In quantum physics, you often deduce that those residents of the micro realm, those elementary particles, have some very strange properties bordering on a quasi-free will. They sort of possess a ‘mind’ of their own. They seemingly have the ability to ‘know’ things about their external world and their relationship to that. They make decisions with respect to those relationships and act accordingly. They are not just little inert billiard balls. There are observations to back this up that include an observation you can make at home to verify this. Look outside your window. What do you see? A very big mystery is what you see, if your window is anything like my window or most windows.
Even if you don’t know or understand very much about quantum mechanics, or quantum physics (same difference), you have probably associated it with weirdness. Unlike the certainty and causality domination of your day-in and day-out macro world, the realm of the quantum is centred on probability, chance and randomness where things happen for absolutely no reason at all and identical scenarios will yield different results. One oft given example you can (and have) witnessed – how light (photons) interacts with a common pane of window glass.
GENERAL DESCRIPTION
Here is a common happening that you have experienced at home or in the office or in the car that you probably never gave a second thought to. That unregistered oddity you experienced is seeing the reflection of AND the passing through of light waves (photons) with respect to a pane of glass simultaneously. What’s so odd about that? Well, what’s odd is that light is both passing through and reflecting from the same pane of glass at the same time. Why both? Why not one or the other scenario? What’s odder still, assuming you are inside, is that not only can you see your reflection or the reflection of what’s in your background but what’s also outside and through your own reflection. You see your reflection and the outside image, both superimposed on top of each other. So photons are both passing through the glass (you can see the outside while you are inside) from the outside to the inside and at the same time reflecting from the inside to the inside (you can see the inside from the inside) both happenings at the same spot on the glass.
And if you go outside the reverse is also true. The outside is partly reflected by the glass surface back to you while you are outside looking in while at the same time light photons from the inside are passing through the entire glass so you can see inside your room though you are standing outside, both inside and outside as superimposed images.
Further, the ratio of pass through to reflection also depends on the thickness of the glass, so presumably the photon ‘knows’ in advance what that thickness is and acts accordingly. If all of that doesn't strike you as odd, nothing will, though it’s so commonplace it probably doesn’t strike you as odd.
OTHER EXAMPLES
This ‘do I or don’t I’ oddity doesn't just apply to panes of glass. This applies to a wide range of transparent, even translucent stuff. The same pass through vs. reflect back applies for example to your eyeball. Some photons enter your eye and deliver their message; some photons hit the identical spot but are reflected back, but can then hit a mirror and reflect back again this time entering your eye so that you see your eye that reflected in the mirror.
Speaking of eyes, you can ‘see’ an external bright light even with your eyelids shut, yet some of the light is also being reflected off the external surface of your eyelids.
Sunglasses are another obvious example. You can see your reflection in the outer side of the lenses, but clearly the sunglasses let through without any obstruction photons too.
You can see your reflection in still water and the bottom beneath the surface too if the water is pretty clear and the bottom is fairly shallow. This should also apply to say a polished diamond or other similar gemstones or crystal(s).
Another visual example – you see sunlight reflected off of the tops of clouds when in an aircraft that’s flying above them. As you descend through them and land, though the day is now overcast, clearly some sunlight photons are passed through the clouds. It’s the same clouds; and the same sunlight; and the same observer; but differing outcomes. So the pass through vs. reflection enigma applies equally to translucent objects (like clouds) too.
Though this is an obviously visual puzzle, well that in itself is obvious since we can only see visible light photons. However, photons come in a wide range of forms, from ultraviolet to radio; infra-red to microwave; gamma rays to X-rays. Presumably this pass through vs. reflection phenomena takes place with non-light photons too. The most obvious example is that radio, TV or cell phone reception tends to be better outside than inside – one reason for your TV aerial or antenna. So, some radio/TV/cell phone photons are reflected off of the outside of your solid building but some pass through too, but this has nothing to do with frequency or wavelength since these transmissions are on a very narrow bandwidth.
In a similar vein, it’s been advocated for decades that the ideal location to do radio astronomy and/or SETI, searching for alien radio signals, is on the far side of the Moon because the Moon’s bulk is 100% opaque to terrestrial and human generated radio signals that just add unwanted noise to the signals the astronomers are looking for.
One clue that the pass through vs. reflection conundrum must be density related, not just thickness related, comes from X-rays. We've all seen X-ray photos of the human hand. The bones stand out; the wedding ring more so, but the flesh is visible too though less so. So some X-ray photons were reflected, greater reflection related to the density of the stuff the X-ray photon was hitting. Yet clearly some X-ray photons passed through since the image of the fleshy bits isn't as strong as the bones and the bones weren't as solid an image as the ring. Yet it was the exact same X-ray dose that hit all three substances – flesh, bone and metal.
THE STANDARD SOLUTIONS
The basic postulate postulated by quantum physicists is that the photon pass through vs. reflection anomaly is an anomaly because it all happens for absolutely no reason at all. It’s all random. It’s all probability. Some photons pass through via the luck of the draw; other photons get reflected by that same random luck of the draw. How is that possible given that we have, in the original example, one identical pane of glass with identical photons impacting? Well, if you don’t invoke causality, you can just about get away with anything anomalous.
The other accepted answer is that any one photon is in a superposition of states. It can be in two places at the same time, so it can both reflect, and pass through the pane of glass at the same time. Either that or the photon has awareness of its external surroundings; it has a mind of its own and decides what it wants to do!
Superposition of state has been experimentally demonstrated via the classic quantum double slit experiment whereby particles, like a photon (but any type of particle will do, like an electron) fired one at a time at two parallel slits, will pass through both slits and thus will interfere with itself and cause a classic wave interference pattern on a target board behind the slits. The only logical conclusion has to be that one particle was in two places at the same time. Personally, I find that absurd, but it’s hard to debate hardcore experimental results.
The one flaw I find in that standard pane of glass situation explanation is that if the photon is in two places at the same time, then both the inside reflected image and the external image – the pass through the glass image – should be equally as vivid. Usually the pass through the glass image is the more obvious of the two superimposed images assuming just one light source, say external sunshine, or the reflected image is the stronger, assuming the prime light source is inside, like say at night.
CAUSALITY & CERTAINTY vs. PROBABILITY & CHANCE
I need state the obvious here – all photons are identical; the pane of glass in question is obviously identical to itself. Therefore, knowing that and only that, one could only conclude that when photon meets window pane, one and only one outcome is possible.
We, the observer say the photon has such and such a probability of going through, or being reflected from, the pane of glass. If seven out of ten photons go through the glass window, then there’s a 70% probability the next photon will go through. Wrong. As far as that photon is concerned, we, the observer, are irrelevant, and it’s 100% certain to either go through the glass or be reflected by the glass. We can be pretty damn sure that a group of photons won’t gather together in the middle of the glass pane and do an impromptu performance of a Wagnerian opera. There’s no probability involved. It’s one or the other. There’s no superposition of state. The photons aren’t in two places at once – passing through and being reflected.
Another way we can be sure causality is operating, albeit going up one level, is that every time you go to the inside of your window pane looking outside, you see both outside and a faint reflection of you and the interior. Not once in a while; not sometimes 100% outside and no reflection; not sometimes a 100% reflection but you can’t see outside (your window isn't a mirror after all), but 100% of the time, each and every time, you see both the exterior outside the pane and the interior reflected inside the pane.
SUMMARY, DISCUSSION & RESOLUTIONS
In summary here, some photons from the inside pass through a pane of glass to the outside; some outside photons pass through that glass to the inside; some photons from the inside reflect off the glass back inside and some outside photons reflect off the glass back outside. The big question is, how does the photon decide what to do? Here comes Ms. Photon heading toward the pane of glass. She has to make up her mind whether to pass on through or reflect back: decisions, decisions. To reflect, or not to reflect, that is the question! IMHO, photons should all go through, or all reflect, from the same pane of clear glass at the same time.
We note from the outset that the glass hasn't been tinted or polarized – not that that would alter the general picture. What we have here is just an ordinary pane of glass.
Further, no external forces are apparently at work here. Both the photons and the glass are electrically neutral. Gravity plays no role and the strong and the weak nuclear forces are only applicable inside atomic nuclei.
To make a long story shorter, causality rules IMHO! Photons are not in a state of superposition; they are not in two places at the same time. Clearly photons are not in a position to ‘know’ anything. Photons have no decision-making apparatus; they have no consciousness of any kind, no free will to be or not to be. That can be demonstrated by adding a little extra thickness and/or density and/or energy.
But first, one could easily suggest that since even seemingly ‘solid’ stuff is 99.999% empty space, that a photon passing through the glass is passing through that entire void, and a photon reflected has hit a glass molecule and bounced back. One exception to that is that the reflection takes place at the surface of the glass pane, none from the interior of the glass. A second exception would be that reflections off of a solid molecular bit in the mainly empty glass pane would be totally scattered in many directions which is what we don’t see. Basic optics – the angle of incidence equals the angle of reflection. Yet clearly if photons are being reflected, they are bouncing off something. Or, perhaps they are being absorbed by the electrons within the glass matrix and then re-emitted, though the photon that’s re-emitted might not be the exact same photon – but that’s of no consequence since all photons are identical.
We note that the greater the thickness or the greater the density the more the pass through to reflection ratio changes. If you look through the exact same pane of glass, but this time edgewise, no photons pass through from one edge to the other edge. The X-ray case study above shows the role of increasing density. Both are an illustration that ultimately things become so thick and/or so dense that while there might not be total reflection, there would be any pass though either. The option for the photon might then be reflection vs. partial penetration. Of course that in itself doesn't explain the either this or that option the photon takes, at least until such time that it becomes one or the other. In a vacuum it’s 100% pass through and 0% reflection; in the case of a metre thick lump of lead, a light photon will 100% reflect and 0% pass through. Restrictions placed in the photon’s way by density and thickness just tends to confirm an earlier notation that stuff is 99.999% void such that pass through equals boldly going through that void; reflection is a collision with that rare bit of stuff that sometimes gets in your way.
But that’s not the entire story. Thickness is also related to opaqueness though they are not the same thing. Photons can pass through Earth’s entire atmosphere from the fringes of outer space to ground level, yet if you dab a smear of black paint on your pane of glass, well that will strop the photons from passing through albeit black paint is a lot less thick than the Earth’s atmosphere.
Energy plays a role too. X-ray photons are more energetic than visible light photons, which is why X-rays are better for detecting structural flaws (like tooth cavities and bone micro-fractures) which are concealed by external surfaces which are opaque to light.
Air and glass are transparent to light photons, but are generally fairly opaque to the less energetic infra-red photons. That’s the general principle or concept behind both the botanical greenhouse and the environmental greenhouse effect, although in the later case not all the components found in air are equally as opaque.
Ultimately invoking variations in properties like density, thickness, energy levels and opaqueness doesn't totally explain why identical particles, with all other factors being equal too, have this Jekyll and Hyde property whereby some do and some don’t; some will and some won’t.
But we see that while things aren't totally explained yet, we’re well on the way to determining the real factors that decide the photon’s fate, and it’s not photon’s free will either.