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Which is the smallest particle that has been actually seen by the scientists? When I say "actually seen", (may be using some ultra advanced microscope or any other man made eye, using any wavelength or phenomena) I really mean it; just like we have seen the red blood cells.

Davidmh's answer is pretty much in line with what I am asking

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Note that it is not straightforward to define 'seen'. Many particles are smaller than the wavelength of visible light, and can therefore not be 'seen', but one can transform smaller wavelength waves to visible light.... Does this still count as 'seeing' to you? –  Danu Mar 21 at 8:59
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Ok. But then, I don't see how this is fundamentally different from using, say, an electron tunneling microscope (which uses tunneling, not some kind of EM radiation) to 'see' something. If detecting a particle in this way also counts as seeing, then I don't see why the things done at the LHC don't. –  Danu Mar 21 at 9:08
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What I'm trying to clarify is that it is not easy to draw a line between the two. In a sense, a particle's interaction with light is also just an effect, and it's a slippery slope. –  Danu Mar 21 at 9:19
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Actually seen, to me means that the photon emitted or reflected by the object is the same photon absorbed by the scientist's eye. Therefore any system not working in the visible spectrum regime will fail this test. Any more advanced observation protocol will not be a direct observation but one based on indirect effects. –  gregsan Mar 21 at 9:23
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Perception is all about observing the effect of something. What is the difference between a man made eye like a ccd sensor (which you seem to accept) and a photographic plate (which you exclude)? Both of them are sensitive to the effect of photons hitting them, as it is the human eye. I really cannot understand the question :/ –  DarioP Mar 21 at 11:39

2 Answers 2

up vote 18 down vote accepted

Taking your question literally, you can see a single barium ion:

The TRIµP group has achieved capturing a single barium ion in a Paul trap. The images show Coulomb crystals formed by a decreasing number of laser-cooled ions as detected with an EMCCD camera. This forms an important step towards the planned experiments on single radium ions to measure atomic parity violation and build an ultra-stable optical clock.

enter image description here

They are in traps like this one:

Barium ion trap

Also, Warren Nagourney from Washingtong University took a picture of a single Barium atom scattering light from a laser:

Single trapped atom, glowing blue

Single trapped atom, glowing blue

Photo credit: Warren Nagourney at the University of Washington, c. 2000

What is this?

Believe it or not, this is a color photograph of a single trapped barium ion held in a radio-frequency Paul trap. Resonant blue and red lasers enter from the left and are focused to the center of the trap, where the single ion is constrained to orbit a region of space about 1 millionth of a meter in size.

What's the red/blue mess on the sides?

Low level out-of-focus laser scatter off of metal trap electrodes and accessories (atom ovens, electron filaments, etc.) as seen in this photo.

How do we know the dot really is an atom?

When one turns off the red laser, the blue dot vanishes. This is because the scattering process requires both laser colors due to a metastable state in the barium ion. If the blue dot stayed around with the red laser off, we might excuse it as being additional laser scatter off some surface.

How was the photo taken?

This is a scanned photo; the camera was a 35mm Nikon (I believe) with a wide open 50mm f/1.8 lens. The exposure time was two minutes. Several shots were taken at different camera positions and this one caught the ion in the very narrow depth of field.

Is this how you normally "view" the ion?

No, we use a 50 mm f/1.8 camera lens to image the blue dot onto a photomultiplier tube. We don't require the focus to be so good when using the PMT.

Where can I see more?

Lots of CCD images of one and several trapped ions are found on the Monroe group site.

Only two minutes exposure time, so probably in a dark enough room, someone with good sensitivity could actually see it.

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How do you prove it is a SINGLE atom? :D Then electron microscope and everything what is visible in it is also a good answer. Question HERE is HOW small, and if its not a single atom but a molecule this changes answer at least by a factor of 2. –  Asphir Dom Mar 21 at 11:47
    
AsphirDom see the edits (thanks to @user11153 for the bulk of that work). If you had actually followed the links you would have found exactly that information. –  Emilio Pisanty Mar 21 at 11:52
    
@EmilioPisanty so exactly which phrase answers my question? –  Asphir Dom Mar 21 at 11:55
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How about the paragraph below "How do we know the dot really is an atom?"? If you're worried that the single dot is actually masking as two atoms, that is so not an issue anymore that ion trappers don't bother making that explicit. The distances between ions in a trap were already well characterized in the early 90s, and the optics are of course perfectly well understood. If there were indeed two atoms, you'd see two dots, as in the first image. –  Emilio Pisanty Mar 21 at 11:59
    
@EmilioPisanty How simple everything is! AH! :) Ok. Lets assume your arguments are plausible. –  Asphir Dom Mar 21 at 12:30

Obviously, the smallest particle that scientists have ever seen directly is a photon.

The question is a bit silly because it tries to eliminate any simple device like a photographic plate. But the human eye, its nerves and the visual cortex together are far more complicated.

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I was tempted to say this, but since the typical definition of "see" means to detect photons reflected or emitted from the object in question, you can't really get away with this answer. :-) . Personally I don't think it counts as "seeing" an atom which just gave off a photon, either. That's "detection" rather than "recognition." –  Carl Witthoft Mar 21 at 11:40
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Well, the photon is obviously ~seen~ and is therefore a pretty darn small particle that is "observable using only the naked eye" :) –  bbozo Mar 21 at 14:03
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You definitely do not see photons. First of all, sight is optical detection of photons that have reflected off of the subject; photons do not bounce off one another to the best of my knowledge. Second, if photons hitting your eye counted as you seeing those photons, then a blind person could see sand when it hits them in the eyes. Detection using your eyes does not constitute seeing, unless that is your comprehensive definition of sight. Actually, if you define detection of photons hitting your eyes as sight of those photons and not their origin, then you have never seen a physical object, eve –  John P Mar 21 at 22:47
    
When there is so much light that you just see white. Are you seeing something or nothing? –  jinawee Mar 21 at 22:55
    
@JohnP : So you can't see the sun? Or a light bulb? They don't exactly reflect photons. That there's more to seeing than just eyes is true; see the last line of my post. –  MSalters Mar 24 at 11:14

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