From: Rowan Crawford 29/09/99 19:37:43 Subject: Quantum Mechanics post id: 41065 I recently read http://www.newscientist.com/nsplus/insight/quantum/genious.html this very well written article about quantum mechanics and managed to gain a much clearer understanding of it than I ever achieved at uni. While I'm willing to accept that a partical really doesn't have a defined existance (bad word?) until you actually look at it, I'm curious about how this conclusion was reached? I mean, if you only know what it's settings are when you look at it, how is that different to anything else in the world? Just because I haven't looked to see if there is/isn't a tiger in my other room doesn't mean that there really IS one in there, along with probabilities of being everywhere else that I haven't looked. Or does it? :} I guess what I mean is, if you only see the settings of the particle when you look, how could you ever know that they didn't have those settings prior to looking? Another example; if someone hands me a pool table ball that I haven't looked at (and lets assume no one else did either), and then I open my eyes and see that it's green, was it not green before I looked at it too? The thing I found interesting about this idea that reality only resolves itself when you look at it is that it sounds like a really high tech virtual reality, where the program only resolves the detail you're looking at. Maybe our whole universe is god's Holodeck ;). Rowan. From: Matt 29/09/99 21:29:45 Subject: re: Quantum Mechanics post id: 41090 The best experiment to date to investigate elements of reality is the GHZ experiment. Check this out! From: Chris W (Avatar) 29/09/99 22:09:38 Subject: re: Quantum Mechanics post id: 41102 Here are some thoughts from a decidedly under-qualified author (who's nevertheless going to inflict himself on you): The interaction of observer and observed in the quantum world is different to our everyday experience. Our ability to accurately measure quantum quantities is driven by a principle espoused by Werner Heisenberg in 1927: The Heisenberg Uncertainty Principle. The principle is an upshot of thought on the proposition that you cannot measure anything without interacting with it in some way. The interactions involved in measuring quantities have a small impact on the properties of the observed object that limits the absolute accuracy of the measurement. The limited accuracy is inherent in the system and cannot be avoided. By way of example, let's measure the position of an electron by shining light (photons) on it. Every photon that hits the electron will impart a small amount of energy that causes the electron to move. This motion (change in momentum), a direct result of observation, is the source of uncertainty in the object's position. Using photons of shorter wavelength allows more accurate measurement of the position of the electron but at a cost; shorter wavelength photons have higher energy thereby changing the momentum of the object by more. The better we know the position, we less we know about the momentum. Heisenberg worked out that there was a relationship between the uncertainties: DxDp > h/2p Planck's Constant h = 6.33 x 10- 34 Joules-seconds x and p are position and momentum respectively. Without actually measuring the position of the electron, the best you can do is to describe the probability that it will be in a certain area at any given time. An electron in an atom is very highly likely to be in one of the locations dictated by the Pauli Exclusion Principle (another story) but has a very small but non-zero probability of being found on the far side of the room. I guess what I mean is, if you only see the settings of the particle when you look, how could you ever know that they didn't have those settings prior to looking? Another example; if someone hands me a pool table ball that I haven't looked at (and lets assume no one else did either), and then I open my eyes and see that it's green, was it not green before I looked at it too? I think you could get into a serious philosophical argument over whether the particle does or does not have properties independent of observation. The particle can be of thought of as having a superposition of all possible properties before observation, collapsing to a single set when measured (the basis of quantum computing). Alternately, you could argue that the particle has no real existence until observed, so the question of its properties is moot. I really can't expound the consensus on this topic (if indeed there is one). You might want to check the Oranges thread in the FAQ before opening this can of worms. From: Dr. Ed G (Avatar) 30/09/99 5:38:42 Subject: re: Quantum Mechanics post id: 41147 A lot if it derives from the wave-particle dilema of light. Late last century and in the early part of this century there was much debate about whether light consisted of waves or particles. A clinching piece of evidence that it was a wave was Young's two-slit experiment in which light is shown to interfere with itself, something that ony a wave-like entity can do. However, a clinching piece of evidence that it was made of particles was the photoelectric effect, in which the total energy of the light is irrelevant, and only the wavelength, which turned out to be the equivalent of the energy of individual packets or quanta (photons), of light mattered. So, with evidence on both sides which was it/is it? The answer is that it is both... or rather neither... or rather light is made up of entities that behave differently depending on how you measure them. If you use a double-slit, they act like waves. If you use a photo-detector (which works via the photoelectric effect) they act like particles. Now, the point you're making is that how can we know what they're doing between measurements? We can now this because the way a measurement is made can be used to control the probability of the outcome of successive measurements in a way that only makes sense if "all bets are off" about the nature of these particles, in between the measurements. Let's take the two-slit experiment again. This works by shining a single source of light onto two slits which are very close together, and which the light can pass through. The effect of this is that if you put a screen some distance further along from the slits, the beams that go through each of the slits interfere with each other and generate an interference pattern. Now, there are two extremely subtle and interesting things about the details of this experiment when you alter it slightly. You can organise the light source to be sooo dim, that at most only one photon passes though the slits at any one time, and this doesn't change the result. This means that single photons must be able to interfere with themselves (not to put too fine a point on it), and this means that a single photon must be able to pass through both slits at the same time! No problem, of course, if you accept that it is a wave. However, if you try and catch an individual photon going through both slits at the same time, it won't. If you put a light detector at the exit of both slits you will only see a photon going through one of the slits at any given time. Furthermore, if a photon only goes through one slit at a time, then it cannot possibly interfere with itself, and an interference pattern should not form... and this is exactly what happens. If you try and be clever and try and catch a photon going through two slits at once, not only will you fail, but the interference pattern will disappear. This is because you've messed with their probability. And what is more, this doesn't only occur with light and photons. Exactly the same things happens for all quantum entities. You can form two-slit interference patterns from everything from electrons, to protons, to atoms, to molecules!!! And the interference patterns always disappear as soon as you try to the individual particles going through both slits at the same time. The dilema is, if you think only in terms of classical concrete reality, then these entities can only logical be either waves or particles, not both. They can either be smeared out over some wide region of space (like waves) or they exist at sharp strictly defined points in space (like particles). It is simply illogical, in a classical sense, for them to be both. The solution that quantum mechanics provides is that these things don't exist as strictly defined concrete realities but as waves of probability, and as soon as you measure them removing or limiting the meaning of such probability - i.e. you've made a definite measurement - they immediately take on a particle-like reality, based on this wave-like probability. What's more, back to the two-slit experiment again, if you rig up your experiment so that you're only, say, 50% sure about whether a photon (or electron, proton, atom, or molecule) went through one or other of the slits (or otherwise both), then there is a chance that the 50% that you missed could indeed pass through both and therefore interfere with themselves, allowing them to generate a pattern that is half interference and half not interference (two blobs on your interefence screen opposite the slits)... and this is exactly what happens. If you can control the probabilities, you control the outcome. It's not about reality, it's only about probability. Soupie twist, Ed G. From: Rowan Crawford 30/09/99 19:53:49 Subject: re: Quantum Mechanics post id: 41530 Thanks very much for the great responces, I can see it (fairly) clearly in my head now. I wonder how much confusion could be removed from quantum mechanics if the scientific community were to make up a totally new word for photons (and their breed) that removed the mental relationship you immediately make when you hear "wave" or "particle". Lets call them "bizos" or something, and say that they "have the property of being a probability wave unless they are interfered with at which point they resolve into a particle", rather than saying "they are sometimes waves and sometimes particles". A 'wave' would be confusing too if it were only ever talked about in relation to other - conflicting - similarities. Sorry, but I hate the silly way a lot of things are taught at school :}. If you put a light detector at the exit of both slits you will only see a photon going through one of the slits at any given time. What if the back wall is the detector, or made up of lots of detectors? Would it then "feel" just the one photon, sorry, bizos :), hit itself, or would it be able to feel the interference now? Also, for the interference to be visible, does that mean that the 'probability wave' must collapse into lots and lots of photos right at the last second, or is the wave enough to get the visible effect? Another thought; if you fire a single photon at the slits and there is a detector at both slits, then is the wave property removed the instant it's 'measured', or does the removal of the wave dissipate at the speed of light, starting at the point where the particle resolved and moving out to the edge of where the probability effect ended? Back to billard balls, they often seem to be used as an example of 'classic' behavior (rather than quantum behavior), but isn't it just a level of accuracy we're talking about? To be as accurate in predicting the final location of the billard ball - as accurate as we are talking in QM - then isn't that just as impossible as determining where a particle is and whats it's doing? Finally, I'm curious to know what some of the implications are from some of these quantum behaviours. Like the ability to link a particle pair in such away that when they are light years apart, if one is measured then the other one instantly knows what to become? Does that imply something about the way the universe is put together? perhaps that particles can be linked together outside the universe or something? And these probability waves that collapse into a single particle when interfered with - is that implying that reality isn't real, or something? Or do all these things instead imply that we are only beginning to see the deeper levels of detail at work in our universe, and that the real way things work is not really 'quantum mechanics', but something else altogether, and QM is only a way of describing some of the things but doesn't really describe them all? -- I should explain that I'm writing a sci-fi short story and am trying to get a decent grasp of the sci(enc) side of things so I make it at least semi-credible :). I've got some really tricky black hole questions coming up, you've been warned ;). Cheers, Rowan. From: Matt 30/09/99 21:33:41 Subject: re: Quantum Mechanics post id: 41558 Light still behaves as a wave - not only a probability wave. When we are talking about single photons then a "probability" wave is associated with this. And this prob wave gives the interference pattern as if it was a real wave. The exp was done with light and then extrapolated back to "what if this light was a single photon". This issue is still debatable I would say - many people still don't talk about "photons" as particles. From: Chris W (Avatar) 30/09/99 23:53:43 Subject: re: Quantum Mechanics post id: 41593 Or do all these things instead imply that we are only beginning to see the deeper levels of detail at work in our universe, and that the real way things work is not really 'quantum mechanics', but something else altogether, and QM is only a way of describing some of the things but doesn't really describe them all? Physics (science) is about coming up with theories which fit the observations we can make, and can be used to predict future behaviour. Theories are evolving entities that are adapted, or abandoned, as new evidence comes to light. If there's one thing that history teaches, it is that even the most entrenched theory, which explains everything you've looked at, can be only part of a picture. Newton's universal gravitation was pretty close to unchallenged until Albert Einstein's General Relativity better explained the very slight difference between Newtonian predictions of Mercury's orbit and measurements. Newton's work stood for hundreds of years, and is still taught today, because it is useful in the realm of everyday experience even though we know it to be imperfect. Quantum theory makes a range of predictions for which we are still seeking confirmation. Some of these are particles that require bigger and better colliders to create. We may not find the particles, in which case the theory will have to change or reasons found. In the meantime, it is a useful tool that is closer to the whole picture than the classical billiard balls view of the atom. From: Dr. Ed G (Avatar) 1/10/99 4:37:30 Subject: re: Quantum Mechanics post id: 41618 If you renamed photons then you'd have to rename electrons, protons, etc., etc., as they all exhibit both particle and wave characteristics. What if the back wall is the detector, or made up of lots of detectors? Would it then "feel" just the one photon, sorry, bizos :), hit itself, or would it be able to feel the interference now? You'd still only see one photon at one location at a time. Also, for the interference to be visible, does that mean that the 'probability wave' must collapse into lots and lots of photos right at the last second, or is the wave enough to get the visible effect? Well, in order to see an intference pattern you'd have to collect many many photons over some amount of time. Like I said, on the screen you'll only ever see one photon at one precise location at a time. The spooky thing is that the photon which is allowed to go through the slits without you knowing which slit it went through, will interfere with itself in a way that only effects the probability of where you will see it on the screen... but it will still only show up at one point on the screen. You get an interference pattern only when you have many individual photons interfering with themselves, effecting the probability of where they will land, and the interference pattern is a statistical distribution reflecting these probabilities. If you know which of the two slits each of the photons passes through, however, they cannot interfere with themselves (they must be given the choice to pass through both slits), and the statistical distribution on the screen will not be an intereference pattern, but two blobs, corresponding to the two slits. It can't be stressed too much that although you need many photons to build an intereference pattern, these photons do not need to pass through the slits together in order to interfere. You will still eventually get an interference pattern (and I stress eventually) even if you have only one photon passing through the slits per month! Light is an electromagnetic wave which is a solution of Maxwell's equations of electromagentism. In this sense light's particle like behaviour can for most intents and purposes be ignored. However, photons are well and truly particles... but particles in the quantum mechanical sense that (i) if you look for them individually (such as with a photo-detector) they will only ever be observed by a measurement at one point in space, i.e. as particles (like electrons, protons, atoms, etc.), but (ii) the probability of where you might see them is governed by their quantum mechanical wave-function (again, like electrons, protons, atoms, etc.). So light is a wave, but the photons that make up the light are particles (albeit in a quantum mechanical, not classical, sense). Similarly, electron beams, proton beams, and atom beams can also (in the right circumstances) be considered as waves, though the elements of which they are made up are particles. Soupie twist, Ed G. From: Dr. Ed G (Avatar) 1/10/99 5:14:16 Subject: re: Quantum Mechanics post id: 41621 Finally, I'm curious to know what some of the implications are from some of these quantum behaviours. Like the ability to link a particle pair in such away that when they are light years apart, if one is measured then the other one instantly knows what to become? Does that imply something about the way the universe is put together? perhaps that particles can be linked together outside the universe or something? Yes... in a sense. The problem with the phenomena of spatially separated correlated systems, like the two coupled particles you refer to is not so much about influences/information travelling across the galaxy at faster than the speed of light, but as you allude to, the way we understand the Universe. Indeed, a misunderstanding of the Universe is what results in this sort of phenomena being presented as a physical dilema. In the case of your correlated particles the problem really involve a subtly incorrect description of the system. If you have a system of to correlated photons heading at light speed in different directions it is incorrect to describe it as two distinct photons which are far apart from each other, but which are connected by this spooky action at a distance. In fact the photons are not distinct at all until you measure one of them. It is more correct to describe the situation as a single system which contains two photons worth of information. When you make a measurement you are not measuring one of the photons but the whole system, and the whole system has not concrete reality until a measurement is made, only a probability distribution of all the realities that it might have when it is eventually measured. And these probability waves that collapse into a single particle when interfered with - is that implying that reality isn't real, or something? Interfered with is not a particularly good word to use in this instance, probably better would be either "perturbed" or "measured". But yes, their properties do not have a concrete reality until those properties are measured. And here's the crunch. If you measure N photons with Young's slit experiment (without detectors at the slit exits) the photon will exhibit wave-like properties (as will electrons, protons, etc.) i.e. an interference pattern will form on the screen. If you measure N photons with the same setup but with particle detectors at the exists of each of the slits, they will all exhibit particle-like properties. The principle take-home point is that the measurement itself determines the properties that are exhibited. Since classically they can't be particles and waves at the same time, the only thing that can exists between measurements is their probability, which is manifest in their quantum wave-functions. Now, while it is true that Quantum Mechanics as we know it today is probably not the whole story of the Universe, a classical view of the Universe is well and truly dead, and anything that succeeds QM is very likely to contain most of the important elements of QM and is unlikely to be very much different (QM explains most physical phenomena very very well). Soupie twist, Ed G. This forum is un-moderated. The views and opinions expressed are those of the individual poster and not the ABC. The ABC reserves the right to remove offensive or inappropriate messages.