Does a single atom have color? What about a subatomic particle?

Factual Questions
1

Not a scientist–I’m sure some of this is going to be offbase, but just see if you can fix it up and help me out anyway, okay? Thanks.

What the human eye calls color results from light reflecting/refracting off of objects. If you get a big enough hunk of pure element (gold, carbon, silver), you can see it and say that the element has a color (indeed, for gold, silver, etc., the element is the archetype of the color). When you want to analyze a unknown chunk of material, you do various chemical things to it to split it into its elemental parts. One of the ways you can identify specific elements in this process is by looking at the spectrum signature of hunk of material (is this right?).

Question #1 is whether this posited spectrum signature (which I may not have correctly described) is always in the visible range or sometimes only registers as ultraviolet/x-ray or something else. Does every element have a specific visible to the human eye color when in solid/liquid/gas form?

Question #2 is whether this holds for the very smallest particles of an element, or whether at some small atomic size light energy changes the atom itself and we can no longer tell what color it is (assuming we otherwise have the ability to see the color or measure the spectrum signature for an object that small). In other words, if you are doing some physics experiment which requires you to capture and identify random single atoms generated by some operation, will color/spectrum cease to be useful in identifying an element?

Question #3 is whether each subatomic particle (proton, neutron) has “color” in the sense used above. What about an electron, which may be properly characterized as energy rather than matter? Does an electron have a color? What about mesons and neutrinos etc.? Do they have color and is every neutrino the same color? Is every proton the same color?

Color being in the eye of the beholder, substitue “particular wavelength of refraction/reflection of light” as needed, and again be generous about the muddled terminology.

2

You’re confusing a couple of things here.

Most of the time, when you see an object as being a certain color, you’re seeing reflected light bouncing off it- it absorbs light of some wavelengths and reflects others. That’s what gives silver and gold their characteristic colors.

Spectra are different. Spectra are made up of photons emitted from atoms, not reflected off of them. These kinds of spectra (there are some other kinds) are made up of a series of bright lines. The colors given off in a spectrum don’t necessarily have anything to do with what color a hunk of material would appear. For example, the spectrum of sodium is dominated by two bright yellow lines, while a hunk of sodium metal appears gray, not yellow.

For a material to reflect light, visible light needs to be shining on it. It will appear a different color if you shine a different color of light on it. What you think of as the true (reflected) color of an object is the color it would appear if white light were shining on it. If you have an object that looks white in white light, for example, and you shine a red light on it instead of white light, it will look red.

For a material to emit light, something has to happen that causes the electrons in the atoms to go from their ground states to an excited state. Heating up a material is one way to do this. There’s a difference from reflected light. Either sodium will emit its spectrum with the two yellow lines or it won’t emit anything at all- there’s nothing you could do that would make it emit green instead.

Each element does have a characteristic spectrum that it will emit. These spectra are used in physics and astronomy to determine what elements are present somewhere.

The spectra of elements can and do include lines in the ultraviolet and infrared. But most physicists and astronomers who are using spectra to identify elements are working in visible light (because it’s easier), so they’ll usually describe a spectrum in terms of visible light.

Each element or compound does have one (or possibly more) characteristic appearances in reflected light. These are generally less useful than spectra for identifying materials, though- for example, a hunk of gold and a hunk of brass look a lot alike.

Individual atoms don’t reflect light- they’re smaller than a wavelength of visible light, so you couldn’t look at an atom by bouncing visible light off of it, even if you had something to magnify the image enough times. Subatomic particles such as protons, neutrons, and electrons don’t reflect light for the same reason. (Electrons are matter, incidentally) Individual atoms, if their electrons are excited out of the ground state, will emit spectra, though.

(There’s something called “color” that is an attribute of quarks, but that has nothing to do with colors that you see, other than having the same name)

3

There will be many spectral lines, at many different frequencies. I think that most elements have at least some lines in the visible, but most of them also have lines in the ultraviolet, and all of them have lines in the infrared and below, down to arbitrarily low frequencies. Highly ionized atoms can have lines up into the hard X-ray region.

The spectral signature, in fact, only applies to individual atoms of an element. Most of the color we see on macroscopic scales depends on molecular structure of the substance, and some things that we think of as “color” aren’t even spectral at all, but a function of things like texture (such as the difference between glossy and matte).

Anything which has internal structure will have a spectral signature, though for subatomic particles, it’ll be insanely high energy (high enough that you’ll never see them excited outside of a particle accelerator). Protons, neutrons, and mesons are made up of quarks, and so have spectral lines, but electrons, muons, neutrinos, and quarks themselves, so far as we can tell, are truly fundamental and have no internal structure, so they don’t. Quarks do have a quantum mechanical property called “color”, but that’s just a label for that particular property, and has no relationship whatsoever to what’s normally called color.

4

I guess this depends how you are defining a spectrum. Certainly raman spectroscopy is based on reflection. Generally, what determines a materials color would be the visible absorption spectrum.

I don’t know about this. Certainly single molecule detection is possible using 830 nm reflected light. I’m not certain how big the coloidal silver is though. In the case of fluorescent single molecule detection the excitation wavelength is almost certainly larger than the molecule. This is not the same as single atom detection of course, but I’m not certain that the wavelength determines the smallest size that can absorb a photon.

5

If I may be so bold as to reprase the OPs question. If I could blow up a proton, neutron, or an atom of gold to the size of a baseball, what color would they be?

6

There’s a trick that lets you see individual atoms visually; IIRC it works by exciting the atom like you say. It involves a Penning trap ( magnetic trap ) to hold the atom, a laser, and a telescope mirror. I don’t recall the color.

7

What color do you want them to be?

That’s the only real answer to this question. Or in other words, you’re asking a meaningless question.

8

The proton and neutron wouldn’t have a color. The atom of gold would be gold colored.

9

Such an atom would have serious problems holding onto its electrons. Forget the two-photon absorption that usually requires a laser, we’re talking thousand-photon absorption. Cut and paste the laws of physics where necesary.

10

If I may be so bold… Ahem…could/does an element have spectral lines strictly of the non-visible variety? And if I an enlarged an atom of it to the size of a cricket ball, could I still catch it?

11

They’d be invisible?

12

No it would not. The color of gold is an aggregate property of the metal. An individual atom does not have that property. At best you could say that a single atom has the color of whatever photon it happens to be emiting at the time you said it, but the next photon may have a different color.

13

Gold is a composite of many colors, from brown to yellow, with subtle but contrasting hues of green and red. It is a damnably difficult color to render as realistic looking.

14

If you blew up an electron to the size of a baseball, you’d have trouble seeing it at all if it still followed the uncertainty principle. Once you saw it in a definite location it would have an extremely uncertain momentum and go zooming off somewhere. But thinking about it that way is flawed, since the electron isn’t a ball that photons bounce off of. We just think of these things as particles and waves to make it easier to visualize, but they don’t behave like macroscopic objects at all. The problem is that elementary particles aren’t really “things” in a sense, they’re more like bits of information. It’d be like asking if I made a thought the size of a baseball, what it would look like. Yeah, it’s hard to get your mind around. If these fundamental units of matter are just mystical waves of probability that don’t actually exist, how can things like baseballs exist? Well, we still haven’t really figured that one out.

15

From a philosophical point of view, that’s probably the best post I’ve ever seen in GQ. And your observations jibe with those of quantum mechanics pioneers and other great scientific thinkers.

“The atoms or the elementary particles are not real; they form a world of potentialities and possibilities rather than one of things or facts.” Werner Heisenberg

“There is no quantum world. There is only an abstract quantum mechanical description.” Niels Bohr

“Stuff is made of atoms. Therefore, atoms can’t be made of stuff.” Raymond Hall of Fermilab

And finally, my personal favorite:

“Something unknown is doing we don’t know what.” Arthur Eddington

16

I don’t think that any unionized atoms have only non-visible lines (though I could be wrong). For some highly ionized atoms, though, the lowest energy lines are all in the UV or higher, so those would satisfy your criteria.

Ad for these questions concerning enlarging an atom to macroscopic size, there are no known physical means to do that. Tell us what rules govern your magical enlargement process, and then we can tell you what (if anything) such a super-atom would look like.

17

They would. They’re smaller than the wavelength of visible light.

Things work differently at that scale than they do at the macroscopic scale we’re used to dealing with. The laws of physics are different at that scale (well, not really, but effects that are so small you don’t notice them at human scale dominate over more familiar effects at that small a scale). You can’t really talk about scaling a subatomic particle up to the size of a baseball- if you did, it would act totally differently than it does at its normal size, because the laws of physics act differently at different scales.

There are a number of popular science books that talk about changing the laws of physics in some way so you could observe effects you don’t normally see at a human scale. George Gamow’s Mr. Tompkins books are classics of this genre. Alice in Quantumland and The Wizard of Quarks by Robert Gilmore are also good.

18

Thanks for the discussion.

So, looking solely at spectral signature caused not by reflection but by photon emission, does every “isolated” proton emit the same spectral lines? [In other words, are all protons absent their participation in an atom the same “color” (if we describe spectral signature as color)?] How can a proton emit a photon (excited electrons) if it does not have an electron? If a single atom is emitting a series of photons why would these emissions ever have a different spectral signature? Are these misguided questions?

19

Almost any system with charged particles that are bound together will have distinctive spectral lines. Such a system doesn’t spontaneously spit out radiation, of course – something has to add energy first so that it can then be radiated. This might be an incoming photon.

The lowest spectral line for a proton has an energy/frequency over a hundred million times higher than visible light. It’s enough energy that the excited state, before it spits that energy back out, isn’t even called a proton any more, since the rest mass is so much larger (31% larger). Also, the energy is only occasionally spit back out in the form of a photon. More commonly, a pion (a bound quark-antiquark pair) comes out, since pion emission is governed by the strong force which dominates the electromagnetic force when it can. (And it can here because there’s enough energy around to make a pion.)

20

I’ll answer what I think you’re asking here:

Every single proton is identical to every single other proton (except for spin, and there are two choices there- spin-up or spin-down). So if protons did have a color, they would all be the same color. See Alice in Quantumland by Robert Gilmore for a somewhat freaky discussion of this (with electrons, but it works the same way for protons).