Wednesday, 16 November 2011

Time travel

Let's try a thought experiment on time travel.

It is a grey and cloudy day. From an upstairs window of her house Alice sees a large, grassy field. Yesterday she saw a woman in a red dress walk through the grass towards the centre of the field. There were no animals or other people in the field - only the woman in the red dress. She carried a brown cotton bag. She stopped in the middle of the field, took a baguette from her bag and dropped it on the grass. Then she walked away without looking back.

Today Alice notices that the baguette had gone. Presumably the birds ate it.

But what really happened to it? Can we go back in time to see if it was eaten by birds? The fate of the bread was not recorded, but there are a few clues and we could make some obvious assumptions. Maybe the baguette was eaten by birds, but for all we know a man in blue shorts cycled across the field on a penny-farthing and picked it up. Or perhaps a fox ran off with it.

Where is the information we need to reconstruct this missing piece of history? Does the information exist somewhere in the present configuration of the universe? It was a cloudy day, so there are no hypothetical images of the field whizzing up into space at the speed of light like the frames of a movie film. 

Does the information exist in that paradoxical place we call the past? Is that a meaningful place it could be? The most likely scenario is that birds pecked away at the baguette until it was all eaten - then they flew away and digested it. But this tiny slice of history has been misplaced because nobody saw it so nobody has first-hand knowledge of the fate of the vanished baguette. 

My personal philosophy leads me to say that the fate of the baguette is lost information which even in principle cannot be reconstituted. The baguette may have met a number of possible fates but possibilities are all they are. If birds ate it, then the baguette will have ended up in an avian digestive tract to be broken down into simpler molecules. Eventually it would emerge as new bird tissue, carbon dioxide and splodges of something else on the ground. 

There is no way to reconstruct the baguette via the tissues and digestive products of unidentified birds which may or may not have eaten it. Even in principle there is no way of distinguishing one excreted carbon dioxide molecule from another. It is not possible that those carbon dioxide molecules could carry with them for all time, their previous baguette-related identity. For me, the universe made an unrecorded change and the history of the grassy field has a baguette-shaped hole in it.

We can't travel back in time. The past has gone forever and information about it will become more and more incomplete. There is nothing to travel back to - the past has holes in it.


David Duff said...

A nice metaphor but where would we all be and what would pass for conversation if we couldn't argue about history?

Also, I'm not sure that you are right in suggesting that information about the past will become more and more incomplete given all this internet-thingie-stuff. 'Too much information' might actually be the future problem.

Demetrius said...

I wish I could go back to 1951.

A K Haart said...

DD - we'll always argue about history because it's incomplete.

D - I don't think I'd go back in case nothing changes.

rogerh said...

Allegedly it takes energy to create 'organisation' above and beyond the cooking of the baguette and said energy is liberated when a bird digests the baguette - in addition to the calories consumed.

But who is to say what represents 'organisation' - and if I 'disorganise' this computer do I get free electricity? Got to lie down now, my brain hurts.

Sam Vega said...

I think that in principle, the baguette could be tracked via a comparison of bird poo etc. with that of other birds. But it would need a colossal amount of knowledge and intellect, which might be why we invented God.

Generally, I find a state of mind in which I can remain equanimous over the fate of baguettes is preferable.

James Higham said...

It also comes down to priorities - how much do we need to reconstruct it?

A K Haart said...

rogerh - you get free electricity from wind if you disorganise your thinking(:

SV - I'm equanimous too, even though it's the first time I've ever used the word (:

JH - yuk (:

Chuckles said...

Nice pic AKH, I remember that issue...:)

A K Haart said...

C - the pic led to the post really, I just had to use it.