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Do we observe the orignial photon?


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Arising from another thread I pose this question in the hope of attracting a Mighty Physicist like George Jones. (Hi George!!) And/or others. BrianB, are you there?

Seriously now; the incident photon from x million LY away arrives at a reflector and buries itself in the aluminium held in place by the glass. It is absorbed and out pops another photon which, for reasons statistical, heads in a direction equivalent to angle of incidence equalling angle of reflection, though no real bouncing has taken place. (This is what happens when an English teacher reads Feynman's QED so be kind.)

In the unlikely event of this being correct my next question is; what happens in a refractor? Is the original incident photon preserved or does some similar exchange occur?

Forgive my ignorance.

Olly

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I see where you're going....this is another 'refractors are better than reflectors because' attack :D

seriously though, what a top question - I'll watch the replies from the boffins with interest.

No no no, absolutely not!! Okay I am a refractor nut as an imager but not as an observer. I do have a twenty inch dob. Honestly, hand on heart, the question is scientific. You will never believe me but it really is!!

Very best,

Olly

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Is the original incident photon preserved

It's a meaningless question, the photon has no existence until it's detected ... just as you can't tell which slit it goes through in the two slit experiment (it interferes with itself) ... the only existence it has when passing through a lens or being reflected from a mirror surface is as a probability function.

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No no no, absolutely not!! Okay I am a refractor nut as an imager but not as an observer. I do have a twenty inch dob. Honestly, hand on heart, the question is scientific. You will never believe me but it really is!!

Very best,

Olly

ha ha I was also honestly joking (no seriously no sarcasm at all - oh no I mentioned sarcasm and now you are thinking of sarcasm being inferred in my unsarcastic response) - I've been watching too many Frasier repeats obviously.

please don't mention 20" dobs around me......must.....resist....aperture.....fever.......

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It's a meaningless question, the photon has no existence until it's detected ... just as you can't tell which slit it goes through in the two slit experiment (it interferes with itself) ... the only existence it has when passing through a lens or being reflected from a mirror surface is as a probability function.

this sort of principle is outstanding. I love the quote (not sure who it was) which was something along the lines of ......'if you think you understand quantum physics, then you don't understand quantum physics'....

the other thing that always amazed me was that an electron is nowhere and everywhere simultaneously? (I think that's right). Sounds like me with my scope......

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this sort of principle is outstanding. I love the quote (not sure who it was) which was something along the lines of ......'if you think you understand quantum physics, then you don't understand quantum physics'....

That's along the same lines as...

“If quantum mechanics hasn't profoundly shocked you, you haven't understood it yet.” - Niels Bohr

Fascinating thread this

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That's along the same lines as...

“If quantum mechanics hasn't profoundly shocked you, you haven't understood it yet.” - Niels Bohr

Fascinating thread this

ha ha

I think that's the one I meant..in my best [removed word] Emery voice "I fink I got it wrong again Dad"

unfortunately I cannot even remember the quotes let alone what it all means. :D

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I have say, that I am (generally) blissfully ignorant of the physics involved in the universe. I have read and understood various books on the early universe and happily forgotten most of what I read, as I find it of no practical use what so ever. That does not stop me from enjoying all the wonders that my eyes can see through the EP.

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what happens in a refractor? Is the original incident photon preserved or does some similar exchange occur?

Why is the effective speed of light in a refractor's objective lens less than the speed of light in a vacuum? Yes, the answer is very similar to

It is absorbed and out pops another photon which, for reasons statistical, heads in a direction equivalent to angle of incidence equalling angle of reflection
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Am I to take it then, that what I am seeing in my Newts, is NOT the actual light from the star? But rather, a synthetic image of the star? I find this most disheatening indeed! I may have to buy an APO!

No, don't do that because, rather as I suspected, both Brian and George's answers mean there is really no difference!

The whole idea of photons refuses to mesh with our common sense view of the world.

Olly

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I always thought the energy of light quantum was given by E=hf where h is plank's constant and and f is frequency. Why this equation should hold is beyond my understanding, but it states how photons can be regarded as both particles and waves at the same time. But to my mind it also gives a photon mass. Unlike aplha and beta radiation, gamma radiation does not carry a charge and is a form of electromagnetic radiation rather than positively charged helium nuclei or negatively charged electrons. ionising radiation at these level will affect the materials in your optics and your own cells On the other hand sunlight falling on optics having less energy will radiate energy in the infra red. When photons of sufficient energy hit a material, the electrons can jump to another shell when they loose energy and fall back to a lower orbit the extra energy is seen as a line emission in spectra (which thankfully is always the same for a given element).

If you have the spectra for AI3+ O2- ionic bonds Al203 then wouldnt the absorption lines indicate the frequency and energy needed to kick electrons in the compound. You could always put any element with an atomic number > 83 near your optics and see what happens, or on your skin for that matter. I believe depleted uranium is a good source of gamma rays.

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Take a look at

Richard P. Feynman - Nobel Lecture

it tries to explain Feynmans Quantum Electrodynamics, get comfortable, it's a long read.

Steve..

You have reminded me about a set of Feynman videos about this.

The Vega Science Trust - Richard Feynman - Science Videos

The second lecture looks like it is particularly relevant to this thread. The lectures were turned into a very popular little book,

QED - The Strange Theory of Light and Matter Penguin Press Science: Amazon.co.uk: Richard P Feynman: Books.

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First thing to consider is whether the photon that hits the telescope mirror or objective is the same photon that left the star/nebula/whatever. Question is, how would we identify it? I can say that a person who leaves a certain house is the same as the one who subsequently gets on a bus because there are various properties that identify this particular person. In the extreme case, I could put a sign on the person saying "this is the guy", and see that the person getting on the bus wears this sign. You can't do that with photons. You can't label them, and any two photons of the same wavelength are identical and indistinguishable. So we can't really say it's the "same" photon. At best we can say it's an identical photon.

All of the above is leaving aside the fact that light from the star or other source has passed through the interstellar medium and Earth's atmosphere, and has undergone interactions while doing so. If a photon gets redshifted, is it the "same" photon, or has it been turned into a different photon? If a photon is scattered off a water molecule, without change of frequency, is the outgoing photon the "same" as the in-going one? Etc.

Alright, the photon bounces off the telescope mirror (is it then the "same"?) or passes through the objective lens (same problem), and eventually through the glass of the eyepiece. It enters the eye, passing through lots of fluid (again, the same problem). It hits the retina - and nothing happens. The human eye doesn't respond to single photons (I've heard it claimed that frogs' eyes do - good for them). It takes a bunch of photons arriving at the same spot on the retina within a certain short period of time for the cell to fire and send a signal to the brain. We do not see single photons: we "see" in response to stimulus from many photons, none of which can unequivocally be identified with photons that were produced at the original source.

So my answer to the question would be no, we don't observe the original photon. But it's nice to think of it as if we were.

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The human eye doesn't respond to single photons

Actually it does - there is a burst of nerve impulses when a photon is detected. Whether the brain "sees" this is a matter of training. That's why skilled experienced observers can see such faint objects. Approx. 4 photons / sec from a static point source is enough to be able to glimpse the star intermittently by averted vision.

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If a photon gets redshifted, is it the "same" photon, or has it been turned into a different photon?

Like I said above, this is much easier to understand if you stop thinking of a photon as an object & start thinking of it as a probability function, the object (wave, particle or some combination of the two) not actually existing until it is detected.

The probability function is not affected by red shifting - whether the cause is doppler shifting or warping of space/time by strong gravitational field - this is what Einstein's Theories of Special & General Relativity are actually telling us. And Maxwell's Equations tell us how the probability function is affected by interaction with matter.

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I believe depleted uranium is a good source of gamma rays.

Nope. Uranium 238 (depleted of the small fraction of U235 which occurs in natural uranium) decays to Thorium 234 by alpha decay with a half life of 4.468 billion years.

Depleted uranium is actually used for radiation shielding in medical radiotherapy equipment, where it will be contaminated by the strong gamma ray sources used (e.g. cobalt 60) and become secondarily radioactive as a consequence. It is also used extensiveley for making couterweights for control surfaces in aircraft, and for ballistic munitions (bullets), where its high density is a valuable property. The radioactivity of depleted uranium is not a significant hazard but its chemical toxicity is.

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I can say that a person who leaves a certain house is the same as the one who subsequently gets on a bus because there are various properties that identify this particular person. In the extreme case, I could put a sign on the person saying "this is the guy", and see that the person getting on the bus wears this sign.

No guarantees there...

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Approx. 4 photons / sec from a static point source is enough to be able to glimpse the star intermittently by averted vision.

So does astronomy improve night vision?

I ask simply because since I took an interest in it, my night vision seems greatly enhanced - or is that just practice from all my fumbling about in the dark? :D

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So does astronomy improve night vision?

I ask simply because since I took an interest in it, my night vision seems greatly enhanced - or is that just practice from all my fumbling about in the dark? :D

Practice enhances, I would say.

Olly

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