How much does a shadow weigh?

It’s an odd question, because just a shadow cannot be weighed. however the material that the shadow falls on can be weighed. We all know that light has energy, in fact, when light lands on a object it pushes that object ever so slightly. On the surface of earth when sunlight is hitting it, every square inch is being pushed with a billionth of a pound which is pretty much nothing. But, over a large surface area the results can be pretty awesome. On a nice sunny day the city of Chicago weighs three hundred pounds more than at night. Just because the light is pushing it.

In outer space, where solar wind isn’t being filtered by earths atmosphere or magnetic field, the results are even more awesome. A space craft traveling from earth to Mars, would be pushed one thousand kilometers of its course, just by light. So these things have to be factored in on journeys to Mars. We’ve actually made spacecrafts that move just by being pushed by light. So, in a way that is calculable but very hard to measure, an area covered by shadow technically ways less than an area being pushed by light.

There are three astronomical bodies that can cast a shadow bright enough for humans to see: one is the sun; the other is the moon; and the third is weird, the answer is Venus. Pete Lawrence investigated this over a digital sky, to make sure the shadow he saw was from Venus, he used a tube that could be pointed at specific regions in the sky. Inside the tube he put a cut out shaped like the astronomical symbol for Venus.

Here is light pointed just adjacent to Venus, At a point in the sky relatively dark and empty to the human eye.

But, here is what came out of the tube when pointed at Venus.

A Venusian shadow.

We know that light moves quickly, two hundred and ninety nine million, seven hundred and ninety two thousand, four hundred and fifty eight meters per second, or C. But the light coming off of your screen and going into your eyeballs is moving slightly slower than C. Because C is the speed of light in a vacuum. And the light coming off your screen and going into your eyeballs right now is having to travel through a medium. In this case, air. The speed of light in air is ever so slightly slower than C. In a vacuum the speed of light is 299,792,458 meters per second. But the speed of light in air is 298,925,574 meters per second.

This is interesting, because light travels more slowly through different materials. But C still remains the universal speed limit. And as long as an 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 propagating off of an object is in the form of compression waves, and as the object gets closer and closer to the speed of sound, the speed that those waves are moving away from it at, each new wave has less time to get out of the way of the next, until eventually those waves collapse all into each other, and the density and pressure is enormous, causing a sonic boom.

Normally, when a charged particle moves through a material who’s molecules can be polarized, the molecules give off photons, but each photon has room to fly away and the waves all destructively interfere with each other, so no radiation is given off. But the faster the particle goes, the less room the photons have away from each other, and their waves start to constructively interfere, giving off a photonic boom, also known as Cherenkov radiation. Astronauts, especially those that have gone all the way to the moon have reported seeing flashes of light. Many people attribute this to high speed particles moving through the liquid inside their eye faster than light normally would, causing photonic booms right inside their body.

Speaking of the speed of light, here’s an interesting theory of a way to go faster than light: let’s say i want to push a button a light year away, which takes light the fastest thing in the universe a year to get there. Now let’s say i built a board a light year long all the way from me to the button, and pushed. Would the other end immediately follow in which case did i just send information faster than light?

Well, we’re not talking about the speed of light anymore, we’re talking about the speed of push. When you push a rigid object, you’re really putting a series of compression waves through the object which move at the speed of sound, in the object’s material. The information about “Whoa, we’ve been pushed, you should move,” is sent by that compression wave, and it only travel at the speed of sound. So when pushing a normal day to day type object, it feels almost instantaneous, but when pushing a light year long board it take a lot longer. It would take about seventy five thousand years to travel a light year.

But here’s the point: the speed of push is not instant, and it’s certainly not the speed of light, but light can push you…in fact, technically you weigh more when the lights are on then when the lights are off.