Linked by Thom Holwerda on Thu 23rd Feb 2006 18:09 UTC
Geek stuff, sci-fi... Ok, this one is just plain scary. "By combining quantum computation and quantum interrogation, scientists at the University of Illinois at Urbana-Champaign have found an exotic way of determining an answer to an algorithm - without ever running the algorithm. Using an optical-based quantum computer, a research team led by physicist Paul Kwiat has presented the first demonstration of 'counterfactual computation', inferring information about an answer, even though the computer did not run." The research team published their results in Nature.
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Member since:

Its right up there with string theory which at least sounds a bit more plausible otherwise they wouldn't put it on PBS Nova or BBC Horizon?. I've never seen this quantum stuff try to be explained to the layman on any of these shows yet, perhaps if it were, it would disappear.

Reply Parent Score: 1

DigitalAxis Member since:

Well, I don't pretend to understand it very well, but here goes. Please don't blame me if I'm wrong (and I probably am)

Basically, the problem with Quantum Mechanics as opposed to Regular Mechanics is that Quantum Mechanics is "non-deterministic". What non-deterministic means is that you can't actually just determine what state something is going to be in, just a probability.

One other bizarre related prediction of this theory is that observing something changes it.

It's Schrodingers' Cat: Imagine a cat in a box, with a bottle of poison and a radioactive source. If the radioactive source happens to decay (there's only a probability that it will or it won't, like flipping a coin), a hammer will smash the bottle and kill the poor kitty.
Until you open the box, you won't know if the radioactive source decayed and killed the cat. The catch is, according to Quantum Mechanics, the cat is BOTH until you check.
Once you check, the "waveform collapses" into one possibility at random. This has actually been tested and observed (the Stern-Gerlach device, I can't find a good explanation of that actual experiment online), fortunately not with cats (that thought experiment was invented to highlight how absurd the whole idea was)

Albert Einstein hated the idea ("God does not play dice with the universe"), and spent a large portion of his later years trying to prove, essentially, that you really could tell whether or not the cat was dead without checking. (The Einstein-Podolsky-Rosen Paradox) Unfortunately, he wasn't successful.

Reply Parent Score: 4

John Nilsson Member since:

I've never liked the Shrödingers cat example. It's to philosophical.

A more direct example of quantum mechanics is the experiment with lasers and mirrors.

The setup is a laser fireing into a semi-transparent mirror, where the resulting rays are later collected in another semi-transparent mirror with two detectors on either exit path.

As long as you don't measure anything except the detectors at the end both detectors get 50% of the laser photons.

The quantum weirdness starts when you block one of the two paths. Common sense dictates that 50% of the photons would be lost and the other 50% would be divided in by the last mirror so that the detectors get 25% of the photons each.

But what happends is that 100% of the photons goes to one of the detecors.

I don't know if quantom logic is involved but it certainly seems similar.

In quantum logic there is a concept of a sqrt!() (that is square root of not) function with the properties that

sqrt!(x) == random(2) (where x is 0 or 1)
sqrt!(sqrt!(x)) == !x

This also shows why it's impossible to measure quantum systems. If the result of the inner sqrt!() was know the result of the outer sqrt!() would be random, and thus unknown.

Edited 2006-02-25 06:06

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someone Member since:

That's one possible intuitive explanation of QM. However, not all interpretations require the wavefunction collapse.

Personally, I like to keep the intuitive part to a minimum and stick to the equations: What's wrong with probability patterns that just happen to match the wavefunction? We will probably need the String Theory to really explain what really is a photon/electron/etc. (In the mean time, I am hoping the LHC will be finished soon)

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