Thursday, 11 September 2014

Gravity, a disappearing sun, big G (not google)...

There was a recent post about an article in Gizmodo outlining what might happen if the Sun suddenly disappeared. 

Fortunately, it's not going to happen (or least very unlikely!). The article's last comment is rather funny 'it would be interesting to live through' - I doubt if living through would be an option. An interesting thought experiment though, especially around the nature of gravity. 

I always pictured the gravity of the sun as being a bit like having a heavy ball on a sheet of rubber ('fabric of space time":). The weight causes things to sink in towards the sun, unless of course there are things preventing that (eg due to other objects in space having enough of an impact to prevent that, having their own dimples in this rubber sheet, & the speed of an object moving around the sun's dimple being sufficient to avoid it falling inexorably to the bottom of the dimple caused by the sun's weight. 

Whilst it's a simple analogy it does have the advantage of being easy to picture. And if the sun is removed, the question then is how quickly the sheet looses its dimple rather than a question about how fast light travels (though looking at it one way you could see how they could relate). 

A potential issue with my initial thought around having a rubber sheet is that take off a heavy ball and the sheet will have some vibrations (which isn't unrealistic) and could fling other objects affected by the sun's dimple in all sorts of directions eg one way and then the other! Perhaps into the dimple of something else, though of course that dimple could also be impacted by the vibrations (best not to think about what holds the object to the rubber sheet:).

I was listening to a podcast about big G, the universal constant used for calculating the gravitational force between two objects (not to be confused by little g, the local gravitational constant, like at different places on the earth's surface). Big G is sometimes called the Mount Everest of physics because it's proving so difficult to measure accurately (only about 0.05% level of certainty- massive in physics terms). It got me thinking about the analogy of a rubber sheet. You can see how g, local gravity is affected by the mass of the object causing the dimple (as well as by the object falling into the dimple, which creates its own dimple though here the analogy is tricky to maintain). However, picturing the curvature of the dimple you can see how that's going to increase the closer the object gets to the middle of the dimple (as well as getting steeper depending on the mass of the two objects). The bit that gets more interesting though is the stretchiness of the fabric- how far that stretchiness is fixed, how far it varies depending on the mass of what's causing the dimples and how far does it varies at different points on the rubber sheet.

The wonders of analogies- and their limitations!

Back to mobile devices:) 

No comments:

Post a Comment