I've had this question for a really long time, but never found a good answer. It's a physics question, not astronomy, but quantum physics is often a part of the show.

In quantum physics, things remain in an undetermined state until they are observed. However, I've never heard a more meaningful answer about what an observer is than "something which causes quantum states to become determinate".

Example answers that I've heard in various pop-physics venues:

1) A human brain (if a person doesn't see it, it doesn't count)
2) A living brain (cat, dog, etc)
3) A camera (even if the film is never developed)
4) A large molecule

So, in practical terms, what is an observer? And, of course, how do we know?

Yeah, that word "observation" has caused a lot of trouble.
Let me cut and paste something I wrote recently on another thread.So you can take your pick of the interpretation you choose to use: quantum mechanics itself has nothing to say on the matter.

Grant Hutchison

It isn't that what is there isn't true until you observe it. It is that you must observe it to know what state it is in.

True quantum physics just make predictions that are much broader than yes or no.

On large scales, I can throw a baseball and there is a 99.99999~% chance that it will end up in the direction I throw it.

In the quantum world, if throw a particle in one direction, it has a x chance of going here, and y chance of ending up over here, or z chance of being somewhere else.

Then you pick the highest number and hope that's where is goes.

Except... it may end up in both places or nowhere near where I was looking for it.

There are a number of interesting points to add to that "Heisenberg split" idea. First of all, it existed in classical mechanics also, it just never mattered to science. In other words, we never had a way to describe what happens to a system that is not observed, but we never found any evidence that what happens was different, so we simply made the simplifying assumption that it was not. It is perfectly good science to make such simplifying working assumptions, the only difference with quantum mechanics is that the targeted precision of the measurements is so spectacularly higher that we discovered the principle of "non-commuting measurements", which means that if you prepare a system in a state with a known value of some measurable, you might not simultaneously be able to know the value of some other measurable. In that light, we see that "measurement" happens at both ends of a scientific prediction-- it happens when you prepare the experiment, and again when you test the result.

This was unknown in classical physics, where it was thought that objects "carried with them" complete information about everything you could measure, but note that was only pure assumption in classical physics-- we simply did not have precise enough measurements to ever test if it was true. Physics made the simplifying assumption, but it should never have taken that assumption as seriously as it did for centuries (it was "magical thinking" to extrapolate a successful hypothesis to a level of precision that was untested). Ironically, many people see quantum mechanics as having a kind of "magical" quality not present in classical physics, but I argue that QM actually represents jettisoning a magical aspect of classical physics in a regime where it could actually be tested.

The second thing about the "Heisenberg split" I'd like to point out is that measurements applied from one side of the divide to quantum systems on the other side do not have some kind of "coincidental" effect of replacing the uncertainty of the possible outcomes with a particular one. Instead, the act of measurement is expressly set up to accomplish precisely that physical influence. So it's not a byproduct of a measurement that a quantum system goes from a "superposition state" to an "eigenstate" of the measurement-- the measurement is designed to create decoherence between the different eigenstates so that the system will behave as if "one or the other actually happened", which is just how classical systems behave. In short, measurement is the act of coupling quantum systems to classical systems so that they will behave classically and we can apply our standard scientific norms. Ironically, this is exactly the opposite from what many people (not Schrodinger, by the way) think happens when you connect a cat to a quantum system that can be used to kill the cat-- they think this puts the cat into a quantum state, when in fact the whole point of measurement is to get a quantum system to behave classically! If you could really put a cat in a quantum state that way, then measurement no longer serves its purpose, and suddenly we are awash in an ephemeral domain where science's norms fail and we have no idea what we are doing any more. We should only go there kicking and screaming, and so far there just isn't any good reason to.

as i understand it the modern view solves the observer problem with

decoherence....(you will find a better explanation than mine if you google it)

a quantum supposition is lost when an object interacts with its enviroment... it does not have to be a human observer..

if you could isolate a big object from its enviroment it would display quantum weirdness..so far some guy in vienna is up to a 60 atom buckeyball!
they are aiming at a virus next..(wtf!?)

quantum computers would take advantage of supposition to do many things at once...but the isolation thing is hard..

i dont understand this but things deconhere ....like this...

an overlap of 2 particals = 1/2x1/2= 1/4

3 particals 1/2x1/2x1/2=1/8

a water drop contains millions of particals ..the overlap between there combined waves is the sum above a million times....basically ZERO

so essentially there is no overlap in their quantum waves and hence no interference...and maybe no observer...

the world becomes the world we know when huge amounts of particals interact with themselves...

?

lol

evil said.....It isn't that what is there isn't true until you observe it. It is that you must observe it to know what state it is in....

if only that was the case....it is in all states until the wave collapses...there are trickey experiments with lots of mirrors that show this ...thats what freaked everyone out ....

its not that is in a certain spin state that you just dont know...it isnt

hence the whole problem with an objective underlying reality..

A lot of people think quantum indeterminacy is an obstacle to the question of "objective reality", but frankly I just don't see it. I think it comes from an overinterpretation of classical physics, the idea of determinism. There was never any evidence that classical physics is completely deterministic, because classical physics does not use sufficient precision to address the issue. Furthermore, we have sensitivity to initial conditions, and the "butterfly effect" in weather forecasting. These are all perfectly classical-- why doesn't anyone seem to think these compromise the notion of "objective reality"? In fact there is no connection-- determinism messes with objective reality just as much as indeterminism does. If we say we can predict the future using some algorithm, how do we include our making of that prediction into the algorithm? This implies that science could never, in the context of an objective reality, provide a complete description of a truly deterministic universe-- so what sense does it make to try and use science to argue that ours is? The acceptance of indeterminacy is a sigh of relief for science, it means we don't have to base our concept of objective reality on a requirement for completeness. Many of the ancient philosophers already understood that, and quantum indeterminacy adds nothing fundamental to this state of affairs. I've never understood why so many claim otherwise.

well i kinda agree with you but i think the problem came about because of the observer problem stated earlier....if by objective reality you mean....

an underlying reality which exists independently of our perceptions and thoughts.

are you saying that you can define an electron in its uncollapsed state as objective reality?

dont ask me!!

i think by no objective reality people were recognizing that evils statement..

''It isn't that what is there isn't true until you observe it. It is that you must observe it to know what state it is in.''

is not true...

ps i think deconherence solves this problem as things do exist (collapse) when we are not looking....

Yes that's right, and in fact we already do that-- the definition of "objective" is satisfied by an electron in an "uncollapsed state" (to wit, different observers will get the same results from experiments on them). So I'm saying an electron in its "uncollapsed state" is no less a part of objective reality than one in a collapsed state. It is we who demand that objective reality have certain properties, based on our own interpretation of our experiences with it. That's not a very "objective" approach to demand reality conform to our requirements, rather than to modify our requirements to conform to reality. We get to define what "objective" means, but reality decides how it will interact with our definitions.

A truer version is, "the idea that the electron is in a 'state' is a requirement that we created for it, so it is up to us to gather the information to be able to describe that state".

yup i agree ...but i guess its a modern way to look at it .. the old days an object was meant to be an object?

i for one am very happy to believe that the universe would still be there if we were not...

That was precicely what I meant when you said I was wrong.

Quantum actions only take place at the subatomic level.

My wife is not in a dual-state when I can't see her. She is not both alive and dead. She is most certainly in one state or the other. If she were, she would have memories of both when I next saw her.

your wife is a Macroscopic object who is fully deconhered so yes

but an electron in a superstition is not the same....it is not that it is say spin up and then you look and say oh its spin up

its not spin up till you look...it is all possible states till its waveform collapses

hence it can go thro both slits...or bounce off a mirror..(light takes all paths thro a mirror)


''The conclusion of all this is that there is no experiment that can tell us what the electrons are doing at the slits that does not also destroy the interference pattern. This seems to imply that there is no answer to the question of what is going on at the slits when we see the interference pattern. The path of the electron from the electron gun to the screen is not knowable when we see the interference pattern. As Heisenberg said, "The path [of the electron] comes into existence only when we observe it."

?


dont ask me but thats the way it is

sorry if i misunderstood you....its coz you put state at the end and that only happens when the waveform collapses...*brain melt*

I find wives are most definitely dual state, at least, even when we do observe them.

There's a chance I will understand Quantum Mechanics one day. Wives, I dunno...

(Honey, I love you if you read this when I'm not looking!)

__________________
If something is in me which can be called religious then it is the unbounded admiration for the structure of the world so far as our science can reveal it... of the manifestations of the profoundest reason and the most radiant beauty, which are only accessible to our reason in their most elementary forms...
Albert Einstein

hey steve hows it going down under?

hear you on that last post !

(if your not looking she aint there !!)....like my 2 year old... if she cant see you you cant see her..lol

my wifes got one state....RIGHT

:/ *gulp*

Hey Damian

Life is good, Rugby season is starting.

My wife dislikes Rugby as much as Astronomy.

But less than Cricket.

But she does love our boys, so she manages our team in spite of.

(...and you do a great job princess!)

__________________
If something is in me which can be called religious then it is the unbounded admiration for the structure of the world so far as our science can reveal it... of the manifestations of the profoundest reason and the most radiant beauty, which are only accessible to our reason in their most elementary forms...
Albert Einstein

Damien, did you hear any more about that Garrett Lisi deal? I followed it for a while but it seemed to die... you were sceptical all along right?

I'm picking you're counting the days to LHC light off! I've listened to most of their podcasts - those guys rock. John Barrowman was a laugh (you could never accuse him of being in the closet!).

Here's the link if anyone has missed them.

http://www.cernpodcast.com/

__________________
If something is in me which can be called religious then it is the unbounded admiration for the structure of the world so far as our science can reveal it... of the manifestations of the profoundest reason and the most radiant beauty, which are only accessible to our reason in their most elementary forms...
Albert Einstein

In the old days they tended to overestimate their ability to conceptualize reality, some even thought what was in their minds was closer to reality than what was accessible to their senses. I think there are three things: what is accessible to our minds, what is accessible to our senses, and what is real.
Yes, I agree, and no experiment indicates otherwise. But what would clearly not be there without us is the way we think about the universe-- what we think is real.

There's only one real problem with this statement.

When I thought I could fly... I fell and bustedededed my noggin.