General Relativity and Grade-9 Astronomy

0) Gravity causes time to slow down.

Everyday Einstein: The GPS and Relativity

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1) Space is curved. Curved space is gravity.

2) We can`t see it - but we can measure it.

3) Gravity makes light bend.

4) Space is expanding.

PART 1: How does the sun make the planets orbit?

Masses cause spacetime to curve and the curve in spacetime tells other masses how to move.

Orbiting objects bend space as they move through it and should generate gravitational waves. These waves will radiate energy, so the objects should spiral into the center – just like they did with the spacetime fabric.

In 1993 Taylor and Hulse got the Nobel prize for measuring a binary pulsar spirally in as predicted.

We live in a 4-dimensional curved space which we cannot see or experience directly. We can get a sense of how it works by consdiering a space with fewer dimensions.

http://www.youtube.com/watch?v=BWyTxCsIXE4

PART 2: How can we detect the curvature of space?

Newton’s gravity predicts simple orbits like the one on the left. Einstein’s curved space predicts the pattern on the right. Careful measurements have confirmed Einstein’s predictions every time.

PART 3: What is the effect of curved space on light?

Mass curves spacetime into gravitational lenses.

The yellow cluster is 5 billion light-years away and it is causing multiple images of a blue galaxy that is twice as far. Gravitational lenses are a new type of telescope.

The universe appears to have a large amount of dark matter which does not reflect, absorb or emit light. This means we can’t see it. But, we can detect it because it will bend light.

http://www.youtube.com/watch?v=7xKFrdzhM2Y

Black holes are regions where gravity is so strong that light cannot escape so we can’t see a black hole directly. How can we detect them?

Inside the central white spot is a supermassive black hole. We can’t see it but there is so much matter is rushing into it, that a lot of it splashes out in two jets moving near the speed of light.

More direct evidence is expected soon using gravitational lensing. The images below simulate a star field with and without a black hole. http://jila.colorado.edu/~ajsh/insidebh/lensearth_640x480.gif

PART 4: How do we know the universe started in a Big Bang?

The galaxies are moving away from us and the more distant ones are moving faster.

There is a simple relationship between velocity and distance. This suggests that everything was once gathered in one point that exploded around 13.8 billion years ago – the Big Bang.

As the universe expanded - it cooled. It is now at -271 degrees Celsius, emitting microwave radiation in all directions.

Looking at distant objects means that you are looking back in time. You are looking at the past. When you look at a star “Is that star really there?”

There is further evidence for an expanding universe when you consider “ Why is it Dark at night?”

The microwave radiation is the farthest back – in time and space – that we can see.

http://www.amnh.org/news/2009/12/the-known-Universe/