posted
From what I understand, the light we see from other planets is very old, since while it is travelling at the speed of light the distance between galaxies is very great. I was wondering, does light travel from one end of the universe of the other, or does it fade after a certain distance? If the light wave was not deflected, would it travel endlessly?
posted
There are no "ends" of the universe, or at least, that's the current thinking.
Light doesn't "fade", but because the very fabric of the universe is expanding (that is, space and time themselves), a given quanta of light will be redshifted, or "stretched", as it travels.
And yes, light would travel endlessly if undeflected.
posted
But doesn't light have properties of waves and particles? Wouldn't the particular(errmm.. whats the adjective form of particle) properties of light cause it to slow down by...I don't know, friction or something?
Posts: 2756 | Registered: Jul 2002
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posted
You know, I get annoyed at the whole "light is a wave and a particle". Yes, it has aspects of both, but in reality, it isn't either. It's a whole separate type of entity. It isn't sometimes one thing, some times another; it always is what it is.
/rant
BTW, physics, in high school and 1st 2 semesters of college don't really teach you anything about light, except basic refractive/reflective type stuff.
-Bok
EDIT: kai, I'm not annoyed at you, by the way, I was taught the same thing. But reality is a bit more complex, and I wish it were taught differently.
quote: BTW, physics, in high school and 1st 2 semesters of college don't really teach you anything about light, except basic refractive/reflective type stuff.
I havne't taken ANY physics course. And I'm glad youre not annoyed
Posts: 2756 | Registered: Jul 2002
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quote: BTW, physics, in high school and 1st 2 semesters of college don't really teach you anything about light, except basic refractive/reflective type stuff.
Not true in many courses -- including the one I've taught, where we do discuss the nature of light, and I always emphasize this:
quote: Yes, it has aspects of both, but in reality, it isn't either. It's a whole separate type of entity. It isn't sometimes one thing, some times another; it always is what it is.
OTOH, it is a difficult concept, and many students refuse to accept it. Some are convinced -- regardless of how I attempt to convince them otherwise -- that it must take turns being one, then the other.
Don't assume the fault lies with the teacher -- or the text -- please.
Posts: 32919 | Registered: Mar 2003
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posted
In my case, it was the teachers (notice the plural). If the student doesn't accept it, that's fine (if frustrating, no doubt). At lower levels I think simply saying "light is a unique substance, but in some circumstances it acts like this, which is similar to a wave, etc." My biggest issues with understanding light was trying to understand how light was both things, since that was what I was taught. I'm a bit dense that way
posted
Can't take all the credit -- we also use an excellent text.
But yeah, I get annoyed when I get students who were taught (as I was) in elementary school that electrons have planetary orbits. So I sympathize with your frustration.
Posts: 32919 | Registered: Mar 2003
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posted
I was just watching an aid at my middle school try to explain “electron orbits” to a student. He said electrons orbit protons/neutrons much like planets orbit the sun. The student looked confused and I kept thinking, "How does that explain that only 2 electrons fit into the first "orbit?"
Posts: 1034 | Registered: Mar 2004
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posted
My first highschool physics class had nothing on light at all. The second had a section of bouncing lasers through a series of mirrors and prisms, but that was an independent, self-structured course (read as: doesn't count).
Second semester physics was E&M where I was, with some light theory at the end. Third one was intro to quantum mechanics. Lots of light in that one.
Posts: 3956 | Registered: Jun 2001
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posted
My first year at Waterloo we had to take a course on Spectroscopy. For some reason the fact that we're the only school that forces that level of quantum mechanics on first years makes us "prestigious". We didn't even have the calculus required to do most of the calculations yet.
And lo, all my interest in quantum mechanics were quashed in one 4 month swoop.
Posts: 3243 | Registered: Apr 2002
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posted
Heh, the spectroscopy/microscopy side of my research is what really does it for me. The biology part is just a requirement. My email must have bored the hell out of you. Posts: 3956 | Registered: Jun 2001
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posted
I remember getting a lot of that wave/particle stuff in AP Chemistry in HS. Had to do with the behavior of electrons if I recall correctly. And I'm pretty sure we did only a little bit of quantum mechanics in Physics that same year. But I don't remember really getting into the nature of light back then.
Now, as to the original questions. My understanding (which may very well be far from correct): Yes, light will travel from one "end" of the universe to the other. One of the reasons it appears to fade is the number of photons ('packets' of light, for lack of a better word) actually reaching your eye diminishes with distance. It takes a lot of photons for the human eye to detect light, and the closer you are to the source, the more likely it is that a high enough number of photons will travel in a parallel enough path so that they reach your eye, and you can see them. The farther from the source you get, even slight variations in initial vector can cause photons to be very far apart when they get to you.
As for red shift, I thought the idea was that the farther away something is, the higher the difference in speed (talking vast differences here). So light of a certain wavelength that leaves an object traveling away from you will be stretched out from your frame of reference. On the flip side of that, if two objects are getting closer to on another at high velocities, the light leaving one will be 'scrunched up' from the frame of reference of the other and there would be a blue shift.
Confused? Yeah, me too. It's interesting stuff once you wrap your mind around it. I especially like the theory that the universe is a big, what was it, dodecahedron? Where if you traveled out one side, you'd reenter on the opposite face, so basically you could go in a straight line and return to the point you started from if you went far enough. Wonder if it's too big to ever be able to see our own galaxy from the outside.
Posts: 609 | Registered: Apr 2003
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1) Acutally, in my college Sense and Perception class (senior spring class, I wanted an easy semester ), it only takes a few photons (like 3-4) to trigger your light receptors in your eyes. It faintness is due to a few factors, including distance (??), interstellar gas clouds, the stretch of spacetime lengthening the wavelength of light beyond our eye's ability to perceive it, and probably a half dozen other things.
The red shift that you talk about exists, but so does mine As we speak, the very spacetime within which we exist is expanding at a certain rate.
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You see, I knew I didn't know that much about it. That photon thing is a bit of a surprise to me though. I was sure it took more. Learn something new every day. Posts: 609 | Registered: Apr 2003
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posted
Happy camper, so was I. It actually takes more than that, in practice, since the eyelid and spacing of your receptors means some light misses, but it only takes a couple hits on the receptors to trigger your brain.
posted
Its rather funny, the whole ideas of "particle" and "wave" are really just approximate descriptions of aggregate phenomena of the real things which are out there, which definitely aren't either, and we have to keep describing these things that are real in terms of things which aren't, at least in any exact sense.
Posts: 15770 | Registered: Dec 2001
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In it the Australian Einstein says, "Crikey, sometimes light acts like a wave. At other times it acts as a particle."
Hey, what teacher can compete with such witty comedy?
Of course, this is another entertainer that the country of Australia would like to see deported to New Zealand.
Posts: 11895 | Registered: Apr 2002
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posted
Skipping ahead from the talk of light being what it is and not changing, can you deny that it ACTS like one or the other sometimes?
It is my understanding that in the two slit experiment, a single photon acts like a wave or a partical depending on whether you observe which slit the photon went through or not.
That an interference style pattern emerges when you dont record what slit it went through (even though its a single photon) like a water or sound wave, and if you do observe which slit it went through, it doesn't make an interference pattern as if it was not a wave at all?
posted
It acts like one or the other only in a very narrow and vague sense. It just always acts like a "real thing", for which our particles and waves are just approximations to aggregate behaviors of.
Posts: 15770 | Registered: Dec 2001
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posted
For instance, if one does not observe which slit is taken, the distribution of electrons will be as if there was interference. But if you look at the sheet after each one has gone through, there will still have only been one additional "impact". So even when not observing which slit it "acts like both a particle and a wave".
When observing which slit the "wave-like" behavior is mostly absent, though, yes. Or rather, it just doesn't impact the results (at least, not in any obvious way). You have a point of observation (the slit), but between the origin and that point of observation, and that point of observation and the destination, the photon still behaves "like a wave".
Posts: 15770 | Registered: Dec 2001
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posted
Great book on this is Q.E.D. (stands for quantum electrodynamics) by Richard P. Feynman (one of my heroes).
What's interesting is that a photon which is never absorbed is also never emitted. The probability that it was emitted goes to zero if it's never absorbed, I think. Also every photon is the same exact particle as every other. It's not just that you can't tell them apart, there really is no difference. The universe is very weird and wonderful.
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posted
Which isn't, Russell? The emit / absorb thing or every photon being indistinguishable in the sense that they have no unique identities?
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Some are convinced -- regardless of how I attempt to convince them otherwise -- that it must take turns being one, then the other.![[Smile]](smile.gif)
![[Big Grin]](biggrin.gif)
Can't take all the credit -- we also use an excellent ![[Eek!]](eek.gif)
So I sympathize with your frustration.
