[garadin76]

Noise cancelling headphones reduce the amount of background hum that you can hear whilst listening to music, although I am no physics expert, I understand that they work by monitoring the sound waves of the ambient noise, and produce 'inverted' sound waves, to effectively balance the waves into neutral. (apologies if this isn't totally correct)

So...If we can do it with sound waves, is it possible to do it with light waves to create some kind of darkness emitter?

I suspect total darkness couldn't be created in a real world situation, due to ambient light bouncing off all sorts of different things, but surely it could make things generally darker even if it couldn't be used as a basic cloaking device.

What would the limitations be in the real world? Could it at least work theoretically?

[erchanbold]

Noise cancelling headphones work by sampling the ambient sound and replicating it - but upside down. They invert the sound. When the ambient sound and the inverted copy combine, they cancel.

I guess theoretically, you could do something similar with light, but you would need a light generator capable of reproducing the spectrum of the ambient light and inverting it.

You could test it by simplify the situation a good bit. Use a laser as an illuminating light source. Since you only have to deal with one frequency of light, things should be much easier. Now get a second laser, tuned to the same frequency. You will need to tweak the second laser until it is outputting light exactly 180 degrees out of phase with the first one. I think that it may be possible to create destructive interference and make it impossible to see the scene, i.e., create darkness.

[lifton]

Noise cancelling works only at a few points - in headphones, it is designed so that the cancellation happens at the entry of the ear (canal).

For light, the identical effect can be seen in the famous experiment where light travels through two small holes and cancels itself at certain places on a screen behind the holes - there is actually "darkness" there (only light from other sources will prevent that it's absolutely dark there).
However, as before, the cancellation happens only at certain points.

Moreover (but I'm guessing here!), sound amplitudes simply add up, so different frequencies are no problem. With light, there are different photons for different frequencies, and as far as I know, you'd have to cancel each wavelength in the light separately (that's why the experiments are almost always done with coherent mono-chromatic light - not your typical light in real life).

[yoel]

That's quite an interesting question :)

If we used these terms i would have to guess that the answer is no, simply because "light" refers to...well, light haha :). And so if we created something that gave off darkness and called it a light then that wouldn't be relevant to the definition of light.
But I'm just being picky, I'm sure we could create something like that in the future haha.

[balaraj]

no
if you think about it the 'noice cancelling headphones' are only blocking sound in a concentrated spot, the ear hole. Whereas the 'darkness emitter' as you call it is focusing on a whole area. Darkness is only the absence of light i cant be created.

[dewey]

In order to be invisible one would have to arrange for the the light from things BEHIND one to pass through! (Yes, you could have some kind of "force field" that would bend the light around you just right but I, for one would not like to stand INSIDE such a force field!

[chanan]

You can absorb light, but you can't "create darkness". If you had a magnet strong enough to bend light, then you might be able to absorb it at a common point, and neutralize it.

[brayden]

Yes

When you turn it off ......

[druson3]

No.
Light is nothing but electromagnetic waves of certain wavelength and frequency which our eyes can percieve and understand.
Darkness (also called lightlessness) is the absence of light. Scientifically it is only possible to have a reduced amount of light.
However; light cannot simply be absorbed without limit. Energy, like visible light, cannot be created or destroyed. It can only be converted from one type of energy to another. Most objects that absorb visible light reemit it as infrared light.[2] So, although an object may appear dark, it is likely bright at a frequency that a human being cannot see. For more information see thermodynamics.
Look up darkness in Wiktionary, the free dictionary.
A dark area has limited light sources, making things hard to see. Exposure to alternating light and darkness (night and day) has caused several evolutionary adaptations to darkness. When a vertebrate, like a human, enters a dark area, its iris dilates, allowing more light to enter the eye and improving night vision. Also, the light detecting cells in the human eye (rods and cones) will regenerate more unbleached rhodopsin when adapting to darkness.
The scientific definition of light includes the entire electromagnetic spectrum, not just visible light, so it is scientifically impossible to create perfect darkness. For example, all objects radiate heat in the form of infrared light and gamma rays, extremely high frequency light, can penetrate even dense materials.