- 15 Jun '10 08:21This discussion came up in another thread "Time paradox". The question is about the observation paradox. A molecule can be in a superposition of states. However, as soon as we measure it, it will

always collapse in one of the states. Does the observer has to be a human(or animal) or can a machine just as well do the measurement?

I would say that if the machine and the specific molecule are completely isolated from the outside world the whole system should be described by a single wavefunction: i.e. i) Molecule is in state A and machine measures state A or ii) Molecule is in state B and machine measures state B.

As soon a we try to read out the output of the machine the whole thing collapses. That is the wavefunction describing the molecule and the machine as a whole. This suggest that the collapse only occurs by an observation of a conscious being.

see also http://en.wikipedia.org/wiki/Consciousness_causes_collapse - 15 Jun '10 10:23The observer does not need to be human, an animal, or even a machine. In quantum physics, an "observer" is a metaphor for a large external system that we don't know everything about (in a mathematical sense). The "observer" is the "exterior" of the system you're considering - an "observation" in this context is some kind of physical interaction with the "exterior", but it turns out you can still describe such interactions to some degree using statistics even if you don't know "everything" (the Hamiltonian) of the system including the exterior (the "observer" ).
- 15 Jun '10 11:24

In this view QM can only describe an isolated sub-system. Hence there is no wavefunction for the universe??*Originally posted by KazetNagorra***The observer does not need to be human, an animal, or even a machine. In quantum physics, an "observer" is a metaphor for a large external system that we don't know everything about (in a mathematical sense). The "observer" is the "exterior" of the system you're considering - an "observation" in this context is some kind of physical interaction with the ...[text shortened]... hing" (the Hamiltonian) of the system including the exterior (the "observer" ).** - 15 Jun '10 12:11 / 1 edit

QM can describe non-isolated systems too, and that is where the "observer" comes in. If you have an isolated system, and you know the Hamiltonian of the system, the time evolution of the system is described deterministically (i.e. without statistics). The question is whether it is possible to calculate a wavefunction for the universe (impossible in practise, surely, but maybe a theoretical possibility), and whether the universe itself is an isolated system or not. Anyway, I don't know all the details of how this would work out.*Originally posted by TitusvE***In this view QM can only describe an isolated sub-system. Hence there is no wavefunction for the universe??** - 15 Jun '10 13:42

I cant really put it better than KazetNagorra has already done.*Originally posted by TitusvE***As soon a we try to read out the output of the machine the whole thing collapses. That is the wavefunction describing the molecule and the machine as a whole. This suggest that the collapse only occurs by an observation of a conscious being.**

There is nothing unique about consciousness when it comes to being an observer.

If we choose to look at the whole universe and our unique position in the 'now', then all past events are a quantum wave that we observe from 'now', and it collapses down to all possible states that are compatible with the current 'now'. But again, this is true whether we are intelligent beings, computers, or mere objects interacting with the universe.

I am sure that any possible experiment investigating quantum mechanics, can be carried out equally well by a human scientist or a robot with no differences appearing in the results. - 15 Jun '10 15:15

If the whole univers is a multidimensional quantum wave then there can't be a collapse unless*Originally posted by twhitehead***I cant really put it better than KazetNagorra has already done.**

There is nothing unique about consciousness when it comes to being an observer.

If we choose to look at the whole universe and our unique position in the 'now', then all past events are a quantum wave that we observe from 'now', and it collapses down to all possible states that are compat ...[text shortened]... equally well by a human scientist or a robot with no differences appearing in the results.

the universe is observed by some external entity (God?? ;-) )

Or can a quantum system collapse by an internal "observation" - 15 Jun '10 16:50 / 2 edits

Any 'observer' if taken individually is 'external' to the rest of the universe. From the observers point of view, all incoming information is what causes the wave function for the rest of the universe to collapse.*Originally posted by TitusvE***Or can a quantum system collapse by an internal "observation"**

Again, this applies regardless of whether or not said observer is conscious.

Schrödinger's cat was an observer for the rest of the universe. To it, you could be dead or alive. - 15 Jun '10 17:50You seem to introduce relativistic principles in QM. Do you really want to imply that a wavefunction can be in a superposition or not depending on the observer? Can a proton that sees me make my wavefunction collapse? You say it is so simple, but it isn't. It is a paradox that has bothered the brilliant minds of our time.
- 15 Jun '10 17:51

It may be our universe is only one of a possible infinity of other universes, each with its own time and space, maybe separated from us by some kind of membrane and in our regular space but separated by distance or separated from us by multiple dimensions, either way if that is the case, the rest of these universes would be the external viewer, still connected in a QM sense. Of course as of right now that is supposition but there are hints of other universes even in the CMB data. News at 11*Originally posted by TitusvE***If the whole univers is a multidimensional quantum wave then there can't be a collapse unless**

the universe is observed by some external entity (God?? ;-) )

Or can a quantum system collapse by an internal "observation" - 16 Jun '10 10:02

The problem is we don't really know what collapse is; there is no adequate mathematical description of wave collapse in a time-dependent way (AFAIK).*Originally posted by TitusvE***If the whole univers is a multidimensional quantum wave then there can't be a collapse unless**

the universe is observed by some external entity (God?? ;-) )

Or can a quantum system collapse by an internal "observation" - 16 Jun '10 16:22

Yes. It is that simple.*Originally posted by TitusvE***Can a proton that sees me make my wavefunction collapse?**

Any interaction between two independent systems essentially rules out any systems incompatible with the interaction thus the wavefunctions of both systems collapse down to the set of all systems that match the interaction. - 16 Jun '10 21:40

????? I cannot make any chocolate from this. Your interpretation of QM seems rather odd. Well, it is just not correct. You suggest that if one molecule, that sees another molecule, can make this molecule collapse. This is nonsense or I wrongly interprete your words. In that case sorry, but what you write is rather incomprenhesible*Originally posted by twhitehead***Yes. It is that simple.**

Any interaction between two independent systems essentially rules out any systems incompatible with the interaction thus the wavefunctions of both systems collapse down to the set of all systems that match the interaction. - 17 Jun '10 06:20

Well, do you have any reason for believing it is incorrect other than an unfounded belief that consciousness plays a part?*Originally posted by TitusvE***????? I cannot make any chocolate from this. Your interpretation of QM seems rather odd. Well, it is just not correct.**

**You suggest that if one molecule, that sees another molecule, can make this molecule collapse.**

Correct.

**This is nonsense or I wrongly interprete your words.**

In what way is it nonsense? What is wrong with it?

**In that case sorry, but what you write is rather incomprenhesible**

Well it can be rather hard to explain something as complex as quantum mechanics.

Lets go back to Schrödingers cat.

Suppose there are two boxes with cats in. they are totally isolated from each other. In each box, the cat may be black or may be white. Suppose the boxes interact - the wall dividing them becomes transparent. This interaction will cause the wave function of (black /white) to collapse and only one color will be observed by each cat.

Now this will happen regardless of whether we use cats or robots.

One of the many effects of quantum mechanics is the fact that light behaves as a wave. Now if consciousness is required to make quantum mechanics work, then it would imply that light would only behave like light when people are watching. Now whose not making sense? - 17 Jun '10 08:07Skip the conscious part and skip the cats. I am not saying I advocate myself the "conscious makes the wavefunction collapse". It is an interesting theory, just like multiple universe that split after each measurements.

If we leave out complexities like consciousness and cats/humans, I think your interpretation of QM is wrong. KazetNagorra is right that there is not a quamtum-mechanical description (in the sense of a time-dependent Schrodinger equation) of the collapse.

To make things simple, suppose we 2 molecules in a box so that they are completely isolated from the outside world. Now you say that one molecule can collapse due to the interaction/"measurement" of the other molecule. This is wrong. In QM the 2 molecules are

described by a single wavefunction. This wave function will only collapse as soon as we open the box. - 17 Jun '10 11:51

I think you are misunderstanding where the boxes are. Again, you are giving too much credit to your own consciousness.*Originally posted by TitusvE***To make things simple, suppose we 2 molecules in a box so that they are completely isolated from the outside world. Now you say that one molecule can collapse due to the interaction/"measurement" of the other molecule. This is wrong. In QM the 2 molecules are**

described by a single wavefunction. This wave function will only collapse as soon as we open the box.

1. A box exists that contains two molecules. We are not interacting with anything inside the box.

2. You are correct that from our perspective, the two molecules can be described by a single wave function.

3. Molecule A is not interacting with molecule B.

4. We can therefore say Molecule A is in its own box and Molecule B is in its own box, and that from Molecule A's perspective Molecule B can be described by a wave function and from Molecule B's perspective Molecule A can be described by a wave function.

5. Upon interaction, they each 'observe' each other and their relative wave functions collapse (for each other).

6. We being isolated do not know of their interaction and thus for us it both has / or hasn't taken place.

7. Only when we open the box and interact with the molecules does the wave function collapse.

8. What is important to note though, is that if we only interact with molecule B, we can see when it interacted with molecule A (in the past, before our interaction), and molecule B would have a record of Molecule As wave function collapsing at the time they interacted, not at the time we interacted with the overall box.

Quantum mechanics is a nested box system.

The important thing to note, is that what is special about quantum mechanics is not wave functions collapsing, but rather when they don't. Light and many other 'particles' behave as waves because their wave function does not collapse, because for a certain period of time they remain in a black box, and when they come out and interact they do not carry enough information about their past to determine their exact history.