I have read about the double slit experiment in Hawking's books and a friend of mine directed me to a YouTube video (search for double slit experiment if you want to watch it). The video really helped me to understand the actual process of the experiment. What I don't get is why does the particle behave like a wave when unobserved and like a particle when under observation?Please Register or Log in to view the hidden image!Please Register or Log in to view the hidden image! Google turned up nothing for me, but then again I really dont know what I'm even looking for!
Nobody knows why. What's more, if you observe it AFTER it passes the slits, it acts like it was a particle when it when through the slits.
It depends entirely on the concept of measurement. You have to allow that an observer is really a part of the wavefunction. That's a little (no, it's a lot) nonintuitive, but it appears to be true. So understanding the process of measurement, delayed choice and so on, the use of 'non-sentient' measuring equipment that only stores a result until an observer looks at it at some later time, even what time means in this context, are a sort-of requirement. Not that any of the above explains what a wavefunction is so much, as what it "isn't".
Just to extend the wierdness of the quantum world, here's http://en.wikipedia.org/wiki/Delayed_choice_quantum_eraser
Nice reference, Alex, I found the delayed choice eraser article quite fascinating. Doesn't really surprise me though, sounds like the detector itself is simply entangled with the photons it detects, and the wave function collapse for the entire system (photons + detector) is what's actually delayed. Of course, as far as I've last heard, the actual process of wave function collapse itself isn't really well understood either, so I hope experiments like this might help provide some useful insights. A word of caution to newcomers to the world of quantum physics: there's a lot of nonsense being spread around the internet and media involving bogus claims about the whole subject. Indeed, when it comes to quantum physics, it seems like there's more rubbish published by the popular media than actual experimentally-established fact. "What the Bleep Do We Know" is a pretty well-known example of this type of media distortion, and this is where the "Dr. Quantum" double slit Youtube clip comes from. What the Bleep was produced by a religious cult/scam known as the "Ramtha School of Enlightenment", and plays off the layman's misunderstanding of what constitutes a "quantum observation". Remember, in order for an "eye" to see something, that something must first have "light" shining on it, i.e. in actual double slit experiments there is always a physical mechanism involved in making the observation; these experiments have nothing to do with consciousness, i.e. it doesn't matter whether or not someone in the lab is actively watching. If our minds could control remote particles at the quantum scale as some mystics suggest, a simple variation on the Stern-Gerlach experiment could have been used to establish such an effect over 80 years ago, and it would be universally accepted in scientific circles by now. Obviously such suggestions are scoffed at by the vast majority of scientists, for very good reasons such as the one mentioned in the previous sentence. Edit: Forgot to mention, I think it's a damn shame, nay an outright atrocity, that the scientific community can't popularize the subject on the same level as the mystics, frauds and con artists. We need to make sexier movies and pictures about quantum physics so people will start paying attention to our side of the story, otherwise they just get bored and move on.
What do you think about this one (for the layman)? http://www.youtube.com/watch?v=wENqNtgsjEA More of a history than an explaination.
Because to observe the particle you must interact with it, in this case by bouncing a photon off of it and this effect of interaction does something called, colapsing the wavefunction. to explain this you must first understand the properties of an unobserved particle... according to QM, an unobserved particle (one than has NO interactions as of yet with the outside world) is superposed (it exists in many forms all at once) but when something interacts with it, say a photon, it must "choose" and particular state and stick with it... as for the double slit experiment, the unobserved particle is superposed and and so hits the screen in many many places BUT as soon as it hits the screen it must then "choose" which place it will stay, (this is "decided" by the probabilities of it staying in different places, given by the wavefunction) but if you observe it at anypoint before it hits the screen, you colapse the wavefunction and the superposition is destroyed, because you have interacted with it, hopefully that makes sense, its very simplified
What the double slit experiments tell us is that reality is probabilistic. That means, we don't "know" what reality is until a wavefunction produces a definite state. The world we perceive as real is some kind of projection of the state variable(s)--particles or waves are wavefunctions acting on themselves. I think Penrose calls this projection "state reduction", or "the R process", but that's only a convenient kind of terminology, a reference to the apparent reduction of the state from an infinite number of possible outcomes, to just one.
Interesting, I've watched the first few minutes so far and will watch more later. But as far as my initial impressions go, it's nowhere near sexy enough. I can already imagine members of the audience crying "booooorrrrrriiiiiiiiinnnnngggg!!!" We need to make some movies about Quantum Physics for audiences under the assumption that a lot of them have ADHD, whether it's actually true or they just tend to behave that way. We live in the age of the soundbite, so if we want to communicate what scientists do to the ordinary layman, we should be taking this into account, which is one of the few things What the Bleep actually did quite well.
Physicists tend to (or should) leave the "why questions" to philosophers and focus on the how and what questions. By "observed" I assume you mean made a "recordable interaction" with larger matter. For example cause a silver halide (iodide?) crystal in a photographic film to undergo an internal change, which if two photons hit that same crystal will become a stable change for many weeks. If unpolarized light beam is directed towards perfect polarizer, half will pass thru and have their electric field now aligned with the "pass direction" of that polarizer, which we can confirm with a second perfect polarizer but we are not observing their electric field rotate to the direction of the polarizer's "pass direction." - We are inferring that it did, if not by chance already so aligned. I.e. we never really "observe" the photon as a wave, but infer that it is from what it can do. The two most important things it can do for this inference are both exhibited in the double slit experiment. * A large number of them will make a wave's interference pattern than can be seen when the film is developed. Also, because this interference pattern can be made with long exposures and an extremely weak light source, so weak that most of the time not even one photon exists, we infer that EACH photon in some sense goes thru both slits. Each photon interferes with itself! No thing we want to call a "particle" can do that - be at two different slit locations at the same time. Note we did not actually observe that the photon went thru both slits, we inferred that, from the fact the two silt interference pattern was still produced with this very weak light source. SUMMARY: We only observe the photon as a particle. We infer it has the characteristic of a wave from what it can do when it is not being observed (as a particle). If observed, it always acts like a particle. ---------------- * The two slits are not far apart, but one can also make the interference pattern with a two path interferometer with very weak light source. In this case large part of the two paths EACH photon travels can be separated by meters - I don't think there is any limit except practical ones of aligning the "half silvered" beam splitters, avoiding absorption, etc. I.e. one path, in principle to go to the moon, be reflected by a mirror there and then return to Earth. The other path would need to be highly folded by many mirror reflections to make it essentially equally long. ("Essentially" as the difference in path must be significantly less than the length of the photons.)** **I have used a two path interferometer to measure how long some photons were. Mine were about 30 cm long. - I.e. with path difference greater than that, the interference pattern has completely faded away to a uniformly illuminated screen - the "head" of one arrives at the screen after the "tail" of the other has, so can not interfer with itself.
No one seems to have a good answer to this question. Let us first assume that the photon may be a wave. There are various reasons why this may be the case, as we can discuss, but the confusion is how a particle can pass through two slits at the same time. If the photon is wave, then wave interference patterns would be expected on the exit side of the barrier. The wave pattern would be then be an interference between the two exit waves. This pattern would be expected to be similar to that of the Fourier transform of a lens, and it appears that this is indeed the case. Isn't that a good reason to investigate this possibility further, rather than rejecting it because it disagrees with particle theory? I am new to this site, so I would like to know whether or not the above questions are not allowed here simply because they might have some disagreement with current accepted theories.
It's pretty simple really. The photon is a wave, not some billiard-ball particle. It's all to do with displacement current. Imagine a wave travelling through a stiff lattice. As the wave passes on through, the lattice is displaced and distorted. This distortion is not localized to one particular region of the lattice, just as a seismic wave isn't localized to one particular region of the earth's crust. Instead the displacement / distortion diminishes with distance from the line of propagation. If you planted something solid in the ground to form two slits, you wouldn't express amazement that a seismic wave passed through both slits at once. In similar vein you shouldn't be amazed when a photon passes through both slits at once. It's an electromagnetic wave. That's what waves do. And what's important to note here is that we can use pair production to make electrons (and positrons) out of photons. Then we can annihilate electrons and positrons to get photons. We can also annihilate protons and antiprotons to get photons. Since material bodies are made out of electrons and protons etc, and these are made out of photons, and photons are waves, in the end they're all made out of waves. Or wavefunction if you prefer. Or "quantum field". I kind of like spacewarp myself. Quantum physics is only mysterious when people insist on photons and material bodies as being made out of little point-particle billiard balls rather than waves. Then people talk about dual-slit electrons and say Woo! Two places at once! Parallel worlds! They forget that electrons are made out of waves, and that they are waves. That's why we can do electron diffraction. They're just waves going round and round, hence pair production and spin angular momentum and magnetic dipole moment and the Einstein-de Haas effect. Like I said, pretty simple really.
Prometheus: If the photon is a wave, which is the most likely answer, then it is not a particle and the double slit experiment correlates with the real world. We can also now predict the results that will be obtained from varying the slit parameters. There are other real world measurements that tend to support this concept. Saying that this reasoning is simply not true is not a very good answer. As to the other comments by Farsight, I find them very difficult to accept, such as the "distortion of a lattice", "similarity to a seismic wave", etc. Where did that reasoning come from?
The photon is not a wave, it's a quantum particle. In a little more detail, the photon is an excitation of the photon field, which is a spin 1 quantum field. The reason I know that's what it is is because quantum electrodynamics models the photon as a quantum field and it is the most accurately tested theory of physics ever - theoretical predictions and experimental measurements of the anomalous magnetic moment of the electron agree to 1 part in 10^12.
btw misunderstandings of the double slit experiment probably arise because you're trying to analyse it using quantum particle mechanics rather than quantum field theory.
Pointing out that Farsight is wrong is always a very good answer. The "photon is a wave" oversimplificationlie ignores the phenomena which show the photon to be a discrete packet of momentum, angular momentum and energy, notably Einstein's 1905 paper on the photoelectric event and Compton's experiment on photon-electron interactions. Nowadays we have experimental evidence that all observed particles obey the same rules, including having periodic properties in space and time relating to their momentum and energy. Waves get more energy as you increase the amplitude continuously. A stream of co-moving identical particles get more energy in discrete chunks. Nature validates the latter view, but the chunk sizes are pretty small by everyday standards. Particles are classified today by their mass, m, and their interactions. Photons are massless (m=0) and interact with electrically charged particles (like electrons). The details are best described today in the theory of quantum electrodynamics. But like all freely traveling particles, between interactions the following equations hold: p is momentum of an entity with respect to an inertial observer h is Planck's constant λ is the de Broglie wavelength as measured by that same inertial observer \(|p| = \frac{h}{\lambda}\) E is the relativistic energy with respect to an inertial observer f is the de Broglie frequency (cycles per second) as measured by that same inertial observer \(E = h f\) v is the velocity of an entity with respect to an inertial observer c is the speed of light in vacuum \(\vec{v} = \frac{\vec{p}c^2}{E}\) m is the invariant mass of an entity \(E^2 = m^2c^4 + p^2c^2\) So while massive particles can have a variety of speeds, massless particles only travel at c.
The Schroedinger wave equation, along with appropriate boundary conditions, develops some elegent standing-wave solutions that agree quite well with the electron bonding capabilities of atoms.
Saying the probability density of finding the electron is described by the square of the amplitude of a complex-valued wave is not the same thing as saying the electron is a wave. This becomes clear as you add electrons. You don't get a higher amplitude wave, but a wave described in a higher dimensional space.
As few readers here have ever used the Schroedering equation (Or the matrix form of QM) for anything and even less know anything about QED, I think for them my post 10 reply is best. It tells the facts in terms they can understand. If anything stated there is incorrect, please tell what that is.
