Atoms

Hi sidelined, Ok, cool. It appears, then, that I was on the right track, although I didn't know the specific figures.Also, thanks for the link. A couple of the terms went over my head but it was interesting, none the less.I enjoyed the application of "dimensions" in the explanation, even if I didn't entirely understand it. In fact, I think I've read something similar to this before; an article discussing the "dimensionality of colour," or something to that effect. I wish I could remember where. Ok, if I'm reading you correctly, this is what I was thinking.Something I'm not sure I understand, though, is the difference between visible light with a wavelength of a certain colour and visible light with a wavelength stretched or compressed (that is, red-shifted or blue-shifted) to a certain colour. For example, what is the difference, physically, between light-waves that we see as "red" and light-waves that are Doppler-shifted into the red?Am I correct in thinking that "red-shifted" does not actually mean "appears red in colour"? That is, the colour displayed by a given light (at least, as far as our eyes are concerned) is not actually related to its red or blue shift, correct?As I understand it, so called "red-shift" and "blue-shift" are opposite extremes of the Doppler Effect on light. However, these shifts in "colour" don't actually manifest as visible red/blue light; they are determined by separating a light's component colours by diffracting it through a prism, and analysing its spectral lines. Is this correct?If so, then what is it that's different about the wavelengths (or frequencies...sorry, I'm still not sure which one applies) that we see as visible colours? For example, what is the difference between light-waves with a wavelength of 700 nanometres emitted from a stationary source, and light-waves stretched to 700 nanometres emitted from a receding source?Presumably, the former would actually appear red to our eyes, while the latter would not (at least, not necessarily). But why? Physically, what is the difference between the two? So do nuclei actually contribute any of the properties that electrons emit as visible colour, or are said properties a sole product of the electrons?Actually, I think I'm missing something here. I'm pretty sure that the nucleus does indeed have an effect on the overall atom's colour. I think I've applied your statement the wrong way. I realize that you weren't saying the nucleus has nothing to do with an atom's colour (just that it isn't responsible for the actual emission of light). However, if the nucleus doesn't interact with the light being received/released, just how does it contribute to the properties of visible colour in an atom? Indeed. And this is something I've always struggled with. Lam asked me, once, if I understand wave/particle duality, and the short answer is; I understand the principle that it refers to, but I most certainly do not "understand it." Oh, I know. I'm familiar with quite a number of ideas that stretch my mind to its limits (and beyond ). One of my most beloved pastimes is researching concepts that I have little to no chance of ever understanding...*cough*...well, it's worth a try. Yes, I am aware of the degree of empty space within atoms. This is something else that has always interested me; the notion that apparently "solid" matter is nothing of the sort.As I understand it, neutronic matter is the densest substance known. In fact, I've read that it actually contains no empty space. But is this even possible, given that neutrons themselves are composed of quarks? I can understand there being little empty space, but none at all? Or perhaps it simply means that there is no distance between the neutrons; that they are actually pressed together (that is, in physical contact, literally touching each other)?Actually, I recently read of a (possibly) new type of star, composed entirely of quarks. I'm not sure how new (or old) this information is, but if it's true, would it constitute a substance even denser than that of a neutron star? That's cool. Perhaps Melchior will know. Believe me, I know how it feels to stray from your area of expertise. Actually, as I don't have an area of expertise, I guess I don't really know how it feels.Being little more than a curious scientific layman, I come across many things that I don't entirely understand. But I think I manage to get my head around most concepts to at least some degree, providing they don't require any deep familiarity with the mathematical theory behind them. Well, I think you're doing a good job already. Thanks for your help, too. It is much appreciated.

No, red-shifted light would actually appear redder than unshifted, as long as you weren't shifting ultraviolet light up into the visible .... There aren't any deep-sky objects that are redshifted significanly that are bright enough to see any color in at all with the naked eye, and very possibly not even with a BIG telescope in front of your eye. If you had a sodium-vapor streetlight on a fast rocket (sodium-vapor because it emits almost all its light at one wavelength) it would appear red instead of yellow if it was moving away from you at, say, 20% of the speed of light, and pure blue if it was moving toward you at some similar speed. Only in that the nucleus determines, through its total charge, how many electrons you have and the energy levels they can occupy. Electrons hopping between levels do all the "doing" that has to do with light in the energy range that we can see. Nuclei give of x-rays when they swap energy levels. There's no difference between them - light has no memory.

I've read about quark stars too - apparently it's still a hypothetical. I don't know just how you could distinguish a quark star from a neutron star anyway. I'm not going to volunteer to go scoop up a sample.

When it comes to the size of light, there isn't really a clear answer, because;A photon of light (particle) is modeled to be a point that has no volume, but travels at the speed of light. A source of light sends out a specific limited amount of these, but they would in total have a volume of zero.A wave of light (wave) is modeled as a sphere that is filled up with a changing electro-magnetical field. It has a volume that depends on how long time it was since the source was 'turned on'.You're going to spend a lot of time getting confused over this; I still am even if I know how the models work. I can't tell you a way to turn this into something that makes sense, because they don't make sense to me. I know they work because I know how to apply them to experiments with electronics, but that's about it...Wavelenght does not have anything to do with the physical size of the wave. The diagrams where you see a sinus-wave that represents a ray of light is missleading because it doesn't show location or displacement, but, and this is important, the strenght of the electrical field. There should be another sinus-wave that goes in and out of the page that shows the strenght of the magnetical field.This message has been edited by Melchior, 12-03-2004 02:46 PM

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Something I'm not sure I understand, though, is the difference between visible light with a wavelength of a certain colour and visible light with a wavelength stretched or compressed (that is, red-shifted or blue-shifted) to a certain colour. For example, what is the difference, physically, between light-waves that we see as "red" and light-waves that are Doppler-shifted into the red?

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I think the first thing you need to realize is that visible light is no different than radio waves (except for the amount of energy in each wave). There is nothing "special" about the wavelengths that the human eye is able to detect. Non-visible light, such as infra-red or ultraviolet, can become visible light through the Doppler effect by changing the wavelength. Also, some organisms are able to detect infra-red wavelengths (eg rattlesnakes) and (IIRC) some are able to detect ultraviolet wavelengths (eg bees). Through instrumentation, humans are able to detect all of the EM wavelengths, from radio waves on up to x rays.Another interesting problem caused by the Doppler effect concerns spaceflight. If we humans are ever able to build a craft that travels at high speeds, say 0.6 c, normal light will actually shift into the range of cosmic radiation (eg gamma rays, I think). In other words, humans would actually have to build shielding to protect themselves from normal visible light.

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However, if the nucleus doesn't interact with the light being received/released, just how does it contribute to the properties of visible colour in an atom?

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The nucleus determines the ground and excited states that electrons are allowed to move between. That is, the nucleus determines the position of the electrons, and light is produced when electrons move between these positions. The nucleus "sets the table" as it were.As to being able to "see" a single atom, this is a problem unto it's own. For a human to see something it has to be emmitting light. Therefore, without amplification or instrumentation, all we really see is the light produced by the atom, not the atom itself. Of course, this could be said of any object of any size so I don't know if this explanation helps or hurts. This is the same problem that Maxwell ran into with his Demon. Maxwell came up with the idea that he could create "free" energy by separating low energy and high energy particles into separate containers. However, the problem he ran into was that it takes energy to measure the energy content of a particle. This is the same type of problem that we run into when trying to "look" at atoms.

Tony650 The red shift for the spectra of atoms refers to the doppler shift that occurs due to the motion of stars relative to us.I picked this up from the hyperphysics website at http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html

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"Calling Atheism a religion is like calling bald a hair color."
--Don Hirschberg

Hi Coragyps, Really? Well, there you go. Another of my misunderstandings corrected.So the reason that distant galaxies don't actually appear red in telescopes and photographs is simply because their red-shift isn't significantly large enough to show up to a degree that we can identify as visibly "red"? And here I was thinking there is something fundamentally different about visible red and red-shift. Thanks for the correction. So the nucleus doesn't have any direct interaction, all it does is balance the atom's overall electrical charge? Is it true, then, to say that as far as electromagnetic interaction goes, there is no difference between an element and any of its isotopes?Something else which occurs to me is that if it is solely the electrons that interact with the light that we see, what are the visual properties of substances which are ionized and have their electrons stripped away?Or perhaps neutronic matter itself is a better example. If it contains only neutrons, does it directly interact with light? So nuclei do have some direct electromagnetic activity (despite x-rays being, of course, invisible to our eyes)? Ok, thanks for clearing that up. Well, I already knew that light has no memory; I meant thanks for the rest of it. Ah, ok. Thanks for the confirmation. Yes, that was one of my first thoughts. As neutrons themselves are comprised of quarks, it could conceivably be very difficult to tell quark and neutron stars apart. I'm sure they'll come up with something, but I can't imagine what. Perhaps we've already observed quark stars which we've unwittingly assumed were neutron stars. Who knows? I certainly don't. Heh, not unless you're the strongest person in the universe...or you just have a very, very small scoop. Oh, and also assuming, of course, that you don't mind having your body gravitationally crushed into a volume the size of a hydrogen atom.

This may very well be incorrect, but I think there is a very, very slight difference in the spectrum of the different isotopes. If it is there it is very small. We need a physicist to clear this up.

Hi Melchior, To be honest, I suspected this might be the case. From what I understand, things at this level tend to be kind of "fuzzy," and I had a feeling that the unique properties of light wouldn't make clear, precise definitions any easier. No size at all? I didn't know that. I knew that photons were massless (hence their ability to travel at the speed of light) but I had no idea they had no volume.This is a concept I have trouble getting my mind around. Not zero size, in and of itself, but how any extant body can be said to have zero size. Doesn't this, in some sense, negate its very existence? I've always wondered how point-like particles can truly be "point-like" (in that they have no volume, whatsoever) and still be said to "exist" in any meaningful way.When you say "zero volume," do you actually mean zero in the strict, mathematical sense, or are you talking about the universe's lower limits of quantized space? Perhaps you mean that a photon's size is on the level of the Planck scale? I'm not trying to put words in your mouth, by the way; I'm just trying to clarify your meaning. So is a light-wave not actually a "wave" in the classical sense (that is, in the way that sound, for instance, is a wave)? I've always wondered exactly how it is that waves of light can travel through empty space.For the most part, I just assumed that it's because light also has a particle-like nature that allows it to do this. Unfortunately, though, this never really made it any clearer to me, as I still can't see how the word "wave" can have any real meaning without a medium through which to propagate. That's ok. I'm learning plenty. I understand this better now than I did before, so I'm heading in the right direction. Thanks again for your help. Yes, which is a whole new problem altogether. Fields are another concept that I've never completely grasped. Oh I know what fields are, but I don't really know what they are...if you get my meaning.I'm not even sure if all "fields" are the same type of physical phenomenon. I understand a "gravitational field," for example, to be a curvature of space-time. But are all fields the same? Is a magnetic field, for instance, also a warpage of space-time geometry? I've only ever seen this explanation related to gravity. I've always likened the concept of a field to that of a "force," but that still doesn't explain what a field actually is, if that makes sense.

Hi Loudmouth, Yes, I understand this. But just to clarify (as I've already uncovered several misunderstandings that I had), am I right in thinking that, in reality, there is really only the electromagnetic spectrum, and what we call "light" is simply a thin strip of wavelengths somewhere near the centre, between both extremes? Am I also right in thinking that the entire spectrum shares the dual particle/wave nature of visible light? Now, there's an interesting thought; I'd never considered that. I assume you mean the visible light coming from outside the ship? That is, the light that is massively blue-shifted from all approaching sources? It determines the electrons' excited states as well? Ah, I knew there had to be more to it than determining the number of electrons. Thank you; I understand. Heh, it's ok. It basically just confirms what I already thought.I realize that, technically, we don't actually see anything we look at; only the light it emits/reflects. I was just wondering if it was possible (at least, in principle) to "see" an atom in the same sense that we "see" regular, macroscopic bodies.My thinking was that if we could somehow isolate a single atom (say, in a magnetic field, in a perfect vacuum), perhaps it would be possible to simply shine a light source on it as we do to "see" any regular object.Of course, I have my doubts as to whether or not it's quite that simple in the case of a single atom. For one thing, an atom is obviously far too small to see with the unaided eye, and as I said in another post, we would still require some means of "enlarging the image," which rather defeats the purpose of trying to see what the atom "actually looks like," doesn't it?

Hi sidelined, Yes, I understand that. What I was asking was, in a nutshell, is there a difference between red (visibly red) light and red-shifted light?For example, is there any difference between the light received from, say, a red giant sitting stationary, relative to the Earth, and a star receding from the Earth, and hence heavily red-shifted? If we assume that both are seen from Earth to have a similar wavelength (say, the red giant actually emits light at close to 700nm, while the receding star emits light of a shorter wavelength which is, however, measured as almost 700 nm on the Earth, due to its recession), is there any physical difference between them? Will they both "look red" from the Earth?Or to put it another way, would there be any way to determine which was which, or if there was indeed any difference at all, based purely of their wavelengths? Would there be any way to tell which was actually emitted at 700 nm and which was merely shifted to 700 nm?Thanks for the link, too. I've been there before but not since discussing this. I might try the "Light and Vision" section and see what I can find. Thanks again, sidelined.

Hi Ned, Perhaps so. I don't actually know; I'm just speculating based on what I've learned so far. Loudmouth said that the nucleus determines not only the number of electrons but their excited states, too. Perhaps the number of neutrons directly affects these states, in some way?

Yes, but not by looking at just one frequency. Different elements emit light in very specific patterns. That is what spectroscopy is all about. Have a look at this reference:http://hyperphysics.phy-astr.gsu.edu/.../quantum/sodium.htmlIf you see the same pattern but in the "wrong" place you can tell it has been shifted. (you could Google "hydrogen alpha line" too )IIRC, it was the pattern of lines from hydrogne (alpha and so on) that was first recognized as being in the "wrong" place.One frequency at a time I don't think one could tell.

Hi TonyThis is a very interesting thread. Here is my input from an analytical point of view. I have never really thought about this too hard until now but my conclusion is that the number of neutrons doesn't have a measurable (if any) effect on the wavelength of the photons emited by excited isotopes of the same element.In spectrometry, these wavelengths are used precisely to identify elements within a compound but they are unable to differentiate between different isotopes of any given element.It is possible that there may be an extremely small difference but it would most likely be lost in the noise of the system. The problem is that atoms in an excited state are far more energetic than those in an unexcited state. This means that the individual movement (brownian motion) of the atoms will always result in a slight spread in the emission spectra of each isotope (possibly due to red and blue shift doppler effects). The spread is extremely narrow but does make spectral lines a little fuzzy around the edges. In short there is no way to resolve the spectra tightly enough to distinguish any difference between isotopes of the same element.PY

There is a small, but measurable difference between the emission spectra of hydrogen (only a proton) and deuterium (a proton and a neutron), see http://hyperphysics.phy-astr.gsu.edu/hbase/hyde.html , bottom of the page.