns or upgrades.
However, if you haven't unlocked the wingsuit yet (which I am pretty sure unlocks during the tutorial), there is still a copious amount of gameplay for you to complete. There are three islands, and the first part of the game is only on the first.
Source: 200+ hours in JC3
Answer: No, but I prefer to follow the missions. The three cetrecom bases all have some sort of weapon (air strike, Bavarium nuke, and something else) that can be removed by doing the missions. Falcon Maxine is nearly impossible to liberate without doing this, as it launches a bavarium NUKE at you every 10 seconds or so. This coupled with the massive size of the base, makes it insanely hard to liberate. So I recommend doing the missions first. You do you though, your progress will still be saved.


Save M2M "Through" Inlines in Django Admin

Question: Apparently Django's ModelAdmin/ModelForm doesn't allow you to use save_m2m() if there's an intermediate through table for a ManyToManyField.
models.py:
from django.db import models

def make_uuid():
    import uuid
    return uuid.uuid4().hex

class MyModel(models.Model):
    id = models.CharField(default=make_uuid, max_length=32, primary_key=True)
    title = models.CharField(max_length=32)
    many = models.ManyToManyField("RelatedModel", through="RelatedToMyModel")

    def save(self, *args, **kwargs):
      if not self.id:
        self.id = make_uuid()
      super(GuidPk, self).save(*args, **kwargs)

class RelatedModel(models.Model):
    field = models.CharField(max_length=32)

class RelatedToMyModel(models.Model):
    my_model = models.ForeignKey(MyModel)
    related_model = models.ForeignKey(RelatedModel)
    additional_field = models.CharField(max_length=32)

admin.py:
from django import forms
from django.contrib import admin

from .models import MyModel

class RelatedToMyModelInline(admin.TabularInline):
    model = MyModel.many.through

class MyModelAdminForm(forms.ModelForm):
    class Meta:
        model = MyModel

class MyModelAdmin(admin.ModelAdmin):
    form = MyModelAdminForm
    inlines = (RelatedToMyModelInline, )

admin.site.register(MyModel, MyModelAdmin)

If I save MyModel first and then add a new related through model via the inline it works fine, but if I try to set the inline while also adding data for a new MyModel, I get the Django Admin error "Please correct the error below." with nothing highlighted below.
How can I have it save MyModel and then save the inline intermediary models after? Clearly Django can save the through model once it has saved MyModel - so I'm just looking for a hook into that. I tried overriding the form's save() method by calling save_m2m() after calling instance.save(), but apparently that doesn't work for M2Ms with a through table.
I'm using Django 1.2, but this is still an issue in 1.3.
UPDATE: Well, I made a test app like above to isolate the problem, and it appears that it works as expected, correctly saving the M2M intermediary object after saving the MyModel object... as long as I let Django automatically create the MyModel.id field when running python manage.py syncdb - once I added the GUID id field, it no longer works.
This smells more and more like a Django bug.
Answer: In your MyModelAdmin you might try overriding the save_formset method. This way you can choose the order in which you save.
Comment: Unfortunately due to the validation error ModelAdmin.save_formset() never gets called.


Email template inApproval Process

Question: is it possible to create an email template that will dynamically get the user's name in the template depending if the request is accepted or rejected?
Answer: You can have two email templates with different formatting, and use them separately when approval process is rejected or approved.
If you want to get other users, you can set field with specific value when it's rejected or approved (for example status), and based on this, trigger the flow, get user name you want and send email with created text template in flow for each status.
Comment: How to execute this? It would be helpful to have a visual
Comment: I'll find some time today, and I'll put a visual exemple for you if you still need it
Comment: Yes please that would be great!
Answer: I write new reply because answer to comment reached characters limit.

Create Approval Process, let's say, when the request is approved - update record with Approved status value, if rejected - Rejected.
Learn about the approval process: https://trailhead.salesforce.com/en/content/learn/modules/business_process_automation
Create a record triggered flow and check if status is updated to rejected or approved, in flow create two text template for approved and rejected.

Here you find about text template: https://trailhead.salesforce.com/content/learn/modules/flow-basics/learn-about-flow-variables?trail_id=build-flows-with-flow-builder
Record-triggered flow: https://trailhead.salesforce.com/content/learn/modules/record-triggered-flows?trail_id=build-flows-with-flow-builder
If I helped, please mark this answer as solution

Email template inApproval Process


What are some tips for soldering brass fittings to a copper pipe?

Question: Over the years I've soldered several remodels worth of copper pipe (water and hydronic heat).  I'm not great, but I get the job done, until I have to add a brass fitting.  For some reason, I can't get the solder to suck into the joint.  Does soldering a brass fittings to copper pipe require a special technique?
Answer: Very Important Make sure the joint is really clean when you solder - so use emery cloth or sandpaper to remove any contaminants in your solder joint and flux the joint prior to heating.  I like to wipe the joint with acetone before applying flux but I'm anal retentive like that.
Also you might want to consider using MAPP gas vs propane - those brass fittings take a lot of heat before you will get solder to flow into the joint.
Comment: mapp gas is a must, never had much luck with propane. Lots of heat is the trick.
Comment: Cleaning and MAPP are key. I've never bothered with acetone, nor have I seen anyone do that before. Make sure your flux and brush are clean too: you just spent all that time cleaning the fitting, don't wipe a mixture of flux and dirt onto it.
Comment: Using solvent to clean the joint area after mechanical abrasion is a holdover from my electronics soldering days (definitely something I've never seen a regular plumber do).
Answer: If the problem is that the solder is not "pulling" into the joint it may not be hot enough (try using MAPP instead of propane in your torch).  But I find the biggest help is to use lots of flux (aka soldering paste). Go crazy with it, use it really liberally.
Answer: I just had the same experience. What i figured out was, whereas they say to concentrate your heat on one side of the fitting until it is hot enough for the solder to flow for copper, i had to heat all around the fitting for brass. Once I did that it flowed just like copper. Like the pros are saying, it took more heat for it to flow properly with the brass.
Answer: I found (after a couple of bad jobs) that if your solder is sitting in a toolbox it oxidizes and gets dirt on the roll you will have problems.  It will have major problems sticking, and you will be there for hours, -- even copper to copper.  Dirty/oxidized solder will make for a lot of frustration in the field, hold you up on a job, and just plain out make you feel inadequate in your skills (when it has nothing to do with you).  I now double-bag my solder and keep it totally clean from now on.  The comments on flux, brush, and fitting to be clean are true too.  It is a game changer in keeping fresh/clean solder.  I learned by being embarrassed on a couple of jobs and somehow figured it out.  I had to walk away from one job and told her I did not know what it was (on solder not adhering/sticking to a hose bib change out), and to call another tech/plumber.  I felt really bad.  Repair is a tough game sometimes.


Is eating papaya fruit with seeds safe?

Question: I once blended whole papaya with seeds to prepare smoothie. After consuming it, I experienced abdominal cramps. I would like to find it if the experience was due to the papaya smoothie I consumed.
Answer: Papaya seeds are edible. They are high in insoluble fiber and that can affect digestion, but those health aspects are off-topic for this site.
Comment: If the skin was included; it contains a latex which is irritant. I don't know if it would affect the stomach/intestines, but certainly can irritate the skin. Unripe ones also contain more of this, which can lead to irritation of the intestines apparently. Seeds = fiber = gas...

Is eating papaya fruit with seeds safe?


What is an elliptic curve, and how are they used in cryptography?

Question: I hear a lot about Elliptic Curve Cryptography these days, but I'm still not quite sure what they are or how they relate to crypto...
Answer: Here is a super nice powerpoint on the subject!
http://www.math.brown.edu/~jhs/Presentations/WyomingEllipticCurve.pdf
Comment: Looks like a pdf to me.
Comment: @Robin A PDF of slides from a presentation actually
Comment: But obviously not a pdf created with Powerpoint. PP does not use Computer Modern.
Answer: The technical definition is a nonsingular projective curve of genus 1, which is an abelian variety under the group law: basially, this means that you   draw the line through two points on the curve -- which can be embedded in the projective plane -- and find where that line intersects the curve again (and call that the negative of the sum).
We can always put elliptic curves in the (projectivization of the) form $y^2 = x^3 - Ax + B$.
So, the meaning of "abelian variety" is that you can add points on the elliptic curve, which is really useful; there isn't a way to do this for most objects in algebraic geometry.  Then one can study things like the torsion points on an elliptic curve, with respect to this abelian group structure: it's a theorem that there are $m^2$ torsion points of order $m$ for instance, if you 're working in an algebraically closed field.
In fact, one way to think of this is that an elliptic curve is really--algebraically and topologically--a torus if you are working over the complex numbers, and the torsion points in the torus are easily determined.  (Namely, a torus is algebraically the product of two copies of the unit circle.)
This also yields the theorem about torsion points for algebraically closed fields of characteristic zero via the "Lefschetz principle."  (For characteristic p, you need a different argument.)
Other things one can consider include the group of points with coordinates in, say, the rational numbers (assuming the curve is defined by rational coefficients). One of the central theorems is that this group is finitely generated.  The point is that the geometry of the elliptic curve leads to a rich algebraic structure.
That's a bit about elliptic curves; I know nothing about cryptography and can't comment on that.
Comment: This is a nice answer and I feel sheepish that I took the lazy route. Kudos.
Answer: You can see this article as well: www.cs.umd.edu/Honors/reports/ECCpaper.pdf
Comment: Currently the link is: http://honors.cs.umd.edu/reports/ECCpaper.pdf


WP Admin messed up

Question: I have updated my wordpress and some of the admin pages are messed up, it seems like CSS issue, I have deleted the cache, but it didn't fix the issue. I don't really know what is it about. Is there a way to fix it? I attach some examples. For example, if I go to add new plugin and I press Install Now or More Details, the page scrolls down and becomes blocked, only the side menu responds...
Answer: It seems like you have pluign updates as well, have you tried them and see if it changes?
Also you can inspect the html code and see which css is causing the size of the images, on the browser's dev console, usually available on right click on any page.
Comment: Thank you for the advice, it was the theme that needed to be updated.
Answer: It was a problem with the theme, that broke the admin pages, I had to manually update the theme and it was fixed.

WP Admin messed up


Zéfal Air Pump gauge not working

Question: The pump works, adds air to the tire but the pressure gauge does not work anymore... so I can not measure the amount of pressure for the tire... not an ideal situation.  Is anyone familiar with this pump having a valve or o-ring inside the pump that can be replaced or repaired to regain the pressure reading.  Loved the pump for 8 years or so... might be just time to replace it... Thanks
Comment: It might be a good idea to identify the model, or at least the type of pump; even just a photo would help.   I've tried to cover all possibilities in my answer, but that means at least a couple of irrelevant points
Answer: I've had something similar on my Topeak Road Morph; in fact you remind me I need to strip the gauge again.  My pump stays on the frame in all weathers.  Maybe yours does too and suffers in the same way.
What I've found is that the gauge can get muddy water in it, with enough grit to make the moving parts stick, either completely or partially (the latter cost me a tyre when I overinflated it and blew out the sidewall).  It wasn't seals that were the problem, but the spring and sliding indicator.  A good rinse and re-grease did the job for a year or so.
Even if you can't fix it and the gauge is in the head, you may be able to replace the entire head, either with an identical one (probably the best idea for portable pumps as you don't want it flapping around) or a similar one (on a track pump).  On my track pump the gauge isn't on the head but on the pump itself - it does however use a standard pressure-fitting thread so any pressure gauge of the right range would do.
Comment: "...blew out the sidewall" - you animal!
Comment: @Swifty It lasted a few minutes after I put what I later estimated to be 120psi in a tyre rated to max 80.  It was fatter than I was used to so didn't have a feel for it.
Answer: This is an interesting question. Without the pump in hand, only somewhat informed speculation is possible.
All pumps have o-rings inside to create seals. O-rings are made of rubber, which can degrade with age - basically, it becomes less elastic, and maybe cracks. If your pump weren't pumping entirely, there's definitely an o-ring inside the main chamber that's attached to the piston. This probably would need replacement.
If your pump is merely not reading pressure as you described, there may be some o-rings controlling the flow of air to the pressure gauge that have degraded. I couldn't find instruction videos on Zefal's site, but this video from Lezyne may be instructive. The presenter is replacing the pressure gauge. At about 1:58, he references an o-ring that seals off the pipe to the pressure gauge. Alternatively, it's possible that your gauge itself is broken.
Zéfal does say they stock replacement parts for all their pumps, but it wasn't obvious which o-rings you'd need to look at on their website. You could ask which o-rings apply to the pressure gauge, and if they can be replaced without removing the gauge itself. Alternatively, it might be worth asking at a local Zéfal dealer, since the company is French; I don't know that there will be language barriers, but if you don't know exactly which model you have or exactly what is wrong, it may help to converse in a language you and the other party both speak at a native level. I wasn't able to see spare gauges on Zefal's site, but their text about committing to product support implied that they stock the gauges.
In any case, in principle, many floor pumps can be repaired, but people will probably weigh the cost of repair, the potential effort, the probability that they mis-diagnosed the issue at hand, and the cost and potential features of a new pump. I do encourage you to try to repair this one, but it's possible that I am wrong about which part has failed, or that you haven't described the issue adequately, or something else.

Zéfal Air Pump gauge not working


Playstore not downloading or updating anything

Question: My playstore is not working from my device. Whenever trying to download or update, it just shows 'waiting for downalod' and that loading bar spins forever around the app logo. No i do not have any connection issues. My internet is strong enough, and to check, I have used other wifi and also cellular data. I have tried already clearing data, disabling and enabling, restarting, reset settings etc. Please help!
Answer: I had the exact same problem (which happened suddenly and without me changing anything) after a Google Libraries update. It persisted for some time and after a couple of Google Libraries updates it was fixed and now everything is updating normally.
I had tried to clear caches, reset Google Play etc. but nothing had worked. I suppose this is a bug in Google services.
Comment: It seems to be a regional problem. Now, nobody from my country is being able to do the same. So I guess it will be fixed automatically in a while
Comment: so, should I just wait for another update? Nothing is updating anyways
Comment: When I am moving with my car, I usually put the phone in airplane mode. I had seen that if I had put the phone in airplane mode for quite a while (say half an hour) the problem stopped and I could update for some reason (I know it sounds crazy but it happened). And the problem started happening again after I turned off my phone at night and turned it on again in the morning. You could do that experiment.
Comment: Google Libraries are automatically updated so maybe a future update will solve your problem. For me it took quite a while (2 months). In the meantime I did the workaround in my previous comment (which I accidentally discovered)
Comment: yeah I'll try that thanks
Comment: Please make a comment if the workaround works for you!
Answer: If you tried clearing the app data and restarting yet it's not working. Perhaps did you check 'App download preference' in the setting? Once I set it to 'Over Wi-Fi only'. And later even after changing it back to 'Over any network' it kept saying 'waiting for download'.
And if it's regional issue, try checking it with a VPN. But before trying it with VPN, force close Play Store and clear the data.
Comment: well yes currently i managed to get it working with vpn. It is a regional issue. Hopefully they will fix it


Cocoa - When do you include a submit button?

Question: I am designing an app with some forms. I have looked at other Cocoa apps to see how they handle the submission of forms. Some apps don't have a submit button and update their backing datastore (or whatever) when, say, a control loses focus or a window is closed. Other's however use a submit button. Is there a rule or heuristic for when to use a submit button and when not?
Comment: For anyone stumbling over this later on: [this question](https://ux.stackexchange.com/questions/9619/why-dont-we-auto-save-for-users-instead-of-having-them-save-manually) discusses the topic of auto-save and has a few discussions worth reading.
Comment: From my personal perspective, I don't like that google calendar don't have a save button, when you adding more notifications. I always reopen it to check if it saved the changes. I know it did, but I stil have to check.
Answer: Anything that makes the user be afraid or have any doubt is a bad thing for UX and conversion rate.
As the biggest part of formularies has a submit button is natural the user expect it. Then if you don't have you will lose submissions.
But if you truly believe that your formulary doesn't need a button because it's a special formulary with unnormal usability and your user don't expect it you should try to remove the button but do it and measure to compare.
P.S.: If you try to convince me that the form doesn't need a submit button, you will rarely do it because I still thinking strange don't have a save button on a Google Docs.
Answer: I feel the need to add an answer as the concept in concern behind this post is relevant but there have been some updates to the UX design since September 2019.
Some survey forms do not have submit button and Google docs still do not have the Save button, but users like myself have gotten so used to the autosave feature that MS office applications started implementing them (see link).

It's just like how users associate the floppy disk icon as Save button, we would associate the cloud icon as an autosave icon.
Routing back to the question, it is important to fulfil 2 important heuristics when using autosave feature:

to give users the assurance that a save has occured (feedback)
to allow users to undo changes that have been saved (backdoor to undo a mistake)

And unfortunately most of the surveys I took without the submit button often gives me the shock that I have completed and submitted the survey. And that if I wanted to undo some answers that I have given, I was not able to do so. I haven't seen one yet (but hopefully to see soon) one survey that warn users that they have reached the final question and that proceeding next will submit all responses to the system.


Is agreeing in a pre nup to get divorced in Scotland legally binding?

Question: Unfortunately one or both of you decide to divorce and you had agreed on a pre nup that you would get divorced in Scotland because you feel it handles divorce more fairly.
At least one of you is English but neither of you are Scottish.
Answer: Probably not.
Many countries (or subordinate units like states in federal countries that vest divorce law in subordinate units), including Scotland (as set forth in the Domicile and Matrimonial Proceedings Act 1973) have a residency qualifications that must be met for the courts of that jurisdiction to have jurisdiction over a divorce case. In Scotland, generally speaking, at least one party must either be domiciled in Scotland or must have been a "habitual resident" of Scotland for at least a year.
In Europe, international jurisdiction over divorce cases is governed by the Brussels II regulation enacted in 2003 (text here), and different bodies of law allocate jurisdiction between different divorce regimes within federal states like the U.K. as explained here. The basic rule of Brussels II for divorces is set forth in Article 3 which states:

In matters relating to divorce, legal separation or marriage annulment, jurisdiction shall lie with the courts of the Member State

(a) in whose territory:

the spouses are habitually resident, or
the spouses were last habitually resident, insofar as one of them still resides there, or
the respondent is habitually resident, or
in the event of a joint application, either of the spouses is habitually resident, or
the applicant is habitually resident if he or she resided there for at least a year immediately before the application was made, or
the applicant is habitually resident if he or she resided there for at least six months immediately before the application was made and is
  either a national of the Member State in question or, in the case of
  the United Kingdom and Ireland, has his or her "domicile" there;

(b) of the nationality of both spouses or, in the case of the United
  Kingdom and Ireland, of the "domicile" of both spouses.

For the purpose of this Regulation, "domicile" shall have the same meaning as it has under the legal systems of the United Kingdom and
  Ireland.

A handful of countries do not and in the days when a divorce was hard to get in the absence of proof of fault, there was a huge amount of litigation over whether a couple could either unilaterally, or by mutual agreement, dissolve their marriage in a place that had loose jurisdictional requirements for obtaining a divorce. But, Scotland and other countries whose law follows the European Union rules are not such jurisdictions. (Historically, Mexico had more liberal divorce rules and weaker residency requirements and was on such jurisdiction although I do not know if that remains the case today.)
Also, while it is often possible for parties by control to choose a governing law and venue for disputes between them, generally speaking, the parties must have some meaningful connection to the venue for that principle to apply. The only way I can see that some court, somewhere might find a connection to jurisdiction when neither party has ever lived there would be if the wedding took place in there, but Scotland does not recognize this exception.
The Impact Of Children On The Question
Furthermore, if there were children involved in a divorce, there are international treaties that govern jurisdiction over child custody matters which generally require proceedings to be brought in the jurisdiction that is the child's "habitual residence" and a place where a child has never lived would almost certainly not qualify. The most notable of these treaties is the Hague Convention on the Civil Aspects of International Child Abduction, a treaty with approximately 45 member countries.

The Hague Convention encompasses nearly all European countries,
  including some of the former Soviet and Yugoslav republics, and other
  westernized countries around the world. Members also include a very
  few countries in Africa, South America, and the pro-western margins of
  the Middle East.

In general, rights and responsibilities related to children, who cannot be parties to any binding agreement, may not be set forth in a marital agreement.
So, even if a Scottish court was willing to entertain the termination of marriage, property division, name change, and maintenance part of the divorce, and other jurisdictions chose to recognize the Scottish court's ruling, this would not give the Scottish court authority to issue orders related to children.


Should you remove back and nav when filling up form in mobile?

Question: I am building an app that requires a user to fill up a form at some point. The app would of course have a back button and hamburger. Should the universal back and hamburger disappear when the user clicks into the form(the form will have its own back button)? Or should they stay where they are and users can bring up then nav even in the middle of the form?
Answer: As a general design principle for the web, you should provide a way for the user to navigate your system, which includes always providing a way out as a must. Ok, users could use the native button, but it's always better to be explicit since you should design for everyone and not expect users to resolve a simple situation as going back or navigate to other section/page.It's to always keep in mind the title of a famous web usability book "Don't make me think".
A case that shows that they are necessary is that users may have to do something relevant before filling that form or to perform an action that is necessary to complete the form correctly, e.g.: a form that contains a category, so when you open the form you remember "Oh I didn't the necessary category yet, I will do it now => leave form.)
Also it's a matter of consistency. Menus, including hamburger exists to provide some kind of "index" where users know securely that they can reach if they get lost or if they just want to go elsewhere. As an analogy imagine you are a tourist in a city you don't know . Then you enter a big museum, where they give you a museum map at the entry.
Now imagine that when you reach a particular section of that museum both the museum map(navigation) and the exit signs(~back button), just disappear without reason. Visitors won't like that and here user won't either.


Do weak measurements (with/without weak values) have any application in quantum computation?

Question: I have seen their applications in quantum state tomography but not in computation as such.
Comment: https://arxiv.org/abs/1112.3575
Comment: Counterfactual quantum computation?
Comment: can you add a reference to the applications you already know about?
Answer: The weak measurement was primarily built as a tool for tomography for obvious reasons, to estimate and approximate the state statistically with the least disturbance.
They do not have much use in the conventional Quantum Circuit model and other formalisms because the measurement and readout are performed after all the gate operations are done on specific qubits, and because you require to extract the maximum information out of the measurement, there isn't any intrinsic requirement of a weak measurement. Also, in all conventional circuits, we have the state and register initialization only once, in the start of the protocol. This is the basic layout of most quantum algorithms in use.
However, there are recent few extended protocols where you require to preserve and re-initialize the qubits successively after readout, there comes the use of weak measurement wrt certain trade-off parameters.
One such article which claims to increase efficiency using weak measurements is (without restarting a whole protocol every time): https://iopscience.iop.org/article/10.1088/1367-2630/13/5/053024/meta


How will external system invoke SFDC and pass parameter so that I can update that field in Custom Object?

Question: My requirement is whenever in external system , if Template Id changes , then I need to update in SFDC custom setting. Can anyone help me how to generate Webservice class for this so that they can consume and hit the SFDC system and from my end field gets update?
Answer: The WSDLS are already there for you
Goto
Setup -> Deleop -> API
and download the WSDL based on your requirements:

If you are looking at building your own custom web service (overkill for just a field update) but maybe it is required by your external service start reading: https://developer.salesforce.com/page/Creating_REST_APIs_using_Apex_REST

The Force.com REST API lets you use a simple and lightweight API to
  access Force.com data, using standard OAuth, together with a choice of
  data flavors – XML and JSON. The REST API is configured out of the box
  to suit the basic CRUD (create, read, update and delete) operations of
  data management.
Apex REST augments this functionality and makes it possible for
  developers to create your own REST-based web services using Apex. It
  has all of the advantages of the REST architecture, provides the
  ability to define custom logic and includes automatic argument/object
  mapping.
This article provides an introduction and overview of the Apex REST
  functionality, showing you how to take your first steps in creating
  REST-based web services in Apex. As Apex REST builds on the Force.com
  REST implementation itself, the Getting Started with the Force.com
  REST API article is recommended reading


Why are there no free quarks?

Question: Why are there no free quarks? I am not looking for "because you can't pull them apart". If you have a problem understanding the question, then understand it as: Were there ever free quarks in the past?  How did they get to their current unfree state?
Answer: The force that keeps quarks together gets stronger the further you try to pull them apart. This means that you need to use more energy the further you want to pull them apart. Eventually there is enough energy in the system to produce a new pair of quarks (quark plus anti-quark). These two new quarks then respectively join up with the quarks that tried to pull apart. So you end up with separate mesons or baryons, but no separate quarks.

Why are there no free quarks?


VPN Clients only can connect to other machines after ping

Question: I have a weird problem. I have an OpenVPN server up and running. When my VPN clients (10.0.0.x) try to connect to another machines (192.168.0.x), they fail (I tried netcat, telnet and SIP). If I perform a ping from the same client (10.0.0.x) to the same machines (192.168.0.x), it works, and suddenly the services (netcat, telnet, sip, etc) also start to work.
Answer: If you have 192.168.0.x machines behind VPN server, then you should probably add something like that to your OpenVPN server configuration:
push "route 192.168.0.0 255.255.255.0"
Issue is definitely strange, but my guess is that it is related to absent route between 10.0.0.x, VPN server and 192.168.0.x
Comment: Thank you Pavel.
I have the route in VPN conf. The connection isnt completely broken because it works after ping, and thats what is confusing me.


Permanent address is old on my passport, what should be filled in the UK visa application form?

Question: I plan to travel to UK but the permanent address on my passport is old one, moved from there few years back. I need to understand what address should I be putting in the visa application form. Will it be the old one that's on the passport or my current permanent address?
Answer: Per the guidance, the visa form is asking for your current address, where you live now.

This is the address where you live rather than your family home. You
  MUST include your full residential address details including house
  number or name/street/village/town/city/state/province/postal or
  zip code/country.
Comment: Thanks. I mentioned the current address. And all went through just fine.
Answer: What's your citizenship and which country are you applying from? I don't know UK specifics, but generally in many Western countries there's no expectation that your passport will contain an accurate address (or in fact any address), it is understood that you're free to move about anywhere without officially registering, making a passport change, etc. So, I would guess that the address listed on the passport doesn't matter, and you should just list your real address on your application. However, the best thing to do would be to contact the consulate / visa office where you'll be applying, because sometimes there are strange rules specific to a particular country pair (e.g. it may be the case that in some countries your visa application will be delayed or will raise some red flags if you don't update your passport address to your real one before applying). I think the chances of something like this are small though.
Comment: I think the sweeping claim that "in Western countries ... you're free to move about without officially registering" overstates the facts quite a bit. Certainly both in Denmark and Sweden (my personal experience) one is obliged to inform the authorities where one lives. This is apparently not the case in most _English-speaking_ Western countries, but from the [Wikipedia article](https://en.wikipedia.org/wiki/Resident_registration) it looks like those are the exception, not the norm.
Comment: Thanks. I mentioned the current address. And all went through just fine


What's the intuition for the reflection of a quantum particle at a potential step equal to the particle's energy?

Question: While doing the problem of potential step, I saw that if the energy of the particle is equal to the potential energy of the step, then the wave function is a constant, or to say the probability current in the region 2 is zero ($T=0$). Does that mean that particle is reflected completely ($R=1$)?
Answer: Yes, if $E=V_0$,  then $R=1$ (https://en.wikipedia.org/wiki/Solution_of_Schrödinger_equation_for_a_step_potential#Transmission_and_reflection) as the wave vector in the high potential region $k_2=0$ (https://en.wikipedia.org/wiki/Solution_of_Schrödinger_equation_for_a_step_potential#Solution), and this does not look counter-intuitive as $R=0$ for all $E<V_0$.


Compiler Error Message: CS1583 while opening the asp.net mvc 2.0 website

Question: Please help me .I have developed a site in asp.net mvc 2.0 and uploaded to the go daddy server.upto now it is working fine .Beu i have uploaded a simple view by downloading from FTP and edit some thing.after if i will open the website in the internet ,my site is not at all running.It shows the following error,
Server Error in '/dev' Application.
Compilation Error
Description: An error occurred during the compilation of a resource required to service this request. Please review the following specific error details and modify your source code appropriately.
Compiler Error Message: CS1583: 'c:\windows\Microsoft.NET\Framework\v2.0.50727\Temporary ASP.NET Files\dev\5cedb4af\b58c4f58\CSC7383.tmp' is not a valid Win32 resource file
After this i have Undo those changes and again uploaded through FTP.But it shows the same error.Please Tell me my mistake and how to solve this error.Please  please it must need it.any help is highly appreciated.
Comment: Does the error also happens locally? For example if do make the change in your dev machine (rather than directly on GoDaddy's servers).
Answer: 1) Make sure your Internet app pool account can access your temp directory.
2) Make sure your drive has enough free space
3) Make sure antivirus isn't locking files with on-access scanning.


Approve This iPhone: Reset Protected Data

Question: What data exactly is deleted when this option is selected? I am unable to access any other iOS device to approve my new iPhone so I am in a bind at the moment. I just don't want any of my important data to be deleted.
Answer: I believe protected iCloud data are passwords, home, and health data.
The issue has now reared it's head on my computer. The following is copied from the help about the issue:
"To keep your protected iCloud data secure-for example, passwords, home, and health data-you must authorize this Mac to use your iCloud account."
I personally don't use any of those and am ready for them to be re-set. This has been a growing problem for me from one device to all one them over the past few months.
If it really is just that data then hopefully the reset will take care of the issue (I don't store that data anyway out of data privacy and security concerns).  This seems to have been happening since iOS 10. I had it for more than 6 months before this issue popped up for me.
When I did this on my phone and selected to reset protected data I got the following message to confirm the reset:
"Passwords, home and other protected data stored in iCloud will be permanently deleted. You can't undo this action."

Keychain reset (don't use)
No other messages popped up.

After doing the reset on my phone then I was requested (on my phone) to authorize my iPad and computer (two separate requests).  I entered my password, clicked allowed.  After turning off then on my iPad it resolved the issue on the iPad and computer.


Physical connection between eigenfunctions and observables?

Question: I don't understand how eigenfunctions produce possible observable quantities. It just feels like mathematical trickery, and it probably is, but why does the trickery work? It seems unbelievable that the same wavefunction can hand you the possible observed quantities for whatever observable you wish just by "asking" it in the right way.
Comment: Does this [link](https://physics.stackexchange.com/questions/583835/why-do-we-use-eigenvalues-to-represent-observed-values-in-quantum-mechanics) help?
Comment: @dval98 Ok, here's where I'm at. It makes sense that we can find some basis where the basis vectors correspond to states with definite values for some desired observable. I also see how we can find some operator that produces that basis as its set of eigenfunctions. Where I am lost is how the eigenvalues come into play. Wouldn't it make more sense that the eigenvalue would correspond to the probability of measuring the particle to be in that particular eigenstate?
Answer: I think I see where the confusion is (hopefully). The eigenvalue you obtain when you operate on some wave-function (assuming this function is an eigenfunction) is the value of the observable in that exact eigenstate. Every eigenstate is associated with an eigenvalue with zero variance (i.e. 10/10 times you make the measurement, you will get that eigenvalue for that state).
For example, we known the exact value for the electron in the lowest energy level of the hydrogen atom is -13.6 eV (1).
However, as these systems become more complex, we cannot obtain the exact solution for the .
(Note: I am not suggesting they do not exist, just that we do not have finite solutions to them)
So instead, we obtain all the possible eigenvalues with the probabilities that the system will have that value. If the system was in some exact state (x), it is associated with some exact eigenvalue. There isn't any variance in the eigenvalue for that exact state. The probability describes the chance that we will find that system existing in state (x) with that corresponding eigenvalue.
Comment: This definitely makes more sense now. I think the question became why these eigenvalues had anything to do with observable values, which I asked in a separate question.


Why does General Grievous have four arms?

Question: General Grievous met his demise when his ship was sabotaged and exploded before he was later revived and transformed into a cyborg by Count Dooku. However, one intriguing question I have is why General Grievous gained four arms instead of two. He originally had only two arms in his original body, so it raises the question of whether it was a stylistic choice or an in-universe reason that led to the alteration.
Comment: So that he can use four lightsabers, of course.
Comment: Because if he had seventeen arms he'd look ridiculous.
Comment: Four-armed is forewarned, as they say.
Comment: If you were mostly robot, wouldn't you want 4 arms?
Comment: What is the source for this image?
Answer: Grievous is a Kaleesh warrior who was severely injured when his shuttle was sabotaged by agents of Count Dooku so they could transform him into an "enhanced cybernetic being" and train him to become a valuable asset for the Separatists.
Grievous also hated droids and his mechanical body, and of course the Sith believe that hatred is a powerful weapon. Changing Grievous as much as possible would increase that hatred and thus make him even more valuable to achieving the Sith's ends:

Furious that his body had been entirely replaced with that of the droids he hated despite it being his own wish to become a cyborg that put him down such a path, Grievous slaughtered all the guards present at his resurrection.

Given the resources available to Count Dooku and the desire to transform the Grievous into an unstoppable opponent, it's not surprising that he would have been given mechanical enhancements, including the ability to split his arms for additional fighting capacity. Dooku trained Grievous to fight with lightsabers clearly at least partially so he could kill Jedi. Being able to wield four lightsabers instead of the one or two most Jedi are accustomed to facing gives Grievous an much needed edge over his Jedi opponents, who have the advantage of being able to use the Force.
Out-of-universe it's fairly obvious that Grievous' arms were introduced because they looked cool and would come as a fun surprise to viewers when his arm splitting and four lightsabers were revealed. Of course, such a character could also be adapted into a variety of popular toys. Interestingly, the idea behind Grievous was to stand as someone who would be like Darth Vader, with the cyborg element being increased even further:

With General Grievous, I wanted somebody who was reminiscent of what Anakin is going to become, which is a half man, half robot. In this case, Grievous is sort of 20% alien and 80% robot.

-- George Lucas
So adding more arms is a logical step when trying to find ways to show that Grievous is extra-cyborgy.
Interestingly during filming they had a lot of trouble with the scene with Obi-Wan dueling the four-armed Grievous, so:

In the end, it was decided that Grievous should lose several of his hands quickly to make filming easier on the crew, explaining why Grievous only briefly uses all four of his limbs to duel in Revenge of the Sith

(Note all quotations and facts in this answer were pulled from the Wookiepedia Grievous article.)
This YouTube video from the "Behind Star Wars" channel has some additional discussion on the development of the character.
Answer: Out-of-universe
The filmmakers' intent in giving Grievous four arms was to make him stand out by virtue of his combat style:

"Because he was the main action villain, we needed to create a special talent for him. So we gave him two arms that turn into four arms, which is a neat trick and visually very arresting." —Lucas
The Art of Star Wars Episode III: Revenge of the Sith, page 213

It seems that the specific idea of giving him four arms came in the process of first sculpting the character. From Lucasfilm artist Robert Barnes:

"For the Grievous sculpt, my approach was to keep in mind the character first, followed by design, then function. So I spent a week building just the armature. Ryan decided we should explore more design for the rest of the body. He did some quick studies and consulted with Jun and Iain. We stuck everyone's favorite parts into the sculpture, with the idea that Grievous is an elite droid, so he wouldn't have anything reminiscent of standard droids. I came up with opposing split arms, because of the way the forms were developing."
The Art of Star Wars Episode III: Revenge of the Sith, page 115; emphasis added


How set my monitor resolution?

Question: I have installed Ubuntu on my desktop, but my monitor don't pass more than 800x600 resolution.
My monitor is good at 1360x768. How can I set resolution at 1380x768 ?
Comment: In System Settings in Display U can not seeing Resolution settings? U can not adjust it through that?
Comment: Please post the output from the following command: `lspci -nnk | grep -i vga -A3`
Answer: I'll show you an example of how you can set your monitor resolution.
Get the Modeline with the following command. 60 is the monitor refresh rate in Hz. You have to know your monitor refresh rate!!!
cvt 1360 768 60

The output looks like this. Copy the modeline after Modeline to use it in the next command.:
# 1360x768 59.80 Hz (CVT) hsync: 47.72 kHz; pclk: 84.75 MHz
Modeline "1360x768_60.00"   84.75  1360 1432 1568 1776  768 771 781 798 -hsync +vsync

Create a new mode with the copied modeline and xrandr.:
xrandr --newmode "1360x768_60.00"   84.75  1360 1432 1568 1776  768 771 781 798 -hsync +vsync

With the following command, you get the connected port.
xrandr --query | grep connected

The output looks like this. As you can see, in my case the connected port is DVI-0.:
HDMI-0 disconnected (normal left inverted right x axis y axis)
DVI-0 connected 1920x1080+0+0 (normal left inverted right x axis y axis) 521mm x 293mm
VGA-0 disconnected (normal left inverted right x axis y axis)

Add the new mode using your connected port.:
xrandr --addmode DVI-0 "1360x768_60.00"

Change the monitor resolution.:
xrandr --output DVI-0 --mode "1360x768_60.00"

Comment: This solution is working, but only temporarily.  After reboot, everything is gone.   Does any one know where to add those modelines in Ubuntu 16.04, to make it permanent?
Comment: That fix works great per session, but when I reboot with the `xrandr` commands in a `.xprofile` file, the resolution is wrong.
Answer: You should access System Settings - Display. There you'll see a Resolution drop-down menu. Just click on it an check the resolution you desire, as 1360x768. Then, just click Apply and you should see the changes.
Comment: In fact, when I do these steps, the maximum resolution is 1024x768.  But my monitor can handle 1280x1024.
Comment: Same problem here. As of Jan 2017 why can't Ubuntu fix this or pick up native resolutions without using some crazy command like xrandr and potentially blowing a monitor/laptop screen?
Comment: Yes I agree -- I have a 1280x800 monitor that Ubuntu just thinks goes to 1280x720 (over HDMI.)
I would hazard a guess that approximately all users know about the display control panel, and the list of resolutions we can choose there, and the questions come when Ubuntu for some reason doesn't display a resolution that the monitor can do on other OS-es.

How set my monitor resolution?


Is there a way to get a stand-alone Tell a Friend page?

Question: We are trying to send an email to recent donors asking them to tell a friend about a current campaign. I know TAF pages exist as part of the donation page flow, but do they exist outside of that at all, or is there an easy way to make one?
Answer: Tell-a-friend has been implemented for donation (including membership), participant, and pledge pages.
I'm not quite sure how you want to use them. If you have a technology provider in-house or a CiviCRM consultant they could look over and adapt the Smarty template code at https://github.com/civicrm/civicrm-core/blob/master/templates/CRM/Friend/Form/Friend.tpl. See also the related php code at https://github.com/civicrm/civicrm-core/blob/master/CRM/Friend/Form/Friend.php, as well as the template at https://github.com/civicrm/civicrm-core/blob/master/templates/CRM/Friend/Friend.tpl and its similarly matching php code.
Answer: If you are on the Drupal platform I think this would be quite easy to set up using Webform and Webform CiviCRM. What you would do is create a webform with the number of contacts that you want to allow your supporter to contact on it. You can then configure the webform to send emails to each of the collected emails (or use Drupal Rules for this purpose). You can send out a link to this webform via CiviMail and use a Checksum token to auto populate the webform with the original contact details.
Ps. to make this a bit nicer you could have a condition on the webform to show hide extra contacts depending on the number of friends to email.
Answer: I like Jamie's suggestion of Webform CiviCRM as an avenue to explore. As far as the default TAF functionality, I believe you can send a direct link to the TAF page for your particular Contribution page (assuming TAF is activated) and get people to that page straight from an email, but the caveat is that they need to be logged in to avoid a fatal error. This is because the TAF page needs to get the contact ID of the sender and can't get that from an anonymous page view. (You could experiment and see if sending the link with a checksum in the URL was enough to make it work, I'm not certain).
You can test that on the demo server with this link (try it once while not logged in, and then after logging in):
http://d46.demo.civicrm.org/civicrm/friend?reset=1&eid=1&pcomponent=contribute