1342: "Ancient Stars"

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Re: 1342: "Ancient Stars"

Postby addams » Sat Mar 15, 2014 6:19 am UTC

Existentialism wrote:I registered on these forums just to posit that the person on the right is Michio Kaku. That is all.

OK.
http://en.wikipedia.org/wiki/Michio_Kaku
That's funny.

cronjob wrote:Electromagnetic radiation is pure energy with no mass. This covers the entire spectrum. Both visible and non-visible.
Electromagnetic radiation actually consists of vibrating/pulsating waves of electrical and magnetic energy. Additionally, electromagnetic radiation can be thought of as a stream of packets of energy. These packets of massless energy are called photons. Each photon particle still "travels" in a wave form type of pattern. Each photon has a certain amount of energy. All electromagnetic radiation consists of photons. The only thing that is different from one type of electromagnetic radiation to the next is the amount of energy contained within the actual photons themselves.

This much I already know.

I also know that electromagnetic radiation travels in a straight line at the speed of light (3 x 108 m/s). And yet packets collide. They radiate. This is how they move and they exchange information when they collide. This much I already know.

I also know that from the day we are born until the day we die, almost every single photon that strikes the surfaces of our eyes is local due to Earths proximity to something the called the Sun. To see Andromeda, those photons have to wade through the overwhelming amount of photons that our entire solar system has been bathing in since our own star went online 4.5 billion years ago. Photons must therefor be able to pass on "information" (of a kind) when they collide, for us to see Andromeda.

So, while the idea of a single photon traveling 250 million light years just to be absorbed by the human eye may seem like a romantic notion to some, that's not actually how it works.

p.s. I still want to know if anybody out there does know and can explain it to the unwashed among us...

It is such a beautiful question.
Both ideas are charming.

Maybe it is a little of both or neither.
I don't know the answer.

I like the question.
I, also, think other posters might want to explain the both a particle AND a wave thing.
One more time.
Life is, just, an exchange of electrons; It is up to us to give it meaning.

We are all in The Gutter.
Some of us see The Gutter.
Some of us see The Stars.
by mr. Oscar Wilde.

Those that want to Know; Know.
Those that do not Know; Don't tell them.
They do terrible things to people that Tell Them.

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Re: 1342: "Ancient Stars"

Postby cronjob » Sat Mar 15, 2014 6:53 am UTC

It is such a beautiful question.
Both ideas are charming.

Maybe it is a little of both or neither.
I don't know the answer.

I like the question.
I, also, think other posters might want to explain the both a particle AND a wave thing.
One more time.


thanx...

In the back of my mind I knew I had read something about this. It's bugging me now, right? So, I racked my brain on this for awhile and I got it. I don't have time to read "The Complete Idiots Guide to Astronomy (2nd Edition): Chapter 7 Electromagnetic radiation: what it is, how it travels, and what it does", but that's it! That's the book! and that's the Chapter! I can't believe I remembered that book and actually found it.

I will read the chapter and report back but not now. It's almost 2:00 am.

~tayl
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Re: 1342: "Ancient Stars"

Postby PM 2Ring » Sat Mar 15, 2014 12:45 pm UTC

chenille wrote:If it's a dark night and you want to see something genuinely far away, though, as snowyowl says there's always the Andromeda galaxy; the light from that is older than the first Homo habilis fossils. Not much chance it's gone away in the mean time, though.

And for those of us in the southern hemisphere, there are the Magellanic Clouds. True, they aren't as far away as M31, but they're pretty easy to spot, as long as there isn't too much background light. But of course, it's pretty hard to resolve the individual stars with the naked eye. :)


rmsgrey wrote:
cronjob wrote:Light is a form of electromagnetic radiation. Radiation defines motion. Light does not move as does the arrow to the tree. It radiates. The motion of a photon consists of bumping into another photon. In this way, information is passed from one photon to another. But what are the odds that a single photon would come all the way from Andromeda and actually strike the surface of the eye ball of someone standing here on Earth?

Earth, being so close to the Sun to begin with, is completely and totally awash in local photons, and has been for quite sometime. So, I would think that almost every photon that strikes the surface of your eye was in fact emitted by our Sun (Sol). So, how does the eye (or the human being it's attached to) make sense of or, create an image of, electromagnetic radiation that radiated information through a process of bumping it along from one photon to the next?

p.s. I find this concept to be quite fascinating so, please don't prove me wrong. I want to know how the actual information arrives by way of radiation to the surface of the eye.


Photons don't bump into each other - in fact, they don't interact with each other at all - if you have two photons on a collision course, they just pass through each other without noticing. What stops a photon is encountering a charged particle (or a magnetic monopole if they exist) - and only if the particle can absorb all of the photon's energy. When astrophysicists say that a photon takes thousands of years to escape the Sun, what they actually mean is that the original photon gets absorbed by a charged particle, which then emits another, similar photon in an unpredictable direction some time later, and if you follow the energy, then it takes thousands of years and trillions of collisions for that original bit of energy to be wrapped up as a photon that's leaving the Sun...

Sorry, cronjob, you're theory is way off-beam.


ucim wrote:Yes, but after all that bouncing around, are they still really the "same" photons?

As rmsgrey said, photons don't really bounce, although it can often be convenient to pretend that they do.


FWIW, the electromagnetic radiation liberated by fusion in the core of the Sun & other stars is not visible light: it's in the form of gamma rays. Some energy is also given out in the form of kinetic energy of the particles produced in the reaction; that kinetic energy agitates the material in the vicinity, raising its temperature. (Some energy is also given off in the form of neutrinos, but neutrinos have such a low probability of interacting with anything that they quickly leave the star without significantly contributing further to the stellar processes. (However, neutrino effects are important in supernova explosions, as Randall alluded to a while back)).

The absorption + emission process that rmsgrey described gradually spreads & downshifts the spectrum of the EM radiation traveling out from the stellar core, mostly due to the Doppler effect, so that by the time the energy reaches the surface of the star much of it's in the visible part of the spectrum, and even lower: infra-red, microwave and radio frequencies. Of course, there's also quite a bit that's in the UV range, and up into the X-ray part of the spectrum, especially for big blue-white stars.

So we can say that photons are emitted in a star's core and they take many thousands of years to bounce there way out to the surface, by which time they've been Doppler-shifted into a broad spectrum that includes visible light. But to be more accurate, we should say that the energy liberated by the processes in the stellar core drive the stellar "machinery", eventually causing the stellar surface & atmosphere to emit photons in a broad spectrum.

Also see http://en.wikipedia.org/wiki/Magnetohydrodynamics


Paulmichael wrote:Well, I imagine that what we consider to be light that is bright enough to be seen from a star is a cluster of many billions of photons, and at least a few of those photons are likely to be thousands of years "old." Not being an astrophysicist, I have no actual idea. :lol:


The human eye can detect rather low light intensity.
From http://en.wikipedia.org/wiki/Absolute_threshold#Vision
The researchers found that the emission of only 90 photons could elicit visual experience. However, only 45 of these actually entered the retina, due to absorption by the optical media. Furthermore, 80% of these did not reach the fovea. Therefore, the human eye can detect as few as nine photons. Moreover, as the chance of any one rod receiving more than one photon is very small, we can assume that it only takes one photon to excite a rod receptor.

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Re: 1342: "Ancient Stars"

Postby Earthling on Mars » Sat Mar 15, 2014 2:45 pm UTC

hercynium wrote:I should have come back to post more hints like I did on facebook, but I didn't so here's the answer:

Spoiler:
On 2008-12-01, Florence + The Machine released the single "Dog Days Are Over".
It received heavy airplay in Great Britain but did not reach worldwide popularity
for over a year.

Eight-point-six years later (Earth time) the signal of these broadcast transmissions
reached Sirius. Upon hearing of its loss of purpose and relevancy it promptly and
unexpectedly exploded.

Earth, of course, did not know this had happened for another 8.6 years, in early
2026, when the first waves of radiation hit. Due to the supernova's proximity, it
burned our planet to a cinder, ironically bringing about the fiery cataclysm the
star believed it had failed to fulfill years before.


(a reminder: I did say this is horribly un-funny...) :roll:

Ah, I would never have gotten that one. :P In case anyone was wondering, my logic was this: Pluto used to be called a planet, but was demoted to dwarf planet. Pluto is also the name of a cartoon dog. So, the Dog Star wanted revenge on Earth for demoting the planet which is remotely connected to a dog. By the time I checked Wikipedia and found that Pluto had been demoted quite a while before Dec 2008, I liked the idea too much to give it up.

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Re: 1342: "Ancient Stars"

Postby Zassounotsukushi » Sat Mar 15, 2014 3:27 pm UTC

I made this graph from Wikipedia because I knew people would be arguing over this:

Image

You could say that most stars are within 100 light-years of Earth. But that would only be true for a sky that isn't sufficiently dark, or with vision that isn't sufficiently good.

Estimates of the number of stars you'll be able to see vary wildly. On the upper end, 5,000 or so may be visible. In those ideal conditions, the vast majority of stars would predate industrial society. In most conditions, the average probably looks more like the 1950s or something like that.

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Re: 1342: "Ancient Stars"

Postby mfb » Sat Mar 15, 2014 4:11 pm UTC

rmsgrey wrote:Taking the 1400 systems within 50 LY figure and extrapolating up to 1000 LY, that gives 8000 times as many systems, or about 10 000 000 stars. So the million brightest (by apparent magnitude) stars in the sky are probably mostly within 1000 LY - on the other hand, if you can see over a million different stars, it's a reasonable estimate to say that the light from some of them is going to be about a thousand years old.

On the other hand, there are only about 10 000 stars visible to the naked eye, so both the millions and the many thousands are indeed substantial overestimates.
At this distance you see the thickness of the galactic disk, and the number of stars does not follow a cube law any more.

Anyway, there are stars visible to the naked eye thousands of lightyears away. Deneb at >2000 LY is in the top 20 of the brightest stars (by apparent magnitude), Eta Carinae is visible and at a distance of ~8000 LY.

Trick question: Is it really an overestimation of the distance, or an underestimation of the speed of light?


cronjob: Learn some physics please and don't try to spread your misconceptions.

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Re: 1342: "Ancient Stars"

Postby Zassounotsukushi » Sat Mar 15, 2014 4:17 pm UTC

I made this graph from Wikipedia because I knew people would be arguing over this:

Image

You could say that most stars are within 100 light-years of Earth. But that would only be true for a sky that isn't sufficiently dark, or with vision that isn't sufficiently good.

Estimates of the number of stars you'll be able to see vary wildly. On the upper end, 5,000 or so may be visible. In those ideal conditions, the vast majority of stars would predate industrial society. In most conditions, the average probably looks more like the 1950s or something like that.

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Re: 1342: "Ancient Stars"

Postby addams » Sat Mar 15, 2014 5:01 pm UTC

On Topic/Off Topic:

"Confuse them with The Facts."
That is a funny thing to say.
I thought it was funny.

What is the Comic about?
How far we can see?

yes. How far we can see is fun and important.
As I was reading. Random Reading. (shrug)

I ran into This:
Le1bn1z wrote:[b]@ Zamfir

As a matter of fact, America has asked for little more than symbolic support from NATO in Afghanistan. The contributions of any given country there, minus the U.K., is the equivalent of a rounding error in American strength numbers.

I got those words, Here:
posting.php?mode=quote&f=8&p=2073281

If we are off by a factor of conversions in our outer space calculations.
The StarLight might sill get here. Stars are great to share.

Do you own a Star, yet? no? Me, neither.
Stars are so Romantic.

Wars are good for Romance.
I, Still, Disapprove!

Who the heck types like that post?
Do The People read that sort of thing and think,
"Those are The Facts. I'm Confused."

No! They think they know it all, Now.
People often Type the way they talk.

The contributions of any given country there, minus the U.K., is the equivalent of a rounding error in American strength numbers.
That is a Math Sentence.
Math People!

(I Know You Are Out There.) Look at that sentence.
Make it Math. How much sense does it make?

Is it, Just, a conversion error?

Spoiler:
StarLight. StarBright.
First Star I see Tonight.

Wish I May.
Wish I Might.

Have the wish
I ...Wish..Tonight.....

Are you done with that Math, yet?

How far is that Star?

As a matter of fact, America has asked for little more than symbolic support from NATO in Afghanistan.

(deep breath...sigh) Politics? Symbolic Support is Politics?
Support for What?

I go visit some people.
EveryWeek they have some Guy show up and say,
"Grant Us Peace In Our Days"

Have you finished the Math, yet?
We can make some good guesses about Outer Space.

Where is the Star of Peace?
I know it has wandered far away in the sky.
Those are Superstitions!

We don't do Superstitions.
We listen to Experts that will say things like.

"shhh. They don't know, we don't know."
Funny World..... Science Guys?

Will you Stop the World?
I want to get off.
Life is, just, an exchange of electrons; It is up to us to give it meaning.

We are all in The Gutter.
Some of us see The Gutter.
Some of us see The Stars.
by mr. Oscar Wilde.

Those that want to Know; Know.
Those that do not Know; Don't tell them.
They do terrible things to people that Tell Them.

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Re: 1342: "Ancient Stars"

Postby cronjob » Sat Mar 15, 2014 6:48 pm UTC

mfb wrote:cronjob: Learn some physics please and don't try to spread your misconceptions.


helpful much?
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Re: 1342: "Ancient Stars"

Postby Mikeski » Sat Mar 15, 2014 7:07 pm UTC

cronjob wrote:
mfb wrote:cronjob: Learn some physics please and don't try to spread your misconceptions.

helpful much?

You started out in your first post with photons "bumping into" other photons, implying that light from other galaxies never gets to us. That's how lightsabers work, not how light works. Educating someone with that view of the universe is well beyond the scope of a comment thread, so "learn some physics, please" is about as helpful as he is required to be.

You further said, in that same post, "please don't prove me wrong", which means you would rather remain ignorant than be corrected. So why should mfb be required to make the attempt to educate you?

And, when others had already tried to correct you, you doubled down on the "bumping photons" thing in your second post, calling the actual behavior of light (that photons from distant places actually get to our eyes) "a romantic notion".

Based on that, mfb is being very helpful to everyone in the thread who isn't you, by attempting to remove erroneous information at the source, with "don't try to spread your misconceptions".

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Re: 1342: "Ancient Stars"

Postby cronjob » Sat Mar 15, 2014 10:57 pm UTC

I want to know how the information gets here. In the short amount of time since my most recent post I started looking around elsewhere. Most of the replies I got here (except for one) where not really very encouraging. Try to remember that this thread is being posted under "Individual XKCD Comic Threads" not "Particle Physics". I don't really want to hijack or step on the thread. So, I wanted to keep my post light hearted and fairly brief, but some people have not grasped the spirit in which it was intended.
Even though one person did, statistically that's not so good. So I'll take the credit for sloppy writing and try to explain things more clearly.

So far, what I have come across says that light as energy conveys information. Light can be defined as electromagnetic radiation. The word electromagnetic means that energy is conveyed in the form of fluctuating electric and magnetic fields and, these fields require no medium to support or sustain them. The word radiation can be used to describe a type of wave-like motion. I am still going to spend a bit more time looking for explanations online regarding the definition of radiation dealing strictly with motion. Electromagnectic radiation "travels" as waves through the vacuum of space and, these waves impart information when they collide causing whatever they collided with to oscillate.

Now for me, this is the next step. Understanding that Atoms consist of electrons, protons and neutrons. Protons and neutrons are made of quarks. Electrons carry a negative electric charge and protons carry a positive electric charge. Neutrons are neutral. Hence the name. They are neither - or +. When you charge particles like protons and electrons that are not moving they become surrounded by an electric field however, doing so to particles that are already in motion causes electromagnetic radiation.

According to James Clerk Maxwell, moving a charged particle creates a disturbance that "travels" through space without the need for any medium. So, particles in a vacuum are getting bumped and banged and knocked around all the time. Atoms collide, electrons become accelerated by magnetic fields and when all this happens, these particles take their fields with them where ever they go thus, sending out electromagnetic waves in space.

So, the information about particle motion must be transmitted through space by the actual changes in the electric and magnetic fields. Fields are not made of matter. They do not consist of any elements. They have no substance. These fields are a way to transfer certain properties? or certain aspects? of certain forces that could be considered information? over vast distances without any physical connection between the two points.

Now here's the thing that gets me. And by that, I mean I really like it. I love the idea of it for some reason. When these waves get to Earth they are going to come face-to-face with the charged particles of Earth. The charged particles in our eyes are of particular interest. They will oscillate in response to the disturbances or fluctuations in the electric field. There is a whole lot about absorption that I have not yet had time to look at but, when the fluctuations from locally charged particles excite the optic nerve we "see" the image of a galaxy or a star.

So, we have this long distance particle relationship or, a shaking of hands if you will however, that statement still infers that all of our "seeing" is still dependent apoun everything our local sun has been spewing out for 4.5 billion years. So I believe I am right when I say that we are awash in the photons of our own sun. Distant photons are not striking the surface of our eyes. Local particles are being excited by the waves caused by other particles that never actually "traveled" to get here. At least not like a frisbee or a lawn dart.

See the part that gets me? The form of motion allowed for light is radiation. Light does not move the way an archers arrow moves from the bow to the tree or the way a soccer ball or golf ball moves when it is struck. The meaning of the word "travel" seems ambiguous to me and, with the kind of imagination I have, I need to nail down exactly what is meant by that. So I will read up on radiation as a form of motion but, I think I will actually need a particle physicist before I'm through. :idea:


~tayl
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Re: 1342: "Ancient Stars"

Postby rmsgrey » Sat Mar 15, 2014 11:54 pm UTC

A picture you may find useful, though it's wrong in almost every respect, is to think about two people holding a rope stretched between them. One of them flicks his end of the rope, and a pulse of displacement travels along the rope to the far end. That pulse traveling along a rope is analogous to a photon traveling through emptiness - a little coherent packet of energy that travels as a unit.

If the people at each end of the rope each flick their end, you get two pulses, one traveling in each direction, that pass through each other un-changed when they meet - the same thing happens with photons - they don't collide with each other; they just pass through each other like ghosts (you can have any number of photons occupying the same place at the same time, unlike electrons and quarks, which can't get too close to others of the same type).

You can see this for yourself if you have two laser pointers and a dark room - each laser pointer will make a spot on the wall, but even if you cross the beams, you won't be able to get them to interact - the two beams will pass through each other as happily as through any other part of the room's interior. If you tried the same thing with two beams of electrons (also known as beta-radiation) and a large phosphor screen (so you can see where the beam lands) you'd find that when you cross the two beams, you get a lot of scatter - instead of two spots, you get a lot of mess. Of course, you'd also need to be conducting the experiment in a vacuum since beta radiation is scattered by air, which makes it rather harder to do at home... You can also experiment with streams of water (I suggest a couple of garden hoses and a warm summer's day) - the point is that while ordinary matter interacts with ordinary matter, light doesn't interact with light.

Photons are the individual ripples in the electric and magnetic fields, and the ripple that was emitted from an excited charged particle in the photosphere of Sirius in late 2005 is the same ripple that strikes my retina, exciting a charged particle in one of the rod cells, being consumed in the process, but starting a chain reaction that leads to my mind perceiving a star. So far as photons exist at all, it's the photon from Sirius that enters my eye.

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Re: 1342: "Ancient Stars"

Postby da Doctah » Sat Mar 15, 2014 11:55 pm UTC

Zassounotsukushi wrote:I made this graph from Wikipedia because I knew people would be arguing over this:

Image

You could say that most stars are within 100 light-years of Earth. But that would only be true for a sky that isn't sufficiently dark, or with vision that isn't sufficiently good.

Estimates of the number of stars you'll be able to see vary wildly. On the upper end, 5,000 or so may be visible. In those ideal conditions, the vast majority of stars would predate industrial society. In most conditions, the average probably looks more like the 1950s or something like that.


But of course, in order for the comic to work, you pretty much have to limit yourself to stars that have at least somewhat familiar names, and that pretty much sets the cutoff at magnitude 2.5 or so. You can't just have a couple of random characters pointing at the sky and saying "actually, that's Zubenelgenubi, and the light you're seeing only goes back to WWII".

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Re: 1342: "Ancient Stars"

Postby Mikeski » Sat Mar 15, 2014 11:56 pm UTC

cronjob wrote:Distant photons are not striking the surface of our eyes. Local particles are being excited by the waves caused by other particles that never actually "traveled" to get here. At least not like a frisbee or a lawn dart.

You were wrong the first time you said this.

You were wrong the second time you said this.

You continue to be wrong when you continue to repeat it. Please cease doing so.

If you want a concrete example of why you must be wrong, consider this. We know what other stars are made of because we can examine the spectral content of the photons they shoot at us; different elements fire off photons of different discrete frequencies. If those exact photons did not reach us (if they were soaked up by other "stuff" in the vacuum of space, or our own atmosphere, and then re-emitted), we would see the spectra of space dust, or of our own atmosphere, and not of stars. (If you think that is what is happening, consider why the same spectrometer on the same telescope pointed at different stars will get different results. It's the same air right in front of it. And there's not much iron in air, either, though there's a fair amount in some stars, which we can see by doing this.)

but, I think I will actually need a particle physicist before I'm through.

You need an introductory physics text, and possibly an introductory calculus text if you want to understand the math in the physics text.

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Re: 1342: "Ancient Stars"

Postby cronjob » Sun Mar 16, 2014 12:56 am UTC

rmsgrey wrote:A picture you may find useful, though it's wrong in almost every respect, is to think about two people holding a rope stretched between them. One of them flicks his end of the rope, and a pulse of displacement travels along the rope to the far end. That pulse traveling along a rope is analogous to a photon traveling through emptiness - a little coherent packet of energy that travels as a unit.

If the people at each end of the rope each flick their end, you get two pulses, one traveling in each direction, that pass through each other un-changed when they meet - the same thing happens with photons - they don't collide with each other; they just pass through each other like ghosts (you can have any number of photons occupying the same place at the same time, unlike electrons and quarks, which can't get too close to others of the same type).

You can see this for yourself if you have two laser pointers and a dark room - each laser pointer will make a spot on the wall, but even if you cross the beams, you won't be able to get them to interact - the two beams will pass through each other as happily as through any other part of the room's interior. If you tried the same thing with two beams of electrons (also known as beta-radiation) and a large phosphor screen (so you can see where the beam lands) you'd find that when you cross the two beams, you get a lot of scatter - instead of two spots, you get a lot of mess. Of course, you'd also need to be conducting the experiment in a vacuum since beta radiation is scattered by air, which makes it rather harder to do at home... You can also experiment with streams of water (I suggest a couple of garden hoses and a warm summer's day) - the point is that while ordinary matter interacts with ordinary matter, light doesn't interact with light.


Hi There:

I understand that a photon has no mass and that an electromagnetic field has no substance or matter. (and that garden hoses on a warm summers day require no explanation at all whatsoever) But when I think of electromagnetic radiation the model (or thought experiment) that comes to my mind is one of two giant electromagnets. Set them on a table and at the flick of switch you will create a magnetic field. It comes into existence. Flick the switch again and the field goes out of existence. Nothing traveled from point a to point b. A magnetic field went into and then out of existence. Nothing more. Nothing less.

Now, move those magnets to opposite ends of the universe. They will be bigger and draw more juice but hey, this is a thought experiment. (It's like asking, "What IF?") So now our magnets are at opposite ends of the universe and we flick the switch and a magnetic field instantly comes into existence. It does not "travel" from one magnet to the other. That field would have to exceed the speed of light to be created instantly by "traveling" between magnets and that's not possible. Flick the switch and you create a magnetic field. It comes into existence. Flick the switch again and the field goes out of existence. Nothing traveled from point a to point b. A magnetic field went into and then out of existence. Nothing more and nothing less, regardless of the distance.

It's this image of that thought experiment in my mind that makes me think that the image in my mind of the image generated by a distant star (an image delivered by an electromagnetic field over vast distances) does not "travel" in the traditional sense of the word.

Photons are the individual ripples in the electric and magnetic fields, and the ripple that was emitted from an excited charged particle in the photosphere of Sirius in late 2005 is the same ripple that strikes my retina, exciting a charged particle in one of the rod cells, being consumed in the process, but starting a chain reaction that leads to my mind perceiving a star. So far as photons exist at all, it's the photon from Sirius that enters my eye.


So, The elctromagnetic field or waves or radiation (since it's moving) impart information in the form of the oscillations of the photons and, those are the ripples to which you refer?
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Re: 1342: "Ancient Stars"

Postby Klear » Sun Mar 16, 2014 1:21 am UTC

rmsgrey wrote:If you tried the same thing with two beams of electrons (also known as beta-radiation) and a large phosphor screen (so you can see where the beam lands) you'd find that when you cross the two beams, you get a lot of scatter - instead of two spots, you get a lot of mess.


Of course, everybody knows that you are not supposed to cross the beams.

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Re: 1342: "Ancient Stars"

Postby speising » Sun Mar 16, 2014 1:35 am UTC

Klear wrote:
rmsgrey wrote:If you tried the same thing with two beams of electrons (also known as beta-radiation) and a large phosphor screen (so you can see where the beam lands) you'd find that when you cross the two beams, you get a lot of scatter - instead of two spots, you get a lot of mess.


Of course, everybody knows that you are not supposed to cross the beams.

that were protons, though.

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Re: 1342: "Ancient Stars"

Postby cronjob » Sun Mar 16, 2014 1:45 am UTC

Mikeski wrote:
cronjob wrote:Distant photons are not striking the surface of our eyes. Local particles are being excited by the waves caused by other particles that never actually "traveled" to get here. At least not like a frisbee or a lawn dart.

You were wrong the first time you said this.

You were wrong the second time you said this.

You continue to be wrong when you continue to repeat it. Please cease doing so.

If you want a concrete example of why you must be wrong, consider this. We know what other stars are made of because we can examine the spectral content of the photons they shoot at us; different elements fire off photons of different discrete frequencies. If those exact photons did not reach us (if they were soaked up by other "stuff" in the vacuum of space, or our own atmosphere, and then re-emitted), we would see the spectra of space dust, or of our own atmosphere, and not of stars. (If you think that is what is happening, consider why the same spectrometer on the same telescope pointed at different stars will get different results. It's the same air right in front of it. And there's not much iron in air, either, though there's a fair amount in some stars, which we can see by doing this.)

but, I think I will actually need a particle physicist before I'm through.

You need an introductory physics text, and possibly an introductory calculus text if you want to understand the math in the physics text.


Don't cop an aneurysm Mikeski. It's just the XKCD Comic Thread. :mrgreen: Personally, for me anyway, I would say this is a personal best when it comes to understanding physics. I'd say I've also covered quite a lot of ground in very short time. So, I give myself around of applause for that. :D

From what I've read so far I'm am left wondering if the electromagnetic radiation which causes the photon to oscillate could get the same result from any photon.
I also wonder what the probability of photons from distance stars striking the surface of the eye would be as opposed to those from our own sun.
...and that makes me wonder if the particles in the eye which are excited by local processes would even care which photon it is, as long as the information imparted by the electromagnetic radiation was the same as when the initial wave form propagated?

Anyway, I have no doubt but what my imagination can out strip my education but those are the things I think about when I'm stargazing. I think that was kind of the basic premise of the cartoon to begin with. :wink:

Also, read the thought experiment I posted above. It might seem overly simplistic but I would like to know what you think of it.
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Re: 1342: "Ancient Stars"

Postby cronjob » Sun Mar 16, 2014 1:47 am UTC

Klear wrote:
rmsgrey wrote:If you tried the same thing with two beams of electrons (also known as beta-radiation) and a large phosphor screen (so you can see where the beam lands) you'd find that when you cross the two beams, you get a lot of scatter - instead of two spots, you get a lot of mess.


Of course, everybody knows that you are not supposed to cross the beams.


It's O.K. to cross the streams though. ...as long as they're just garden hoses and it's warm out... 8-)
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Re: 1342: "Ancient Stars"

Postby chenille » Sun Mar 16, 2014 3:51 am UTC

cronjob wrote:From what I've read so far I'm am left wondering if the electromagnetic radiation which causes the photon to oscillate could get the same result from any photon.

The point is that electromagnetic radiation and photons aren't different things. More specifically it was discovered that there can't be just any amount of electromagnetic radiation, but it always comes in discrete quantities linked to frequency, and the name photon is what was given for one of those units. Talking about what one does to the other is like talking about the interaction between matter and atoms; it's a very confused description because matter is atoms.

cronjob wrote:So now our magnets are at opposite ends of the universe and we flick the switch and a magnetic field instantly comes into existence. It does not "travel" from one magnet to the other. That field would have to exceed the speed of light to be created instantly by "traveling" between magnets and that's not possible. Flick the switch and you create a magnetic field. It comes into existence. Flick the switch again and the field goes out of existence. Nothing traveled from point a to point b. A magnetic field went into and then out of existence. Nothing more and nothing less, regardless of the distance.

This is not the way it works, though. Whether you regard it as traveling or not, doing something in one place and instantaneously having effects somewhere else is exceeding the speed of light. The magnetic field is something whose value is defined for each given point, but changes in it do not happen everywhere at once. Instead, a change that starts in one place only propagates from there at the speed of light. rmsgrey's rope is a good analogy: it's true the rope doesn't move from one person to the other at all, but the little ripples in it do.

Electromagnetic radiation is one type of this. Maxwell's equations give that a changing magnetic field creates an electric field, and a changing electric field creates a magnetic field. So oscillations are self-sustaining: fluctuations in each field gives the other one. And again, the specific values of the fields refer to single positions, but the changes themselves propagate outward at the speed of light. So, radiation, which given time could travel infinitely far away if there were nothing to block it.

And by the way, that's how everything works in quantum theory: fields and particles are the same thing. Just as a photon is a unit ripple in the electromagnetic field, so an electron is a unit ripple in the electron field, the main difference being the type of components (spinor stuff instead of electric and magnetic potential). So this isn't really a different type of traveling than anything else, it's simply what describing travel using fields looks like, which you maybe haven't seen for matter before.

I hope that helps you understand what is happening a little better. If not, and you want to learn more, you might also try the science subforum.
Last edited by chenille on Sun Mar 16, 2014 4:24 am UTC, edited 1 time in total.

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Re: 1342: "Ancient Stars"

Postby addams » Sun Mar 16, 2014 4:21 am UTC

Mikeski wrote:
cronjob wrote:Distant photons are not striking the surface of our eyes. Local particles are being excited by the waves caused by other particles that never actually "traveled" to get here. At least not like a frisbee or a lawn dart.

You were wrong the first time you said this.

You were wrong the second time you said this.

You continue to be wrong when you continue to repeat it. Please cease doing so.

If you want a concrete example of why you must be wrong, consider this. We know what other stars are made of because we can examine the spectral content of the photons they shoot at us; different elements fire off photons of different discrete frequencies. If those exact photons did not reach us (if they were soaked up by other "stuff" in the vacuum of space, or our own atmosphere, and then re-emitted), we would see the spectra of space dust, or of our own atmosphere, and not of stars. (If you think that [i]is what is happening, consider why the same spectrometer on the same telescope pointed at different stars will get different results. It's the same air right in front of it. And there's not much iron in air, either, though there's a fair amount in some stars, which we can see by doing this.)[/i]

but, I think I will actually need a particle physicist before I'm through.

You need an introductory physics text, and possibly an introductory calculus text if you want to understand the math in the physics text.

We don't want to take Physics.
That is, kind of, snotty.

If you asked us about something we know,
we wouldn't tell you to read a book.

That explanation makes sense.
Even if you did top it off with a snotty remark.

We are seeing the very same light that left that star?
So; We are back where we were. ok.

Eight minutes from Sol to eye? ok.
And; Some much larger number from Alpha Centauri?

The very same photon?
Carrying the very same data, from its creation?

And; We get to see it.
And; With a spectrometer we can know what kind of atmosphere....?

Wait. wait... When we look at the atmosphere of Jupeter or Mars,
We are seeing bouncing photons. right?

What is not absorbed from the full spectrum tells the guy with the machine what is There. right?

What was the Question?
How does light travel?

Does "Fast" still work as an answer?

It has been said, "If you can't explain it to a child or your grandmother, you don't know it."
One guy said, "I don't understand porn. I can't explain it to my grandmother."
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Re: 1342: "Ancient Stars"

Postby cronjob » Sun Mar 16, 2014 5:01 am UTC

chenille wrote:I hope that helps you understand what is happening a little better. If not, and you want to learn more, you might also try the science subforum.


Thanks for taking the time to try and unwash this particular "unwashed heathen". I am feeling a little dizzy at the moment though, considering that my giant electromagnet thought experiment just got tossed. I was absolutely certain that the reasoning behind that was sound. :shock:

p.s. I am guessing that what you meant to say was that I should try reading the science subforum. I wouldn't want it to be as convoluted as this comics thread.

~tayl
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Re: 1342: "Ancient Stars"

Postby cronjob » Sun Mar 16, 2014 5:12 am UTC

addams wrote:
Mikeski wrote:
cronjob wrote:Distant photons are not striking the surface of our eyes. Local particles are being excited by the waves caused by other particles that never actually "traveled" to get here. At least not like a frisbee or a lawn dart.

You were wrong the first time you said this.

You were wrong the second time you said this.

You continue to be wrong when you continue to repeat it. Please cease doing so.

If you want a concrete example of why you must be wrong, consider this. We know what other stars are made of because we can examine the spectral content of the photons they shoot at us; different elements fire off photons of different discrete frequencies. If those exact photons did not reach us (if they were soaked up by other "stuff" in the vacuum of space, or our own atmosphere, and then re-emitted), we would see the spectra of space dust, or of our own atmosphere, and not of stars. (If you think that [i]is what is happening, consider why the same spectrometer on the same telescope pointed at different stars will get different results. It's the same air right in front of it. And there's not much iron in air, either, though there's a fair amount in some stars, which we can see by doing this.)[/i]

but, I think I will actually need a particle physicist before I'm through.

You need an introductory physics text, and possibly an introductory calculus text if you want to understand the math in the physics text.

We don't want to take Physics.
That is, kind of, snotty.

If you asked us about something we know,
we wouldn't tell you to read a book.

That explanation makes sense.
Even if you did top it off with a snotty remark.

We are seeing the very same light that left that star?
So; We are back where we were. ok.

Eight minutes from Sol to eye? ok.
And; Some much larger number from Alpha Centauri?

The very same photon?
Carrying the very same data, from its creation?

And; We get to see it.
And; With a spectrometer we can know what kind of atmosphere....?

Wait. wait... When we look at the atmosphere of Jupeter or Mars,
We are seeing bouncing photons. right?

What is not absorbed from the full spectrum tells the guy with the machine what is There. right?

What was the Question?
How does light travel?

Does "Fast" still work as an answer?

It has been said, "If you can't explain it to a child or your grandmother, you don't know it."
One guy said, "I don't understand porn. I can't explain it to my grandmother."


...i think you might just appreciate this...

Richard Feynman wrote:

"He [Feynman's father] was happy with me, I believe. Once though, when I came back from MIT and been there a few years, he said to me, "Now, you've become educated about these things, and there's one question I've always had that I've never really understood very well, and I'd like to ask you, now that you've studied this, to explain it to me." And I asked him what it was, and he said that he understood that when an atom makes a transition from one state to another it emits a particle of light called a photon. And I said "That's right.". And he said "Well now, is the photon in the atom ahead of the time that it comes out, or is there no photon in to start with?".
I said "There's no photon in, it's just that when the electron makes a transition it comes out." He says "Well then where does it come from, and how does it come out?".
So I said - of course I couldn't answer him, the view is that photon numbers aren't conserved, they're just created by the motion of the electron - I tried to explain it to him something like that the sound that I'm making now wasn't "in" me. It's not like my little boy said, when he was just little one day he was talking and suddenly said that he could no longer say a certain word - the word was cat - because his word bag had run out of the word cat. So, there's no "word bag" that you have inside where you use up the words as they come out, you make them as you go along, and in the same sense there is no "photon bag" in an atom, and when the photons come out they don't come from somewhere. But I couldn't do much better. He was not satisfied with me in that respect and I never was able to explain any of the things that he didn't understand. So he was unsuccessful, he sent me through all these universities in order to find out these things and never did find out!"

...I love the language of Feynman. 8-) It's so technical.
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Re: 1342: "Ancient Stars"

Postby addams » Sun Mar 16, 2014 6:01 am UTC

I think the man made sense.
Thank you for bringing that paragraph here.

I would have never found it on my own.

Each particle is an endless pool of light.
Like internet posters are an endless pool of words.

I'm no better than Feynman's Dad.
Life is, just, an exchange of electrons; It is up to us to give it meaning.

We are all in The Gutter.
Some of us see The Gutter.
Some of us see The Stars.
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Re: 1342: "Ancient Stars"

Postby gmalivuk » Sun Mar 16, 2014 6:03 am UTC

cronjob: You said you might need a particle physicist, but there are already many of them on the forums, and some of them have already responded to your posts here. We are a pretty highly knowledgeable bunch on these forums, and we tend to take our science seriously, even in the Individual Comic discussions (especially when the comic being discussed is one of the more scientific ones).

If you want to talk about an interesting idea you've had about how the universe works, and you don't want pesky facts to get in the way and disprove your idea, we have a Fictional Science subforum for just that. Folks there can propose a science fiction sort of idea they've got, and everyone can have a jolly old time running with it and thinking about how the universe might look if that thing were true.

But unless you acknowledge from the start that you want a hypothetical discussion about a cool idea, when you ask a seemingly straightforward question (such as how information gets here from distant light sources), people are going to give you a straightforward, fact-based answer, whether or not it fits your own preconceived notions about how the world ought to work.
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Re: 1342: "Ancient Stars"

Postby Zassounotsukushi » Sun Mar 16, 2014 12:48 pm UTC

da Doctah wrote:
Zassounotsukushi wrote:I made this graph from Wikipedia because I knew people would be arguing over this:

Image

You could say that most stars are within 100 light-years of Earth. But that would only be true for a sky that isn't sufficiently dark, or with vision that isn't sufficiently good.

Estimates of the number of stars you'll be able to see vary wildly. On the upper end, 5,000 or so may be visible. In those ideal conditions, the vast majority of stars would predate industrial society. In most conditions, the average probably looks more like the 1950s or something like that.


But of course, in order for the comic to work, you pretty much have to limit yourself to stars that have at least somewhat familiar names, and that pretty much sets the cutoff at magnitude 2.5 or so. You can't just have a couple of random characters pointing at the sky and saying "actually, that's Zubenelgenubi, and the light you're seeing only goes back to WWII".


While I see your point, I wonder if you really meant to type 2.5, or something lower. Perhaps 0. Realistically, if we're close to somewhere urban, there might only be 10 stars visible in the sky. If we're talking about average people, that's fairly average. But that's a fun question. I would like to see a distribution function of the number of stars that people can see. Given that, whether they point to a star that's 8 light-years away or 360 light-years away is purely up to chance.

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Re: 1342: "Ancient Stars"

Postby rmsgrey » Sun Mar 16, 2014 2:15 pm UTC

Zassounotsukushi wrote:While I see your point, I wonder if you really meant to type 2.5, or something lower. Perhaps 0. Realistically, if we're close to somewhere urban, there might only be 10 stars visible in the sky. If we're talking about average people, that's fairly average. But that's a fun question. I would like to see a distribution function of the number of stars that people can see. Given that, whether they point to a star that's 8 light-years away or 360 light-years away is purely up to chance.


Most people recognise words as the names of stars because they've heard someone else talk about them, not because they associate that word with a particular dot in the night sky.

Sirius has a fair chance of being picked as an example since it's much closer than anything brighter, and much brighter than anything closer, so has the brightest apparent magnitude of any star in the night sky by some way (4 planets, the moon, the sun, and assorted man-made objects are brighter).

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Re: 1342: "Ancient Stars"

Postby gmalivuk » Sun Mar 16, 2014 3:17 pm UTC

Wikipedia wrote:There is even variation within metropolitan areas. For those who lives in the immediate suburbs of New York City, the limiting magnitude might be 4.0. This corresponds to roughly 250 visible stars, or one-tenth the number that can be perceived under perfectly dark skies. From the New York City boroughs outside Manhattan (Brooklyn, Queens, Staten Island and the Bronx), the limiting magnitude might be 3.0, suggesting that at best, only about 50 stars might be seen at any one time. From brightly lit Midtown Manhattan, the limiting magnitude is possibly 2.0, meaning that from the heart of New York City only approximately 15 stars will be visible at any given time.


The map information here combined with the detailed descriptions of each color here might get you what you're looking for, Zassounotsukushi.
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Re: 1342: "Ancient Stars"

Postby ucim » Sun Mar 16, 2014 3:25 pm UTC

cronjob wrote:Now, move those magnets to opposite ends of the universe. They will be bigger and draw more juice but hey, this is a thought experiment. (It's like asking, "What IF?") So now our magnets are at opposite ends of the universe and we flick the switch and a magnetic field instantly comes into existence. It does not "travel" from one magnet to the other.
That's the mistake. It happens very fast, but not instantly. And if you move the two magnets far enough away, there will be a measurable amount of time before the second magnet responds. In the meantime, the first magnet did its thing, and the second magnet is blissfully unaware. The energy, or information, or whathave you, however, is "somewhere", and that "somewhere" is in the photon, or electromagnetic ripple, that is traveling from one magnet to another... at the speed of light.

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Re: 1342: "Ancient Stars"

Postby Zassounotsukushi » Sun Mar 16, 2014 3:47 pm UTC

Interesting. Those are higher magnitude cutoffs than what I had expected. I thought that starless nights were fairly common in cities, but I guess that was just due to lack of education on it.

I took the brightness-weighted average of distance. This would be by converting the apparent magnitude into a brightness metric, and then averaging the distance using that weighting. Doing this, I found 13.2 light-years. This would mean that out of the 91 brightest stars, the photons are on average 13.2 years old.

I'm not sure if going to more clear skies would budge this number very much either, since the brightest star accounts for such a large fraction of the total light.

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Re: 1342: "Ancient Stars"

Postby gmalivuk » Sun Mar 16, 2014 3:54 pm UTC

It's entirely possible that standing on the street in the middle of Times Square means you won't be able to see a single star, because in addition to skyglow you're contending with all the city lights directly visible to you as well. I suspect the figures given there amount to something more like what you'd see from the top of a building.
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Re: 1342: "Ancient Stars"

Postby cronjob » Sun Mar 16, 2014 4:37 pm UTC

addams wrote:I think the man made sense.
Thank you for bringing that paragraph here.

I would have never found it on my own.

Each particle is an endless pool of light.
Like internet posters are an endless pool of words.

I'm no better than Feynman's Dad.


...totally agree.
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Re: 1342: "Ancient Stars"

Postby cronjob » Sun Mar 16, 2014 5:07 pm UTC

gmalivuk wrote:cronjob: You said you might need a particle physicist, but there are already many of them on the forums, and some of them have already responded to your posts here. We are a pretty highly knowledgeable bunch on these forums, and we tend to take our science seriously, even in the Individual Comic discussions (especially when the comic being discussed is one of the more scientific ones).


Yes there are. I appreciate their time and effort in responding and I thanked them. And then I tried to ingest what they said and RTFM'd. To those of you who were sincerely helpful, thanx again.

gmalivuk wrote:If you want to talk about an interesting idea you've had about how the universe works, and you don't want pesky facts to get in the way and disprove your idea, we have a Fictional Science subforum for just that. Folks there can propose a science fiction sort of idea they've got, and everyone can have a jolly old time running with it and thinking about how the universe might look if that thing were true.


Just posting a link to the fictional science subforum would probably have made for a great post if it was done in good humor.

gmalivuk wrote:IBut unless you acknowledge from the start that you want a hypothetical discussion about a cool idea, when you ask a seemingly straightforward question (such as how information gets here from distant light sources), people are going to give you a straightforward, fact-based answer, whether or not it fits your own preconceived notions about how the world ought to work.


You are absolutely right. But you have to understand that all of the ideas that I have (the ideas that germinate when I'm out looking at the stars at night) are seeds planted from reading books written by people with Ph.Ds in front of their names to begin with, particle physicists included.
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Re: 1342: "Ancient Stars"

Postby bigchiefbc » Sun Mar 16, 2014 9:10 pm UTC

gmalivuk wrote:
Wikipedia wrote:There is even variation within metropolitan areas. For those who lives in the immediate suburbs of New York City, the limiting magnitude might be 4.0. This corresponds to roughly 250 visible stars, or one-tenth the number that can be perceived under perfectly dark skies. From the New York City boroughs outside Manhattan (Brooklyn, Queens, Staten Island and the Bronx), the limiting magnitude might be 3.0, suggesting that at best, only about 50 stars might be seen at any one time. From brightly lit Midtown Manhattan, the limiting magnitude is possibly 2.0, meaning that from the heart of New York City only approximately 15 stars will be visible at any given time.


The map information here combined with the detailed descriptions of each color here might get you what you're looking for, Zassounotsukushi.


I'm a few miles northeast of Providence, on the line between red and dark-red in that map. Last night my son and I went our for a bit and we could clearly see a lot of the minor stars in Orion, which are all between 4 and 5 in magnitude.

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Re: 1342: "Ancient Stars"

Postby addams » Mon Mar 17, 2014 1:28 am UTC

gmalivuk wrote:It's entirely possible that standing on the street in the middle of Times Square means you won't be able to see a single star, because in addition to skyglow you're contending with all the city lights directly visible to you as well. I suspect the figures given there amount to something more like what you'd see from the top of a building.

I was in a city.
The night skies were clear.

I was up high on the outside of a building.
Night after night; One Star.

Some nights there would be a handful of stars.
I could count on that one star. Nothing else.

Cities drown out the stars.
Don't feel bad, City People.
Rain Clouds drown out the stars, too.
Life is, just, an exchange of electrons; It is up to us to give it meaning.

We are all in The Gutter.
Some of us see The Gutter.
Some of us see The Stars.
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Those that do not Know; Don't tell them.
They do terrible things to people that Tell Them.

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Re: 1342: "Ancient Stars"

Postby gmalivuk » Mon Mar 17, 2014 1:52 am UTC

bigchiefbc wrote:
gmalivuk wrote:
Wikipedia wrote:There is even variation within metropolitan areas. For those who lives in the immediate suburbs of New York City, the limiting magnitude might be 4.0. This corresponds to roughly 250 visible stars, or one-tenth the number that can be perceived under perfectly dark skies. From the New York City boroughs outside Manhattan (Brooklyn, Queens, Staten Island and the Bronx), the limiting magnitude might be 3.0, suggesting that at best, only about 50 stars might be seen at any one time. From brightly lit Midtown Manhattan, the limiting magnitude is possibly 2.0, meaning that from the heart of New York City only approximately 15 stars will be visible at any given time.


The map information here combined with the detailed descriptions of each color here might get you what you're looking for, Zassounotsukushi.


I'm a few miles northeast of Providence, on the line between red and dark-red in that map. Last night my son and I went our for a bit and we could clearly see a lot of the minor stars in Orion, which are all between 4 and 5 in magnitude.
Which fits well with the map's figure of 5.0-5.5 for dark red.
Unless stated otherwise, I do not care whether a statement, by itself, constitutes a persuasive political argument. I care whether it's true.
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addams
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Re: 1342: "Ancient Stars"

Postby addams » Fri Mar 21, 2014 5:42 am UTC

cronjob wrote:
Klear wrote:
rmsgrey wrote:If you tried the same thing with two beams of electrons (also known as beta-radiation) and a large phosphor screen (so you can see where the beam lands) you'd find that when you cross the two beams, you get a lot of scatter - instead of two spots, you get a lot of mess.


Of course, everybody knows that you are not supposed to cross the beams.


It's O.K. to cross the streams though. ...as long as they're just garden hoses and it's warm out... 8-)

Yes. Yes. We all know, "Don't cross the steams, Men."
That is, almost, a law of the universe.

Do you know Why?

Think. Go a head. Think.
Remember where we got the words?

http://www.youtube.com/watch?v=9wrEEd1ajz4
The Truth. And; How I know?

You take a guess or two.
Spoiler:
Boys.
I swear. Boys.

Don't cross the streams, because Eddies Mom might kill you if you do it, again.
Do you know what the bathroom looks like after three eleven year old boys Piss together?

You did? Then why did you let them go in there, together?
Girls, almost, never do that.

Lock themselves in for hours and hours? Yes.
Cross the streams? No.
Life is, just, an exchange of electrons; It is up to us to give it meaning.

We are all in The Gutter.
Some of us see The Gutter.
Some of us see The Stars.
by mr. Oscar Wilde.

Those that want to Know; Know.
Those that do not Know; Don't tell them.
They do terrible things to people that Tell Them.


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