On Saturday, 28 December 2013 06:18:26 UTC+13, Edgar L. Owen wrote:
> Many worlds is probably the most outlandishly improbable theory of all 
> time, and should have been laughed out of existence as soon as it was 
> proposed. Do

Fortunately, science is not decided on what seems probable to humans, or we 
would never have realised that there is anything except the Earth and some 
lights in the sky. The MWI is very far from the most outlandishly 
improbable theory of all time, I can name a dozen ontological theories that 
are more outlandish without even asking WIkipedia, such as the idea that 
the world was created by the shenannigans of various gods.

you actually understand what it says or implies? Basically that every 
> quantum event that ever occured in the history of the universe spawns an 
> entire new universe of all its possible outcomes and every event in every 
> one of those new universes does the same. This immediately exponentially 
> escalates in the first few minutes of the universe into uncountable new 
> universes and has been expanding exponentially ever since over 14.7 billion 
> years! Just try to calculate the

The MWI is a straight interpretation of our best theory of matter - an 
interpretation that removes any extra assumptions (wave function collapse, 
pilot waves, wave-particle duality etc). It is simply what the relevant 
equations say, converted without interpretation to human language (if one 
leaves aside the actual phrase "many worlds", which is misleading). The 
equations imply that all possible outcomes occur for a given quantum event, 
or to be exact that the entities we regard as particles are in fact waves, 
capable of interfering with themselves, but only detectable (I suppose 
"entanglable" would be a better word) by a process of localisation that is, 
I'm told, neatly explained by decoherence. This implies that the universal 
wavefunction is constantly spreading and differentiating. This is generally 
characterised as "parallel universes coming into existence" but that isn't 
a completely accurate description (and in any case it is quite possible 
that space and time are emergent properties of the universal wavefunction).

> number of new universe that now exist. It's larger than the largest number 
> that could ever be imagined or even written down. There is not enough paper 
> in the universe, or enough computer memory in the entire universe to even 
> express a number this large! Doesn't anyone ever use common sense and think 
> through these things to see how stupid they are? And it violates all sorts 
> of conservations since energy eg. is multiplied exponentially beyond 
> counting. Geeez, it would be impossible to come up with something dumber, 
> especially when it is completely clear that decoherence theory falsifies it 
> conclusively.

If that was a correct description of the MWI, you might have a point, but 
it isn't. Oddly enough clever people *have* thought about this, some of 
them on this very list. Have you read "The Fabric of Reality" by David 
Deutsch? That's what Americans would call "MWI 101" or "The MWI for 
dummies". If you have, you will know that the MWI posits a continuum of 
"worlds" which can only ever differentiate, not "split" or "branch" or any 
of the other common misconceptions. The fact that the universe can generate 
greater and greater detail indefinitely (or possibly only to certain 
physical limits, like the Bekenstein bound) is no more surprising than the 
fact that in GR a finite universe can expand to infinite size (under 
certain conditions), or that the centre of a black hole (according to GR) 
is a singularity of infinite density. These are all properties of the 
continuum, a mathematical object that may or may not describe space-time 
(if it doesn't, it does so to very high precision, apparently many orders 
of magnitude smaller than the Planck length). The idea that the MWI 
violates the conservation of energy was laid to rest a long time ago. A 
simple example is a quantum computer factoring a 500 bit number. The 
equations of QM say that this is physically possible, even if we have 
trouble doing it in practice - it requires 500 qubits to be suitably 
prepared and then shaken down somehow (with Shor's algorithm, I think) to 
obtain the result. QM says this happens by generating a superposition of 2 
to the power of 500 quantum states, which according to my trusty calculator 
is quite a lot. These superpositions are in fact capable of decohering into 
2^500 possible states, although Shor's algo or whatever ensures that 
99.999...% of these give the right answer. The question is, how or where do 
all these states exist? QM says they all exist right here, in "our 
universe" (which the MWI claims is a convenient fiction, of course) - but 
how can 2^500 states exist at the same time for the same qubits (which are 
normally atoms, but could in theory be photons, electrons, etc) ? Where is 
the calculation performed? This is a massive parallel computation, carried 
out inside an object that could in theory be the size of a sugar cube. If 
it was carried out using the computational resources of our universe, it 
would need a *lot* of Hubble spheres - around 10^70 of them - to supply the 

So is QM wrong? Is a quantum computer impossible, or impossible beyond some 
cutoff well below 2^500 qubits? If not, that qc contains 2^500 
mini-parallel-universes. But if you accept that, what happens when you 
decohere them? Do you 2^500 slightly larger parallel universes, say 
including the scientist who does the measurement? Or if not, why not? The 
onus is on rival theories to explain where the cutoff occurs, and why.
If for some reason you *haven't* read FOR, go away and do so. Then you will 
be in a position to discuss this topic without relying on a mistaken 
interpretation of the MWI.

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