weight of shadow

Hey, MadMax here.

And I'm sure that we all love to have fun with hand shadows, but how much does a shadow weigh?

It might sound like a silly question, because it is. I mean, a shadow cannot be put on a scale and weighed. But the material that it falls on top of can be weighed. And we know that light has energy. In fact, whenlight encounters an object, it pushes that object just a little bit. On the surface of Earth, when sunlight is hitting it, every square inch is being pushed with a force of about one-billionth of a pound, which is basically nothing. But, over a large enough surface area, the results can be pretty fun.

On a sunny day, the city of Chicago weighs 300 pounds more, simply because sunlight is falling on it, pushing it. In outer space, where solar wind isn't filtered by Earth's atmosphere or magnetic field, the results are even bigger. A space craft, travelingfrom Earth to Mars, would be pushed by light 1,000 km off course. So these things have to be factored into journeys to Mars. We've actually already created things that can sailwith light: giant reflective solar sails that are pushed by the Sun's light. So, in a way that is calculable, though difficultto measure, an area covered in shadow technicaly weighs less than surrounding areas being pushed by light. But enough about the Sun.

There are 3 astronomical bodies that can cast shadows on the surface of Earth bright enough for us humans to see.One is obviously the Sun, and the other is the Moon.But what's the third? Venus. Pete Lawrence investigated this over a digitalsky. Now, to make sure that the shadow he saw was caused by Venus, he used a tube thatcould be pointed at specific regions in the sky.

Inside the tube, he put a cut out shapedlike the astronomical symbol for Venus. Now, here is light coming through the tubewhen pointed just adjacent to Venus at a point in the sky relatively dark and empty tothe human eye. But here is what came out of the tube when pointed at Venus - a Venusian shadow. We all know that light travels quickly - 299,792,458 metres per seconds = c. But this light right here, in fact, the lightcoming off your screen into your eyeballs right now, is moving slightly slower than"c" because "c" is the speed of light in a vacuum, but all of this light if having to travelthrough a medium, in this case air. The speed of light in air is ever-so slightlyslower than "c"- 298,925,574 m/s.

This is interesting because light travels more slowly through different materials, but "c" remains the universal speed limit, and as long asan object doesn't go that fast, it can still outpace light in a material. A charged particle, for instance an electron,can travel through water faster than light does, but never faster than "c". When this happens, we get something analogous to a sonic boom. We get a "Photonic Boom." In a sonic boom, the sound information propagatingoff of the object is in the form of compression waves, and as the object gets closer and closerto the speed of sound, the speed that those waves are moving away from it at, each newwave has less time to get out of the way of the next, until eventually the waves collapseall into each other and the denisty and pressure is enormous, causing a sonic boom.


Future reading