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0811: "Starlight"

Posted: Wed Oct 27, 2010 4:04 am UTC
by jspenguin
Image
alt:"Don't worry! From the light's point of view, home and your eye are in the same place, and the journey takes no time at all! Relativity saves the day again."

Interesting question: when a photon hits a mirror, does the original photon bounce or does it get absorbed, then a new photon is emitted? Is there, physically speaking, a difference?

Re: 0811: "Starlight"

Posted: Wed Oct 27, 2010 4:06 am UTC
by Troger64
I always assumed the photon bounced, but my major is in mathematics so I'm not exactly qualified to say...

Re: 0811: "Starlight"

Posted: Wed Oct 27, 2010 4:07 am UTC
by BlazeOrangeDeer
hmmm I think since photons are in constant interaction with the quantum vacuum, they can be replaced with identical virtual photons at any time. Idk?

Re: 0811: "Starlight"

Posted: Wed Oct 27, 2010 4:09 am UTC
by syko_lozz
surely it bounces? wouldn't absorption and reemission mean that mirrors were made out of stuff that can emit many different wavelengths of light? I dont think they're that special. But then again I'm a biology major, so who am I to comment?

Re: 0811: "Starlight"

Posted: Wed Oct 27, 2010 4:11 am UTC
by BlazeOrangeDeer
I think the photon interacts with the electron, bringing it to a higher energy state, then the electron falls back and re-emits it. The difference probably doesn't matter

Re: 0811: "Starlight"

Posted: Wed Oct 27, 2010 4:12 am UTC
by Meem1029
That's an interesting question. As a physics major I will eventually be able to say probably, but as a freshman I have no idea. I guess I have always thought of it as just bouncing with the same photon, but I'm not sure. As far as a difference, I think there is some sort of a difference because of the concept of quantum cryptography making it so listening can be detected, but I don't know. Of course thinking about it if the photon always moves at c it would have to have infinite acceleration to turn around completely and instantly. That or I just don't really understand what goes on at those levels (OK, I know I don't fully understand it, but I think I have a bit of a clue).

Edit: Reading through the alt-text again made me realize something I want to confirm. Because of relativity, wouldn't a photon technically only exist for an instant (or actually no time) in it's own reference frame? Or would it be forever? Asking random questions about relativity is so much more interesting than the lab I'm supposed to be doing.

Re: 0811: "Starlight"

Posted: Wed Oct 27, 2010 4:13 am UTC
by madock345
Science is fun but i have to say
Dawwwww
beret-guy is soooo cute remindes me of the one about his mom

Re: 0811: "Starlight"

Posted: Wed Oct 27, 2010 4:18 am UTC
by glasnt
HI JOEE

Beret guy is cute.

I would suggest that solarpanels could be a happy reflecty bed, I'm sure that could be marketed somehow.

Re: 0811: "Starlight"

Posted: Wed Oct 27, 2010 4:20 am UTC
by BlazeOrangeDeer
Meem1029 wrote:Of course thinking about it if the photon always moves at c it would have to have infinite acceleration to turn around completely and instantly. That or I just don't really understand what goes on at those levels (OK, I know I don't fully understand it, but I think I have a bit of a clue).

As far as I understand (high school senior with a physics obsession) spacetime is quantized too so infinities don't occur, and newtonian mechanics are not accurate at that level. Aaaaaand newtonian physics doesn't describe entities with no rest mass.

Re: 0811: "Starlight"

Posted: Wed Oct 27, 2010 4:29 am UTC
by Faranya
I doubt that bouncing is the solution to this. I would hypothesize that the photon is absorbed by the material, and then the material of the mirror emits another photon of approximately the same, but slightly less energy.

Re: 0811: "Starlight"

Posted: Wed Oct 27, 2010 4:31 am UTC
by BlazeOrangeDeer
Faranya wrote:I doubt that bouncing is the solution to this. I would hypothesize that the photon is absorbed by the material, and then the material of the mirror emits another photon of approximately the same, but slightly less energy.

At that scale there is nowhere for the energy to go, so the photon would be basically identical in energy (frequency)

Re: 0811: "Starlight"

Posted: Wed Oct 27, 2010 4:32 am UTC
by Sam Knight
I don't get this comic at all.. Why the mirror?

Re: 0811: "Starlight"

Posted: Wed Oct 27, 2010 4:35 am UTC
by Me321
Then use gravity to bend it arround the planet, you just need a very massive object at the correct location.

Re: 0811: "Starlight"

Posted: Wed Oct 27, 2010 4:37 am UTC
by BlazeOrangeDeer
Sam Knight wrote:I don't get this comic at all.. Why the mirror?

He wants the photons to be able to travel back home. He probably should have used three mirrors at right angles (corner retro-reflector) so they would actually reflect straight back at their source ;)

Re: 0811: "Starlight"

Posted: Wed Oct 27, 2010 4:40 am UTC
by LucasBrown
Ruined by the title-text. I like the comic and I like the title-text, but the combination of the two is awful.

Re: 0811: "Starlight"

Posted: Wed Oct 27, 2010 4:45 am UTC
by Kaijyuu
Relativity saves the day again. Yay!

Then use gravity to bend it arround the planet, you just need a very massive object at the correct location.

You'd need a black hole to do that. Plus the vast majority of the photons would get sucked in, which is a fate far worse than being absorbed by your eye.

Re: 0811: "Starlight"

Posted: Wed Oct 27, 2010 4:46 am UTC
by joee
I interpreted that as a photo frame and thought it was van gogh going "okay, which section of sky am I going to paint"

HI GLASNT

Re: 0811: "Starlight"

Posted: Wed Oct 27, 2010 4:46 am UTC
by Singulaire
I've actually discussed that question with my high school physics teacher and a couple of friends (hurray for end-of-the-year lessons when you're not really studying anymore). Our final verdict was that- barring some unknown quantum effect that makes thermodynamics go wonky at that level- the photon should be absorbed, excite an atom, and as the electron returns to its base orbit, a slightly less energetic photon will be emitted.
Ofcourse, despite my use of the word "verdict", a bunch of student and a high school teacher, addorned by whatever degree they may be, are by no means an authority.

Re: 0811: "Starlight"

Posted: Wed Oct 27, 2010 4:51 am UTC
by Meem1029
If the effect does come because of absorption, why does the light always reflect back with equal angles of incidence. Actually, why does it either way? On a macroscopic scale it seems logical that it would, but when looking on an atomic scale there doesn't seem to be any good reason for this.

Also, I wonder if scientists actually know the answer to these questions or if they are open questions that are just mostly irrelevant.

Re: 0811: "Starlight"

Posted: Wed Oct 27, 2010 4:59 am UTC
by Mapar
The photon is absorbed, I read that about a year ago in a scientific article. It was in Knack, one of our local "weeklies".

Re: 0811: "Starlight"

Posted: Wed Oct 27, 2010 5:02 am UTC
by BlazeOrangeDeer
Singulaire wrote:I've actually discussed that question with my high school physics teacher and a couple of friends (hurray for end-of-the-year lessons when you're not really studying anymore). Our final verdict was that- barring some unknown quantum effect that makes thermodynamics go wonky at that level- the photon should be absorbed, excite an atom, and as the electron returns to its base orbit, a slightly less energetic photon will be emitted.
Ofcourse, despite my use of the word "verdict", a bunch of student and a high school teacher, addorned by whatever degree they may be, are by no means an authority.

I thought thermodynamics were a macro effect of quantum events, just as newtonian mechanics are basically the total of quantum events in a large system. So yeah, they basically "go wonky" at the quantum scale and the photon retains the energy.

Re: 0811: "Starlight"

Posted: Wed Oct 27, 2010 5:03 am UTC
by Meem1029
Do you have a link or title for that, because that would be a very interesting article to read?

(O.T., but does it seem awkward to anyone else when you ask a question with a side-note at the end as part of the sentence because the punctuation just seems bizarre for that? And I just realized that I did the same thing in my question right there.)

Re: 0811: "Starlight"

Posted: Wed Oct 27, 2010 5:06 am UTC
by Arariel
Singulaire wrote:I've actually discussed that question with my high school physics teacher and a couple of friends (hurray for end-of-the-year lessons when you're not really studying anymore). Our final verdict was that- barring some unknown quantum effect that makes thermodynamics go wonky at that level- the photon should be absorbed, excite an atom, and as the electron returns to its base orbit, a slightly less energetic photon will be emitted.
Ofcourse, despite my use of the word "verdict", a bunch of student and a high school teacher, addorned by whatever degree they may be, are by no means an authority.

But why lower energy (and thus frequency)? I don't think light gets any redder when it hits a mirror. Could it be intensity instead?

Re: 0811: "Starlight"

Posted: Wed Oct 27, 2010 5:09 am UTC
by Airbuilder7
I assumed that the rectangular object was a picture frame, in order to refute the woman's cynicism with the beauty of the stars... But now that I look at it, there seem to be specks in the center of the rectangle when he's carrying it, so it might be a mirror as some have suggested. I don't know; I lean toward the poetic picture frame. After all, if this particular starlight did not end its journey in our eyes, it would fly past our small planet and travel onward in the black for another 14 billion years, never to be known and appreciated.

This comic reminds me strongly of a YouTube video about the Hubble Ultra Deep Field I came across last year. The girl's same argument could be made about the photons finally coming to rest against the image collecting plate. Amazing stuff.

EDIT: Found the link. http://www.youtube.com/watch?v=oAVjF_7ensg

Maybe it's the music, but the sheer implications of the image make me tear up a little. Go science. Go mankind. :)

Re: 0811: "Starlight"

Posted: Wed Oct 27, 2010 5:12 am UTC
by Time Kitten
Hmm... I just figured that if anything shiny is that way because of the electron sea thingy that also makes it conductive and everything, and traditionally to bounce photons you use electron streams, it'd be bouncing off of the reflective surface... No clue how glass works, still.

Re: 0811: "Starlight"

Posted: Wed Oct 27, 2010 5:13 am UTC
by Subject17
I was under the impression that atoms could only emit photons at a certain wavelength/frequency, hence emission spectra. Since mirrors can reflect all of visible light (along with infrared and ultraviolet) essentially "perfectly" (barring impurities, etc) back, or at least nearly true to the photon's original energy, wouldn't that mean they have to bounce? Isn't it similar to how colors work (ie blue reflects blue/absorbs all others, etc), or was that just a simplification to the real effects?

Ugghh. We need someone on a grad level of physics before I start to trust any information suggested here.

Re: 0811: "Starlight"

Posted: Wed Oct 27, 2010 5:14 am UTC
by Rilian
Why does it say the light dies here?

Re: 0811: "Starlight"

Posted: Wed Oct 27, 2010 5:15 am UTC
by Eternal Density
Pi Cheers for Relativity!
(Or just 3 if you want to keep things rational.)
Arariel wrote:
Singulaire wrote:I've actually discussed that question with my high school physics teacher and a couple of friends (hurray for end-of-the-year lessons when you're not really studying anymore). Our final verdict was that- barring some unknown quantum effect that makes thermodynamics go wonky at that level- the photon should be absorbed, excite an atom, and as the electron returns to its base orbit, a slightly less energetic photon will be emitted.
Ofcourse, despite my use of the word "verdict", a bunch of student and a high school teacher, addorned by whatever degree they may be, are by no means an authority.

But why lower energy (and thus frequency)? I don't think light gets any redder when it hits a mirror. Could it be intensity instead?
Yeah, the electron is going to fall back to the energy level it started from, thus the same colour of light. When looking at an 'infinite' reflection between parallel mirrors, it gets darker, not redder.
In other words, HighSchoolPhysicsTeacherFail.

Re: 0811: "Starlight"

Posted: Wed Oct 27, 2010 5:16 am UTC
by Retsam
Definitely a GOOMMR moment. I was trying to procrastinate my astronomy essay by reading xkcd... darnit.

Re: 0811: "Starlight"

Posted: Wed Oct 27, 2010 5:16 am UTC
by leifbk
I believe that reflection happens to the light in its wave-form. That said, that dual nature of light has always bothered me.

Re: 0811: "Starlight"

Posted: Wed Oct 27, 2010 5:20 am UTC
by big boss
If the mirror absorbed and re-emitted the photons there is no reason why they should all be re-emitted in a coherent way to recreate the incident image. So i dont think that that is a valid explanation. Also the thermal motions of the atoms in the mirror would make recreating the originally image after reflection impossible because thermal vibrations would distort the paths of the reflected photons.

Edit: If one looks at the mirror as a high potential barrier (a Dirac delta function possibly?) in the path of the wavefunction of the photons. When the wavefunction of the photons hits this barrier they have a nonzero probability of reflecting and going in the opposite direction (this is not to say that they were absorbed by the mirror, they were just reflected).

Re: 0811: "Starlight"

Posted: Wed Oct 27, 2010 5:22 am UTC
by rpgamer
Me321 wrote:Then use gravity to bend it arround the planet, you just need a very massive object at the correct location.

Is this the setup for a "Your mom!" joke?

To other posts: I also thought the object was a picture frame at first. It kinda works both ways, the artistic beauty of those pretty dots viewed in frame, or trying to send the starlight back home.

On the subject of starlight, I've always been intrigued by the idea that everything we see out there happened so long ago. Some of it maybe not so long ago, but there's still plenty of pre-Earth light finally making it here. And also the whole space expansion, and how beyond a certain point, light out there will never reach us, with space expanding away faster and faster and so forth.

Re: 0811: "Starlight"

Posted: Wed Oct 27, 2010 5:27 am UTC
by Ego
Mirror doesn't absorb-and-then-emit photones. It seems like photons are bounced from free electrons in silver surface of mirror.
http://en.wikipedia.org/wiki/Metallic_b ... properties
Yeah, mining physics knowledge from wikipedia is lame.

Re: 0811: "Starlight"

Posted: Wed Oct 27, 2010 5:27 am UTC
by Singulaire
wait, why are we talking about intensity? I'm pretty sure that's a result of the same number of photons being spread over a larger area, which doesn't work with a single photon.
Also, a mirror reflectd all wavelengths? hmm, maybe that would apply to all wavelengths in a certain range, but if my mirror can reflect X-rays, that would lend itself to some interesting pranks. Maybe I'm just getting confused because we're using different definitions of "mirror".
Also, if anyone could provide an article about thermodynamics not working at such a microscopic level, I would love to read it. That was pretty much the thing that stumped us actually- we couldn't agree whether energy could or couldn't be lost in the interaction.

Edit: Ego, I love you.

Second Edit: To elucidate, the discussion was about whether photons lose energy when interacting with atoms (even if infinitsimally small) as part of a larger discussion that can be described as "What happens when light travels, and other interesting minutiae".

Re: 0811: "Starlight"

Posted: Wed Oct 27, 2010 5:31 am UTC
by rcox1
Very cute comic.

As far as the rectangular thing, I think it is a mirror to reflect the light back into space.

As far the pedants, the hypothetical individual photon has probably interacted with many atoms in it's trip to your eye, not the least of which is all the gunk we call the atmosphere. It has been absorbed and reemitted many times so it is a philosophical argument.

To be pseudo philosophical, the starlight is composed many photons which we can wave our hands and say that it forms some sort of wave function(ugh, that has to be wrong), and that the density decreasing cubically as the it travels, which means by the time it hits only very few photons of the star will hit the earth, while most will travel through empty space, unhindered. As such the perturbation to the original function would be minimal. All this, of course, is complete crap.

What the comic did remind of is the Feynman one electron in the universe notion, and by extension one photon, up, down, top, bottom, charm and strange quark, etc. When I took my QM course and perused the lecture notes and popular work, this along with the diagrams seemed a very elegant way to look at the universe.

Re: 0811: "Starlight"

Posted: Wed Oct 27, 2010 5:34 am UTC
by ijuin
For elections that are bound to individual atoms, that would be true (and it would be why non-reflective materials have colors instead of being all white or silver). However, the "electron sea" in metals is not bound to orbitals, and thus can absorb/release a much wider band of energies.

AFAIK the whole "reflects at the same angle" part is an example of the dual particle/wave nature of photons--it's a wavelike property.

Re: 0811: "Starlight"

Posted: Wed Oct 27, 2010 5:43 am UTC
by coder0xff
"In the classical electrodynamics, light is considered as electromagnetic wave, which is governed by the Maxwell Equations. Light waves incident on a material induce small oscillations of polarisation in the individual atoms (or oscillation of electrons, in metals), causing each particle to radiate a small secondary wave (in all directions, like a dipole antenna). All these waves add up to give specular reflection and refraction, according to the Huygens-Fresnel principle." http://en.wikipedia.org/wiki/Reflection_(physics)#Mechanisms_of_reflection

That said, this comic made me go, "awwww." :D

Re: 0811: "Starlight"

Posted: Wed Oct 27, 2010 5:45 am UTC
by Sam Knight
BlazeOrangeDeer wrote:
Sam Knight wrote:I don't get this comic at all.. Why the mirror?

He wants the photons to be able to travel back home. He probably should have used three mirrors at right angles (corner retro-reflector) so they would actually reflect straight back at their source ;)


Haha, trying to quickly cram today's XKCD comic in while working on my neuro paper = me thinking the people had travelled trillions of miles to die under the starlight, and so mirror.

Re: 0811: "Starlight"

Posted: Wed Oct 27, 2010 5:45 am UTC
by Hypoon
Wikipedia seems to have an intelligent section on this: http://en.wikipedia.org/wiki/Reflection_%28physics%29
Mechanisms of reflection

In the classical electrodynamics, light is considered as electromagnetic wave, which is governed by the Maxwell Equations. Light waves incident on a material induce small oscillations of polarisation in the individual atoms (or oscillation of electrons, in metals), causing each particle to radiate a small secondary wave (in all directions, like a dipole antenna). All these waves add up to give specular reflection and refraction, according to the Huygens-Fresnel principle.

In case of dielectric (glass), the electric field of the light acts on the electrons in the glass, the moving electrons generate a field and become a new radiator. The refraction light in the glass is the combined of the forward radiation of the electrons and the incident light and; the backward radiation is the one we see reflected from the surface of transparent materials, this radiation comes from everywhere in the glass, but it turns out that the total effect is equivalent to a reflection from the surface.

In metals, the electrons with no binding energy are called free electrons. The density number of the free electrons is very large. When these electrons oscillate with the incident light, the phase differences between the radiation field of these electrons and the incident field are π, so the forward radiation will compensate the incident light at a skin depth, and backward radiation is just the reflected light.

Light–matter interaction in terms of photons is a topic of quantum electrodynamics, and is described in detail by Richard Feynman in his popular book QED: The Strange Theory of Light and Matter.


More corroborating evidence from Scientific American: http://www.scientificamerican.com/article.cfm?id=how-do-mirrors-reflect-ph
To understand how such mirrors work, let us first describe the interaction of light with some media in the semiclassical view. Light consists of electromagnetic waves, which induce some oscillation of electrons in any substance hit by the light. In an insulator such as glass, the electrons are firmly bound and can only oscillate around their normal position. This movement influences the propagation of light so that its wave velocity is reduced, while there is only a small loss of energy. This is different in a metal, where some of the electrons are free to move over large distances, but their motion is damped so that energy is dissipated. The wave amplitude decays very quickly in the metal--usually within a small fraction of the wavelength. Associated with that decay is a loss of energy in the wave and some heating of the metal. Most of the incident optical power, however, is reflected at the air/metal interface. In other words, the power is transferred to another wave with a different propagation direction (opposite to the original direction for normal incidence on the surface).


The what I've learned while getting my undergraduate physics degree so far supports the above. Applying conservation of momentum to the particle nature of light will show that the angle of reflection must be the same as the incident angle (on the other side of the normal, of course). The wave nature of light covers most other aspects.

Re: 0811: "Starlight"

Posted: Wed Oct 27, 2010 5:56 am UTC
by fow
Don't worry! From the light's point of view, home and your eye are in the same place, and the journey takes no time at all! Relativity saves the day again.


Actually, from the light's point of view, everything is in one of two places. Everything behind it in one, everything in front in the other.

So your eye and home are as far apart as they can be. So, so sad.