Hey everyone, I did A Level physics at high school and have had a passing interest in Physics since then. I recently read Professor Stephen Hawking's recent book "The Grand Design" and was astounded by something that I read, which I have since been trying to get my head round. I was wondering if some of the physics experts here could clear this up for me. According to the Heisenberg Uncertainty Principle, I think I am correct in saying, that it is impossible to accurately measure both the position and velocity of a particle because the very act of observing the particle effects the particle's behaviour. From the double-slit experiment, I think it is also correct to state that when a particle moves between two points in spacetime, A and B, there is uncertainty about where the particle is and which route the particle takes as it travels between A and B. In other words, the particle doesn't usually travel between A and B in a straight line, as Sir Isaac probably would have expected. Now, Hawking states, referencing work done by the great Richard Feynman, that there are two possible ways in which you can interpret this: "According to the quantum model, however, the particle is said to have no definite position during the time it is between the starting point and the end point. Feynman realized one does not have to interpret that to mean that particles take no path as they travel between source and screen. It could mean instead that particles take every possible path connecting those points." He goes on to say: "This, Feynman asserted, is what makes quantum physics different from Newtonian physics. The situation at both slits matters because, rather than following a single definite path, particles take every path, and they take them all simultanously!" It was at this point that my jaw dropped and I entered a period of genuine astonishment. He then continued: "That sounds like science fiction, but it isn't. Feynman formulated a mathematical expression - the Feynman sum over histories - that reflects this idea and reproduces all the laws of quantum physics." "In the double-slit experiment Feynman's ideas mean that particles take paths that go through only one slit or only the other; paths that thread through the first slit, back out through the second slit, and then through the first again; paths that visit the grand design restaurant that serves that great curried shrimp, and then circle Jupiter a few times before heading home; even paths that go across the universe and back. This, in Feynman's view, explains how the particle acquires the information about which slits are open" Now, please excuse my possible ignorance of high level quantum physics, but how can a particle take every possible path between two points, A and B, simultaneously? It's clearly not practical for a particle to do this in 3 or 4 dimensional space. Is Hawking suggesting that the particle is able to do this by travelling through other dimensions that link together different positions in 4 dimensional spacetime. I believe current thinking is that there are probably 11 dimensions in total. Or have I completely mis-interpretted what he is saying?
Quantum Mechanics does not imply there exist extra dimensions of space-time as does string theory. The path integral formalism treats the particle's position as a probability distribution controlled by a wave, and utilizes the fact (as should be well understood from the central peak in the double slit experiment) that waves which take very close to the same path interfere constructively and a trick well known to students of calculus -- that a smooth function with a minimum is close to flat very close to its minimum. In optics, where this technique started, a minimum in path length translates in a lot of waves interfering constructively, and so light typically takes the minimum path between points -- aka a straight line in empty space. So no extra dimensions required. But this path integral formalism and the "particles take no path between observations" formalism both tell the same story -- the quantum reality of the universe at the scale of attojoule, nanometer, and picosecond is pretty weird by the standards of humans and their scale of joule, centimeter and millisecond. http://en.wikipedia.org/wiki/Path_integral_formulation http://www.quantumfieldtheory.info/Path_Integrals_in_Quantum_Theories.htm
In light's travel, the eye can block the course. The eye becomes the target, yet the source remains the source. --Keith1
The Two-Slit Experiment In the case of one electron interfering with itself in the two-slit experiment, modern physics has unhinged itself from causal reality by having it that electrons do not exist until they are observed at the phosphor screen, the pattern being caused by the superposition of all possible paths the electron might take to the screen, yet, it happens, and even electrons hundreds of kilometers apart will form an interference pattern that accurately corresponds to the source’s diameter. The Two-Slit Mystery Revealed In the two-slit experiment, in which one photon (or electron) is sent through one at a time, still forming an interference pattern, the solution to the mystery is that any given photon sets the aperture’s state based on its exit trajectory, for it has a particular momentum and energy based on the aperture’s state and the incident photon’s momentum; the next photon to follow encounters this set state, which is that casual relationship between the two, although widely separated in time. The material of which the aperture is composed is a system with an existing quantum that that interacts with an incoming photon. The three states that the aperture must occupy are 1) photon is incident, 2) photon merges with aperture, and 3) photon is transmitted. Since energy and momentum are always conserved and there is angular variance caused by the width and spacing of the slits, the aperture’s quantum state changes depending on the direction of the transmitted photon. The aperture, although macroscopic, still exists in only one state at a time, and charts the path for the next incident photon. The results of the photon passages, due to the conversation of momentum (to infinitely fine resolution), must be preserved in the aperture’s current state as a complimentary response. So, there is no observer-created reality with none before it. Heisenberg was wrong to say that “atoms are not things”. Reality remains intact.
The delayed quantum eraser experiment shows that your explanation is incorrect. http://en.wikipedia.org/wiki/Delayed_choice_quantum_eraser
Yes, most likely. I like to look at things both ways. Here's the other, latest view: RECENTLY, AT BOLTZMANNGASSE 3 IN VIENNA Institut für Quantenoptik und Quanteninformation (IQOQI) Does the moon still exist when we aren’t looking at it? Yes. LOCALITY AND/OR REALISM (AND ‘IT FROM BIT’?) One (or both) of these assumptions is inadequate to describe the physical world; however, Bell’s theorem does not say which to abandon. However, lately it has been confirmed even more conclusively by Ziellinger and associates that entangled particles do not have preexisting properties, such as polarization, that are independent of any observation. So, there goes naive realism. Now, what about at the classical level? Well, although there, too, we transform reality, or I could even say, create reality, although it’s consistent among all individuals, for we see the same trees and buildings, for example. Two particles are called entangled if they share the same fuzzy quantum state, meaning neither of them begins with definite properties such as location or polarization (Which can be thought of as a particle’s spatial orientation). Measure the polarization of one photon, and it randomly adopts a certain value, say, horizontal or vertical; oddly, the polarization of the other photon will always correlate to that of its partner. Zeilinger, whose group invented a common tool for entangling polarization, likes to illustrate the idea by imagining a pair of dice that always land on matching numbers. Equally mysterious, the act of measuring one photon’s polarization immediately forces the second photon to adopt a complementary value. This change happens instantaneously, even if the photons are across the galaxy; the light-speed limit obeyed by the rest of the world can take a leap, for all that quantum physics cares. I’d like to come to the second freedom: the freedom of nature. You said that for example the velocity or the location of a particle are only determined at the moment of the measurement, and entirely at random. I maintain: it is so random that not even God knows the answer. For me the concept of “information” is at the basis of everything we call “nature”. The moon, the chair, the equation of states, anything and everything, because we can’t talk about anything without de facto speaking about the information we have of these things; in this sense the information is the basic building block of our world. In your last book you wrote: “Laws of nature should make no distinction between reality and information.” Why? We’ve learnt in the natural sciences that the key to understanding can often be found if we lift certain dividing lines in our minds. Newton showed that the apple falls to the ground according to the same laws that govern the moon’s orbit of the Earth. And with this he made the old differentiation between earthly and Heavenly phenomena obsolete. Darwin showed that there is no dividing line between man and animal. And Einstein lifted the line dividing space and time. But in our heads, we still draw a dividing line between “reality” and “knowledge about reality”, in other words between reality and information. And you cannot draw this line; there is no recipe, no process for distinguishing between reality and information. All this thinking and talking about reality is about information, which is why one should not make a distinction in the formulation of laws of nature; Quantum theory, correctly interpreted, is information theory. And can you explain all these strange quantum phenomena conclusively with your information concept? Not all of them yet, but we’re working on it; with limitation it works excellently. How? I imagine that a quantum system can carry only a limited amount of information, which is sufficient only for a single measurement. Let’s come back to the situation of two particles colliding like billiard balls, and in so doing entering a state of limitation. In terms of information theory that means that after the collision the entire information is smeared over both particles, rather than the individual particles carrying the information. And that means the entire information we have pertains to the relationship between both particles; for that reason, by measuring the first particle I can anticipate the speed of the second, but the speed of the first particle is entirely random. Because the information isn’t sufficient? Exactly. Its randomness is ultimately a consequence of the finiteness of the information. Quantum breakdown: To investigate where quantum mechanics breaks down and classical mechanics begins, the team is investigating two weird quantum properties: entanglement and superposition. When two particles become entangled, they become inextricably intertwined, so that changing the properties of one has an immediate effect on the properties of its partner. Superposition is another feature that is peculiar to quantum systems; before a quantum object is measured, it does not have definite characteristics; instead, it exists in a superposition of multiple mutually contradictory states—allowing it to be in two places at once, for example. Thus, if information is the most fundamental notion in quantum physics, a very natural understanding of phenomena like quantum decoherence or quantum teleportation emerges. And, so, quantum entanglement is then nothing else than the property of subsystems of a composed quantum systems to carry information jointly, independent of space and time; and the randomness of individual quantum events is a consequence of the finiteness of information. The reduction of the wave packet is just a reflection of the fact that the representation of our information has to change whenever the information itself changes as a consequence of an observation. … a few months ago Zeilinger reported implementing a new kind of statistical bell test, devised by Leggett, that pits quantum mechanics against a category of theories in which entangled photons have real polarizations but exchange hidden particles that travel faster than light. In principle, such faster-than-light theories might have perfectly mimicked quantum strangeness and let realism go unmolested. Not so, according to the experiment: the results could be explained only by quantum unreality. So what idea replaces realism? The situation calls to mind one of Zeilinger’s favorite books, the humorous novel ‘The hitchhiker’s guide to the galaxy’, by douglas adams, in which a mighty computer crunches the meaning of life, the universe and everything and spits out the number 42. So its creators build a bigger computer to discover the question. If quantum indeterminacy is like the number 42, then what idea makes it intelligible? Zeilinger’s guess is information, just like a bit, can be 0 or 1; a measured particle ends up either here or there; but if a particle carries only that one bit of information, it will have none left over to specify its location before the measurement. Unlike Einstein, Zeilinger accepts that randomness is reality’s bedrock. Still, “I can’t believe that quantum mechanics is the final word,” he says. “I have a feeling that if we get really deep insight into why the world has quantum mechanics”—where the 42 comes from—“we might go beyond. That’s what I hope. Then, finally, would come understanding.”
It's statements of this kind which provide the underlying instability of your ideas. Velocity and position of a particle are not at all, in any way, RANDOM. They are just mutually exclusive when it comes to measurement. But by trying to ascribe the element of Randomness, you show that you don't understand quantum theory. Since you don't understand it, it must be incorrect.
This is an interesting explanation. The first thing to pop in my head was that this is testable*: run the dual-slit experiment "one photon at a time". On the odd-numbered photons, run the experiment normally; on the even numbered photons, block one of the slits and switch to a second receptor screen. Standard QM theory suggests that the odd-numbered screen would show an interference pattern, while yours would not. *Testable and practically testable are of course not the same thing. I have no idea if technology could actually run such an experiment.
The delayed quantum eraser experiment also seems to conflict with SciWriter's novel take on the nature of reality.
The experiment is run over many iterations, isn't it? If so, the quantum state of the apparatus is set by the previous photon, so the "delayed" aspect becomes a non-factor. Right?
Thanks for the responses. I've been doing a bit more studying and although I don't understand a lot of the maths, I quite like the idea of sum of histories. It seems to me like a really neat solution to a problem. Albeit one that seriously challenges my perceptions of reality, but I suppose that's the case with a lot of quantum physics. Fairplay to Mr Feynman for this one. I've been trying to follow your posts re the double-slit experiment, SciWriter, but I'm struggling a bit I must admit, largely due to my limited knowledge in the area. I'm trying to read around a bit more now though. Are you basically saying that you think Professor Hawking and Richard Feynman's interpretations are wrong? I've now re-read a couple of the chapters in "The Grand Design" and my mind has been blown for a second time by Hawking's top-down approach to Cosmology, where he applies Feynman's sum of histories method to the history of the universe: "In cosmology, in other words, one shouldn't follow the history of the universe from the bottom up because that assumes there's a single history, with a well-defined starting point and evolution. Instead, one should trace the histories from the top down, backwards from the present time. Some histories will be more probable than others, and the sum will normally be dominated by a single history that starts with the creation of the universe and culminates in the state under consideration." "This leads to a radically different view of cosmology, and the relation between cause and effect. The histories that contribute to the Feynman sum don't have an independent existence, but depend on what is being measured. We create history by our observation, rather than history creating us." I've since then been looking at things like retrocausality, which I think is kind of equivilent to what Hawking is talking about? Where cause and effect can be reversed and the effect can influence the cause? It led me to a question about time. Does time only exist in the macroscopic world and not in the quantum world? Or is it simply possible to travel in 2 directions on the time line at the quantum level?
I do go for standard QM, but always still try to see if it can be accounted for in a less mystical way, plus that how could anything be truly random, being like a miniature first cause in and of itself. Hawking feels that everything, like the whole universe, is still within the wave function, some of the more probable and/or selected paths dominating.
While the 'quantum state' may have been set by the previous photon, what that state consists of is altered by the delayed aspect of the quantum information eraser. But it is altered AFTER the wavicle has passed through the slit and 'set' the 'quantum state'. BTW, my discussion of the 'quantum state' of the slit, which is something which is totally undefined in any way, should in no way imply that I agree at all with the explanation. In fact, I think it is simply a way of trying to rationalize that which the poster just can't understand. But even on that level, it is disproved by experimentation.
SnowSportSid: It is worse than you think.According to the Heisenberg Uncertainty Principle, I think I am correct in saying, that it is impossible to accurately measure both the position and velocity of a particle because the very act of observing the particle effects the particle's behaviour.The bolded (by me) part of the above is true, but it is misleading. It is the basic idea behind what is called a disturbance model of quantum reality. The Uncertainty Principle asserts that a quantum entity cannot have a precise position & a precise momentum at the same time. It is not a statement claiming some inadequacy of measurement technology. A Bose-Einstein condensate seems to be experimental evidence that precise velocity implies imprecise location. At close to absolute zero temperature, it is known (without making measurements) that atoms are almost motionless. In this state, they seem to be smeared over a large (by atomic level standards) volume of space. Many intelligent people (including some quantum theory experts) believe in a disturbance model. For example: Nick Herbert, from the preface to his book Quantum RealityI regarded the Copenhagen Interpretation as sheer mystification compared to the clarity & common sense of my disturbance model. Blissfully ignorant concerning the real issues surrounding the quantum reality question, I got my degree & continued my career as an industrial & academic physicist.Later in the book he describes how he stopped believing in the disturbance model. One view that helps is to imagine that quantum entities arrive & depart as particles, but travel as waves. There is good reason to believe that quantum entities do not have continuous paths. No valid model of an atom can be constructed using electrons as particles with continuous paths. Bohr once compared the wave/particle duality problem with a well known illusion. He said something like the following.There is neither a vase nor a pair of facing profiles in that drawing. What is really there is black ink on white paper. I can safely say that quantum entities are neither particles nor waves. Unfortunately, I have no idea of what is really there.I think that the human mind is incapable of constructing an intuitively satisfactory model of quantum reality because that mind is the result of millions of years of evolution in an environment that for all practical purposes obeys the laws of classical physics. Developing an intuitive understanding of quantum reality seems analogous to developing an intuitive understanding of geometric objects in 4D, 5D, et cetera spaces.There is considerable mathematics providing a knowledge of the properties of 4D objects. There are formulae for the volume & surface area of 4D & higher dimensional hyperSpheres & other extraDimensional objects. However, I cannot create in my mind a satifactory image of the corner of 4D cube, which is one of the simplest hyperSpace objects. Similarly, physcists can apply mathematics to the quantum level reality & design lasers (& other wonderful gadgets), but cannot create a mental image of that quantum reality.
I like that. There is the particle and there is the wave—either one forced on us by our observations, being jointly known as the ‘wavicle’, all three states of which are truly not the actual reality.
Interesting to learn that neither particle or wave reflect the true quantum reality. This sounds like a good book, so I think I will invest in it. Also pleased to hear that the quantum reality is probably incomprehensible to my human brain. I've wondered before whether it was actually possible or not to visualise all those extra supposed dimensions that string theory implies. Perhaps not.
The analogue of this has already been done in Bell experiments that close the so-called "locality loophole". Bell inequalities are violated by interference terms, and Bell experiments which aim to close the locality loophole alter the detector settings constantly throughout the experiment.
Hi there, I was doing some reading about the double slit experiment and the uncertainty principle and while googling came across this web site so registered. The first post on this site I found, the poster in question was suggesting that in the double slit experiment, the person carrying out the experiement influenced the result, but surely this could easily be resolved by having the machine switch on the particle detectors at random without anyone having knowledge of whether the detectors were on or off? Other people suggested either the particles were being influenced by the aperture of the equipment, or they had knowledge of the other slit or paths taken by the other particles (or photons or whatever). Well, that leads me to the question, can photons or particles in a wave be in a state of quantum entanglement one with another? If a wave could be described in such a way (made up of entangled photons for example), this would not only explain how one particle has "knowledge" of the other particles \ paths \ slit, but could also be used to explain how the wave breaks down upon observation just as it does when the spin of seperated entangled particles are examined?
