h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

As far as I know, there are several things on a subatomic level that exists in a kind of undecided state, only picking a state once observed/measured. For example the spin of an electron. I assume that a machine standing by itself in a lab somewhere could do such a measurement, causing the objects quantum state to collapse. Is that true, or are humans somehow required to observe it? I would assume the former, the common use of the word "observer" just gives the impression of the latter. Anyone know exactly what is required?

It needs a human. If a machine observes it, then the machine's state just becomes combined with the particle's state to be in a superposition of two states: "Machine detected A" and "Machine detected B".

That's the paradox.

But surely if the machine performs a measurement it is, in essence , the observer.

t.

But the reading on the machine's screen where is says "yes" or "no" is itself unknowable until someone looks at it...

Damn it Gnomon, that was not the answer I was looking for.

If the sign on the machine saying yes or no is unknowable untill you look at it, couldn´t you also say the answer the scientist has read off it is unknowable untill you ask him about it? What is the difference between those two links?

I'm not an expert but I believe it's still not decided what constitutes an observer in quantum physics. There's no reason to make people special, but the theory doesn't say what is special. At the moment, as I understand it, the theory just works if you don't worry about it too much, but it's basically unresolved.

I think it is this unresolved issue that makes physicists unhappy with what is known as the Copenhagen Interpretation although it works, so they came up the alternative explanation: the many worlds interpretation in which the wave function never collapses and all possible outcomes occur simultaneously in parallel universes.

If it needs a human(or a squirrel?) that just seems to indicate that consciousness is something special, rather than just an effect of materialistic properties of the brain. That last bit I don´t know how to formulate, but I think you know what I mean. That´s the part that doesn´t sit well with me.

You're right, Xanatic, it doesn't sit well with a lot of people.

I should imagine this lot struggled with the observer problem...

http://www.theregister.co.uk/2011/05/04/qubit_characterization_breakthrough/

t.

I love those qubit computers. You set up a physical switch which is in a superposition of two different states, effectively running your computer in two different parallel worlds. With 20 qubits, you can run your program in 2^20 = 1 million parallel worlds, each one running on a slightly different version of a problem, so you can crack the problem 1 million times as quickly.

"It needs a human."

That is only one interpretation though, and one both with its own problems and, I think, wrong. If you're interested there are various places on the web more learned than I which cover them. Not least of which are what levels of consciousness are required (a cat to use a particularly appropriate example? Or just humans?) and the implication that the entirety of reality could not have existed as a physical reality until the first conscious human came into being to observe it.

But it the conscious human required is also demonstrably wrong.
Taking the classic double slit experiment, there is a modification whereby detectors are placed by the slits to register which slit the photon comes through. But this detector is not itself observed by a human. The outcome though is the same, the photon only comes through one or other slit. So a conscious human is not required, merely a detector (which can, indeed, be a human). There's also stuff about quantum erasure which further complicates things a bit but I don't know much about it.

The next stage on from the copenhagen interpretation is to do with quantum decoherence and the many worlds interpretation. This apparently does away with observer/non-observer distinctions but I ain't even going to pretend to have any proper level of understanding of it, still trying to process it. Worth checking out though.

But, from what I have read and (mis?)understood, the idea of a conscious human being required is not the case any more.

I know that the many worlds interpretation does away with the need for an observer, but at the price of having billions of parallel universes. Is there another interpretation as well, with just a single universe and no observer?

Hmmm, not sure it does away with the need for an observer - wavefunction collapse occurs though interactions (as I understand it) even ones which do not involve physical contact. It just means that any system can be an observer so the observer/non-observer distinction isn't required. Basically anything which requires there to be a decision made can act as an observer, is my understanding.

Like I say, I don't understand quantum decoherence, so I shan't carry that discussion on.

As for single universe, no observer - I'm sure there are such interpretations, but I don't see anything much about them in what I've read.

That is also more what I would imagine, that things don´t decide on a state untill they have to. However the interaction that requires one or the other state can be done by any kind of object, and doesn´t require anything conscious. We just wouldn´t notice the wave function collapse unless we were around to see it happen.

My understanding was that that was the way the physicists expected it to work, but when they did the experiments, they got different results depending on whether a person or a machine did the observing.

Damn you Universe!

"but when they did the experiments, they got different results depending on whether a person or a machine did the observing."

what happend when a machine AND a person did the observing???

I may be wrong. It´s a long time since I read up on all this, and I don´t really remember it well.

I've certainly not seen or read/heard anything that suggests a difference in outcome between conscious and non-conscious (whatever that definition may be) observer. Would be very interested in anyone can find something like that.

The early interpretation was the one that a conscious observer was needed - you can find this question asked in all sorts of places - but the later hypotheses and experiments (such as the double slit modification I mentioned above) showed that 'observer' is perhaps the wrong interpretation, and that interaction is a better term.

Of course, the important thing to remember is we know very little about this, and no doubt the reality is far stranger than what is currently being thought about.
I mean, already this year we have reports of strange results from the Tevatron and LHC that could indicate particles that don't fit the standard model. And possibly a 5th force. Both very early days and could still well be mistakes or something else entirely. But it just shows how much is up for grabs.