# Why can't people be invisible?



## kythe (Feb 22, 2014)

Sorry if this is an elementary question, but I've never taken a physics class.  

In "The Invisible Man", H.G. Wells explains the refractive index.  His character invents an apparatus which can change the refractive index of all the parts of the body so light simply passes through and he is invisible.  But that book was written well over 100 yrs ago and clearly no one has found the secret to invisibility so far.  There is obviously a glaring error somewhere.  

We can be "partly" invisible, in the sense that we have x-rays which can see through the skin. But I suspect that works differently since its still not with the naked eye - the x-ray image must be translated into a visual form.  Also, it still doesn't penetrate the entire body.

Can anyone elaborate?


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## Nick B (Feb 22, 2014)

Refraction isn't like going through an object, it is light being bent by an object.  An electromagnetic field will bend light and at the moment it is the most likely candidate to be able to create an invisibility cloak, you create an strong electromagnetic field around the object and light literally bends around said object so what you see is light reflected from behind it.


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## chrispenycate (Feb 22, 2014)

An invisible man would also be blind. And while Wells used an albino hero (to avoid the problem of pigmentation) there are lots more components of the body, such as the iron in red blood corpuscles, that would go on being essential. Humans are not designed for a jellyfish's metabolism with all transparent components.

So far our best hope for a tarnhelm is a sophisticated camouflage with sensors on one side, amplified and retransmitted from the other. Not as elegant as changing your internal light absorptive/reflective characteristics, but remarkably effective - when seen from one angle. The invisible tank. So far, measuring the directive characteristics of the incoming light and duplicating on the emitters is beyond us, but there is no theoretical reason why this can't be beaten, giving us full 360° coverage.


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## Grimward (Feb 22, 2014)

*Grins*

I'm both invisible and inaudible to my teenagers when they don't want something; does that count?


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## Mirannan (Feb 22, 2014)

Another route to this is the one played for laughs by both Douglas Adams and Terry Pratchett - the "somebody else's problem" field. Difficult to do; it would require ubiquitous nanotech or some similarly powerful future tech. You're in plain sight but people simply don't notice you. With more work, this could apply to machinery as well.

There is even precedent for this phenomenon, called "attention blindness". An amusing experiment was done a few years ago, in which people were asked (and given monetary incentive) to pay close attention to the number of passes to and from a particular person on a basketball court. Something like 90% of the experimental subjects completely failed to see the man who was dancing around on the court in a gorilla suit for about 2 minutes during the experiment.


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## Stephen Palmer (Feb 22, 2014)

In one of his many fantastic books, Daniel Dennett pointed out research that shows much of what we think we're seeing is actually being "filled in" for us by our unconscious minds.

I think this was in 'Consciousness Explained' ?


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## Nerds_feather (Feb 22, 2014)

Actual invisibility may be impossible, but cloaking devices are not.


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## kythe (Feb 22, 2014)

Nerds_feather said:


> Actual invisibility may be impossible, but cloaking devices are not.



These are some interesting developments, but even this does not explain Star Trek's cloaking device.  Of course, that is more of a plot device whenever they want a ship to appear out of nowhere.  The cloaked ship is invisible to the naked eye as well as all scanners and sensors.  Yet even Star Trek does not tackle invisibility for individual people.


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## Parson (Feb 25, 2014)

kythe said:


> These are some interesting developments, but even this does not explain Star Trek's cloaking device.  Of course, that is more of a plot device whenever they want a ship to appear out of nowhere.  The cloaked ship is invisible to the naked eye as well as all scanners and sensors.  Yet even Star Trek does not tackle invisibility for individual people.



But "My Favorite Martian" did.


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## SevenStars (Feb 25, 2014)

It's not really making people  invisible, however, Michio Kaku has an alternative idea in this clip 


http://www.youtube.com/watch?v=Za72ZFwjkjUhere


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## Gramm838 (Feb 25, 2014)

How do you know that the people replying to this thread aren't invisible? You can't see any of us, but text appears on your screen...


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## Aquilonian (Mar 1, 2014)

Mirannan said:


> Another route to this is the one played for laughs by both Douglas Adams and Terry Pratchett... You're in plain sight but people simply don't notice you. With more work, this could apply to machinery as well.
> 
> There is even precedent for this phenomenon, called "attention blindness". An amusing experiment was done a few years ago, in which people were asked (and given monetary incentive) to pay close attention to the number of passes to and from a particular person on a basketball court. Something like 90% of the experimental subjects completely failed to see the man who was dancing around on the court in a gorilla suit for about 2 minutes during the experiment.



Have experienced the above at one of Derren Brown's live shows. Why bother with being "invisible" if you can just be "non-noticeable" or "instantly forgettable?" There are occult techniques for this- as practiced by the Japanese Ninja, though doubtless they exaggerated their powers for intimidatory effect. (A good excuse also for bodyguards who had failed to protect their lord from assassination!)  I have used these techniques with some success, although it's very hard to maintain if anyone appears whom you really care about, as your attention is automatically drawn to them and theirs to you. A busy public bar is a good place to learn about this. 

As regards technology, a light-sensitive suit that would simply collect light falling on your back and display it on your front seems most plausible, but it would need a lot of computing to prevent distortion. Alternatively, a beam focused on the viewer's retina to create an additional "blind spot" that exactly covered your image. Advantage would be that the brain would probably automatically "pull the edges together to fill the gap" as with the normal blind spot. Disadvantages are that (a) the technology does not yet exist at all, and (b) you'd need to target every potential viewer seperately.


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## BigBadBob141 (May 12, 2014)

One unwanted side effect of true invisibility (if such a thing is possible) is complete blindness.
Your sight relies on the fact that the lens in your eyes focuses light and the retina at the back of your eyeball absorbs it.
But with both being invisible your eye can neither focus or absorb light, therefore you would be sightless.
And if the tissue of your body was 100% invisible what about the food digesting in your stomach or the waste matter in your bowels.
Yuk!, I don't old H.G. thought of that.


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## Mirannan (May 12, 2014)

Aquilonian - Agreed about the public bar part. Unfortunately, I seem to have the power of attentional invisibility which only works on bartenders.


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## Rafellin (May 12, 2014)

Who says some people can't be invisible?

Because if they can, do you really think they'll be telling anyone?


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## kythe (May 13, 2014)

Rafellin said:


> Who says some people can't be invisible?
> 
> Because if they can, do you really think they'll be telling anyone?



Yeah, like that one guy in the movie "Mystery Men".  He could turn invisible - but only when no one was looking!


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## JoanDrake (May 14, 2014)

"That ******* hacked my eyes!!"


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