[chris (OP)]

James got in touch with me today to say this:

I want to thank you for your podcast and I enjoy listening and increasing my knowledge base and learning so many things.

My question deals with 'gravity wells' -- when seeing on TV or searching the Interweb they always show a picture similar to a ball sitting on a net and the "well" only depressed in one direction. This gives the illusion that it is only 'one way' (kinda one way) whereas would not the gravity well actually encircle the entire 'ball' / planet / star in all directions?

If so how could this be 'pictured' to show the proper view.

Regards -- James

[yor_on]

heh, as a hole in a vacuum?

[evan_au]

The 2-dimensional "ball on a net" is a metaphor for the way that spacetime is distorted near a mass. But it's a circular metaphor because it says "Gravity is like this net that is distorted by gravity pulling down on the ball". To someone who had grown up in zero-g, this metaphor would fall flat (fortunately, this is a rather small population, at this point in time).

The vertical scale is arbitrary in a drawing. But the physical implementation used by charities to attract children rolling coins into a well need some mathematical accuracy. Otherwise the coins would not circle for as long. My vague recollection is that to produce something like an inverse square law (1/r²), you need a shape that is like 1/r.

However, you are asking for a 3-dimensional visualization of 4-dimensional spacetime. And we are not so good at describing 4-dimensional objects.

I think you should just stick with the 2-dimensional metaphor, which can be described as a 3-dimensional shape.

[PmbPhy]

James got in touch with me today to say this:

I want to thank you for your podcast and I enjoy listening and increasing my knowledge base and learning so many things.

My question deals with 'gravity wells' -- when seeing on TV or searching the Interweb they always show a picture similar to a ball sitting on a net and the "well" only depressed in one direction. This gives the illusion that it is only 'one way' (kinda one way) whereas would not the gravity well actually encircle the entire 'ball' / planet / star in all directions?

If so how could this be 'pictured' to show the proper view.

It can't be. When such a picture is draw it can only be drawn in two dimensions since that's how we see. When pictures are drawn they can necessarily not perfectly represent reality since all three dimensions cannot be illustrated in a picture.

[jeffreyH]

Take a tennis ball to the top of a tower and film it with a high speed camera as it is dropped. Now do the same with several tennis balls dropped at regular intervals. The distance between each successive ball will get larger as they fall. This is what the rubber sheet analogy is trying to describe.