[Eternal Student (OP)]

Hi.

Something else to think about, if it's a quiet day.

The Question:
    Do you think we might be able to see anything that is located anywhere while looking in any direction and regardless of any (blocking) objects that might be between us and the thing we want to see?

Further elaboration on the question because we will probably need special equipment.
   Could we do this if we use a suitable diffraction grating and some suitable computer software to reconstruct the normal (line of sight)  image of that thing from the interference patterns that we might obtain?


Background for consideration:
    Let's just keep it simple to start with.....   
(i)   Suppose we want to see the light just from a simple LED or laser pointer.   We'll assume that emits light of one frequency, say blue light at 620 THz (or wavelength  450nm).
(ii)  Let's keep everything else dark, so we'll be in a big room with no windows or light bulbs except that one LED or laser pointer that is switched on.    However, the room will have other stuff in it.   There will be walls, tables, wardrobes and stuff,  it's just that the only light is from the laser pointer so all of that stuff, inlcuding most of the empty space in the room will look black.   None the less, that other stuff is still there,  if the laser pointer is shining on it, then it will absorb that light and effectively block its progress as usual.   We'll be quite Gothic in style and have all our furniture and walls painted black - we'll just assume they abosrb any of the blue light shone on them and don't really reflect any of it.
(iii)   If we set the laser pointer (or LED) up in some place so that we have a perfectly good direct line of sight to it, then we can see it,  that should be obvious.   Alternatively, we can set the laser up so that we have no direct line of sight to it  (because the wardrobe or something else is blocking it).  It's this situation that we'll be interested in - we'll have no direct line of sight to the laser pointer because something (a wall or the wardrobe) is between us and the laser pointer.
(iv)  Now, without the wardrobe, there was a path for the light to travel from the laser pointer to us.   However, Feynman states that light actually travels along ALL paths between those two points.   We only see it as moving along the straight line path because that's the only place where we get constructive interference of the wave function from a subset of possible paths that are either exactly along that line or else close to being that straight line.   Along any other path, even the sligthest change in that path effectively returns a completely random phase shift in the wave function.   The sum of all these random wave crests and troughs gives us a net 0 amplitude for the wave function in that region and hence 0 probability of finding the light (or even a single photon) in that region.

- - - - - - - - - -
   That's a lot of words and you'll probably find it's been better explained in this popular science video because there are diagrams, pictures and a practical demonstration.
   The video is called  Something Strange Happens When You Trust Quantum Mechanics  and is produced by the PopSci YouTube channel known as "Veritasium".      It's 33 minutes so you may not want to watch it all.   
   You can take your pick of either of these two 5 minute sections:
You'll find the theory of light taking all paths at timestamp  20.00  (approx), the 7th section of the video,   which then leads in to the practical demonstration at timestamp  25.00 (approx) and is the 8th section of that video.
   

Best Wishes.

 - - - - - - -

[alancalverd]

Feynman states that light actually travels along ALL paths between those two points.

Er, no. We can model propagation that way, but there's a difference between a model and reality.

As for a conundrum, you have painted all the surfaces black so any light not travelling in a straight line from A to B will be absorbed by something.

And a second conundrum. Consider a single photon travelling from A to B. It transfers energy  hν to B. If you put your wardrobe in the way, any photon reaching B cannot have energy hν because some of the energy will have been absorbed by the wardrobe.

Your choice of a laser source is important. In principle a laser system could emit single photons with no dispersal angle, so Huygens' construction, which is pretty much the same as the "all paths" model, clearly doesn't make sense - a bullet cannot split into multiple parts and recombine into the same bullet if  the multiple paths have different lengths, and some of them involve sandbags.

So you won't see whatever emits the radiation, but you could see a degraded spectrum of scattered radiation, from which you could deduce the existence of something behind the wardrobe.

[Eternal Student (OP)]

Hi.

Er, no. We can model propagation that way, but there's a difference between a model and reality.

     How would we know what the reality is?   One reasonable approach is to find the best model, one that explains and predicts most (preferably all) the final outcomes.   That model is then likely to be the actual reality of what is happening.   However, yes, in principle it may not be the actual reality of what is happening, just the best model we have so far.
     Let's avoid the philosophy of the nature of reality:   I don't need to know what the reality is.   All we need is that the model works and predicts outcomes.   If we can model light as taking all paths between two points, then let us do that and consider the paths that go in some curve around the wardrobe.

- - - - - - - - - - -
    I could spend more time addressing each sentence you posted but then you'd have to read all of that and it's not an efficient use of your or anyone else's time.
    I suspect you haven't been able to glance at practical demonstration given in the YT video I posted in post #1 and that's OK.   Your time is precious and anything that isn't written onto the forum doesn't have to be looked at, it's just optional and it might even be harmful to click some link etc.
   So, I'll just try and put the experiment into words and pictures here:

We want a laser and a wardrobe but there is only one person who has to be using all the equipment at the same time.   So we'll use a mirror to bounce the light off,  then one man can hold the laser and just point it at the mirror,  since the mirror will bounce the light rays they can still be effectively at the other side of the wardrobe to see what can be seen from beyond it.    It's hard to put a wardrobe on the mirror, so we'll just put a black sheet of card over about one half of the mirror,  that will do the same job as the wardrobe.

Diagram:


laser-mirror.jpg (24.62 kB . 540x507 - viewed 879 times)

    The light ray bounces off what we'll say is the middle of that mirror.   Now we put some card over a bit more than half of the mirror, so that we certainly cover up that middle point on the mirror where the ray was assumed to be hitting and bouncing off.
    The camera mounted above this equipment will then see no light at all, as you would expect from a classical understanding of what is happening.   The laser was sending light only along one narrow beam and the place on the mirror where it hits is being blocked.   (Well, to be honest, the card they used in their demo is a bit reflective and you do see a spot of light on the card where the beam hits it  - but the main point is that you cannot see the laser emitter or the spot of light that would be way over to the left as far as the camera is concerned and is the reflection of the laser emitter in the mirror).
     However, if we put a fine diffraction grating over the left hand side of the mirror,  i.e.  to the left of the wardrobe / card then we actually do see something on the camera.    Classically, there was no light being sent to this region (that is the region of the mirror that is to the left of the middle point where the incident ray seemed to strike), the light was only travelling in a single straight line along a narrow beam. 

Diagram:


laser-mirror-wardrobe.jpg (42.36 kB . 540x507 - viewed 871 times)


    The experiment is an attempt to illustrate that, regardless of the classical model, the light from the laser has actually been there or travelled that path over to the left of the card / wardrobe even though we do not ordinarily see it coming from there  (all we usually see is just the light along a staright line path from the laser emitter).   The diffraction grating assists in reducing the destructive interference that usually occurrs so that there is only some constructive interference in that region and we can see some spots of light.
      The last 5 minutes of the YT video I linked to in post #1 has that demonstration, if you have just 5 minutes to spare,  I'd strongly advise watching just that bit of the video.

Best Wishes.

[Eternal Student (OP)]

Hi again,

   This was only intended as something to pass the time.   It seemed quite incredible to me but I'm not everyone.
Some people might prefer to consider the practical applications.  So, one example might be that the defence forces don't have to use surveillance drones because a line of sight may not be required.    With a few hundred years to get all this tech working,  then a military intelligence officer can just sit in his/her office and hold up a diffraction grating to view the enemies forces over on the other side of the world.   More importantly, a parent actually will be able to see their children steal the cookies from the jar, even if the child thought they were out of sight.

Best Wishes.

[alancalverd]

One reasonable approach is to find the best model, one that explains and predicts most (preferably all) the final outcomes. 

Problem is that we don't have a single model that works for photons - or, it seems, anything else! Our best model for the propagation of light, electrons, and even buckyballs, seems to be a wave model, but when any of these things interact  with a detector, we need a particle model.

Anyway, I've now looked at the parts of the video that you highlighted. Whilst the theory is clear, it's actually a poor demonstration and doesn't, for instance, explain where the refracted dot appears, or why its movement isn't obviously correlated with the movement of the primary dot. I think our colleague Hamdani could provide a more convincing demonstration.

That said, there were plenty of experiments  in "Over The Horizon" radar in the Sixties, attempting to pick out small disturbances in a signal backscattered from the ionosphere, caused by a rapidly moving object like a plane or a missile.....

[Eternal Student (OP)]

Hi.

   Yes, I actually do see what you're saying.
   It could be explained as some single edge diffraction that is occurring.   Specifically, the incoming ray was in the vicinity of the edge of the wardrobe / card.   The diffraction around the card is potentially as important as anything happening in the diffraction grating.   The single edge diffraction around the piece of card may be sufficient to explain how some light would be found over to the left of the card.

    I still quite like the notion of light taking all paths    BUT   I hadn't thought of single edge diffraction and the subsequent possibility of explaining the outcome using only classical models of light propagation.   Thanks for your time and input.

Late Editing:   When I watched the video again carefully, there actually is a relationship between the the "primary spot" as you called it  (where the laser was pointed to) and the behaviour of the dots over to the left of the card.   While they don't move much, they do become more intense as the laser is pointed closer to the edge.

Best Wishes.

[alancalverd]

Which makes me think that the diffraction grating is actually generating an image of the "loom" of the laser - the light scattered off-axis by the structure of the laser unit, atmospheric dust and moisture....all the stuff that allows us to "see" a lighthouse that is below the horizon, or know that a laser pointing away from us is switched on.

[Petrochemicals]

It seems the equivalent of fluorescent tubes lighting up beneath power lines, I suppose with the right equipment it is possible.

Years ago during the reign of analogue there was a rumour that the CIA could read your TV screen from an adjoining room, I cannot find anything due to the modern world clogging it up with spying smart TVs etc.