The thing about Quantum Theory is that you never actually understand it, as nature on a microscopic scale is qualitatively different than the mesoscopic scaled world in which we live. There is no problem in specifying the location of a particle (or a car, or a person) and its velocity at the same time. This is impossible on a quantum scale. A perfect precision in the velocity of a particle means you have zero knowledge of its location and vice versa.
People who understand quantum theory understand it in the sense that they know what calculations to perform to explain what phenomena. It'll never become a part of you as for example knowing how to cook or knowing how to walk are.

Yes.

I once debugged an intermittent problem on an electronics circuit. I wanted to see the setup/hold time of the troublesome signal. The moment I put the probe on, the circuit no longer showed any intermittent problem. The capacitor of the probe changed the circuit.

I showed my boss the signal on the oscilloscope and said, this is not the signal which caused the problem. We can never observed the true signal since any probe which touch it, would alter the signal. That's an example of the Heisenberg uncertainty principle at play.

Excellent - we have an expert, because I have a few people here struggling with it

http://www.bbc.com/future/story/20130124-will-we-ever-get-quantum-theory

They will be so pleased you've got it all worked out :-)

that article is stupid and that questionnaire is stupid. nobody cares how you interpret things. even interpreting things by itself is stupid. like that "many worlds" theory. retarded and useless and not science. theories are meant to make predictions, that is all. if you are arguing that quantum theory means that the universe splits off into all these other universes whenever a choice is made... well then youre being an idiot. unless this conjection can be proven in some way then it is of no value to science.

actually i should differentiate what i call science and what people here think science is. i mean the search for objective truth, not the ability to conform.

Quantum physics makes sense when you consider it 'engineering' rather than 'science'. The formulas work to a purpose, but they don't make sense in a proper way. Feynman makes good explanations using "little ropes" and "waves in a swimming pool", highly recommended to listen to what he has to say .

I agree with you, the questionnaire is a joke. Most people are no where near the level of Bohr and Einstein to say they believe one or the other.

It's boiled down to the fundamental belief, is our fate has been predetermined or it's unknown and we write it as we go. Most people don't like to be controlled so clearly they would reject Einstein's deterministic notion.

I read hundred and hundred of articles and still never come close to make any choice on such questionnaire.

Look, the people who really understand Quantum Physics are very, very unlikely to be on this forum, in my humble opinion.

"Look, the people who really understand Quantum Physics are very, very unlikely to be on this forum, in my humble opinion."

http://yumceleb.com/2013/01/scarlett-johanssen-popcorn/

Sure I understand it.

It was an American TV show where Sam Beckett jumped around in time. He usually said "Oh boy" at the end of each episode. Good programme. Let me know if there is anything about the Big Bang theory that you don't understand.

I studied computer science & physics for about four years, passed exams on the basics of quantum physics, understood the early experiments that began the break with classical physics, but when it got into solid state physics etc I hit a wall.

I ended up dropping physics at that point and switching to plain computer science. I do feel a bit weird about people e.g. my coworker who see some documentaries on quantum physics (which I also can enjoy) and think they understand it.

On the other hand I see nothing wrong with people being interested in it, and I also like to discuss things I have little real understanding of and like some pop-science, but I think the way people draw too close a connection between pop-science and practical science it what irks me; they're pretty different in my experience.


Also +1 to the Feynman lectures; maybe if he had been teaching me I wouldn't have failed quantum physics (.. who am I kidding)

@Nigee, you're now asking a different set of questions -- you're asking about how to interpret quantum mechanics. That's also a useful set of questions, but it's really something that would fall under the philosophy of physics (a burgeoning area of research and one with a lot of interesting questions) more than physics.

The basic idea of physics is that it's supposed to be predictive. On that level, quantum mechanics works, and I use it on a regular basis (I've written papers in particle phenomenology, so it's kind of hard to avoid quantum mechanics, field theory, etc.) The reason that quantum mechanics admits multiple interpretations is that those interpretations do not make different predictions in a way that can be tested. Quantum mechanics definitely gives counter-intuitive results, or, rather, it takes doing a lot of calculations to build up an intuition for what the answer might be. It also has the unfortunate property that very few problems can be solved exactly; most calculations are instead done approximately, which makes it even harder to build up intuition using the underlying principles. [Hartree-Fock is probably the messiest canonical problem I've ever worked through while learning something]

@MeepMeep: The example you gave is not actually an example of the uncertainty principle. What you're referring to instead is the idea that the observation can influence the result. For an example such as the one you gave, there are usually ways to build a probe that has less influence and measure what you're trying to measure. However, in quantum mechanics, that's often not the case.

Let me try and give a slightly more abstract example (thanks to Allan Adams for suggesting this way of explaining uncertainty to me). Imagine two black boxes that take particles in and have them come out in one of two different holes. If we send a bunch of particles into the first box (let's call it "color"), half of them come out the first hole ("red") and the other half the second hole ("blue"). Similarly, if we send a bunch of particles into the second box ("shape"), half of them come out "triangle" and half come out "circle".

It turns out that the following all end up being true:

Experiment 1: Take a new set of particles and put them into a color box.

Result: 50% red, 50% blue

Experiment 2: Take a new set of particles and put them in a color box. Take the red ones and put them into another color box.

Result: 100% are red.

Experiment 3/4: The same is true for a shape box.

Experiment 5: Take a new set of particles and put them in a color box. Take the ones that come out red and put them into a shape box.

Result: 50% triangle, 50% circle.

Experiment 6: Take a new set of particles and put them in a shape box, then take the triangles and put them in a color box.

Result: 50% red, 50% blue

OK, so far we get what we'd expect. Half of particles have each color and half have each shape. Now the fun part.

Experiment 7: Take a new set of particles and put them in a color box. Take the red ones and put them in a shape box. Take the triangles and put them in a color box.

Result: 50% red, 50% blue

Experiment 8: Take a new set of particles and put them in a color box. Take the red ones and put them in a shape box. Take the triangles and put them in a color box.

Result: 100% red.

So, what should we conclude from all this? Being red is not a permanent property. Neither is being a triangle. After all, a red particle that is then checked for shape is only sometimes red. However, a red particle that is then checked for color is still red. In other words, the boxes that we've put these particles through are changing the properties of the particles. This example is related to the uncertainty principle; you can think about the example I gave as measuring, say, the spin of a particle in the x- and y- directions, which cannot be measured simultaneously because of uncertainty. But, it's actually a different effect. And since I've probably written far too much and nobody's going to read this, I suppose I'll stop here.

Yeah too much words. Basically the same particle could have different color based on how/when you observe it.

Now, if you would be so kind, talk about the Quantum entanglement. It appears the speed of the 'information', the corresponding spin of the complementary pair particles transpire at speed thousand of times faster than the speed of light.

Some time I wonder if the scientists cook up a bunch of unverifiable Quantum Physics to get the tax money to support their habit. Hmm..I am going to make up a new field Quantum Diplomacy, LoL. Sometime the player is good, sometime the player is evil depends on the game and time he plays.

I'm not sure what my motivation is to write a thoughtful reply when your answer is that my previous reply was too long to bother reading.

So I win.

I understand bits and pieces of quantum theory.

@CSteinhardt _ Liked your example above and would love to hear more. As an armchair idiot, I am always up for learning. So please post more. I would love to read it (and yes, I read all the way through the other post). Question were steps seven and eight supposed to be identical with different results? Just wantt o make certain I understood. And if you put them in shpe first then triangles in color then red in shape, would you possibly get 50/50 different shapes soemtimes and 100% triangles sometimes?

And more to the point, could all of them potentially change color so that you got red triangles that suddenly came out the other color?

Argh - sorry, 7 and 8 were not supposed to be identical. And I don't see a way to edit that. 8 should instead be:

Experiment 8: Take a new set of particles and put them in a color box. Take the red ones and put them in a shape box. Take the triangles and put them in a color box. Take the red ones and put them in another color box.

And now they come out 100% red.

Yes, I could get the other color the same way. So, for example, take a color box and select the red ones. Then, take a shape box and select the triangles (or circles). Then take a color box, and since it's 50% red and 50% blue, take the blue ones. Now if you put those blue ones in a color box, they're all blue.

So the idea is a repeat test (back to back color boxes) will come out the same, but with a different test in between, they have a chance to alter their color. So thorugh repeatedly alternating the tests, you will eventuallyu come down to a true red triangle particle if one exists?

Could this be a way of separating some kind of static known particles from quasar like particles (say ones where the alternate both shape and color or just color or just shape) by using a preset pattern of tests and results?

Remember, I'm a software dev, so I think in purely logical terms and algorithms. Hope that will work OK for this or my brain proabably won't wrap around it well.

The idea is that particle have a color, particles have a shape, but uncertainty tells us that the better we know one of them, the worse we can know the other. In other words, a particle can be definitely red, a particle can be definitely a triangle, but it can't be both definitely red and definitely a triangle at the same time.

So, you can put a color box in and pick the blue ones, say, then they'll be 100% blue, 50% triangle, and 50% circle. And you can keep putting more and more color boxes in there, and they'll still be 100% blue, 50% triangle, and 50% circle. But then if you measure the shape (let's say you pick triangles), then you'll have 100% triangles, but by uncertainty, you don't have any information about their color any more, so they'll be back to 50% red and 50% blue.

Example:

blue, blue, blue, blue, blue, then another color box -> 100% blue
blue, blue, blue, blue, blue, triangle, then another color box -> 50% blue, 50% red

Or, to put it another way which illustrates how counterintuitive this is, let's say I put a stream of particles through four boxes:

Particles -> Color -> Shape -> Color -> Shape

1/16 of the particles will be measured as red, triangle, blue, and circle.

OK - I'll see what I can do with an outline on entanglement. Then I'm getting some sleep.

Entanglement is a counterintuitive result which arises from a seeming collision between several laws of physics that are well-known even to high school physics students: (1) Conservation of [energy, momentum, charge, angular momentum, etc.], (2) That information cannot travel faster than the speed of light, and (3) the superposition of states and observer effect that we just discussed

We know that just as an electron and positron (anti-electron) can annhilate and produce a pair of photons, similarly a pair of photons can produce an electron-positron pair. Electrons and positrons have spin (angular momentum with some restrictions; let's call it "up" or "down"). By conservation laws, we know that the total angular momentum will remain 0 after the pair of particles is produced. So, if we produce an electron-positron pair, measure the spin of the electron, and then measure the spin of the position, we should expect one to be up and one to be down. But, we don't know which is which before measuring. So, let's consider three experiments. [Note: Yes, I'm slightly lying in the way I've presented a couple of these because the wavefunction can spread again; feel free to PM me if you want to quibble and I'll give you a more rigorous answer if you're willing to do some math]

Experiment 1: Produce an electron-positron pair, and move them a large distance apart. Emily measures the spin of the electron, and tells Paul her result. Paul then performs a test that measures whether the position is spin-up, spin-down, or in a superposition of both states.

Result: On average, Emily gets 50% up and 50% down, as expected. In our example, let's say Emily turns out to get an electron that is spin-down. Paul now performs his test, and discovers that the positron is spin-up. This is what we expect, because of conservation of angular momentum.

Experiment 2: Emily does not measure anything about the electron. Paul then performs a test that measures whether the position is spin-up, spin-down, or in a superposition of both states.

Result: Since nothing has been measured, Paul finds that the positron is in a superposition of both states. This is the result we expect from quantum mechanics.

Experiment 3: Now it gets tricky. Emily and Paul synchronize their clocks. Emily measures the spin of the electron. In this case, it's spin-up. Paul is a long distance away, and before information travelling at the speed of light would be able to travel between Emily and Paul, Paul performs a test that measures the spin of the positron. From a relativity point of view, this means that neither Emily's test nor Paul's test happens *before* the other, and neither one should be able to affect the other.

Now, we have a problem. What results do we expect? Well, conservation laws tell us that Emily and Paul should get opposite results. So, one will always be spin-up and the other will always be spin-down.

However, quantum mechanics tells us that before measurements are done, the electron and positron are in a superposition of both states. And, relativity tells us that the measurements done by Emily and Paul cannot affect each other. Thus, Emily and Paul should each get a random result, which means that 50% of the time they should have opposite spin and 50% of the time they should have the same spin.

Actual result: Emily and Paul always measure opposite spins. In other words, the conservation laws are correct, but either relativity or the superposition of states is violated. Since we tested the superposition of states (Experiment 2), it appears that relativity is wrong -- somehow, information is indeed travelling faster than the speed of light. Experimentally, a minimum of something like 10^4 times faster than light, and possibly instantaneously.

Note that this isn't unverifiable; in fact, the reason we've adopted this strange theory of quantum entanglement is precisely because as strange as these results are, they're experimentally verified.

Yes, I understand Quantum Mechanics.

When Feynman said we can't understand QM, he was referring to an intuitive understanding (which I certainly don't have) that is common in Classical Physics. However, there are still rules and with the help of math, there's really no reason a scientist or engineering should it's OK to not understand QM.

It seems like CS is on a roll, but if you have any more questions, I'd be happy to help.

The only thing to "understand" is that the universe is so weird and we have such a lack of true understanding about what is going on that we might as well be gophers discussing democracy.

The difference is that gophers don't have democracy, but we do have machines that run because of our understanding of QM.

Re: filters of color and shape

OK, so it isn't logical in the sense that the filters filter state at that moment but the state is in flux. Therfore the same particles as they are reduced down continue to change their state. In your example we would have 16 possible combinations any given particle could be and the more we test it, the more combinations come out, meaning that after a color test we could never predict the next color test if we should happen to run it through a shape test. Got it.

This actually can happen in IT with poorely designed software when the test of state actually alters other states on an object so one time it may be property A turned on but in testing property B, proerty A gets randomly turned on or off so a retest of proerty A (which happens to randomly turn property B on or off) may or may not result in a different value and may or may not alter the value of property B. OK, that makes sense.

I'll have to read the rest and wrap my head around it in a bit.

The world will always be full of idiots who think Universe ultimately follows classical physics. Nothing can save their souls.