h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Good to see this on the FP at last.

Anyone reading this for the first time should be told that this had a very difficult journey through PR, and we are hoping that some of the excellent arguments raised there can be continued here.

There's one model of the motion which I suggested early on, but which didn't survive the red pen. You can see the squirrel and the photographer as being fixed to either end of a solid bar which is rotating at some point along its length.

Depending on where this point of rotation is, you can have some very different scenarios, but all of which are described by the same model:

Assume that the bar lies along the x-axis, the squirrel is at point S and the photographer is at point P. We'll also mark the midpoint between the squirrel and the photographer as M.

Consider the motion of the bar as the point of rotation R moves from minus infinity, through to S, then through to the midpoint M, then to P, and onwards beyond P to positive infinity.


R = minus infinity: the tree is of infinite size and the trunk is like an infinitely long straight wall. The squirrel moves along the trunk and the photographer is moving parallel to it. The photographer can get his picture!

Minus infinity < R < S: the squirrel and the photographer are circling the tree but on the same side of the trunk. Again the photographer can get his picture.

R = S: The tree trunk is infinitely narrow. The squirrel is effectively motionless in mid-air and the photographer circles it - photography is again easy.

S < R < M: This is the classic situation described in the article. The squirrel circles the tree trunk on the opposite side to the photographer. No photographs are possible from this point.

S = M: Both squirrel and photographer are clambering on the tree trunk, on opposite sides.

M < R < P: Opposite sides again, but the photographer is inside the tree trunk. If this is too surreal, then you can always consider the tree to have a hollow trunk.

R = P: The squirrel runs around the tree while the photographer is motionless at the centre of it.

P < R < plus infinity: Both squirrel and photographer are encircling the tree trunk on the same side, but the photographer is inside the trunk.

P = plus infinity: the tree is of infinite size and the trunk is like an infinitely long straight wall. The squirrel moves along the trunk and the photographer is moving parallel to it, but inside the trunk.

There, I'm happy with that. *dons tin helmet*

Icy

What if the squirrel had a camera?

at the risk of bypassing all the terribly interesting maths, why doesn't said photographer either (a) use a telephoto lens or (b) set the camera up on a tripod on one side of the tree and chase the squirrel until it's in front of the camera then remote fire it. job done, get the kettle on.

this is why i wasn't a terribly good philosophy student. but i can take a photo of a squirrel.

Or he could have a mate with another camera.
When I first heard this riddle it wasn't a photographer. I think it was a dog. More of a reason for the squirrel to be the other side of the tree.

The squirrels round here come and take food from your hand - even if you've got a camera.

One problem with the first post:

"S = M: Both squirrel and photographer are clambering on the tree trunk, on opposite sides."

Surely you mean R = M?

What's this? Peer Review?

Yes, you're correct of course, R = M.

Ah, now then, that's why I could make neither head nor tail of it.
Did you see a flash just them?