Is "visible" light, actually visible? Claiming that visible light is called visible light and therefore it must be visible, is circular reasoning. This question is not about the definition of visible, because in that regard, light shows none of the characteristics of visible objects. Light is in fact, what makes objects visible.

Now I'm not talking about wavelengths we can't detect or even light that doesn't strike our eyes. I'm specifically referring to detectable light that strikes our retina. Many will see this as a futile argument about definitions until they actually grasp the differences and realize the implications.

The purpose of vision, what gives us an evolutionary advantage, is that it allows us to see things. For those unfamiliar with the concept of indirect realism, here's a link. http://cns-alumni.bu.edu/~slehar/webstuff/book/chap1.html . What we see are objects, like predators, food and possible mates. These objects exist in objective reality (outside our heads) but we perceive our brains representation of these objects in our subjective reality (the reality we perceive inside our heads).

DETECTION IS NOT PERCEPTION.

Our eyes detect light, but detection is a mechanical process of which we are not directly conscious of. We can deduce that the process is occurring due to the fact that we see objects and understand the visual process. Seeing something, on the other hand, is a conscious process. We sometimes don't even see things which are right in front of our eyes and I don't mean figuratively. Have you every moved something out of the way while looking in the fridge, when the thing you're looking for is the thing you moved? Seeing is not perceiving. Perception is consciously seeing something. Detection is not perception.

DO WE SEE OBJECTS OR LIGHT.

The most common held belief is that we perceive light, not objects. As far as detection goes, that's true. Our eyes detect light that strikes the retina. This begins the physiological aspect of vision. Phototransduction, electrochemical impulses travelling to the visual cortex via the optic nerve, the subconscious creating visual representations and sensations. All these processes are subconscious. It's only then that conscious perception comes in. And what we consciously perceive are the objects, from which the detected light, originates. So we do not see light, we see the (brains representation) objects.

LIGHT, LIGHT AND LIGHT.

Light, the word, has many meanings and this adds to the confusion of whether we see light or not. We have heavy and light, darkness and light, electromagnetic radiation and figuratively, seeing the light, which represents comprehension. We can see if something is heavy or light. We can see brightness. But brightness is not a property of electromagnetic radiation. Brightness is a sensation, like colors. If colors are the interpretation of lights wavelength, then brightness is the brains interpretation of lights amplitude. Now many people believe seeing light (brightness) is seeing light. That if you shine a laser into your eye, you're seeing light. In a sense, they're right. We are perceiving brightness. But this brightness is a result of our cones being -saturated. If you look at a 60w globe and hold your gaze steady, your eyes will adjust to the brightness. After a few seconds you begin to see the element from which the light emanates. Brightness is an obstruction, preventing us from seeing the object itself. Brightness is phenomenal in nature. Not a property of light itself.

SEE AND SEE.

The word see, too has different meanings. The expression "to see the future " has two interpretations. One as a psychic, having visions of the future, and one as a visionary, able to predict future trends. To consciously perceive or to consciously conceive. So we can see "conceive " light but not perceive light.

WHAT DO WE ACTUALLY DETECT?

A photon is a boson particle. It can't actually be detected. What we detect is when a photon strikes something. It's the collision we detect. At the moment of collision, a photon no longer exists. Everything we know about light, is deduced by detecting these collisions. Light itself is undetectable. It neither emits, nor reflects anything which would allow us to detect it.

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Even after reading this, it seems like a futile argument about definitions. Do you mean that the way looking into a source of light makes us think of light is different from the way looking at a door makes us think of doors?

I'm saying detection is a mechanical process of which we aren't conscious. Even image creation in the visual cortex is subconscious. We only become conscious of a few objects. We conclude that our eyes detect light, but we do not actually perceive light itself.

This seems ... uncontroversial; I would be surprised if more than a tiny fraction of LW readers thought otherwise. Your post seems to me to be making at length a point that's already well understood around here. (That's not necessarily a bad thing -- it depends on what your goals were in writing it.)

(Though I think there is some scope for definitional arguments around "we do not actually perceive light itself". In normal circumstances we perceive (say) an apple by detecting the light emanating from it that reaches our eyes; but suppose some cunning configuration of optical elements produces a light field near-identical to the one an apple would have produced without there being any apple-like object involved; we might want to insist that there's something in the real world that we're perceiving even if it isn't really an apple, and that light field might be a candidate for that something.)

Incidentally, I think the last paragraph of your post is simply wrong; I would say that a photon can be detected, and the way we detect it is by giving it the opportunity to be absorbed by an electron in suitable circumstances. So no, light isn't "undetectable" even though photons can't be detected without absorbing them. This, again, is a disagreement about definitions rather than about the underlying facts.

As for your last point, I understand that you consider the absorption of the photon as detection. By your logic, a rock detects light. Do rocks also feel cold?

(It feels to me as if I'm trying to have a serious discussion while you're trying to score points. I hope this impression is wrong.)

The absorption-by-an-electron mechanism is exactly the same one you describe as how photons (or, as you would prefer to say, collisions between photons and other things) get detected. Obviously that absorption is only the first step in the process, and obviously there are instances of photon-absorption that don't lead to anything we would want to call detection. What I was saying (and I apologize if I was insufficiently clear) is that the process we both agree happens, where a photon excites an electron and the electron then does other things whose effects end up including, e.g., certain kinds of effects in a person's brain or a digital camera's memory circuitry, can just as well be called "detecting a photon" as "detecting a photon collision".

I think your final question is more snark than actual argument, and I hope the foregoing paragraph has indicated why the snark is not appropriate. But I'll answer it anyway: rocks don't generally "detect light" in any useful sense, but in any case they come closer to "detecting light" than to "feeling cold" because feeling is a term that we use specifically to denote processes in an actual brain. One could reasonably say that a thermostat "detects low temperatures" but not (other than as deliberate anthropomorphism, perhaps for fun) that it "feels cold". Similarly, I would be happy to say that a rod or cone cell in a human retina "detects light" but not that it, say, "sees the sun"; or that whatever organs in the human body respond to cold -- I realise that I have no idea offhand what they are -- "detect low temperatures" but not that they "feel cold".

I'm trying to point out the difference between detecting something and detecting it's effect. We detect the spike in energy resulting from light striking something. https://books.google.co.za/books?dq=do+we+see+or+feel+photons%3F&hl=en&id=rPNHAwAAQBAJ&lpg=PA109&ots=z-SPeSNkqN&pg=PA109&sa=X&sig=DNk__1lCk-GcaYSeMXSSoBeUsFs&source=bl&ved=0ahUKEwiytpf1m43TAhWBCMAKHZ9vAUEQ6AEIQjAH#v=onepage&q=do%20we%20see%20or%20feel%20photons%3F&f=false

That gives me a "you have reached your limit and can't read any more" message. I found what seems to be the same book on Amazon UK and tried their "look inside" feature but failed to find anything saying anything to do with seeing or feeling photons.

Anyway. Whether to say "we detect photons" or "we detect photons striking our retina" or "we detect photons interacting with electrons in rhodopsin in our rod and cone cells" or "we detect electrical impulses in our retina arising from photon-electron interactions" or whatever is, it seems to me, a matter of terminology only. We're describing the same process in any case. You (if I'm understanding you right) consider it definitely wrong to say that we detect photons, and I don't yet understand why. (I can think of some possible reasons but I don't find any of them convincing and I would rather not argue against a straw man.)

Am I correctly understanding your position? If so, why do you consider it wrong to say that when a photon interacts with an appropriate electron in a rod or cone cell in a human retina, that photon has been detected? What bad consequence ensues from using the word "detect" like that? (Or, if your objection isn't about bad consequences: how is using the word "detect" like that inconsistent with other usages we're attached to? Or ... whatever it is that's wrong, what's wrong?)

The point is that a photon is a boson particle. At the moment we detect a collision, the photon ceases to exist. Prior to the collision a photon existed. We can only ever detect where and when a photon has struck something. Never the photon itself.

I know that photons are bosons. I know that they cease to exist when they interact with electrons. What I don't understand is why you think that those facts (which are not in dispute) make it wrong to say that we detect photons.

About your example, would a reflection from a mirror qualify? How about a rainbow? Isn't that why the images produced are called virtual images? And don't we see objects?

With a reflection from a mirror, there is an actual object responsible for that light field, which is (at least) very similar to what we seem to be seeing; I think it's obviously reasonable to say in that case that what we are seeing is the thing in the mirror. But suppose the actual optical apparatus consists of a bunch of lasers and diffractive optical elements and things, and there is no good candidate for "what we're perceiving" that in any way resembles the apple we seem to be seeing. Personally I'm happy just saying that there isn't any thing we're perceiving in that situation, or perhaps that we're perceiving a non-existent apple, or that we're perceiving an illusion of an apple, or something. But I can see how someone might find "we're perceiving the light" a less unsatisfactory answer than those.

A rainbow is an interesting example. When I "see a rainbow", would you say I'm actually perceiving a rainbow? If so, just what is the rainbow? What kind of thing is it, in your account of the world?

I agree that we see objects. That's the usual case. That's why my weird example in which we might want to say something unusual is one which in some ways greatly resembles the usual case of seeing an object, but in which it's not clear that there's any good candidate for "the object we're seeing".

A rainbow consists of millions of tiny reflections of the sun, off the inner, concave , surface of raindrops, having undergone refraction. You're seeing multiple reflections of the sun.

When I look at a wall illuminated by the sun, I am also seeing lots of tiny reflections of the sun. So if you're saying that when I look at a rainbow, the object I am actually perceiving is the sun, why does that not apply equally to the wall of a building?

An obvious alternative would be to say that when I look at a rainbow, what I'm seeing is lots of water droplets. That seems unsatisfactory too, because if I look at the rainbow for a minute then typically there are no water droplets in common between the starting and ending configurations. So maybe I'm seeing an assemblage of water droplets which is stable in something like the way our bodies are stable even though their material is mostly replaced every few weeks. I don't much like that either, though I have no very concrete objection to make to it. The alternative I actually prefer is to say that these questions are mostly about words rather than about things, and that there's not much value in picking specific answers to them.

Light from a building is dispersed, light from a mirror is reflected. With dispersed light, we see (in our minds ) the object dispersing the light. Light disperced by raindrops, makes the raindrops visible. Light reflected by raindrops makes the sun visible.

I think you may be making a distinction that isn't really there. The way in which light from a building is dispersed is by being reflected in different directions off lots of little bits of surface. ... Having written this, I wonder whether it's quite right. If you point a laser at a rough surface, you get a "speckle" effect that derives from interference between different paths the light can take from the laser to your eye; perhaps the appearance of ordinary rough surfaces owes something nontrivial to such interference effects and it's therefore not adequate to think of it as the result of lots of tiny reflections. It seems to me that the answer to the question "what object, if any, am I looking at when I see a rainbow?" should probably not depend on this level of careful physical analysis.

Let me ask a slightly different question. It seems like you're very sure that there is a Right Answer to that question. Why? I would say that there probably isn't a Right Answer, because I don't think the terms in the question are precisely enough defined for there to be one in difficult cases; and I would say that of course we can make more precise definitions that make there be a definite Right Answer, but I don't see much need to do so. In any tricky case where that answer really matters I would expect to find that the question we really need an answer to is something else that's better addressed directly.

No, not like that. That's a question about words more than about things. (At least, it seems so to me.) I can't offhand think of any situation in which answering the rainbow question would really matter, or really seem to matter; but if there were one, the sort of "something else" I would expect to be more fruitful to address would be a question about the underlying physics.

How about this, where does a rainbow exist? Is a rainbow the process which results in a light pattern? Or is a rainbow the arch of colors we perceive in our indirect version of reality?

There is no such thing as "where a rainbow exists". Rainbows aren't (in so far as they're properly considered things at all) things that have locations. What a rainbow has is more like a direction: it is located around the cone at an angle of 42 degrees from the line between the sun and you. (Which means that in some sense differently located people see different rainbows, though in practice it's more convenient to express things differently and treat rainbows seen by two nearby people looking in similar directions as "the same rainbow".)

I wouldn't say that a rainbow is a process; I don't see any way in which that makes anything clearer. I don't think I'd say it's "the arch of colours we perceive in our indirect version of reality" either, because if you make a rainbow strictly an artefact of human perception then you have trouble dealing with the fact that e.g. a camera looking in the same direction as you will record "the same" rainbow as you do.

So what? A camera records a light pattern which it later emits to our eyes, resulting in a visual representation. A rainbow is link any other image in a mirror. A virtual image. It exists in the mind of the observer. The same way two people see two different images in a mirror. Technically, we each see two images. One for each eye. We also see two rainbows, uncles we are looking at an image on a screen.

If a rainbow is something that happens in the mind of the observer then it is not possible for a camera to take a picture of a rainbow. At best, it can take a picture that will strike a human observer as rainbow-like, or something like that.

And, sure, you can choose to define "rainbow" that way, as referring to what happens in a person's mind when they look towards a region where there are lots of water droplets illuminated by a light source behind that person. But I don't see why we should define "rainbow" that way.

(I don't think we have any disagreement about what's actually happening in the world when someone "sees a rainbow".)

That's my point, very few people understand the process, but they can all See the rainbow. It is common usage that a rainbow is the perceived arch of colours, not the process.

A tiny fraction, you say? We'll see! Having argued this point I can tell you from personal experience that a large portion of readers believe we can see actual light.

A large portion of what readers? LW's population is a bit unusual.

Going by the first comment, I'll reserve judgement go now.

https://www.lesserwrong.com/posts/7X2j8HAkWdmMoS8PE/disputing-definitions