I'm sure variations on this come up a lot, but I was specifically interested in a response to a claim made by a particular article:     The article is aiming to disprove the idea that an 'observer' collapsing a wave function has anything to do with consciousness or human observers.  At one point it argues that the reason an electron in the double slit experiment has its wave function collapsed when detected before entering one slit or the other is due to the photons fired at it to detect its position having momentum and physically interacting with it. The author uses the analogy of tennis balls being fired into a dark room and using a tennis racquet to detect them - the method of detection causes a change in their behaviour.  Now in this instance that's all well and good, but firing photons is not the only means of detecting something. In the Schrödinger's cat thought experiment, for instance, a Geiger counter is used. As far as I understand it, these simply detect passively rather than firing out photons with momentum.  I know many people disagree with the wave function collapse theory and prefer decoherence (which I'm still wrapping my head around) but I'm interested to know if this is something that's explained within collapse theory or if it's something that's seen as disproving it.
122698
I'm working off the article, . I think it's well-written, but I'm not convinced about this part:     So what is causing the change in the particle’s behaviour?      In the example given above, when electrons are fired at a fluorescent screen, when we bombard the electrons with photons the interaction changes the state of the system catastrophically as, despite lacking mass, photons do carry momentum.   Here the author strongly implicates the momentum of photons as being responsible for the collapse. But is that correct to say?  I have read that the  verified that obscuring which-way information restores wave-like behavior. So it feels to me it's more about some "outer" state (perhaps well-characterized as "information") that we don't yet understand that collapses the system, not per se the photon-electron interaction.  Or am I wrong, and in fact for example, the process of obscuring in the quantum eraser experiments somehow "undoes" the photon-electron interaction or restores the quantum state through another interaction?
121205
According to various sources on the web, it seems like the general concensus is that there isn't actually any wave-particle duality with quantum particles. For example,  implies that diffraction patterns in  were interpreted as wave interference due to apparatus limitation at the time they were first performed.  Does this mean that all those sources and animations showing two waves interfering are simply incorrect, classical conclusions which don't have anything to do with (quantum) reality?  What's actually the most confusing is that most sites which state that it is now possible to pass individual photons through these slits, also claim that these individual photons somehow interfere with themselves resulting in the observed patterns. That seems like a rather thin explanation, doesn't it?  So, is there actually any need to use wave interference to explain the phenomena, or can we simply state that the pattern is probabilistic in a certain way, without involving the "spooky" explanations?
17748
While I was hobby-reading about quantum mechanics I came across Heisenberg's theory. But while I was trying to understand it I thought of this: if the speed of light (as well as the momentum of a photon) is well-defined, then if in an experiment the position of the photon was found, the researchers would know the exact position of the photon, but also its momentum which is constant. Is this an exception from the rule, or am I just ignorant?  PS, 12 grade physics education, please try to explain in simple words
121503
Is the uncertainty principle a technical difficulty in measurement or is it an intrinsic concept in quantum mechanics irrelevant of any measurement?  Everyone knows the thought experiment of measuring the position of an electron. One can detect electron's position by hitting it with a photon, due to Compton scattering the collision of the photon with electron will change electron's momentum. This experiment is used to explain uncertainty principle to layman, but it is over simplified, isn't it? It also gives an impression that if there was a better suited experimental method the uncertainty principle becomes irrelevant.   I personally think it is intrinsic as it arises from the non-zero commutator of position and momentum operators irrespective of the measurement process. Am I right?  EDIT: My question is similar to certain extent to  and . The answers there are nice but they focus on explaining basics of quantum mechanics more than they comment on the technical difficulty part. In the answers of question 2 there are statements like "So, it's not a knowledge limit" and "you're sort of correct when you say it's an observational limit" without further comments  To summarize, assume hypothetically we managed to find a way in the future where we can have a look at an electron without disturbing it by measurement or causing its wave function to collapse, would the uncertainty principle still hold in such a case?? Why/Why not?
20926
Imagine you're teaching a first course on quantum mechanics in which your students are well-versed in classical mechanics, but have never seen any quantum before.  How would you motivate the subject and convince your students that in fact classical mechanics cannot explain the real world and that quantum mechanics, given your knowledge of classical mechanics, is the most obvious alternative to try?  If you sit down and think about it, the idea that the state of a system, instead of being specified by the finitely many particles' position and momentum, is now described by an element of some abstract (rigged) Hilbert space and that the observables correspond to self-adjoint operators on the space of states is not at all obvious.  Why should this be the case, or at least, why might we expect this to be the case?   Then there is the issue of measurement which is even more difficult to motivate.  In the usual formulation of quantum mechanics, we assume that, given a state $|\psi \rangle$ and an observable $A$, the probability of measuring a value between $a$ and $a+da$ is given by $|\langle a|\psi \rangle |^2da$ (and furthermore, if $a$ is not an eigenvalue of $A$, then the probability of measuring a value in this interval is $0$).  How would you convince your students that this had to be the case?  I have thought about this question of motivation for a couple of years now, and so far, the only answers I've come up with are incomplete, not entirely satisfactory, and seem to be much more non-trivial than I feel they should be.  So, what do you guys think?  Can you motivate the usual formulation of quantum mechanics using only classical mechanics and minimal appeal to experimental results?  Note that, at some point, you will have to make reference to experiment.  After all, this is the reason why we needed to develop quantum mechanics.  In principle, we could just say "The Born Rule is true because its experimentally verified.", but I find this particularly unsatisfying.  I think we can do better.  Thus, I would ask that when you do invoke the results of an experiment, you do so to only justify fundamental truths, by which I mean something that can not itself just be explained in terms of more theory.  You might say that my conjecture is that the Born Rule is not a fundamental truth in this sense, but can instead be explained by more fundamental theory, which itself is justified via experiment.  Edit:  To clarify, I will try to make use of a much simpler example.  In an ideal gas, if you fix the volume, then the temperature is proportional to pressure.  So we may ask "Why?".  You could say "Well, because experiment.", or alternatively you could say "It is a trivial corollary of the ideal gas law.".  If you choose the latter, you can then ask why that is true.  Once again, you can just say "Because experiment." or you could try to prove it using more fundamental physical truths (using the kinetic theory of gases, for example).  The objective, then, is to come up with the most fundamental physical truths, prove everything else we know in terms of those, and then verify the fundamental physical truths via experiment.  And in this particular case, the objective is to do this with quantum mechanics.
105820
So, 2 days ago I learned that quantum entanglement is just about our perception. There is not an active link between particles and they are not communicating with each other. But then, while surfing on the internet this page came up   It says: ""If I jiggle one electron, the other electron 'senses' this vibration instantly, faster than the speed of light. Einstein thought that this therefore disproved the quantum theory, since nothing can go faster than light," Kaku wrote."  and then I read about EPR paradox and really confused. If there is not an active link between quantum particles what Kaku is talking about ? Can someone explain?
109995
My Young's slits experiment doesn’t work. I mean I tried to see wave effect of light but I didn’t see any thing expect particles! I’ve sent you a pic of my laser and a short video of the experiment. Is there any problem you think?
58234
Apologies if this has been asked before (I did check and I believe it wasn't). I have a question about the  of photons (or other particles). Depending on what and how we measure the photon turns out to be either a wave or a particle. Recently I saw some web page (and I can't remember where) that maybe neither is true. Both the wave perception and the particle perception are just that: perceptions, strengthened by our perhaps limited ability to observe. What if the reality of the photon is something else, something "on top" of our two perceptions?? Could someone direct me to a site that could tell me more about it (or maybe debunk the whole idea?)?
79481
There are a lot of questions about this topic on this site, none of them answer my question specifically. I have read this question:   I think of the emitted photon as a point particle (but with a polarization vector) traveling in a straight line from the atom to the measuring device.    For some people, a photon is a dimensionless point traveling on a world line (Eugene Wigner's definition of a particle).    The photons do not have a well defined trajectory. The diagram shows them as if they were little balls travelling along a well defined path, however the photons are delocalised and don't have a specific position or direction of motion.    because photons take all paths, but since their underlying physics is &quot;wavey&quot; different paths can &quot;interfere.    This calculation assumes that light simultaneously travels over all possible paths. To what extent this is just a calculational device and to what extent it reflects an underlying physical reality is a matter of opinion.    Indeed, nothing in nature moves around on all possible paths, in reality it's a field that permeates the vacuum which has quantized solutions.  There are mainly two thoughts:  the photon travels in a straight line, and that explains why it only interacts with a certain atom, meaning, that the photon can only be detected once, and the photon will not interact with the other atoms because they are out of its trajectory  the photon takes all paths, and that is why it truly goes through all possible avenues, explaining experiments like interference     Now these lead to two different propagation pictures. On the top, you can see the photon (EM wavepacket) spreading spherically in all directions. On the bottom, you can see a photon traveling from A to B, taking all possible paths. But which picture could be experimentally proven to be correct? The picture on the top cannot explain why the photon misses all the other atoms, that is, why it is not interacting with other atoms, because all atoms are in its way basically (it spreads spherically). The picture on the bottom cannot explain diffraction, because the photons is shown not to spread like a wave (cannot interfere), but just like a billiard ball on different paths. After the question was closed, I am editing to clarify (to reopen), that (as I understand), the question was closed due to the word &quot;really&quot; and what it physically means for the photon to take all paths, or whether the picture on the top (spherical spread) physically describes what is happening. I now revise these words, and the &quot;physically&quot; is meant here to say &quot;experimentally provable&quot;. Question:  Do photons take all paths or not?
6880
One of the problems with Bohr's theory to describe the hydrogen atom, was that the electron orbiting around the nucleus has an acceleration. Therefore it radiates and loses energy, until it would collapse with the nucleus.  Now Schrödinger describes the electron as a wave function. His theory is able to describe all atoms (in contrast to Bohr's model), but how is the radiation problem solved? I understand that the wave has not an exact position in the time anymore. But the electron still "moves", so it has an acceleration anyway (because of vibrations or so).   Why is in this theory the electron not radiating anymore? And if it is, why doesn't the atom collapse?
104718
My somewhat basic understanding of the concept comes from lectures I've attended about the Bohr-model, which explains the phenomenon as arising from the fact that certain configurations of an atom can only absorb certain wavelengths of light and other configurations can emit the same wavelength and change into the first configuration.  Now what I cannot understand is why these effects do not cancel out, and why in some instances absorption wins out and we observe absorption lines, and in other instances it is the other way around. Also, if I haven't misunderstood there are also some instances where we observe both absorption and emission at once.  So my question boils down to this: Why?
81619
As is usually said, measurement of an observable $q$ leads to collapse of wavefunction to an eigenstate of the corresponding operator $\hat q$. That is, now the wavefunction in $q$ representation is $\psi(q)=\delta(q-q_0)$ where $q_0$ is result of measurement.  Now, in reality measurements are never exact, so the wavefunction should not have that sharp peak. Instead it should be broadened depending on precision of measurement. In this case, can we still introduce an operator, an eigenstate of which would the new wavefunction appear? Is it useful in any way? Or does the new wavefunction depend too much on the way it was measured so that each instrument would have its own operator? How would such opeartor look e.g. for a single-slit experiment?
47253
Is there a phenomenon visible to the naked eye that requires quantum mechanics to be satisfactorily explained? I am looking for a sort of quantic Newtonian apple.
3247
From what I understand, the  states that there is a fundamental natural limit to how accurately we can measure velocity and momentum at the same time. It's not a limit on equipment but just a natural phenomenon.  However, isn't this just an observational limit? There is a definite velocity and momentum, we just don't know it. As in, we can only know so much about the universe, but the universe still has definite characteristics.  Considering this, how do a wide range of quantum mechanical phenomena work? For example, quantum tunneling - its based on the fact that the position of the object is indefinite. But the position is definite, we just don't know it definitely. Or the famous light slot experiment? The creation of more light slots due to uncertainty of the photon's positions?  What I am basically asking is why is a limit on the observer, affecting the phenomenon he is observing? Isn't that equivalent to saying  because we haven't seen Star X, it doesn't exist? It's limiting the definition of the universe to the limits of our observation!
120681
I'm trying to understand quantum physics. I'm pretty familiar with it but I can't decide what counts as observing to cause particle behave (at least when it's about lights). So the question is what do we see with our eye-balls?   We point a laser (or any kind of light source) to the wall. We see its way from point A to B. Do I "see" a particle or a wave? Let's see an average object. It pretty much looks like their "pieces" a observing that influences their behave. Does this means while we're watching a light it acts like particle in quantum level?
53904
Schroedinger's equation, as we all know, is time symmetric. In quantum field theory, we have to come up with a more sophisticated CPT reversal, but the essential point remains unchanged. However, the collapse of the wave function in the Copenhagen interpretation is manifestly time asymmetric. Correct me if I'm wrong, but can you uncollapse a wave function, converting it from an eigenfunction to a superposition of eigenfunctions?  Is this asymmetry connected with the thermodynamic arrow of time and the second law in statistical mechanics, or are they independent? How would an uncollapse look like, and can we experimentally arrange for an uncollapse? Why are there more collapses than uncollapses? If an observer unobserves a quantum thingie, does that thingey uncollapse?
19391
I bet the automatic response to my question would be "Bell's theorem" and of course I am not disputing Bell's proof. I am however uncertain of one of his assumptions.  The so called "no conspiracy" assumption states that we somehow posses this magical thing called free will which makes us able to break free from the causal chain of events that make us measure the $y$-axis instead of the $x$-axis and so on in experiments. Obviously this makes little sense once reviewed under the light of logic. What this naturally implies is that, if a theory is fully deterministic, it is instantly super-deterministic.  Simon Kochen and John Conway published a theorem in 2009, , which simply-put states: either everything is deterministic (super-deterministic) or every particle has free will.  Among the few who accept this to the fullest extent is Gerard 't Hooft. He has proposed that what we call sub-atomic particles are really just templates and that there is a realm beneath the quantum where the true fabric of reality is "hidden". By accepting determinism he escapes Bell's theorem (due to not having to accept the "no conspiracy" assumption) and can construct a local, deterministic and realist hypothesis of reality. In Gerard 't Hooft's model there is cellular automata somewhere near the Planck scale which gives rise to the "emergent quantum mechanics".  This means that there exist no superpositions in objective reality, the cat is always either dead or alive, no collapse of wave functions, no branching universes, non-locality or retro-causality.  The big question for me is basically: Why do people have such a hard time accepting determinism? Some will object and say "oh but it's SUPER-determinism", but that makes no sense whatsoever, either everything is determined (determinism) or only somethings are (quasi-determinism). By accepting it we escape Bell's theorem. Bell himself was well aware of this and mentioned it a few times in interviews in the 80's.  I know some are concerned that if we accept complete determinism we can no longer do science because it's all a HUGE CONSPIRACY. Gerard 't Hooft has answered critics who bring up this here: , section 6.  Additionally, another author who has taken a deeper look into the worries of "conspiracies" in QM interpretations is Peter J. Lewis in his paper  (also ).  So to all of you who dismiss local hidden variables I ask: Why?