Exploring the Unseen Forces

"Do you want to see the most illegal thing I own? It's a penny from 2027. That's right, a piece of counterfeit US currency. Or is it? There are no 2027 pennies today, which means this is a counterfeit of an original that doesn't exist yet. I mean, sure, if you didn't look at the date, this could pass as real. But will it not truly be counterfeit until 2027? Well, what we do know is that it's a novelty item with a lifespan. It's really cool to show people today – a penny from the future. But in 2027, it will become indistinguishable from a common penny and cease to be as interesting.

I cannot tell you where I got this, but the point is, I can break the law. I am free – perhaps not free from punishment, but nonetheless, here we are. I cannot break the laws of physics; nothing can. Today, I want to explore the lawful behavior of spinning.

Let's start with a physics classic: as a spinning ice skater pulls their body in closer, they spin faster and faster. It's really cool, but what is accelerating their rotation? What is pushing them around faster and faster? You can try this at home with a chair that spins and some really heavy books in both hands.

Now, to get started spinning, I'm gonna have Jake give me a push. No, I don't need Jake. Watch this – with him gone, I can nonetheless speed myself up by just pulling the books in. If I move the books out, I slow down. Somehow, pulling the books in speeds me up. Now, if you look up why this happens, you'll probably be told that it is because of the conservation of angular momentum.

Oh, the conservation of angular momentum – I love demonstrations of it, and this is one of my favorites. This is a Hoberman's sphere, a toy that collapses and expands. Doesn't it sort of remind you of me with the books far from my body and then the books close to my body? It behaves in much the same way. I need to give it some angular momentum to start, but once I do, if I bring all of that mass towards the axis of rotation, it speeds up and then it slows down, speeds up, slows down. I love this – thank you, conservation of angular momentum.

But what are you? If you look it up, this is what you will find for a particle in circular motion around some center of rotation. See, the angular momentum of the particle is defined as the product of the particle's mass, the particle's instantaneous velocity V, and the particle's distance from the center of rotation, R – MVR. This is angular momentum. If you divided me, the chair, and the books I'm holding into a bunch of tiny little particles and found the MVRs for every one of those particles and summed them all up, you would have the angular momentum of the entire system.

But notice that this is just a mathematical expression combining three different measures. It's not a physical substance you could pull out of a particle and hold in your hand or put in a jar and study. It's kind of like taking my weight, multiplying it by the number of countries on Earth, and then multiplying it by time. However, unlike something like that, this concept is really useful because we have found that in our universe over time, it is conserved.

If the particle is pulled towards the center of rotation, its R value will go down, but angular momentum is conserved. So, one of these other variables must go up. At low speeds, mass is essentially constant, so the only option is for the velocity of the particle to increase. For the particle to spin around faster than it was before. If the R value gets smaller, the velocity must get larger, or else a law has been broken.

But how do atoms and molecules know to follow this law? I mean, is there some kind of physics police force in the universe bullying everything into compliance? How do all the atoms I pull in know to speed up, and why do they always obey our laws like embodied presences guiding matter around?

No, I have with me right now three things – a lamp, a nail, and a shadow. Gotcha, I actually have four things with me. The fourth is Inks astonishing ruler, designed by Vsauce. If you're subscribed to the Curiosity Box, you already have one or it's on its way to you. And if you're not subscribed to the Curiosity Box – well, you're missing out on the fruits of my mind.

I have always wanted a ruler like this, so I made one. And now I – and you – can have one. It is exactly one light nanosecond long, which means that this is the distance light travels in a vacuum during one billionth of a second. If I hold my hand just that far away from my eye, I am seeing my hand as it was a billionth of a second in the past. Pretty cool.

Now, this ruler allows you to measure lengths of things in light picoseconds, sound microseconds, Micro Everest's. That's one millionth of the height of Everest, beard fortnights, decimal inches, and of course, good-old centimeters. Now, if you asked me, 'Hey Mike, why is the shadow of the nail four and a half centimeters long?' Well, I would say, 'Who's Mike?' But then I would be helpful and answer that the length of the shadow is caused by the height of the nail and the position of the light source. And that's a pretty good explanation because if the nail were taller, and/or if the light source were lower, the shadow would be longer. Or if the nail were shorter, and/or the light source was higher, the shadow would be smaller.

Now, these three measures – the nail's height, the light's position, and the shadow's length – are all related mathematically. If you gave me only two of them, I could figure out the third – always. So, I could declare this relationship to be a law. But that does not mean that the law causes them to all have the measure that they do. For example, if you ask me, 'Hey, why is the nail six centimeters tall? What causes the nail to have that height?' Well, if I said, 'The nail's height is caused by the shadow's length and the light's position,' you'd be like, 'As if!' Alright, I mean, sure, we can figure out the nail's height by knowing about the shadow and the light, but that does not mean that they are causing the nail to have the height that it does. To know why the nail is six centimeters tall, we'd have to ask whoever manufactured it, or whoever sets nail standards.

Or whatever – the point here is that we should not confuse relationships, laws, with causes – explanations that involve the causal reasons for things are often better.

But what is an explanation? Well, that's easy to answer – explanations help us understand things. But what does it mean to understand? Does it mean to not stand up fully, or does it mean to stand underneath and look at from below? If I can understand something, can I also overstand something? Well, as it turns out,