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Posted: July 27 2013 at 14:53
Dean wrote:
BaldFriede wrote:
I just want to mention that it is equally impossible to become invisible. Or rather, it is possible but with a serious "but": Anyone who is invisible automatically becomes blind.
There is another form of invisibilty and that's the Klingon cloaking device, a mechanism which bends light around the object being concealed. The prinicples of this have been demonstrated at single EM frequencies and it does work, however broadband versions may prove to be impossible due to refraction and masking every single footprint the ship leaves behind may also prove to be impossible.
Sorry, but the same principle applies here. If light is bent around an object then the object will certainly become invisible to the world, but the world will also become invisible to anyone within the object. How should the visual information reach anyone in the object when the light is moved around it? You are literally "being left in the dark". "Seeing" always means "Interacting with the light"; it has to be absorbed at least partially. But this very absorption is what makes an object visible.
So a person who wanted to become invisible would at least have his eyes appear to be visible. or else he or she would be blind. Would be a funny sight indeed to have a pair of eyes floating around in the air.
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Posted: July 27 2013 at 16:12
BaldFriede wrote:
[QUOTE=Dean][QUOTE=BaldFriede] Sorry, but the same principle applies here. If light is bent around an object then the object will certainly become invisible to the world, but the world will also become invisible to anyone within the object. How should the visual information reach anyone in the object when the light is moved around it? You are literally "being left in the dark". "Seeing" always means "Interacting with the light"; it has to be absorbed at least partially. But this very absorption is what makes an object visible.
So a person who wanted to become invisible would at least have his eyes appear to be visible. or else he or she would be blind. Would be a funny sight indeed to have a pair of eyes floating around in the air.
Sorry but this is not correct. Invisibility can be one-sided only. If someone can not see you it means that the light coming from behind you is manipulated so as to get around you and out again in front of you. Whether you can still perceive the light coming from your observer in front of you does not necessary follow. Transparent materials are invisible and of course they interact with light, they receive light and can detect that light before refracting it. Does not glass get warm when you place it in sunshine? that heat is light photons being absorbed by the glass molecules, yet the glass is still transparent and invisible.
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Posted: July 27 2013 at 16:42
Gerinski wrote:
BaldFriede wrote:
[QUOTE=Dean][QUOTE=BaldFriede] Sorry, but the same principle applies here. If light is bent around an object then the object will certainly become invisible to the world, but the world will also become invisible to anyone within the object. How should the visual information reach anyone in the object when the light is moved around it? You are literally "being left in the dark". "Seeing" always means "Interacting with the light"; it has to be absorbed at least partially. But this very absorption is what makes an object visible.
So a person who wanted to become invisible would at least have his eyes appear to be visible. or else he or she would be blind. Would be a funny sight indeed to have a pair of eyes floating around in the air.
Sorry but this is not correct. Invisibility can be one-sided only. If someone can not see you it means that the light coming from behind you is manipulated so as to get around you and out again in front of you. Whether you can still perceive the light coming from your observer in front of you does not necessary follow. Transparent materials are invisible and of course they interact with light, they receive light and can detect that light before refracting it. Does not glass get warm when you place it in sunshine? that heat is light photons being absorbed by the glass molecules, yet the glass is still transparent and invisible.
Sorry, but the error is on your side. Your assumption that only the light coming
from behind the visible object is important for an object to become
visible as a kind of obstruction to it is wrong. ANY light interchanging
with the object that is visible, and that means also any light coming
from the front falling on the object and either being reflected or
absorbed by it counts. And then the principle explained by me comes into
a action.
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Posted: July 27 2013 at 16:42
BaldFriede wrote:
As to speed of light: It can be argued that everything moves at the speed of light, only in four dimensions, the fourth being what we notice at time. The Lorentz transformations make immediate sense when one thinks of them as the projection of four-dimensional movement into three-dimensional space.
This is correct in some certain sense.
BaldFriede wrote:
What's more: This view can also explain the duality of wave and corpuscle; you just have to think of a different concept of corpuscle. Instead of "mass bends time-space" you have to think of "mass IS bent time-space". We don't get the fluctuation of these waves though because as we move through that fourth dimension we always stay at the peak of the wave, therefore it appears to be steady.
This is crackpot. Particle-Wave duality has nothing to do with General Relativity (which was the domain of the first sentence and also of the 'mass bends spacetime' one), and certainly nothing with mass either.
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Posted: July 27 2013 at 16:46
BaldFriede wrote:
Your assumption that only the light coming
from behind the visible object is important for an object to become
visible as a kind of obstruction to it is wrong. ANY light interchanging
with the object that is visible, and that means also any light coming
from the front falling on the object and either being reflected or
absorbed by it counts. And then the principle explained by me comes into
a action.
Nope. There are significantly transparent animals who can see. There's no reason why an artificial eye could not be made with transparent materials. One thing has nothing to do with the other.
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Posted: July 27 2013 at 16:47
Gerinski wrote:
BaldFriede wrote:
As to speed of light: It can be argued that everything moves at the speed of light, only in four dimensions, the fourth being what we notice at time. The Lorentz transformations make immediate sense when one thinks of them as the projection of four-dimensional movement into three-dimensional space.
This is correct in some certain sense.
BaldFriede wrote:
What's more: This view can also explain the duality of wave and corpuscle; you just have to think of a different concept of corpuscle. Instead of "mass bends time-space" you have to think of "mass IS bent time-space". We don't get the fluctuation of these waves though because as we move through that fourth dimension we always stay at the peak of the wave, therefore it appears to be steady.
This is crackpot. Particle-Wave duality has nothing to do with General Relativity (which was the domain of the first sentence and also of the 'mass bends spacetime' one), and certainly nothing with mass either.
Not true. Do you know the famous Einstein thought experiment about riding on a photon?
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Posted: July 27 2013 at 16:54
Gerinski wrote:
BaldFriede wrote:
Your assumption that only the light coming
from behind the visible object is important for an object to become
visible as a kind of obstruction to it is wrong. ANY light interchanging
with the object that is visible, and that means also any light coming
from the front falling on the object and either being reflected or
absorbed by it counts. And then the principle explained by me comes into
a action.
Nope. There are significantly transparent animals who can see. There's no reason why an artificial eye could not be made with transparent materials. One thing has nothing to do with the other.
"Transparent" is by no means the same as "invisible". You can still see those animals, don't you, however transparent they may be. That means that a lot of light passes through them, but still a little part is absorbed, else you would not be able to see them at all.
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Posted: July 27 2013 at 17:05
BaldFriede wrote:
Not true. Do you know the famous Einstein thought experiment about riding on a photon?
What about it? that was in the domain of Special Relativity, you are mixing yet a different thing. Particle-Wave duality has nothing to do with it either.
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Posted: July 27 2013 at 17:08
BaldFriede wrote:
"Transparent" is by no means the same as "invisible". You can still see those animals, don't you, however transparent they may be. That means that a lot of light passes through them, but still a little part is absorbed, else you would not be able to see them at all.
An object does not need to absorb zero light to be perfectly transparent (in fact there is absolutely nothing (consisting of ordinary baryonic matter) which does not absorb a single photon, even the single electron in an hydrogen atom absorbs a single photon (and re-emits it when it goes back to its previous energy state), so according to you nothing could ever be invisible).
Is air not invisible enough for you? do you think air does not absorb light photons? lots of them, more than enough for it to see (if it could) much better that our eyes can (our eyes have detection limited to the optical part of the spectrum, air absorbs any kind of frequencies so it can 'see' radio waves, ultraviolet, infrarred or whatever you want).
I repeat, you could build an artificial eye which was more than transparent enough for being invisible for all practical purposes. It would still be able to perfectly detect light, why shouldn't it?
If you are interested the materials for the 'cloak' Dean talked about are called metamaterials and are an active field of research, their multiple potential applications are more down to Earth that achieving invisibility but their ability to make something within them invisible has been experimentally confirmed (within a limited frequency range).
Incidentally, they are one of the many examples in science and technology which teach us the lesson to be very cautious in saying that anything is 'impossible', they have the property of having a negative refractive index, which was for a long time considered impossible in classic optics.
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Posted: July 27 2013 at 17:32
And Lorenz transformations do not really say what you say about moving through spacetime, they just mean that relativistic physics is frame dependant so are free to use whatever frame of reference you want. For any process you can change the frame of reference to whatever other you want and the transformations show that the physics are the same. The statement you used is just an easy way to grasp this concept, but it's not strictly correct.
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Posted: July 27 2013 at 17:44
BaldFriede wrote:
Dean wrote:
BaldFriede wrote:
I just want to mention that it is equally impossible to become invisible. Or rather, it is possible but with a serious "but": Anyone who is invisible automatically becomes blind.
There is another form of invisibilty and that's the Klingon cloaking device, a mechanism which bends light around the object being concealed. The prinicples of this have been demonstrated at single EM frequencies and it does work, however broadband versions may prove to be impossible due to refraction and masking every single footprint the ship leaves behind may also prove to be impossible.
Sorry, but the same principle applies here. If light is bent around an object then the object will certainly become invisible to the world, but the world will also become invisible to anyone within the object. How should the visual information reach anyone in the object when the light is moved around it? You are literally "being left in the dark". "Seeing" always means "Interacting with the light"; it has to be absorbed at least partially. But this very absorption is what makes an object visible.
So a person who wanted to become invisible would at least have his eyes appear to be visible. or else he or she would be blind. Would be a funny sight indeed to have a pair of eyes floating around in the air.
So the concept of a two-way mirror is impossible? A cloaking device is an illusion - smoke and mirrors - it is not true invisibility. Some flatfish mimick invisibility by changing their skin pigment to match the surface they rest on yet they can still see perfectly. A cloaking device could perform the same illusion for a single view, but it gets complicated when there are multiple viewpoints; bending light around an object suffers from the same problem, so as I said - it may prove to be impossible to achieve perfect broadband invisibilty. However, that does not mean the hidden object will be blind (because it is not true invisibility, it is still a camouflage trick)
To produce the illusion of invisibilty you do not have to produce perfection, all you need to do is be able to fool the sensors of whatever it is you are hiding from. This we have demonstrated using stealth technology on aircraft where we only have to fool a radar into not seeing the aircraft: The F-117 Nighthawk is invisible to radar, yet we can see it withthe naked eye. If we are hiding in the visible light spectrum from things with visible-light eyes we could still "see" in other frequencies such as infra red or ultraviolet using suitable cameras, and yes, we would be detectable in those frequencies but we would be invisible to the naked eye.
You can only see something that emits, reflects or blocks light; to give the appearence of invisiblity to an observer all you need to do is bend the light around the object you are trying to hide and not reflect back any of the light coming towards it. In single plane cloaking (like a flatfish) you only need to fool a viewer in one direction: the light coming towards him from behind you would need to be bent around you, and all the light coming from him towards you would have to be absorbed completely which means you would still see him.
To be invisible in multiple planes simutaneous is more complex (probably too complex, but hey-ho), but since you are creating an illusion rather than perfect invisibility you would still be able to see. Like a two way mirror which reflects most of the light but allows some to be transfered throught the half-silver backing, the bending of the light around you does not have to be 100% for you to appear to be invisible to multiple observers, you could permit some small fraction of the light to be absorbed (ie not bent) and, by using photo-multipliers if necessary, use this to "see".
In the case of a transparent object we can detect the object because the light is refracted, not because some of the light is absorbed. Our eyes are not sensitive enough to detect that a small portion of the light is absorbed by the transparent creature, (or by a sheet of glass come to that - we see transparent objects by refraction, to prove that submerse a small Pyrex™ bowl in a larger bowl filled with vegitable oil - because the Pyrex and oil have the same refractive index the smaller bowl becomes invisible). In a light-bending invisiblity illusion device the refractive index is unchanged and any small fraction of the light that is absorbed rather than bent would be hard to detect yet would allow us to see.
Only the most sophisticated imaging equipment would be able to detect that a fraction of the light has been aborbed by the "invisible" object so to all intents and purposes the object is invisible and it can see.
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Posted: July 27 2013 at 17:44
Gerinski wrote:
BaldFriede wrote:
"Transparent" is by no means the same as "invisible". You can still see those animals, don't you, however transparent they may be. That means that a lot of light passes through them, but still a little part is absorbed, else you would not be able to see them at all.
An object does not need to absorb zero light to be perfectly transparent. Is air not invisible enough for you? do you think air does not absorb light photons?
I repeat, you could build an artificial eye which was more than transparent enough for being invisible for all practical purposes. It would still be able to perfectly detect light, why shouldn't it?
"Invisible" means "You can't see it". Period.
The question should rather be: Why should it? And how efficient would such an eye be? The more transparent it is the less it does react with light. But reaction with light is important for seeing.; it is actually all that seeing is about. There is a reason why night-active animals like cats have eyes that especially reflect the light.
The more transparent an eye becomes the less efficient it is. You would need a lot of light to be able to see with such an eye.as you describe..
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Posted: July 27 2013 at 17:48
BaldFriede wrote:
Gerinski wrote:
BaldFriede wrote:
"Transparent" is by no means the same as "invisible". You can still see those animals, don't you, however transparent they may be. That means that a lot of light passes through them, but still a little part is absorbed, else you would not be able to see them at all.
An object does not need to absorb zero light to be perfectly transparent. Is air not invisible enough for you? do you think air does not absorb light photons?
I repeat, you could build an artificial eye which was more than transparent enough for being invisible for all practical purposes. It would still be able to perfectly detect light, why shouldn't it?
"Invisible" means "You can't see it". Period.
The question should rather be: Why should it? And how efficient would such an eye be? The more transparent it is the less it does react with light. But reaction with light is important for seeing.; it is actually all that seeing is about. There is a reason why night-active animals like cats have eyes that especially reflect the light.
The more transparent an eye becomes the less efficient it is. You would need a lot of light to be able to see with such an eye.as you describe..
Nope, read the edit of my previous post, I thought you were off-line so I elaborated a bit. Again, air is invisible and I have no doubt that the quantity of photons it absorbs is more than enough to see perfectly.
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Posted: July 27 2013 at 18:00
And transparency or opacity has nothing to do with absorbing light or not. Every object, transparent or opaque absorbs and re-emits light, whether the light will be re-emitted in a way that will still get out from the back of the object rather than reflected simply depends on the way the molecules and atoms are arranged.
Most solids are opaque because their atoms are arranged in complex irregular patterns so it's very difficult that any wave can cross much of them without being excessively absorbed and reflected. Many liquids are transparent because their molecules are arranged in a pattern, so light of the correct wavelenght can find stable distances of the required fixed lenght so as to navigate around the particles (still much light is absorbed, but many is re-emitted in the correct direction so goes through. This is also why crystals, even being solid are often transparent. The molecular density of many gases is very low so light waves have little trouble getting through and that's why many gasses are transparent.
Incidentally, this is also why sometimes you may hear people say that 'glass is a very dense liquid' which sounds fanciful but is not true. In glass the correct arrangement of the molecules is achieved by heating them and them cooling them very quickly, then they arrange themselves in a fixed pattern. Glass is more like a crystal than a 'dense liquid'.
If you put a piece of a bit thick glass and a piece of iron in the sunshine they will both absorb and re-emit the same number of photons (possibly glass a bit less since a few may achieve passing through without really hitting any atom, which if the glass is a bit thick is really unlikely too, and with the possible difference of the 2 pieces ending at slightly different temperature), the only difference in that in glass a big part of the the re-emission is successfully directed in the desired direction (although it certainly reflect a significant part too in the direction it came from), while iron will reflect them all back to the side they came from.
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Posted: July 27 2013 at 18:54
BaldFriede wrote:
The question should rather be: Why should it? And how efficient would such an eye be? The more transparent it is the less it does react with light. But reaction with light is important for seeing.; it is actually all that seeing is about. There is a reason why night-active animals like cats have eyes that especially reflect the light.
Not sure what your cats eyes example is getting at because it tends to support Gerard's assertion rather than yours - the tapetum lucidum layer is behind the cat's retina and reflects light back through the retina to allow the rod photoreceptors a second chance to react to the photons of light entering the eye. This increases sensitivity without the need to make the eye bigger and so absorb more of the incoming light - like an image intensifier that works on photon multiplication it allows the cat to see at light thresholds seven times lower than the human eye - its interaction (or reaction) with the incoming light is significantly reduced. In normal circumstances the cats eyes do not glow (otherwise that would be a dead giveaway to any prey they are stalking)
The thing with eyes is they do not need to be very big in proportion to the size of the body to be able to see the entire scene in front of you, which means you don't have to "absorb" very much of that light to see it (in the case of the cat, because of the tapetum lucidum layer that can be seven times less light than a human). You could compute the geometry of this based upon the pupil size and the total viewing area, which would give you a figure that represents the proportion of light coming towards you that actually enters the eye. My guess is the amount of light that the eye absorbs in seeing in relation to the total "area" of light in the scene is too small to measure. Another way of computing it would be to calculate the pupil area in relation to the whole body area since the body area would represent the amount of light that needs to be bent in order to be invisible and the pupil area would represent the amount of that light that would have absorbed in order to see. A crude estimate would give 250mm² for the eyes and 1m² for the body, or a ratio of 0.025% - again I'm guessing that is too small to measure.
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Posted: July 27 2013 at 19:13
BaldFriede wrote:
This model very nicely explains why nothing that has a mass can ever reach the speed of light in our three dimensions, because no matter how you increase thhe spedd in our three spatial dimensions there will always be a rest of speed along the fourth axis time. The relativistic corrective SQRT(1- v"/c") , or as I prefer to write it (SQRT((c"-v")/c") actually is the sinus of the angle the wave has with the time axis (any mathematician worth his salt should immediately spot the similarity of that corrective and the Pythagorean theorem with c being the hypotenuse and v one of the cathetii of a rectangular triangle).
This may all sound strange at first, but calculate it through. It works!
I have to admit I've not given this any thought and simply read it and moved on this afternoon, but the use of Pythagorus' theorem here reminds me more of simple vector addition than it does orthogonal sine waves, and since we are talking about the resultant effect of two velocities, then the former looks more convincing.
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Posted: July 27 2013 at 19:20
Dean wrote:
BaldFriede wrote:
Dean wrote:
BaldFriede wrote:
I just want to mention that it is equally impossible to become invisible. Or rather, it is possible but with a serious "but": Anyone who is invisible automatically becomes blind.
There is another form of invisibilty and that's the Klingon cloaking device, a mechanism which bends light around the object being concealed. The prinicples of this have been demonstrated at single EM frequencies and it does work, however broadband versions may prove to be impossible due to refraction and masking every single footprint the ship leaves behind may also prove to be impossible.
Sorry, but the same principle applies here. If light is bent around an object then the object will certainly become invisible to the world, but the world will also become invisible to anyone within the object. How should the visual information reach anyone in the object when the light is moved around it? You are literally "being left in the dark". "Seeing" always means "Interacting with the light"; it has to be absorbed at least partially. But this very absorption is what makes an object visible.
So a person who wanted to become invisible would at least have his eyes appear to be visible. or else he or she would be blind. Would be a funny sight indeed to have a pair of eyes floating around in the air.
So the concept of a two-way mirror is impossible? A cloaking device is an illusion - smoke and mirrors - it is not true invisibility. Some flatfish mimick invisibility by changing their skin pigment to match the surface they rest on yet they can still see perfectly. A cloaking device could perform the same illusion for a single view, but it gets complicated when there are multiple viewpoints; bending light around an object suffers from the same problem, so as I said - it may prove to be impossible to achieve perfect broadband invisibilty. However, that does not mean the hidden object will be blind (because it is not true invisibility, it is still a camouflage trick)
To produce the illusion of invisibilty you do not have to produce perfection, all you need to do is be able to fool the sensors of whatever it is you are hiding from. This we have demonstrated using stealth technology on aircraft where we only have to fool a radar into not seeing the aircraft: The F-117 Nighthawk is invisible to radar, yet we can see it withthe naked eye. If we are hiding in the visible light spectrum from things with visible-light eyes we could still "see" in other frequencies such as infra red or ultraviolet using suitable cameras, and yes, we would be detectable in those frequencies but we would be invisible to the naked eye.
You can only see something that emits, reflects or blocks light; to give the appearence of invisiblity to an observer all you need to do is bend the light around the object you are trying to hide and not reflect back any of the light coming towards it. In single plane cloaking (like a flatfish) you only need to fool a viewer in one direction: the light coming towards him from behind you would need to be bent around you, and all the light coming from him towards you would have to be absorbed completely which means you would still see him.
To be invisible in multiple planes simutaneous is more complex (probably too complex, but hey-ho), but since you are creating an illusion rather than perfect invisibility you would still be able to see. Like a two way mirror which reflects most of the light but allows some to be transfered throught the half-silver backing, the bending of the light around you does not have to be 100% for you to appear to be invisible to multiple observers, you could permit some small fraction of the light to be absorbed (ie not bent) and, by using photo-multipliers if necessary, use this to "see".
In the case of a transparent object we can detect the object because the light is refracted, not because some of the light is absorbed. Our eyes are not sensitive enough to detect that a small portion of the light is absorbed by the transparent creature, (or by a sheet of glass come to that - we see transparent objects by refraction, to prove that submerse a small Pyrex™ bowl in a larger bowl filled with vegitable oil - because the Pyrex and oil have the same refractive index the smaller bowl becomes invisible). In a light-bending invisiblity illusion device the refractive index is unchanged and any small fraction of the light that is absorbed rather than bent would be hard to detect yet would allow us to see.
Only the most sophisticated imaging equipment would be able to detect that a fraction of the light has been aborbed by the "invisible" object so to all intents and purposes the object is invisible and it can see.
A two-way mirror is indeed impossible; so-called two-way mirrors have a very brightly lit room on one side and a darkened on the other. But with a simple trick you can look through it from "the wrong side": Lean your eyes against the glass and put your hands around your eyes to shut out the light. You can then easily look through that two-way mirror from the wrong side.
I won't comment on the flatfish since it is clearly visible, just indistinguishable from its surroundings.
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Posted: July 27 2013 at 19:30
BaldFriede wrote:
A two-way mirror is indeed impossible; so-called two-way mirrors have a very brightly lit room on one side and a darkened on the other. But with a simple trick you can look through it from "the wrong side": Lean your eyes against the glass and put your hands around your eyes to shut out the light. You can then easily look through that two-way mirror from the wrong side.
Yep. But the trick does work, and I did say it was an illusion. Smoke and Mirrors.
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Posted: July 27 2013 at 19:46
BaldFriede wrote:
I won't comment on the flatfish since it is clearly visible, just indistinguishable from its surroundings.
I did say it mimicked invisibilty, by way of illustrating how light bending cloaking would work.
Here is a crude example of a non-bending technique that produces a similar effect (sure it's not invisible, but that's just a limitation of technology )
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Dean wrote:
