# Counter intuitive? Weird? Quantum Physics?

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I have had an interest in Quantum Physics for a couple of years now.

Originally I was interested in the theory of Relativity. I was young (around 8 - 12) at the time. My brother insisted it meant it was possible to travel in time which, naturally, interested me very much.

At that age I had great difficulty understanding the basic concepts. Over time I have come to understand the basic idea behind the theory through reading web pages and Stephen Hawkings book "A Briefer History of Time".

I still do not have a clear understanding of the theory.

Anyway, after reading Quantum Physics is not consistent with the theory of relativity I began reading up on this theory. I asked around.

What I heard excited me. Things like "it is really weird", "counter intuitive", "defies common sense".

So away I went, trying to learn this new theory (without maths). I've learned quite a bit but in the end, it still seemed like I was missing something.

Finally I decided to try and learn the Maths behind the theory.

So I have started learning maths: Linear Equations, Quadratic Equations, Probability, Algebra.

I have two maths (no physics background) tutors.

I have now learned some of the relevant maths (calculations for Electromagnetic Waves, Photoelectric Effect).

I have also looked at the maths behind the Uncertainty principle (struggling here, any help on breaking down the equation would be appreciated) and have a limited understanding of the maths regarding "The Balmer Series".

This has taken me a lot of time.

The problem I have: I still don't get what is so weird about it. Or counter intuitive. What exactly is so weird about it? The fact that light has properties of both waves and particles?

The unpredictability of particle?

The hype I have come across in the past suggested to me there was something really unusual about the topic... I have not come across anything big yet.

Is anyone able to point out to me where to find the part in Quantum Physics that amazes everyone I come across? Or is it just the uncertainty principle?

Any particular equation I should direct my attention too?

As I said above, if anyone can help out with the Uncertainty Principle, that would be great. I have not been able to find a web site that clearly explains the mathematics (that is my version of clear).

My current understanding of the equation:

Uncertainty in position multiplied by the uncertainty in momentum is larger or equal too: planks constant (6.626068 × 10-34) divided by (2 x pi)

I have a sheet in front of me that gives me some questions and answers.

The thing is, I don't know how I'm suppose to get to those answers. My current understanding of the the equation doesn't give the right answers.

Thanks for any help

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"Weird" is subjective, and we don't know what you already know or how well you understand it. Thus, I don't know what I could say that you would find weird. One anecdotal observation I've had is that people who understand quantum mechanics poorly tend to either not see what the big deal is (where you seem to be right now), or go to the opposite extreme and attribute anything "weird" they can think of to QM (what several recent threads have been about). Anyway, manipulating equations is never going to "blow your mind." The counterintuitive parts come from trying to intuitive grasp what those equations actually represent.

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"Weird" is basically "not behaving classically."

Entities do not have well-defined trajectories — they behave as waves. Their energy is quantized, which makes certain interactions (where we determine their location to some degree) make it seem like that they are particles. They can interfere with themselves.

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That's all pretty wierd if you ask me.

I can't think of anything more wierd actually, can you?

'Anyone who is not shocked by quantum theory has not understood it' - Neils Bohr

He was right then, and still is now.

The whole universe can be thought of as a delayed choice experiment in which the existence of observers who notice what is going on is what imparts tangible reality to the origin of everything - John Gribbin 1984

Edited by bombus
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I actually almost completely agree with the OP. I constantly tell students that there is (almost) nothing wierd about the QM. They have all seen the Heisenberg Uncertainty principle before, they have seen that operators don't commute, and they have seen waves obeying wave equations with boundary conditions. None of that is new to QM.

However, I do say "almost" because there is one weird bit of QM: the wavefunction collapse. This is where a measurement causes a non-local change in the physical system. This is pretty weird.

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Severian, I am just learning about QM.

The bit you mention about measurement causing a non-local change in the physical system seems to be the wierd bit that has the disturbing implications. From what I have read , ultimately it means that 'nothing is real' and that stuff only becomes real when people start looking at it. Is that right? I think I kno where bombus got that quote from as I have been reading a book by John Gribbin called in search of schrodingers cat and it has that quote in it. Talking abou most physicists John Gribbin writes that 'They learn to think of the waves as real, and few of them get through a course in Quantum Theory without coming away with a picture of the atom in their imagination. People work with the probabilistic interpretation without really understanding it...'

How would you explain the 'wierd thing' you just mentioned above?

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How would you explain the 'wierd thing' you just mentioned above?

If I could explain why wavefunction collapses (rather than just saying it does), I would win a Nobel Prize. So your question may be a little beyond the scope of this forum.

I wouldn't say it means that 'nothing is real' though. It is just saying that when we measure things which are very small, it is impossible for us to make a measurement that doesn't alter the experiment in a fundamental way. This is actually a pretty obvious conclusion to reach, but the surprising thing is the way in which we disturb the experiment in a completely non-local way. This sets the wavefunction collapse apart from any other phnomena we observe, and seems to provide the observer with some kind of special significance, since it appears to happen only when an observer makes a measurement.

There are lots of people who, pretty much for this reason, refuse to accept wavefunction collapse, and try instead to construct hidden variable theories.

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I heard it doesn't even have to be a living observer. It could be just a mechanical detector/observer that collapses the wavefunction.

But I'm not entirely sure it's true.

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I heard it doesn't even have to be a living observer. It could be just a mechanical detector/observer that collapses the wavefunction.

But I'm not entirely sure it's true.

Any measurement causes wave function collapse. The rest is semantics and philosophy (about whether or not observer must be "conscious"), and can safely be dismissed.

This was covered recently and extensively over here (starting around post #7): http://www.scienceforums.net/forum/showthread.php?t=41124

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Any measurement causes wave function collapse. The rest is semantics and philosophy (about whether or not observer must be "conscious"), and can safely be dismissed.

This was covered recently and extensively over here (starting around post #7): http://www.scienceforums.net/forum/showthread.php?t=41124

I think you are confusing two things. Quantum decoherence is not necessarily the same thing as wavefunction collapse.

The experiments to which I think you are referring show that after a 'robotic' measurement interference effects are lost. This is not very surprising, since interaction with a macroscopic object will greatly increase the phase space of allowed configurations, making overlap of wavefunctions much smaller. Indeed, we see these effects in particle physics with relatively few particles, nevermind macroscopic objects. In other words, one would expect to lose interference between different final states when macroscopic objects become involved, but (in pricniple at least) the system could still not have 'decided' between distinct non-interfering states.

Now, some people have tried to argue that this somehow is wavefunction collapse. That somehow decoherence leads to the wavefunction collapsing to a particular eigenstate. I personally don't think this is convincing simply because there seems no mechanism to do this. Interference effects never truly go away (without wavefunction collapse) - they just become so small as to be unobservable, so there needs to be some discontinuity in the system.

That is not to say that it is wrong. It may well be correct, and decoherence may somehow lead to wavefunction collapse. I don't know. Also, to many people it is much nicer to have a mechanical source of wavefunction collapse rather than relying on something as nebulous as an 'observer'.

But I think the point is that it is an unsceintific question, or as you put it "semantics and philosophy". Since I can never know the outcome of an experiment until I look at it (by definition) I can never say when the wavefunction collapsed - I can say that it has (since I have measured a distinct value) and I can say whether the system had experienced decoherence. I cannot tell the difference, in principle, between the copenhagen interpretation, many worlds, or a quantum decoherence induced collapse.

That is why the Copenhagen Interpretation is just an interpretation. There is no way to test it, so it is really just philosophy.

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Thank you for that, Severian. I appreciate your insights on this greatly, as well as your willingness to share your experience and understandings here.

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Any measurement causes wave function collapse. The rest is semantics and philosophy (about whether or not observer must be "conscious"), and can safely be dismissed.

I'm sorry to harp on about this, but no they cannot be safely dismissed, and the interpretation of the results is only 'semantics and philosophy' because many (most?) QM physicists simply don't engage with the implications of QM experimental results. It's perfectly possible to 'cook with quanta' and produce lasers, processors, solid state electonics etc without ever having to assess what one is 'actually' doing. What one IS doing is the mystery and does have profound implications that result in very 'unbelieveable' scientific explanations.

I suggest all who disagree with me read John Gribbins 'In search of scrodingers cat'. The author very well sums up the background, nature and implications of Quantum Theory. If you are familiar with QM you might wish to skip to Chapter 10: The Proof of the Pudding.

Since I can never know the outcome of an experiment until I look at it (by definition) I can never say when the wavefunction collapsed - I can say that it has (since I have measured a distinct value) and I can say whether the system had experienced decoherence. I cannot tell the difference, in principle, between the copenhagen interpretation, many worlds, or a quantum decoherence induced collapse.

That is why the Copenhagen Interpretation is just an interpretation. There is no way to test it, so it is really just philosophy.

This is for the most part correct, but the interpretations have come about from rigorous scientific experiment, so it is perhaps a little unfair to say its simply 'philosophy'. Also one should remember that science is a form of philosophy anyway...

Edited by bombus
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I'm sorry to harp on about this, but no they cannot be safely dismissed, and the interpretation of the results is only 'semantics and philosophy' because many (most?) QM physicists simply don't engage with the implications of QM experimental results. It's perfectly possible to 'cook with quanta' and produce lasers, processors, solid state electonics etc without ever having to assess what one is 'actually' doing. What one IS doing is the mystery and does have profound implications that result in very 'unbelieveable' scientific explanations.

I suggest all who disagree with me read John Gribbins 'In search of scrodingers cat'. The author very well sums up the background, nature and implications of Quantum Theory. If you are familiar with QM you might wish to skip to Chapter 10: The Proof of the Pudding.

What do you think Chapt 10 is saying?

The quote from Feynman is particularly apt: The 'paradox' is only a conflict between reality and your feeling of what reality 'ought to be.'

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I'm sorry to harp on about this, but no they cannot be safely dismissed, and the interpretation of the results is only 'semantics and philosophy' because many (most?) QM physicists simply don't engage with the implications of QM experimental results.

[in response to iNow's comment about conscious observation]

I don't want to beat a dead horse, or hijack a thread, but I think the single best reason why it can be safely dismissed as nothing but a philosophical oe semantical issue is simply because there is no definite scientific definition of a 'conscious' agent, anyway. Consciousness is a gradient, not a binary phenomenon.

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the interpretations have come about from rigorous scientific experiment

I would dispute that. The mathematical formalism on which the interpretations rest has come about from rigourous scientific experiment, but the interpretations of the formalism themselves have not.

Describe to me an experiment which can distinguish between the Copenhagen interpretation and, say, the many worlds interpretation, and maybe I will change my mind.

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The problem I have: I still don't get what is so weird about it. Or counter intuitive. What exactly is so weird about it? The fact that light has properties of both waves and particles?

As the famous quote implies, if you don't think that quantum mechanics is weird or counter-intuitive, you haven't understood it. Also I managed to go through physics master's and post graduate studies without seeing anything so weird in quantum mechanics - I didn't think that in the probabilities, uncertainties and such there would be anything weird. That is right, there isn't.

But nowadays I do think that quantum mechanics is counter-intuitive. The issue in my mind is the non-commutativity of the observables, which is different than in the ordinary probability theory, where observables correspond to random variables. Note that probability theory is a mathematical formulation of human thinking and in that sense it is the natural way to think. But quantum mechanics does not work according to it. The uncertainty relation also arises from here and the ERP paradox and Bell's inequalities (which are very weird!): either there is no world when we don't look at it or the world is not local.

Still I do not understand quantum mechanics. I do have enough maths training to understand all the background operator and quantum probability theory and such, but the more I study QM, the weirder it seems.

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