aet.radal ssg wrote:

From the initial page from the included link to the archive: "I'm no physicist so I don't know for sure that >these implications would
follow, but I am very doubtful that interworld communication is consistent
with the basics of quantum mechanics.  The fact that this paper has not
been published in peer reviewed journals in 7 years indicates that it
probably doesn't work."

Back when I wasn't long in the field of video production I was well aware of the insistance and belief of >TV engineers that a single tube industrial color video camera was not broadcast quality. Working in >cable, where they were used for cablecast, I had plenty of opportunity to look at picture quality, etc. and >came to the conclusion that it shouldn't be a problem. 2 years later I got the chance to prove it when a >local news station sent a crew out to cover something that I was shooting. In the end I gave them >the editied sequence I had shot (now down two generations), and they took it and edited it into their >story, which would have taken it down a third. Then they broadcasted it over the air. I taped it off-air and >the results were conclusive - I was right, all the nay-sayer engineers were wrong. A $40,000 Ikegami  >vs a $1,500 Panasonic and it was a tie except for one slight red bleed from a costume due to the >Saticon tube bias toward red in the camera I used, which could have been color corrected with a time >base corrector, but whoever dubbed the tape left the red level a little too hot.

My point being that that was the first in a long line of "you can'ts" that I've faced which I eventually >proved, "you can". Thus I have a dim view of such positions when they aren't backed up with >experiments that prove so *conclusively*. As long as the possibility exists, I keep an open mind. >Besides, if unbriddled skepticism was right all the time, we wouldn't be using computers, flying, or >even have phones of any kind, just to name a few things.

There is a fundamental difference between claims that we can never do something because the engineering problems are too great, and claims that we can never do something because the laws of physics themselves say it's impossible. For example, I've heard people say things like "I'm sure we'll eventually break the light-speed barrier, after all, once people thought it was impossible that we'd ever break the sound barrier but they've been proved wrong". But the two are not really comparable, because no one ever thought the laws of physics said breaking the sound barrier was impossible, they just thought the technical challenges to doing so would be too difficult, whereas the light-speed barrier is built into the basic structure of relativity (although there are possible loopholes in general relativity like wormholes, where you get to distant destinations quickly without ever *locally* exceeding the speed of light).

Similarly, when Hal Finney suggests he thinks interworld communication is impossible, I think he's suggesting that it would violate basic principles of QM, not that it's too big of a technical challenge. I also saw this suggested in the book "Schrodinger's Rabbits" by Colin Bruce, a pop science book about the MWI (p. 137):

"If only we could do a clear and unambiguous communication-between-worlds experiment. Then there would be no room for argument about the reality of many-worlds. Unfortunately, the laws of physics do not seem to allow such a thing.

"This is frustrating because two potentially useful methods of harnessing the power of many-worlds, which we will look at in detail shortly, can be described in terms of sharing resources between worlds, or even sharing information between worlds. For example, a loose way of describing the operation of a quantum computer is as follows: As worlds start to diverge, hundreds of billions of different copies of the computer come into existence. Each of these computer copies can work on a different calculation. The shared results of their labors, however, can be made available to all the diverging worlds created when the bubble of Hilbert space describing the computer is systematically collapsed by measurement at the end of the calculation.

"This makes it sound as if Hilbert space might possibly be used as a kind of mailbox for communicating between worlds. Unfortunately, the mathematics that describes Hilbert space rules this out because it implies that everything that goes on in Hilbert space is reversible. As soon as you try to take information out of Hilbert space, that reversibility is destroyed. Such acts of measurement, by definition, cause decoherence. You can preserve multiworld access to a bubble of Hilbert space only by allowing it to evolve undisturbed. It reminds me of C.S. Lewis's "Wood Between the Worlds" described in the Magician's Nephew. Any Hilbert space accessible from more than one world line must be a timeless place, in which we can leave no permanent mark."

If this is correct (and I don't understand this stuff well enough to say for sure it is), then inter-world communication would only be possible if it turned out that the existing principles of QM were wrong, and that they ended up being modified by some future theory (quantum gravity, perhaps) in a way that removes these fundamental obstacles.

Jesse


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