I have to dash out. Let me know when you've got the cable. It's worth getting a good cable. I have some cables with gold plated contacts that I use for stuff like this. How much difference it makes I'm not sure, but the gold plating means the contacts don't corrode.
@SirCumference well, the hubble radius is usually a good approximation of the cosmic EH and is denoted as $\frac{1}{aH}$. A universe dominated by a cosmological constant has $H\propto const$, which makes $d_{EH}\propto\frac{1}{a}$, but phantom energy is not a cosmo. const. It causes more excessive expansion
so, you'd probably find it proportional to a function that decreases faster than $\frac{1}{a(t)}$
I have tried very hard to comply with the spirit of this forum but obviously still have not got the hang of it. I write a short comment and then am told it should be an answer.
I answer a question and it is down voted but no reason is given.
Here is an example http://physics.stackexchange.co...
@SirCumference That'd be a good approximation, but the actual event horizon would be a bit more complicated. After all, the Hubble radius is only a good approximation for the event horizon
@Jim So, my question is: if the scale factor constantly increases in a de Sitter universe as well, and $d \propto \frac{1}{a(t)H}$, then shouldn't the event horizon be increasing in distance over time?
Today, the size of the close-votes queue is 55k questions and growing. Is that a problem?
Which action could be taken to clean it up? It seems to be growing faster than Stack Overflow members are capable to handle it.
There are (at the time of writing this; see edits at bottom of post) 54.2k questions with close votes on the review page on Stack Overflow. Unlike the other review tasks, which all usually return to 0 quickly, the number of questions with close votes continues to rise. According the this Meta Stack
I know this question is going to make me unpopular here – but I feel I have no other options.
I have been concerned by the size of the close votes review queue for quite a while.
When I started reviewing (less than a year ago) this queue already had ~30K questions pending review. Time passes an...
@Emilio, remarkable, was not aware of that. but have noticed that reviews are timeconsuming and generate no "gamification" incentive. now looks like a bigger problem than with one site, more like a SE architecture/ scalability glitch. but must say dont have huge sympathy because sometimes seems like SE makes it too easy to/ puts too much emphasis on closing questions (aka "FGITW")...
This got me thinking...
Not about a contest though. I don't want people reviewing questions so that they'll get something in return; folks should be doing it because they care about the questions, about their community. If folks don't care, then what's the point?
But maybe folks do care, maybe...
@SirCumference sorry, my $\frac{1}{aH}$ is the comoving hubble radius. I'm so used to working in comoving coordinates. The proper hubble radius is $1\over H$
This is just depressing:
And I don't mean it's depressing that there are that many questions that might need to be closed. There are nearly 6 million open questions on Stack Overflow - if under 2% of those are crap, we're doing great. Realistically, that number is probably going to get a lot b...
::shrugs:: i just don't think that we need to argue over the noise from pinging, that's all. besides, as a member of SE, you know that rep doesn't necessarily decide problems.
Stage 2 completed: The initial phase of the burndown was completed on 2014-03-03 10:48:49; the second phase completed on 2014-03-08 at 19:41. Detailed statistics on how these played out can be found on Phase 1 results and Phase 2 results.
There remains much work to be done! You can find r...
Interesting thing I just noticed, it's been 2 years since I got hit by a car, and only know is the difference in muscle on my right and left calves evening out
This might seem as a stupid question but I would like to upload a word document as an answer to a question. I can convert the document to html and copy its contents but most of the formula disappear when pasting into the stack exchange answer box.
I dont have a website, which could refer the inte...
@JohnathanRalls at very, very high energy densities quantum electrodynamics predicts there will be some interaction and scattering. However I think the energies at which this happens are experimentally inaccessible at the moment.
Scattering of light by light does not occur in the solutions of Maxwell's equations (since they are linear and EM waves obey superposition), but it is a prediction of QED (the most significant Feynman diagrams have a closed loop of four electron propagators).
Has scattering of light by light in ...
Yes, but the energy densities involved are astronomically high. Even if you could increase the interaction by a factor of 100 or 1000 you'd still be talking about vast energy densities.
Actually you do get non-linear effects in some dielectrics. This is called non-linear optics. However it is the interaction of the light with the medium that is non-linear not the interaction of light with light.
I asked him/her as a favour to stick to a standard procedure on this chatroom and s/he refused, so I withdrew from the conversation and indicated I was doing that .
@EmilioPisanty I asked a few people I know and neither of them had either - I'm kinda puzzled how it might generate so much traffic for us if no one really knows it.
@JohnRennie i feel i need to apologize for the nonsense going on, and would request to steal some of your attention towards an inquiry you may have knowledge about.
@JohnathanRalls There is lots of stuff on self-focussing of lasers in plasmas. But that's still just the interaction of the light with the medium not of light with light.
On p94 of Fetter & Walecka's Quantum Theory Of Many Particle Systems, it is stated that since propagators with both arguments at the same time such as $G^0_{\alpha\beta}(x, x)$ arise from contractions of field operators in the 2-body potential, which always appear in the order $\psi^\dagger_\beta...
oh that's fine. i'm not all that interested in light on light interaction, but was curious. if i am able to reproduce a self-focusing effect through a plasma that'd be sufficient
@JohnathanRalls I don't know of anyone in the site who does experimental work on light-plasma interactions. To be honest we tend to the theoretical end of the subject.
@JohnathanRalls No. The density of ions in a spark is pretty small and they recombine almost instantly. I'd be surprised if the density were anywhere near high enough to affect light significantly.
what would be the difference in using an electrode embedded in the container to make contact with the gas as opposed to using an electric field through a coil?
Putting the electrodes inside creates a discharge tube. But these only work at low pressures where the plasma density would be too low to do much to the light. I can't see how putting coils outside would do much.
If the Hubble parameter truly changes with time, will the event horizon's proper (and comoving) distance be time variant in a de Sitter universe, considering $d \propto \frac{1}{H}$?
@JohnathanRalls The problem is the massive amounts of energy required. Ionisation energies tend to be 5-10 electron volts per atom. Multiply that by Avagadro's number to get the molar energies required and the results are scary.
@SirCumference I can't remember the details I'm afraid.
@JohnRennie could you point me in the direction of some literature? something more directed than bouncing from article to article would be ideal, if you don't mind.
I am 21 years old and in my 3rd year of b.tech. Though I have a huge interest in Computer science but from previous 5 months I want to pursue Physics and want to become a Physicist.
so is it too late for that?
Can i still pursue physics and if it is possible then how should I learn it from basi...
@JaimeGallego Yeah, it's not a very commonly advertised feature.
We do prefer to have people earn their access to chat the "normal" way, by getting 20 reputation. But not if it means upvoting a post that doesn't deserve the upvotes.
Incidentally, what @Oswald was proposing is technically full-fledged vote fraud - one of the cardinal sins of SE. (But we genrally don't punish it in cases like this because it's an honest mistake and we'd really rather just say "hey don't do that" than get into official warnings and suspensions and so on.)
A non-standard cosmology is any physical cosmological model of the universe that was, or still is, proposed as an alternative to the then-current standard model of cosmology. The term non-standard is applied to any theory that does not conform to the scientific consensus. Because the term depends on the prevailing consensus, the meaning of the term changes over time. For example, hot dark matter would not have been considered non-standard in 1990, but would be in 2010. Conversely, a non-zero cosmological constant resulting in an accelerating universe would have been considered non-standard in 1990...
@SirCumference So, given that the article explicitly mentions the $\Lambda$-CDM model as the currently standard model of cosmology, and that that model does not include the possiblity of a Big Rip, what's the question?
@ACuriousMind My guess is a disagreement between us and the reviewers over the extent to which editors should add original content to someone else's post.
which is precisely why I'm sceptical of the claims that whatever they measured is actually the "wavefunction" because standard QM also has phase freedom, i.e. for any quantum state the real and imaginary part of the "wavefunction" is not actually defined.
@ACuriousMind ā question on that... do you know any refs that discuss this "phase freedom" esp wrt weak measurement(s)?
@vzn Every non-beginner quantum mechanics text should discuss that quantum states are actually rays and not vectors in Hilbert space, i.e. $\lvert \psi\rangle$ and $\mathrm{e}^{\mathrm{i}\phi}\lvert \psi\rangle$ denote the same state (which is what I meant with "phase freedom").
@ACuriousMind ok. anyway, think your phrasing did get to the crux of the issue. the pt is, maybe there is something new here wrt weak measurement. std qm theory posits only |psi|^2 can be measured and not psi exactly. suppose weak measurements are giving statistical measurements on psi itself. that would not actually contradict std QM would it?
@vzn You can't "measure" $\lvert\psi\rvert^2$ either - best you can do is do a lot of position measurements and infer it from that. (There are ways to know $\lvert \psi\rvert^2$ exactly, but in that case you know the states of the system exactly and figured out what state the system is in, so you know its wavefunction, that's not what I'd call measurement). I don't know what a "statistical measurement on psi itself" is supposed to be.
@ACuriousMind ok, am not an expert on qm so this is difficult, but the general idea is that (to borrow some of your own framing) suppose a new theory removed the phase freedom, in such a way that it can measure phases, but not in a way that contradicts (std) QM. that seems to fit the bill of a "deeper theory"... & it seems like weak measurements nearly or precisely make that claim...
@vzn Measuring the phases would contradict standard QM because it is an axiom of standard QM that vectors belonging to the same ray stand for the same physical state. If you could measure the phase this would mean they are physically distinct states.
let me try to phrase this another way. theory I proposes A, B can only be measured in combination. theory II proposes A, B can be measured separately but not ever in any way that contradicts measuring them in combination by theory I. therefore, the theories are not in contradiction. but theory II is "deeper" than theory I.
@DanielSank "Measure" in the sense of "figure it out to sufficient accuracy", yes. "Measure" in the sense of "there's a self-adjoint observable whose eigenvalues correspond to the possible measurement results for this quantity", no.
@vzn You are misunderstanding the problem. (I assert that) standard QM asserts that the phase is not a property of the physical state. It's not "unmeasurable", it has no physical meaning and is only there because the mathematical description in terms of Hilbert space instead of projective Hilbert space (where all rays are collapsed to point) is more convenient. Anything that "measures" "the" phase contradicts standard QM because there is no unique phase associated to a state in QM.
@ACuriousMind ok but think you are overextending the actual claims/ content of QM and getting more into the copenhagen interpretation with that assertion. what is the proof (from axioms) that measuring unique phase must be impossible? and plz dont cite bells thm. & dont think the no cloning thm works here either... what if there is a way to measure it that doesnt contradict any of the axioms of QM? that seems to me what weak measurement is getting at...
@vzn I already told you, it's an axiom of QM that states are rays (and by definition a ray is the set of vectors differing only by a phase and scaling). Please learn the basic mathematics of quantum mechanics before suspecting interpretational issues and Bell's theorem behind every little thing.
@vzn I have to say that you have a bit of a habit of hearing about a buzz-topic and then coming here to ask if that new topic somehow revolutionizes our entire understanding of Nature.
This is extremely unlikely to happen, ever.
Science is more incremental than the fairy-stories might have you believe.
@vzn Please believe me when I say that a large fraction of the quantum community already understands the new theory.
Weak measurement isn't new.
I think you might feel that weak measurement is revolutionary simply because you've been taught an archaic, and unfortunately present-in-textbooks version of QM.
@vzn Uh, "phase freedom" was my argument for why I would be cautious about the interpretation of the results of one particular measurment, weak measurement itself never "abandoned phase freedom", where did you get that from?
@ACuriousMind think abandoning phase freedom may be the step toward Boldly Going Where No One Has Gone Beforeā¢. maybe "abandon" is too loaded/ dramatic a word for some though :|
@DanielSank dont know how to be more specific. am basically proposing that (some) weak measurement experiments are actually statistically measuring/ sampling the real/ imaginary parts of psi. & that there is some currently hidden evolution ("mechanics") of these components that is not contained in std qm. ie/ aka maybe a quantum submechanics
@DanielSank think some of the "trajectories" being measured by weak measurements have (physical) rules that are not predicted by (but dont contradict) std qm.
← dont know a lot of things ... anyway can look up a favorite "more axiomatic treatment of QM" ref for you DS (not near my books right now) but the contents wont be any surprise to you...
@DanielSank I know, but I am genuinely curious how one might know the von Neumann axioms but not what a ray is, given that one of the von Neumann axioms is either "states are rays in Hilbert space", "states are vectors up to scalar multiplication" or something equivalent. It's like someone saying they don't know how to make dough and then casually referring to their experience baking bread later.
← have very many QM refs of various qualities, havent read all of em yet
anyway the "new" (weak measurement) theory seems to be measuring points on a bloch sphere so to speak, something std QM has "nothing to say" about. aka trajectories
What you are calling "std QM" is obsolete. Comparing modern experiments to an old theory is not particularly interesting.
Trajectories are very well understood.
Also, making a plot of points on a bloch sphere doesn't even require trajectories. You can do it with projective measurements in what is called "state tomography".
@DanielSank what if the real/ imaginary parts of psi are the legendary "hidden variables"? staring everyone in the face since the very beginning? that looks more to me like the case, & weak measurement describes its "sub-"mechanics....