• Question: What is general relativity?

    Asked by lisaloo to Claire, Kate, Matt, Rob, Sam on 24 Jun 2013.
    • Photo: Kate Husband

      Kate Husband answered on 24 Jun 2013:

      General relativity is a theory that explains gravity almost perfectly (it’s ridiculously nearly perfect). It’s a pretty complicated theory but I’ll give explaining it a go. It says that space and time form a 4D web and that matter can distort this so that for example when light goes past a massive object like the Sun it has a longer distance to travel and it bends around the Sun. We have seen this happening – in solar eclipses we can see some stars near the edge of the sun which should really be behind the sun but there light has bent around the sun.

    • Photo: Claire Lee

      Claire Lee answered on 24 Jun 2013:

      I think if you could summarise General Relativity (GR) in one sentence it would be:

      Gravity is geometry.

      We learnt from Newton that objects with mass have gravity. But what exactly *is* gravity?

      That “web” that Kate wrote about is called spacetime – it is the universe itself, and it can be flat or curved, like a giant sort-of trampoline.

      If nothing is on the trampoline, it is flat. I could roll a tennis ball across the trampoline and it would just roll to the other side. But if you sit in the middle of the trampoline, then you’d make a dent in the middle. And if I put a tennis ball on the edge, it will roll down the trampoline until it gets to you.

      Now imagine that the trampoline was invisible. All I would see is a tennis ball rolling towards you! It would look like you were exerting some sort of mysterious “force” on the tennis ball, and I would call that force “gravity”.

      What Einstein did was invent the mathematics that allowed us to see the trampoline. Now, gravity doesn’t look so mysterious, now what’s happening is obvious – you are causing a dent in the trampoline, and the tennis ball is rolling down the dent.

      It’s the same thing with the way the universe works – everything that has mass (everything made out of matter) causes dents in the universe-trampoline, which we call spacetime. And as other objects move through spacetime, they follow the curves and dents, just like that tennis ball did.

      That, basically, is general relativity: matter tells spacetime how to curve; spacetime tells matter how to move.

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