Black Friday

By Phil Plait | November 26, 2010 6:51 am

"How much more black could this be?
And the answer is none. None more black."

— the eminent cosmologist Nigel Tufnel, PhD.

Today in the U.S. is (only semi-jokingly) called Black Friday, because it’s traditionally a big shopping day for the holidays, which means crowds and madness.

blackSo I was thinking about the term, which made me think about the adjective: black. What does it mean?

When I was a kid we’d argue if black was a color or not. Of course it is, some kids would say: there’s a crayon called black, and that is — for a kid — clearly the definitive source of evidence. But then someone would point out that black is the lack of color, the lack of light.

That’s correct. Black isn’t a color any more than male pattern baldness is a hairstyle. The lack of something usually isn’t something. I’ll leave it to you to argue over whether 0 is a number*.

So if black is the lack of light, can something every truly be black?

wiens_plotThat’s an interesting question. First, I’ll note that in science there is this thing called a blackbody: an object like this would absorb all radiation that hits it, and warms up. As it warms up, it actually re-emits the absorbed energy in a specific way. Most of the energy is emitted at a certain wavelength of light depending on the temperature (the hotter an object is, the shorter the wavelength of the peak; see the plot shown here or read about Wien’s Displacement Law), with some given off at shorter and longer wavelengths. The graph of this looks a bit like an off-kilter bell curve. This idea is a very powerful one, since many objects behave in a manner pretty close to blackbodies: stars, planets, your oven top, and even you (your peak wavelength is well out into the infrared, at about 10μ roughly 12 times the reddest wavelength your eye can see).

So, paradoxically, to a scientist something that is black actually does emit light and color. But scientists are weird, and we’ll leave them to their theories. What about everyday folk? If I wanted to point out something really black to them, could I?

I think the answer is no. Look at it this way: what’s the blackest thing you know?

Space. Everyone says that as an analogy: "black as space", or "dark as night". The sky is black at night, right?

Well, no, not really. The air above us actually does glow a bit, and even though it’s very difficult to see with the eye, telescopes are plagued by it. It’s one of the main reasons we launch telescopes into space. Above the air, the sky is much darker.

OK, fine. So then surely we can say outer space, deep space, is black. Yes?

Well, again, no. Even space itself isn’t really, and can never really be, black.

First of all, space isn’t ever truly empty. Even in deep space, the void between galaxies, isn’t totally empty. There is an average of roughly one subatomic particle per cubic meter out there, which is incredibly rarified — at sea level here on earth, a cubic meter of air has 1025 molecules in it: that’s 10,000,000,000,000,000,000,000,000 molecules! So space is pretty empty, but not completely empty.

OK, fine, I hear you say. Then let’s pick a cubic centimeter of space (roughly the size of a mini-marshmallow) that really is empty. Literally, zero subatomic particles. What about that?

Marshmallow of Science, sacrificing allWell, even then it’s not empty. Sure, it’s empty of matter, but it still has energy! On average, every cubic centimeter of even deep space has photons passing through it — including light left over from the Big Bang itself. I remember calculating that number back in grad school; I can’t verify it, but I remember it being the equivalent of about one photon of visible light per cc (or more if they are lower energy; we did the math to get the energy per cc, so there are more photons if they all have lower energy).

Then you might argue that we could block the light from passing through our particular cubic centimeter of otherwise empty space, making sure it has no photons in it. OK fair enough. You might also argue that this isn’t really energy in the volume of space itself, it’s just passing through, and doesn’t count. What then? Is our little cube of the heavens black yet?

Well… no. Sorry. There’s one more thing to consider, and if your brain isn’t stretchy and tired yet, this might do it. Basically, space itself has energy. Not passing through it, not contained in it like a fence, but part of the very fabric of space. It’s less like space is a box confining energy, and more like space is a thing itself, a physical substance, that can have energy.

Yeah, I know. We’re used to space being a framework, a mathematical construct against which we measure things like distance, velocity, and so on. But in reality, space is a thing, with physical properties. It can even change shape: it can be warped, distorted, stretched and compressed!

whydoese=mc2This is a consequence of Einstein’s relativity theory, and pardon me if I don’t go into details here; I don’t want to get too far off in tangents. But you can read a book by my friend Brian Cox called Why Does E=mc2 if you want more on this. But for now, let’s just say that space itself can have energy in it. That’s where all the current thinking in physics is leading.

How much energy does space have? Ah, that’s the million dollar question (literally; whoever answers it successfully will probably win a Nobel Prize). Some estimates put it at a very tiny 10-15 Joules per cc. A Joule is a pretty small unit of energy in human terms; it’s about the same amount of energy you expend to pick up a shoe off the floor. So 0.000000000000001 of them is small indeed… until you remember how many cubic centimeters of space there are out there. If you enclosed the solar system in a sphere, using Neptune’s orbit as the radius, space would contain roughly 1030 Joules of energy, enough to power the entire United States for tens of billions of years.


And that’s the lower limit. Some theories of how much energy is stored in the vacuum are far, far larger than that. The problem here is that we’re not sure how this works. The physics is relatively new, and it’s very difficult to figure out (let alone measure) how much energy is stored there.

And it gets worse, sorta. Dark energy is the term given to a kind of pressure exerted on space itself, causing the Universe to expand ever-faster every second. It was only discovered in 1998, and we know very little about it. For example, it may be related to vacuum energy, or it might be something different. It’s possible it might be a finite amount of something spread out over the Universe, or it’s possible every little parcel of space gets its own amount. If the former is true, then it gets spread out over time, getting more and more diffuse as the Universe expands. If it’s the latter, it actually increases, because there’s more space every day as the Universe gets bigger! So it’s possible the amount of dark energy in each cc of space is constant, or it might be getting smaller.

Arg! OK, enough! What the heck does all this mean?

Beats me. Except I think that no matter where you go, no matter how far you travel, space will always have something in it. Matter, energy (dark of otherwise), marshmallows (mini or otherwise), whatever.

It also means a true vacuum, something literally empty, is also something literally impossible to achieve.

So the real answer to what black is — excluding the Friday after Thanksgiving — may only be theoretical. But we do know the answer to Dr. Tufnel’s query: lots.

Things can be lots more black.

Image credit: Wikipedia, NASA/ESA/Hubble

* Oh, I can’t leave that hanging. My opinion: it depends on context. Zero is a number if it’s being used as one, as in "2 – 2 = 0". But it’s not a number if it’s used to describe something that isn’t there, like "I have zero dollars." In that case it’s an amount, or really the lack of an amount.


Comments (94)

Links to this Post

  1. BaslerCast - Is anything really black? | November 29, 2010
  1. Kirk Aplin

    I think zero is a number. A number answers the question “How much?”. Nothing is just a valid an answer to that question as something, so zero is just a valid answer as π. What’s your definition of a number?

  2. Pete Jackson

    So if empty space has energy, then it has mass according to E = mc**2. Then this mass should cause space to attract to itself by gravity, or curve all by itself as per general relativity. Wouldn’t the self-gravity then cause space itself to ‘clump up’?

  3. Hey, the colors in that Wikipedia graph are backwards. That made the physics part of my brain hurt!

  4. xander

    Zero is a member of the set of real numbers. It is also a member of the set of integers and the set of rational numbers (which can both be viewed as subsets of the reals). It is not, normally, considered to be a member of the set of natural numbers (the counting numbers). So perhaps context is important, as if we are considering the natural numbers, then 0 is not part of the set that we are considering. But in that context, negative numbers are not numbers, either.

    I also don’t really understand your distinction between a number and an amount (or, rather, I disagree with the way in which you characterize it). Amounts can be described by numbers. Amounts themselves are probably not numbers—amounts are rather concrete, while numbers are abstract—but amounts can still be described as numbers.

    The example that you give of money is actually a perfect example of this. When I get paid, I have some number of dollars. When I spend that money, I have fewer and fewer dollars (all amounts, all described by numbers). When I spend all of my money, I have $0.00. If I have a credit card and keep spending, I now have a negative amount of money, which is also an amount described by a number.

    Put another way, if I have no money, I can describe this as follows: the amount of dollars that I have (or don’t have) is nothing, nada, zilch; the number of dollars that I have is 0.


  5. In theory, black is not a color. In practice it is.

    What I’m curious about is what something would look like if it were truly black.

    For example, if somebody wore a black pair of pants you would not be able to see them, correct? Black being the absence of light means nothing gets reflected back to your eyes.

    But it’s also a solid object which means you can’t see through them.

    So what I think is you’d see nothing with the outline of a pair of pants.

    One thing that amuses me as a model builder is showing them a model and asking them what color a black part is.

    When they answer, “Black, of course!” I show them a black part that is darker. I can do that three or four times because I never use black to represent black. It’s too black. :)

    So I use dark grays which come across as black in context. Kind of like the optical illusions that Phil posts where two colors are identical but look completely different in an image due to surrounding colors.

    Anyway, my understanding of black in a technical sense is that in light it is the absence of light but in pigment it’s actually very, very dark blue and that black can’t even be achieved. Not sure about that last part but it’s probably true.

  6. Nigel Depledge

    What could be blacker? Easy. Disaster Area’s stunt-ship.

    “It’s this wild colour scheme that freaks me. Every time you try to operate one of these weird black controls, which are labelled in black on a black background, a small black light lights up black to let you know you’ve done it. What is this, some kind of galactic hyper-hearse?”

  7. Nigel Depledge

    Cafeen Man (4) said:

    Anyway, my understanding of black in a technical sense is that in light it is the absence of light but in pigment it’s actually very, very dark blue and that black can’t even be achieved. Not sure about that last part but it’s probably true.

    Yes, in chemistry class at school, our introduction to chromatography had us separating “black” ink into its component dyes using water and filter paper. It was a bit of a surprise to see that it contained, among other things, dyes that were blue, purple-ish and yellow.

    A single chemical substance that absorbs all visible wavelengths of light would be very hard to make, because of the required bandwidth. Far easier to mix up lots of dyes that, in combination, absorb most of the visible wavelengths.

    Then again, isn’t the remnant of a burned-out comet nucleus (i.e. one with no ices remaining) pretty darn black?

    And what about the dark side of Iapetus? That’s pretty black, too.

  8. Terry

    “What is the blackest thing you know?” Um… buckminsterfullerenes?

  9. caprica six

    Wonderfully probing article. Much to cerebrally digest without going into extreme tech terms. I know one thing for certain – I won’t be contributing any joules to ‘black friday’ in the vacuum of any stores. The vacuum of our atmosphere will have to steal some from me at home researching this, lol. Thanks for this.

  10. Weird: when I clicked on the footnote link in the RSS, it linked to this (from the previous post):

    “Though you have to be careful; the two images show different things in different colors, so a cloud layer that is dark in one image may be bright in another.”

    Which sort of made sense, but seemed to contradict your point.

    Personally, I’ll keep using ‘black’ to mean ‘anything very dark’ which means that sometimes the same thing will be black and sometimes ‘light’ or even ‘white’.

    I recall an excellent rant by Stephen Fry not long ago about pedantry…

    If you enclosed the solar system in a sphere, using Neptune’s orbit as the radius

    Meee-ow! What did Alan Stern ever do to you?

  11. Joe

    Did someone say “black”?

  12. Chris

    Don’t forget the neutrinos that are passing through every cm^3 of space.

  13. RL

    This post reminds me of the difference between a scientist and an engineer.

  14. Brian

    I’d argue that color is more a function of perception than it is of wavelength; different species and even different individuals perceive colors differently, and there are limits to human vision, hence false color images. So from that perspective, everything below the point at which the eye detects no light is ‘black’, because our brain doesn’t see anything (hallucinations aside).

    If you were locked up in a completely sealed room with no ‘light sources’, yes there would be molecules, and energy, and you and the wall and the air would be emitting light in the IR, but it’s still black.

  15. John

    The NYT this morning claimed that “Black Friday” refers to the hope that many retailers’ profits for the year switch from a loss (“red”) to a gain (“black”) as the Xmas shopping mania gets underway.

  16. Terry

    @John (15):
    That’s the story I’ve heard. Unfortunately, the name “Black Friday” predates that explanation.

  17. Alex

    Zero isn’t a number; you can’t divide by it.

  18. Charles Schmidt

    Yes John, it is the change in the color of ink in book keeping and had no other point than that.

  19. Dunc

    What about the event horizon of a black hole? That’s pretty black.

  20. Romulus Von Flex


    John is right that is the reason it is called black friday. I don’t know where you heard about it being because of being crowds and madness.

  21. …what’s the blackest thing you know?

    My cold, hard heart the morning after the mongrel* hordes have gone back to their far-off homelands, leaving behind overstuffed trash cans, picked-clean turkey carcasses, clanking recycle bins filled with empty bottles of cheap domestic beer, and the lingering tang of too many people whose names I don’t remember clinging to the linens and window shades, now drawn tight against the chilly November air.

    *Trust me on this one.

  22. Daniel J. Andrews

    It isn’t the ink, but what the ink colour refers to–i.e. a profit versus a loss, which in a hyper-capitalist society is something to ‘worship’ and build vast ideologies around. So it is reasonable to imagine that is how the name Black Friday came about. But Terry points out the name predates that explanation so I’m still in the black…err…dark….

    Edit to add: Kuhnigget, lol! Good one!

  23. Kimpatsu

    Phil, if black isn’t a colour, then neither is red, because bees can’t see it. (To them, it’s “infra-yellow”). What you mean is “is black a colour WITHIN THE SPECTRUM VISIBLE TO HUMANS?”, which is not the same question at all.
    Shame on you for being so anthropocentric.

  24. CS

    I don’t think it is correct to say that black is definitely not a color. It depends on which definition you use: the scientific sense in terms of wavelengths or the visual sense in terms of our perception of how on object looks. In the scientific sense, black is not a color because color is defined by frequency. In the visual sense (which is what most people are referring to when speaking of color), black is a color. OED even explicitly states black and white are included in the latter definition.

    Saying a black object does not have a color is just conflating the two related but non-identical definitions.

  25. I have been deep underground in caves. When you turn out the light, it is so black that the random noise in your retina becomes distracting.
    That’s black.

  26. Perfect! Bravo! I’ve been waiting for a physicist or other scientist to admit this for a long time! People always talk about “nothing” or “empty space”. But of course, as you’ve shown here there is no such thing. This is why I’ve oft been heard to say “I don’t believe in nothing”. People try to talk about nothing like it EXISTS, they even give it properties. I of course think the term “nothing” is about as useless as “supernatural”. But then, I’m a philosophical naturalist. This also would be a good thing to bring up when creationists and other theologians bring up the whole “Something from Nothing” argument. There is absolutely no evidence that the universe started from “nothing”. In fact, I would argue that that is a non-sensical concept.

    Thanks for the awesome post Phil. :)

  27. noen

    Zero is the empty set.

    Colors don’t exist.

    “When I was a kid we’d argue if black was a color or not.”

    Black is the color that some pigments can have. What we call color is simply “the way that some things interact with light.” Because the way that some things interact with light is usually very consistent we are justified in giving that interaction a name, blue for instance, but we shouldn’t think of color as a property that somehow inheres in things.

    “Except I think that no matter where you go, no matter how far you travel, space will always have something in it.”

    Therefore it is incorrect to refer to virtual particles as an example of how something can come from nothing. Since they are not generated from “nothing” but come from space itself, which you say is itself a substance. That eliminates some replies to the cosmological argument. Interesting.

  28. Black isn’t a color any more than male pattern baldness is a hairstyle.

    Is that your fancy way of saying that you have black hair? 😉

    Great post, by the way!

    (Oh, and I finally watched the second TiVoed episode of Bad Universe, yesterday. It was awesome! Anyone know if there’s any news on the series?)

  29. Gary Ansorge

    The blackest black must be a large black hole, which, as long as it’s potential Hawking radiation is “cooler” than the CMB, must absorb ALL radiation/matter.

    So black you can’t see it with anything less than a gravity wave detector,,,darn, I guess it’s white in the gravity wave range,,,

    ,,,never mind,,,

    Gary 7

  30. Brown

    A good astronomy question is: what color is Earth’s Moon?

    Most people would say “white,” since it certainly LOOKS white against the blackness of a starry sky. And yet, the Moon reflects about as much light as asphalt, which most people would call “black” without hesitation.

    The astronauts who went to the Moon, and who knew in advance that the Moon was a relatively dark body, were nevertheless surprised by the blackness of the rocks that they found on the “white” Moon.

    Every now and then, someone says that “Black is not white and white is not black.” And yet, the Moon is an object that could be truthfully be called either black or white.

  31. Daffy

    One of the classic movie lines!

  32. Black is a colour, if you define colour as “some combination of R G and B components”, or “something with hue, saturation and luminance”. That’s good enough for me (you can tell I use computers a lot…).

    Black is not a colour if you define colour as “something with a definite hue”, for the same reason that the North and South poles don’t have a longitude; but then, by that definition, white is not a colour either – and neither is any shade of grey (i.e. a flat spectrum in the visible range), for the same reason that a circle of latitude (e.g. the equator) doesn’t have a longitude. I think that’s stretching it, so this line of reasoning leads you down a sticky trail indeed.

    As for zero not being a number – that’s just absurd! This kind of talk has always bewildered me (and I’m a maths graduate – but perhaps that’s WHY it bewilders me. Hmmm..). Zero is an integer: integers are not NATURAL numbers, but they are numbers nonetheless. To say zero is not a number is like saying that the equator is not a latitude! Insane.

    But anyway, let’s cut to the chase here: what’s the blackest thing that exists? Well, yes, I agree with Nigel in #6 that there’s Disaster Area’s stunt ship (or the Hagunenon Admiral’s ship, depending on whether you follow the radio version) – but I think it might be a toss-up between this and PRIESTS’ SOCKS. 😀

    P.S. Apologies to Americans for “grey”, “colour” and “maths” :)

  33. MadScientist

    Proof that black is *not* a color:

    1. get a shoebox (or any large box)

    2. paint the inside a nice bright color – white will do nicely. Alternatively, glue white paper to the inside of the box.

    3. make a small hole in the middle of one side of the box

    4. peek into the hole – what’s the inside of the box look like?

    Other tricks:

    1. Get a long(ish) narrow box like a toothpaste box.

    2. line the inside with black paper

    3. poke a small hole in both ends so you can peer through

    4. in the daylight look around for a distant open window – the window would appear black (unless you have very bad smog or fog)

    5. peer through the box and at the window, allowing your eye a little time to adjust to the lower light level. What color is that open window compared to the inside of your black box?

    When working with light we sometimes create a “blackbody” – an object which is meant to emit (or absorb) light according solely to its temperature. For thermal infrared work this blackbody can be at room temperature, so you don’t even see a glow since no visible light at all is emitted. A small (well-made) handheld infrared blackbody is a pretty neat thing to have – you can hold it in your hand and it appears as though you have a black hole in your hand. It’s somewhat disconcerting when you know what the actual 3D shape of the object is, and yet from over 120 degrees of viewing angles no features can be seen at all – nothing but a black hole in space.

  34. Blizzzzzaaaarrg!

    For what it’s worth, I always thought of black & white like this:

    In reference to light, white is the presence of all colors, and black is the absence of all colors.

    In reference to pigment, white is the absence of all colors, and black is the presence of all colors.

    Phil’s post was great…but there’s 2 ways to think of colors – light or pigment.

  35. Mark McAndrew

    Great article! Talking of black… Can anyone tell me if this is an accurate image?

    If that’s real and not just a graphic, two things really puzzle me. The colours are opposite their ‘real’ opposites – at least, to human eyes (stare at any one of them, look at a white wall, you’ll see the opposite colour). Why is that? Must be a clue to how we sense this little range of EMR, surely?

    Also, whether or not the colours are really a smooth, continuous grading of wavelengths which only appears to be discrete bands by the limitations of our vision… why do the bands get thicker according to the Universe’s favourite ratio (Phi)?

    Any optical physicists in the room?

    (PS. The real meaning of black! )

  36. Chris

    Forget ‘Black Friday’, what about ‘Cyber Monday’? It made a little sense when people had far faster internet connections at work than at home (and hence it was easier to shop image-laden online stores), but that’s been a long time and my home broadband connection has been faster than the (shared) work connection for many years now. And I don’t have to worry about corporate firewalls and proxy servers, monitoring, or any of that other crap. So you would expect the heavy shopping to occur this weekend (as people avoid the crowds at the stores) and a huge drop-off on Monday.


    I remember seeing some science show where the expert said that he prayed that there was no extractable energy in the vacuum. The reason was simple – if we could extract energy then it meant that the vacuum wasn’t the lowest possible energy state and it could spontaneously collapse to a lower state at some point. This would undoubtably be a Bad Thing that would literally mean the end of the world – think of the inflationary period after the collapse of the false vacuum in the first few instances of the universe.

    If it is the lowest energy state then it’s a little hard to understand what significance there is in the energy of a vacuum. It’s like the gas tank in my car. There’s still a significant amount of fuel in the tank when my car “runs out of gas”, but it’s unreachable due to the construction of the tank. So does it matter to anyone other than the mechanic who works on my car and has to remember that it’s a fire or even explosion hazard?

    On the other hand there are some real consequences….

  37. Andy

    The etymology of the name “Black Friday” comes from the large amount of sales that stores make, showing a large amount of profits on a balance sheet. Net losses on a balance sheet are marked in red, whereas net gains are marked in black. The cynical recasting of the name came later…

  38. KAE

    The monolith from the novel “2001 a Space Odyssey” by Arthur C. Clarke (not the film version ).
    Yeah – I know it’s fictional, but whenever I try to imagine something perfectly black, that’s what I think of.

  39. mike burkhart

    I think it depends to an artist black is a color so it depends on the defination of color.Zero is a number its important to our base 10 number system many ancient systems didnot have 0 .Black has also symbolized evel or negitive in the Bible evel is refered to as darkness good as light .In Star Wars the dark side of the force

  40. John Paradox

    So, if there’s no real Black in the Universe, that means I don’t have to worry when a ‘black’ cat crosses my path?



  41. mike burkhart

    It depends on your defination of color to an artist black is a color .Zero is a number and it is very important to our base 10 number system. Many ancient number systems did not have 0 (the Romans did’nt there no Roman numeral for 0) .The frist to have 0 were the Myans who used a base 20 number system

  42. 30. Brown Says:
    November 26th, 2010 at 10:47 am

    Every now and then, someone says that “Black is not white and white is not black.” And yet, the Moon is an object that could be truthfully be called either black or white.


    Just to mess with people I’ll point to something white or black and say it’s really light black or really dark white. Not that’s it’s particularly clever but you can tell who has an imagination pretty quickly.

    Those without one don’t give it a thought. The rest pause for a moment.

  43. Keith (the first one)

    I like (30) Brown’s* question about the colour of the Moon. It’s just one example of how we percieve colour. *Brown is definitely a colour 😉

    Now, is white a colour? If black is a lack of light, then white is an equal presence of light in all wavelengths. So if black isn’t a colour, I don’t think white should be either.

    Having said that, in normal conversation I always consider both colours, especially as I often talk about how things are painted, and things are often painted white or black.

    Anyway, as they say, once you go black…

  44. Clay Erickson

    “….can something every truly be black?” Who is your editor?

  45. Trebuchet

    My favorite example of the relative nature of “black” is the TV screen.

    Turn the set off. What color is the screen? Sort of a not terribly dark gray-green, I think. Turn it on. See any black? Sure you do. But it can’t physically be any blacker than that screen with the set off. It’s all contrast.

  46. el jefe

    I’ve always understood that “black Friday” was the day that ledgers went “in the black” or positive balance yet a quick web search shows that the origin in regards to the day after Thanksgiving refers to police-observed stupidity from shoppers. They called it “black Friday” because it was a crappy day to patrol the shoppers. There were other historic Fridays of blackness in the past but they have no relation to our annual day of madness. I get that sensitivity to the word “black” has lead people to come up with nefarious origins for the term but they just aren’t true.

    I Likewise have always understood that zero is a fine number on paper or in equations yet it has no value in most practical, daily applications. If my wallet is empty I saw that I don’t have any cash rather than “I have zero dollars”. I guess it all boils down to phrasing but I make fun of people who have zero cans of haggis as opposed to not having any.

    In regards to black and white, it depends on projected vs. reflected light. It’s all about perspective.

  47. Michel

    As a teacher once said “You are a zero! That´s what you are A ZERO!! And I am a thousand times better than you!”
    So I said (I know now: not smart) “Ok so you are. But 1000 times zero is still…”

    BTW In those days theachers were allowed to be a bit more fysical than nowadays.

  48. chris j.

    everybody knows that black doesn’t get any blacker than the northside dubliners.

    “Do you not get it, lads? The Irish are the blacks of Europe. And Dubliners are the blacks of Ireland. And the Northside Dubliners are the blacks of Dublin. So say it once, say it loud: I’m black and I’m proud.”

  49. noen

    MadScientist Said:

    “Proof that black is *not* a color:
    1. get a shoebox (or any large box)
    2. paint the inside a nice bright color – white will do nicely. Alternatively, glue white paper to the inside of the box.
    3. make a small hole in the middle of one side of the box
    4. peek into the hole – what’s the inside of the box look like?”

    I’d say it looks black. Black is the color that we humans assign to those things that look “black” to us. That is why black is a color…. because we say so that’s why. We are allowed to do that. That is what language is. We assign phonemes to things that exist in external reality. “Things” being pretty loosely defined (in this case) as perceptual qualia.

    Now imagine this… Imagine that I have color reversal. I perceive red things as green and green things as red. It isn’t a physical defect, I experience the same color spectrum you do, just reversed. So when I look at a red light my personal experience is that it looks green to me. BUT… I had this condition all my life and when I acquired language I learned to label as “red” the things that look to me as “green”. So when you point to a red light and ask me what color it is I say it’s red, even though my internal experience of it is that it is green. You could never know simply by talking to me whether or not I had this color reversal visual experience.

    In fact, YOU could never know if you also experienced color reversal. How could you? There is no possible test that could detect it. In fact, there could be millions of people with color reversal all around us and we would never know it.

  50. hugo

    @noen, I thought I was the only one guy crazy enough to ponder the possibility of colour reversal, or colour flipping (perceiving different colour than others do).

    Perhaps everyone has the /same/ favourite colour, even….

  51. alfaniner

    Things can be lots more black.
    I, for one, would really like to have had a “Click to enebonize” link on the first picture.

  52. jmt

    Bad, bad astronomer using strange units like “µ” and “cc”!

    BTW, after reading this page, the hair on the top of my head seems to be black… or white… or zero…??? Is zero a color???

  53. mike burkhart

    One more thing 0 is important for is the binary system that every computer uses .It only has two numbers 1and0. Meaning on and off all of a computer switching ,comunications ,and even memory use the binary system even dvds just have a 1and0code that a laser reads.A message sent into space by the Aricebo raido telescope shows a human and the solar system in 1sand0s all these comments including mine were 1sand0s that flew thro the air to this web site .

  54. Brian Too

    Fundamental color theory addresses 2 different colour origins. One is radiative and the other is reflective. There are different colour spaces for each one, and there are more than one space per fundamental origin.

    For instance, the RGB system is for radiative colours. A pure black is when no energy is emitting from the colour surface. Your computer monitor is a radiative colour surface.

    Pantone and CYMK colour systems are for reflective origins. A pure black is when all ambient energy is absorbed by the colour surface (i.e. none is reflected). A printed page is a reflective colour surface.

    Pure blacks, either radiative or reflective, are more of a concept than an achievable physical reality. In real world use you are more concerned about ‘close enough’ than perfection.

    If I remember correctly, radiative colours have more possible values than reflective colours. They can be brighter and have more tonal range.

  55. Douglas Troy

    Black is a color, because something that is absent of all color is transparent.

    Also, zero is the number used to describe the absence of value.

    That’s just my two cents.

  56. If no visible light’s reflected, it’s black. It could reflect 100% of all invisible light, but it’d still be black. And zero isn’t nothing; it’s an additive identity.

  57. LoboLoco

    Awesome Phil, you always make it sound so easy.

  58. This is why atheism isn’t a religion.

  59. James Ruiz

    I dont think zero is a number… actually if you just put it as “how much”, you would nevere answer square root of two right? Because well, where do you end? And squareroot of two IS a number!! What i think is how close to zero is the zero we take for granted? We can ALWAYS ONLY have aproximations, positive or negative, but just aproximations…..
    Zero is not a number, lets leave it like a changeable value :)

  60. Messier Tidy Upper

    …what’s the blackest thing you know?
    Space. Everyone says that as an analogy

    Except there’s always an exception or two and here its me – I would’ve said coal, especially the anthracite variety or oil! 😉

    Or Emperor Palpatine / Darth Sidious’es heart & Darth Vader’s outfit! 😉

    At least in part because of moonlit nights, light pollution, skyglow, the zenithal light and nebulosity I know space is not always all that black.

    Also the blackest areas with the leats amount of illumination are in mines underground if you turn the lights off and theer are no bioluminescent or geoluminescent objects around.

    Nice article though – such pedantic technicalities aside. :-)


    PS. I like the paradox J.K. Rowling creted with Sirius Black – pairing the brightest white star in the sky (apparent magnitude terms natch) with the colour of darkness.

    Some black dogs & cats can be pretty good examples of blackness too .. 😉

  61. Messier Tidy Upper

    See :

    Although that photographic illustration doesn’t exactly do the colour justice.

    See also :

    for the confectionary liquorice which can often be very black in colour as can Cocoa Cola. 😉

    Plus off topic but of possible interest at the opposite end of the darkness / light spectrum is this quirky development via an online news site :

    I guess you have to admire her chutzpah while at the same time being slack-jawed at the ridiculousness of what she’s done there. :-)

  62. Messier Tidy Upper

    BTW. My all-time favourite references to “blackness”, however, would have to be Fireflies use of space as “the Black” eg. as sung here by Marian Call :

    Plus also Carl Sagan’s use of the term “Sacre Noir” or Sacred Black (after the French expletive Sacre bleu lit. “sacred blue”) also for space and the night sky in his Pale Blue Dot book. (Chapter 10 Pages 167 & 156, Headline Book Publishing, 1995.) :

    Then too there’s some great SF allusions and poetry-like metaphors and imaginings regarding darkness and light in Somtow Sucharitkul’s

    beautifully written and rather durn awesome Inquestor series. :-)

  63. Robin

    Interesting, Phil, but some of it I just don’t buy. For one thing the presence of something doesn’t mean that something is emitting anything in the light spectrum (light in the more pure electromagnetic sense). Moreover, having energy doesn’t mean that something emits light. I think you have to talk about color in terms of light. That’s almost always been its context.

    I suppose someone could you use a “tree falling when you weren’t there” argument to say that if light isn’t detected, then it’s black, but I think that’s easily circumvented by placing a thought-detector in a convenient place to detect photons that refused to travel in a more convenient, observable direction.

    I accept that just like light can be a particle and a wave, black can be a color (and thus be visible) and can also be the absence of light. Whichever it is depends on the context and is mutually exclusive in that context to the other option.

  64. katwagner

    In my Zone System book (you know, original b&w photography) Zone Zero is defined as “photo paper black; maximum black representing emptiness, nothing; openings into unlit rooms. Seems solid rather than mysterious.” And then Zone One is “nearly black, the beginnings of a sense of empty space. Unlit rooms; forests; depths; shadows in dim light.” Rather poetic, don’t you think? Which is why I love the craft of black and white photography and hunkering down in my darkroom.

  65. HMS

    CafeenMan wrote:
    “So I use dark grays which come across as black in context. Kind of like the optical illusions that Phil posts where two colors are identical but look completely different in an image due to surrounding colors.”

    This of course comes with certain pitfalls of its own. I have had people arguing that the color denoting black in an illustration is actually gray, when any artist would tell you it’s not meant to be literally seen as gray. This is also why it’s a bit of a pitfall to open up CG art in photoshop and use the hexadecimal values as proof a color is really another color. It completely ignores artistic intent and illusionary effects.

  66. Markle

    Zero not a number? Zero is not null. Zero is part of the set of Natural Numbers. Puh-lease, Dr. Phil, stop trolling. 😀

  67. Rick

    Blackest thing I know?

    Wesley Snipes.

  68. fred edison

    Oh, that’s just great. Now RCH will see your picture and claim it’s a derelict alien roasted spacecraft sending a weather report back to Earth. Thanks a lot, Phil.

    On the subject of how black is black, at least in Milky Way terms, it’s been passed down for generations that the center of the galaxy emits enough light in certain Earthly dark places, and with the right atmospheric conditions, to cast a visible shadow. Weird and wacky stuff.

  69. Try putting “none” against colour on the order form for a new car and see what reaction you get from the salesman :-)

  70. Messier Tidy Upper

    @ ^ dave from manchester England : LOL. 😀

    @1. Kirk Aplin Says:

    I think zero is a number. A number answers the question “How much?”. Nothing is just a valid an answer to that question as something, so zero is just a valid answer as π.

    “Nothing? Nothing shall come of nothing!” To quote Shakespeare’s King Lear – both play and eponymous monarch. Yet something *shall* come of nothing after all & that’s the debate over whether nothing is some thing or rather some quantity! 😉

    Is zero a number? Well yes & no. It is mathematical and arithmatical concept and it does count and help you count so it isn’t just nothing! Even though it is and refers to the absence of everything and cannot be divided or multiplied.

    I’ll leave someone else to do a number on how you define, well, “number” except to note that it could be ennumerated as a number of things! 😉

    @40. John Paradox Says:

    So, if there’s no real Black in the Universe, that means I don’t have to worry when a ‘black’ cat crosses my path?

    Of course not! Black cats are good luck as well as good mousers, good acrobats and good lap-warmers – as I can attest having owned a couple of them, my curent cat being one. 😉

    @32. Mike Torr :

    P.S. Apologies to Americans for “grey”, “colour” and “maths”

    Er, actually I think *they* should be apologising to *us*! It is the English language, after all, not the American one! 😉 😛

    @30. Brown Says:

    A good astronomy question is: what color is Earth’s Moon?
    Most people would say “white,” since it certainly LOOKS white against the blackness of a starry sky. And yet, the Moon reflects about as much light as asphalt, which most people would call “black” without hesitation. The astronauts who went to the Moon, and who knew in advance that the Moon was a relatively dark body, were nevertheless surprised by the blackness of the rocks that they found on the “white” Moon. Every now and then, someone says that “Black is not white and white is not black.” And yet, the Moon is an object that could be truthfully be called either black or white.

    Great point & well said. :-)

    Reminds me of the recent post here on how “brown dwrafs” are actually mauve and how our Sun & other “yellow” G type stars are actually *white* rather than yellow – although their peak radiation is actually emitted mostly in the green frequency.

    Plus how seemingly dark sunspots would be brilliantly white or red if they were seen separately off the solar (or stellar) surface and black holes – as Stephen Hawking pointed out aren’t really black either!

    Our eyes *do* decieve us on many things and impressions of colours are certainly among those things.

  71. Messier Tidy Upper

    Reminds me of the recent post here on how “brown dwrafs” are actually mauve

    Which would be this one :

    Another really black object one that is literally jet black is here :

    While Pitch Black :

    is something somewhat different again! 😉

  72. “Black” requires spectral absorption plus surface patterning for physical entrapment. Degrease then lightly bolt together in registration a couple of hundred fresh double edged razor blades. Look at the sharps face. Light goes in, bounces between converging edges, and cannot get out. It is black until a half wavelength will not fit into the surface gaps. Martin Black Paint N-150-1. An MgO reflectance standard sprayed with flat black. Micronized manganese ferrite black, CAS 68186-94-7, CI #77494. That will suck your eyes out with black.

    There is an average of roughly one subatomic particle per cubic meter out there, which is incredibly rarified There are 336/cm^3 Big Bang neutrinos: 56 each neutrinos and antineutrinos each in three flavors. Given rest mass-equivalent not exceeding 0.2 eV, only Big Bang neutrinos red-shifted by cosmic inflation to 1.945 kelvin or 1.68×10^(-4) eV are non-relativistic, possessing helicity but not chirality. Neutrinos decoupled about one second after the Big Bang at 2.5 MeV temperature. Photons decoupled about 377,000 years after the Big Bang at 0.5 eV temperature. Primordial neutrinos are colder than cosmic microwave background at 2.725 K because the neutrino transparency point came earlier in the expansion.

    How much energy does space have? …Some estimates put it at a very tiny 10^(-15) Joules per cc Vacuum zero point fluctuations do not appear Doppler shifted and are Lorenz-invariant: intensity varies as the cube of ZPF frequency. The grain of space appears near 10^(-33) cm, the Planck length. Integrating intensity over allowed frequencies gives 10^94 gm/cm^3. Nuclear density is 2×10^14 gm/cm^3.

    What the heck does all this mean? Beats me. GOOD MAN! Orthodox physics has huge problems with observation versus derived theory: 1 biological homochirality, 2) chiral beta-decay rate annual modulation (arxiv:1004.1761), 3) divergence of chiral neutrino-antineutrino reaction channels (arxiv:1007.2923, 1007.1150v3), 4) no SUSY partners, 5) Weak interaction chirality, 6) matter absent antimatter, 7) false vacuum decay powering cosmic inflation, 7) all quantum gravitation formulations are empirical crap.

    Physical chirality is emergent from aggregation. Fundamental symmetries from which physical theory rigorously derives – classical and quantum gravitation, Standard Model and SUSY – exclude chirality. Symmetry breakings are inserted by hand to rationalize the observed universe. Let’s strop Occam’s razor: The Big Bang was intensely chiral with a false vacuum pseudoscalar background (left foot) active only in the massed sector. Generic weak interactions are left and right shoes but strong interactions blur chirality into socks. A contemporary vacuum background chiral remnant – active only in the massed sector – is testable in a geometric parity Eotvos experiment. Here are two such experiments, chemically and macroscopically identical left- vs. right shoes,
    Somebody should look. The worst they can do is succeed.

  73. Svlad Cjelli

    “Male pattern baldness” is a flippant example. There are other baldnesses, you know. People DO – in the real world, that is – balden themselves for style, and in many cases quite creatively.

    On a more relevant point, in which contexts is the proposed definition of colour supported and/or not in conflict? And where it is in conflict with common language, to what extent would the rest of the language need refurbishing for it to fit? Related: What term or change in formulation would replace the old, now unwanted, uses of “colour”?
    (E.g. What would be proposed in paintery as a more accurate category name for encompassing the now-colours? I do not doubt that there are synonyms, such as “tinge”, though one would have to justify why black would be a tinge if not a colour. A new constuct or another changed usage would be able to fill the role.)
    More importantly, what does the new definition have to offer in terms of communication or other usefulness?

  74. Anonym

    It’s called “Black Friday” because, traditionally, that’s the day of the year when lagging product sales turned profitable and moved business accounts from ‘red’ (loss) to ‘black’ (profit). (Check with any business accountant to verify).

  75. I’ve heard the same as Anonym #75 said. Though, to be precise, it’s the time in the financial year, which starts at the beginning of October, not the day in the calendar year. Retail would be a far more precarious business than it already is if businesses couldn’t count on being in the black until 46 weeks into the year.

  76. MadScientist

    @noen #49:

    “In fact, YOU could never know if you also experienced color reversal. How could you? There is no possible test that could detect it.”

    To say that there is no possible test to detect it is quite an amazing claim; I would put it in the same category as “humans cannot build machines which fly”. However, I will not argue a case unless you can point out how such a reversal can occur in the first place. After all, it will require a substantial violation of the laws of heredity. I do not like to speculate on the characteristics of imaginary things which defy nature such as gods.

    Now back to blackness: I disagree with the B.A. – of course you can have perfect blackness. The radiation which our eyes is not sensitive to can hardly be called ‘color’. If we perceive no light, then we have black. The fact that photons are everywhere is immaterial since we cannot detect them with our eyes.

  77. Fenchurch

    See, I think one could rather describe black the same way you described zero. You can have your coffee black, or buy a pair of black trousers, but a room with all the lights turned out is not black. It may appear to be black for the first two or three seconds after the light turns out, but that’s just quite literally a trick of the light. After a few moments, you can make out shapes. And if everything were truly BLACK, you would not be able to see one shape against the other. The simple ability to see shapes in the darkness proves that there’s light in that room somewhere.

    So, yes. I’ll take that pair of shoes in black, because they’ll look rather nice with my black jacket, I should think.

  78. Reidh

    You wouldn’t know that feces smelled if you didn’t have a nose. Therefore would we know black or light if we couldn’t see? Is what is between your ears black enough to qualify as the utter absence of light?

  79. Ben

    For your average person, black is absence of light that is perceptible with the unaided eye.

    Infra red, ultra violet, speculative popular hypothesis’s like dark energy and the “big bang” – – not relevant at all.

    In that context, blackest black I’ve experienced was deep in a twisty cave system in Westbrookville NY, where I turned off my lights for about an hour for a nap (it was a very effort-intensive excursion); when I woke up, I literally couldn’t see my hands in front of my face. Black? Yep.

    Aside from that, I consider the regions between magnitude 13 stars essentially black, because I haven’t been able to get my camera (at ISO 12800 and f/1.4, about 4 seconds) to register any light from there, though I can image magnitude 13 stars. So there’s an instrument-based perception of mine. Is there more light there? Yes – but since I can’t see it, you know, it’s kinda irrelevant to me. The day I get an ISO 102k camera in my hot little hands, that will change. I wouldn’t mind an 85mm f/1.2 lens, either; had one, sold it, regret it.

  80. Kaleberg

    re: #2 Pete Jackson on space having mass: That’s actually true. The mass of curved space varies, so the gravitational field itself has an attractive mass. That’s one of the big differences between Newton’s theory of gravitation and Einstein’s and accounts for the precession of Mercury.

    re: black as a color: It definitely is a color. Just about every color gamut includes black or a set of blacks. If you’ve ever bought a large screen television, one of the specifications is how black a black it can produce, that is, how it renders the pixel RGB(0, 0, 0). Of course, there are an awful lot of blacks. I used to have a Pantone guide to black on black printing. It was a 200+ page looseleaf with thousands of samples of how one could print in black ink on black paper. There are also an awful lot of reds, and blues and greens and so on, though I don’t think Pantone sells similar X on X printing books for other colors.

  81. MadScientist

    @Fenchurch#78: If you can make out shapes in the dark, then there is indeed some light (visible to humans). However, with some small effort you can block out all visible light and I guarantee you won’t be able to see anything. Humans also lose color perception in dim lighting because the rod photoreceptors in the eyes are far more sensitive than the cone (color) photoreceptors. So with low enough light you turn colorblind and in even lower light you can’t see anything. For older people, protein strands floating freely about in the eye fluid can occasionally strike the retina and cause (the perception of) bright flashes even though there are no visible photons.

  82. Svlad Cjelli

    @77 “After all, it will require a substantial violation of the laws of heredity.”

    “Require” is an absurdly strong word. We don’t know enough about the heredity of colour perception in the first place. We do know, however, that brain structures are only hereditary in the form of vague and general tendencies and show noticeable variation and plasticity.

  83. @mike burkhart, #41: The Mayans may have been the first to discover/invent 0, but because they had no contact with Old World natural philosophers until after the 15th century, the Mayan 0 does not appear in modern mathematics. The origin of the 0 that we all know and love came into mathematics from India, by way of the Middle East. The first European mathematician/natural philosopher to use the Indian 0 was probably Fibonacci some time in the 13th century (at least 200 years before contact with the Mayans).

  84. MadScientist

    @svlad #83: Nonsense. The photoreceptors are excited by photons with different energy ranges and will send signals along developed pathways. The construction is ultimately governed by heredity and the mechanics of specific signals are governed by physics. Unless you have mechanisms by which lower energy light can excite and produce the same signals as higher energy light or else somehow affect development to consistently reroute all the sensors differently you cannot get such hypothetical color switching. Things simply do not develop that way. The color switching hypothesis requires a fundamental and somehow widespread inconsistency in the development of an organism which is on par with the Crocoduck evolution.

  85. Dionigi

    If I cut my hair 1cm long it is a hair style. If I cut my hair 1mm long it is a hairstyle. If I shave my head with a razor it is a hairstyle and if I cut my hair on top and left it at the sides, like male pattern baldness it would be a hairstyle and used to be in China. If a skirt is 50cm long it is a skirt. If it is 20cm long it is a skirt. The absence of something definitely is something.

  86. What about Black Holes? Not even light can escape those. I know about black holes being able to emit radiation, but this radiation is emitted from around the black hole, really close to it actually, as far as i understand. Between this limit and the event horizon, can’t we say that nothing gets out? That its surface would be truly “black”?
    I’m not an expert on this matter, so if someone could explain this better, i’d be really greatful.

  87. In art, we’re taught that black isn’t a color nor is white. You use black or white to change the value of other colors by making them lighter or darker. It’s been a while so I forget the exact lingo… tint and hue? I dunno. :)

  88. The Beer

    Why didn’t you set up the black pitcure so it can be enlarged?

  89. Hannes

    The darkest thing is a photon.

  90. dg

    @ Pete Jackson #2

    The important thing to know is that energy (including mass) is not the only thing that curves space and affects the evolution of the universe – the Pressure that comes with the source of energy also needs to be taken into account. See this:

  91. Keith Bowden

    I remember my 8th grade science teacher (who loved throwing out bombastic statements) began the year with the statement “There is no such thing as cold,” let us wrestle with that concept for a while, explained his argument and gave us the following summation: “The condition known as ‘cold’ can be described as the absence of heat.” That was a fun year (1977-8).

    And I’d guess that death is pretty black. 😉

  92. Phil wrote:

    “… space will always have something in it. Matter, energy (dark of otherwise), marshmallows (mini or otherwise), whatever.”

    Wait … so, marshmallows aren’t considered “matter” now?

    Perhaps they’re made of antimatter. I hesitate to think what all those antisugar molecules must be doing to the atoms lining my stomach and intestines….


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