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I was chatting with my grandson the other day (he’s 30 with a Master’s in Chemistry and an interest in Astronomy – and Yes, I am incredibly old!) and we lighted on a puzzling question: when observing a celestial object, a stream of photons enters the eye, stimulates the optic nerve and prompts the brain to form an image. But what happens to the photons thereafter?
Can anyone provide an answer, preferably one that does not require a PhD in Physics to understand?
Merry Christmas!
AlanP.S. I would appreciate it if answers do not refer to Schroedinger’s Cat, singularities or higher-order infinities.
The photons hit a receptor in the retina (not the nerve directly). They are absorbed and their energy causes a chemical reaction. A long chain of subsequent reactions eventually reach the brain.
The photoreceptor is slowly (hence a long time taken for dark-adaption) returned to its initial chemical state, with the photon’s energy ultimately converted into heat.
An explanation in terms of QED would not be illuminating!
Hi Alan,
The simple answer is that the photons are “absorbed” by the process which stimulates the receptors in the retina. This process requires energy, which is supplied by the photons – which are basically just little packets of energy. In a similar way, photons incident upon a green leaf are absorbed by the chlorophyll molecules in the leaf to excite them to the higher energy level required to begin the cascade of processes involved in photosynthesis.
The reason the photons are able to travel through the eye without being absorbed is that quantum theory says they must be entirely absorbed, or not at all. For something to absorb a photon thus requires the absorber to have two energy levels separated by exactly the amount of energy contained within the photon. This is not true for the “interior substance” of the eye but is true for the retinal cells – something which evolution has honed throughout aeons. Moreover, because light of different colours corresponds to photons of different energies, discrete sets of retinal cells have evolved to respond to different energies/colours – hence our ability to see in colour. “Colour” is merely an artifact of our perception though – much like the false-colour renderings of the images from Hubble and JWST. Given this, different eyes see colour differently. Even within the human population increased sensitivity to both infra-red and ultra-violet light is not unknown. For example, evolution has given Schrodinger’s Cat greater sensitivity to low light, which is important for hunting at dusk, but a lesser sensitivity in the red. Assuming said Cat is actually alive, that is. Oh, sorry, I wasn’t supposed to mention said feline, was I? I promise no mention of singularities or higher-order infinities though.
Hope that is both non-PhD understandable and generally helpful!
Steve
Thank you very much Paul and Steve for taking the time to tackle my query. That has made a grand start to the Christmas festivities. Are you involved in setting the Christmas Quiz?
We had speculated that the photons, as bundles of energy, must be absorbed by the body, but were unclear about how that takes place. I am somewhat clearer now, although that cat reference almost confused the picture!
I take it then that we do, in a sense (the philosphers’favourite caveat – though I am not a philosopher), ’embody’ every star, galaxy, planet, meteor, etc that we ever observe in that we receive some miniscule parts of them – photons – and incorporate them into ourselves. I think that’s quite an interesting thought.
What happens to the photons that are not absorbed, such as those hitting my hand? Or are they absorbed by a different mechanism?
However, I notice the quotation marks around “absorbed”, Steve, which perhaps indicates that you don’t want this word taken too literally . . .
Am I heading for a singularity?
Alan
Those hitting your hand are either absorbed (converted to heat) or reflected. Longer wavelengths are preferentially reflected than shorter ones which is why your skin looks reddish rather than greyish.
Hello again Alan,
I put the quotation marks in because the word absorbed is usually employed in the sense of water being absorbed into a sponge, and that is not really what happens in the case of retinal stimulation or photosynthesis as it implies that the absorbed object stays within the absorber, albeit perhaps in some altered physical form. The danger here is in thinking of photons as objects, like little snooker balls. They in fact have no corporeal form, being merely packets of pure energy. When the amount of energy a photon carries matches the energy required to excite a molecule it encounters to a higher energy state the photon ceases to exist and the energy it carried is transferred to the molecule (all molecules vibrate and flex to some exent and the higher energy states correspond to modes of greater vibration or flexion). In its turn, the molecule can then pass on that energy to another molecule (dropping back to its original state as it does so) and so on. The end result of such a chain of transfers is often the production of a new molecule, the additional energy perhaps being used to overcome a repulsive force between constituent parts of the new molecule which would prevent that molecule being formed were it not for the intervention of the photon down the line.
Therefore, because a photon ceases to exist when it is “absorbed”, we do not embody every star etc. that we observe because the very act of seeing it destroys the photons coming from it. Again, the danger is to imagine photons as little balls which somehow lodge within us – not so! We do, of course, embody “star stuff” a different way, in that we are constructed from elements only produced in supernovae, but that is another subject entirely!
Steve
P.S. No, not involved in setting the Christmas Quiz but I am always open to being asked!
Thanks, Steve. That is a very helpful clarification.
I hardly dare pursue the question of the non-corporeal character of photons as I suspect this may be taking us ever-closer to the dreaded feline. Nor to delve into further questions about how the brain processes signals to yield images.
The general topic of the ‘mechanics’ of observing (what happens after the inputs are received from the object by the eye) are perhaps not as frequently highlighted in discussions of observational astronomy as they might be, at least among amateurs. So there is a lot about observational instruments, techniques, etc. but rather less about the observer, whose status within the observing process seems largely taken for granted as a kind of unproblematic sensor. (An exception is Norman Davidson’s (1985) Astronomy and the Imagination)
With your permission, I will pass on what you have told me to my grandson. Also, I wonder if you might want to post something on this topic under the ‘Tutorials’ banner. Surely other members would be interested? Of course, I have no influence on this nor the Christmas Quiz – if only I did I might get more of the answers!.
Regards
AlanIf the Xmas quiz is not challenging enough for you you might like to investigate the modern view of what a photon is.
They certainly don’t stream from a star to your eye and most states of the quantum em field don’t have a number operator (you can’t say how many photon it has) and to cap it all they don’t have a position operator (so can’t say where they are).
Try Googling “The concept of the Photon revisited ” for an interesting discussion. The text is quite accessible without the maths. The paper us by Muthukrishnan · 2003 ·
Regards Andrew
Hello Andrew
Many thanks indeed for your reply.
I read the abstract of the paper you mentioned and that was enough to convince me that our discussion is moving into the realms of metaphysics, Heisenberg etc.
Rather than get lost there (I am not a Physicist), I am happy to live with the conclusion so aptly stated by the late Patrick M: “We just don’t know.”
Like most things, if you look too closely they disappear!
AlanHello yet again Alan,
The ultimate problem in understanding photons is that, as implied by Andrew and sort of stated in the Muthukrishnan paper, they don’t actually exist! Not as usually understood in explanations of optical phenomena, anyway. They are just one way of looking at light, which can be useful in some situations but not in others. In the end, as is also shown in the paper, everything depends on a) the maths and b) what you are trying to deduce from your observations. However, mere mortals who are not quantum physicists need models on which to base their understanding, so as long as you don’t think too hard about the ultimate nature of light and carefully pick the scenarios in which you use one or the other, the wave and particle (i.e. photon) models provide reasonable, if not exactly rigorous, explanations of such things as the excitation of molecules involved in retinal detection or photosynthesis. So don’t despair entirely!
But yes, as you say, often the closer you look the less definite things become – an example of the Heisenberg Uncertainty Principle!
Please do pass on anything I have said to your grandson. I believe it to be accurate at a “non-specialist” level so hopefully it won’t lead him down incorrect paths. However, I doubt whether a more detailed discussion than that in this forum thread would be considered appropriate as a Tutorial, as it is really in the realm of physics rather than astronomy.
As to the status of the observer in the observing process, I have read articles discussing the effect of variation of colour discrimination between observers on the observation of coloured binary stars but, as you say, little else. Still, now that most observations are done with electronic devices perhaps the poor old human observer will become a seriously endangered species!
Best regards,
Steve
Many thanks Steve. All very interesting, and I am certainly better informed now than at the start of this thread.
I am reminded of Dr Samuel Johnson’s response to Bishop Berkeley’s claim that everything is an idea without material substance – Johnson kicked a large stone, saying “I refute it thus!” On a pragmatic level, so long as I can continue to see the light, that will do for me!
Merry Christmas
AlanAnd a Merry Christmas & Happy New Year to you also Alan!
I hope that the light you see over the coming week will be from not just stars & planets but also from twinkly illuminations, glowing candles, burning brandy on the plum pudding and the glow on the cheeks of family & friends! Just don’t start talking about all the photons flying about!!
With very best regards,
Steve
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