What is Light?

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What is Light?

Post by BDanielMayfield » Sun Jan 25, 2015 3:43 pm

The recent apod discussion between starsurfer, geck and Chris re whether ALL of the electromagnet (EM) spectrum should be referred to as "light" prompts this question: What is light? Is it best to just think of light as the parts of the EM band that include and are near the visible range? Or is light equal to the whole spectrum from microwaves clear to gamma rays?

Also, deeper spinoff questions: What IS light, really? By this I'm asking, is the dual particle/wave nature of light understood well enough that an explanation can be given that is understandable to non scientists? And why is it called "electromagnet" radiation?

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Re: What is Light?

Post by Chris Peterson » Sun Jan 25, 2015 4:13 pm

BDanielMayfield wrote:The recent apod discussion between starsurfer, geck and Chris re whether ALL of the electromagnet (EM) spectrum should be referred to as "light" prompts this question: What is light? Is it best to just think of light as the parts of the EM band that include and are near the visible range? Or is light equal to the whole spectrum from microwaves clear to gamma rays?
"Light" has historically meant visible light, for the obvious reason that nobody knew there was anything else until the 17th century. Near IR was the first non-visible EM radiation to be discovered, and was associated with light because of its proximity to red, and the fact that it could be manipulated optically just like visible light. As our knowledge of the EM spectrum expanded, different parts were given different names. Sometimes this was because they were not initially associated with the EM spectrum, sometimes simply because of behavior very different from other parts.

Today, the phenomenon itself is universally referred to as "electromagnetic radiation". Naturally defined regions within the spectrum are also universally recognized, e.g. radio, x-rays, visible light, etc. "Light" remains mainly used for visible light and parts of the UV and IR spectrum. But there are no hard and fast rules, no formal definition. For instance, the ultrabright synchrotron UV/soft x-ray source at Berkeley is called the "Advanced Light Source". You would be more likely to find the term "light" applied to wavelengths shorter than visible light than much longer. That's because from mid-IR and shorter, we mainly see EM radiation as particles. At the longer end, we see it by its electromagnetic properties. So longer wavelengths behave like radio- we create it or detect it by the motion of electrons in conductors, by manipulating fields. I don't think I've ever heard a radio astronomer call what he is studying "light".
BDanielMayfield wrote:Also, deeper spinoff questions: What IS light, really? By this I'm asking, is the dual particle/wave nature of light understood well enough that an explanation can be given that is understandable to non scientists? And why is it called "electromagnet" radiation?
There are four fundamental forces: the weak force, the strong force, gravitation, and the electromagnetic force. The latter is at the core of electrical fields and magnetic fields- properties of electromagnetic radiation. Photons are the particles which carry the electromagnetic force (in modern quantum theory, all forces have associated particle carriers).

Particle/wave duality is perfectly understood. It's just that you need to understand this in terms of the mathematics that describes it. Unfortunately, that makes it difficult to explain to most people, as the duality is contrary to physical intuition.
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Re: What is Light?

Post by BDanielMayfield » Sun Jan 25, 2015 5:45 pm

Chris Peterson wrote:"Light" has historically meant visible light, for the obvious reason that nobody knew there was anything else until the 17th century. Near IR was the first non-visible EM radiation to be discovered, and was associated with light because of its proximity to red, and the fact that it could be manipulated optically just like visible light. As our knowledge of the EM spectrum expanded, different parts were given different names. Sometimes this was because they were not initially associated with the EM spectrum, sometimes simply because of behavior very different from other parts.
Thanks for that historical framework Chris.
Today, the phenomenon itself is universally referred to as "electromagnetic radiation". Naturally defined regions within the spectrum are also universally recognized, e.g. radio, x-rays, visible light, etc. "Light" remains mainly used for visible light and parts of the UV and IR spectrum. But there are no hard and fast rules, no formal definition. For instance, the ultrabright synchrotron UV/soft x-ray source at Berkeley is called the "Advanced Light Source". You would be more likely to find the term "light" applied to wavelengths shorter than visible light than much longer. That's because from mid-IR and shorter, we mainly see EM radiation as particles. At the longer end, we see it by its electromagnetic properties. So longer wavelengths behave like radio- we create it or detect it by the motion of electrons in conductors, by manipulating fields. I don't think I've ever heard a radio astronomer call what he is studying "light".
So this mild debate you and geck were having amounts to semantics, essentially?
BDanielMayfield wrote:Also, deeper spinoff questions: What IS light, really? By this I'm asking, is the dual particle/wave nature of light understood well enough that an explanation can be given that is understandable to non scientists? And why is it called "electromagnet" radiation?
There are four fundamental forces: the weak force, the strong force, gravitation, and the electromagnetic force. The latter is at the core of electrical fields and magnetic fields- properties of electromagnetic radiation. Photons are the particles which carry the electromagnetic force (in modern quantum theory, all forces have associated particle carriers).
Yes, this should be commonly understood by the scientifically literate.
Particle/wave duality is perfectly understood.
Perfectly? Wow, that sounds like a very lofty standard. I don't consider myself as understanding anything "perfectly".
It's just that you need to understand this in terms of the mathematics that describes it. Unfortunately, that makes it difficult to explain to most people, as the duality is contrary to physical intuition.
But I like math, so I hope this explanation delves deeper.
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Re: What is Light?

Post by Chris Peterson » Sun Jan 25, 2015 6:09 pm

BDanielMayfield wrote:So this mild debate you and geck were having amounts to semantics, essentially?
Semantics and convention, yes.
BDanielMayfield wrote:
Particle/wave duality is perfectly understood.
Perfectly?
Well, yes. Effectively. By that I mean that the behavior of electromagnetic radiation appears fully described by theory. We don't see any holes- things that EM does which we lack the theory to explain. That describes perfect understanding to me. Of course, it doesn't if you want to get into some sort of philosophical discussion about the nature of knowledge, or whether a completely predictive theory actually relates to the true nature of the Universe. But in terms of the science, we don't see basic research into the nature of particle/wave duality, because that theory is very well developed (perhaps fully developed).
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Re: What is Light?

Post by BDanielMayfield » Sun Jan 25, 2015 6:42 pm

Chris Peterson wrote:
BDanielMayfield wrote:
Particle/wave duality is perfectly understood.
Perfectly?
Well, yes. Effectively. By that I mean that the behavior of electromagnetic radiation appears fully described by theory. We don't see any holes- things that EM does which we lack the theory to explain. That describes perfect understanding to me. Of course, it doesn't if you want to get into some sort of philosophical discussion about the nature of knowledge, or whether a completely predictive theory actually relates to the true nature of the Universe. But in terms of the science, we don't see basic research into the nature of particle/wave duality, because that theory is very well developed (perhaps fully developed).
Ah, that I can accept, and I do.

For the record, I wasn't attempting to steer this into a philosophical direction. I seek physical enlightenment upon the nature of physical, photonic light. I'd like to be able to explain to others what light is. A thurough understanding of something should include the ability to correctly explain it to others, even if the explanation is challenging.

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Re: What is Light?

Post by geckzilla » Sun Jan 25, 2015 6:53 pm

I ask myself this question at least a few times a week. I think I understand it a lot better than many if not most people but then I end up learning something new and it feels like I know almost nothing at all.
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Re: What is Light?

Post by BDanielMayfield » Sun Jan 25, 2015 10:02 pm

geckzilla wrote:I ask myself this question at least a few times a week. I think I understand it a lot better than many if not most people but then I end up learning something new and it feels like I know almost nothing at all.
I agree with your sentiments about this geck.

The universe is vast enough to keep us humble for a long long, really long time to come, I'd predict.
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Re: What is Light?

Post by Speculator » Sun Jan 25, 2015 11:33 pm

What is light travelling through a vacuum and darkness? What is light without the brains of creatures or organisms to interpret and use it? When does light become warmth and then heat? Physical science, it seems to me, always borders on or is entwined with philosophy.

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Re: What is Light?

Post by Chris Peterson » Mon Jan 26, 2015 12:44 am

Speculator wrote:What is light travelling through a vacuum and darkness? What is light without the brains of creatures or organisms to interpret and use it? When does light become warmth and then heat? Physical science, it seems to me, always borders on or is entwined with philosophy.
Electromagnetic radiation is subject to the laws of nature, and those laws care not in the least if anything is there to interpret things.
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Re: What is Light?

Post by geckzilla » Mon Jan 26, 2015 2:43 am

Philosophy is its own quagmire. I think one can adopt a simple philosophy and get along well enough without too much effort. You can try to figure out the figuring out all you want and run yourself in circles but I wouldn't bother. I think my favorite thing about space is that even while it appears to be a vacuum of nothingness it is itself a medium.
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Re: What is Light?

Post by THX1138 » Mon Jan 26, 2015 5:50 am

This is probably not the wisest of questions but i really and genuinely would like to know
Light / One single photon
Once this single photon comes in to existence it will by it's own nature start traveling at C in one direction or another
SO if one were close enough to see this single photon's light it would have to be traveling towards you? It couldn't be seen from two different directions like

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Re: What is Light?

Post by Chris Peterson » Mon Jan 26, 2015 6:06 am

THX1138 wrote:This is probably not the wisest of questions but i really and genuinely would like to know
Light / One single photon
Once this single photon comes in to existence it will by it's own nature start traveling at C in one direction or another
SO if one were close enough to see this single photon's light it would have to be traveling towards you? It couldn't be seen from two different directions like
What does "see this single photon's light" mean? A photon doesn't emit light. If the photon strikes a detector of some kind (such as your retina), it's energy will be observed. If it doesn't strike a detector, you have no way of knowing it even exists.
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Re: What is Light?

Post by THX1138 » Mon Jan 26, 2015 10:24 am

Rephrased, but i think what you said already answers my question

Two people in a dark room, one single photon in that room
That photon happens to run in to one persons eye / retina and so for an instant the person will see a single flash through one of his eyes while the other person
in the room observed no flash of light
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Re: What is Light?

Post by Chris Peterson » Mon Jan 26, 2015 2:54 pm

THX1138 wrote:Rephrased, but i think what you said already answers my question

Two people in a dark room, one single photon in that room
That photon happens to run in to one persons eye / retina and so for an instant the person will see a single flash through one of his eyes while the other person
in the room observed no flash of light
Yup. A flash of light is an interaction with a photon. No interaction, no flash.
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Re: What is Light?

Post by geckzilla » Mon Jan 26, 2015 4:34 pm

Lots of interactions and no flash, too. Even one visible light photon has a low chance of being seen as a flash of light but it's pretty darn cool that the human retina is sensitive enough that it's even possible.
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Re: What is Light?

Post by THX1138 » Tue Jan 27, 2015 3:37 am

I was hung-up on this light thing with the idea that if a single photon were to strike the ground right in front of two people and they could both visibly see this happen then / well what would that mean about the structure of a photon.
Seems the closest thing to this ever happening though is persons noticing that a photon must have stricken a detector on one device or another.
Thank you all for taking the time to explain the matter for me
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Re: What is Light?

Post by BDanielMayfield » Tue Jan 27, 2015 12:41 pm

THX1138 wrote:I was hung-up on this light thing with the idea that if a single photon were to strike the ground right in front of two people and they could both visibly see this happen then / well what would that mean about the structure of a photon.
Seems the closest thing to this ever happening though is persons noticing that a photon must have stricken a detector on one device or another.
Thank you all for taking the time to explain the matter for me
But if the single photon in the dark room was energetic enough (a gamma ray, for example) two or more observers should be able to notice secondary effects, at least some of the time, depending on how the energy dissipates.

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Re: What is Light?

Post by Chris Peterson » Tue Jan 27, 2015 3:00 pm

BDanielMayfield wrote:But if the single photon in the dark room was energetic enough (a gamma ray, for example) two or more observers should be able to notice secondary effects, at least some of the time, depending on how the energy dissipates.
It doesn't even have to be particularly energetic. Every time we look at an image we're seeing secondary effects of a photon interaction. But certainly, photons interact with matter in ways that produce secondary photons- by simple ionization of atoms at lower energies, by producing bremsstahlung or synchrotron radiation at higher energies. But in all such cases, the original photon is absorbed in an interaction and is not directly observed.
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Re: What is Light?

Post by Beyond » Tue Jan 27, 2015 4:21 pm

Chris Peterson wrote: ...bremsstrahlung...
Chris, you are too well educated for my own good. :lol2:
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Re: What is Light?

Post by Chris Peterson » Tue Jan 27, 2015 4:32 pm

Beyond wrote:
Chris Peterson wrote: ...bremsstrahlung...
Chris, you are too well educated for my own good. :lol2:
Ha. Radiation is strahlung. Thanks for the repair.
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Re: What is Light?

Post by Beyond » Tue Jan 27, 2015 5:29 pm

I had absolutely no idea, but glad to have been of help. :lol2:
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Re: What is Light?

Post by Ron-Astro Pharmacist » Wed Jan 28, 2015 11:43 pm

The trouble I have in understanding the nature of light comes from a human nature to question things that seem unnatural. For instance with light – if a photon interacts with an atom, its electron moves to a higher energy level. Then when the electron settles to a lower energy level, a photon is emitted. It is tempting to the make the easy corollary that the area surrounding the nucleus is "something" not quite understood yet. That "something" which creates light seems to come and go from the area that’s called the electron cloud. It's almost like when it's close to the nucleus, it creates the space for the electron but when away from the atom's nucleus - the only part that is detectable is its movement as a wave.

A simplistic conjecture but it’s the only way I can see light in a way that makes sense to me. Naturally, I’ll continue to question it explicitly. :D
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Re: What is Light?

Post by Chris Peterson » Thu Jan 29, 2015 12:22 am

Ron-Astro Pharmacist wrote:The trouble I have in understanding the nature of light comes from a human nature to question things that seem unnatural. For instance with light – if a photon interacts with an atom, its electron moves to a higher energy level. Then when the electron settles to a lower energy level, a photon is emitted. It is tempting to the make the easy corollary that the area surrounding the nucleus is "something" not quite understood yet. That "something" which creates light seems to come and go from the area that’s called the electron cloud. It's almost like when it's close to the nucleus, it creates the space for the electron but when away from the atom's nucleus - the only part that is detectable is its movement as a wave.

A simplistic conjecture but it’s the only way I can see light in a way that makes sense to me. Naturally, I’ll continue to question it explicitly. :D
Photons are carriers of energy. When a photon is absorbed by an atom, that energy is conserved, meaning the atom is more energetic. It makes sense that the energy is stored in the behavior of the electrons. Similarly, it makes sense that if an electron gives up energy, it should be released as a photon.

Wave-particle duality is not limited to photons or light. Every elementary particle exhibits it.
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Re: What is Light?

Post by MargaritaMc » Thu Jan 29, 2015 8:48 pm

By chance, I've just come across this article in a December 2014 issue of a Science Daily
Quantum physics just got less complicated: Wave-particle duality and quantum uncertainty are same thing

It's tangentially related to this discussion. Maybe. Interesting, anyway.

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Re: What is Light?

Post by bystander » Thu Jan 29, 2015 9:11 pm

MargaritaMc wrote:By chance, I've just come across this article in a December 2014 issue of a Science Daily
Quantum physics just got less complicated: Wave-particle duality and quantum uncertainty are same thing

Equivalence of wave–particle duality to entropic uncertainty - Patrick J. Coles, Jędrzej Kaniewski, Stephanie Wehner
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