Key concepts
Physics
Mass
Gravity
Spacetime

Introduction
Do words like “general relativity,” “gravity well” and “spacetime continuum” sound intimidating? Don’t worry, you don’t have to be Albert Einstein to understand them! Try this fun activity to learn about these concepts and black holes—using some common household materials.

Background
We experience gravity all the time. It’s such a regular part of our lives that we might not give it much thought. It keeps our feet planted on the ground so we don’t float off into space. If we throw a ball up, it comes back down. Pretty easy, right?

In classical, or Newtonian, mechanics, gravity is described as a force that acts between two objects, pulling them together. Technically any object with mass exerts a gravitational force on other objects. This force between objects, however, is usually too small for us to feel. You don’t feel a sideways gravitational pull from the person standing next to you, for example, or even from bigger objects such as cars or buildings. We only feel the gravitational force from huge masses, such as Earth.

Einstein's theory of general relativity describes gravity a little differently. First it describes space and time as a “fabric,” or “continuum” called spacetime. (If you think of space as three-dimensional and you add time as another dimension, then you get something that has four dimensions.) General relativity describes gravity as the curvature of this four-dimensional spacetime, which is caused by mass. So large masses such as Earth cause a large curvature in spacetime whereas smaller masses such as our bodies barely cause any curvature. If that sounds confusing, don’t worry—this activity will help you visualize it.

Although Earth's gravity is very strong, we can build rockets and spaceships that can escape it and fly off to the rest of the solar system. We can also send electromagnetic radiation such as light or radio waves out into space. A black hole, however, is an object in space with so much mass and with gravity so strong (that is, it curves spacetime so much) that almost nothing can escape it—not even light. Anything that passes too close to a black hole (inside a region called the event horizon) will be sucked in. You might wonder how astronomers know black holes exist. After all, if light cannot escape them (meaning they do not emit light or reflect light), how can we see them? Scientists know black holes exist because we can observe their effects on other celestial bodies such as stars. For example, a star’s orbit may be affected by the gravity of a nearby black hole, and we can observe matter being sucked into a black hole (before it disappears behind the event horizon).

In this activity you will use some simple household materials to demonstrate how a large mass such as a black hole curves spacetime—and what effect this has on other nearby objects.

Materials

• Stretchy fabric, such as a polyester/Lycra T-shirt (not cotton) or a large athletic bandage
• Round, heavy object, such as an apple, orange or pool ball
• Two marbles or ping-pong balls
• Two helpers
• Marker and ruler (optional)

Preparation

• If you are using an athletic bandage, cut a large rectangular piece out of it.
• Have your two volunteers hold your stretchy fabric by the corners, pull it taut and hold it flat. This fabric represents spacetime.

Procedure

• Put a marble somewhere on the fabric and watch it. What happens?
• Now roll a marble across the fabric and watch how it moves. What shape does its path take?
• Place your heavy, round object in the middle of the fabric. Make sure your volunteers still pull the fabric tightly enough that it does not have any wrinkles or bumps in it—try to make sure it’s as smooth as possible. What happens to the fabric?
• Next place the marble near the edge of the fabric and let it go. What happens?
• Now try to roll the marble from one side of the fabric to the other. Is it difficult? What shape does the marble's path form?
• Try rolling the marble across the fabric at different speeds. What happens if you roll the marble slowly? Faster? How does the shape of its path change? Is there a minimum speed you need to roll the marble to prevent it from getting sucked in toward the larger ball?
• Try rolling two marbles on the fabric at once. Do the marbles appear to affect each other’s motion (assuming they do not collide)?
• Extra: If you don’t mind drawing on your fabric, lay it flat on a table. Use a ruler and marker to draw a grid of squares on the fabric. The grid should consist of lines that intersect each other at right angles. Now repeat the activity. What happens to the grid?

Observations and results
You might think the results of this activity are obvious at first. When you hold the fabric flat and put a marble on it the marble doesn’t move. If you roll the marble across the flat fabric, it moves in a straight line. No surprise there! Remember, however, that your fabric represents spacetime. When there is no large mass present on the fabric, it doesn’t curve at all, which is why your marble can just sit there or move in a straight line when you roll it. This is how a beam of light would travel through space if there were no black holes nearby.

When you place a heavy ball in the middle of your fabric, it causes the fabric to curve downward, just like the presence of a large mass (such as a planet, star or black hole) causes spacetime to curve, forming a gravity well. It’s now nearly impossible to get the marble to hold still—it always rolls toward the middle. When you try to roll the marble in a line, it follows a curved path. As long as the marble doesn’t get too close to the middle you could probably still roll it from one end of the fabric to the other—similar to how a beam of light that passes near a black hole (but not inside the event horizon) will curve but then keep going. If it gets too close to the center, however, the marble gets sucked in and cannot escape—just like light cannot escape if it ends up inside a black hole’s event horizon.

When you rolled two marbles at once you probably noticed that (as long as they didn’t collide) they did not really affect each other’s motion. This is because the marbles are not heavy enough (they do not have enough mass) to introduce any additional curvature in the fabric. All of the curvature is due to the much heavier ball in the middle. This demonstrates why we only notice Earth's gravity pulling us down and we don’t get pulled sideway by other objects such as people, cars or buildings—their gravity is much too weak compared to Earth's.

More to explore
What Is a Black Hole, from NASA
Showing Science: Watch Objects in Free Fall, from Scientific American
A Brief History of Time
, by Stephen Hawking
Astrophysics for People in a Hurry, by Neil deGrasse Tyson
Science Activities for All Ages!, from Science Buddies