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

There is a branch of physics that believes in the MWI. In it there are an infinite number of worlds each playing out our decisions / consequences.

So by going 'back' in time, the consequences of shooting Adolf Hitler in 1938 mean that in universe B WWII never occurs, while in universe A it does.

bbs

That's the theory I subscribe to, assuming Time Travel is possible. Sadly this means you couldn't really return to your original "timeline"

The problem with this is that it simply *isn't* time-travel. What would you see if someone stepped into a 'multiple-worlds-time-machine'? Well, basically they'd be annihilated. They'd just disappear. There would be no evidence that they'd travelled through time, they'd just cease to exist. The only person to which anything more would happen would be the time traveller, and all he'd experience was the sensation of being in a world which was a lot older (or newer) than his own. But this would be no more time travel than if they went to a huge theatre, in which everyone was pretending to be people from the traveller's past, but weren't actually.

I guess I can see what you're saying; as far as the first universe is concerned, the time traveler ceased to exist. However, as far as the time traveler was concerned, he had gone into the past, or the future, or whatever. I personally believe a vairation on this theory, which I call the quantum multiverse theory.

PhysicsMan

part of this confusion comes from there being different meanings of the multiple worlds hypothosis.

in one variant, there are multiple simultaniously existing worlds in which every choice that has happened went all ways, creating extra new worlds at each decision point.

another variant has there being multiple possible timelines, and when you impinge on a timeline by entering it using a time machine, you switch immediately to a different world and thus a different timeline, but only one timeline exists.

As both of these variants (and a small number of others) are refered to using the same labelling, it makes it very difficult to make assumptions as to which variant people are using, while still having some expectation that you are all talking about the same thing.

Well since the 'one existent time-line at a time' theory doesn't is *any* way survive grandfather-paradox attacks - since it would still have to be the case in the existent universe that the time traveller either existed or didn't, we have to go for a 'changing possible world where they all already exist' theory...

Another possibility is that there's a self-consistent universal solution. To illustrate this, just take one of the simple universes you'll find scattered around the floor of the average lab, some scissors and sticky back plastic. Initially, this universe just takes its initial conditions and propagates them forward in time, and causality is fine.

But now, snip and tuck, stick two separated bits of the universe together, to generate a simple time machine. Sometimes, everything is still fairly normal - but where there are closed time-like loops (i.e. when the solution could kill its grandfather...does a solution have a grandfather?...well, never mind), there are parts of the universe in which the initial conditions don't determine the total reality.

So, how does the universe work out what actually happens? There are various possible ways of sorting things out (some are oddly similar to quantum interpretations). As we've heard, multiple versions of the universe may appear out of nowhere to take account of the entire range of possibilities (so you might find yourself squeezed somewhat uncomfortably into a corner of the lab).

Alternatively, just enough extra internal conditions can be specified on the time-like loop, perhaps using a suitable Brownian Motion generator (a nice hot cup of tea, or even a Wiener sausage [see, e.g., Phys Rev E 62, 3116]). This all seems fine, until you start to look in detail at the mini-universe. Away from the time machine, events appear to have their usual consequences. Close to it, however, the requirement of global self-consistency begins to take effect: increasingly unlikely and bizarre things happen in order to make sure everything is set up OK before it appears at the other side.

Sadly, the unlikely events will almost certainly consist not of infinite numbers of monkeys proffering their manuscripts of Hamlet, but rather more trivial coincidences with upsetting (not to say fatal) consequences for the metabolism of the intrepid would-be time traveller (cf A467714).

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This message is the random burbling of the author, and in no way represents the views of the Damogran Galactic Laboratory.
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