/sci/ - Science & Math

Observing means a process of consciousness. Only qualia can collapse the wave function.

>>5292684
Oh I see, that really make sense, thanks anon.

Observing means in a state that can be measured through some means. Typically this means being in a state that can interact with an electromagnetic field.

>>5292705
EM causes wave function collapse?

What if you did the two slit experiment in water using high energy neutrons, and have photo multiplier tubes in the slits to detect the Chernenkov radiation emitted by the particles as they pass through the slit. You wouldn't be "observing" the particles themselves, would the interference pattern remain?

>>5292743
No. The neutrons would not be delocalised because they would be emitting light. Their position would be measurable and thus they cannot be delocalised.

The radiation has interacted with the particle, it doesn't matter that the interaction wasn't directly with the detector.

>>5292684

>>5292743
Actually that kind of reminds me of another question that I had.

Are all the Hadrons in the nucleus in a constant state of wave function collapse? Is wave-particle duality something only elementary particles experience?

>>5292764
So what exactly causes the collapse? Not the fact that in collapse they emit photons, but what actually causes the collapse.

>>5292799
The interaction that results in photons. There isn't a clearer picture than that.

>>5292857
Is that because no explanation is known or is it because you don't understand the concept of circular logic?

Is it actually possible for the partice not to be observed? Isn't it always affected by the gravitational force, no matter how small?

Wavefunction collapse as you understand it -- with the state vector being updated in a special way when a "measurement" is performed -- probably doesn't exist.

There are plenty of ideas about what's actually happening, but they more or less predict the same experimental results. Different people will tell you different "interpretations" are the best.

The simplest, and therefore best, interpretation posits a single set of possible final states for the universe, one of which is chosen randomly in accordance with the Born rule. To see what happened in between, you don't just look at the initial state vector; you have to use a combination of the initial and final states. The "weak value" of a quantity is:
<div class="math">A_w = \frac{\langle f|A|i \rangle}{\langle f|i \rangle}</div>(working in the Heisenberg picture for convenience)
You can think of this as a probability-amplitude-weighted average over all paths from the initial to the final state. Alternate branches of the initial state wavefunction, like the one with Schrodinger's cat's dead twin, don't need to be collapsed away; they simply don't correspond to anything real because they don't overlap with the final state wavefunction.

The second simplest interpretation would probably be many-worlds. In many-worlds, any spacelike hypersurface gives you a decomposition of the initial state vector into eigenstates of some set of local quantities on the hypersurface. Then the parts of the decomposition with a larger <span class="math">\langle \psi|\psi \rangle[/spoiler] are in more "real" than others, and you are subjectively more likely to find yourself in them. It's technically okay with special relativity because even though you need to choose an arbitrary notion of simultaneity to decompose the state vector into worlds, the worlds resulting from any choice can be equally real.

>>5293528
>>5293480
I hope you planned on mentioning deBroglie-Bohm theory before you left this thread, anon. Don't make me do it myself.

>>5293480
>>5293528

Yeah, I think I'll stick with the first one that doesn't contain make believe and magic.

It's also possible to pick out a special set of hypersurfaces as real -- in other words, introduce a coordinate that represents absolute time -- then pick one of many-worlds' worlds as being real in each slice. You also introduce a rule which tells you, based on which world was real in the previous time slice, which one is real in the next time slice. The rule has to be specifically constructed to agree with the probability distribution predicted by quantum mechanics, but that's not difficult to do.

This is the pilot-wave approach, and even though it's very close to the way people thought of the world before learning any physics, you can see that it's one of the most complicated interpretations, and it includes lots of arbitrary choices: an arbitrary choice of absolute time and an arbitrary choice of the rule for updating which world is real.

>>5293568
dBB/pilot-wave can be fully conceptualised without all that many-worlds bollocks, you know.

>absolute time
http://arxiv.org/abs/1002.3226
>It is often argued that hypothetic nonlocal reality responsible for nonlocal quantum correlations between entangled particles cannot be consistent with relativity. I review the most frequent arguments of that sort, explain how they can all be circumvented, and present an explicit Bohmian model of nonlocal reality (compatible with quantum phenomena) that fully obeys the principle of relativistic covariance and does not involve a preferred Lorentz frame.

>arbitrary choice of the rule for updating which world is real
See my first point.

It's also possible that collapse in the traditional sense really is real. But you have to introduce an objective rule for when the state vector collapses -- it isn't enough to wave your hands and say "measurement" or "macroscopic." But this isn't hard to do. The trick is to make collapse a very rare process. Once you have a quantum state in which a huge number of particles have different positions in each of two branches, it becomes likely that one of those particles will collapse to one of the two possible positions. When this happens, since the wavefunction is an entangled state, all of the other particles end up in a position consistent with the particle that caused the collapse.

GRW is an example of such a theory. It's not a relativistic theory, but I don't see anything stopping the construction of a relativistic theory along these lines.

The nice thing about these kind of theories that if it actually happens this way, we may eventually be able to discover it.

>>5293607
>http://arxiv.org/abs/1002.3226
I don't think his <span class="math">\int d^4x |\psi|^2 = 1[/spoiler] idea makes any sense, but the idea of a wavefunction with a separate time variable for each wavefunction seems promising.

>http://arxiv.org/abs/1002.3226
also
>each particle has its own trajectory <span class="math">X_a^\mu(s)[/spoiler]
>in spacetime, where s is an auxiliary scalar parameter

What happens when the particles inevitably get out of sync?

>>5293420
Not possible. At least we can't observe that things aren't being observed.

>>5293787
Also, s represents an arbitrary choice of absolute time, so >>5293607 was wrong to claim this model as a counterexample.

If I could observe an atom without my observation collapsing the wave function. What would I see?

>>5294023
Who says you can't?
http://en.wikipedia.org/wiki/Weak_measurement

>>5294023
Photons being reflected by the grrove of the electrons then being absorbed by the neutrons

On a side note, can anybody explain to me,aside from why they seperate protons why neutrons are there?
That's my imo anyway

>>5294080
Because nucleons are fermions and can get closer together if they're not all the same type of particle. Also, electrostatic repulsion makes it easier to be a neutron in a large atom. "Easier" meaning "less energy." If a proton can decrease its potential energy enough by turning into a neutron, it does so, creating a positron+neutrino pair that carries off the energy.

This video explains it nicely:
http://www.youtube.com/watch?v=0NkdEfvmr5g

Observing something means you're interacting with it to get some sort of feedback. If you're interacting with it, it means you're altering the system, and your experiment is now ruined as you have no fucking idea what happened exactly before you checked.
Basically you see something going into a black box, and something coming out of it, but the only way to open the lid and see what's inside the box is to break it open.

>>5294951
>your experiment is now ruined
Not for all (macroscopic) measurements which ultimately depend on microscopic phenomena, e.g. 1H NMR. Sure it depends on interaction of the spectrometer (Rf pulses sent and detected) with the sample but it doesn't fuck the sample up. Wait longer than roughly 5*T1 (a matter of seconds at most) and it's pretty much like nothing happened.

observing means knowing something happened

You cannot observe something without interacting with it in some way.

All interactions alter the thing being interacted with on some level. This "collapses" the wave function, breaks entanglement states etc...

Interactions are comparatively destructive at very small scales.

For example- in measuring a particle you perturb it with your instruments and irreversibly alter all of its future states from the ones it would have had had you not interfered. Or let's say you need to measure a photon- you have to absorb it with your measuring device before you can know anything about it.

>>5294961
So it is qualia after all?

>>5294023
Isn't this exactly what this year Nobel Prize in Physics was for? Manipulating quantum states without disturbing them?

>>5292782
no, it even works with fullerene
http://www.univie.ac.at/qfp/research/matterwave/c60/index.html

>>5295859
^NO MAN; HE JUST RESTORED THE INITIAL STATE

sorry caps lock