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StevenO
2009-Mar-31, 12:14 AM
I have always wondered how the mechanism works that an electron, a particle that is now measured to be < 10-22 m emits a photon ( size unknown) for instance in a nearby star, 10 lightyears away. Then that photon first expands as a wave to a sphere with a size of 10 lightyears and then gets absorbed again by a tiny electron in our eye.

Is that a form of ultra-precise targeting? Do the wavefunctions of photon and electron collapse?? How many photons would need to be emitted by that star to even make it statistically plausible that this is even possible???

Sam5
2009-Mar-31, 12:58 AM
How many electrons are in a star? How often do they emit photons?

slang
2009-Mar-31, 07:24 AM
I have always wondered how the mechanism works that an electron, a particle that is now measured to be < 10-22 m emits a photon ( size unknown) for instance in a nearby star, 10 lightyears away. Then that photon first expands as a wave to a sphere with a size of 10 lightyears and then gets absorbed again by a tiny electron in our eye.

(Bold mine) I don't understand where that part comes from, if we're talking about a single photon. Is this Heisenberg on steroids, or am I missing some basic (or advanced) particle physics?

StevenO
2009-Mar-31, 10:17 AM
How many electrons are in a star? How often do they emit photons?

From what I have seen here (http://brucegary.net/MTP_tutorial/MTP_ch1.html), a star modeled as BB with a surface temp of around 10K would emit in the order of 1040 photons / sec / micron2.

That's a lot. But, assuming the diameter of the star is about 109 meters, then at our location 10 lightyears(~1016m) away, it still would add up to about 1040 / (1016/109)2 = 1026 photons/sec/micron2.

If the size of the electron is about 10-22m we would have to scale again with (pi*10-22/10-6)2, that would amount to ~1031, which would amount to only 1 photon per 105 sec hitting an electron?

StevenO
2009-Mar-31, 10:41 AM
(Bold mine) I don't understand where that part comes from, if we're talking about a single photon. Is this Heisenberg on steroids, or am I missing some basic (or advanced) particle physics?
That's from the classical treatment of light as EM waves. I would'nt know how to explain it any other way. Otherwise could you please explain me what the 'size' of the photon 'wave-packet' is and how the photon 'wave' remains with the photon 'particle'.

stutefish
2009-Mar-31, 04:43 PM
That's from the classical treatment of light as EM waves. I would'nt know how to explain it any other way. Otherwise could you please explain me what the 'size' of the photon 'wave-packet' is and how the photon 'wave' remains with the photon 'particle'.
The "wave packet" is an expression of the energy level of a single photon. It isn't a constantly-expanding thing. It's the photon itself, and it interacts with other stuff more or less like a particle, and less or more like a wave. But that's all having to do with the specific behavior of individual photons.

The expanding sphere of light you're talking about is the spray of all the photons being emitted in every direction, from a single source. A single photon doesn't expand in all directions. It's a discrete packet with a discrete energy level, traveling in a discrete direction.

Smoke Ring
2009-Mar-31, 05:04 PM
I have always wondered how the mechanism works that an electron, a particle that is now measured to be < 10-22 m emits a photon ( size unknown) for instance in a nearby star, 10 lightyears away. Then that photon first expands as a wave to a sphere with a size of 10 lightyears and then gets absorbed again by a tiny electron in our eye.

Is that a form of ultra-precise targeting? Do the wavefunctions of photon and electron collapse?? How many photons would need to be emitted by that star to even make it statistically plausible that this is even possible???

Where did you find this value for the size of the electron?

Sam5
2009-Mar-31, 05:37 PM
The "wave packet" is an expression of the energy level of a single photon. It isn't a constantly-expanding thing. It's the photon itself, and it interacts with other stuff more or less like a particle, and less or more like a wave. But that's all having to do with the specific behavior of individual photons.

The expanding sphere of light you're talking about is the spray of all the photons being emitted in every direction, from a single source. A single photon doesn't expand in all directions. It's a discrete packet with a discrete energy level, traveling in a discrete direction.

That’s what I was taught in school and in my photography studies. This seems to jive with the Feynman symbol for a single photon. (Scroll down to the second illustration, which is the photon diagram.)
http://en.wikipedia.org/wiki/Photon

The “particle” concept seems to be associated with the way a single photon interacts with a single electron, as if that photon is a “particle”. I was taught that this could be the result of a “resonation” reaction on the part of the electron, caused by the “bump, bump, bump” of the oscillating EM waves.

The following illustration could represent a group of photons leaving a light source at the same time, which essentially would create an “expanding sphere of light”, but with each photon being a separate entity. This would illustrate what you call a, “spray of all the photons being emitted in every direction, from a single source.”

http://i33.tinypic.com/2i7o0lg.jpg

This basically goes back to the concept of the Faraday/Maxwell EM light wave theory.

In a 1929 paper, Einstein attributed the original concept to Faraday:
http://www.rain.org/~karpeles/einfrm.html

"Faraday also had the bold idea that under appropriate circumstances fields might detach themselves from the bodies producing them and speed away through space as free fields: this was his interpretation of light."

And he made this comment about Maxwell and light:

”Maxwell then discovered the wonderful group of formulae which seems so simple to us nowadays and which finally build the bridge between the theory of electro-magnetism and the theory of light. It appeared that light consists of rapidly oscillating electro magnetic fields.”

Here’s Maxwell’s EM light-wave illustration in his 1876 book:
http://i37.tinypic.com/2cfz9c5.jpg

This would be one full wave:

http://i34.tinypic.com/hx0m75.jpg

This would be a Maxwell “beam” containing a “wave train” of photons linked together:

http://i35.tinypic.com/a2u35x.jpg

If I am wrong about any of this, perhaps someone can explain how and why. Thanks. :)

StevenO
2009-Mar-31, 06:33 PM
Where did you find this value for the size of the electron?
In Wikipedia's electron article:

Observation of a single electron in a Penning trap shows the upper limit of the particle's radius is 10−22 meters.[66] The classical electron radius is 2.8179 10−15 m.


This is the original paper about the new electron radius. A Single Atomic Particle Forever Floating at Rest in Free Space: New Value for Electron Radius (http://www.iop.org/EJ/abstract/1402-4896/1988/T22/016/)

trinitree88
2009-Apr-01, 02:08 PM
In Wikipedia's electron article:


This is the original paper about the new electron radius. A Single Atomic Particle Forever Floating at Rest in Free Space: New Value for Electron Radius (http://www.iop.org/EJ/abstract/1402-4896/1988/T22/016/)

StevenO. In particle physics the electron is treated as a point source of no extent physically....therefor no structure. Scattering studies infer this.pete

Rocky1775
2009-Apr-01, 03:06 PM
The "wave packet" is an expression of the energy level of a single photon. It isn't a constantly-expanding thing. It's the photon itself, and it interacts with other stuff more or less like a particle, and less or more like a wave. But that's all having to do with the specific behavior of individual photons.

The expanding sphere of light you're talking about is the spray of all the photons being emitted in every direction, from a single source. A single photon doesn't expand in all directions. It's a discrete packet with a discrete energy level, traveling in a discrete direction.

My understanding, if you could call it that, is that the photon doesn't exist until the light wave interacts with something. Light travels as a wave and interacts with things like a particle. So the wave expands in all directions, and then it disappears when it is absorbed at a point location as if it were a particle. As they say in quantum physics, if this makes sense, then you don't understand. :doh:

trinitree88
2009-Apr-01, 06:11 PM
My understanding, if you could call it that, is that the photon doesn't exist until the light wave interacts with something. Light travels as a wave and interacts with things like a particle. So the wave expands in all directions, and then it disappears when it is absorbed at a point location as if it were a particle. As they say in quantum physics, if this makes sense, then you don't understand. :doh:

Rocky. Not quite. If I take a laser pointer and point it at you, the red photons emanating in your direction have a wavelength, and some momentum. They don't have a momentum in every direction around a sphere encompassing the tip of the laser....only in your direction. since momentum is conserved, that's where they go. They have an interaction area prortional to their wavelength...so they might hit you head on, or a partial wavelength to either side of head on...so it's pretty straight. But if I switch to microwaves, they have a wider error bar where they can be absorbed, but a lower energy per photon.
If a source of photons sprays in all directions at once...like the sun, then you can have effects on one side of the solar system coincident with effects on the other. Such a source is not monochromatic....like the laser pointer...nor directional. pete

StevenO
2009-Apr-01, 10:43 PM
StevenO. In particle physics the electron is treated as a point source of no extent physically....therefor no structure. Scattering studies infer this.pete

That is the point that bother's me. I was trying to estimate the probability that a photon from a star 'hits' an electron residing in our eye. Seen the 'undefined' size of both the electron and photon that is apparently not possible. Now, can anybody describe how to calculate the number of photons from a star we should see with our eyes? Should I take the size of the atoms instead? Should I take the wavelenght of the photon?

According to QM theories photons are absorbed by electrons, so my first guess was to estimate the probability from the number of photons and the 'size' of the electron. Since there are many electrons in our eyes, even a average of 1 photon hitting an electron per 3 hours could still mean the star is pretty well visible. They say five to nine photons hitting our eye per 100ms triggers an observable response.

Ken G
2009-Apr-04, 02:10 AM
That is the point that bother's me. I was trying to estimate the probability that a photon from a star 'hits' an electron residing in our eye. That's a fine thing to do, but the question is, what is the "cross section" (area) of the electron that you should use? In some cases (free electrons), you can indeed use the "classical radius" of the electron to figure that out, but free electrons are a poor way to absorb light, so your eye doesn't use those. It uses bound electrons, which are incredibly sensitive at certain frequencies (your ear does something similar, it is the magic of the "resonance"). So the cross section is millions of times larger at those frequencies. Does this mean the electron "grows" at those frequencies? Not really, it just means it interacts very strongly with light. As has been pointed out, this has to do with the wave nature of light, and means you can either interpret the light as made of tiny particles and then the bound electron has to have a variable size, or you can imagine the bound electron has a particular size and then you have to treat the photon "wave function" like a wave with a very large spread. As usual, it is important not to take any of these pictures too literally, this is just how physics works.
Now, can anybody describe how to calculate the number of photons from a star we should see with our eyes?
See a quantum mechanics text, it's not easy.