Why Quantum Physics is Cool Pt 3 (Updated Pt 4 Link)
Created on: September 26th, 2006
Part 1 at http://quantumiscool1.ytmnd.com
Part 2 at http://quantamiscool2.ytmnd.com
Part 3 at http://wqpic4.ytmnd.com/
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The uncertainty principle is mainly about being able to know the location or the velocity of a particle, but never both at the same time. Also it includes not being able to know the spin of more than one axis per particle. Neither really play into this, as we simply need to know the location at a given time.
The problem is: If the results (wave pattern/no wave pattern) are correlated to the presence of a detector, then the detector has an effect on the results. If the detector changes the results, then you are talking about 2 different experiments, not one. In one expiriment you produce a wave pattern because no detector is present. In another, you do not produce a wave pattern because a detector is present. The results are not duplicated. It is not the same.
No. I just showed how having the detection devices ON, yet erasing the evidence brings back the wave pattern, just as if the devices were not on. I can go further into this with the Delayed Choice Quantum Eraser experiment with laser splitting to further illustrate the point. But the point remains, we can even leave the particles in question unaltered, without any measurements, and only measure their idler photons, and the only factor that determines an interference pattern is us knowing the path or not.
It's not peek-a-boo, though... you can't just say that cognition is the determining factor. The determining factor is ___________. We may not know what it is, but it certainly isn't "us knowing the path". Actually, I'll even give you that it could merely be a cognitive faculty. They haven't isolated the cognitive faculty and shown that this is the cause. The results change. There is a cause for this. The cause is unkown. That seems to be all that the evidence points to.
The photon knows where it is at all times. The photon knows this because it knows where it isn't. If we subtract where the photon is by where the photon isn't, or where the photon isn't by where the photon is - whichever's greater - we get really confused. X3 Just kidding around. I love your sites, SCIENCE RULES
Let us consider a free particle. In quantum mechanics, there is wave-particle duality so the properties of the particle can be described as a wave. Therefore, its quantum state can be represented as a wave, of arbitrary shape and extending over all of space, called a wavefunction. The position and momentum of the particle are observables. The Uncertainty Principle of quantum mechanics states that both the position and the momentum cannot simultaneously be known with infinite precision at the same time. However, we can measure just the position alone of a moving free particle creating an eigenstate of position with a wavefunction that is very large at a particular position x, and zero everywhere else. If we perform a position measurement on such a wavefunction, we will obtain the result x with 100% probability. In other words, we will know the position of the free particle. This is called an eigenstate of position.
If the particle is in an eigenstate of position then its momentum is completely unknown. An eigenstate of momentum, on the other hand, has the form of a plane wave. It can be shown that the wavelength is equal to h/p, where h is Planck's constant and p is the momentum of the eigenstate. If the particle is in an eigenstate of momentum then its position is completely blurred out.
Usually, a system will not be in an eigenstate of whatever observable we are interested in. However, if we measure the observable, the wavefunction will immediately become an eigenstate of that observable. This process is known as wavefunction collapse. If we know the wavefunction at the instant before the measurement, we will be able to compute the probability of collapsing into each of the possible eigenstates. For example, the free particle in our previous example will usually have a wavefunction that is a wave packet centered around some mean position x0, neither an eigenstate of position nor of momentum. When we measure the p
After we perform the measurement, obtaining some result x, the wavefunction collapses into a position eigenstate centered at x.
Wave functions can change as time progresses. An equation known as the Schrödinger equation describes how wave functions change in time, a role similar to Newton's second law in classical mechanics. The Schrödinger equation, applied to our free particle, predicts that the center of a wave packet will move through space at a constant velocity, like a classical particle with no forces acting on it. However, the wave packet will also spread out as time progresses, which means that the position becomes more uncertain. This also has the effect of turning position eigenstates (which can be thought of as infinitely sharp wave packets) into broadened wave packets that are no longer position eigenstates.
Some wave functions produce probability distributions that are constant in time. Many systems that are treated dynamically in classical mechanics are described by such "static" wave functions. For example, a single electron in an unexcited atom is pictured classically as a particle moving in a circular trajectory around the atomic nucleus, whereas in quantum mechanics it is described by a static, spherically symmetric wavefunction surrounding the nucleus.
The time evolution of wave functions is deterministic in the sense that, given a wavefunction at an initial time, it makes a definite prediction of what the wavefunction will be at any later time. During a measurement, the change of the wavefunction into another one is not deterministic, but rather unpredictable, i.e., random.
The probabilistic nature of quantum mechanics thus stems from the act of measurement. This is one of the most difficult aspects of quantum systems to understand. It was the central topic in the famous Bohr-Einstein debates, in which the two scientists attempted to clarify these fundamental principles by way of thought experiments. In the decades after the formulation of quantum mechanics, the question of what constitutes a "measurement" has been extensively studied. Interpretations of quantum mechanics have been formulated to do away with the concept of "wavefunction collapse"; see, for example, the relative state interpretation. The basic idea is that when a quantum system interacts
In the mathematically rigorous formulation of quantum mechanics, developed by Paul Dirac and John von Neumann, the possible states of a quantum mechanical system are represented by unit vectors (called "state vectors") residing in a complex separable Hilbert space (variously called the "state space" or the "associated Hilbert space" of the system) well defined upto a complex number of norm 1 (the phase factor). In other words, the possible states are points in the projectivization of a Hilbert space. The exact nature of this Hilbert space is dependent on the system; for example, the state space for position and momentum states is the space of square-integrable functions, while the state space for the spin of a single proton is just the product of two complex planes. Each observable is represented by a densely defined Hermitian (or self-adjoint) linear operator acting on the state space. Each eigenstate of an observable corresponds to an eigenvector of the operator, and the associated eigenvalue corresponds t
The time evolution of a quantum state is described by the Schrödinger equation, in which the Hamiltonian, the operator corresponding to the total energy of the system, generates time evolution.
The inner product between two state vectors is a complex number known as a probability amplitude. During a measurement, the probability that a system collapses from a given initial state to a particular eigenstate is given by the square of the absolute value of the probability amplitudes between the initial and final states. The possible results of a measurement are the eigenvalues of the operator - which explains the choice of Hermitian operators, for which all the eigenvalues are real. We can find the probability distribution of an observable in a given state by computing the spectral decomposition of the corresponding operator. Heisenberg's uncertainty principle is represented by the statement that the operators corresponding to certain observables do not commute.
The Schrödinger equation acts on the entire probability amplitude, not merely its absolute value. Whereas the absolute value of the probability amplitude encodes information about probabilities, its phase encodes information about the interference between quantum states. This gives rise to the wave-like behavior of quantum states.
It turns out that analytic solutions of Schrödinger's equation are only available for a small number of model Hamiltonians, of which the quantum harmonic oscillator, the particle in a box, the hydrogen-molecular ion and the hydrogen atom are the most important representatives. Even the helium atom, which contains just one more electron than hydrogen, defies all attempts at a fully analytic treatment. There exist several techniques for generating approximate solutions.
If you can convince me that if you fire one photon, it travels BOTH paths for sure, but if you use the detector, it only travels one, then you win. But what you SEEM to be saying is that you put a steady beam without observation, and half go each way. And then, you observe, and half go each way. I dont see how your observations changes this!!! Probability != positiion.
I've studied quantum physics myself and still think it's quite mind boggling. For the rest of everyone else: instead of calling the author a liar, how about you do some research yourself. This stuff is obviously too much for the simple minds of most YTMNDers. But then again, most of them are about 13 years old. Look up the information yourselves...read this to start off: http://www.fortunecity.com/emachines/e11/86/qphil.html
The thing is that this isn't shocking. Most people think of photons as infinitessimal THINGS being flung about through space, like microscoping tennis balls or atomic nuclei. They are not. Photons are "packets" with no mass; they're not physical things. They're a way of visualizing a subatomic force. We think of them as tiny dots being shot out of a laser and find is mind-boggling that they could be in two places at once, but the reality of the situation is they're NOT tennis balls, they don't have any mass
...to speak of and they're not OBJECTS. That a photon is representative of all of its possible paths (think the electron cloud model) is bizarre only because we try to compare them to things they are not. If we inacurately compare a photon with an object, then of course it is shocking. But photons are something else entirely. it is the model that is faulty. It's similar to how molecules are constructed for observation with little bars between them; they're actually nothing like this, but it helps to think
of them that way. Likewise, it helps us to think of photons as particles moving in predictable patterns, but this is not what they actually are.
On another note, the idea that OBSERVING a photon's location forces it to act like a particle is not accurately represented. It is the MEASUREMENT of their location which causes it, because in measuring a photon its potential path has changed. If a photon has a series of infinite paths between the laser and the paper, it is considered to be in all of those
places. However, when a device used to measure their location is introduced into the system, they can no longer represent "all potential paths between laser and paper" because there are two distinct sets: "All potential paths between laser and paper" (the one slit), and "All potential paths between laser and measuring device". It is the device which interferes with their paths and causes them to behave as independent systems, rather than a single system. The measuring device isn't invisible, THAT'S the
relevance of Heisenberg. As to the spin experiment, I may not iunderstand exactly how it was carried out, but if my comprehension serves me it is the fact that a tool is used to "erase" them that causes them to behave like waves again, not the thing being erased. See, they would not hit the erasing tool in a wave pattern, but leaving said tool they change, this time representing the set of "All possible outcomes between erasing tool and paper".
yes, technically the photon has no mass "at rest"... but the point is the same, which is that it is not an object in the conventional sense. It has mass for certain functions, but it is not a THING, in that its size is directly related to its motion. The point is not technicalities, it's to demonstrate that a photon is not an object (and I'd assume you'd agree), but can best be represented that way in our imaginations. I guess what I mean is the sooner you stop visualizing a tennis ball as your photon ...
It's really sad when the ignorance of people gets in the way of actual scientific data. How many of these any-sayers have actually been in a quantum physics class? How many of them would argue the same arguments with the professor? The answer is ZERO. Because they would be wrong. Just as they are wrong now. ----- If anything, the whiny nay-sayers are just looking to refute something using their own perception of logic...morons.
@thegoat: You don't argue with professors because in lower-level physics courses (not Masters Thesis stuff, just the basic prereqs) you'd rather pass that pick a fight. I've had too many foolish professors in too many subjects to take their word as golden. I'm not trying to refute any experiments here. I know full well what the double-slit experiment is and says. I'm simply trying to put it in a different perspective, void of all the stoner psychological implications. We're NOT talking about one thing
being in two places at once because it's not a conventional "THING". And we're not talking about the very act of observing the photon, we're talking about how "observation" in the world of quantum particles means "measurement", and placing any object in the potential path of a photon is going to alter its behaviour. Heisenberg isn't a principle only for subatomic particles: If you go into the wild and study primitive tribes, don't you think that the fact that you are there is going to change the way they
Again, though, "Look at it" only applies because in order to look at it, you must interfere with it. If, as the author said (in either this or the last one), you could shrink down to a microscopic size and watch it fly by, it would never have a wavefunction collapse because your observation does not interfere with it, causing its nature to change.
OOH! Do Schroedinger's cat and quantum suicide for the next one.
Einstien never did believe Quantum Mechanics was anything more than a probability-representation of something more fundimental that we don't yet understand. "God does not play dice." Being a determinist, myself, I'm in that same boat with him. Quantum physics makes some good points, but it is not going to be a part of the unifying theory of everything, if and when we wrap our minds around that one.
Excllent series of sites. Though if you want my advice (which you probably don't), I'd leave off with attempting to convince skeptics. People who're 1-ing you are going to continue to 1 you, and those of us who can grasp the simplified concepts expressed here are going to continue to learn and be amused.
Its like saying "There is a ball in one of two boxes... at any given time,
half is in each box. But when you put scales under them, the ball is only
in one box! OMGWTF!" Probability is NOT reality - Pettitman
On the contrary, it is.
FTL. Quantum Physics has done nothing for the science community. We need
another great scientist to throw these quantum people in the garbage. - w1ngzer0
It's been proven. It's helped us understand the universe on the most basic scales.
I'm freakin curious here. I can't seem to find any information about this or not, but, were these experiments (the lazer one and the double slit experiment) performed in a vacuum enviroment? If not, then the molecules of air floating around should theoretically cause some sort of interference, correct? or am i on some sort of wrong track, here?
"It is the device which
interferes with their paths and causes them to behave as independent
systems, rather than a single system. The measuring device isn't invisible" Here's the point I was making all along... if a = b and a = c then b = c. b =/= c, therefore a =/= b. By saying, "simply by obseriving the path of the particle we change the results" is misleading when you actually mean, "simply by using a measuring device we change the results".
I'm considering making another. But 2 things give me pause. 1st, each one of these I do drops in numbers of views from the last one by about half. 2nd, I'm trying to think of another cool fact in Quantum Physics/Mechanics that wouldn't take me a 2 hour long gif to describe in a way that everyone could get it.
It really saddens me that there are so many people who are just skeptical and don't even give quantum physics a chance, or because they don't understand it they plead ignorance and just reject it. I pity these people, because it's things like this which show how amazing and mysterious the universe really is. 5'd
Quantum physics....if i were smart enough i would major in it. We know so little about the things that everything is made of. Look at humans in the present day and the way we think, then look back 1000 years and observe how they thought. Could we be as clueless as we once were 1000 years ago, still? Absolutely.
Is it the act of measuring the results, or the act of observing the measurements, that causes the wave to collapse? (The latter implies that sapience is required, while the former could be done in the absence of any life at all.) Is there any way to tell the difference? We can't very well analyze our measurements without observing them first...
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