How Much Does a Shadow Weigh?

Hello, Vsauce. Miguel here. And I'm sure we all love to have fun with hand

shadow

s, but how

much

does

a

shadow

weigh

? It may seem like a silly question, because it is. I mean, a shadow can't be put on a scale and

weigh

ed. But the material on which it falls can be weighed. And we know that light has energy. In fact, when light encounters an object, it pushes it a little. On the Earth's surface, when sunlight hits it, every square centimeter is pushed with a force of about a billionth of a pound, which is basically nothing.

But, on a large enough surface, the results can be quite funny. On a sunny day, the city of Chicago weighs 300 pounds more, simply because the sunlight falls on it and pushes it around. In outer space, where the solar wind is not filtered by the Earth's atmosphere or magnetic field, the results are even greater. A spacecraft traveling from Earth to Mars would be pushed by light 1,000 kilometers from its trajectory. So these things must be taken into account when traveling to Mars. In fact, we've already created things that can navigate with light: giant reflective solar sails that are pushed by light from the Sun.

So, in a calculable, if difficult to measure, way, an area covered in shadow technically weighs less than surrounding areas. driven by light. But enough about the sun. There are 3 astronomical bodies that can cast shadows on the Earth's surface bright enough for humans to see. One is obviously the Sun and the other is the Moon. But what is the third? Venus. Pete Lawrence investigated this over a digital sky. Now, to make sure that the shadow he saw was caused by Venus, he used a tube that could be pointed at specific regions of the sky. Inside the tube, he placed a cutout in the shape of the astronomical symbol for Venus.

Now, here is light coming in through the tube as it points right next to Venus at a point in the sky that is relatively dark and empty to the human eye. But this is what came out of the tube when it was pointed at Venus: a Venusian shadow. We all know that light travels quickly: 299,792,458 meters per second = c. But this light here, in fact, the light coming from your screen into your eyeballs right now, is moving a little bit slower than "c" because "c" is the speed of light in a vacuum, but all this light

does

have to travel through a medium, in this case air.

The speed of light in air is slightly slower than "c": 298,925,574 m/s. This is interesting because light travels more slowly through different materials, but "c" is still the universal speed limit, and as long as an object is not going as fast, it can still outpace light in a material. A charged particle, for example an electron, can travel through water faster than light, but never faster than "c". When this happens, we get something analogous to a sonic boom. We get a "photon boom". In a sonic boom, the sound information propagating away from the object is in the form of compression waves, and as the object gets closer and closer to the speed of sound, the speed at which those waves move away from it, each new wave has less time to move out of the way of the next, until finally the waves collapse into each other and the density and pressure are enormous, causing a sonic boom.

Normally, when a charged particle moves through a material whose molecules can polarize, the molecules give off photons. But each photon has room to fly and all the waves interfere with each other destructively, so no radiation is emitted. But the faster the particle moves, the less space the photons have between each other and their waves begin to interfere constructively, emitting a photonic burst: "Cherenkov radiation." Astronauts, especially those who have traveled to the Moon, have reported seeing flashes of light. Many people attribute this to high-speed particles moving through the fluid inside your eyes faster than light normally would, causing photonic bursts within your body.

Speaking of the speed of light, here's a great question some of you have sent me. This is a possible way to go faster than "c". That's how it goes. Let's say I want to press a button that is one light year away from me, which means that light, as fast as possible in the universe, would take a year to get from me to the button. Ok, what if I built a board, one light year long, from me to the button and then pushed one end of the board? Would the other end immediately press the button? And if so, did I just break the speed of light?

Did I just send information faster than light? Well, we're not talking about the speed of light anymore, are we? We are talking about the speed of the thrust. When you push a rigid object, what you are really doing is putting a series of compression waves through the object, which move at the speed of sound in the material of the object. The information about "wow, we've been pushed, you should move" is sent through that compression wave and it only travels at the speed of sound. So when you push a normal sized, everyday object, it feels almost instantaneous.

But pushing a board one light year long would take

much

longer. An interesting way to see this in action is to look at an object in which compression waves travel more slowly. For example, what Veritasium has done: leave people speechless by showing Slinky's abandonment. The information that tells the Slinky that "hey, we're moving" travels through the Slinky slowly enough that a slow-motion camera can see what's happening. If you haven't seen all of Veritasium's Slinky videos, you missed out. In fact, you should watch all of his stuff, it's magnificent. But to summarize, this is the point. The speed of the thrust is not instantaneous and certainly not the speed of light.

But the light can push you. In fact, you technically weigh more when the lights are on than when they're off. I've put links in the description to all kinds of cool stuff that you should definitely check out for fun and science. And as always, thanks for watching.