From the brains behind brains on, this is the Moment of Um.

Moment of Um comes to you from APM Studios. I'm Anna Goldfield.

Um.

When I was a kid, I really liked thinking about what ifs, you know, imaginary or impossible

ideas. Like, what if there was a cow who made chocolate milk? Would she have to eat

chocolate grass? If she jumped on a cow trampoline in winter, would she make chocolate

ice cream? Hmm. How high could you bounce on a cow-sized trampoline? And so on. Our listeners

have a lot of what-if questions, too, like Dax, who asked, what would happen if everyone on Earth

went to the same place? And then, what if they all jumped in the air

at the same time? Well, let's see if we can get the scoop from an expert.

So what would happen if everyone on Earth went to the same place and jumped at the same time?

My name is Dr. Kylie Kennedy, and I am a physicist at Princeton University,

where I do research on the tiniest particles in the universe. I'm really interested in

understanding what the building blocks of the universe are and how they interact to form the world

we know today. So when I think about this problem, I want to paint the picture of two ice skaters on an ice skating rink.

So if you and your friend go to the ice skating rink and you stand in the middle and you then push off of each other,

both of you are going to go in different directions.

Now, if you go and stand with a big group of people, so there's like 10 of your friends and family and just you,

and then you push off of them, what's going to happen is you're actually going to go farther away than the group of 10 people.

They might move just a little bit, but they're not going to go as far apart as if it were just you and your friend.

The reason why it's easier for you and your friend to push each other away and why when you push off your friend you guys go

in equal and opposite directions and why that's different from when you push off your group of

...