Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Lewis Thomson on 17/10/2022 15:19:42
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Daniel is looking for answers to this question.
"It is my understanding that photons are massless and can act as either particles or waves. None of this information helps me understand how a photon relays information across various distances. How is the color red stored and transmitted from a stop sign, across a void, and into my eyeball?"
Discuss your scientific findings in the comments below...
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Colours are determined by the frequency of the emitted light.
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how a photon relays information across various distances
To send information via photons (or any form of communication) requires:
- A sender
- A receiver
- A communications channel between them
- A common understanding between the sender and receiver of how to encode and decode the signals over the channel
- A common understanding between the sender and receiver of how to interpret the signals
- And because communication channels are not perfect, there is usually some mechanism to detect and correct lost or corrupted communications (eg "say that again?")
how a photon relays information across various distances
There are many ways to encode information on photons, and many things that you can do with it. The common methods we use with photons are:
- Writing
- Television & screens on computers and phones
- Mobile Phones, WiFi and radio
- Optical fibers
- In medical applications we use X-Rays, Gamma rays, infra red and radio waves, which are photons of different wavelengths, to communicate information about disease states outside of the body to a radiographer
The main advantages are that:
- It travels very quickly: 300,000km/second in a vacuum, pretty much the same speed in air, and around 200,000 km/second in water
- It travels very far in a vacuum (we can see the Sun, and distant stars), pretty far through air (on a clear day), and even penetrates a few hundred meters into the sea.
- It carries energy - almost all of our biosphere is powered by photons from the Sun.
- Plants use this energy to turn CO2 into food (for themselves, and us)
- Many organisms are sensitive to light, because it helps synchronise their day/night circadian rhythm.
stop sign
Light from the Sun contains photons of many wavelengths (or frequencies, if you prefer).
- The white reflective pigment in the "STOP" letters reflects all these wavelengths
- The red pigment in the stop sign absorbs the green and blue wavelengths, reflecting the red wavelengths.
- Your eye focuses these wavelengths into a pattern onto the retina, where it is carried to your brain
- The translation of this pattern into the word "STOP" requires years of schooling.
- The translation of this word into you pressing the brake pedal in your car requires even more training
See: https://en.wikipedia.org/wiki/Communication_theory
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Typically, information is not modulated on individual photons, because it is too easy for an individual photon to get "lost" (sent in the wrong direction, absorbed, reflected, refracted, etc).
- Information is usually modulated on many photons at once, more often treating it as a "coherent electromagnetic wave"; losing 99.9% of the photons still allows the information to be decoded, modulated on the wave
- One of the challenges of quantum computers is that they rely on individual quanta (eg individual photons), and they are easily disturbed.
Treating light as a wave, modulation is often done by:
- Changing the reflectivity and/or colour across a surface: Reading a book, watching TV
- Changing the amplitude of a carrier wave: AM radio
- Changing the wavelength of a carrier wave: FM radio
- Turning the light on and off: ship-to-ship morse code, or optical fiber communication up to 10Gbps
- Modifying the phase and/or amplitude of the wave: Fax machines, DSL broadband, optical fiber communication up to 50Gbps
- Using all of the above at once: Modern WiFi and cellular mobile can carry 100Mbps (4G) - Gbps (5G);
- Using multiple wavelengths to transmit in parallel: current commercial optical fiber systems can carry Tbps
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Daniel is looking for answers to this question.
"It is my understanding that photons are massless and can act as either particles or waves. None of this information helps me understand how a photon relays information across various distances. How is the color red stored and transmitted from a stop sign, across a void, and into my eyeball?"
Discuss your scientific findings in the comments below...
Light comes in various wavelengths. For visible light this ranges from 380 to 700 nanometers. This encompasses the colors from violet to red.
Typical "white" light is really just a mix of light made up of all these wavelengths. When white light strikes the stop sign, most of these wavelengths are absorbed by the paint, with only those wavelengths in the red region reflected back.
When that light enters your eye passes through the lens, and on to the retina. The retina is finely covered with receptors; rods, which are just sensitive to intensity and cones which come in three types: Ones sensitive to the red wavelenths, ones most sensitive to Blue wavelengths, one most sensitive to red ones, and ones most sensitive to Green wavelengths
With the stop sign, only the red-sensitive cones will detect anything. and it will be signaled to the brain that you see red.
For other colors, the three types combine their signals to reproduce the color detected.
If you want to think of it in terms of photons, the wavelength determines the energy of each photon (red photons are less energetic than violet one)
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It comes from Einsteins theory of black body radiation.
https://physics.stackexchange.com/questions/208942/why-does-wavelength-determine-the-energy-of-a-photon
But it doesn't answer how we see colors as red. Although there is a inverse equivalence between photons and wavelengths a lot of it will have to do with interpreting. And that's about our brains, the extension of it being our senses.
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the point there is that we 'mostly' agree on any color chosen, so we should have a same interpreter genetically coded in us, presumably. But even then it becomes mind boggling. I think it's a very good question.
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we 'mostly' agree on any color chosen
If a child in kindergarten is asked "what colour are the leaves on a tree?", and they answer "blue" or "red", they are corrected to say "green".
If a child in kindergarten is asked to draw a tree, and they draw blue or red leaves, they are referred for colour-blindness testing.
- However, the reverse does not happen - some girls are natural tetrochromats, and can distinguish colours that everyone else says are just "red". So they learn from all us trichromats that all these colours (which they can see are distinctly different) are actually all the same.
Language has a lot to do with it - some languages don't have separate words for "blue" and "green" - which produces problems for those kids when they suddenly go to an English-speaking kindergarten, and are told that they are giving the wrong answers...
https://en.wikipedia.org/wiki/Tetrachromacy#Humans
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Yeah, it's one of my standing questions too. And one I find very intricate Evan. If we define it by 'photons' then we apparently are able to 'see' even single ones, well, statistically defined. So would they also trigger a 'color' depending on their energy? 'White' is everything we can't 'sort out' whereas the other colors becomes more selective. And then we have form, the shape of things, very important as we otherwise only would stumble around, constantly hitting them. Where is the threshold for discerning forms? How many 'photons' for that?
https://www.nature.com/articles/ncomms12172
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And btw, defining it from 'photons' becomes quite correct if one think of black body radiation, and the theory behind that. Max Planck was the first defining a hypothesis on how it might work, from where Einstein built on it further, into a theory. It's called ' the ultraviolet catastrophe ' and was something wave theory failed to explain.
https://www.webassign.net/question_assets/buelemphys1/chapter27/section27dash1.pdf
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And that's the problem, what is 'light'? And how do we define it? I think I will link this one too, where ES makes some definitions clearer
https://www.thenakedscientists.com/forum/index.php?topic=85773.0
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And you're perfectly correct Evan, group pressures, socioeconomics, and hierarchies defines a lot , maybe all, of our behaviors.
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The light has all spectrums the object itself reflects light in the case of the stop sign it would reflect red and white. The stop sign is simple reflecting back the red and white light which it can't absorb.
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'photons' ...'see' even single ones. So would they also trigger a 'color' depending on their energy?
Individual photons do have a colour - if you put it through a diffraction grating, and see where it comes out.
- The human eye is more complex - cone cells have different sensitivity to photons of different energy, but they only work in bright light, so they can't detect down to individual photons.
- The rod calls are much more sensitive, and can detect down to low numbers of photons, but they are not colour-sensitive.
https://en.wikipedia.org/wiki/Cone_cell
'White' is everything we can't 'sort out'
For humans, white means multiple photons which trigger all of the types of cone cells.
For night-time vision, any light hitting the rods would be regarded as "white" or "bright".
But if you were using a diffraction grating, there is no such thing as a "white" photon.
There are tricks used to produce ultrashort laser pulses with a very wide spectrum - this could be called a "white" laser pulse, but not a white photon.
This won the Nobel Prize in 2018
https://www.forbes.com/sites/chadorzel/2018/10/02/nobel-prize-in-physics-2018-how-to-make-ultra-intense-ultra-short-laser-pulses/?sh=35aca4ca6bca
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Individual photons do have a colour - if you put it through a diffraction grating, and see where it comes out.
Photons only have a colour relative to some frame of reference. It's not an objective thing.
So the frequency, energy, direction of travel, and deflection due to a diffraction grating are all frame dependent.
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All good points, Evan and Halc, and an interesting link too, to it. What I'm wondering about more specifically is the 'information density' one might be able to put to a 'isolated photon'. 'Energy' and the rest an interaction between source and sink?
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The retina is finely covered with receptors; rods, which are just sensitive to intensity and cones which come in three types: Ones sensitive to the red wavelenths, ones most sensitive to Blue wavelengths, one most sensitive to red ones, and ones most sensitive to Green wavelengths
You mentioned red twice, wich might be confusing to some readers.
The diagram below shows normalized spectral sensitivity of human light receptors. They are blue cone, rod, green cone, and red cone, respectively.
The red cone is also slightly sensitive to violet. That's why violet looks like a combination of red and blue.
(https://midimagic.sgc-hosting.com/eyespect.jpg)
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Sort of relevant. I was in a west London pub with a colleague, explaining to an American visitor how to get to Liverpool Street. Simplest route was by Tube, Piccadilly Line to Holborn (as it then was) then Central Line. He was concerned to remember the unfamiliar names of the lines so said "dark blue northward, then red eastward". "Hold on" said Kate "there are two red lines". She had reached the age of 30 without knowing she was color blind.