Loophole-free Bell test ‘Spooky action at a distance’, no cheating

Another question along the same lines: If I give my friend one of my entangled particles, can I tell if he has measured the spin by examining my half of the pair of entangled particles?

*edit missed a word

Those words provoke little controversy when spoken about matter beyond the visible universe. Why? Because the simplest mathematical description we have of the objects we can see also includes those which in principle we cannot. We grant them the same ontological status as the Earth we stand on -- few would claim them as mere fictions to ease our task of prediction.

Occam's Razor properly applied limits the amount of information in the description, not the amount of things the description describes. For example, pi's infinite decimal expansion is spared by this information theoretic razor. If you reject the multiverse, your full description of the universe must contain one bit for every binary "choice" the universe made, every qubit that collapsed. Quite a few bits that turns out to be. The multiverse description suffers no such flaw, as it contains only the initial conditions and the rules by which those conditions change over time.

It doesn't create worlds. There's always only exactly one fuzzy-quantum-thing. It's just that some parts of it no longer interact with other parts of it.

[1] http://plato.stanford.edu/entries/qm-manyworlds/#5

Let's start a thread where we talk about how tcp/ip works ... but with a twist. The only admissible references in the discussion are color pictures aimed at non-programmers. Anybody tries to talk about what actually works, we label them the "shut up and program" crowd and decry their lack of imagination.

It seems to me that you have a valve that instantly opens/close a different valve at an arbitrary distance.

So essentially you have an ever growing set of merging and splitting history branches, always growing for every point in space, continously.

I've got no idea how to make that work for your example of polarizers, or for the example of delayed quantum eraser experiments, etc... The surface of the bubbles apparantly have to carry all of that information and there must be some mechanism for them to decide which ones interact with which others.

That's a pretty uncharitable interpretation of it. If you look at the actual physics, MWI is incredibly simple: the wavefunction just evolves as the wavefunction evolves. That's it. There's some amplitude over here, and there's some amplitude over there, and this possibility and that possibility are mutually exclusive etc (with some "weird" quantum correlations, of course.)

The Copenhagen interpretation's "wavefunction collapse" is just like a Bayesian update -- the wavefunction you're left with is like a conditional probability. Maybe it makes you happy to throw away the "other universe" bits of the wavefunction and normalise probabilities on your own branch, but you obviously don't have to, and any discussion of when the collapse happens (or worse, whether it's a physical process that propagates across the universe faster than light) is obviously very silly business.

QM is like lazy evaluation. The result is not computed until someone actually forces it. This is different from the result being computed but you just don't know what it is.

Another way is to think of entangled states is like two virtual addresses which alias to the same physical address. The addresses could be very far apart (similar to entangled particles being very far apart), and their contents are coupled but "uncomputed", until one of the virtual addresses is accessed, faulting in the physical page. At this point, both addresses instantly point to the same contents.

The classical interpretation needs so-called hidden variables which determine the state of the cat.

In the quantum mechanics interpretation the state is determined when the interaction (measurement, looking at it) takes place. Before that there are several parallel realities of what could be.

In the case of the cat we cannot actually determine which interpretation is correct. We need a different kind of experiment:

As far as I understood (no physicist here) they use two entangled objects. E.g. two photons which are generated by a special process so that one polarized one way and the other one polarized the other way but we don't know which is which before the measurement.

Then you need to measure both photons separately and determine (statistically) which has which polarization.

These measurements need to be spaced out far enough such that faster than light speed communication would be required for the photon measured first to tell the second photon what was happening.

So far it still doesn't help to exclude the possibility of hidden variables since the measurement setup is static.

Another trick is needed.

They use a random generator to determine at both ends what to measure (which polarization direction) shortly before doing the actual measurement. So, when the photon is generated it doesn't know what would be acutally measured.

When you tabularize the all the possible combinations of how the photons could be measured and calculate the possible probability for the measurement results, there is a difference between what we would expect from classical physics when the status is determined at photon generation and stored inside the photons in a hidden variable and the result we would get from quantum mechanics without a hidden variable but with (faster than light) spooky entanglements.

The experiments show that there is no hidden variable. This is a BIG thing because this means that classical physics are not enough to explain the universe.

https://vimeo.com/126833477

The science part here, is that a hypothesis has been proposed about the inner workings, and the "so what?" objection is a valid question. If the "so what?" is nothing, it's just an idea.

The "so what?" part is where the science happens. In trying to promote the idea (hypothesis) as a working idea with evidence (theory) We look for side effects of the idea that are expected to be different than our existing ideas (theories). We expect to see a cat when we open the box, and it's alive or dead. That's not interesting. But let's actually ask a science-like question of the consequences. These aren't related to the quantum mechanics questions really, but it's the sort of "consequences" questions that do get asked.

Maybe we can do the experiment over a longer time, like a week. We put the cat in the box with food and water. In the original experiment the poison is released at a random time during the week. So under our existing understanding (theory) we expect, if we run the experiment a hundred times we would see that random amounts of food and water are left pretty much evenly from 0% to 100% depending on when the cat randomly actually died.

Here is a different idea (hypothesis) about the side effects: If the cat really is alive and dead as hypothesized, it will use only the part of the food the 'alive' part would use, which is reasonably predictable. It changes by percentage over the week from 100% (alive) to 0% (fully dead). This means we should always see the same, or similar amounts of food and water left at the end. This is different than the usual prediction!

Now we have a new idea, a hypothesis that relies on consequences of the first hypothesis. So we run 100 experiments, and unlike our current understanding of cat mortality, we find that in this case the remaining food and water values a grouped up at about 50%! We can even do a control where the cat is observed the entire week, and we find that under these "observed" conditions our expected spread of values does happen. We've proven that our previous idea isn't enough to explain everything, and we have tested a prediction of the new idea, so our previous theory isn't "wrong" but it isn't entirely accurate, and the new hypothesis has gained evidence to become a new more complete theory.

Now in the real world at the cat scale, we definitely would NOT expect this to be the case, and real science is much more complicated. It's easy to make errors in predictions, or have "confounding" factors that throw off our investigations. But the initial investigations into spooky action were every bit as bizarre and unexpected. We're learning how things work at the atomic scale, and it turns out that while they mostly behaved as we though they did, there are some odd edge cases that really turn things on their head. Much like we don't poison cats in our day to day life, just in thought experiments (I hope), we don't interact with spooky action in science a whole lot either. But as we learn more, it informs our understanding, and frequently leads some unexpected concequences.

For an excellent real-world consequence, under the classic model, photosythesis is too efficient. It requires quantum entanglement to explain the efficiency! http://io9.com/new-evidence-that-plants-get-their-energy-usi...

It's simple, logical and wrong.

The idea with the cat was to magnify (in the macro world) a quantum phenomenon from the micro-world were the state of the object is absolutely and measurably NOT "either/or". It's both (all) states at the same time.

>In the Schrödinger's Cat example, there the only reason to assume the cat is in a dual state is if you want to make a more complex interpretation of the situation.

No, the reason to assume the cat is in a dual state is that the particle determining its fate IS in a dual state (superposition), a state contrary to everyday intuition.

Now, you could say that the cat is dead or alive in some split one of "infinite universes" or a few other explanations, e.g. that we have collapsed to a single state earlier for some reason, but not the crude dead-or-alive-we-just-don't-know-yet thing in a single physical universe...

(The answer to the thought is that yes, indeed, the cat would already be in a single state prior to observation, because the large macro system would have decohered and collapsed into a single state prior to an observer checking the status of the system.. unless I'm mixing something up.)

That point has to be hammered on, rather than neglected, because in reality it is completely true that no such box exists, and almost certainly no such box could exist, and it's very important when explaining this to people that it's a thought experiment, a metaphor, not something that actually happens in the macroscopic world.

http://www.amazon.com/Quantum-Mechanics-The-Theoretical-Mini...

This is Leonard Susskinds attempt at writing a book that tells you the bare minimum you need to know to really understand what's happening. (experience with calculus and linear algebra extremely helpful but not necessarily required)

Pretend instead that we measured the cat with a different measurement that was only somewhat correlated with the cat being alive, such that this test would return "true" with a 75% chance. A subsequent second measurement using the original alive/dead criteria would now return "alive" with a 70% probability! That is, we have altered the in-between state of the cat (in the cat's favor!) by our first measurement. It is this that is unique to quantum mechanics.

A pseudo-classical analogue of this phenomenon can be observed with polarizing filters. Analogous to the cat's initial radiation dose is a 45°-polarized light beam. Analogous to a measurement of the cat being alive/dead is a 0°-polarized filter – 50% of light makes it through the filter. However if a 15°-polarized filter is placed between these, one will instead observe that 70% of light makes it through the 0° filter! (You can also test this easily at home with three polarizing filters; two held at 90° to each other, and a third placed in between at 45° – the third will allow light to pass through the others.) This is solely due to the nature of electromagnetic waves, which very closely parallels that of quantum fields.

Replace "light beam" with "single photon" in the above paragraph and you get a fully-quantum analogue of my modified Schrödinger's cat experiment ;)

In modern times, when we observe nonlocal effects between objects (such as with gravity), we first measure the effect, and only later do we invent something called a "field" whose definition consists of nothing other than the observed measurements themselves.

This is still nothing other than the same "Hypothesis non fingo" as before, except that now, the question of whether fields can possibly be explained in terms of classical physical pictures does not exist anywhere in the entire physics profession. That is, those kinds of hand-waving, natural language questions just don't have any place in the discourse of those who can rightfully be called credentialed, working physicists.

To such physicists, a particle is simply a set of quantum numbers that exists in n-dimensional Hilbert space (or "mathspace"). Again, whatever field is said to exist at the particle's location is nothing more than the measurement that is recorded by some apparatus that exists in [classical] spacetime. So, it is of course necessarily true that the particle and the [part of the] field [that affected it] must exist at the same location.

But to then say that this is therefore a meaningful account of something called "local physics" is, IMO, fairly absurd, because the entire point of the inventions called fields is to say: "Hey, I have no idea what accounts for instantaneous, space-bridging forces like gravity, so I am just going to leave it to the philosophers to offer up speculations so that the masses may be entertained."

Of course, this can be countered by the argument that gravity is fundamentally different from the electromagnetic force, because electromagnetism necessarily depends on the supposed propagation of things called photons. But this argument depends crucially on the notion that our mathematical shorthand for the quantization of the EM field (ie, photons) is at the same time a meaningful physical description that exists in the framework of a classical spacetime picture that is based fundamentally on the quality of locality.

However, QED (quantum electrodynamics; the quantum description of electromagnetism) in no way exists in any such classical framework. It is all rather the very same "mathspace" previously mentioned. The "field" is nothing but whatever bit of data that some classical measuring device recorded there.

p.s. Thanks for the upvotes :)... sometimes it pays to complain, I guess!!!

IANAP, but that is probably not a correct picture. Photons carry energy and therefore if you imagine electrically or magnetically charged objects sending out a constant stream of photons in order to mediate electric or magnetic forces you quickly get a problem with conservation of energy.

In fact, magnetism is a realtivistic effect; it's how the electrostatic force behaves when things are moving.

This also seems at least not accurate to me - relativity turns electric forces into magnetic forces and vice versa when you change reference frames, but they are not the same thing, they are two different aspects.

The basis of measurement here is analogous to the angle of a polarising filter in a photon detector, but the detectors in this experiment are directly measuring the spin of the nitrogen vacancies (so what is the physical implementation of the different basis of measurement?).