On 11/06/2017 1:31 am, Telmo Menezes wrote:
On Sat, Jun 10, 2017 at 1:11 AM, Bruce Kellett
<[email protected]> wrote:
On 10/06/2017 2:36 am, Telmo Menezes wrote:
On Fri, Jun 9, 2017 at 12:37 AM, Bruce Kellett
The idea that the explanation is epistemological rather that ontological
has
been my preferred position for a long time. If the wave-function is
merely
an epistemological device for calculating probabilities and not a really
existing object, all worries about collapse and action-at-a-distance
vanish.
Of course, multi worlds also vanish, but in my opinion that is no bad
thing.
So what's your position on Deutsch's argument about quantum computers?
Where does the extra computing power come from?
It has long been understood that Deutsch is out to lunch on this.
He appears
to assume that a quantum computer is just using the same algorithms that a
classical computer would use, only executing them in a massively parallel
manner.
I find it very hard to believe that David Deutsch does not have a good
understanding of quantum computers.
This is manifestly false. Quantum computers operate in a completely
different way -- that is why there are so few actual algorithms for quantum
computers to execute that gain massive speed improvements.
I think you built a straw man and now you're attacking it. When I
heard Deutsch make the argument, he was referring explicitly to Shor's
algorithm. This is sufficient to demonstrate an increase in
computational power that would be impossible in the classical world.
No one is denying that Shor's algorithm on a quantum computer would
factorize numbers exponentially faster that a classical Turing machine
could do it. But that does not mean that a quantum computer is just lot
of classical Turing machines acting in parallel.
As for more general speed improvements, there is for example Grover's
algorithm, that offers a quadratic improvement in searching unsorted
lists. This has wide applicability in software engineering.
Of course, building more complex quantum computers is still beyond our
technical abilities. I don't think that's news for anyone...
As Brent says in his recent post, Scott Aaronson points out:
"The way a quantum algorithms work is that they arrange for wrong answers to
destructively interfere while the desired answer interferes constructively.
Interference requires that they take place in the same world."
Yes, but this is not classical interference, it's interference between
superpositions of states. So how can this computation happen in the
physical world?
No one is suggesting that this is classical interference. Interference
between superpositions of states does happen in the physical world! Are
you suggesting that two-slit interference does not happen in the
physical world? Interference between qbits happens in the physical world
just as much as two-slit interference. If you define a "world" as
something closed to outside interactions, then interference can only
take place in the one world.
For me, that gives more credence to the claim that the
wave function describes a real object.
I don't see how that could follow. The wave function exists in complex
configuration space -- that is not the "real world".
Classical computers do not have quantum interference. Quantum computing does
not prove the existence of parallel worlds -- there is no need for other
worlds in which to find the computational power, you just need a modicum of
insight into how quantum computing algorithms work.
You might claim that Deutsch is a known expert on quantum computing, but
more commonly, Deutsch is known for having way out, non-standard ideas on
quantum mechanics.
Oh no! Everyone should be kept in line!
Physics is full of people who take non-standard positions. On the whole,
that is a good thing, because it provides an opportunity for real
advances in understanding. But that does not mean that all non-standard
positions are 'true' or valuable.
Bruce
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