Quantum Mechanics

[vincent]

Can someone give me a thorough explanation on this field of physics/science

I know the definition for it is basically.: The study of sub atomic particles.

I never really took the time to read up on it and posted this question here for the purpose of gaining an understanding of it.

And don't tell me to google it.

[Whiffle]

Quantum Mechanics - Easy:

http://en.wikipedia.org/wiki/Quantum_Mechanics_-_simplified

Quatum Mechanics - Technical

http://en.wikipedia.org/wiki/Quantum_mechanics

[MonkeyClaw Veteran]

are u pulling a raid here cause i really am afraid that i dont grasp it so i dont want to seem stupid, lol. but from what i understand it deals a bunch with the uncertaintiy principle, the principle that one can not know the exact position and velocity of an object at the same time, therefore instead of electrons having a set orbital in an atom, they have a sphere of uncertainty, an area at which they could be at any given time.

also i believe it encompases wave mechanics such that particles of matter can act as a wave, like photons

again, i think im right, but i am not sure, we havent gotten there yet in physics and my chemistry touched on it but i may be getting it confused with other stuff, lol

[vincent]

are u pulling a raid here

586630048[/snapback]

I posted this for my understanding of it and also to contribute an intellectual discussion to neowin's science section. nothing more. Knowledge cannot harm . :)

[raid517]

Can someone give me a thorough explanation on this field of physics/science

I know the definition for it is basically.: The study of sub atomic particles.

I never really took the time to read up on it and posted this question here for the purpose of gaining an understanding of it.

Seriously?

It's a big topic. Why don't we start with what you do know and then pick a couple of topic from there?

What specifically would you like to know most - and and what level? By thorough do you mean you want all the math's too? Or are you just looking for some book recommediations?

It would be pretty hard to give you a full and thorough breakdown of such a huge field in a single forum post. But we could certainly make a start with some of the more interesting topics.

Like I said, it depends on which part of QM interests you the most?

GJ

[vincent]

Seriously?

It's a big topic. Why don't we start with what you do know and then pick a couple of topic from there?

What specifically would you like to know most - and and what level? By thorough do you mean you want all the math's too? Or are you just looking for some book recommediations?

It would be pretty hard to give you a full and thorough breakdown of such a huge field in a single forum post. But we could certainly make a start with some of the more interesting topics.

Like I said, it depends on which part of QM interests you the most?

GJ

586630059[/snapback]

Well mainly two things right, now.

The sub atomic level of particles, how small are we talking here? How can we observe them and study them on the "Quantum" level? Also the uncertainty principle in which it deals with.

[MonkeyClaw Veteran]

I posted this for my understanding of it and also to contribute an intellectual discussion to neowin's science section. nothing more. Knowledge cannot harm . :)

586630057[/snapback]

i was just messing and alluding to the gravity thread, no harm intended, lol

[davemania]

The uncertaintity princple basically says that you can't know a particle positions without distorting it velocity and vice versa.

The sub atomic level of particles, how small are we talking here?

Electron, proton etc

[remix17]

Well mainly two things right, now.

The sub atomic level of particles, how small are we talking here? How can we observe them and study them on the "Quantum" level? Also the uncertainty principle in which it deals with.

586630068[/snapback]

The uncertainty is by far the most interesting aspect of quantum mechanics. In a nutshell you can't simultaneously measure particle's position and momentum with exact accuracy. The more you know about position, the less is known about momentum, and vice versa. This is because the very act of observing a particle will change its momentum. Measurement of position means sending a light (generally speaking) of certain wavelength at the particle which changes particle's momentum.

[ev0|]

This is because the very act of observing a particle will change its momentum.

586630346[/snapback]

Yeah, and this change happens faster than light speed and across huge distances (which shouldn't be possible) unless you explain it that these particles are all connected somehow in another dimension we can't understand, probably one of the "shrunken" dimensions that make up String and M-Theory.

[raid517]

Well I also think that the uncertainty principal is one of the most interesting aspects of physics as there is even some considerable experimental evidence to suggest that even the merest conscious intention to observe a system can fundamentally alter the state of that system. It is almost in some cases as though the very process of consciousness and intent to act can have a direct impact on the nature of the Universe. It is again this issue of interacting with a system and that interaction appearing to happen 'faster than light' because the Universe appears to know what you intend to do, even before you do it, which is most perplexing to modern physics.

GJ

Edited October 6, 2005 by raid517

[dreamz Veteran]

The uncertainty is by far the most interesting aspect of quantum mechanics. In a nutshell you can't simultaneously measure particle's position and momentum with exact accuracy. The more you know about position, the less is known about momentum, and vice versa. This is because the very act of observing a particle will change its momentum. Measurement of position means sending a light (generally speaking) of certain wavelength at the particle which changes particle's momentum.

586630346[/snapback]

it is interesting to note this because the heisenberg paper had an ambiguous title

anschaulich can mean perceivable, visual, intuitive, etc. it's important because it indicates what heisenberg and co. were going after with regards to the physical theory.

also, heisenberg used the word "relations" as opposed to principle, though it is possible to argue that principle can be used to indicate empirical uniformity.

and, of course, anyone who has read about the uncertainty principle will ask: well, is it a fact of the experiment (i.e. we're physicall unable to know both quantities) or is it a fact of nature (i.e. we could have the perfect setup, but nature is itself indeterminate)? heisenberg appears to dismantle this question by positing that what is real coincides with knowability.

some more info: http://plato.stanford.edu/entries/qt-uncertainty/

Well I also think that the uncertainty principal is one of the most interesting aspects of physics as there is even some considerable experimental evidence to suggest that even the merest conscious intention to observe a system can fundamentally alter the state of that system. It is almost in some cases as though the very process of consciousness and intent to act can have a direct impact on the nature of the Universe. It is again this issue of interacting with a system and that interaction appearing to happen 'faster than light' because the Universe appears to know what you intend to do, even before you do it, which is most perplexing to modern physics.

GJ

586632644[/snapback]

it is good to bracket our concepts and make sure that we understand them. some people (thinkers included) have interpreted the copenhagen interpretation as meaning that consciousness, as a human product, necessarily alters physical states. that's not a very precise way of putting it, and it often leads to people conflating consciousness with experimentation and systemic effects with causal relations. this leads to very strange theories that, to be exact, are pseudo-scientific. it is measurement that alters it, not a consciousness.

[remix17]

and, of course, anyone who has read about the uncertainty principle will ask: well, is it a fact of the experiment (i.e. we're physicall unable to know both quantities) or is it a fact of nature (i.e. we could have the perfect setup, but nature is itself indeterminate)?  heisenberg appears to dismantle this question by positing that what is real coincides with knowability.

586632871[/snapback]

The answer is somewhat is between. It is a fact that in order to see something, one must send a beam of light at it of wavelength no greater then the object itself. But a light is a stream of particles and each particle has energy. This energy is always sufficient enough to change particle's initial momentum upon collision. So no matter how we got about it, we can never know both quantities at the same time. It doesn't even matter if an electron has definite position, because we will never know what it is. So as far as nature and physicists are concerned, the particle doesn't exist until it is observed.

I reject the idea that nature is random and that physical sytems have purely probabilistic nature. But I am willing to accept that nature doesn't provide the ability to make exact deterministic measurements. Physical reality is determined by concious observers. There is no such thing as objective reality. Physical objects do not have a shape, or color, unless someone is watching, they do not make a sound unless someone is listening.

[dreamz Veteran]

The answer is somewhat is between. It is a fact that in order to see something, one must send a beam of light at it of wavelength no greater then the object itself. But a light is a stream of particles and each particle has energy. This energy is always sufficient enough to change particle's initial momentum upon collision. So no matter how we got about it, we can never know both quantities at the same time. It doesn't even matter if an electron has definite position, because we will never know what it is. So as far as nature and physicists are concerned, the particle doesn't exist until it is observed.

I reject the idea that nature is random and that physical sytems have purely probabilistic nature. But I am willing to accept that nature doesn't provide the ability to make exact deterministic measurements. Physical reality is determined by concious observers. There is no such thing as objective reality. Physical objects do not have a shape, or color, unless someone is watching, they do not make a sound unless someone is listening.

586633911[/snapback]

that's not quite what i meant. we could perform the thought experiment and ask, is it possible to devise an experiment such that there is no energy that would move the particle? it is to ask if it is possible, in any conceivable manner, to know both values. to answer this question, we need to move away from experimental setups (and their related bias) and ask if uncertainty is empirical or natural.

heisenberg argued that the question is malformed. what is empirical (i.e. what is knowable empirically) constitutes the whole of reality. hence, there is no point in asking what could be out there (even if it were true) if it is unknowable.

[raid517]

it is good to bracket our concepts and make sure that we understand them. some people (thinkers included) have interpreted the copenhagen interpretation as meaning that consciousness, as a human product, necessarily alters physical states. that's not a very precise way of putting it, and it often leads to people conflating consciousness with experimentation and systemic effects with causal relations. this leads to very strange theories that, to be exact, are pseudo-scientific. it is measurement that alters it, not a consciousness.

Well we might agree, or disagree on that. But to grasp this and to allow the reader to decide for themselves, one must first consider the subject of wave particle duality.

The subject of the dual nature of energy and mass equivalence first emerged with Einstein's famous E=MC^2 equation. This had many implications - but essentially it said that the constituent elements of energy and matter were at their fundamental level exactly equivalent. Einstein believed in a highly deterministic universe - a universe with definite outcomes where indeed there was no uncertainty. (Hence his famous statements when asked if light was a particle or a wave, that 'Light is a particle and that is that!' and that 'God does not play dice with the Universe. The idea that two different things could behave in the same way seemed counter intuitive to him - even though his own calculations clearly showed that they were not really two different things at all and that they really undoubtedly could behave in the same way.

Energy and matter we have learnt from Einstein's theories therefore are are analogous, matter can be simply described in terms of energy and energy in the same terms of matter. So far we have only really considered two ways in which energy can be transferred. These are particles and waves. One it seemed was always distinct and unique from the other. For example, an electron is a particle (because they are observed to behave in particle-like ways ) - and light is a wave (because it behaves in wave-like ways, such as Interference). But Einstein fundamentally changed that previously safe assumption (unwittingly perhaps, because in the end he never really liked it's implications). Shortly after Einstein released his work on special relativity, a number of other physicists began to ponder the possibility that if matter and energy were equivalent, that this should also mean that under certain conditions they should show identical properties too. That is that if energy (or light) were ever seen to behave like a wave, that it should also therefore logically be capable of behaving like a particle - and if matter was ever seen to be behaving like a particle, it should also be capable of behaving like a wave. This categorisation was duly applied even to objects below the scale of direct observation, essentially by analogy with macroscopic phenomena.

However, more problems emerge with Einstein's viewpoint: since further experiments later showed (in support of the earlier theory) that electrons can indeed also be made to interfere and thus appear wave-like; and light (especially in the photoelectric effect, as analysed in 1905 by Albert Einstein) could indeed possess particle-like properties. Quantum mechanics emphasises the primacy of measurement and not attributing properties to objects beyond what can be measured. Hence the concept of wave-particle duality arose: it is not necessary, or useful, to say that an electron is a particle - or a wave - just that in certain circumstances it behaves like a wave and in others like a particle. The exact outcome of whether a measurement to decide whether light (or matter) is to be viewed as a particle, or as a wave appears only to depend almost exclusively on the experimental conditions. In other words it really just depends on how you choose to look at it.

So how can this be? How can light and/or matter behave like a wave on Monday, Wednesday, Friday; while on Tuesday, Thursday, Saturday light behaves like a particle; while on Sunday you can pretty much take your pick? Moreover what exactly is the deciding factor between these two opposing states? Many scientists believe that because it takes a conscious choice on behalf of the experimenter to decide which experiment to run (and therefore which outcome is likely to be found in the experiment), it is often proposed that somehow the Universe must be 'aware' of what choices we intend to make - after all it is not until we act upon the experiment (or decide which experiment to run) that light or energy is said to be forced to make a choice - because until that choice is made by us light and energy is envisaged to exist within what is known as a 'superposition of states' ( a key phrase that is worth looking up) where it exists as neither quite matter nor as quite energy - but something in between this. So why should the conscious choices we make force the Universe to make a choice, what is it about us and our intentions that can force the Universe to do this? After all the evidence is very clear that this same choice is not made by the universe until we choose to look - so is it possible perhaps that the universe might be aware of our intentions? What is it exactly about our interaction with the universe that forces it to make a choice that it might not otherwise normally make?

To this extent this is where I would tend to side with dreamz (although I wouldn't quite go as far as to dismiss this particular perspective as 'pseudo science').

What I think may be true is that we are possibly missing the bigger picture, that there is something amiss or incomplete in our understanding that is leading us to make these kinds of (possibly) erroneous assumptions. (Although don't take my word for it - as a lot of very respectable scientists do believe that conciousness can have a direct impact on the Universe). Why for example, if on the quantum scale can we see that somethings can exist in two separate states simultaneously is this pattern not repeated on the larger/macro scale? (And btw the maths very much does show this - there is nothing pseudo scientific about this at all). In quantum mechanics, the wave-particle duality is described by state functions which encode the probability distributions of all measurable variables. The position of the particle is one such variable. Before an observation is made the position of the particle is described in terms of probability waves which can interfere with each other. Hence a photon (and/or electron or any other quantity of matter) is said to be capable of existing every possible state simultaneously - which incidentally is also the basis for what is known as quantum computing. The difficulty with this perspective is given by the analogy of Schrodinger's cat. In this scenario a cat is placed in a sealed box. Attached to the box is an apparatus containing a radioactive nucleus and a canister of poison gas. The experiment is set up so that there is a 50% chance of the nucleus decaying in one hour. If the nucleus decays, it will emit a particle that triggers the apparatus, which opens the canister and kills the cat. According to quantum mechanics, the unobserved nucleus is described as a superposition (mixture) of "decayed nucleus" and "undecayed nucleus". However, when the box is opened the experimenter sees only a "decayed nucleus/dead cat" or an "undecayed nucleus/living cat." The question is: when does the system stop existing as a mixture of states and become one or the other? The purpose of the experiment is to illustrate that quantum mechanics is incomplete without some rules to describe when the wavefunction and the cat becomes dead or remains alive instead of a mixture of both. And therein lies the problem for modern physics, because clearly the world isn't filled with half dead and half alive cats. Or in other words unlike on the quantum scale where the world appears to be fundamentally inderministic, on the macro (or large scale) the world appears to be almost completely deterministic, exactly as both Newton and Einstein (initially) described it. While on the quantum scale God very much does play dice, on the macro scale it appears he does not - because we are able to make very precise and deterministic measurements of almost everything we see. So the challenge for physics in the future is how exactly do we reconcile these two seemingly incompatable perspectives?

In any case as I said this is a very big topic and it is impossible to cover everything in a single forum post, but hopefully I have said enough here to wet your appetite and make you want to read more. I hope I've managed to be clear and haven't confused anyone too much, but if anyone does want any more clarification, feel free to let me know and I will do my best to help out.

Best regards,

GJ

Edited October 7, 2005 by raid517

[vincent]

:| :|

I understood what you were saying in regards to undeterministic factors regarding particles. But why does Quantum mechanics come to the conclusion that particle behavior is not deterministic? and why classical physics says otherwise? So basically nothing in this universe is predictable? I find that kinda hard to believe, if given the information on a certain situation on any body of matter i think you can predict on what will happen.

I think my brain just fried :wacko:

But im getting a clearer picture on this, thanks raid.

[raid517]

:| :|

I understood what you were saying in regards to undeterministic factors regarding particles. But why  does Quantum mechanics come to the conclusion that particle behavior is not deterministic? and why classical physics says otherwise? So basically nothing in this universe is predictable? I find that kinda hard to believe, if given the information on a certain situation on any body of matter i think you can predict on what will happen.

I think my brain just fried :wacko:

But im getting a clearer picture on this, thanks raid.

586634728[/snapback]

Well it's late and I'm tierd. But a couple of things you could look up are the Heisenberg uncertainty principal, the Einstein Podolsky Rosen (or EPR) thought Experiment, Bell's theorem - which is used as an elegant proof (through a clever attempted disproof) of Quantum Mechanics and anything at all you can find on quantum computing. (Wikipedia is a good resource for stuff like this - I personally encourage my own kids to use it).

But essentially the answer to your question is, no it is not possible on a quantum scale to ever really know anything with 100% certainty. Firstly because by simply interacting with that sytem you are invarably going to change it in some way (for example shining a light on it in a microscope, or firing electrons at it to make your measurement) by altering either the energy, or the position of the observed sample - and secondly because energy and matter on the quantum scale are said to exist in a spooky state of being neither quite 100% energy, or 100% matter and only ever become either of these pretty much when we decide that that is what we want to see. (Or in other words until we construct an experiment designed to look at matter and energy in terms of particles or waves). Secondly there is a a strong suggestion in QM, if it is right (and so far it has proved to be one of the most spectacularly succesful theories ever devised) that at it's most fundamental level, the Universe is almost entirely indeterministic in that it exists in a state of wave paticle duality, which never quite makes iup it's mind which it is, until a conscious observer decides to interact with it and forces it to make a choice.

It's pretty spooky stuff - but a lot of physics is like that. It can drive you a bit mad at times thinking about it all.

GJ

[raid517]

Man I wish I had more time to edit my posts... Secondly is thirdly... But I'm sure you get the picture...

GJ

[raid517]

But essentially the answer to your question is, no it is not possible on a quantum scale to ever really know anything with 100% certainty. Firstly because by simply interacting with that sytem you are invarably going to change it in some way (for example shining a light on it in a microscope, or firing electrons at it to make your measurement) by altering either the energy, or the position of the observed sample - and secondly because energy and matter on the quantum scale are said to exist in a spooky state of being neither quite 100% energy, or 100% matter and only ever become either of these pretty much when we decide that that is what we want to see. (Or in other words until we construct an experiment designed to look at matter and energy in terms of particles or waves). In this sense, there is a a strong suggestion in QM, if it is right (and so far it has proved to be one of the most spectacularly succesful theories ever devised) that at it's most fundamental level, the Universe is almost entirely indeterministic in that it exists in a state of wave paticle duality, which never quite makes iup it's mind which it is, until a conscious observer decides to interact with it and forces it to make a choice.

There that's better... Edited for clarity. The lesson is I guess, don't write posts concerning the subject of quantum mechanics when you get up to check your email because you can't sleep at 4:30 in the morning). Man I wish you were a mod rigput and then I could ask you to fix stupid mistakes like that for me. :blush:)

GJ

[dreamz Veteran]

as fo the pseudo-science bit, i'm talking strictly about "quantum consciousness" or mysticism and whatnot.

schrodinger himself never meant for wave function collapse to be mystical. that was a misinterpretation appropriated by non-scientists.

i talk about it here: https://www.neowin.net/forum/index.php?show...l=consciousness

[raid517]

Well I don't go much into the 'mysticism' of it myself. I'm the least mystical person you are ever likely to meet. Like I said I think possibly (while QM has undoubtedly been an amazingly successful theory) that the true answer to these puzzling phenomena might be that we are simply still missing large parts of the puzzle. QM might provide a part of the answer - but it may not (by any means) be the whole answer initself. Moreover consciousness can only really play a limited part in the great cosmological scheme of things - since we know that in actual fact it did not emerge (in the form of modern human beings) until very late on in the history of the Universe. (Indeed if you were to measure it in real terms, human being didn't emerge until only a few seconds before midnight on the astronomical clock. The Universe therefore it seems, existed quite happily throughout the vast duration of it's history, without feeling the need for any of us to exist at all).

However (while I might like to be able to) I do not rule out that conciousness can have a direct impact on the nature of the Universe - and indeed the current state of QM makes this very hard to refute anyway. We can already see that conciousness can have an impact on the Universe, because clearly we are already here and are already capable of altering nature - but it is to what extent we can have an impact that remains very much open to debate. I think that insomuch as consciousness might be linked to 'quantum entanglement', it is perfectly possible that even the intention to act upon an experiment can have an impact on the outcome - but only to the extent that it is part of a distinct and explainable physical process. I do not like to attribute any more relevance to it than this - as some people do tend to (mis)interprit this phemomena as being almost 'spiritual', or religious in origin - when in fact there is no need for it to be that way at all.

GJ

[vincent]

ok, so let me ask this.

What is Quantum Mechanics' role in the Big bang theory?

[raid517]

Man that is a very big question rigput. That is almost a whole book in its own right. Are you seriously looking to learn all of this stuff just from forum posts? Because if not, there are probably some non technical books I could recommend for you. :)

The biggest problem though that confronts us is that when we look at the orderly nature of the world on the macro scale, everything seems fairly straightforward and predicable. We have an elegant and orderly mechanism where the stars and galaxies orbit each other, like all of the wheels and cogs in some stunningly beautiful and unimaginably large cosmic clock.

But things get a whole lot less orderly on on the micro and sub atomic scale. Here everything seems much less predicable - and they often even seem almost completely random. Nothing here is certain - for example if you could shrink yourself down to the quantum scale it would be possible for you to exist in all places at once, or in no definite place, or in a definite place at a definite time, but with a different colour of hair and different clothes than you would have had if you had been observed at a different place or time (in fact you could be said to posses an infinite variety of different hair colourings and different sets of clothes). It is a place where you could be visiting your aunt in Virginia, while at the same time lying on a beach in California, while watching the latest Hollywood movie at your local cinema, without ever really needing to leave home to do any of these! On the quantum scale even the very concepts of up, down, backwards and forwards and of time themselves literally have no meaning and have no baring at all on the outcome of an event. The only way we can explain the weirdness of things on the quantum scale is if somehow all possible outcomes could be said to exist simultaneously. (And indeed experiments do appear to show that this very much is the case).

So what we appear to have is two fundamentally different models of the Universe. On the one hand we have Einstein and his elegant and orderly universe that he proposed in his General Theory of Relativity, and then we have quantum mechanics which seems almost chaotic and utterly unpredictable by comparison. The question is how can that be the case? How can we appear to have what on the surface seem like two distinctly different models of the Universe? After all we only have one Universe, so how can we have two different models of how the Universe works? Trying to mesh quantum mechanics with Einstein's Theory of General Relativity is currently a bit similar to trying to push a round ball into a square hole. They just don't fit together very well at all.

The biggest problem we face right now then is in trying to unite all of the 4 fundamental forces - the weak, the strong, electro magnetism and gravity. The only problem is that the standard model (not an accidental phrase, you should possibly look it up) that we have only allows us to unite 3 of these. Gravity it seems 9just like Einsteine) just does not want to play ball with Qunatum Mechanics.

String theory on the other hand does offer a solution to these issues and is capable of uniting all of the 4 fundamental forces - since essentially what it does is describe gravity as a particle (or a graviton), which means that in this instance we do indeed appear to have a theory (which is often referred to as The Theory of Everything) that can unite all of the forces of nature under one single model. The problem is that at it's core, string theory is an almost wholly untestable hypothesis. Over the scales at which we are talking about, it is almost impossible to envisage any experiment to test it's validity, or any possible means of actually observing the tiny strings that string theory predicts everything is made of. Naturally therefore this proved highly controversial, since no theory that does not hold out the prospect of being subject to testing, or to scientific observation has ever previously been considered to be a branch of science. Moreover worse still is that since since the first example of string theory was uncovered, there are now 4 other competing string theories which all appear equally valid and equally convincing. So problem is if there are currently a total of 5 string theories, then perhaps there might even be 10, or 15, or even hundreds of different possible versions of the theory. And if we can't test it, how can we know which of these if any is ever likely to be correct?

Does this mean that we should abandon string theory? Possibly not. String theory after all does offer some extremely interesting and seductive solutions to many of the problems that confront physics today. It is possible that someone might even come along at some point in the future and sort all of these different problems out for us - and perhaps they might show how all of the different string theories are simply different aspects of the same even deeper underlying theory. But until then I think it is important to keep it in perspective and to regard it with a healthy scepticism. It may not be physics (at least not yet) but as a purely intellectual pursuit and as a hypothesis, it is certainly worth a great deal more investigation.

Best regards,

GJ

[remix17]

The Elegant Universe explains it best.

Read a book or watch a 3-hour video :

http://www.pbs.org/wgbh/nova/elegant/program.html

[raid517]

Thankyou Xraizer4000, that is a truly excellent link. :) I wish it would get pinned! We don't have anything pinned in the science section yet....

GJ

Edited October 11, 2005 by raid517