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Comment by dgritsko | Hacker News Reader

This is an insanely cool picture.

Some quick napkin math (please let me know if I'm way off base here): in the second pic, the electrodes are ~74 pixels apart and the strontium atom is ~2 pixels wide.

The electrodes are ~0.078 inches apart, so these 2 pixels represent ~0.0021 inches (or ~0.05334 mm).

Google tells me that the Van der Waals radius of a single strontium atom is 255 pm.

So the diameter of the actual atom is something like 10,000x smaller than the space represented by these pixels. Crazy!

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I guess it's basically a point source compared to the camera resolution, so what you're seeing as the "atom size" is the diffraction limit of the camera lens.

Analyze this photo instead: http://content.assets.pressassociation.io/2018/02/11175054/f...

While the manual for my DSLR doesn't specifically refer to this use case, reading between the lines it sounds like using a jpg encoding for atomic-scale image capture isn't recommended.

I was just thinking that the atom has to be letting off (reflecting) some serious quantity of light in order for just one to appear with off the shelf camera equipment.

I still don't get it, 10,000 times more light than its size?

They say it was a long exposure.

The atom was illuminated and irradiated light for a long enough time to register on the sensor.

Even if the exposure were for a year, if the amount of light being emitted is less than the hardware can perceive, it wouldn't register - hence the need for electron-microscopes and such. There's a story here about the size of the light being emitted that doesn't seem satisfactorily answered (not by you, the article and stuff in general).

> Even if the exposure were for a year, if the amount of light being emitted is less than the hardware can perceive, it wouldn't register

That is not true. It might be noisy, but it would show up.

In point of comparison the human eye can see a single photon.

A single atom can emit a single photon, ergo you can see a single atom.

You won't get any detail out of it obviously, but you'll see it as a dot of light.

That's what's happening here.

> There's a story here about the size of the light being emitted that doesn't seem satisfactorily answered

You are misunderstanding the "size". The size "10,000" is the resolution needed to differentiate between two atoms sitting next to each other.

But in order to see the atom (detect the atom), you just need to be able to capture what it emits. And it emits photons, and it's not that hard to capture and record a single photon.

So you do, and it shows up as a single dot in the image.

If you had two atoms near each other, both emitting photons, you would not be able to distinguish them from each other (i.e. you couldn't say if there were one or two), unless they were farther apart.

If we're seeing the single photon emitted by a single atom, then how much larger is the spot of light in the image compared to the atom that emitted it?

Why isn't there a ruler somewhere in the picture? My brain is absolutely failing to make sense of the scales and sizes in this image.

> then how much larger is the spot of light in the image compared to the atom that emitted it?

The atom is about 255 picometers, the light is around 400 nanometers. That makes the light around 1500 times larger than the atom.

(Which also helps explain why visible light can not distinguish atoms placed close together - it's so much larger than them.)

> Why isn't there a ruler somewhere in the picture? My brain is absolutely failing to make sense of the scales and sizes in this image.

The two electrodes are about 2mm apart. About the width (not length) of a sesame seed. It's really small. The photo was taken through a microscope.

Another point of reference: the width between the electrodes is about 5,000 times the wavelength of the light being emitted.

In theory, if you had a good enough camera, the dot of light would be around 1/5000 of the width. (However the diffraction limit of your lens might enter into play, blurring the image.)

And finally, the width between the electrodes is about 8,000,000 times the size of the atom.

Thank you so much; that was exactly the explanation I needed =)

Maybe it's light reflected by the atom at multiple points? The article mentions that it's "held nearly motionless" but over a long exposure perhaps the degree of movement allowed created a composite large enough to show up in the photo.

No, that's not it. Just because it is small, doesn't make it invisible.

You can't see single atoms because they normally don't send any light toward you. But if they did, and this one is, you would see that light.

You aren't seeing the atom exactly, you are seeing what it sends toward you, and the human eye can see single photons. (And so can cameras.)

> No, that's not it.

That has to be at least partially it for the rest of your comment to be right (which it is!). To say the atom was "held nearly motionless" is to say it moved a little during the exposure. It sent the light toward the sensor from all of its positions.

> That has to be at least partially it

No, not at all. The wavelength of the light is about 1,500 times larger than the atom emitting it. So the atom would need to move at least 1,500 diameters to make any difference in the light.

It doesn't move that much.

> It doesn't move that much.

Source?

Here: https://en.wikipedia.org/wiki/Laser_cooling

Think about it this way: every photon your eyes perceive came from a single atom.

CCDs can detect single photons. It isn't much of a challenge. I would be suspicious that the surrounding apparatus is Photoshopped in from a separate exposure.

Nope, not photoshopped. It's a single exposure; the apparatus is illuminated using flashes.

The latter made lighting the shot in a controlled fashion a bit easier than if I had used continuous sources – you'd be looking at using a torch with a bunch of filters or a computer monitor on the lowest brightness otherwise.

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