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Basic questions about QM

  1. Feb 17, 2015 #1
    Hi guys,

    I'm a newbie in this area of physics and like many other unfamiliar territories, I've had anxiety and confusion when I first heard the intriguing concepts of this theory. I hope someone can help me understand it better though it is evident that it's very difficult and counter-intuitive.

    1) Do objects and particles exist even when they are not measured, just in an undefined state?

    2)If small particles can be almost everywhere according to their probability function, how come the objects on the macro level seem fixed, with a pretty defined position (for instance we can calculate how far away from the Earth is the moon etc.)

    3)Does QM obey the laws of causality and is it consistent with SR and GR?

    4)Long story short, can somebody illustrate to me the basic principles of QM and how they correlate to everyday world and objects that we see, and how the apparent stability of macroobjects can emerge from 'chaos' of electrons etc.

  2. jcsd
  3. Feb 17, 2015 #2


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    1) You open a topic of philosophy that causes many people to shout at each other in unproductive fashion.

    People will argue about every word in your question. They will hold positions that seem to be consistent with QM, and not consistent with every-day experience, for words such as "exist" and "measure." And they will argue about what exactly QM implies about each of these and about what Bell's theorem means. It can be a lot of fun to have these discussions, but in the end, meaning is elusive.

    I follow Feynman's attitude which is summed up more or less as follows: Shut up and calculate. QM says that you calculate things this way and that way. Then you get out values that can be compared to experiments. And so far those experiments match the calculation just as well as can be performed. So arguments about what QM really means can be fun, but probably do not make much difference. The useful things are, what can be observed and how do those things compare to what can be calculated? And they match. So relax.

    2) Get Feynman's book _QED_. He explains this quite well. The explanation is kind of long, so I will let you read it there.

    3) QM and SR are completely consistent. QM is causal in SR. And it has been tested to a truly impressive extent and been successful.

    We do not yet know how to fit GR and QM together. There are a lot of tantalizing crazy things in various attempts to do so. And string theory makes a big attempt but so far has not got any predictions to test it. So as of this time, QM and GR are a happy place to have fun with new theories.

    4) Again, Feynman's book _QED_.
  4. Feb 17, 2015 #3


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    Most of your questions (and many closely related ones) are answered in:
    http://lanl.arxiv.org/abs/quant-ph/0609163 [Found.Phys.37:1563-1611,2007]
  5. Feb 17, 2015 #4


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    two answers

    Quantum mechanics doesn't say they do, but it also doesn't say they don't. The theory works either way because it doesn't say anything about what happens to a particle that's not interacting and being measured.

    This isn't just a QM thing. I put my hand in my pocket and feel a coin. I take my hand out, wait a while, then put my hand back in my pocket and feel the coin again. Naturally I assume that the coin was here in between... but I'd find it very hard to disprove the hypothesis that the coin disappears when I take my hand out of my pocket and reappears when I put my hand back in. But even though I can't prove it, no one would seriously argue that it's not there when my hand isn't in my pocket. with particles, sometimes it's easier to think about them one way and other times it's easier to think about them the other way.... so try not to get too hung up on this question.

    This also isn't just a QM thing. The air around you is made up of an enormous number of molecules bouncing around at random, and air pressure comes from the impact of these particles hitting and bouncing off of a surface. So how come the pressure on the underside of the tabletop is equal to the pressure on the top side? It is possible, just by random chance, that every one of the molecules underneath the table just happened to be moving up at the same time that all the molecules above the table also happened to be moving up - and the table would take off like a rocket. This doesn't happen because the molecules are moving randomly, so on average the there are as many down-movers as up-movers hitting the table at any moment and it all cancels out... or if we're off by one or two molecules at any moment, the effect is too small to notice.

    Something similar happens with quantum uncertainty in macroscopic objects. Each atom as a probability distribution, but the probability of all the atoms in the earth or the moon being out of place in the same direction by enough to affect the earth-moon difference is near as no never mind zero. (It's hard to explain just how unimaginably small that probability is... But by comparison entire squadrons of flying tables would be an everyday occurrence).
  6. Feb 17, 2015 #5


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    Well first you have to distinguish between the formalism and interpretations. Everyone agrees on the formalism and it is silent about what's going on when not observed. However interpretations have their own differing take. That's what creates the arguments a previous poster mentioned.

    You are falling into a semantic trap. Remember the answer to question 1 - the formalism is silent on what properties like position (that is being somewhere) etc when not observed. Your query should be how does our everyday classical world emerge from a theory that is about observations that appear in an assumed everyday classical world? That is a very deep question on which much progress has been made, but a few issues still remain. I cant really give a full answer here but here are some links to start you on that journey:

    The paper linked to by Demystifier is also excellent on that and many other issues.

    Added later:
    I have had a bit more of a look at it and still think its excellent - but its says a few things I do not agree with eg Schroedinger's equation is local. Its based on the Galilean transformations which are non-local. But discussing that will take us too far afield. If anyone wants to discuss it starting a new thread would be the best bet.

    That depends on what you mean by causality. The theory is about predicting the probabilities of the outcomes of observations. Central do doing that is thing thing called the state - once you know the state you can predict the probabilities of the outcome of any observation you might do. The state evolves causally - but all its able to do is help in predicting probabilities. You can decide if that causal or not - for what's it worth I think it is - but its one of those argument things that are pretty pointless really - it simply semantics on what you mean by causal.

    It is perfectly consistent with SR and its combination, called Quantum Field Theory (QFT), is the deepest formalism about the the quantum world - uniting both particles and fields. In QFT everything is a quantum field and particles are excitations of that field - photons are excitations of the EM field, electrons excitations of the electron field etc etc. But exactly what that means can only be explained by the theory itself - its quite advanced and requires many years of study. But an interesting lay book has been written:

    Also there is Feynmans classic:

    But if you want to actually study the real deal some good books have started to appear that tackle it after a first course in QM:

    Regarding GR the issue is subtle - and not what many popularisations will tell you:
    'Effective field theory shows that general relativity and quantum mechanics work together perfectly normally over a range of scales and curvatures, including those relevant for the world that we see around us. However, effective field theories are only valid over some range of scales. General relativity certainly does have problematic issues at extreme scales. There are important problems which the effective field theory does not solve because they are beyond its range of validity. However, this means that the issue of quantum gravity is not what we thought it to be. Rather than a fundamental incompatibility of quantum mechanics and gravity, we are in the more familiar situation of needing a more complete theory beyond the range of their combined applicability. The usual marriage of general relativity and quantum mechanics is fine at ordinary energies, but we now seek to uncover the modifications that must be present in more extreme conditions. This is the modern view of the problem of quantum gravity, and it represents progress over the outdated view of the past.'

    Unfortunately that will require many years of study. If you wish to undertake it here is THE book on the issue:

    Last edited by a moderator: May 7, 2017
  7. Feb 17, 2015 #6


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    1) QM does not answer.

    2 and 4) You see a classical world because of effects like decoherence and wave function collapse :)

    The surprising thing is that Question 1 is a deeper question than Questions 2 and 4. Questions 2 and 4 are about your observations, which QM can certainly answer. Question 1 is about what is happening when you don't observe, which QM does not answer. Question 1 is a deep problem, and requires a theory beyond QM such as Bohmian Mechanics, or an approach like Many-Worlds.

    3) If by causality you mean that no classical information can be sent faster than light, then yes, QM obeys that. But if by causality you mean that QM is a theory of localized things travelling on spacetime, then the answer is no - Bell's theorem says that no theory that obeys local causality can reproduce all the predictions of QM (but there are loopholes like retrocausation and superdeterminism).
    Last edited: Feb 17, 2015
  8. Feb 17, 2015 #7


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    QFT yes - it's based on the Lorentz transformations.

    Basic QM - no - its based on the Galilean transformations.

    This is the exact reason I have issues with saying Schroedinger's equation is local - but I don't want to hijack this thread with the issue - a new thread would be better.

  9. Feb 17, 2015 #8


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    Sure I agree.
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