In February of this year, articles started popping up describing the results reported in this paper by Mahler et al:   Taken from  article discussing the results, the following claims are related:     "Indeed, Steinberg and colleagues identified cases in which photon-1   begins on a trajectory from the lower slit, but then swerves upward   into a trajectory that appears to be from the upper slit. Using a   technique called quantum-state tomography, they were able to monitor   the polarization of photon-2 during this swerve, and saw its value   rotate from horizontal (indicating the lower slit) to vertical   (indicating the upper slit). As a result, a measurement on photon-2 at   the end of the trajectory gives the "wrong" slit."   While any theory that wants to be taken seriously as an alternative to Quantum Mechanics must agree with validated experimental predictions /results of QM (and indeed this study was done specifically to address predictions of BM that seemed at odds with QM), I'm having trouble seeing how the quoted series of measurements above can be reconciled with QM as I understand it.  Meaning:   A) My understanding of how QM would describe the above series of measurements is lacking (most likely)   or   B) the above series of measurements imply which-way knowledge while preserving interference can be demonstrated.  Which one of these seems to be the case, and can you help me view these results within the framework of classical QM (no pun intended)?    EDIT:   After reading the linked question marked duplicate, the answers do no address my specific question of how the authors claim to show which-way information while preserving interference is either erroneous, or in keeping with the validated postulates of QM.  Being peer-reviewed, I'd assume they (and their reviewers) at least believe they are on somewhat stable ground to make these claims.
120718
The following paper was recently featured in a German science magazine (Spektrum der Wissenschaft):  (DOI:10.1126/science.1501466)  The abstract reads     Weak measurement allows one to empirically determine a set of average trajectories for an ensemble of quantum particles. However, when two particles are entangled, the trajectories of the first particle can depend nonlocally on the position of the second particle. Moreover, the theory describing these trajectories, called Bohmian mechanics, predicts trajectories that were at first deemed “surreal” when the second particle is used to probe the position of the first particle. We entangle two photons and determine a set of Bohmian trajectories for one of them using weak measurements and postselection. We show that the trajectories seem surreal only if one ignores their manifest nonlocality.   To what extent does this show Bohmian mechanics is correct in the sense that it explains things normal QM does not explain?  Unfortunately I had to realize I don't know enough about the subject to understand the full paper. I would just like to know if they actually claim to have experimentally shown that an interpretation of QM is distinctly different from standard QM.  I am particularly asking in light of thoughts like .  EDIT  I first accepted the answer given by @Timaeus below. There are two reasons I removed the acceptance tick again:   I discussed with a friend who knows a lot more than me about weak measurements. He said they are not really completely understood yet, nevertheless give surprising empirical results. These seem to be hard to reconcile with the standard interpretation of quantum mechanics, though a lot easier to reconcile with things like Bohmian mechanics. It is hard to read from the papers if it actually shows distinguishing features between the interpretations. Now Timaeus argued that they can't because the "interpretations" by definition only predict the same results. Well, apparently they don't so I will repeat my question slightly differently: Does this paper show that Bohmian mechanics is correct and that the standard interpretation is not? There has been  by the same group that in fact won the "Breakthrough of the year" award. From the abstract:      A consequence of the quantum mechanical uncertainty principle is that one may not discuss the path or “trajectory” that a quantum particle takes, because any measurement of position irrevocably disturbs the momentum, and vice versa. Using weak measurements, however, it is possible to operationally define a set of trajectories for an ensemble of quantum particles. We sent single photons emitted by a quantum dot through a double-slit interferometer and reconstructed these trajectories by performing a weak measurement of the photon momentum, postselected according to the result of a strong measurement of photon position in a series of planes. The results provide an observationally grounded description of the propagation of subensembles of quantum particles in a two-slit interferometer.
121521
Yeah, I haven't quite understood, or been told, what happens to, for example an electron and its wavefunction, when you stop to measure it.  I mean, an electron has a wave function describing its position and so on in the region of space it is confined. When I measure it, I get an output, and the wavefunction collapses - as far as I understand. But then, when I stop measuring, the electron has not been "destroyed" has it? So does it resume its original wavefunction, and continue as nothing has happened, or have I totally misunderstood something? :)
51015
I'm a mathematician, so I am not familiar with experimental physics.  I am watching MIT course on Quantum Mechanics, and they describe the two-slit experiment with electrons passing through the slits.   Here is my question: How do we measure the position of an electron with a light source?  AND, why does longer wavelength mean we can't be as accurate in our measurement of the position.  What is the apparatus?  How does this actually work in a lab?  What I describe below is described in the video at 59:50 ()  They talk about trying to "cheat" and observe which slit the electron went through.  They say to shine very weak light at the electrons before they pass through the slit.  Sure, the light will impart some momentum and alter the path of the electron a little bit, but if the light is low energy enough, we could perhaps know which slit it went through, yet not change the diffraction pattern by very much.  Great!  Except weak light means low frequency, long wavelength.  Then they say that long wavelength means we can't tell which slit the electron went through.  We need wavelength on the order of the distance between the two slits to know which slit it went through.  But increasing frequency by this amount is just enough to wash out any interference pattern at all, and so our experiment fails.  My question:  How do we measure the position of an electron with a light source?  AND, why does longer wavelength mean we can't be as accurate in our measurement of the position.  What is the apparatus?  How does this actually work in a lab?
69284
When quantum entanglement is explained in "layman's terms", it seems (to me) that the first premise, that we have to accept on faith, is that a particle doesn't have a certain property (the particle is not IN one state or another) until that property is measured.    For example, I have read that a particle may have "spin up" or "spin down" but -- here's where I get lost -- it's not that we don't KNOW which spin the particle has until we measure it, but actually the particle has NEITHER spin until we measure it.  Now, if we accept that premise, then we can see how spooky it is, when two particles are created with correlated spins -- if one has spin up, the other has spin down, but neither particle "has" either spin until one of them is measured.  At that instant, the other particle "assumes" the other spin.   At least that's how I interpret things I have read.  Certainly, if we accept that a particle doesn't actually HAVE the property until it is measured, but rather the particle exists in a superposition of states until the measurement is made, then we "regular citizens" can understand what is meant by "spooky action at a distance".  (As soon as one particle is measured, the other particle assumes the "other" spin, no matter how far away it is.  How did the second particle know the first particle was measured?  That, I think, is the spooky action at a distance (and the information transfer can be faster than light).)  However, from a layman's perspective, I want to cry out "but the particle does have a definite spin, we just don't KNOW what it is, until it is measured!  Duh!"  Now, I can't believe that all of the physics community hasn't thought of that objection -- but -- here's my point -- although the spooky action at a distance can be explained to us, once we accept the premise of superposition -- Why isn't the fact of superposition explainable also, in terms mortals can understand?  I have looked, but I haven't found a layman's explanation of why we should accept the fact that a particle doesn't have a particular spin, or other property, until it's measured.  It sure seems that the particles "have" the properties, even though we haven't measured them yet.    I know that much of quantum mechanics is not "intuitive".  If anyone can explain why particles don't have a definite property, even before we measure them, I would be grateful.
43959
Recently, I have heard of extensions of the double slit experiment that propose to use a bacterium as the particle. I've also heard that the double slit experiment, in principle, would still show an interference pattern if it were done with a human or a car, as long as there is no observer.  This made me wonder: if this is true, and I exist in a superposition, do I have a mass which exists in several locations at the same time and with an associated weight of 200lbs in each of my superposition locations?  If that were true and a fact, wouldn't that account for the missing mass which we attribute to dark matter? Also, even if on some other level this is true about particles only, wouldn't this be where we would find a link between quantum mechanics and gravity, linking it with relativity?  Forgive me if this is a silly question as I am almost 63 years old and have been almost 50 years out of school, but I just love to read and think about science and learn as much as I can.
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?)?
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
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?
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?
19156
When a photon of light hits a mirror does the exact same photon of light bounce back or is it absorbed then one with the same properties emitted? If the same one is bounced back does it's velocity take all values on $[-c,c]$ or does it just jump from $c$ to $-c$ when it hits the mirror?  Or, is the phenomenon of a mirror better explained using a wave analogy? If so, what is this explanation?
92568
I'm curious about QM and spent the last 3 days thinking about Delayed Choice Quantum Eraser (DCQE) experiment, but I couldn't solve this issue:   Assume we do the DCQE experiment so that for the whole experiment (let's say, 1000 particles) finishes before the first particle reaches the detectors 1-4 (using the notation from ). Assume also we're using a switch to control if the idler particle goes to which-way-detector or to the eraser.  shows what we should see in D0, even though we don't know yet R01-04 Now, knowing D0, use the switch to send the 100 particles that reached the most-left side of D0 to the eraser detectors (D1 and D2). It seems much more likely that they reach D2 than D1, because it's on peak on D2 (and on rest on D1) even though it's going through a half-silvered mirror. This looks like a contradiction to me.   So, am I missing something or QM is just that weird?
68703
I've always hit two big stumbling blocks in conceiving of the proof or disproof of  as being even valid idea, let alone an answerable question... I feel I must be misunderstanding some very fundamental things.   Quantum mechanics is deterministic, ignoring the bit where we take our complex number and fudge it into a probability. We use computers to make calculations to provide the predictions of quantum mechanics - how is this not itself a hidden variable theory?   From what I understand the famous Bell theorem and work around it doesn't disprove hidden variable theories, just one specific flavour of them, and even then I am not convinced. I feel like I must be missing something again, because this result seems to me in no way profound... although it certainly fits into the picture of the EPR 'paradox' and is a good demonstration of the validity of QM.  I guess the crux of my problem is that this 'area' really addresses wave function collapse and the problems it causes, but I don't see the need for some collapse mechanism to begin with... again I feel like I am missing something really fundamental.  Apologies if this is a bad question. I have had a look around and there are similar questions, but I don't feel that either of my points are addressed...
73549
I understand how one associates the spin of a quantum particle, e.g. of a photon, with intrinsic angular momentum. And in electromagnetism I have always understood the polarization of an EM wave as the oscillations of the E and M field, not necessarily being  aligned with the direction of propagation of the wave.     Questions:   But when one talks about the polarization of a photon in Quantum Mechanics, how does it really differ from its spin?  Are they somehow related and what's the physical idea behind photon polarization in contrast to photon-spin? Feel free to use mathematical reasonings as well if you see it fit!
78737
I tried searches and the question has been posed in other fora, but no experiment came up.   Track chambers (cloud chambers, bubble chambers , time projection chambers, solid state detectors like the vertex detectors at LHC)  give the track of the particle as it ionizes the  medium, and could be carried out in geometry after the particle has passed the double slit. The straight track should be pointing back to the slit it came from and its record could be used as the points on the screen in the classical double slit experiment.  The set up as I see it would be the classical setup for single electron through the double slits but instead of a "screen" one has a detector and detects the track.  It should be a long enough detector to get an accuracy less than the slit difference so it could point back to the slit, as the interslit distance is of the order of 100 microns and detectors are giving order of microns accuracies.  This experiment, if possible,  would resolve the controversy whether the detection of the slit destroys the interference pattern or the  at the slits change the boundary conditions and destroy the interference pattern.  An expert's opinion is necessary whether the experiment is possible, whether the energies of the electrons to show interference with a specific d separation is enough to create an accurate track in a solid state detector. If not a cloud chamber would do , but again the energy of the electron would be important because it would have to pass the barrier air/chamber.  It could succeed if the double slits were within a cloud/ bubble chamber; the beam count was low  (10 to twelve per picture) but it was spread in the vertical direction. If the beam could be focused on the slits , it should be doable.
89742
Quantum Mechanics is very successful in determining the overall statistical distribution of many measurements of the same process.  On the other hand, it is completely clueless in determining the outcome of a single measurement. It can only describe it as having a "random" outcome within the predicted distribution.  Where does this randomness come from? Has physics "given up" on the existence of microscopic physical laws by saying that single measurements are not bound to a physical law?  As a side note: repeating the same measurement over and over with the same apparatus makes the successive measurements non-independent, statistically speaking. There could be a hidden "stateful" mechanism influencing the results. Has any study of fundamental QM features been performed taking this into account? What was the outcome?    Edit: since 2 out of 3 questions seem to me not to answer my original question, maybe a clarification on the question itself will improve the quality of the page :-)  The question is about why single measurements have the values they have. Out of the, say, 1000 measure that make a successful QM experiment, why do the single measurements happen in that particular order? Why does the wave function collapse to a specific eigenvalue and not another? It's undeniable that this collapse (or projection) happens. Is this random? What is the source of this randomness?  In other words: what is the mechanism of choice?    Edit 2: More in particular you can refer to chapter 29 of "The road to reality" by Penrose, and with special interest page 809 where the Everett interpretation is discussed - including why it is, if not wrong, quite incomplete.
61421
I'm still struggling a bit with some ideas around double slit experiments. One that keeps cropping up for me is the role of observers.   Imagine a classic double slit experiment with a hidden observer who has arranged an apparatus to detect which slit the electrons are passing through. This person and their measurements are hidden to you and you have no interaction with them.  So the question is, do you see an interference pattern or not?  Additionally: And if the answer is Not, then is the reason because they "disturbed" the electron (by say firing photons at them) or is it for another reason? And if it is because they "disturbed" the electron, then how is it that unobserved electron's are not disturbed since they certainly interact with other objects, for example other atoms in the matter around the slit(s) will feel a slight gravitational tug as it passes through.
11381
In a double-slit experiment, interference patterns are shown when light passes through the slits and illuminate the screen. So the question is, if one shoots a single photon, does the screen show interference pattern? Or does the screen show only one location that the single photon particle is at?