The h Bar

6:19 AM

@EmilioPisanty I have a question..you free?....very basic one!

Emilio Pisanty

@ayc not very, but ask anyway

is white light a superposition of its constituent colors?

6:34 AM

@ayc it depends on how you define white

There is a technical definition to do with the frequency spectrum of the light, and there's also what looks white to humans - they aren't the same.

the usual one..the light that is used in dispersion experiments with prism

That would mean the light is a sum of plane waves of all frequencies in the visible range. I wouldn't call it a superposition because that has a specific technical meaning in quantum mechanics.

@JohnRennie my basic thought is this:....in waves on string we used superposition principle to get the resultant wave of two different waves......in this case would we get the equation of white light by adding the constituent colors

@ayc yes, it's exactly the same. For each frequency you have a wave $E(t,x) = A\sin(\omega t - kx)$ and you just add them all up to get the white light.

Though that isn't how white light is actually generated.

Typically white light would come from black body radiation, e.g. in a filament light bulb.

The EM radiation isn't generated as discrete frequencies that are then added together. The black body radiation generates a continuous range of frequencies.

and since we can only see the visible part...the frequencies in the visble part,all together appear to us as white?,,,is that right?

but in that case a black body should appear to us white in colour...but I dont think it is so

6:46 AM

The eye has three types of receptor cell that are sensitive to red, green and blue light. When all three receptors receive equal signals the brain interprets this as white.

This could be due to genuinely white light, but it can also be due to a mixture of just the three colours and the eye can't tell the difference.

TVs work by generating just three colours that fool the eye into thinking a continuous range of colours is present.

@ayc the colour of a black body depends on the temperature. Black bodies of the right temperature look white. Cool black bodies look red, very hot black bodies look blue, and in between they look white. Around 6000K looks white.

That's because the black body spectrum isn't flat so it doesn't contain equal amounts of all frequencies. But at the right temperature the black body spectrum contains approximately equal amounts of all frequencies in the visible range.

@JohnRennie blue stars are hotter than the white and red ones right?...just like black body!

@ayc stars are basically just black bodies i.e. the spectrum of a star is very close to the black body spectrum.

@JohnRennie Ohk,...thank for your time!!! cu later

@ayc bye

@JohnRennie but..wait I still dont understand dispersion ..If I combine VIBGYOR equation and write the equation of white light..then you see white light is now just a wave like a monochromatic light only.......why cant it simply undergo difraction like a monochromatic light would?

7:00 AM

@ayc the diffraction equation for e.g. double slits is $n\lambda = d\sin\theta$. Yes?

yes

White light has a range of wavelengths $\lambda$ so it gets diffracted by a range of angles $\theta$

@JohnRennie if I take equations of VIBGYOR and then add those eqautions then would the final equation be in this form:$E(t,x) = A\sin(\omega t - kx)$

@ayc no

That equation is for a plane wave of a single frequency $\omega$

oh then its my bad!...I thought that you can write it in this way!...if you could indeed write in such a way then there shouldn't be any splitting right?

7:05 AM

@ayc yes, there wouldn't be any splitting if white light could be written as an equation of that form, but it can't.

@JohnRennie ohk then..I get it now!..cu later!

Q: Why is making a laser that shines light over the full spectrum, i.e. a 'white laser', so difficult?

Bye

Emilio Pisanty

7:44 AM

3

Why is it that, while there are red, green, and blue laser out there, I can't seem to find lasers that shine light over the full spectrum of colors"white lasers"? How hard is to make one? Is it possible?

All I have time for right now, I'm afraid

8:07 AM

$$E=mc^2$$

$$E=h\nu$$

$$m=\frac{\nu}{c^2}$$

I can collect my nobel later

2019 Avnish Kabaj proves that photons have mass

8:21 AM

@AvnishKabaj You dropped the h.

I guess there's no point arguing with someone like this:

@knzhou If you take the trouble to actually read it, you will find that the real deception is GR — Alan Gee 28 mins ago

I expect that he's a little annoyed by those downvotes, though. ;)

8:38 AM

@JohnRennie can you revive frozen room if you are room owner etc? Or only moderators can do

@Zerix can't remember. I think you have to ask a mod. I'm fairly certain I've had my own rooms frozen and I couldn't unfreeze them myself.

@PM2Ring oh no

My nobel

Is it quantum mechanics mumbo jumbo because of which photons don't have mass?

Okay. I thought you had that power

@AvnishKabaj $E=mc^2$ is wrong

It's actually $E^2 = p^2c^2 + m^2c^4$

P is momentum?

8:43 AM

For a photon $m=0$ so the equation reduces to $E=pc$

Yes, $p$ is momentum.

@JohnRennie Why is the mass 0 ?

It's pure energy ?

@AvnishKabaj the word energy is largely meaningless. Photons aren't energy, they are photons - massless particles.

Are photons visible?

Q: If photons have no mass, how can they have momentum?

146

As an explanation of why a large gravitational field (such as a black hole) can bend light, I have heard that light has momentum. This is given as a solution to the problem of only massive objects being affected by gravity. However, momentum is the product of mass and velocity, so, by this defini...

special-relativity electromagnetic-radiation photons mass momentum

@JohnRennie Thanks

Can we see them using scientific equipment?

8:46 AM

@Zerix that's not a well phrased question.

The word visible tends to mean interacts with light, but photons are light so they don't need to interact with light.

Ohh.

m.imgur.com/gallery/isyAirD

Not a photon though

@Zerix Only mods can unfreeze rooms. See meta.stackexchange.com/questions/70198/…

9:06 AM

@AvnishKabaj Some of the answers on that page mention that photons have relativistic mass, however relativistic mass is a deprecated concept because it's confusing and misleading. However, in some situations it can be useful to ascribe mass to a collection of photons. Eg, a box containing a bunch of photons has more energy than the same box without the photons, and so it has higher mass, although it's not easy to measure the difference.

Loong

@JohnRennie and now try to explain it here: astronomy.stackexchange.com/q/29620/8021 ;-)

@Loong

In case it isn't obvious, we can't see the photons that are in the photon sphere, only those that happen to spiral out of it & head our way. — PM 2Ring 6 hours ago

Q: Applying Kramers-Kronig relation to a simple damped oscillator

morning

anderstood

@EmilioPisanty Just in case you missed this question (since you seem to know the Kramers-Kronig relation):

1

I just discovered the Kramers-Kronig relation and am trying to apply it to a simple damped oscillator of the form subjected to an impulse at $t=0$, which is a causal system: $$m\ddot x + c\dot x + k x = \delta(0).$$ In the time-domain, the response can be decomposed in odd/even parts as $x(t) =...

classical-mechanics signal-processing linear-systems analyticity

9:39 AM

@PM2Ring Oh I see

9:53 AM

How do we flag something like this? physics.stackexchange.com/questions/463239/… Blatantly off-topic?

Q: Is there any simple proof of the no-ghost theorem?

10:40 AM

I'm always glad when mathematicians have to deal with our stupid terminology because we basically discovered it

Like how they have to drag the word "spinor" around

Even though it has nothing to do with the mathematical structure involved

or "timelike" for pseudoriemannian manifolds

bolbteppa

10:58 AM

Or even velocity and acceleration for derivatives :p

6

Is there any simple proof of the no-ghost theorem in string theory?

string-theory hilbert-space unitarity ghosts

:(

@bolbteppa Or energy

The ~ energy functional~

bolbteppa

That's a good one, I really have no idea how a math person doing intro PDE's could make sense of that language

1:18 PM

samuel-lereah.com/posts/2019/02/27/…

New post baby

and it's what everyone loves

Robert Geroch!

1:32 PM

Gonna try to prove the statements in the quiz

they are tricky

Anonymous

@Chair Yeah, could've given it a bit more time...

Anonymous

@Chair The trusted user privileges are nice. You'll also need some considerable rep if you ever run for mod. ;)

Anonymous

A 40/40 candidate score makes you a formidable candidate on a site like Physics (or any site of comparable magnitude). Though, beyond 20k the rep is not counted in the candidate score. The rest you earn from the participation/editing/moderation badges.

10k is nice, but it can be a bit frustrating having to wait to cast delete votes. 20k gets rid of that delay, and lets you delete bad answers.

user351417

1:50 PM

@Slereah Hmmm, as funny as that is, I don't think we should talk about him...

user351417

Like one follows proposed-faqs :P

I'm the physics guy who makes people with the True Theories disappear

part of the physics illuminati

2

Cowards!

Anonymous

@Slereah I chuckled before deleting those. Last time you got flagged for similar message(s). Careful. :P

How do you get the job in the shadow physics mafia

user351417

@Blue Meh I don't think I'd ever do that... a while back, I noticed that I was spending more time than I should be on meta, and I was spending similar amounts of time in the first posts/close review queues and the main site. It wasn't fun to spend so much time just looking at the lowest-quality content, so lately I've slowed down my reviewing. I think now's the time for me to focus on learning that physics instead, as fun as mod-power may be

Loong

1:55 PM

@Slereah xkcd 2113

@Loong I know, but it doesn't have like a forward address!

To send a resume

user351417

I am super pissed off because lately windows has taken to auto-correcting them emojis on SE chat. For no apparent reason, it kicks in only when I edit old messages.

Anonymous

@Chair That is true. I'd choose learning more physics over modding any day. "Meta discussions" (in meta as well as the Teachers' Lounge) do drain me.

user351417

I should totally switch to linux sometime soon.

user351417

I don't even know why I still use windows.

user351417

2:01 PM

It is difficult to answer without the exact crystal structure of both compounds (which I didn't even try to get), but the C2-symmetry of 1,2-dimethylhydrazine (1,2-DMH) is really stinking, compared to unsymmetrical dimethylhydrazine. If you consider the plane "H–N–N–H" in 1,2-DMH, it is obvious that close molecule can form a sheet just by hydrogen-bonds. It is not the alpha and omega of the crystal structure, but I am not surprised at all by the melting points you gave. — SteffX 22 hours ago

user351417

::sigh::

user351417

The only interaction so far is a comment saying that it's very difficult.

Anonymous

As a matter of fact, the TL turned out to be a bit different than what I expected. You'll still spot lots of heated debates there almost every other day and mods talking past each other...just like normal users on any other meta site. We have running gag that: "a user is worthy of being a mod only if they can survive the TL". So yeah, modding is not always fun. Although I do enjoy the "powers"... :P

user351417

How many spam/offensive flags does the TL room see every day? :P

Anonymous

@Chair Unfortunately, flagging is useless there...

user351417

2:06 PM

Mods flaggin' each other to hell is certainly something I'd love to see.

Anonymous

Mods can't be suspended or kicked. Well, technically they can. But that might turn things very ugly. I can distinctly remember one case where a mod had to be temporarily kicked, but that was an extreme case.

Unsymmetrical dimethylhydrazine? Sounds like someone's been reading John Clark's Ignition!

user351417

10/10 would watch scenes and tantrums generated by mod-power wielders.

2

user351417

@PM2Ring Yep, multiple people here have been looking at that one :P

user351417

(top few messages here)

user351417

2:09 PM

It's quite fun, though I've been finding less time to read it lately.

Anonymous

@Chair Trust me, it's pretty ugly. :P

Anonymous

But well, there are some fun folks there too.

user351417

@Blue 10/10 would still watch (perhaps with popcorn)

Q: Requests for Solutions Manuals

3:10 PM

That Geroch quiz is some tricky shit

Physics Meta

3:27 PM

0

In a recent meta post, 20 most viewed questions that need answers, there was a discussion on whether a couple questions (specifically Cohen Tannoudji solutions to exercises and Is there any solution manual to A.P. French's 'Vibrations and Waves'?) that requested solutions manuals for specific tex...

discussion scope

@PM2Ring it is a cracking good book and highly recommended for all of us who would actaully quite like to be rocket scientists :-)

"If, for $S$ and $S'$, three-dimensional manifolds, $S \times \mathbb{R}$ and $S' \times \mathbb{R}$ are diffeomorphic then $S$ and $S'$ are diffeomorphic. (FALSE)"

Jesus

apparently a standard counterexample is punctured manifolds of genus $g$

the horror

Q: Can a person free fall onto a big block of jelly and survive disregarding the eventual suffocation?

3:45 PM

0

Assuming the height is sufficient enough to achieve terminal velocity how high should the block of soft jelly be to dissipate the impact energy safely for an average human?

kinematics free-fall

I'm writing the grant proposal right now!

Though I anticipate problems with the ethics committee when I ask for volunteers.

There's a few cases of people surviving falling from airplanes into thick layers of snow

not sure how similar snow is to jelly

@JohnRennie Definitely! Although Max Gergel's memoir (which I also learned of via Derek Lowe) is more entertaining, IMHO.

@PM2Ring link?

Feb 15 at 13:30, by PM 2Ring

You also need to read Max Gergel's Excuse Me Sir, Would You Like to Buy a Kilo of Isopropyl Bromide? It's hillarious. And a little scary to read about the blasé attitude to safety and environmental protection they had back then.

See blogs.sciencemag.org/pipeline/archives/2010/05/27/…

People can die falling at terminal velocity into water, I don't think jelly would be any better. OTOH, falling onto a big chunk of aerogel should be fine, although you'd probably want a dust mask.

4:00 PM

@PM2Ring thanks, I've grabbed a copy :-)

Enjoy!

Anonymous

6:19 PM

Differentiation and Integration

Anonymous

Ah, this certainly brings back some memories... :P

fun times

Anonymous

Heyo!

Sup :D

(if ACM comes in here and says "inf", I'm gonna glare at him angrily for a few seconds)

ACuriousMind

inf :D

Q: Should I prefer the most up-voted answer or the accepted answer?

6:34 PM

glares

Anonymous

ACM has anti-glare glasses...

Physics Meta

0

I came across this question, where the accepted answer had a score of $-7$, but there was also an answer with a score of $31$. As a student who has absolutely no idea about quantum mechanics, or the"correctness" or "wrongness" of the accepted answer, which answer should I prefer?

discussion answers voting accepted-answer best-practices

Anonymous

(Probably, literally.)

glare intensifies

Anonymous

You emitting laser now?

6:37 PM

soon

Anonymous

@PhysicsMeta Sigh.

I got that gold badge thing where my answer was more upvoted than the accepted answer by some margin

Anonymous

Of course, the populist badge. :)

tbf though, especially for questions that get to the hot networks queue, highest voted answer is still not always best

Anonymous

Yeah, there's no clear-cut general answer to "Should I prefer the most up-voted answer or the accepted answer?".

6:48 PM

or some other answer...

or no answer because none of the answers to the linked questions really answered what a measurement really is indepth

or all the answers?

what other permutation is there...hmmm

The accepted answer

The most voted answer tends to be

The most politically correct one not really useful but the guys on the site savvy it because it's cool

Oh God if it's -7 +31 then go for +31

Lol

should one "prefer" any answer if he did not understand it?

Anonymous

That's a nice philosophical question.

6:55 PM

LMAO my description was 10/10 accurate

Observer is a special person (or a system that contains such person) which does not obey the usual laws of quantum mechanics. While it is much easier to define observer from a philosophical point of view, the mathematical answer is that the observer is a system which manifests subjective decoherence when observed. For the definition of subjective decoherence and precise mathematical formulations, refer to this work

^-7

Are we talking quantum mechanics? Then I'd say that a "measurement" is any operation that entangles orthogonal states of the system under consideration with orthogonal states of the environment. "Measurement" is the important thing in most formulations of QM. Colloquially speaking, an observer is something that performs measurements.

The only other place in physics I can think of where "observer" shows up is in the oft-used phrase "This is obvious to the casual observer". This is just shorthand for "I can't be bothered to write out the mathematical proof".

^+31

To me the pleb the first one looks more useful but I guess it's technically incorrect because i know nothing about quantum mech

As i see it, we have various theories and physics strives towards predicting our subjective experiences. With quantum mechanics it is more of give us the probabilities of experiencing something in the future. Every theory also includes the theory of the space we are within, be it however many dimensions and whichever structure. But there is no way for us to even know if the space is the way we assume it to be. All we can do is guess based on our subjective experiences

If a theory predicts our subjective experiences more often and more accurate than another theory, then we consider it a better theory more often than not

Anonymous

I'd replace "subjective experiences" with "experimental results". But yes, that's what physics is.

Anonymous

Unlike mathematics, there's no notion of "absolute truth" in physics (albeit, mathematics too does begin with a set of axioms...which are well, supposed to be true).

If you replace subjective experiences with experimental results you would be wrong though, because those results have to reach someone in the end.

Anonymous

@pZombie Depends on how you define "subjective".

7:09 PM

@Blue Yeah, you're telling me. I went to write like a two line answer suggesting to go with the upvoted since the person asking likely doesn't know what a good answer looks like. Ended up turning into several paragraphs basically winding up with "probably, but if you're not sure research yourself"

~~whoops, meant to reply that to blue's post right beside it~~

you could imagine devices like computers storing those results and analyzing them, but the devices themselves, their structure and their functionality are based on some theory of space or spacetime and other theories on top of that for which you can never be certain if they are accurate. You can be certain that some are good theories and some are better depending on your method of evaluating them but you can never be certain of any theory being the correct one

Anonymous

@JMac Done. Abused my powers again. ;)

Anonymous

@JMac You did manage to frame it nicely. :)

Anonymous

I'm hoping someone will add a comment along the lines of "the accepted answer sucks", just in case the OP is still confused. :P

hopefully OP will accept it so that everyone can downvote it into oblivion

7:13 PM

For example, there is a theory which assumes a special reference frame but has exactly the same predictive power like special relativity. They are interchangeable if all you care about is accurate predictions

7:25 PM

So how are we supposed to figure out which theory is the correct one if they both have the same predictive power, yet work with completely different concepts of space itself?

@pZombie If they have the same predictive power, why do you need to figure out which one is correct? they are functionally identical

This might have you question what measurement devices really are. Because the measurement devices themselves change depending on which theory you assume to be the correct one. Their whole method of operation

JMac - because you would have to consider all this if you were to answer the question about what an observer is

that and much more which i am not capable of considering right now, which why i won't even attempt to answer the question

knzhou

@PM2Ring I tried engaging with him on a different question.

It's a bad habit of mine.

I keep trying to show random programmers why their shower-thought theory of everything isn't being taken seriously.

Despite 25 attempts over the years, I've never gotten any to display even a moment of self-reflection.

@JMac Also, it depends on how you define functionality identical. They both have the same "local" predictive power as in giving yo the same results for any experiment you perform locally but they paint a completely different picture of how "the world out there" is while you are "here and now"

@JMac with special relativity for example, two observers meeting at a single spacetime location but moving at different velocities cannot even agree on if their cat is dead or alive. The question is nonsensical to be asked in the frame of SR. There is no agreed upon "same time" for any such two or more observers when moving at different velocities

@JMac whereas with the other theory having the same predictive power as SR but allowing to assume a special frame of reference, two observers meeting at some point in spacetime can always agree on if their cat is alive or dead "at the moment"

Q: Am I allowed to call this guy a kook?

@knzhou At this point, I think I only ever do that to see what ridiculous reasons they are going to come up with to support what they say. It's one of those areas where the Dunning Kruger effect is really pronounced. At the same time, it takes a pretty large dose of arrogance to assume that as an outsider, you have a novel thought that solves the current issues (night and day difference for me between "this is really what's happening" and "why does this theory i came up with not work?")

@pZombie Do they have any method to test the difference, or determine what the difference would look like? If they do, then the theories don't have the same predictive power, if they don't, why would physicists care?

Physics Meta

7:39 PM

0

Is it against the rules to call this guy a kook? Where is the hard drive to our reality?

discussion comments ethics

knzhou

@JMac Usually I ask how their theory accounts for contradictory observed results, or how their interpretation is supposed to be more useful than the standard ones. But they always quickly dodge questions of that type and get back on the conspiracy train.

Anonymous

@PhysicsMeta I see, today's a fine day on meta-land. :P

knzhou

So much mental energy in the world is getting stuck in infinite loops.

@JMac no method to test the difference, no. Should physicists then care? Depends i guess. If the job of a physicists is just to build devices that work and predict local experiences accurately, then i guess they should not care. But if the job of a physicists is also to come up with a theory that describes a "world out there" that makes sense to humans then we are not done yet

@pZombie No physicist would argue that we are done. There is plenty we don't understand and are unable to adequately predict with our models. The point is though, physics is about physical results. The point is, there's not much benefit in trying to determine which of two models is correct if you can't even show how they lead to different testable outcomes. If we can't test it, it's just guesswork anyways, not science

Anonymous

7:47 PM

@knzhou It's really not worth engaging with such users. Downvote and move on. Not only you; I notice a lot of users have this habit of trying to debate with those who're clearly here to spread non-mainstream propaganda of their own making.

@JMac For example, we could consider spacetime to be a static hypercube with all events fixed. If we wanted to add QM to the mix, we could consider some higher being throwing some dice, creating the hypercube in the process and let it be. A physicists then could tell you that this is it. Give you the rules of how the hypercube was created and since all events have been mapped, give you the measurement results for any given spacetime location/volume of space. But would we be done then?

@JMac what about our experience of change or what we call time? Wouldn't we also have to at least assume some kind of equivalent to a laser read head, reading through an already written/static DVD movie? Us experiencing that static hypercube by moving through it like that laser read head does through the DVD?

It's such a waste most of the time. I'll usually give benefit of the doubt if it seems like they could just have a misunderstanding about pop-sci stuff; but even then, a majority of the time when I explain a concept or give them a chance to clarify,
they go down crazy lane. I've definitely gotten caught up in their chain of bad arguments before, but when I'm sure it's going in a downward spiral I try to get myself to disengage

@JMac Is it the job of a physicist to also come up with a theory that accurately maps our subjective experience of time through that static hypercube or is it enough to just describe the hypercube?

Anonymous

@JMac Yeah, that's what I'd recommend in general. The main point here is: no matter how hard you try, you won't be able to "convert" someone who religiously believes that they're on the right. There are far better places to utilize your spoons.

@pZombie You wouldn't need to assume anything. You would need to state what predictions your model makes, why it makes them, and how that can be applied to future predictions. It doesn't matter if your technique winds up acting like you're reading information off a hypercube with a laser head. All that matters is the usefulness of the predictive power. Physics is about the how, not the why

Anonymous

8:13 PM

@JMac Re: this. Physics has its own CoC page.

Anonymous

Although, yes...the content is the same. :)

Anonymous

They just copied it over to all the sites.

Q: The second draft of our Code of Conduct is available for feedback and review

@Blue Yeah, I looked at our first actually; but it didn't say "be nice" anywhere, so I googled for the be nice one, saw it was the exact same, and linked it

Anonymous

Basically, we call the new CoC the "Be Nice" policy these days. They made some changes in the language a few months ago. Which is why you don't see "Be Nice" explicitly written there.

Anonymous

Be Nice.

Whether you've come to ask questions, or to generously share what you know, remember that we’re all here to learn, together. Be welcoming and patient, especially with those who may not know everything you do. Oh, and bring your sense of humor. Just in case.

That basically covers it. But these three guidelines may help:

Rudeness and belittling language are not okay. Your tone should match the way you'd talk in person with someone you respect and whom you want to respect you. If you don't have time to say something politely, just leave it for someone who does.

Anonymous

8:19 PM

This was the earlier CoC. ^

Anonymous

142

You provided us with some really helpful feedback on our first draft attempt to expand our 'Be nice' policy into a formal code of conduct, and we're extremely grateful for your time, patience and insights. This was not an easy discussion to have and we are extremely proud of the civility and insi...

discussion announcements code-of-conduct

yeah, I'm aware of most of that, I just always see it called the "be-nice" policy, so I was second guessing if the Physics CoC was updated to comply with the changes that were made network wide recently

Anonymous

@JMac Yup, that's the case indeed. :)

all we need now is a highly intelligent AI to implement the policy as it was intended

Or, you know, regularly intelligent people who already exist and have to deal with the direct consequences of the rules anyways, and so should be capable of enforcing them

8:30 PM

that never really works

Anonymous

Okay, LOL. They even accepted your answer. Certainly a law-abiding individual. :P