Many-worlds Interpretation

[Rob Lister]

So, so far we're up to factoring a two digit number using Quantum Computers.

Just 9998 digits to go before we prove the many world theory?

I ask in honesty. I don't know Quantum from Quantis, but to surmise that because one can construct a quantum computer to factor a two digit number is a far cry from one that can factor a 10000 digit number.

 

[Myriad]

The analogy with conventional computers might be leading Deutsch to unjustified over-optimistic conclusions. The idea of easy scalability is so universal at all levels of computer science, theoretical as well as practical, that it tends to get taken for granted. If you can etch one logic gate on a chip, you can etch an array of ten million logic gates on a chip. If a program performs an operation three times (and the code is written with good "style"), it's trivially easy to modify it to perform the operation three million times. The same algorithm that factors a two-digit number can factor a 10,000-digit number. (It might take way longer than the age of the universe to run, but you can be certain it works.) And so forth.

If quantum computing shared that aspect of conventional computing, then surely getting the first few qbits to work was the hard part, and scaling it up to gigaqbits is just a matter of patient engineering and efficient mass-production.

But instead, quantum computing might be dealing with hardware design for which (if certain ideas about decoherence are valid) some fundamental aspect of causality fights against your attempts to scale it up. This is unfamiliar territory to say the least.

The whole point about the various "interpretations" of quantum mechanics is that the theory itself does not differentiate between them. If quantum computers offered a way to differentiate between them, then that would mean that quantum computation added some fundamentally new physics, and it would be at the absolute CENTER of modern physics, rather than just one more specialty among so many.

Because it's worth repeating.

Respectfully,
Myriad

 

[Unnamed]

http://www.hedweb.com/manworld.htm#believes

Anyways the long and short of it is that, to the extent that quantum mechanics accurately describes the observer as well as the observed, many worlds is true. Even if a future theory replaces quantum mechanics, that statement will be true.

The whole point about the various "interpretations" of quantum mechanics is that the theory itself does not differentiate between them. If quantum computers offered a way to differentiate between them, then that would mean that quantum computation added some fundamentally new physics [...]

This part of that Everett FAQ linked by Ben proposes a way to differentiate the interpretations:

What unique predictions does many-worlds make?

[note: edited for length]
Many-worlds makes at least three predictions, two of them unique: about linearity, quantum gravity and reversible quantum computers.

Assuming that we have a reversible machine intelligence to hand then the experiment consists of the machine making three reversible measurements of the spin of an electron (or polarisation of a photon). (1) First it measures the spin along the z-axis. (2) Second it measures the spin along the x-axis and records either spin "left" or spin "right" and notes this in its memory. The machine now reverses the entire x-axis measurement - including reversibly erasing its memory of the second measurement. (3) Third the machine takes a spin measurement along the z-axis.

According to the Copenhagen interpretation the original (1) and final (3) z-axis spin measurements have only a 50% chance of agreeing because the intervention of the x-axis measurement by the conscious observer (the machine) caused the collapse of the electron's wavefunction. According to many-worlds the first and third measurements will always agree. Hence when the machine reversed the second measurement these two worlds merged back together, restoring the original state of the electron 100% of the time.

Only by accepting the existence of the other Everett-worlds is this 100% restoration explicable.

 

[wipeout]

http://www.hedweb.com/manworld.htm#believes

This strongly suggests that "wiser physicists" do not include people like Hawking and Feynman.

Not necesarily. Gell-Mann and Feynman were in very close agreement on quantum mechanics and Gell-Mann gets included as a many-worlds believer when that is not what he believes at all. He believes in Everett's approach but not all the well-known associations.

Everett's approach leads to many possible past/present/future decohering histories of the universe (the "worlds" of many-worlds) but you can remain agnostic as to the reality of the worlds other than our own and also allow that some addition to the theory could show that only some worlds or only one world occurs.

It may be that, like Gell-Mann, when Hawking says he supports Everett's approach, people misinterpret that to mean he believes in many-worlds, all equally real, when he might not mean that as well.

 

[Ziggurat]

Assuming that we have a reversible machine intelligence to hand then the experiment consists of the machine making three reversible measurements of the spin of an electron (or polarisation of a photon).

Your source defined a necessary condition for reversibility, but it didn't actually define reversibility. But let me note that a Stern-Gerlach apparatus does, in fact, rely upon a reversible process.

(1) First it measures the spin along the z-axis. (2) Second it measures the spin along the x-axis and records either spin "left" or spin "right" and notes this in its memory. The machine now reverses the entire x-axis measurement - including reversibly erasing its memory of the second measurement. (3) Third the machine takes a spin measurement along the z-axis.

According to the Copenhagen interpretation the original (1) and final (3) z-axis spin measurements have only a 50% chance of agreeing because the intervention of the x-axis measurement by the conscious observer (the machine) caused the collapse of the electron's wavefunction.

You can already do this experiment, by putting multiple Stern-Gerlach apparati in sequence and at angles. In fact, exactly this experiment has probably already been done. And the answer WILL be that the first and third measurements will only show 50% correlation.

According to many-worlds the first and third measurements will always agree.

Contrary to the opinion expressed here, you can construct a many-worlds "theory" in which the first and third measurements do not, in fact, agree with each other 100% of the time. Once again, that's what the whole fuss is about: quantum mechanics tells you quite definitively and unambiguously that there will only be a 50% agreement between the first and third measurements, and the various interpretations remain only interpretations because they do NOT make differing predictions. The only way they ever WILL make different predictions is if some fundamentally new physics comes along. Nonlinearity of the wave function (one of the other methods noted) would qualify as fundamentally new physics.

 

[Ben Tilly]

Not necesarily. Gell-Mann and Feynman were in very close agreement on quantum mechanics and Gell-Mann gets included as a many-worlds believer when that is not what he believes at all. He believes in Everett's approach but not all the well-known associations.

Everett's approach leads to many possible past/present/future decohering histories of the universe (the "worlds" of many-worlds) but you can remain agnostic as to the reality of the worlds other than our own and also allow that some addition to the theory could show that only some worlds or only one world occurs.

It may be that, like Gell-Mann, when Hawking says he supports Everett's approach, people misinterpret that to mean he believes in many-worlds, all equally real, when he might not mean that as well.

I believe that Hawking really does believe in many-worlds, all equally real.

As for Feynman, he is quoted in that FAQ as saying that Everett's approach is how you have to model the collapse process. Which logically leads to many worlds. But whether those other worlds are truly real is a philosophical question. Given Feynman's well-known antipathy to philosophical questions, I doubt he'd particularly care to have any conversation on that topic.

But let me add John Baez as a fairly well-known physicist who is a very vocal advocate of many worlds.

Cheers,
Ben

 

[CurtC]

I've heard it said that quantum computers (will) steal processing power from those other universes. All I have to say is, those a**holes in other universes better not be stealing any of my processing power!

Or maybe they already are. Would be a bitch to go to all the trouble to make a large-scale quantum computer only to find out that other worlds had already used up their own processing power plus some of ours.