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

I once saw a juggler who juggled an axe, a meat cleaver and a chainsaw. During his spiel he said "This is the axe that George Washington used to chop down the cherry tree. I've had to replace the blade... and the handle. *Pause* It occupies the same space."

This throws up all kinds of deep, deep questions.

How much of something do you have to replace before it's not original any more? 'All of it' sounds like the simple answer, but if you replace all of something and it's still in the same place is it still the same thing? Suppose we're talking about a structure of some kind. The Mathematical Bridge in Cambridge has been rebuilt a couple of times since it was first constructed in the 1700s, but it's in the same place as it always was and it does the same job. People traverse the same points in space across it as they have since the bridge was first built. If it wasn't there they couldn't do that.

If the bridge was taken away would there be a bridge-shaped hole in space that the bridge used to occupy?

When Windsor Castle burnt down they rebuilt the parts that were destroyed. They occupy the same points in space as the old parts did before they were consumed in the conflagration.

Suppose the object in question is smaller, though, like an axe. That makes it more ephemeral and transient in some ways, but on the same basis, if you have to replace the handle because it rotted away it's still an axe. And then if you have to replace the blade, it's still an axe in the same space as it would be if it was still made of the original parts. Does that mean if you didn't replace the handle and the blade there'd be an axe-shaped hole in space where it used to be? If that's the case then it's still the same axe no matter how many times the parts are replaced.

Isn't it?

No, I'm not drunk, I haven't ingested any pharmaceuticals and I haven't... something something.

I'm doing philosophy next year. I'll come back to you about this then

Ah, but Gosho, is this really something you think of a lot, or is it several similar thoughts which occupy the same space in your head?


Geggs

I'm reading this at the moment, which is full of similar ponderings about paths and routes: http://www.theguardian.com/books/2012/may/31/old-ways-robert-macfarlane-review

He talks about the Icknield trackway in southern England, parts of which have been in use from the Stone Age. That's fairly clear-cut; the paths are still there an still visible. But what about more intangible "paths"? The ones taken by migratory birds every year, where the same air currents guide them on the same route? Or what about rivers? Am I seeing the same River Lagan on my way to work that my grandfather saw on his way to work in 1930? Or sea-lanes. Are these all "ancient tracks" in the same way as the Icknield, and are they the same as the ones that were there in their place 1000 years ago?

Your post also reminded me of a lecture Seamus Heaney gave, where he mentioned an old tree on his aunt's farm from his childhood, now cut down, and how that tree-shaped space still existed in the air which hadn't existed before. He compared it to the presence left by a poet (Kavanagh? Can't remember, someone who influenced him) who is now dead, but whose influence is still there, in the "space" it occupied, if you know where to look.

It's too early to be getting this metaphysical.

It's also true that the human body replaces every single bit of it over 7 years, so we're not the same physical thing we started out as.

that's made me feel a bit stranger than even I normally do...

Geggs, I nominate that post of yours for QOTD. Brilliant And, relatedly...

Lanza, isn't it the case that neurons are the only cells that don't get replaced? Or has new research found that they do? That question is important because that's where our consciousness resides, or at least, our thoughts and memories. If neurons are being replaced the way other cells in the body are, that would open up a whole new area of this conundrum because the new neurons would be occupying the same space as the older ones, *and* so would the memories they encapsulate.

Intangibles, such as sea-lanes, aren't the same as physical objects, including trackways on land, but yes, I'd say that is the same river your grandfather saw, unless it's been altered in some way by humans. Nature will change it over centuries and millennia (very rarely in the space of days if there's a big flood), but a few generations isn't enough to make a difference if natural erosion is the only factor. It's not the same water, obviously, but it's in the same place as the water your grandfather watched.

And I haven't even got around to the issue of what happens when something that doesn't move gets erased by whatever means (demolition, collapse etc) and something else is built in its place, such as London Bridge. One thing I didn't make clear in the first post was that the objects I was talking about were being replaced by something identical to the original. The axe, the bridge. That's not the case with London Bridge though. Each one has been different. as are the buildings that, for instance, now occupy the space in London that used to be the slums of St Giles rookery.

I shouldn't be thinking this hard before breakfast.

I think this is one of those "imponderables", the sort of thing that really isn't worth thinking too hard about as there is no definitive answer. They're fun to think about for a while, but don't get too hung up about them.

Yesterday, I was reading this What-IF? from Randall Monroe of XKCD. http://what-if.xkcd.com/65/

The original question wasn't that interesting at first glance, 'how tall is your twitter feed?'. But then I see a link to Randall opining about why he doesn't tolerate any discussion of the Airplane on a Treadmill problem. http://blog.xkcd.com/2008/09/09/the-goddamn-airplane-on-the-goddamn-treadmill/

That's one that I was absolutely positive that I KNEW that anyone arguing about it was wrong because it seemed so obvious to me (and the Mythbusters who "confirmed" my position on it. http://www.youtube.com/watch?v=YORCk1BN7QY ) I haven't changed my position, but now I'm willing to accept that those opposed to it might be so for other reasons than I assumed.

As to whether it's the same ax? I say yes, but I don't think about it much anymore.

"bridge-shaped hole in space" But what you need to bear in mind quote "space is big. Really big. You just won't believe how vastly, hugely, mind-bogglingly big it is," the bridge would still be in existence in the minds of all those that saw it walked over it ect, but not to those who came after..

physical objects..... suppose you have to look at the rearrangement of Atoms and those bits smaller than atoms that are compressed to make bricks and things that are more tangible to our senses (smell/touch/sight ect) the air that we breathe is a physical object its just that its so small we cant see it but it did exist inside "that bridge, building slums" so there is not a "?-shaped hole in space" just that the space surrounding the space within the bricks has been filled with more space brought about by the absence of the bricks. those bricks must have displaced space elsewhere, other wise there would be a hole somewhere.

I've always thought of memories more of electrical impulses so once created wont dis-create or vanish, even if the cells that create them die, all radio broadcasts are still travelling in space, like television signals some where ? miles away from earth the opera " Ombra mai fu" is just being received broadcast in 1903 ..

There's a couple of good points there, logicus.

If something is destroyed and something else put in its place perhaps the ?-shaped space it occupied (if such a thing ever indeed existed) vanishes when the last person who remembers it dies.

This is, of course, speculation.

Er, now I can't remember now what the other one was, and as hard as I look I can't bring it to mind.

Any road up, what I was leading up to there was "If you've speculated about six things this morning, why not round it off... etc, etc."

I'm trying to remember just which Discworld novel has a Royal/sacred/low king piece of Dwarfen bread, which is stolen. And replaced, just as the stolen piece had been replaced when it got too old and chipped.
Same question.
Mostly I think about this around cars. How much of the original street car is left when you've modified and gutted, and put in roller cage, and replaced the engine and powerline and seats to make it a drag racing car going down the 1/4 mile at 400 km/hour?

Here's a similar question. It occurred to me the first time on what I think is the only occasion I've ever been to the British Museum. Despite having spent about 35 years living in London

I was standing in front of a bust from somewhere in the ancient world, possibly Greece, and the thought occurred to me that in relation to the piece, I was standing in the footsteps of the person who created it millennia ago. Which also occurred to me a year or two later when I was in the National Gallery standing in front of paintings by Van Gogh, Constable, Turner etc.