Gravity Demo1/25/13

This demo/activity in Space Odyssey will give visitors a hands-on feel for how gravity works. Specifically, how Newton interpreted the force of gravity between a planet and a star and how Einstein re-interpreted this same seeming attraction.

The demo/activity uses; a gravity well, various props and the Galaxy Stage computer screens and looks like this:

More on that later but first some background on gravity.

Historical background Gravity is the “force” that causes two objects to attract each other. If there is nothing

to stop them, gravity will cause the two objects to accelerate toward each other. The reason I put quotes around the word “force” is because, as we shall see, Newton interpreted this motion as a force of attraction, while Einstein interpreted the same motion as a curvature of spacetime. More on this later.

Even in the time of the ancients, people knew that objects fell “downwards.” A better way to think of it would be that objects move towards the center of the Earth, but you would never think of phrasing it that way if you didn’t know the Earth was round.

Scientists studied the motions of falling objects to see if they could figure out any patterns. Perhaps the most famous of these experiments is one that probably never even happened, and that is Galileo’s famous Leaning Tower of Pizza Pisa experiment. He determined that, if you disregard air resistance, every falling body drops at the same rate—although he didn’t know why. (He actually did experiments with balls rolling down inclined planes to reach his conclusions.)

Another, seemingly unrelated set of observations was made by Kepler who determined that the planets orbit the Sun in ellipses (ovals) with the Sun at one focus of the ellipse. Again, he did not know why this was happening, just that it was.

Isaac Newton was the person who put it all together. As the story goes, he was sitting under a tree when an apple fell on his head and he had an “Aha” moment. He

realized that the force that pulled the apple down out of the tree was the same force that pulled the Moon around the Earth once a month. In other words, what Galileo and Kepler saw were really instances of the same thing.

You could say that Newton had “forces” on the mind. Forces are things pushing or pulling on other things. But this was different. Gravity was a “force at a distance.” And that bothered Newton. He couldn’t figure out how that worked, but he forged ahead anyway.

He explained Galileo’s experiment this way: Objects have what he called inertial mass, their resistance to being accelerated. A more massive object is harder to “get moving” (accelerate) than a less massive one. But, he reasoned, if the more massive object is falling at the same rate as the less massive one, then the Earth must be pulling correspondingly harder on it. In other words, if an object is harder to move, the Earth makes up for that by pulling correspondingly harder. If it is easier to move, the Earth simply doesn’t pull as hard. The two properties, how hard an object is to accelerate and how much the Earth pulls on it, exactly cancel out. But he didn’t know why.

At the same time, he looked at Kepler’s orbits of planets going around the Sun. He realized that if the Sun were pulling on a planet in such a way that the closer the planet is to the Sun, the harder the Sun would pull on it, the resulting path would be exactly what Kepler observed. (Newton, of course, was more specific. He determined, for instance, that if the planet and Sun were twice as far away from each other, they would pull with only 1/4 the force. This was his so-called inverse-square law.)

So, that’s how he summed up his theory of gravity. The more massive objects are, and the closer they are, the more they will pull on each other with a force of gravity. But, why that force worked that way, and how it acted at a distance, were still mysteries.

Along came Albert Einstein. He, like Newton, was also bothered by this “force at a distance” stuff. What Einstein did in his General Theory of Relativity was to come up with a totally different explanation of gravity. On the surface, things like planets orbiting stars LOOK almost like what Newton described.

But here’s how it works. Imagine you are in a race car speeding along a track. When you get to a turn, you must turn your wheels to keep your car on the track. But, what if it is one of those racetracks with banked turns? The bank of the track itself sort of pushes your car through the turn. If you did it just right, you could almost take your hands off the steering wheel and let the banked track push you around.

What do I mean when I say, “If you did it just right?” If you were driving too fast, and took your hands off the wheel, your car would go flying up and off the track. And, if you were driving too slowly, the bank of the track would push your car toward the inside of the track. But, at just the right speed, even with your hands off the wheel, your car would stay centered on the track. In a crazy way, it is like you are driving your car “straight” on a banked track. In fact, if you did it just right, and watched your steering wheel, it wouldn’t turn to the left or right. It would stay straight, even if your hands were off it.

Now imagine a banked racetrack that makes a complete circle. Once your car was at the right speed, you could take your hands off the wheel and you’d keep going round and round the track. The track, itself, would be sort of pushing you around.

Just to finish our imaginary racetrack, put a hamburger stand right in the center of the circular track with a giant neon sign. What would the situation look like from high above? It would look like the racecars were orbiting the hamburger sign. If it were dark out, and you couldn’t see the banked racetrack, you might even think that the hamburger sign had some force it was using to pull on the racecars. Almost like a force of gravity. How that “force” worked, you wouldn’t have a clue. But, if you couldn’t see and didn’t know about the banked track, what else could you think was going on?

By the way, and this is important, if you were high enough above the racetrack, you might think it was painted on a 2-dimensional surface and not even realize that it had a 3-dimensional thickness caused by the banking of the track. If you only thought it was a 2-dimensional painted track, you really wouldn’t know what was going on.

Einstein was the guy who figured out that the banked racetrack was there, even though he couldn’t see it — smart guy. He was the guy who said that you could have planets orbiting stars with what LOOKS LIKE a force of gravity when really they are just traveling along some sort of “banked track” in space. In the exact way that your racecar is pushed around by the bank of the track while your steering wheel stays straight, he posited that planets were moving “straight” along some sort of banked track around their stars.

Back to the real world.

In Einstein’s formulation of how gravity works, the “banked tracks” don’t actually exist in space itself, but in something Einstein called Spacetime. Spacetime is 4-dimensional. It has an extra dimension that we can’t see with our 3-dimensional perceptions. In the same way that our imaginary observer thought he was looking down at a 2-dimensional painted track and didn’t realize it was really a 3-dimensional banked track, a real observer looking at real planets around real stars might think he or she was looking at a 3-dimensional universe and not be aware that the planets were actually traveling on a 4-dimensional banked track in Spacetime. Traveling on “straight paths” on these 4-dimensional banked tracks, I might add.

One difference between our racetrack example and the real world is this. The banked racetrack sort of “pushed” the racecars through their turns. In the real world, there is no pushing or pulling of objects, like planets, traveling on the “banked tracks of Spacetime.” It is just that the planets simply stay on the banked tracks. And just as when you throw a ball in outer space it keeps traveling forever in a straight line with no additional force needed, planets traveling through Spacetime travel forever on “straight paths” on these 4-dimensional banked tracks, with no additional force needed. From the planet’s perspective, it is traveling in a straight line.

So Einstein concluded that massive objects, like stars, cause these 4-dimensional banked tracks to form, and the planets travel in straight paths along these 4-dimensional banked tracks. If you only knew about 3 dimensions, as Newton did, you might conclude that there was some “force” between the star and planet acting over a distance. But, if

you realized that there was this 4-dimensional banked track, then no force at a distance is necessary.

If all of this is a little confusing, no worries, after all, he was Einstein.

Components of the Demo/ActivityThe idea of this demo activity is to give visitors two different views of gravity:

Newton’s and Einstein’s. We do this by placing a gravity well directly below a downward pointing video camera which is mounted in the ceiling in the audience area of the Galaxy Stage. The imagery from the camera is displayed on the flatscreens above the Galaxy Stage.

Because the camera points straight down, if you look at the screens you don’t see the depth of the gravity well. It flattens the image into a 2-dimensional image. When balls are rolled on the gravity well (in 3 dimensions) it looks, on the screens, as if they are simply orbiting the star (in 2 dimensions).

Thought of differently, the screens show Newton’s view of gravity, as planets orbit the star in a plane, pulled by a “force of gravity.” You can also see what is “really” happening (Einstein’s view) by looking at the gravity well where the balls are rolling in an extra dimension and are not actually being attracted to the star at all. They are simply being pushed around by the curved surface that they are rolling on.

Equipment Required

The Gravity Well Fabric of Spacetime

Props:

Lacrosse Balls (White, Orange, Yellow) Racquet Ball (Red) Hand Ball (Blue) Large balls for Stars (Green, Orange,

Psychedelic) Computer Remote Projector Remote Ball on a String Bag to hold small props

Remotes

Projector Remote (Only for on /off) Computer Remote

Buttons 1 & 2: Scrolls through slide show or the live gravity well projections. Button 3: Toggles between Powerpoint and live camera.Button 4: Toggles the orbit tracks on/off.

Set-Up Procedure

The Gravity Well is stored behind the Galaxy Stage. Please remember to raise the stage’s backdrop so you do not risk damaging the backdrop.

Backdrop Down

Switch

Backdrop Up

Roll the Gravity Well down to the audience area. It should be positioned approximately in the center of the audience.

There is a metal pin that holds the well in its folded position. Removing this will let you fold the well into place. There are pins on both sides to hold the well in place once open.

Metal pin in storage positionMetal pin in demonstration position

Attach the legs.

Turn on the projector.

Eddie aiming at the projector Where Eddie is aiming

Log into the AMX. Stop the Default show and switch the stage computer to the Left and Right Monitors

(Note: It is possible you will need to go to the computer switcher backstage and make sure the Gravity Computer is selected. The red light should be blinking indicating the Gravity Computer is selected.)

Once the projector is on you can position the gravity well. The circular light from the projector should fit the circle of the gravity well.

Lock the Wheels and you are ready to begin.

How to do the activityThere is no particular order to do things in this activity. Especially since people will be coming and going the whole time. Your job is to give visitors a feel for what gravity is, how Isaac Newton explained gravity, and how Albert Einstein explained it.

As a general rule, what the visitors see up on the screens is like Newton’s view. What you do at the gravity well is Einstein’s explanation for how it all works.

So, here is a list of things to try. Pick and choose to tailor things to the visitors. Obviously, what you do for a group of kindergarten kids will be different from what you would do for a group of physicists.

Having conversations with your visitors about what is going on is the most important thing you can do.

Tell visitors that you are going to explain two different theories of how gravity works

Show a video of the Earth going around the Sun and ask why this is happening (Button 3 on the remote allows you to toggle to this).

We are going to come up with two different ways to simulate what is happening in this movie.

Show how you can create what looks like a planet orbiting a star in different ways.

o Newton’s way: twirl the ball connected to the string and explain that the string represents Newton’s “force” of gravity. (If you let go of the string the ball would fly off.)

o The problem is this: there isn’t really any string holding a planet and star together. So what is this force of gravity? How can a force act at a distance? Even Newton didn’t know.

o Einstein’s way: ASK: Is there any way to get the same motion of a planet going around a star without having a force of gravity holding them together? That is what we are going to try to do.

o Roll a ball inside the gravity well and show that the ball goes round and round because the plastic walls push it around.

Give a brief history of gravity

o Things fall down (toward center of Earth)

o Galileo figured out that everything falls at the same rate. (Drop two objects to show this.)

o Newton postulated that all bits of matter are attracted to each other by gravity. The same force that pulls an apple to the ground also holds the Moon orbiting around the Earth. But he didn’t know why.

Have visitor roll several balls at once.

Put one of the large balls in the center of the gravity well to represent a star.

Balls get faster as they get closer to the center of the gravity well, just as they get faster as they get closer to a star.

Remove the ball leaving an empty “black hole” in the center of the gravity well.

Flip through several of the images you can project onto the well. A grid. An elliptical orbit. Etc.

With the ellipse on the gravity well, see if visitors can roll the ball to stay on the ellipse. (Very hard)

Mention that the reason the balls spiral inward is because the friction on our gravity well model. If it were frictionless (as with real planets in space) the balls would stay on their same orbit forever.

What causes this curvature of Spacetime? Einstein said that the presence of matter (like a star) causes the curvature, and the more massive the star, the more spacetime curves.

Put the “fabric of spacetime” (lycra on hoop) onto the gravity well. It is better if you drape the extra fabric over the sides. It makes a better image on the camera.

Roll a ball across “flat” spacetime to show it goes in a straight line.

Put the smallest of the stars in the middle of the fabric to show how it curves spacetime, then roll a small ball to show how it orbits.

o Show how the rolling planet affects the motion of the star. They affect each other.

Put a heavier star onto the fabric to show how it curves spacetime even more.

Project the grid onto the fabric. Even when the grid appears distorted on the curved spacetime, it still seems “flat” on the screens. In other words, what Einstein interpreted as curved spacetime, Newton interpreted as flat space with a force of gravity.

Compare how balls orbit different weights of stars.

There are lots of other things you can try. But, this should get you started.

Other components in Space Odyssey having to do with gravity:

Orbits table (Kepler’s laws)

Galileo and the Leaning Tower of Pizza show (all objects drop at the same rate)

Doppler Demo: Star / Planet mobile: Center of Gravity

Orbits Table: Pluto and Charon rotating around common center of gravity.

Background materials (websites, videos, articles, digital collections links)http://science.howstuffworks.com/environmental/earth/geophysics/question232.htm - An explanation of Newton and Einstein’s theories.

http://csep10.phys.utk.edu/astr162/lect/cosmology/gravity.html -- Web Syllabus, Dept. Physics & Astronomy, University of Tennessee lecture on “Gravitation and theGeneral Theory of Relativity”

http://physics.about.com/od/quantumphysics/f/quantumgravity.htm - Discussion of an evolving gravitation theory based on a theoretical entity, a graviton, which is a virtual particle that mediates the gravitational force.