# Gravity Visualized

Dan Burns explains his space-time warping demo at a PTSOS workshop at Los Gatos High School, on March 10, 2012.

### Transcript:

Relativity, the explanation for gravity, is that matter bends space. And so you put mass in place in space and warp space-time. And objects are not feeling a force of gravity. They’re just following the natural curvature.

And so this is a sheet of lycra; where do you get this is my old bike shorts. There’s a spandex.com, and you can get a sheet like this if you buy the sale stuff, so you don’t care what it looks like, right? For like 20 bucks, maybe less, depending on the sale. And so you put the matter, and it works space-time. And so, if I have another object, it also works space-time. They feel that, and they’re attracted to each other.

And so that’s, that’s Einstein’s picture of gravity objects, work space-time, feel like curvature, encourager and move accordingly. And if you have more mass, you’re out there.

You have more mass; it’s going to bend space-time more. And so if you have objects, they’re going to respond to that, right. And so you put something there that’s attracted. Now, in reality, that big mass would feel the warping of space-time by the marble, too, right? It would move a little bit, but we usually ignore that, you know, the earth makes the sun move a little bit. Still, it’s so small, and you can ignore it. The moon makes the earth kind of wobble around a point three-quarters of the way from the center of the earth, and we usually don’t account for that when we’re looking at satellite motion.

Well, instead of just letting go of one, what if I give it a sideways push? It orbits now. It’s losing energy, which wouldn’t happen. The in the solar system, right? Not noticeably, there are some perturbations from other planets and things. But this one does lose energy and spirals in. If I don’t push it as hard, it will do an ellipse for a while anyway. And more air.

So one of the things I thought to do with this is when you study the solar system, all the planets are going around the sun the same way. Why is God deciding that the only life clockwise or anti, whether it’s above or below the solar system, right? Why is everything going in the same direction? Well, the answer is, it wasn’t different directions. But there was a preferred direction to distant form from had a slight preference one way versus another. And things go oppositely got eliminated. And when it’s all said and done, everything’s going the same way. That usually works 90% of the time.

And so I have slightly more in one hand, and another bio consciously do that. And you want to get them to the spread out a little when you throw it. Because they’re colliding with each other, that you’re the ones going up the wrong way get eliminated, right.

Now, there also would have been, you know, this is where this analogy breaks down. There’s, you know, another dimension, right. But those things are out of the plane that wasn’t near that preferred plane also got eliminated. And so that’s kind of cool.

Now my students use a Ph. D. T PhET. Simulation called my solar system. And so they’ve seen all this on the computer. And when we do this, during our special relativity unit, they’re like, hey, it’s just like the computer that’s kind of cool that a spreadsheet a lycra model saying sit inside just like it but similar to the computer. Now, it’s some things we’ve discovered using this outside system, larger marbles, my friend with them out again, but they’ve been space-time too, right. And so maybe you can get one thing to orbit another. There we go.

So the earth-moon system maybe. And so you can see that this seam, you can’t buy lycra, this bag. You get it in rolls, and you have to sew it together. I’ve been told to use stretchy string stretchy thread. To sew it together. And so next time I do it all, do that because it pulls. And so this is a little bit of thread fixes that. And so in the morning, when I first set this up, I’ll have it leaning against the table, I’ll be underneath fixing holes from last year, one time my principal walked in, and I hear somebody in the room, and I’m underneath I’m in the middle. So in it, I’m going to pause yet.

And then he’s looking around for me, and he knows somebody in the room. He’s Oh, there you are. What are you doing? There I go, I’m repairing a rip in the fabric of space-time. Often to get to say that. Another thing that says you’ve probably seen plastic ones of these, right, the Discovery Center Children’s Discovery Museum in San Jose has one the Exploratorium has one. Sometimes they have been shopping malls to get you to put money in it’s, but you can’t do one thing with those is put to masses.

And so when we went to the moon, when we first started, we set our spaceships on what are called free return trajectories. So they would go to the moon. And this is hard to get to happen. And they would come back and like a figure eight. And so that lets you show. Now, of course, the move was smaller. But if you look at the plots of the free return trajectory back from the Apollo program, you’ll see it has a figure-eight sort of shape.

And so, this takes a little bit of practice to be able to do the figure eight. These clamps around here, you can adjust it to make it tighter if it’s not working. So Steve and I, when we set this up in the morning, we played around with it. And I think it was working better than it usually is. So not easy. And so I go through that one year, a kid who read some stuff about the dark energy or dark energy cold. Light having to have that extra. Oh, I guess any of these words. And so I’m just thinking, energy, what is it that makes everything come apart? Right? And so dark energy would be that
stuff goes apart rather than toward it. So if you can work in dark energy anytime, right?

And so I go through that spiel, maybe it takes me 1015 minutes, talking with the kids, you know, if they’re not familiar with, and then I just let them play. And so that’s where we discovered the earth-moon thing is I just let it play. And they’re like Mr. Burns, come over here look. And they were all excited because they discovered that said, Oh, yeah, let’s go. And so there’s that I decided to throw out a few other things. So they can try this selected stuff over time. And put out all the marbles.

So if you want to try it out, but we do, we probably won’t get back to electricity, magnetism, but we want to play Mr. Burns. So I’d say the whole thing if you bought all the parts, and everything is about \$100. But one of my favorite days, so you’re going to do it Monday, right Stevie. But do it once, and you’ll be hooked. And it will take a little bit of fun to put it together. But I’ll post the plans. And it’s not a whole lot of work or takes a portable thing, right? It takes a bit to come apart, all the PVC pipe, wrap it with a bungee cord, stuff the like are in that box, and I can carry the whole thing myself. This is about the right size. You can get your whole class around it pretty much smaller, what one problem is reaching. Some of you, I think, would have trouble with ads. He gets a student to do it right. When they got to watch, though, some students will think to go underneath the push-up. And then, while they’re underneath, somebody decides to put the big mass down. You know, they get their space-time worked. And so you tell them you can’t go under this.

We will catch we will get things are so new and is dying. And so is general relativity in the state standards. No, probably the crowning achievement of science, not in the state standards. Can you fit in after the star tests? You bet you can find some videos as you know, I show him a thing on gravity waves, and then we do this, and that’s you know, a day, almost a day after. But since I don’t understand it myself, that’s plenty. So let’s go back into the room.