14 ms·

Why have so many physicists shrugged off the paradoxes of quantum mechanics?

87 points by bradstreet 8y ago

gus_massa 8y ago
> Bohr postulated a distinction between the quantum world and the world of everyday objects. A “classical” object is an object of everyday experience. It has, for example, a definite position and momentum, whether observed or not. A “quantum” object, such as an electron, has a different status; it’s an abstraction. Some properties, such as electrical charge, belong to the electron abstraction intrinsically, but others can be said to exist only when they are measured or observed. This is a common error. Macroscopic "everyday" objects don't have a definite position and momentum. Macroscopic objects are quantum objects. But when the mass is big enough, the position and momentum can be defined simultaneously with an error that is so small that you can just ignore the uncertainty and approximate them as classical objects. (Looking at them as classical objects is just a good approximation, like ignoring the gravity force of the Moon in most common situations.) Anyway, the measurement problem is a real problem and nobody knows how to solve it. The current fade is to use decoherence to explain it. It is a promising idea, so I hope that in a few years/decades/centuries we can give a good explanation of the measurements that avoid anything that looks like magic.
- gchamonlive 8y ago
  can we measure that imprecision of momentum/position relation even though small?
  - steve76 8y ago
    h/2pi ? That likely applies to not just light, but everything. Do we really need that much precision to get what we want? Can we flip it, and just say as long as we can do that, there's still hope to spark up another big bang when everything is cold and expanded? Are they doing something useful by asking these questions or just trying to make a name for themselves, or learn on your dime?
  - anongraddebt 8y ago
    I believe gus_massa was just saying that we simply define out of existence this supposed problem with measurement (for macro objects). This is done because even though the 'furniture of the world'/everyday objects/macroscopic objects are still quantum entities, at this level, error rates are such that they can be treated in a classical manner.
  - AnimalMuppet 8y ago
    No. It would take far more digits of accuracy than we can measure. For example, here's an object of 1 kg mass. The product of the uncertainty of its position and momentum is h-bar / 2, or h / 4 pi, which is about 5e-35 kg m^2/s. We're going to measure the position to one wavelength of visible light (say, 500 nm wavelength, so 5e-7 m). That means that we need to be able to measure the momentum to an accuracy of 1e-28 kg m/s, which for a mass of 1 kg means measuring the velocity to within an accuracy of 1e-28 m/s. Good luck with that...
    - aj7 8y ago
      Good reply.
  - spaced-out 8y ago
    One of my homework assignments in a quantum mechanics class was to calculate the uncertainty of a cheetah that was running at 65 mph (and given its weight). You can in theory but the numbers get so small that they're clearly unmeasurable.
- philipov 8y ago
  I thought the current fad was to use holography to explain it. The problem is that to measure something in quantum mechanics requires separating the world into two systems. To make exact measurements, the observing system has to be infinitely large to avoid quantum fluctuation, and the experiment has to be repeated infinitely often because it is probabilistic. To deal with the infinite size, particle physicists push the observer systems out to infinity. This is fine for scattering experiments because collider detectors are so large and far away from a collision it might as well be infinity, but this poses a problem for quantum gravity and cosmology. It seems that the most popular approaches to dealing with it have been based on Juan Maldecena's AdS/CFT correspondance, which is a toy model that allows those observers off at infinity to live on the boundary of spacetime and to describe the behavior of the interior entirely in terms of the projection of that behavior onto the boundary. The challenge is that our universe doesn't look like Anti de Sitter space, but rather like (plain) de Sitter space, and so there is no natural boundary to project onto, which has led some physicists to question the legitimacy of dividing the quantum mechanics need to divide the world into 2 systems. The measurement problem seems like an artifact of that, and what's really happening is that measurement causes the two systems to become entangled. Wave function collapse would then be the subjective result of the two systems entering a mixed state, because the observed system can no longer be described independently of the system that just observed it.
  - whatshisface 8y ago
    A route other than "dividing the world into two categories," has been available since the early days of quantum mechanics, if you believe in many worlds you don't have that problem. It is important to recognize that we are not talking about physics here, we are talking about philosophy. If anything, the greatest shock of quantum mechanics is how it shows that "questions we naturally want to ask about the natural world" and "totally unresolvable philosophical disputes" are not completely disjoint.
    - philipov 8y ago
      And that is exactly why it isn't a paradox in quantum mechanics. Quantum mechanics is consistent for the domain of questions it was designed to ask: particle scattering experiments. It's when people push it into philosophy or other unintended domains that they run into problems.
- andbberger 8y ago
  Check out https://en.wikipedia.org/wiki/Ghirardi%E2%80%93Rimini%E2%80%93Weber_theory https://en.wikipedia.org/wiki/Ghirardi%E2%80%93Rimini%E2%80%...
- aj7 8y ago
  Well put.
- pmiller2 8y ago
  > Macroscopic objects are quantum objects. But when the mass is big enough, the position and momentum can be defined simultaneously with an error that is so small that you can just ignore the uncertainty and approximate them as classical objects. Exactly. And, due to quantum tunnelling, there's a teeny tiny chance I could walk through a wall, but because there are a lot of particles in me that all have to tunnel a relatively large distance, and the probability of even one of my particles tunnelling that far is so tiny, it won't happen. Intuitively, the difference between quantum and classical objects is a lot like the central limit theorem. Add up a bunch of uncertainties, all of which have similar distributions, and you're going to get something with a very small variance (uncertainty).
- Sylos 8y ago
  > This is a common error. Macroscopic "everyday" objects don't have a definite position and momentum. Macroscopic objects are quantum objects. But when the mass is big enough, the position and momentum can be defined simultaneously with an error that is so small that you can just ignore the uncertainty and approximate them as classical objects. To put this into simpler terms: Whenever we measure something, we need to throw something at it and then have that something rebound and hit us again. In most experiments, we throw photons and have them rebound into our eyes. Throwing a photon against a "classical object" - a chair, a ladder, bacteria - is like throwing a tennis ball against a skyscraper. You throwing that does not have no effect at all, but it's very much negligible. But when trying to measure quanta, you're now throwing your tennis ball at a football, or at another tennis ball. You're gonna be lucky, if it rebounds at all, instead of just pushing the object that you're trying to measure out of the way. (You also don't have any smaller balls to throw.) That's why when you measure something in quantum physics, you only know that it has this exact value in the moment that you measure it. It's going to be pushed away because you threw something at it, so after your measurement it has a different value. You also can't observe it over a longer period, so there's no way to know whether it was only in that moment at your measured position or a long time beforehand.
  - david927 8y ago
    That's a nice explanation but doesn't it give the impression that if we could find a better way to do that experiment, we could find a way around the problem, when instead it's a fundamental limit on what we can know about a quantum system?
    - gus_massa 8y ago
      I agree with both. This explanation is easier to understand, but it makes it look like a technological problem that can be solved, instead of a fundamental property of the universe.
      - atomack 8y ago
        I think david927's intuition is more correct here. The uncertainty in the position and momentum is intrinsic to quantum mechanics - it's built into the 'wave function'. The suggestion that if one pushes away something by throwing something else builds on a purely classical intuition and wouldn't require quantum mechanics to explain if this was all we observed. The uncertainty in quantum mechanics is fundamental (to quantum mechanics) and emerges through a different, as yet unknown, mechanism.
- Gibbon1 8y ago
  I skimmed a paper about twenty years ago that pretty much explains the deference to me. Macroscopic objects are subject to quantum mechanical cascades. The end result is the probability constraints for an object composed of an enormous number of interacting particles rapidly goes to zero. The example was a playing card stood on a knife edge. The probability cascade causes it to fall one direction or the other. We can't predict which side it will fall over on. But it will fall.
- paganel 8y ago
  > with an error that is so small that you can just ignore the uncertainty and approximate them as classical objects. I’d say that’s the gist of it, that we cannot “just ignore” the uncertainty because it’s too small, because if you do that then your model and the real world are indeed different. Also, at the end of it all what does “too small” mean? “Too small” compared to what? To a galaxy? To a super-nova? To a planet? To a cat? To the things we try to “discover” at CERN? To things smaller than them? To say nothing of the fact that comparing a number to physical stuff will eventually bring you head on against Zeno’s paradox, one way or the other. I agree though that using the “too small” trick does generally allow us to do great things, like send stuff to the confines of the solar system or to build nuclear bombs, i.e. it allows us to be efficient, but that does not mean that by being efficient our models are also identical representations of what reality really is, so to speak. What I’m saying is that maybe the “mathematization” of the physical world is a leaking abstraction, and that maybe we’d be better off by saying “we’ll never really know what the world is made up of”. But the problem is that they haven’t awarded science Nobel prizes to people saying “there’s really no way for us to learn how the Universe really works”, at least not that I know of, at best you’re seen as a mysticist when saying that, at worst as a know-nothing or a cynic.
  - philipov 8y ago
    "Too small" means either it can not be detected experimentally, or even in principle, depending on whether you are allowing gedankenexperiments. I don't think it is reasonable to expect our models to be identical to reality. Otherwise, they wouldn't be models, they would be reality. A theory is correct if it produces experimentally verified predictions.
  - throwawaymath 8y ago
    > To say nothing of the fact that comparing a number to physical stuff will eventually bring you head on against Zeno’s paradox, one way or the other. Zeno's paradoxes are soluble by basic calculus. Once you distinguish between countable and uncountable infinities, the problem of crossing a bounded interval in finite time ceases to be paradoxical. This is basically to say I don't think this is a particular problem for the resolution of outstanding inconsistencies in theoretical physics.
PaulHoule 8y ago
They aren't paradoxes of quantum mechanics. They are a paradox between how we intuitively expect the world to work and reality. Our intuition is wrong.
- nikofeyn 8y ago
  you could say that about any paradox. it other words, it isn't a helpful position because it doesn't give any insight. why even have the word paradox? plus, our intuition is just as much a part of reality as your "reality".
  - aik 8y ago
    Your definition of reality here is unrelated to the OP's point. Quantum mechanics will work the way it does regardless of what your intuition on how it should work thinks. Also the emergence of an apparently paradox is a great heuristic that raises the question of whether our intuition and understanding is wrong or correct.
    - gowld 8y ago
      It's not a matter of intuition, it's a matter of incompleteness or inconsistence in the theory. There is no theoretrically testable explanation of what happens when states transition in quantum mechanics.
    - nikofeyn 8y ago
      i didn't define reality, although i implied it to be everything, which was a counterpoint to the commenter's implication that quantum mechanics is reality and our intuition is somehow separate from that. > Quantum mechanics will work the way it does regardless of what your intuition on how it should work thinks. i didn't say otherwise, although i think what you've stated is debatable.
  - ChrisSD 8y ago
    A paradox is essentially a logical contradiction. As far as we know such contradictions don't occur in nature. Paradoxes indicate that there is a flaw in either our theory or our understanding of our theory.
deleted 8y ago
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aj7 8y ago
Essentially because of Feynman’s teaching that quantum mechanics works accurately to predict the results of experiments, that that is EVERYTHING, and that “paradoxes” and “intuition” are suspect.
- acqq 8y ago
  And we can actually listen here Feynman's answer to "what is the meaning of..." and "why" questions: https://www.youtube.com/watch?v=36GT2zI8lVA https://www.youtube.com/watch?v=36GT2zI8lVA Note, despite the title of somebody presenting this segment of his talk, he definitely doesn't talk just about "magnets" but really about "the meaning" and "why" questions (the youtube poster got it). About the needed "expected framework" to avoid "perpetual why questions" etc. Even the things that we take for granted, e.g. using electricity in our day to day life, or the eventual "why" we don't fall through the floor can't be properly explained using just our "common" intuition. In another direction, it works, we are "used" to see some effects of the electrical forces all the time, so that's what is "intuitive" to us because we're familiar with it. So back to the original title of the article submission here: "why ... shrugged off the paradoxes..." it can be easily seen that once one is getting an answer inside of the expected framework, one doesn't "shrug it off," one is actually satisfied with the answer.
platz 8y ago
quantum mechanics has two different laws that describe how a system changes in time. Rule 1 says that except during a measurement, the wave evolves smoothly and deterministically, exploring every possibility. Rule 2 says that during a measurement of position, the wave collapses around the position where it's seen, with a probability proportional to the square of the height of the wave, before the collapse. It is only Rule 2 that mentions probabilities at all. And, Rule 1 and Rule 2 are in catastrophic tension with each other. Rule 1 says that there are always all possible outcomes. Rule 2 says there is only 1 outcome and it's picked out with some probability rule. It's pretty contradictory - is there one outcome or all outcomes? > youtube.com/watch?v=Zri9gS1w5ok Perimeter’s Lee Smolin will argue that the problems that have bedeviled quantum physics since its inception are unsolved and unsolvable for the simple reason that the theory is incomplete.
- rohit2412 8y ago
  And I never understand what is a "measurement". Has it something to do with a human being observing it? Isn't any measurement just electromagnetic or gravitation forces acting on it, which act all the time?
  - platz 8y ago
    I think one development that occurred since the late 80's, is that most physicists don't think conciousness is required at all, developing the idea of "decoherence" instead. Not that resolves the measurement problem in any way, of course.
  - cygx 8y ago
    A measurement is an interaction that crosses the Heisenberg cut separating the quantum from the classical world. Eg you prepare an atom in a superposition of states relative to some 'quantization axis' determined by a Stern-Gerlach apparatus. The state will randomly 'collapse' into one of the available final states, resulting in a macroscopic dot on some screen. You and all your physicist buddies will agree on the placement of the dot. There is no agreement on how this happens, complicated by the fact that there is no such thing as a classical world I alluded to above, ie any interactions with the apparatus and the conscious observers supposedly obey the laws of quantum mechanics.
zzo38computer 8y ago
These idea is interesting, but, I had a different idea, is constraint interpretation. Mathematics is the real reality. For example, some equations can have multiple solutions, such as x^2+2x-3=0 can be 1 or -3. If you make the experiment that you have a entangled photons and you are on a different planet, you can measure it and if the people on other planet measure it in the same way they will get the same answer; it doesn't go "instantly" or at any other speed; the constraint is they are same, and this entanglement propagates backward in time from the point of view from both experimenters; it doesn't say if one is "before" or "after" another one. You can trace it backward in time to the source and forward in time to the other experimenter, and that is the line of the constraint.
ttlei 8y ago
Correct me if I am wrong, the "paradoxes" described here are due to our lack of understanding of the theory right and not because the theory is inconsistent.
- throwawaymath 8y ago
  In what circumstance does that not describe a paradox?
  - lou1306 8y ago
    For instance, Russel's paradox arises from an inconsistent naive set theory.
    - throwawaymath 8y ago
      Ah, good example. I guess that's fair.
- yk 8y ago
  The paradoxes are because the theory is formulated in terms of complicated mathematical expressions that do not map neatly onto a naive understanding of macroscopic reality. The entire problem of learning QM is, that one has to learn not to trust ones intuition and instead look at what the math is showing. For example the supposed paradox of being sometimes a wave and sometimes a particle is only there if you insist to use the macroscopic analogies of wave and particle. In the full theory, the object is always a vector in an Hillbertspace, which is a concept that does not map neatly onto some combination of wave and particle.
combatentropy 8y ago
It seems to me that the resolution of the quantum mysteries might come from three sources: (1) the Holographic Principle, (2) Dark Matter, and (3) the Planck Length. 1. The Holographic Principle. The universe looks three dimensional but fundamentally it is different. 3-D space is a projection from some 2-D circuit board. If I believe that, then I have no problem hearing about (a) hidden variables or (b) spooky action at a distance. 2. Dark Matter. The universe is 98% unaccounted for. Could it be that space is not a vacuum? Perhaps we are like fish in water, moving through it but taking it for granted. How can "something" have travelled through "nothing" (a vacuum) anyway? Maybe there's an aether after all. 3. The Planck Length. Space is not an infinitely smooth line but instead there is a fundamental bit size. We thought atoms were it, that's how they got their name ("atom" means uncuttable). Later we found inside them protons and neutrons, and within those, quarks. Someone therefore might imagine that we could go on subdividing forever. But the Planck Length is a hard stop: 0.000000000000000000000000000000016 of a millimeter. If we are immersed in invisible water, then it's unsurprising that there are "waves." And yet if there is a fundamental smallest size, like the grains of photographic film, then it's not all that weird to say that you can think of matter as particles. Perhaps that's all that a "particle" is, a piece of the water.
- gus_massa 8y ago
  2) As far as I know, Dark Matter is totally unrelated to the quantum mysteries (uncertainty principle, measurement problem, ...) 3) In elementary particles, the magnetic moment is twice the expected value of a fake classical particle of the same mass and charge (for a complete accurate explanation see https://en.wikipedia.org/wiki/G-factor_(physics) https://en.wikipedia.org/wiki/G-factor_(physics) ) For elementary particles the number is 2. For non elementary particles, it can be any number. For electrons/muons/quarks the number is almost 2, and the difference is caused because the elementary particle has a cloud of virtual particles around. We can calculate this correction quite well. So there is a high chance that electrons/muons/quarks are actually elementary particles. Anyway, I don't understand how the subdivision of the current elementary particles is related to the quantum mysteries. You get the same mysteries with electrons (that are probably elementary) and with protons (that are definitively not elementary).
q_revert 8y ago
because it's really hard, and they need to publish :) -- phd in physics
- atomack 8y ago
  I feel this -- phd in physics
- tmoot 8y ago
  What, you don't want to be a 10th year grad student then post-doc till your mid-to-late 30's? Also, to be honest, those problems are kind of boring. :) -- phd in physics.
atomack 8y ago
It's a bit ungenerous to physicists to suggest they've simply shrugged off the paradoxes of quantum mechanics. The first couple of paragraphs outline reasons why quantum mechanics has gained so much traction. The various attempts to understand its more difficult results have turned out to be notoriously difficult to substantiate or disprove experimentally. I don't actually dispute the socialogical forces the author describes as they apply to physics research but I don't think apparent lack of progress is due to lack of concern amongst the physics community.
predictions 8y ago
Blah blah blah. Fuck quantum physics. The only reason it works is because it operates within broad statistical margins. Those margins are big because the things being measured are small, cheap and interchangable. We throw tons of billiard balls at the problem until every bucket of all possible outcomes has at least one billiard ball in it. Then the score is tallied up, and a model is fit down onto it. The wave phenomenon and unexplained gravity effects are really just leftovers, and used as social discriminators to ensure doublethink compliance, when indoctinating students into the dogma of social control.
tus87 8y ago
> Pascual Jordan, an important member of Bohr’s circle, cut the Gordian knot: An electron does not have a position until it is observed; the observation is what compels it to assume one. Quantum mechanics makes statistical predictions about where it is more or less likely to be observed. The things physicists will say instead of "we don't know the answer".
gxwmg 8y ago
For the same reason psychologists have shrugged off the fact that most papers cannot be replicated.
- hjk05 8y ago
  Quantum mechanical experiments get replicated time and time again. There is nothing even resembling the reproducibility crisis in quantum mechanics. The issue discussed in the article is only about interpreting why the results always come out exactly like we predict, not any issues with the results or reporting of them.
jcoffland 8y ago
All human models of reality are flawed. It is improbable that our small minds will ever comprehend the nature of the universe in it's entirety. Each answer uncovers more questions, more paradoxes. That is not to say we should not continue to try to understand the world around us. Only that logic dictates that we proceed with humility.
- placebo 8y ago
  > All human models of reality are flawed. Definitely. I wholeheartedly subscribe to this view (https://en.wikipedia.org/wiki/All_models_are_wrong https://en.wikipedia.org/wiki/All_models_are_wrong). The thinking mind will always be confined to models. The way I see it, the questions posed here are an expected wall one will eventually hit when the mind attempts to explain something that is fundamentally inexplicable - i.e the nature of existence. My current intuition is that the hard philosophical questions posed by probing deep into what matter essentially is are deeply linked to the hard problem of consciousness. I'll make an even bolder claim, which is that both of these questions arise from a misunderstanding of what consciousness actually is and that no less than a Copernican revolution in understanding its role is what's needed to put them to rest. Of course, that won't be the end of physics, as the mystery is endless - but it might be the start of a deeper humility and appreciation of that mystery.
  - frolig 8y ago
    The rise in visibility of subjective science (science of consciousness/spirituality) and integration of it with objective science will accomplish that. Luckily the subjective sciences are already highly developed, just look at Kabbalah. You can read a thousand pages of it and you still won't understand a single word!
bhouston 8y ago
I think the observes are in super position and measurement collapses us and the world. Thus there are no nonlocal effects rather we just get rid of multiple universes which were averages together and pick one, or at least the subset of possible universes that correspond to that particular measurement. Thus instead of non local effects one selects from multiple existing solutions at present and discard the rest. I think this makes a lot more sense. The other alternative was we are in a simulation and it has variable levels of resolution (wave verses particle) and it will computer higher resolutions on demand (eg observers), but that these on demand increases in resolution affect the simulation going forward. This seems to be overly complex thus I prefer the first interpretation. I am a layman in this area so I am probably wrong. There must already exist a formal name for this? Isn't this where multiverses comes into play?
danetch 8y ago
I feel someone else may have said that, but simply said, what if the phenomenon is a wave, and its observable effect a particle? Isn't it the fact that the wave hits matter that makes it observable / measurable. Doesn't seem incompatible, even with the monitors, with the double slits experiment
- rohit2412 8y ago
  I think a lot of hidden variables theories have been discredited. https://physics.stackexchange.com/questions/374266/what-are-non-local-hidden-variables https://physics.stackexchange.com/questions/374266/what-are-...
mathgenius 8y ago
There have been several books recently that describe the saga of twentieth century quantum physics, culminating in the realisation that entangelement is a real thing that has consequences. My favourite is "The Age of Entanglement: When Quantum Physics Was Reborn" by Louisa Gilder. It's a fun book. The history is fascinating. I'm waiting for a book that describes the deeper sociological aspect. A book with attitude. For instance, several of the founders of quantum physics had mystico-philosophical things to say about the quantum. But all of this was thrown out the window with the advent of the second world war. Physics became overrun by calculations. It took quite a while for people to get around to thinking about entanglement again. John Bell struggled for decades to get people to notice his work on this. Even the guy that invented the laser was met with stern disbelief when he presented his results to the "elders". Partly this is science just doing its thing, but there also does seem to be a history of science being dragged, kicking and screaming. Especially with quantum physics.
alkonaut 8y ago
What is the general feeling among theoretical physicists when it comes to breakthroughs in resolving these paradoxes? Decades ago there was great optimism about string theory and similar, followed by a long period of disillusionment . What is the guesstimates now for the next big breakthrough such as unifying gravity and QM? Is it seen as something achievable in the near/foreseeable future, or is it considered to be a far future achievement comparable to say interstellar travel (“we’ll get there if we aren’t extinct by then, but our current civilization will seem primitive by the time we do”)?
Muha_ 8y ago
As Wheller said "It from bit". It seems to me that when physicists can to consider observation, collapse and physic phenomenas gennerally as some kind of informatinal processes, then quantumn puzzles about reality will be resolved. I think them not sussessed yet because this is incredible difficult task; not because this wrong way. May be it will be something like informational multiverse. Some principle like multiverse branches selection to keep "information conservation law" (as another manifistation of unitarity) can explain nonlocality ect..

87 points by bradstreet 8y ago

> Bohr postulated a distinction between the quantum world and the world of everyday objects. A “classical” object is an object of everyday experience. It has, for example, a definite position and momentum, whether observed or not. A “quantum” object, such as an electron, has a different status; it’s an abstraction. Some properties, such as electrical charge, belong to the electron abstraction intrinsically, but others can be said to exist only when they are measured or observed. This is a common error. Macroscopic "everyday" objects don't have a definite position and momentum. Macroscopic objects are quantum objects. But when the mass is big enough, the position and momentum can be defined simultaneously with an error that is so small that you can just ignore the uncertainty and approximate them as classical objects. (Looking at them as classical objects is just a good approximation, like ignoring the gravity force of the Moon in most common situations.) Anyway, the measurement problem is a real problem and nobody knows how to solve it. The current fade is to use decoherence to explain it. It is a promising idea, so I hope that in a few years/decades/centuries we can give a good explanation of the measurements that avoid anything that looks like magic.

gchamonlive 8y ago
can we measure that imprecision of momentum/position relation even though small?
- steve76 8y ago
  h/2pi ? That likely applies to not just light, but everything. Do we really need that much precision to get what we want? Can we flip it, and just say as long as we can do that, there's still hope to spark up another big bang when everything is cold and expanded? Are they doing something useful by asking these questions or just trying to make a name for themselves, or learn on your dime?
- anongraddebt 8y ago
  I believe gus_massa was just saying that we simply define out of existence this supposed problem with measurement (for macro objects). This is done because even though the 'furniture of the world'/everyday objects/macroscopic objects are still quantum entities, at this level, error rates are such that they can be treated in a classical manner.
- AnimalMuppet 8y ago
  No. It would take far more digits of accuracy than we can measure. For example, here's an object of 1 kg mass. The product of the uncertainty of its position and momentum is h-bar / 2, or h / 4 pi, which is about 5e-35 kg m^2/s. We're going to measure the position to one wavelength of visible light (say, 500 nm wavelength, so 5e-7 m). That means that we need to be able to measure the momentum to an accuracy of 1e-28 kg m/s, which for a mass of 1 kg means measuring the velocity to within an accuracy of 1e-28 m/s. Good luck with that...
  - aj7 8y ago
    Good reply.
- spaced-out 8y ago
  One of my homework assignments in a quantum mechanics class was to calculate the uncertainty of a cheetah that was running at 65 mph (and given its weight). You can in theory but the numbers get so small that they're clearly unmeasurable.
philipov 8y ago
I thought the current fad was to use holography to explain it. The problem is that to measure something in quantum mechanics requires separating the world into two systems. To make exact measurements, the observing system has to be infinitely large to avoid quantum fluctuation, and the experiment has to be repeated infinitely often because it is probabilistic. To deal with the infinite size, particle physicists push the observer systems out to infinity. This is fine for scattering experiments because collider detectors are so large and far away from a collision it might as well be infinity, but this poses a problem for quantum gravity and cosmology. It seems that the most popular approaches to dealing with it have been based on Juan Maldecena's AdS/CFT correspondance, which is a toy model that allows those observers off at infinity to live on the boundary of spacetime and to describe the behavior of the interior entirely in terms of the projection of that behavior onto the boundary. The challenge is that our universe doesn't look like Anti de Sitter space, but rather like (plain) de Sitter space, and so there is no natural boundary to project onto, which has led some physicists to question the legitimacy of dividing the quantum mechanics need to divide the world into 2 systems. The measurement problem seems like an artifact of that, and what's really happening is that measurement causes the two systems to become entangled. Wave function collapse would then be the subjective result of the two systems entering a mixed state, because the observed system can no longer be described independently of the system that just observed it.
- whatshisface 8y ago
  A route other than "dividing the world into two categories," has been available since the early days of quantum mechanics, if you believe in many worlds you don't have that problem. It is important to recognize that we are not talking about physics here, we are talking about philosophy. If anything, the greatest shock of quantum mechanics is how it shows that "questions we naturally want to ask about the natural world" and "totally unresolvable philosophical disputes" are not completely disjoint.
  - philipov 8y ago
    And that is exactly why it isn't a paradox in quantum mechanics. Quantum mechanics is consistent for the domain of questions it was designed to ask: particle scattering experiments. It's when people push it into philosophy or other unintended domains that they run into problems.
andbberger 8y ago
Check out https://en.wikipedia.org/wiki/Ghirardi%E2%80%93Rimini%E2%80%93Weber_theory https://en.wikipedia.org/wiki/Ghirardi%E2%80%93Rimini%E2%80%...
aj7 8y ago
Well put.
pmiller2 8y ago
> Macroscopic objects are quantum objects. But when the mass is big enough, the position and momentum can be defined simultaneously with an error that is so small that you can just ignore the uncertainty and approximate them as classical objects. Exactly. And, due to quantum tunnelling, there's a teeny tiny chance I could walk through a wall, but because there are a lot of particles in me that all have to tunnel a relatively large distance, and the probability of even one of my particles tunnelling that far is so tiny, it won't happen. Intuitively, the difference between quantum and classical objects is a lot like the central limit theorem. Add up a bunch of uncertainties, all of which have similar distributions, and you're going to get something with a very small variance (uncertainty).
Sylos 8y ago
> This is a common error. Macroscopic "everyday" objects don't have a definite position and momentum. Macroscopic objects are quantum objects. But when the mass is big enough, the position and momentum can be defined simultaneously with an error that is so small that you can just ignore the uncertainty and approximate them as classical objects. To put this into simpler terms: Whenever we measure something, we need to throw something at it and then have that something rebound and hit us again. In most experiments, we throw photons and have them rebound into our eyes. Throwing a photon against a "classical object" - a chair, a ladder, bacteria - is like throwing a tennis ball against a skyscraper. You throwing that does not have no effect at all, but it's very much negligible. But when trying to measure quanta, you're now throwing your tennis ball at a football, or at another tennis ball. You're gonna be lucky, if it rebounds at all, instead of just pushing the object that you're trying to measure out of the way. (You also don't have any smaller balls to throw.) That's why when you measure something in quantum physics, you only know that it has this exact value in the moment that you measure it. It's going to be pushed away because you threw something at it, so after your measurement it has a different value. You also can't observe it over a longer period, so there's no way to know whether it was only in that moment at your measured position or a long time beforehand.
- david927 8y ago
  That's a nice explanation but doesn't it give the impression that if we could find a better way to do that experiment, we could find a way around the problem, when instead it's a fundamental limit on what we can know about a quantum system?
  - gus_massa 8y ago
    I agree with both. This explanation is easier to understand, but it makes it look like a technological problem that can be solved, instead of a fundamental property of the universe.
    - atomack 8y ago
      I think david927's intuition is more correct here. The uncertainty in the position and momentum is intrinsic to quantum mechanics - it's built into the 'wave function'. The suggestion that if one pushes away something by throwing something else builds on a purely classical intuition and wouldn't require quantum mechanics to explain if this was all we observed. The uncertainty in quantum mechanics is fundamental (to quantum mechanics) and emerges through a different, as yet unknown, mechanism.
Gibbon1 8y ago
I skimmed a paper about twenty years ago that pretty much explains the deference to me. Macroscopic objects are subject to quantum mechanical cascades. The end result is the probability constraints for an object composed of an enormous number of interacting particles rapidly goes to zero. The example was a playing card stood on a knife edge. The probability cascade causes it to fall one direction or the other. We can't predict which side it will fall over on. But it will fall.
paganel 8y ago
> with an error that is so small that you can just ignore the uncertainty and approximate them as classical objects. I’d say that’s the gist of it, that we cannot “just ignore” the uncertainty because it’s too small, because if you do that then your model and the real world are indeed different. Also, at the end of it all what does “too small” mean? “Too small” compared to what? To a galaxy? To a super-nova? To a planet? To a cat? To the things we try to “discover” at CERN? To things smaller than them? To say nothing of the fact that comparing a number to physical stuff will eventually bring you head on against Zeno’s paradox, one way or the other. I agree though that using the “too small” trick does generally allow us to do great things, like send stuff to the confines of the solar system or to build nuclear bombs, i.e. it allows us to be efficient, but that does not mean that by being efficient our models are also identical representations of what reality really is, so to speak. What I’m saying is that maybe the “mathematization” of the physical world is a leaking abstraction, and that maybe we’d be better off by saying “we’ll never really know what the world is made up of”. But the problem is that they haven’t awarded science Nobel prizes to people saying “there’s really no way for us to learn how the Universe really works”, at least not that I know of, at best you’re seen as a mysticist when saying that, at worst as a know-nothing or a cynic.
- philipov 8y ago
  "Too small" means either it can not be detected experimentally, or even in principle, depending on whether you are allowing gedankenexperiments. I don't think it is reasonable to expect our models to be identical to reality. Otherwise, they wouldn't be models, they would be reality. A theory is correct if it produces experimentally verified predictions.
- throwawaymath 8y ago
  > To say nothing of the fact that comparing a number to physical stuff will eventually bring you head on against Zeno’s paradox, one way or the other. Zeno's paradoxes are soluble by basic calculus. Once you distinguish between countable and uncountable infinities, the problem of crossing a bounded interval in finite time ceases to be paradoxical. This is basically to say I don't think this is a particular problem for the resolution of outstanding inconsistencies in theoretical physics.

PaulHoule 8y ago

They aren't paradoxes of quantum mechanics. They are a paradox between how we intuitively expect the world to work and reality. Our intuition is wrong.

nikofeyn 8y ago
you could say that about any paradox. it other words, it isn't a helpful position because it doesn't give any insight. why even have the word paradox? plus, our intuition is just as much a part of reality as your "reality".
- aik 8y ago
  Your definition of reality here is unrelated to the OP's point. Quantum mechanics will work the way it does regardless of what your intuition on how it should work thinks. Also the emergence of an apparently paradox is a great heuristic that raises the question of whether our intuition and understanding is wrong or correct.
  - gowld 8y ago
    It's not a matter of intuition, it's a matter of incompleteness or inconsistence in the theory. There is no theoretrically testable explanation of what happens when states transition in quantum mechanics.
  - nikofeyn 8y ago
    i didn't define reality, although i implied it to be everything, which was a counterpoint to the commenter's implication that quantum mechanics is reality and our intuition is somehow separate from that. > Quantum mechanics will work the way it does regardless of what your intuition on how it should work thinks. i didn't say otherwise, although i think what you've stated is debatable.
- ChrisSD 8y ago
  A paradox is essentially a logical contradiction. As far as we know such contradictions don't occur in nature. Paradoxes indicate that there is a flaw in either our theory or our understanding of our theory.

deleted 8y ago

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aj7 8y ago

Essentially because of Feynman’s teaching that quantum mechanics works accurately to predict the results of experiments, that that is EVERYTHING, and that “paradoxes” and “intuition” are suspect.

acqq 8y ago
And we can actually listen here Feynman's answer to "what is the meaning of..." and "why" questions: https://www.youtube.com/watch?v=36GT2zI8lVA https://www.youtube.com/watch?v=36GT2zI8lVA Note, despite the title of somebody presenting this segment of his talk, he definitely doesn't talk just about "magnets" but really about "the meaning" and "why" questions (the youtube poster got it). About the needed "expected framework" to avoid "perpetual why questions" etc. Even the things that we take for granted, e.g. using electricity in our day to day life, or the eventual "why" we don't fall through the floor can't be properly explained using just our "common" intuition. In another direction, it works, we are "used" to see some effects of the electrical forces all the time, so that's what is "intuitive" to us because we're familiar with it. So back to the original title of the article submission here: "why ... shrugged off the paradoxes..." it can be easily seen that once one is getting an answer inside of the expected framework, one doesn't "shrug it off," one is actually satisfied with the answer.

platz 8y ago

quantum mechanics has two different laws that describe how a system changes in time. Rule 1 says that except during a measurement, the wave evolves smoothly and deterministically, exploring every possibility. Rule 2 says that during a measurement of position, the wave collapses around the position where it's seen, with a probability proportional to the square of the height of the wave, before the collapse. It is only Rule 2 that mentions probabilities at all. And, Rule 1 and Rule 2 are in catastrophic tension with each other. Rule 1 says that there are always all possible outcomes. Rule 2 says there is only 1 outcome and it's picked out with some probability rule. It's pretty contradictory - is there one outcome or all outcomes? > youtube.com/watch?v=Zri9gS1w5ok Perimeter’s Lee Smolin will argue that the problems that have bedeviled quantum physics since its inception are unsolved and unsolvable for the simple reason that the theory is incomplete.

rohit2412 8y ago
And I never understand what is a "measurement". Has it something to do with a human being observing it? Isn't any measurement just electromagnetic or gravitation forces acting on it, which act all the time?
- platz 8y ago
  I think one development that occurred since the late 80's, is that most physicists don't think conciousness is required at all, developing the idea of "decoherence" instead. Not that resolves the measurement problem in any way, of course.
- cygx 8y ago
  A measurement is an interaction that crosses the Heisenberg cut separating the quantum from the classical world. Eg you prepare an atom in a superposition of states relative to some 'quantization axis' determined by a Stern-Gerlach apparatus. The state will randomly 'collapse' into one of the available final states, resulting in a macroscopic dot on some screen. You and all your physicist buddies will agree on the placement of the dot. There is no agreement on how this happens, complicated by the fact that there is no such thing as a classical world I alluded to above, ie any interactions with the apparatus and the conscious observers supposedly obey the laws of quantum mechanics.

zzo38computer 8y ago

These idea is interesting, but, I had a different idea, is constraint interpretation. Mathematics is the real reality. For example, some equations can have multiple solutions, such as x^2+2x-3=0 can be 1 or -3. If you make the experiment that you have a entangled photons and you are on a different planet, you can measure it and if the people on other planet measure it in the same way they will get the same answer; it doesn't go "instantly" or at any other speed; the constraint is they are same, and this entanglement propagates backward in time from the point of view from both experimenters; it doesn't say if one is "before" or "after" another one. You can trace it backward in time to the source and forward in time to the other experimenter, and that is the line of the constraint.

ttlei 8y ago

Correct me if I am wrong, the "paradoxes" described here are due to our lack of understanding of the theory right and not because the theory is inconsistent.

throwawaymath 8y ago
In what circumstance does that not describe a paradox?
- lou1306 8y ago
  For instance, Russel's paradox arises from an inconsistent naive set theory.
  - throwawaymath 8y ago
    Ah, good example. I guess that's fair.
yk 8y ago
The paradoxes are because the theory is formulated in terms of complicated mathematical expressions that do not map neatly onto a naive understanding of macroscopic reality. The entire problem of learning QM is, that one has to learn not to trust ones intuition and instead look at what the math is showing. For example the supposed paradox of being sometimes a wave and sometimes a particle is only there if you insist to use the macroscopic analogies of wave and particle. In the full theory, the object is always a vector in an Hillbertspace, which is a concept that does not map neatly onto some combination of wave and particle.

combatentropy 8y ago

It seems to me that the resolution of the quantum mysteries might come from three sources: (1) the Holographic Principle, (2) Dark Matter, and (3) the Planck Length. 1. The Holographic Principle. The universe looks three dimensional but fundamentally it is different. 3-D space is a projection from some 2-D circuit board. If I believe that, then I have no problem hearing about (a) hidden variables or (b) spooky action at a distance. 2. Dark Matter. The universe is 98% unaccounted for. Could it be that space is not a vacuum? Perhaps we are like fish in water, moving through it but taking it for granted. How can "something" have travelled through "nothing" (a vacuum) anyway? Maybe there's an aether after all. 3. The Planck Length. Space is not an infinitely smooth line but instead there is a fundamental bit size. We thought atoms were it, that's how they got their name ("atom" means uncuttable). Later we found inside them protons and neutrons, and within those, quarks. Someone therefore might imagine that we could go on subdividing forever. But the Planck Length is a hard stop: 0.000000000000000000000000000000016 of a millimeter. If we are immersed in invisible water, then it's unsurprising that there are "waves." And yet if there is a fundamental smallest size, like the grains of photographic film, then it's not all that weird to say that you can think of matter as particles. Perhaps that's all that a "particle" is, a piece of the water.

gus_massa 8y ago
2) As far as I know, Dark Matter is totally unrelated to the quantum mysteries (uncertainty principle, measurement problem, ...) 3) In elementary particles, the magnetic moment is twice the expected value of a fake classical particle of the same mass and charge (for a complete accurate explanation see https://en.wikipedia.org/wiki/G-factor_(physics) https://en.wikipedia.org/wiki/G-factor_(physics) ) For elementary particles the number is 2. For non elementary particles, it can be any number. For electrons/muons/quarks the number is almost 2, and the difference is caused because the elementary particle has a cloud of virtual particles around. We can calculate this correction quite well. So there is a high chance that electrons/muons/quarks are actually elementary particles. Anyway, I don't understand how the subdivision of the current elementary particles is related to the quantum mysteries. You get the same mysteries with electrons (that are probably elementary) and with protons (that are definitively not elementary).

q_revert 8y ago

because it's really hard, and they need to publish :) -- phd in physics

atomack 8y ago
I feel this -- phd in physics
tmoot 8y ago
What, you don't want to be a 10th year grad student then post-doc till your mid-to-late 30's? Also, to be honest, those problems are kind of boring. :) -- phd in physics.

atomack 8y ago

It's a bit ungenerous to physicists to suggest they've simply shrugged off the paradoxes of quantum mechanics. The first couple of paragraphs outline reasons why quantum mechanics has gained so much traction. The various attempts to understand its more difficult results have turned out to be notoriously difficult to substantiate or disprove experimentally. I don't actually dispute the socialogical forces the author describes as they apply to physics research but I don't think apparent lack of progress is due to lack of concern amongst the physics community.

predictions 8y ago

Blah blah blah. Fuck quantum physics. The only reason it works is because it operates within broad statistical margins. Those margins are big because the things being measured are small, cheap and interchangable. We throw tons of billiard balls at the problem until every bucket of all possible outcomes has at least one billiard ball in it. Then the score is tallied up, and a model is fit down onto it. The wave phenomenon and unexplained gravity effects are really just leftovers, and used as social discriminators to ensure doublethink compliance, when indoctinating students into the dogma of social control.

tus87 8y ago

> Pascual Jordan, an important member of Bohr’s circle, cut the Gordian knot: An electron does not have a position until it is observed; the observation is what compels it to assume one. Quantum mechanics makes statistical predictions about where it is more or less likely to be observed. The things physicists will say instead of "we don't know the answer".

gxwmg 8y ago

For the same reason psychologists have shrugged off the fact that most papers cannot be replicated.

hjk05 8y ago
Quantum mechanical experiments get replicated time and time again. There is nothing even resembling the reproducibility crisis in quantum mechanics. The issue discussed in the article is only about interpreting why the results always come out exactly like we predict, not any issues with the results or reporting of them.

jcoffland 8y ago

All human models of reality are flawed. It is improbable that our small minds will ever comprehend the nature of the universe in it's entirety. Each answer uncovers more questions, more paradoxes. That is not to say we should not continue to try to understand the world around us. Only that logic dictates that we proceed with humility.

placebo 8y ago
> All human models of reality are flawed. Definitely. I wholeheartedly subscribe to this view (https://en.wikipedia.org/wiki/All_models_are_wrong https://en.wikipedia.org/wiki/All_models_are_wrong). The thinking mind will always be confined to models. The way I see it, the questions posed here are an expected wall one will eventually hit when the mind attempts to explain something that is fundamentally inexplicable - i.e the nature of existence. My current intuition is that the hard philosophical questions posed by probing deep into what matter essentially is are deeply linked to the hard problem of consciousness. I'll make an even bolder claim, which is that both of these questions arise from a misunderstanding of what consciousness actually is and that no less than a Copernican revolution in understanding its role is what's needed to put them to rest. Of course, that won't be the end of physics, as the mystery is endless - but it might be the start of a deeper humility and appreciation of that mystery.
- frolig 8y ago
  The rise in visibility of subjective science (science of consciousness/spirituality) and integration of it with objective science will accomplish that. Luckily the subjective sciences are already highly developed, just look at Kabbalah. You can read a thousand pages of it and you still won't understand a single word!

bhouston 8y ago

I think the observes are in super position and measurement collapses us and the world. Thus there are no nonlocal effects rather we just get rid of multiple universes which were averages together and pick one, or at least the subset of possible universes that correspond to that particular measurement. Thus instead of non local effects one selects from multiple existing solutions at present and discard the rest. I think this makes a lot more sense. The other alternative was we are in a simulation and it has variable levels of resolution (wave verses particle) and it will computer higher resolutions on demand (eg observers), but that these on demand increases in resolution affect the simulation going forward. This seems to be overly complex thus I prefer the first interpretation. I am a layman in this area so I am probably wrong. There must already exist a formal name for this? Isn't this where multiverses comes into play?

danetch 8y ago

I feel someone else may have said that, but simply said, what if the phenomenon is a wave, and its observable effect a particle? Isn't it the fact that the wave hits matter that makes it observable / measurable. Doesn't seem incompatible, even with the monitors, with the double slits experiment

rohit2412 8y ago
I think a lot of hidden variables theories have been discredited. https://physics.stackexchange.com/questions/374266/what-are-non-local-hidden-variables https://physics.stackexchange.com/questions/374266/what-are-...

mathgenius 8y ago

There have been several books recently that describe the saga of twentieth century quantum physics, culminating in the realisation that entangelement is a real thing that has consequences. My favourite is "The Age of Entanglement: When Quantum Physics Was Reborn" by Louisa Gilder. It's a fun book. The history is fascinating. I'm waiting for a book that describes the deeper sociological aspect. A book with attitude. For instance, several of the founders of quantum physics had mystico-philosophical things to say about the quantum. But all of this was thrown out the window with the advent of the second world war. Physics became overrun by calculations. It took quite a while for people to get around to thinking about entanglement again. John Bell struggled for decades to get people to notice his work on this. Even the guy that invented the laser was met with stern disbelief when he presented his results to the "elders". Partly this is science just doing its thing, but there also does seem to be a history of science being dragged, kicking and screaming. Especially with quantum physics.

alkonaut 8y ago

What is the general feeling among theoretical physicists when it comes to breakthroughs in resolving these paradoxes? Decades ago there was great optimism about string theory and similar, followed by a long period of disillusionment . What is the guesstimates now for the next big breakthrough such as unifying gravity and QM? Is it seen as something achievable in the near/foreseeable future, or is it considered to be a far future achievement comparable to say interstellar travel (“we’ll get there if we aren’t extinct by then, but our current civilization will seem primitive by the time we do”)?

Muha_ 8y ago

As Wheller said "It from bit". It seems to me that when physicists can to consider observation, collapse and physic phenomenas gennerally as some kind of informatinal processes, then quantumn puzzles about reality will be resolved. I think them not sussessed yet because this is incredible difficult task; not because this wrong way. May be it will be something like informational multiverse. Some principle like multiverse branches selection to keep "information conservation law" (as another manifistation of unitarity) can explain nonlocality ect..