Can there be black light ? I mean is it possible to devise a machine that outputs darkness ?

Question

I understand there are various colours that light can have. But i was wondering why there is no 'black' light. What is the logical explanation for this ? I mean I am expecting an answer that goes beyond mentioning the spectrum details. All I could think of was a machine as powerful as a blackhole; it could bend the light so hard that all we would see is darkness. But is there any other way ?

P.S. I am a programmer and didn't study much Physics beyond high school. This question is not a goof. I am not asking this question for fun. I seriously have this curiosity.

Answer 1

Such a machine would have to zero out the entropy of the photons it cancels, so it is a form of Maxwell demon. It would have to know exactly which photons it wants to cancel out, and it would have to absorb their entropy and then some. It is impossible to do this at a distance--- you would have to put a black barrier in the path of the photons.

An analogous question: you can make a "heat ray", which heats up objects in its path. Can you make a cold ray which cools everything in its path? Again it is easier to heat than to cool, because cooling requires large entropy production elsewhere, while heating doesn't require anything.

Answer 2

Darkness is the absence of visible light. You can't build a machine to output the absence of something, except insofar as it does not output that thing. So a device that just sits there and doesn't produce any visible light at all (like a lamp that is turned off, or most rocks) is as good an example of this machine as you are going to get.

A black hole is something different: it absorbs all light that falls on it, but it doesn't actually produce a thing called darkness.

Answer 3

This is a very meaningful question in my oppinion. I have heard physicists talk about philosophy of physics classes where they deal with questions like "can light be the absence of darkness"? This might seem like a question lacking concrete implications, but it isn't to at least some extent. Light follows transport phenomena which is distinguishable and test-ably different from darkness. Could you make a device that propagates darkness in the same way that light propagates? No, you can't.

You mention colors, and I think that in the context of colors your question makes more sense. There are multiple combinations of values that can be used to represent a color, and in computer programs you can see some where a saturation value is used, for instance. It would be possible to configure a basis for representing a color where lightness, or conversely, darkness is one of the values involved, sure. But the reality is that colors swaths are not a physical representation of light incident on something in the real world. For the real world we represent light as a spectrum, where there is a specific intensity for every energy/wavelength in the entire real number line. It is more difficult to think of something that represents darkness in a more straightforward sense with the correct physical model.

All I could think of was a machine as powerful as a blackhole; it could bend the light so hard that all we would see is darkness. But is there any other way ?

A black hole always falls short of preventing light from reaching you, with the exceptions of the cases that you are just on the event horizon or below the event horizon. If light is being pulled in, you are being pulled in as well.

Answer 4

There is a kind of quantum optics experiment known as vacuum squeezing. This is accomplished, at its basic level, by sending a strong laser through a sequence of polarizing beam splitters arranged so that light takes only one path (the other paths cancel out). By introducing non-linear elements such as Rubidium-87 vapor, the path with "no light" can undergo something called squeezing. Simply put, the Heisenberg uncertainty relation between position and momentum of photons in the dark state can appear to be violated (although it isn't if you look at the whole Wigner function).

Is this the sort of answer you're looking for? It's kind of a vague question.

Answer 5

my understanding of what you are asking is if there can be light with negative energy. The knowledge we have is that, in the current state of our universe that is not possible. Photons can be either absorbed from a null ray coming from a past cone, or emitted to a null ray going into the future cone. But "black" light would require a photon that will be absorbed from the future cone; this we have not ever observed (or at least, we believe we haven't observed them).

This all goes to the subject of causation; when we make a source to emit light, we believe the light will be there, propagating in the forward cone, both when we measure it and when we don't; like the trees, they will fall regardless if there is someone on the forest. We make the source to emit light by closing a circuit, which flows a current, which heats something.

To an extent, is the 2nd law of thermodynamics that "gets in the way" of us making a target to absorb light, since it would allow us to absorb thermal radiation of objects by throwing "black light" into them. But there are dynamical reasons as well; if we had a source of "negative light" (which is equivalent to a target of positive light), it would be perceived as electromagnetic waves of negative energy, because upon their capture by atoms in the future, they will decrease their energy in order to compensate the energy that was absorbed in the target - Maxwell equations predict a definite positive energy density for electromagnetic waves, so, such solutions are disallowed.

Answer 6

OK.

Define black.

If black is the usual definition of a colour and the expectation is that it has a frequency and energy h*nu then it exists in the infrared and the ultraviolet regions. Human eyes will see a lamp shining in wavelengths outside its receptors in the retina, as black. One can call it a machine generating black light.

Black defined as darkness means no radiative energy

a machine as powerful as a blackhole; it could bend the light so hard that all we would see is darkness

All that would do is create a vacuum , and yes a vacuum would give off no radiation and human eyes would not see anything: darkness.

Answer 7

If one uses a cloaking device, lights flows around the object cloaked so the cloaked space is pure darkness. There is not yet a cloaking device for the visible light, but it has been done for other frequencies as well as for sound. The cloaked room is completly silent with no sound whatsoever.

Answer 8

It may be worth looking into an alternative theory of colour eg. Goethe. He proposes that colour arises from the interplay of light and darkness. Darkness, then, is not the absence of light but the polar opposite. It is my opinion that human consciousness is not ready to arrive at a true understanding of the phenomenon of darkness. We are still grappling with the mystery of light which remains inextricably linked with the phenomenon of fire and the activities of stars - our sun being the most obvious example. Some people believe that thought and consciousness emit a subtle light. It is the earth's atmosphere that possibly provides the most clues in the search for a better understanding of this topic.