Smaller than 1 pixel?

The unsharp mask Radius slider goes down to 0.10 of a pixel. How is it possible for Photoshop to sharpen only one-tenth of one pixel? A pixel is the smallest unit of information, no? Like the Planck constant.

Borrowed from Photoshop Essentials description of Radius control

"Step 5: Drag the Radius slider to adjust the edge width
So far, we've learned that the Amount value controls the amount of contrast applied to the edges. The Radius value in the Unsharp Mask dialog box controls the thickness of the edges. Radius determines how many pixels extending outward from an edge that should be included as part of the edge and affected by the Amount value.

The Radius slider in Photoshop's Unsharp Mask filter
Use the Radius slider to adjust the edge thickness.
With the default Radius of 1 pixel, only a single pixel on either side of the edge (one pixel on the light side and one on the dark side) is affected. But increasing the Radius value to 10 pixels would extend the edge thickness out to 10 pixels on either side.

Thinner edges make the increased contrast from the Amount value less obvious, while thicker edges make it more obvious. But if you increase the Radius value too much, halos will appear along the edges, which are tell-tale signs of oversharpening and something you want to avoid.

Comparing different Radius values
Here's a comparison showing three different Radius settings. In the first image (left), I started with the default Radius value of 1 pixel. Notice that while we can see an increase in contrast from the Amount setting (currently at 500%), the image still looks relatively clean.

But in the second image (center), I've increased the Radius to 5 pixels. And not only is the frog's skin texture looking brittle and oversharpened, but outlines, or halos, are starting to appear. There's a dark green outline along the top of his head, and a light outline around the eyes. This can sometimes create the illusion of added detail, but all we're really doing is distorting the image.

And in the third example (right), I've set the Radius value to 20 pixels. This means that the edge thickness is now extending outward to 20 pixels on either side, and the halos are now clearly visible everywhere:

A comparison showing how the Radius value affects sharpening with Photoshop's Unsharp Mask filter
The default Radius: 1 pixel (left); Radius: 5 pixels (center); Radius: 20 pixels (right).
How to choose the correct Radius value for sharpening
Like the Amount value, the correct Radius setting will depend on the size of your image, your subject matter, and whether the image is being sharpened for print or for the web. Ideally, you'll want to set the Radius to a value just below the point where the halos start to appear.

But remember that pixels are smaller when printed than they are on a computer screen, so the same Radius value will create edges that look thicker on your screen than they will in print. Use the guideline below to find the best setting:

Print: When sharpening for print, try a Radius value of 1 to 2 pixels. For very large images, you may be able to push the value closer to 3 pixels. But anything beyond that will most likely produce an obvious halo effect.
Email/Web: When sharpening for email or the web, Radius values below 1 pixel work best. The ideal range is usually between 0.5 and 0.7 pixels."

Interesting question and waiting to see if someone knows the real answer. However, playing around with it, changes less than 1 do make quite a visible change to the image especially if you crank up the amount to the max. Sample below.

This explains where the Radius control gets factored into a sharpening equation, likely to make your head swim and eyballs bleed, and regret getting the explaination!

https://www.imatest.co​m …0line%20spread%​20function).