Black is the absence of light because it absorbs light, but when we create black paint or black objects, light is always reflected, either in all directions in matte or smoothly in shiny black objects, making it never a true black. Would it be possible to use polarization to create an object that does not reflect any light, creating a truly black substance, without any shadows or reflection of light?
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1$\begingroup$ In theory yes. Just use two perfect polarizes and cross them. But I don't understand why this shouldn't have any shadows since having a shadow is independent of the colour of the object. $\endgroup$– GonencCommented May 11, 2015 at 21:24
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$\begingroup$ The shadows were referring to any shadows that fall on the object, not the object's shadow $\endgroup$– Jack HoltCommented May 11, 2015 at 21:27
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1$\begingroup$ possible duplicate of How can opaque black object occur? $\endgroup$– rob ♦Commented May 12, 2015 at 3:38
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$\begingroup$ However, the photo in Floris's answer is much more impressive than mine in the linked question. $\endgroup$– rob ♦Commented May 12, 2015 at 3:39
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5$\begingroup$ At some point, you will have to deal with blackbody radiation causing the object to glow. How would you like that dealt with when answering your question? $\endgroup$– Cort AmmonCommented May 12, 2015 at 5:43
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4 Answers
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The problem with the suggestion of using polarization is that you now have the reflections off the polarizers to contend with.
I think the short answer is "it depends on how 'black' you want it to be". "Truly black" = reflectance of 0. I am quite sure that is impossible - there will always be some probability of light scattering off a surface. All you can do is make that probability "quite small".
The world record for "blackness" appears to be held currently by Ventablack, a material with a special surface structure (nanotubes) that traps incident photons, and reflects less than 0.04 % of incident light. That is indeed very nearly black (but nowhere near "perfect"). Just look at this picture to get a sense of just how black that is. Of course if most cameras have 12 bit sensors, then one LSB is 1 part in 4000 - and 0.04% is 1 part in 2500. So indeed, this is almost invisibly black for a typical camera. Uncanny.
(Image from the above linked source).
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14$\begingroup$ Cool. That looks like a bad job of photo-shopping. I'd like to see it in person. $\endgroup$ Commented May 11, 2015 at 22:03
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5$\begingroup$ Imaging dressing someone up in a full body suite coated in this. They'd look like a walking shadow. Would be absolutely terrifying... The ultimate ghost-ninja. $\endgroup$– BaldrickCommented May 12, 2015 at 10:25
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3$\begingroup$ @Baldrick Until dust settles on it after approximately 10 seconds of use. $\endgroup$– gerritCommented May 12, 2015 at 13:18
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2$\begingroup$ It's like, how much more black could this be? And the answer is, none. None more black. $\endgroup$ Commented May 12, 2015 at 17:04
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The only thing I can think of being true black would probably be a black hole. As light does not bounce off a black hole.
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4$\begingroup$ Although we all know that block holes do glow, right? $\endgroup$– DWinCommented May 12, 2015 at 6:51
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$\begingroup$ @DWin yeaa, but once you get past the glow.. ;) $\endgroup$– AdsyCommented May 12, 2015 at 15:12
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$\begingroup$ The black hole shadow (the region in your sky where photons would have had to emerge from the black hole to hit your eye) is very, very black... although there is some Hawking radiation making it imperfectly black in a mathematical sense for astronomical sized holes. However, the bending of light around the hole also makes it surround itself with Einstein rings that can get pretty bright when a star on the other side is magnified.. $\endgroup$ Commented Feb 28, 2023 at 13:52
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Just to add to the above answers, and since to did not limit your question to the visible range - if you define black as absence of light (photons emitted or reflected), then there is no such substance, because according to black body radiation model, everything with a temperature above absolute zero (which is essentially truly everything in the universe:) radiates, meaning it's always emitting photons, and thus is not black.
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$\begingroup$ Something to ponder: A glowing coal looks red, but according to physics is "black"... $\endgroup$– FlorisCommented Apr 27, 2016 at 12:46
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$\begingroup$ Why would you say it's black? $\endgroup$ Commented Apr 27, 2016 at 13:13
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$\begingroup$ Perfect emissivity = perfect absorptivity = black body $\endgroup$– FlorisCommented Apr 27, 2016 at 13:35
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$\begingroup$ I would argue that coal is nowhere near perfect absorption. Either way, this does not contradict my answer: since even perfect black bodies emit light, and since the PO asked about the complete absence of light, the answer remains that no such object exists. $\endgroup$ Commented May 1, 2016 at 4:31
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$\begingroup$ OP asked about "An object that does not reflect any light" $\endgroup$– FlorisCommented May 1, 2016 at 11:40
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A true black surface would need a temperature of 0 Kelvins as anything with heat will have excited electrons and will emit photons. However, dark matter is an example of purely black matter it only means that dark matter does not consist electrons. Theoritically, a purely black object would be one that does not radiate light or absorb light. As absorbtion of any kind of light also brings along with it heat which again hypes up the electrons. Hence, it is impossible, even theoritically for an object to be purely black.
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$\begingroup$ Dark matter ought to have been called transparent matter. $\endgroup$ Commented Feb 28, 2023 at 13:54