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Physics: Quantum all the way
nature.com — How does our classical world emerge from the counterintuitive principles of quantum theory? Can we even be sure that the world doesn't 'go quantum' when no one is watching?
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- Tr33fiddy, on 05/01/2008, -2/+20Something I never quite understood about Quantum theory; the probability aspect. Is a quantum object in a superposition of states in reality or is it that way because there is no way to adequately measure which state it is in without altering the state (in a random, unpredictable way). I.e. are quantum objects (quantum size and unobserved) truly indiscreet or merely give that illusion because we can never know their state (I'm aware of the impossibility of resolution here!)
I always read the Uncertaintly Principle in a kind of... certain way; every state at a given moment in any closed system (say, the Universe ;) is the logical resultant of the previous state. However, since we can never know the exacty composition of states in any system, Universe or smaller, prediction of any resultant state is by definition impossible. But that does not mean that there is randomness there, or indeed that every resultant state of any system is not the logical and unforeseeable conclusion of it's previous, unobservable states.
As for the article, surely it is the macro world which is counterintuitive against the quantum since the latter underpins the former. Apply probability and observation to enough of the quantum, then existing theory will predict what we see out or scale.- Scynet, on 05/01/2008, -5/+1Personally, I don't see how we can say if something is truly random or not until we know, for sure, all the forces in play. This means we have to be certain that we know what the basic building blocks of life are and how they affect us. We're talking about quarks and quantum chromodynamics now, but we're not sure if this is what life really rises from or if there's something even smaller. How do you ever know?
- etgryphon, on 05/01/2008, -2/+8Good Question.
At the quantum level, all object do in fact exist at every state as a probability. The smaller you make the area the probability of being in each state normalizes. The Heisenberg Uncertainty Principle is important because since at the smaller levels the probability state is normalized it is tough to figure where something is and how fast the wave is propagating. Think of it in terms of a wave at sea rushing toward the shore, you want to measure the height of the peak and the speed of the wave, but you can only measure from afar and you can do one of two things: take a picture to get the height; or you can throw up a wall and measure the energy of the wave to measure the speed. Both act will destroy the ability get the other calculation.
So for you first question. When you measure the position, the probability states collapse to a fixed position. So at that Fixed Time the position is x, but that is no guarantee that it will be there in the next moment. The other effect that you see is at the quantum level when you measure something you are introducing energy into the system and at the quantum level that alters state either changing the position or changing the vector. - dropbox, on 05/01/2008, -2/+11Dawkins describes this dilemma best in his recent TED appearance.
Taken from: http://www.ted.com/talks/view/id/98
Science has taught us against all intuition that solid things, like crystals and rocks, are almost entirely comprised of empty space...Why then do rocks look and feel solid and hard and impenetrable? As an evolutionary biologist, I'd say this, our brains have evolved to help us survive within the orders of magnitude of size and speed which our bodies operate at. We never evolved to navigate in the world of atoms. If we had, our brains probably would perceive rocks as full of empty space. Rocks feel hard and impenetrable to our hands precisely because objects like rocks and hands can not penetrate each other. It's therefore useful for our brains to construct notions like solidity and impenetrability, because such notions help us navigate our bodies through the middle sized world in which we have to navigate.
Moving to the other end of the scale, our ancestors never had to navigate through the cosmos, at speeds close the speed of light. If they had, our brains would be much better at understanding Einstein.
I want to give the name "Middle World" to the medium scaled environment in which we've evolved...We are evolved denizens of Middle World, and that limits what we are capable of imagining. - Godlike, on 05/01/2008, -1/+5Exactly what I want to know... they say when the state is in quantum 'flux' it can be _considered to be_ one or the other or both, but in reality it can really only be one or the other right? Stupid cat.
- etgryphon, on 05/01/2008, -0/+6No. It is a fundamentally different way of thinking. It doesn't randomly jump from state to state and you just happen to measure it at that time. It literally exists at every state all at the same time at different probabilities. The probabilities are if you continuously measured the position what state would it be in once you measured it and caused it to collapse to a fixed state. This gives rise to some ultra cool principles. Like there is a mathematical probability that you can walk through walls.
I wouldn't try it though...I haven't been able to do it for a few years now.- Godlike, on 05/01/2008, -0/+3I do it all the time, I just use the door.
- Godlike, on 05/01/2008, -0/+1Also it seems like you are saying it is the measuring stick that determines the actual physical size of what you are measuring and not the thing itself. What if the cat loses 5 pounds when it dies? How much does the box weigh?
- etgryphon, on 05/02/2008, -0/+1This only happens if you knew the cat's original weight...the thought experiment has to do with observation and the state of the cat. It would be hard to expand it to mass at that level because then you are just talking about energy. You are measuring position or speed and those are two things that are effected by observation at the quantum level do to preservation of energy.
btw, love the response above...
- etgryphon, on 05/02/2008, -0/+1This only happens if you knew the cat's original weight...the thought experiment has to do with observation and the state of the cat. It would be hard to expand it to mass at that level because then you are just talking about energy. You are measuring position or speed and those are two things that are effected by observation at the quantum level do to preservation of energy.
- etgryphon, on 05/01/2008, -0/+6No. It is a fundamentally different way of thinking. It doesn't randomly jump from state to state and you just happen to measure it at that time. It literally exists at every state all at the same time at different probabilities. The probabilities are if you continuously measured the position what state would it be in once you measured it and caused it to collapse to a fixed state. This gives rise to some ultra cool principles. Like there is a mathematical probability that you can walk through walls.
- JigoroKano, on 05/01/2008, -0/+2The superposition is real, otherwise you couldn't get interference effects. And every state is a superposition of states in another basis. This is relative.
And as for your second paragraph: pure states in quantum mechanics are not restricted by the HUP as they reside in Hilbert space and not, as is classically, in phase space. It is phase space information that is restricted by the HUP. Though this is a wave property and not an necessarily a property of indeterminism. - Shuukyoku, on 05/01/2008, -0/+2I used to live with a physics major, and he had this quote that he loved. I can't remember who coined it, but it was something to the effect of, "We are coming to realize that its not that our theories were impossible, its that we exist in a very strange place."
- Fordi, on 05/02/2008, -1/+3It's odd; I never found QED and classical physics to be irreconcilable, at a purely conceptual level.
I'll explain: At the quantum level, you're dealing with probabilistic waveforms interacting with one another, sometimes collapsing out, sometimes not. An observer causes them all to collapse.
This doesn't bother me much, as an observer necessarily interacts with his observation, no matter how slight. If you're measuring velocity using any force, be it magnetism, light, etc, you're interacting - the particle feels an equal and opposite force to the one you use to detect it.
Particles, obviously, interact with one another. So a particle interacting with another will normally collapse both waveforms. This stops when the particles are bound and acting as a whole.
As you scale up, then, the probability of any given particle existing in a superpositional - uninteracted - state drops close to zero. In short, the more matter you clump together, the more deterministically it behaves, and classical physics emerges, without the influence of 'spookiness'.
Mind you, I'm not inclined to even attempt the crazy math here; this is just my personal interpretation of what happens, not a canonical GUT.- Fordi, on 05/02/2008, -0/+1Note: When I say bound particles act as a whole: I'm talking at the subatomic level. Three quarks in a proton are at 'rest' relative to one another, so exist in simulataneous superposition. Neutrons and protons exist at rest relative to one another, so that the nucleus exists in a single quantum state. When you scale up, however, even the atoms held by chemical bonds have relative velocity to one another (temperature). As a result, macroscopic matter doesn't exist in quantum states - unless you cool them far enough. At this point, you have Bose-Einstien Condensates.
At least, that's what I think.- Fordi, on 05/02/2008, -0/+1Last note, I swear:
I wouldn't be surprised to find that degenerate matter ('nutronium', 'nucleum', or 'quark matter' in colloquial terms) acts in the same way as quanta, with all the weirdness that involves.
- Fordi, on 05/02/2008, -0/+1Last note, I swear:
- JigoroKano, on 05/02/2008, -0/+1"Particles, obviously, interact with one another. So a particle interacting with another will normally collapse both waveforms. This stops when the particles are bound and acting as a whole."
That just simply isn't true. When particles interact, it doesn't generally cause wave function collapse. The hydrogen solution to Schroedinger's equation is an interaction between a proton and an electron. The solutions reveal the energy spectra. No collapse is induced. Similarly with QFT.- Fordi, on 05/02/2008, -0/+1I'm wrong, then. thank you.
- Fordi, on 05/02/2008, -0/+1Note: When I say bound particles act as a whole: I'm talking at the subatomic level. Three quarks in a proton are at 'rest' relative to one another, so exist in simulataneous superposition. Neutrons and protons exist at rest relative to one another, so that the nucleus exists in a single quantum state. When you scale up, however, even the atoms held by chemical bonds have relative velocity to one another (temperature). As a result, macroscopic matter doesn't exist in quantum states - unless you cool them far enough. At this point, you have Bose-Einstien Condensates.
- Andysan, on 05/02/2008, -0/+1There is nothing absolutely true or false about classical or quantum physics. These a just models that try to explain some observed effect. One thing is for sure -- there will be more models in the future that try to explain observations yet to come. Just saying.
- thingnumber2, on 05/01/2008, -3/+10Sometimes I come home after a long day of work and the world went quantum while I was gone and *I* have to clean up the mess.
- rodrigo74, on 05/01/2008, -8/+5 ............................................________
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..................................., - bratterscain, on 05/01/2008, -4/+2I agree. I'm getting tired of the ***** lame, joking comments. Can't we have serious discussion here like most of the other comments on the sites that are dugg?
Sometimes I feel we're playing checkers while the rest of the sites are playing chess.- Godlike, on 05/01/2008, -1/+3Fark sucks now so all the frat boy ***** are bombing Digg with their *****.
BEHOLD THE POWER OF FARK to suck so hard that it ruins a science news website.- InferiorWang, on 05/01/2008, -1/+4Digg has been rolling down hill for a while. I keep holding on in hopes that it might return to a fraction of what it used to be, but the story selection and comments don't seem to be getting any better. I'm gonna go hang out at slashdot for a bit.
- MWeather, on 05/01/2008, -0/+4"Digg has been rolling down hill for a while."
"I'm gonna go hang out at slashdot for a bit."
Imagine a beowulf cluster of those!
- MWeather, on 05/01/2008, -0/+4"Digg has been rolling down hill for a while."
- InferiorWang, on 05/01/2008, -1/+4Digg has been rolling down hill for a while. I keep holding on in hopes that it might return to a fraction of what it used to be, but the story selection and comments don't seem to be getting any better. I'm gonna go hang out at slashdot for a bit.
- thingnumber2, on 05/01/2008, -1/+1My comment certainly isn't stopping YOU from having serious discussion about this topic. I think it's your own lack of motivation.
- Godlike, on 05/01/2008, -1/+3Fark sucks now so all the frat boy ***** are bombing Digg with their *****.
- Shuukyoku, on 05/01/2008, -1/+1I really love quantum theory. Mostly because if the universe is doing all this extra ***** when we're not looking, one's gotta wonder if its just doing it to screw with us. As soon as your back is turned, everything behind you is moving and calculating, and all of a sudden you turn around at particles are like, "*****! I'm a desk!"
And as for the no-joking comment, there are a few serious discussions going on here that you could have joined. Or started your own. Besides, go seek out the "average" internet junkie, then come back and tell me that this thread is below that average.
- rodrigo74, on 05/01/2008, -8/+5 ............................................________
- insanebrain, on 05/01/2008, -4/+7Whow.. only one reply .. I guess quantum stuff is too much for the average digger.
- atact88, on 05/01/2008, -2/+0Well the fact that it ain't even 9 AM yet might have something to do with it too. Personally, I'm still studying up on it for a physics test, lol. I can't say much about it.
- insanebrain, on 05/01/2008, -0/+2It's always 9 AM somewhere . . I posted it at 3 PM ;)
- paperclipsNsoup, on 05/01/2008, -0/+2Heavy reading, my brain is still spinning, but once it stops it'll all soak in.
- atact88, on 05/01/2008, -2/+0Well the fact that it ain't even 9 AM yet might have something to do with it too. Personally, I'm still studying up on it for a physics test, lol. I can't say much about it.
- ImmediateAction, on 05/01/2008, -3/+9It sounds like you have a decent grasp on some of the fundamental concepts, however one can predict the probability of going from one state to another. This is known as the transition rate and it comes from Fermi's Golden Rule. The reason why it's a "superposition of states" is that the solution to the Schrodinger equation, which is an adapted form of the diffusion equation, has solutions, and for anyone who has taken a Diff Eq class, one knows that if there are multiple solutions to a differential equation, then by proof, any linear combination (superposition) of those solutions is also a solution. This is what they mean by superposition of states.
They also used this as an explanation to the problem created by the double slit experiment. When there was a detector present at one of the slits to detect if that slit was indeed the slit that the electron went through, they explained that a quantum object (in this case the electron) explores all possible paths from the radiation source to the interference screen.
Glad to see someone else has an interest in quantum mechanics. Haha. - tafdc, on 05/01/2008, -3/+2Actually, both non-quantum and quantum realities are occurring, have occurred and will occur.
- CheeseburgerBro, on 05/01/2008, -0/+3You're a Cylon.
- ManOfVirtues, on 05/01/2008, -1/+1and you're and idiot, so whats your point?
- CheeseburgerBro, on 05/01/2008, -0/+2Well, I have a better command of punctuation and typing than you do, and my reply was mildly topical in that sort of lazy, au courant, meme-fellating sort of way -- whereas your own comment is just banal, defensive and boring.
I would say my point, therefore, is that you fairly suck, ManOfVirtues. Like Voltaire, however, I would let a Frenchman shoot me with a dirty pistol rather than deny you your right to public clowning.
Til next we bleat!
- CheeseburgerBro, on 05/01/2008, -0/+2Well, I have a better command of punctuation and typing than you do, and my reply was mildly topical in that sort of lazy, au courant, meme-fellating sort of way -- whereas your own comment is just banal, defensive and boring.
- ManOfVirtues, on 05/01/2008, -1/+1and you're and idiot, so whats your point?
- CheeseburgerBro, on 05/01/2008, -0/+3You're a Cylon.
- inajeep, on 05/01/2008, -1/+1I don't pretend to understand Quantum physics but the unstable and unreliable measurements I think are summed up in a quote from the article.
"In the quantum world, says Anton Zeilinger, what you see may depend on how the measurements are made." - J. GODANY- loki49152, on 05/01/2008, -0/+0Don't worry. Pretty much everyone who blathers on about Quantum Mechanics is only pretending to understand it.
- ImmediateAction, on 05/01/2008, -6/+2They're the same ***** realities...
- Dystisis, on 05/01/2008, -2/+3As above so below.
- eighties, on 05/01/2008, -2/+1It's called the law of averages. The only perplexing thing is the quantum-to-classical transition phase.
- bsmang, on 05/01/2008, -0/+2Only on a quantum physics article can you pretty much get away with replying to posts by simply writing a new post.
- empiric, on 05/01/2008, -0/+0Yeah... I'd have put a back-door into physical reality too.
- metapop, on 05/01/2008, -1/+2i don't know a whole lot about physics, nevermind quantum physics- but i do think it's fascinating. i saw a special on string theory a few years back on nova, and they were discussing the difference between classical physics and quantum and how they are applied, and i basically gathered that quantum can only be applied to very small objects- my question is was then and is now: what is the threshold in which quantum stops working and classical takes over? if quantum is applied in very small objects, as the objects get larger, quantum must stop operating someone along the line. what is that point?
- JigoroKano, on 05/01/2008, -1/+2You wouldn't have a personal computer if it weren't for quantum mechanics.
Is that point enough?- metapop, on 05/01/2008, -0/+2how did that answer my question? i wasn't suggesting that quantum isn't useful. i asked at what point (regarding size) can quantum be applied, and what are the largest objects in which quantum can be applied? as you move up the scale of objects (think katamari), when does quantum stop working, and classical physics take over?
- JigoroKano, on 05/01/2008, -1/+3Sorry, I am dyslexic.
It's the amount of environmental entanglement that determines the lack of quantum coherence. Strictly speaking, there is no particular size scale, but Keith Schwab in the article is trying to find quantum phenomena in larger and larger objects. It's an open question. He is looking for it experimentally.
- JigoroKano, on 05/01/2008, -1/+3Sorry, I am dyslexic.
- metapop, on 05/01/2008, -0/+2how did that answer my question? i wasn't suggesting that quantum isn't useful. i asked at what point (regarding size) can quantum be applied, and what are the largest objects in which quantum can be applied? as you move up the scale of objects (think katamari), when does quantum stop working, and classical physics take over?
- Naidel, on 05/02/2008, -0/+2Quantum never "stops working". Judging by some comments here, this seems to be a common misunderstanding. The thing is, in the quantum world everything is ruled by probability. For example, an electron -- and even an atom -- always has a probability to "tunnel" through matter. That probability decreases exponentially with distance however. There is a theoretical probability that you can walk through a wall, but it won't happen because it means that all atoms in your body have to tunnel through all atoms in the wall.
So in a macroscopical world you can still do quantum calculations on objects, and get the correct results, however this is unnecessarily cumbersome since the probability of said object behaving as a quantum particle is virtually nil. Hence we still have classic physics. It makes calculations easier, and is a very good approximation for the scale we live and operate in.
In other words: there is no fine line dividing quantum and classical physics.
- JigoroKano, on 05/01/2008, -1/+2You wouldn't have a personal computer if it weren't for quantum mechanics.
- quill, on 05/01/2008, -0/+3Interested in advanced physics (Relativity, Quantum, String Theory) but don't want to be blasted by math? Read: The Elegant Universe by Brian Green and its follow-up, The Fabric of the Cosmos.
The former blew my mind. Reality is REALLY weird. The latter is not quite as easily digestible, but goes into interesting details about the behaviour of things at a quantum level. It has a great section on Quantum Entanglement ("Spooky action at a distance").- arobicha, on 05/01/2008, -0/+1Those books are indeed good, but are also basically just propaganda pieces for string theory. Don't be fooled! it's an elegant theory, but it's untested. The Feynman Lectures, and QED (while a bit outdated) are an excellent introduction to very complicated physical theories intended for the layperson. Not to mention they're just as funny.
- etgryphon, on 05/02/2008, -0/+1Yay, for the Feynman Lectures!
- arobicha, on 05/01/2008, -0/+1Those books are indeed good, but are also basically just propaganda pieces for string theory. Don't be fooled! it's an elegant theory, but it's untested. The Feynman Lectures, and QED (while a bit outdated) are an excellent introduction to very complicated physical theories intended for the layperson. Not to mention they're just as funny.
- Dukeye, on 05/01/2008, -0/+2This area of study is fascinating, not just for its practical application in quantum computing but for the implications on the foundations of our reality.
- AMSRay, on 05/01/2008, -1/+1In our macro world, what measurements you make are determined by what results you want to see.
- alecks, on 05/01/2008, -3/+1There exists only ONE particle, in infinitely superposed states. We are all made of this on particle. The universe is made of this one particle. Our consciousness collapses it's state into infinitely many reflections, which look and behave as the reality we perceive.
- jaymzdean, on 05/01/2008, -0/+4You're not starting a religion, I hope.
- wickedisnotevil, on 05/01/2008, -1/+0Who says that the quantum theory principles are counterintuitive? Perhaps it is the "classical world's" warped perception of intuition that causes the confusion. This article does a better job of summing up the confusion - http://www.flickspin.com/en/artificial_intelligenc ...
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