Dual nature of light, my mind is melting

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Dual nature of light, my mind is melting

Postby PhantomReality » Wed May 16, 2007 1:35 pm UTC

W00t my first post on xkcd forums, why didn't I think of this before.

So I've got this problem. I'm a highschool student (about to graduate in a few weeks, freakin finally) and I'm stuck in a regular physics class. I could have taken AP, and stimulated my mind...but dying from work is not something I really wanted to do. In any case my regular Physics teacher has a degree and whatnot but isn't very good at explaining some aspects of phsyics and only knows so much.

So here's my question:

We have talked a lot about the dual partical/wave nature of light and I drill this poor guy with questions. The double slit experiment irks me and he won't give me a straight answer about it. How can we say that it is creating an unexplainable phenomenon based on the assumption that we can release one photon at a time? How do we know that the vacuum inside the test area is perfect....argh. Basically does anybody out there know the specifics of how this experiment is conducted?

Sorry about the wall of text, but I'm sure the intelligent community of xkcd can handle it. ;)

Thanks all.
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Postby miles01110 » Wed May 16, 2007 1:55 pm UTC

You aim some [monochromatic] light at two thin slits. Since light behaves like a wave, you get constructive interference and destructive interference when the light diffracts after passing through the slits. That's why you see peaks on the piece of paper or whatever you put behind it.

Now reduce it down to the quantum level. A single photon "knows" to pass through one of the slits, and subsequently "knows" to go into one of the areas of constructive interference. It's really weird. I don't know why or how the photon knows or chooses, though.

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Postby PhantomReality » Wed May 16, 2007 2:08 pm UTC

I understand the explanation for the phenomenon, but my question is how can they justify their claims if they aren't really sure that they really are releasing one photon at a time. I mean how can you say "we release one photon through the slit" when you aren't really sure if light is even a particle. It just doesn't make sense to me.

An atom changing state releases one photon...is that how they do it? I don't know.
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Postby miles01110 » Wed May 16, 2007 2:13 pm UTC

Well, since light is a wave, it doesn't really make sense to release "one" :-)

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Postby NathanielK » Wed May 16, 2007 2:27 pm UTC

You know light is particles because, if you dim your light source enough (just slightly too dim to see), the detector starts giving occasional blips of a certain energy, instead of a constant reading of some fraction of that energy. You can dim the light source in the double slit experiment until your target is only seeing occasional blips. If you plot the blips, however, you see the interference pattern - unless you have some way of telling which slit the photon went through, in which case the interference disappears!
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Postby Hawknc » Wed May 16, 2007 2:42 pm UTC

miles01110 wrote:Well, since light is a wave, it doesn't really make sense to release "one" :-)

Well, a photon is a "packet" of waves in a generalised manner of speaking, so yes, you are only releasing one photon at a time if you can do it. You're releasing a discrete amount of energy. You have to release successive photons to see the wave pattern, but even if you only release one it still acts as a wave.

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Postby Mathmagic » Wed May 16, 2007 2:42 pm UTC

PhantomReality wrote:I understand the explanation for the phenomenon, but my question is how can they justify their claims if they aren't really sure that they really are releasing one photon at a time. I mean how can you say "we release one photon through the slit" when you aren't really sure if light is even a particle. It just doesn't make sense to me.


To answer your first question (How can they be sure they are releasing one photon at a time?):
Wikipedia wrote:If sunlight is replaced with a light source that is capable of producing just one photon at a time, and the screen is sensitive enough to detect a single photon, Young's experiment can, in theory, be performed one photon at a time—with identical results.

And to answer your second part (...when you aren't really sure if light is even a particle)...well, we technically "don't" *know* that light is a "particle" per se, rather it's more like a packet of energy called a photon (in other words, the photon acts as a package of energy. The photon has no mass, so it can't be your ordinary "particle", but it has particle-like behaviour).
Wikipedia wrote:If either slit is covered, the individual photons hitting the screen, over time, create a pattern with a single peak. But if both slits are left open, the pattern of photons hitting the screen, over time, again becomes a series of light and dark fringes. This result seems to both confirm and contradict the wave theory. On the one hand, the interference pattern confirms that light still behaves much like a wave, even though we send it one particle at a time. On the other hand, each time a photon with a certain energy is emitted, the screen detects a photon with the same energy. Under the Copenhagen Interpretation of quantum theory, an individual photon is seen as passing through both slits at once, and interfering with itself, producing the interference pattern.

As you can see, the screen does not "immediately" give an interference pattern like it would if a laser was directed at a diffraction grating. I believe (and I'm just "guessing" here, as I don't really know) that sunlight is able to release one photon at a time because it creates EMR by means of fission/fusion reactions. But that's just my best guess. :smile:
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Postby evilbeanfiend » Wed May 16, 2007 3:03 pm UTC

wikipedia has some nice stuff on this

http://en.wikipedia.org/wiki/Double-slit_experiment

the copenhagen interpretation is that the photon has a wave funtion that gives a probability of where the photon is, it is this that interferes with its self when passing through the slits. when you measure on the screen the wave function collapses and the photon is in one place.

a non perfect vacuum, non perfect emitter and non perfect detector will give you some experimental error (essentially the nodes which should be black will be dark grey instead) but the pattern should still be observable (unless the experimental error is ridiculously high compared to the number of particles you detect)
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Postby Hawknc » Wed May 16, 2007 3:06 pm UTC

Er...I don't think either of you actually managed to answer his question. :P The question was HOW the light source produced those single photons, I think.

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Postby Andrew » Wed May 16, 2007 3:07 pm UTC

PhantomReality wrote:I understand the explanation for the phenomenon, but my question is how can they justify their claims if they aren't really sure that they really are releasing one photon at a time. I mean how can you say "we release one photon through the slit" when you aren't really sure if light is even a particle. It just doesn't make sense to me.

We know light is a particle because of the photoelectric effect.
We know light is a wave because of interference experiments.

Quantum theory is the only model we have that predicts both of these things, and it also predicts the results of the double slit experiment. I think the experiment has been done, under really ludicrously controlled conditions, but I'm not certain it has. Many other experiments have seemed to confirm quantum theory, though, so even if the experiment's never been done with truly one-photon-at-a-time conditions, trust that it would work that way if it was.

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Postby Mathmagic » Wed May 16, 2007 3:18 pm UTC

Hawknc wrote:Er...I don't think either of you actually managed to answer his question. :razz: The question was HOW the light source produced those single photons, I think.


I tried...
mathmagic wrote:I believe (and I'm just "guessing" here, as I don't really know) that sunlight is able to release one photon at a time because it creates EMR by means of fission/fusion reactions. But that's just my best guess.
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Postby Hawknc » Wed May 16, 2007 3:28 pm UTC

Mathmagic: my apologies, I missed that last paragraph.

It's definitely possible to emit only one photon, generally by exciting an atom (although there are LEDs that can emit single photons, apparently). I wouldn't know the specific method used in Young's experiment, though.

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Postby Shadowfish » Wed May 16, 2007 4:28 pm UTC

I don't know how to make a single monochromatic photon, but I do know that
a photomultiplier is able to detect a single photon. I was really skeptical about the idea that a single photon was detectable until I heard about those things. If you've studied the photoelectric effect, it's easy to understand how they work. [/url]

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Postby NathanielK » Wed May 16, 2007 10:47 pm UTC

If you want a more technical explanation (but no equations) of the current theory, check out Richard Feynman's book QED, or watch the lectures it was based on at vega.org.uk. The video is of poor quality, and requires RealPlayer, but it's cool hearing him speak.
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Postby Gelsamel » Thu May 17, 2007 1:04 am UTC

You have to realize that wave/particle duality of light IS JUST A MODEL.

Light isn't actually a particle, and it also isn't exactly a wave. However when we examine how light behaves it seems to exhibit properties of BOTH, therefore we can imagine that it is both, imagining that it is both lets us do calculations which come out correct.

We do this a lot in physics.
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Postby UmbrageOfSnow » Thu May 17, 2007 6:31 am UTC

Gelsamel wrote:You have to realize that wave/particle duality of light IS JUST A MODEL.

We do this a lot in physics.


While you are correct, be careful about "just a model" and "just a theory" comments, it is pretty well established and the best model we have for explaining everything properly. If you can do better, propose your model.

Models and theories should not be knocked for being what they are, they are basically impossible to really prove, you can just support them better or discredit them.

As any creationist will point out, EVOLUTION IS JUST A THEORY. Then again, so is gravity.
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Postby Gelsamel » Thu May 17, 2007 8:08 am UTC

UmbrageOfSnow wrote:
Gelsamel wrote:You have to realize that wave/particle duality of light IS JUST A MODEL.

We do this a lot in physics.


While you are correct, be careful about "just a model" and "just a theory" comments, it is pretty well established and the best model we have for explaining everything properly. If you can do better, propose your model.

Models and theories should not be knocked for being what they are, they are basically impossible to really prove, you can just support them better or discredit them.

As any creationist will point out, EVOLUTION IS JUST A THEORY. Then again, so is gravity.


I didn't mean it that way.

I mean it is "just a model" in that scientists don't claim light -IS- a wave and a particle at the same time, just that it exhibits properties of both at the same time. And that if you think of it as both you can do the math.

I'm not knocking them, or casting doubt. Just reminding the OP that "Light is a particle and a wave at the same time" is an incorrect statement.

Edit: As an example. Liquid Crystals aren't Solid and Liquid at the same time, they're something totally different, but exhibit properties of Solids and Liquids.
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Postby evilbeanfiend » Thu May 17, 2007 8:37 am UTC

Gelsamel wrote:
I didn't mean it that way.

I mean it is "just a model" in that scientists don't claim light -IS- a wave and a particle at the same time, just that it exhibits properties of both at the same time. And that if you think of it as both you can do the math.

I'm not knocking them, or casting doubt. Just reminding the OP that "Light is a particle and a wave at the same time" is an incorrect statement.

Edit: As an example. Liquid Crystals aren't Solid and Liquid at the same time, they're something totally different, but exhibit properties of Solids and Liquids.


yes in fact it would be better to say that it is a quantised wave, and quantised waves behave a bit like waves and a bit like particles. second quantisation is the maths you need to show they are mathematically similar.

of course this is also 'justa model' at a philosophical level you also have postivism vs realism, is the model just a model and any underlying truth is meaningless or are we close enough for all practical purposes that we can state the model is the truth (even if we have to refine it later).
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Re: Dual nature of light, my mind is melting

Postby explorer » Sun Dec 14, 2008 11:15 pm UTC

In the webbin, I got an article titled, 'Everything we Knew about Light is Wrong'. The article discusses double-slit experiment. In fact, article suggests that if we extend the experiment by making a slit in the dark band and peep throught the slit then due to destructive experiment we shall not see anything when we peep through these slits then we find virtually no difference in the view from dark band and the light band.

Is author correct in interpreting that light does not have to reach us for us to see it. Author also correctly observes that human eye can detect a single photon without absorbing it but we cannot see the sun even when our eyes can absorb the photons emitted by the sun.

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Re: Dual nature of light, my mind is melting

Postby explorer » Sun Dec 14, 2008 11:17 pm UTC

The article also suggest that the light and darkness are relative phenomenon and author experimentally proves it. I have downloaded the complete article from http://www.norlabs.org and find that it is quite interesting.

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Re: Dual nature of light, my mind is melting

Postby GBog » Mon Dec 15, 2008 12:32 am UTC

Sunil Thakur (in article referenced above) wrote:When we peep through the holes one by one, we find virtually no difference
in the views from both the slits except that we cannot see light source from the slits in the
dark band
.
(Emphasis mine.)

Well... yeah. No light from the lightsource reaches the dark bands. Virtually no difference, except the essential one. I claim douchebaggery on the article.

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Re: Dual nature of light, my mind is melting

Postby ThinkerEmeritus » Mon Dec 15, 2008 4:43 pm UTC

Edit: Ignore this. I read the wrong blurb.
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Re: Dual nature of light, my mind is melting

Postby Certhas » Mon Dec 15, 2008 6:32 pm UTC

Another way to Gelsamels point: It isn't necessary that things are either wave or particle, the correct statement is "light is neither a wave nor a particle". What is it? To the best of our knowledge a linear state in a hilbertspace upon which operators that in certain regimes mirror particles and in other mirror waves act. Sorry if that is an overly technical answer, that is because QFT is an overly technical subject.
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Re: Dual nature of light, my mind is melting

Postby explorer » Mon Dec 15, 2008 7:38 pm UTC

I am sorry but I am not an expert and therefore I would like to ask a simple question that author raises, "how do we see a single photon" and "why we cannot see the sun even when our eyes can absorb the photons emitted by the sun?'

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Re: Dual nature of light, my mind is melting

Postby explorer » Mon Dec 15, 2008 7:41 pm UTC

and how is image formed in the mirror even when path of light is blocked? I conducted the experiment and found it to be correct.

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Re: Dual nature of light, my mind is melting

Postby GBog » Mon Dec 15, 2008 8:10 pm UTC

Certhas wrote:Another way to Gelsamels point: It isn't necessary that things are either wave or particle, the correct statement is "light is neither a wave nor a particle". What is it? To the best of our knowledge a linear state in a hilbertspace upon which operators that in certain regimes mirror particles and in other mirror waves act. Sorry if that is an overly technical answer, that is because PONIES! Ponies, ponies, ponies!1! I like Ponies! is an overly technical subject.


Ponies are indeed a highly technical subject. (To any mod who reads this: is it possible to deactivate this particular filter on the science forum?)
explorer wrote:I am sorry but I am not an expert and therefore I would like to ask a simple question that author raises, "how do we see a single photon" and "why we cannot see the sun even when our eyes can absorb the photons emitted by the sun?'


Well, the human eye (or more correctly the nerves connecting the eyes to the brain) is not sensitive enough for a human to directly see a single photon. But there exists sensors which can register single photons. I'm not really sure how exactly these work technically, I would assume the photoelectric effect plays a role. A search on google yields 400,000 results for 'single photon detector', but the descriptions on the first few pages seem highly technical. Knock yourself out. I don't understand your second question. When does this happen?

explorer wrote:and how is image formed in the mirror even when path of light is blocked? I conducted the experiment and found it to be correct.


What exactly do you mean? What image is formed in the mirror when the path of light is blocked? Remember that light is also to some degree reflected off most surfaces, so you can have light going around a corner by reflecting off opposing walls. When you conducted the experiment, did you have other light sources in the room? Where the paths from all light sources and all surfaces light might have reflected off, blocked?

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Re: Dual nature of light, my mind is melting

Postby Gammashield » Mon Dec 15, 2008 9:07 pm UTC

explorer wrote:I am sorry but I am not an expert and therefore I would like to ask a simple question that author raises, "how do we see a single photon" and "why we cannot see the sun even when our eyes can absorb the photons emitted by the sun?'

and how is image formed in the mirror even when path of light is blocked? I conducted the experiment and found it to be correct.


Well, this is getting kind of off-track (the dual nature of light deserves plenty more discussion, yet), but it's worth explaining why this guy's 'articles' (also known as: unprofessional, self-published rants) are crap. And, yes, they *are* crap, they're worse than even the usual crank-physics. Forget 'advanced physics', basic logic can disprove all of this, with the starting points of "light generally moves in straight lines unless it bounces off something" and "we 'see' things by having light hit the back of our eyeball." The man who wrote this *completely* misunderstands how our theories say light works, every 'impossible' effect he talks about is *easily* explained by conventional photon theory.

For the sake of not getting *too* off-track with paragraphs of explanations of line-of-sight, I've thrown most of my answer behind a spoiler-tag.

Spoiler:
But, to answer your questions specifically: first, we *don't* see single photons. Our eyes generally need about a hundred or so photons in order to cross a threshold to visibility for us, when we'll see an incredibly, *incredibly* faint dot. But that may be an irrelevant point, if this guy's getting at what I'm afraid he's getting at. We have such things as single photon detectors after all, and they work. I think he's trying to talk about seeing a photon 'on a flyby', or something. But, we *don't* observe photons even electronically unless they hit the detector. Let me phrase that a bit differently, just to make sure it gets across, because it's important. We only ever detect photons by absorbing them. If a photon hits and is absorbed, we detect it. If it doesn't... we don't. So, whatever this dude's trying to get at with this question, it's nonsensical.

As for 'why can't we see the sun even when our eyes absorb photons emitted by the sun'... again, this verges on nonsensical. Photons don't come with little labels saying "I'm from the sun!". Photons fly out of the sun, in straight lines, in tons of different colors (read: energies). When they hit the atmosphere, a few will start bouncing around. Some tiny portion of them ( blue ones) bounce off the air particularly often, and enter your eye. So your brain goes "Hey! Blue! Coming from that direction!". And so you see a blue sky, in that direction. Most photons *don't* end up bouncing, so if you turn your head towards the sun, suddenly that vast majority of sun-photons travelling in a straight line stop hitting the back of your head, and hit your eye instead. So your eye goes "Hey! Lots of photons! Coming from that direction!". And you go blind, because damn it, you shouldn't be staring at the sun. Of *course* photons that randomly bounced around a bit don't make the shape of the sun anymore. They randomly bounced around! So it all mushes out into a general haze (like, you know, the big blue sky) The ones that didn't bounce, *do* stay in an arrangement the same shape as the sun. And we can see them... by looking at the sun. But since they didn't bounce, they'll only enter your eye if your eye lines up with where the sun actually is.


The 'mirror' example is particularly offputting, because this *exact* kind of situation is the example often used in low-level physics courses to show how light and mirrors work in standard physics. There's nothing worth questioning there, if one has any understanding of how photons work with mirrors. It's very obvious the person who wrote this never understood how mirrors worked. But hey, it's an excuse to try to dispel some people's misconceptions about how light works.

To try to explain... first, let's think of a single bright dot, like a tiny LED (this whole analogy works perfectly for complicated shapes too, but let's keep the example as simple as humanly possible to show why 'blocked paths' don't stop mirrors) Think of this point sending off a growing 'sphere' of photons, little particles in every direction. Every point in the sphere, every photon emitted by the dot, moves in a straight line away from the center dot. Now, mentally, stick a person near this dot. Whenever one of those photons in the sphere hits his eye (which only a tiny, tiny portion of the total photons output will do, since most are going the wrong way), it activates a receptor in his eye, and the brain goes "Hey! photons! Coming from *that* direction!" And when the brain interprets this, it has the fellow see a bright point, in that direction. That's sight.

Now, let's set that dot next to a mirror. How a mirror works, is that whenever one of these photons in the giant growing sphere hit the mirror, they bounce off, at the same angle they hit the mirror. Now, mentally stick that person nearby again. Now, the dot itself still does the same thing, sending off light in every direction, so the person sees that single dot in the same way. However, now, since light's bouncing off a mirror, some tiny* portion of the light that hits the mirror will hit at the exact right spot so that when it bounces off, and enters the dude's eye. The eye goes "Hey! Photons! Coming from *this* direction!", and the person sees a second dot, at an 'impossible' place, behind the mirror (since the brain just extrapolates, assuming the photon traveled in a straight line the whole time, resulting in the 'phantom' dot in the mirror.) Realize, the point where it bounces off the mirror is often *much* higher than the level the dot was sitting at. In fact, to find the point where the light's bouncing off? With your finger, reach out to touch the dot behind the mirror. Where your finger hits the glass, and covers the dot? That's where the light's bouncing.

Now, do this experiment with the little wall set up. What light paths did you block? What light paths did you *not* block? I'll answer that now: You blocked the light paths where light went from the dot, hit the mirror, and bounced back towards the dot. But guess what? You weren't looking at those paths anyway, because before the mirror was there? They flew off away from us, and we never saw them. After the mirror was there? They bounced, flew past our feet, and we never saw them. After the wall was there? They flew, hit the wall, and got absorbed. And we never saw 'em. No great loss in any case; we never cared about those photons, no matter what we were doing with walls or mirrors. More importantly, what light-paths *didn't* you block? The one that went up-and-a-bit-towards-the-mirror in a straight line, hit the mirror high up, and then bounced toward your eye. And *that's* why you see the object in the mirror, even after you stuck that low wall there.

Realize, the mirror isn't some special-wall, that takes a picture from its perspective of what's going on, and then displays it. It just bounces light paths. And if it's bouncing light into your eye, it doesn't matter whether the low parts of the mirror get hit or not. It's only the high-parts of the mirror that matter anyway for the dot-image, when you're high up.

So, again, *every* effect he talks about, from solar eclipses, to mirrors, to slit experiments, to double-shadows, are *perfectly* explained by the current photon theory of light. Don't let him con you into thinking otherwise to sell his book.


And that's that.

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Re: Dual nature of light, my mind is melting

Postby explorer » Tue Dec 16, 2008 7:23 am UTC

Gammashield

I read both parts of your post with interest. Let us forget the article. I am curious and hope I can ask simple questions. Do u think light is information or does it carry information. You suggest,

Photons don't come with little labels saying "I'm from the sun!". ....Photons don't come with little labels saying "I'm from the sun!". Photons fly out of the sun, in straight lines, in tons of different colors (read: energies). When they hit the atmosphere, .................Of *course* photons that randomly bounced around a bit don't make the shape of the sun anymore.

Does it mean light is not carrier of information? Then, how is information communicated? Let us say sun exists at point 'x' and emits photons. As per u these photons only make us look in the direction and when we look in the direction then we 'see' the sun'.

second point...

I searched google and find quite a no. of sites that suggest that dark-adopted human eye can detect a single photon but it does not get registered in the brain.

Third Point,

I am still looking for the mechanism that allows image of the object to be formed in the mirror even when path of the light is blocked.

Fourth point,

I made a darawing of eclipses in the excel as per scale and found that earth must remain outside the umbra and then searched google and found that we believe that earth falls within the umbra?

Can u throw more light on this aspect.

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Re: Dual nature of light, my mind is melting

Postby Mr_Rose » Tue Dec 16, 2008 9:55 am UTC

explorer wrote:I searched google and find quite a no. of sites that suggest that dark-adopted human eye can detect a single photon but it does not get registered in the brain
Yes, so? A single pigment molecule in a single rod-cell can absorb a single photon and get excited enough to change shape and trigger the cascade that leads to a detection event in the brain. Except that without the reinforcement of a few tens of other photons the cascade is suppressed by a feedback mechanism built to prevent us going haywire due to too much information; imagine if you could see every cosmic ray or spare neutrino that just happened to interact with your eye - you'd go bonkers trying to sort it all out into a meaningful picture, for values of meaningful that improve your ability to stalk and kill buffalo.
Microevolution is a term — when used by creationists — that is the evolutionary equivalent of the belief that the mechanism you use to walk from your bedroom to the kitchen is insufficient to get you from New York to Los Angeles.

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Re: Dual nature of light, my mind is melting

Postby You, sir, name? » Tue Dec 16, 2008 11:40 am UTC

It is probably easier to understand wave-particle duality from a matter standpoint. If you shoot classical particles (neutrons or whatnot) through a double-slit at some film that registers them, you will get the same sort of interference pattern as you would with light. Even if you shoot them at a rate where you can see the individual dots appearing on the screen. This means that the particles interfere with themselves as they pass through the slit, thus illustrating wave-particle duality in matter (it exhibits wave-like behavior in the slit, then it exhibits particle-like behavior when it hits the screen).
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Re: Dual nature of light, my mind is melting

Postby GBog » Tue Dec 16, 2008 1:32 pm UTC

The questions here were directed at Gammashield, but it seems to me that me and gamma are on the same side here, so:

explorer wrote:Photons don't come with little labels saying "I'm from the sun!". ....Photons don't come with little labels saying "I'm from the sun!". Photons fly out of the sun, in straight lines, in tons of different colors (read: energies). When they hit the atmosphere, .................Of *course* photons that randomly bounced around a bit don't make the shape of the sun anymore.

Does it mean light is not carrier of information? Then, how is information communicated? Let us say sun exists at point 'x' and emits photons. As per u these photons only make us look in the direction and when we look in the direction then we 'see' the sun'.

Individual photons carry information. However a single photon does not carry the information "This photon is from the sun" or anything like that. The only information a single photon can carry is basically its frequency. e.g. "This photon is blue." (EDIT: plus its polarization and phase.)What you see as the sun is only a large collection of photons, in a wide spectrum of frequencies coming from a quite narrow part of your visual field. Your brain identifies this as the sun. The photons carry information, but for you to make anything meaningful from that information, you also need metainformation supplied by your brain. If you had lived underground all your life, and never heard of this thing called 'the sun' that collection of photons coming from a small area in the sky wouldn't mean anything to you.

Third Point,

I am still looking for the mechanism that allows image of the object to be formed in the mirror even when path of the light is blocked.


You still haven't explained exactly what you mean by this. Could you perhaps make a drawing to make it clear what path of light is blocked and what you see in the mirror?

Fourth point,

I made a darawing of eclipses in the excel as per scale and found that earth must remain outside the umbra and then searched google and found that we believe that earth falls within the umbra?

Can u throw more light on this aspect.


Actually, this was kind of interesting, I did some calculations, and found that if the sun and the moon are at their average distances from the earth when a solar eclipse occurs, the umbra does not reach earth, and you have an annular eclipse. If, however, the moon is at its closest point to earth (perigee), the umbra reaches the earth just fine. Try doing your drawings again, but with the moon at perigee, or close to it.

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Re: Dual nature of light, my mind is melting

Postby explorer » Tue Dec 16, 2008 3:29 pm UTC

The moon at perigee is 356000 km away from earth and this is the distance I had taken. It is easy to make a drawing on excel. I hope all of u can throw light on this aspect.


I do not know how to make a drawing hree but it is simple - an object is placed in front of the mirror and u place a screen (I used a newspaper) that is higher than the object and extends beyond object even sideways (as suggested in the experiment) and u view the object from the top of the mirror and find that there is no change even though there is no way light can reach from object to mirror.

Coming back to your reply on the non-visibility of sun. Both of u suggest that light does not carry information but what I fail to comprehend is the fact that if we are in space and look in any direction then all the photons are supposed to be coming from sun directly and therefore we shall not be required to look at the sun directly. Moreover, a person who had no seen the sun before will still see the sun as an object.

After reading reply of gammashield, I went to a site that replies to queries on physics and the repy I received was that these phenomenon are difficult to explain with current theories and he asked more time to think about the observations in the article.

I am not getting a 100% convincing technical analysis like the one presented in the article.

If we can detect a single photon wherever it is then we sure can detect 1000 or a million. I mean more questions come to my mind than raised in the article.

There is somehting about the article that appeals to simple logic but I guess it is too simple.

I do not know. Hope I will get a technical analysis.

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Re: Dual nature of light, my mind is melting

Postby SU3SU2U1 » Tue Dec 16, 2008 4:04 pm UTC

To answer the original question- you can do the experiment with electrons instead of photons. Getting single electrons is easier.

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Re: Dual nature of light, my mind is melting

Postby Minerva » Tue Dec 16, 2008 4:07 pm UTC

As Feynman said, light doesn't behave like a wave on Tuesdays and like a particle on Thursdays. It is not "both a particle and a wave"; it is something else altogether.

It's just like quantum mechanical "spin". The particle doesn't actually spin, it's just a nice analogy with certain consistencies in its properties.

"It might be better to think of it as something else, neither a wave nor a particle, something with no ready counterpart in the everyday world of the palpable, that under some circumstances partakes of the properties of a wave, and under others, of a particle." -- Carl Sagan, Billions and Billions


As for single photons, it is possible to create single-photon sources. Incidentally, some folks at my university are responsible for developing what is perhaps the best single-photon source in the world.

Single photons can be detected with photomultiplier tubes.
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Re: Dual nature of light, my mind is melting

Postby Xanthir » Tue Dec 16, 2008 5:09 pm UTC

explorer wrote:I do not know how to make a drawing hree but it is simple - an object is placed in front of the mirror and u place a screen (I used a newspaper) that is higher than the object and extends beyond object even sideways (as suggested in the experiment) and u view the object from the top of the mirror and find that there is no change even though there is no way light can reach from object to mirror.

Well, usually we just use MS Paint and save it as a jpeg, then attach it. In your case, though, it would probably be more illuminating (no pun intended) to take a few pictures with a digital camera, from the side and from your 'impossible' view, so we can see exactly what you're talking about.

I assure you that basic optics is an absolutely solved problem. There is no simple example that you can set up with a newspaper and a mirror which will disprove it. You're just making a simple mistake, and we can help you find where that is.

Coming back to your reply on the non-visibility of sun. Both of u suggest that light does not carry information but what I fail to comprehend is the fact that if we are in space and look in any direction then all the photons are supposed to be coming from sun directly and therefore we shall not be required to look at the sun directly. Moreover, a person who had no seen the sun before will still see the sun as an object.

Honestly, even after reading both your original posts and other's replies, I'm confused as to what you are talking about. Are you trying to assert that just because we see photons from the sun, we should see 'the sun'? Like if I look at the full moon at night, the photons should form an image of the sun because they come from the sun originally?

This is completely incorrect. As was stated before, photons carry only very limited amounts of information. They do *not* carry information about where they came from - a photon from the sun doesn't "know" that it came from the sun. In order to see 'the sun', the photons we receive have to be relatively undisturbed. If they get scattered around, they won't form an image anymore.

Look into any corner store, and you'll likely find a convex mirror mounted near the ceiling that gives the store employees a wide view of the store. This sort of mirror purposely distorts the light that hits it, but only a little bit. When the reflected light hits our eyes, it's still approximately in the same pattern as when it was originally emitted/reflected from the patrons and shelves and such. If it hits a normal object, though, or just scatters in the air, it reflects in all directions, and loses the pattern it was originally emitted in.

After reading reply of gammashield, I went to a site that replies to queries on physics and the repy I received was that these phenomenon are difficult to explain with current theories and he asked more time to think about the observations in the article.

The person who responded to you was either an idiot, or misunderstood what you were asking (the latter is fairly likely, since you're asking things informed by an article that is so utterly wrong that it's difficult to even grasp what you are trying to get at). Everything you are asking about is well-understood, and in fact most was completely explored by Newton.
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Re: Dual nature of light, my mind is melting

Postby GBog » Tue Dec 16, 2008 5:18 pm UTC

explorer wrote:I do not know how to make a drawing hree but it is simple - an object is placed in front of the mirror and u place a screen (I used a newspaper) that is higher than the object and extends beyond object even sideways (as suggested in the experiment) and u view the object from the top of the mirror and find that there is no change even though there is no way light can reach from object to mirror.

So... like this approximately?
mirror2.JPG
mirror2.JPG (6.78 KiB) Viewed 3021 times

Light can reach from object to mirror.

Coming back to your reply on the non-visibility of sun. Both of u suggest that light does not carry information but what I fail to comprehend is the fact that if we are in space and look in any direction then all the photons are supposed to be coming from sun directly and therefore we shall not be required to look at the sun directly. Moreover, a person who had no seen the sun before will still see the sun as an object.

I see your misunderstanding here. The light is typically reflected of other surfaces or scattered in the atmosphere before it reaches your eye. It doesn't come directly from the sun. The light which comes directly, unreflected, unscattered, from the sun, you see as that bright yellowish sphere in the sky.

Now, back to your concerns about the solar eclipse, I believe you must be getting something wrong.

Distance between earth and sun: [imath]s_{sun} = 1.496 \cdot 10^{11} \rm{m}[/imath]
Mean solar diameter: [imath]d_{sun} = 1.392 \cdot 10^9 \rm{m}[/imath]
Angular size of sun: [imath]\theta_{sun} \approx \arctan \frac{d_{sun}}{s_{sun}} \approx 0.533^{\rm o}[/imath]

Distance between earth and moon: [imath]s_{moon} = 3.56 \cdot 10^8 \rm{m}[/imath]
Mean lunar diameter: [imath]d_{moon} = 3.47 \cdot 10^6 \rm{m}[/imath]
Angular size of moon: [imath]\theta_{moon} \approx \arctan \frac{d_{moon}}{s_{moon}} \approx 0.559^{\rm o}[/imath]

The moon has, at perigee, a larger angular size than the sun. It can cover the sun completely.

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Re: Dual nature of light, my mind is melting

Postby explorer » Tue Dec 16, 2008 5:50 pm UTC

Let me clarify that I am not asserting anything. I am just trying to understand may be the basics once again because I find some logic in someone else's arguments but I am not sure whether it is entirely correct.

I look at the problem of the non-visibility of the sun as follows,

There is an object and there is an observer. How is the information about the object being communicated to the observer?

If eyes or any equipment can detect a single photon then it must detect several photons wherever they are without absorbing these photons? Is this observation incorrect? If it is then why?

Third issue, moon may block the view of the sun as our finger can block the view of an electric bulb without blocking the light.

Fourth issue, the image created is perfectly fine except that in the mirror the image is behind the screen and not over the screen. You may see it at home. I have checked the image in the article and it is right behind the screen and not over it.

I may be wrong in communicating the issues and the technicalities presented in the article and therefore it will be better if you were to read the article completely and then kindly explain to me why the observations are wrong. I have downloaded the article from the SCRIBD. The link is,

http://www.scribd.com/doc/8537121/Every ... t-is-Wrong

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Re: Dual nature of light, my mind is melting

Postby BlackSails » Tue Dec 16, 2008 6:54 pm UTC

Just about everything in that article is wrong. If you want to know why, take an introductory physics course that goes into optics.




Anyway, since we are on the subject, take a look at this: http://cobweb.ecn.purdue.edu/~photspec/ ... nhardt.pdf

The image on the right is when you put a regular white light inside the cup.

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Re: Dual nature of light, my mind is melting

Postby GBog » Tue Dec 16, 2008 7:18 pm UTC

explorer wrote:There is an object and there is an observer. How is the information about the object being communicated to the observer?

Through light coming from a lightsource reflecting of the object (or generated by the object itself) and then reaching the observer. Note however, that the light being reflected of an object gives you virtually no information by itself. It is the light coming from an object, together with light coming from surrounding objects (or lack of it) that makes you able to make out, say, its shape. When your eyes are open, they are constantly being bombarded with a huge number of photons. I mean literally trillions each second. Every single of these gives you virtually no information, but all of these combined, specifically how they vary in intensity and frequency across your field of vision, gives your brain the information that it processes to make the image that you see.

If eyes or any equipment can detect a single photon then it must detect several photons wherever they are without absorbing these photons? Is this observation incorrect? If it is then why?
Your observation is incorrect. As to why... Why should it be correct? How could you possible be able to detect a photons travelling through, say, the Andromeda galaxy? Detecting single photons is a matter of making sensitive instruments. Detecting photons that doesn't hit you is another thing altogether. And to detect a photon, you have to affect it in some way (since it has to affect you, or the equipment, and to any action there is an equal and opposite reaction). Either absorbing it altogether, or in some other way (for instance reflecting it). If you don't affect it, it doesn't affect you, and you can't detect it.

Third issue, moon may block the view of the sun as our finger can block the view of an electric bulb without blocking the light.

Ah, but you see, in space there are no walls. So while light from an electric bulb may be reflected off the walls before reaching you even if your finger is in the way, light from the sun doesn't have anything large to reflect off before reaching earth.

Fourth issue, the image created is perfectly fine except that in the mirror the image is behind the screen and not over the screen. You may see it at home. I have checked the image in the article and it is right behind the screen and not over it.

Look again. Look at the object, then lower your eyes until you see the top of the screen. You see the object behind the screen as if you are looking over the top of the screen, and indeed, you are.

Another experiment: Replicate the setup from previously and remove the mirror and place yourself where the mirror was. Move your head forward until you can see the object over the top of the screen. You can do this before you pass over the screen. (If not, go back, raise your head a little and start again) You are now looking at an object behind a screen without a mirror. Your view now should be very similar to the one you had in the mirror (except that was mirrored, obviously). There is something between you and the object. Is the path of the light blocked in this setup?

I'm sorry, but there appears that you have a major misconception about how mirrors work and/or about simple geometry. Either that or you're a troll. Anyway, I don't think I'll respond any more to this thread. Suffice to say pretty much everything in that article is wrong.

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Re: Dual nature of light, my mind is melting

Postby Gammashield » Tue Dec 16, 2008 7:32 pm UTC

Well, alright, let's try this again. Your point one and point two tie into eachother very heavily, so I'll try to answer them together. This may all need a topic split, since I'm starting to feel a bit guilty about throwing paragraphs at this that aren't actually tied into the dual nature of light. And yes, I've read the article through. It's *also* wrong on all these points.

explorer wrote:There is an object and there is an observer. How is the information about the object being communicated to the observer?

If eyes or any equipment can detect a single photon then it must detect several photons wherever they are without absorbing these photons? Is this observation incorrect? If it is then why?


As for "If our eyes or any equipment can detect a single photon then it must detect several photons wherever they are without absorbing these photons?" Er... that observation is incorrect, in several different ways. And the way you phrase it makes me think you're misunderstanding a fundamental point of light. Very tellingly, you say "detect photons wherever they are." Listen carefully to this next point, I think it's the spot that's confusing you and making this guy's papers sound probable.

It's *impossible*, absolutely, completely, 'don't even think about it' impossible, to detect photons at a distance. The only photons we can ever detect are ones that enter our eye (or, equivalently, enter our photon detector lens). The only photons we can detect in any way are ones that fly through the air, *hit us* in the face, go through the eyeball, and get absorbed.

If you see something, *anything*, across a room? It spat out trillions of photons in every direction, millons of them happened to be going the right way to fly straight into your eye, and all of those millions got absorbed. That's sight. We never 'detect photons over there'. We only ever detect photons that hit our eyes. Yes, we can detect a single photon, by absorbing it in a detector (like, say, the eye). Yes, we can detect two photons. By absorbing both of them at the same time in a detector. Yes, we can detect millions of photons. By absorbing all of them at the same time in a detector. We can *never*, ever, detect a photon without it hitting us and getting observed. Photons that don't get absorbed have absolutely nothing to do with sight."

As for your first point, 'how does information get communicated... a lot of that is answered above, if you think about it. Here's how it works. The object tosses off millions of photons, everywhere. Every single point of the object, every spot in space where it is, every atom, tosses off piles and piles of photons in every direction. A nice big pile of photons from each point hits your eye, getting absorbed. The *only* information each photon has is *color*, and *direction* ( Aka: what direction was it flying, and what color it was.) So your brain gets a mess of info going "Blue, from a quarter of a degree above the middle of our vision! Slight darker blue, from an eighth of a degree above the middle of our vision!, Bluish-green, from an eighth of a degree above our center-of-vision and a little to the left!".

That's all the information we get. That's all the information it's *possible* to get. Our brain takes that set-of-colors-and-directions-the-colors-came-from, does an *insane* amount of processing based on paralax between the eyes, current muscle contractions around the eye, comparision of color-splotches to previously-seen-splotches, color gradations, object recognition procedures, etc, etc, etc, etc, to go "Oh! That's a blue box, five meters away, slightly tilted towards us, with a yellowish light source off to its left shining on it!" That's how 'the info is communicated', as you say. Millions upon millions of photons, each traveling in a straight line unless it bounces off of something or is absorbed by that something, and we put all that together into a picture of the world.

Picture it as every visible object in the universe hurling billions of tiny colored baseballs every which-way. Sometime catching eachother's balls based on the color. Sometimes letting them bounce off. And based on which baseballs hit us in the eyes from which directions, our brain figures out how the pitchers must be grouped together, how far away they must be, and how the pattern hitting us will change if we move, and tells us all the information of how the world is laid out.

It's a hard problem. But our brain's pretty awesome like that.


Third issue, moon may block the view of the sun as our finger can block the view of an electric bulb without blocking the light.



So, onto your third issue, the eclipse. I'm... not sure what to tell you here. I've done the math. Others have done the math. When the moon is as the perigee, parts of the earth are inside the umbra. You're doing the math wrong, and/or drawing the picture wrong. Without seeing your work, it's pretty much impossible to know *how* you're doing it wrong. Maybe you forgot to subtract the distance between the earth and the moon from the distance between the earth and the sun, to find out the distance between the sun and the moon. Maybe you forgot to take into account the radius of the earth. Maybe you don't realize the *whole* earth doesn't have to be inside the umbra, only a few square miles (after all, eclipses only occur over a tiny area at a time, it's why during an eclipse, you have to live in the right place to see it). But, yes, a few square miles of the earth *are* inside the umbra at perigee.

As for the thumb/lightbulb example... gah, I almost dont' want to go into it, as it may get you off on the wrong track here. You *do* block the light when you put your thumb in the way. That's why your eyes stop hurting; the light from the lightbulb stops getting in your eyes, so you don't see the light. The *exact* same thing happens with an eclipse. Except since the moon is so much bigger, instead of the shadow being about the size of your eye, it's several miles across, and so there's no stray 'bouncing' light coming from all the other objects around you, since they're in the shadow too. If you don't understand what I mean by this, then go back to that first discussion about how light works, and think about it some more.


Fourth issue, the image created is perfectly fine except that in the mirror the image is behind the screen and not over the screen. You may see it at home. I have checked the image in the article and it is right behind the screen and not over it.



Fourth issue... the mirror. Here? No. You're wrong. The image is *not* behind the screen. Remember, there's *nothing* behind the mirror, just wall. The only place the light is coming from is the mirror itself (though, remember, the photons aren't *on* the mirror. They're in your eye. They just happened to come from the mirror's direction, and your brain is back-calculating to figure out 'oh, they must be from over there). Like I said before, do your experiment, and then without moving your head, reach forward and *touch* the mirror where the image is, so that your hand is covering it. *That* is where the photons are bouncing. And I promise, *promise* you, that's at a point taller than the newspaper. It has to be. The image looks like it's behind the mirror, and low to the ground. That's an illusion, because our brain doesn't know how to find the depth of a mirror-image. The *actual* projection point is much closer and higher up, that spot you're touching that covers the imagine. And there *is* a line-of-sight between the spot you touch, and the object itself.

The light doesn't 'stay behind the mirror' or 'stays across from the object along the mirror, or anything silly like that. It just bounces. And it bounces high up. The fact that the image looks like it's hanging out low is an optical illusion, because of the way mirrors bounce light. And you can see that by figuring out where on the mirror the image must be coming from (by touching the point where the image is without moving your head, like I said)


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