Which Way Is Down?

Hey, Vsauce. Michael here.

Down here.

But which way is down?

And how much does down weigh? Well, down weighs about a 100th of a gram per cubic centimeter.

It is light and airy, which makes it a great source of insulation and buoyancy for water birds.

But if you let go of down...

It falls down.

So that's which way down is.

It's the direction gravity is pulling everything in.

Now, for someone on the other side of the Earth, my down is their up.

But where are falling things going? Why do things fall?

Are they being pushed, or pulled, or is it because of time travel?

First things first: let's turn the Sun into a black hole.

We can do that using Universe Sandbox 2, this simulator will blow your mind. I love it.

In fact, I love it so much I put a code to get the game for free in the current Curiosity Box.

If you're not subscribed to the box yet, you are missing out!

Okay, look, for the purposes of this video, we want the Solar System.

And here it is. Notice that everything's moving pretty quickly around the Sun.

That's because we currently have the game set so that every second that passes for us,

is 14 days, almost, in the game.

If I change this to one second,

we're looking at the Solar System in real time.

You'll notice that it almost looks like it's frozen.

Even though the earth is traveling around the Sun at about 30 km/s, it barely appears to be moving.

That is how vast space is. Anyway, let's go back to 14 days

I like that motion. Now look at the Sun

It is not, currently, a black hole, but we can change that. What we need to do is compress the Sun.

So let's lock its mass so that it doesn't change while we make its radius smaller.

Let's make its radius as small as we can.

And, oh, where'd it go? Well it's still there, it's just become a black hole.

Pretty spooky, but now, let's look at the rest of the Solar System.

Alright, zooming out and-

huh.

Nothing's... changed. I mean something's changed.

It's colder and darker, but nothing's flying off into space or getting sucked in.

You see, by shrinking the Sun, we didn't change the direction of down for the planets.

They're always being pulled by gravity towards its middle and making it smaller didn't move where the middle was.

But also, the strength of that force pulling them to the middle of the sun stayed the same.

That gives us a clue as to what down is.

The clue is the other thing we didn't change: mass.

Mass is a measure of how hard it is to accelerate something; to change its motion.

Now right now, these two balls have zero motion relative to me.

Slapping around this hollow plastic ball

is pretty easy, but doing the same to this solid steel ball

is a lot harder.

Now gravity and weight have nothing to do with this.

Gravity acts downward, not against my horizontal slapping.

Of course, gravity does contribute to friction, but friction works against me when I start moving the ball,

but works with me when I stop the ball.

And the steel ball is harder to stop than the plastic ball.

The difference is mass. The steel ball is more massive

It's more resistant to having its motion changed.

Mass is an intrinsic property; it does not depend on what's around or change from place to place.

It can sometimes be thought of as the amount of matter something has.

Your mass is the same regardless of where you are.

On the moon, on earth, in the middle of intergalactic space floating around.

But all of this said, mass does seem to care about what's around.

Mass loves company.

Things with mass and/or energy are attracted together by a force that we call gravity.

The feeling of gravity is just you and the earth being attracted to one another.

Now every portion of an object with mass attracts other portions towards it.

The average of all this pulling is an attraction between centers of mass.

Giant things like Earth exert an obvious pull, but everything does. Even a baseball.

These two baseballs are attracted together by their own gravities.

Except their masses are so small, the force is minuscule, and it can't overcome friction or push air out of the way.