The Sciences

Black Friday

Bad AstronomyBy Phil PlaitNov 26, 2010 11:51 AM


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"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.

So 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?

That'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 10^25 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?

Well, 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!

This 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=mc^2

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 10^30 Joules of energy, enough to power the entire United States for tens of billions of years. Wow. 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.

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