How small can something be and still have a shadow?

photo by asilverthorne - public domain

My five-year-old daughter asked this question.  There must be some lower size limit at which you just can’t cast a shadow, right? 

Not necessarily.  As it turns out, you can be really any size and still have a shadow.

All it takes to make a shadow is to block light, and since light is made of photons (basically, consider them the particles that make light waves,) I figure all you technically have to do is keep photons from going through you and – ta-da! – shadow!

Photons?

For my shadow-making purposes, I’m just dealing with whether at least one photon gets knocked off course.  But, it should be noted that light, among other quantum entities, is a particle and a wave at the same time, as was figured out with the classroom-famous double-slit experiment.  Recently, some intrepid scientists at a Swiss university photographed light in both its forms simultaneously.  An article and animated video of how they did it can be found here.

Using the naked eye…

The smallest shadow that you can see would be made by whatever object is so small that you can barely see it, so if you’re, say, me, the smallest visible shadow would be from maybe a grain of sand about the size of this period here.

Using some serious tech…

Some scientists at an Australian university made a bit of a splash when they photographed the shadow made by an atom using only visible light and an extremely powerful optical microscope.  

Says Professor Dave Kielpinski of Griffith University's Centre for Quantum Dynamics:  "We wanted to investigate how few atoms are required to cast a shadow and we proved it takes just one. We have reached the extreme limit of microscopy; you can not see anything smaller than an atom using visible light.”

Check out the atom shadow right here.

Using my basic definition…

And coming at it from the particle physics realm, you could actually be no size and still block a photon and therefore have a one-photon-sized shadow.  That is, if you’re something like an electron.

Electrons are so teeny that physicists consider them to just have no size.  Basically, they’re a one-dimensional point for all intents and purposes, and mathematics involving electrons work out just fine if electrons are sizeless and referred to as a “point particle.”

Handily, photons are similarly teeny, all energy and no mass (like the toddlers of the quantum world perhaps?), which means they go and go at the speed of, well, light, until something stops them.

But point-like as electrons may be, they still don’t usually let photons pass right through.  It's hard to tell because of the wave nature of all parties involved, but we're quite certain that photons can collide with electrons enough that diagrams are made to commemorate the moments. 

When photon meets electron, they can collide with a variety of results, including the creation of totally different particles, but always with some change in energy and trajectory.  Regardless of what exactly happens, it would be rare indeed for you to find the photon continuing on its initial path once a collision has occurred, so technically, the photon didn’t land behind the electron, and therefore – ta da! – shadow!

If this all sounds a little too far-fetched and you need some better insight, ask the Shadow.  He knows.  J