Astronomy

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Video

Materials

• 1 large pot
• Plastic wrap
• Tape
• 1 needle
• 1 heavy ball or marble (or other heavy and small object)
• 1 light ball or marble

Short explanation

In this model, the plastic wrap acts as spacetime - the "fabric" that permeates the entire universe. Each body with mass curves spacetime and causes other bodies to move in the curvature (funnel) that is formed. This is how the Moon is kept in orbit around Earth and how Earth is kept in orbit around the Sun.

Long explanation

Before 1916, it was a mystery how gravity works. Why does the Sun keep Earth in orbit and why does Earth keep us humans on the ground? It was known that gravity existed, but how did it really work? The Sun and the Earth didn't seem to be in contact with each other in any way. How was gravity conveyed?

Einstein provided the solution in his general theory of relativity. The universe consists of four dimensions - the three space dimensions (up-down, right-left, forward-backward) and the time dimension - which are intertwined in what is called spacetime. Earth, like everything else with mass, curves spacetime. It's very difficult, or perhaps impossible for the human brain, to imagine this curvature. But in this model, the four-dimensional spacetime is simplified to the two-dimensional surface of the plastic wrap, and we get an idea of how it works. In fact, there is no major curvature of time on Earth - this is only noticeable in more extreme situations, such as near black holes or just after the Big Bang - so the two-dimensional plastic wrap is really more a simplification of just three-dimensional space, something that is not completely alien to us.

If you look at the plastic wrap directly from above, you will not see that it's curved around the large ball. But still the little ball orbits around it. That's just the way it is in reality. Space is curved around the Sun even though we do not see it, and Earth moves round and around in this cone. However, Earth doesn't slow down as the small ball does, and this is because there is no friction in space, like it is between the ball and the plastic wrap.

Experiment

You can turn this model and demonstration into an experiment. This will make it a better science project. To do that, try answering one of the following questions. The answer to the question will be your hypothesis. Then test the hypothesis by doing the experiment.

1. Pretend that the big ball is Earth and the small ball is an asteroid.
1. What happens to the curvature of spacetime if Earth gets heavier (you can press down on Earth to simulate this)?
2. What then happens to Earth's gravitational pull? Does it get stronger or weaker?
3. What then happens to the risk of being hit by an asteroid? Does it get greater or lower?
4. What would need to happen for the asteroid to end up in orbit around Earth?
2. Pretend that the big ball is Earth and the small ball is the Moon.
1. How is the Moon's orbital period affected if the Moon were to get closer to Earth?
2. What happens if you place the Moon in an elliptical ("oval") orbit?
3. What would happen if Earth and the Moon had the same mass?
4. What would happen if Earth had two identical moons in orbit around it?
5. Can you place the Moon in an eternal orbit in this model?
3. What happens if you put two large balls in the middle, with a small distance between them, and put a small ball in orbit around both?

Gilla:		Dela: