I know I am missing something and this question is probably very silly, but I would like to understand. Quoting an article:     If one photon is measured to be in a +1 state, the other must be in a   -1 state. Since the outcome of one photon affects the outcome of the other, the two are said to be entangled.......when you measure the state of one photon you immediately know the state of the other....If we’re light years apart, we each know the other’s outcome for entangled pairs of photons, but the outcome of each entangled pair is random (what with quantum uncertainty and all), and we can’t force our photon to have a particular outcome.   I just cannot see the "magic" here. Using a stupid analogy: There are two balls, black and white, wrapped in a piece of cloth. You take one and I take the other. Whenever and wherever I unwrap the one I took, I will immediately know which one you have.  What is so special about that in the world of particles, how does the outcome of the first affect the other?
97709
I'm struggling to understand the concept of quantum entanglement. I've distilled my understanding into an analogy, and I need your help to validate it. Here it is:  Let's say I receive two envelopes. Both envelopes have this written on them:     Hello! Open this letter...   If the paper inside is red, then other envelope has blue paper.   If the paper inside is blue, then other envelope has red paper.   So, if I open one letter, and the paper inside is red, then I know the other letter contains the blue paper.  Is this equivalent to quantum entanglement? The fact that I will know the color of the other paper when I open (i.e. observe) one envelope?  In theory, if one of the letters "appeared" in a different galaxy, I would still know the color of the paper inside that letter, just by opening my letter, instantly, correct?  Before I open the letter, is the paper inside each envelope in superposition (??), i.e. 50/50, red or blue?  In this analogy, I am assuming that what's written on the letters is true. Is this a correct assumption in quantum physics? I think so yes?  In real quantum physics, would I be able to change the color of one paper, so that I change the color of the other one? No right?
43121
I understand that faster-than-light communication is impossible when making single measurements, because the outcome of each measurement is random. However, shouldn't measurement on one side collapse the wave function on the other side, such that interference effects would disappear? Making measurements on "bunches" of entangled particles would thus allow FTL communication, by making observed interference effects appear or disappear. How does such an experiment not:  1) Clearly imply that faster-than-light communication is possible?  or  2) (if #1 is rejected) Imply that measurement of one half of an entangled pair does not cause the collapse of the other half's wave function.  Why doesn't this thought experiment clearly show that if we maintain that FTL communication is ruled out, we must also rule out "universal collapse" in the Copenhagen interpretation?  EDIT: Here is an example of an explicit experiment (though I think experts could come up with something better):  You can entangle a photon with an electron such that the angle of the photon is correlated with the electron's position at each slit of a double slit experiment. If the photon is detected (it's outgoing angle measured), then which-path information is known, and there is no interference. If the photon is not detected, the interference remains.   The experiment is designed such that the photon and electron go in roughly opposite directions, apart from the tiny deflection which gives which-path information. You set up a series of photon detectors 100 ly away on one side, and your double slit experiment 100 ly away in the opposite direction. Now you produce the entangled pairs in bunches, say of 100 entangled pairs, each coming every millisecond, with a muon coming between each bunch to serve as a separator.   Then the idea is that someone at the photon detector side can send information to someone watching the double-slit experiment, by selectively detecting all of the photons in some bunches, but not in others. If all of the photons are detected for one bunch, then the corresponding electron bunch 200 ly away should show no interference effects. If all of the photons are not detected for one bunch, then the corresponding electron bunch 200 ly away would show the usual double-slit interference effects (say on a phosphorus screen). (Note that this does not require combining information from the photon-detector-side with the electron-double-slit side in order to get the interference effects. The interference effects would visibly show up as the electron blips populate the phosphorus screen, as is usual in a double-slit experiment when which-path information is not measured.)  In such a way the person at the photon detectors can send '1's and '0's depending on whether they measure the photons in a given bunch. Suppose they send 'SOS' in Morse code. This requires 9 bunches, and so this will take 900 milliseconds, which is less than 200 years. The point is that such an experiment would only work if you assume that the measurement of the photon really does collapse the wave function nonlocally.
104400
Can two particles remain entangled even if one is past the event horizon of a black hole? If both particles are in the black hole?  What changes occur when the particle(s) crosses(cross) the event horizon?  I have basic Physics knowledge, so I request that answers not assume an in-depth understanding of the field. Thank you all in advance!
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!
73681
new finding by Minev et al. seems to suggest that transitions between atomic states are not instantaneous, but continuous processes wherein a superposition smoothly adjusts from favoring one state to another (if I understand it correctly). The authors also claim to be able to catch a system "mid-jump" and reverse it. Popular articles are  and .  I am curious if this finding rules out any interpretations of QM. It seems to generally go against the Copenhagen attitude, which describes measurements as collapsing physical systems into a definite classical state. The popular articles indeed claim that the founders of QM would have been surprised by the new finding.  The link with the asterisk mentions that something called "quantum trajectories theory" predicts what was observed. Is this an interpretation, or a theory? And are they implying that other interpretations/theories don't work?
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!
73200
According to special relativity, a photon traveling at the speed of light, does not “experience” time. It is traveling into the future at an infinite rate, right?  That would mean that everything that could ever happen (my death, Trump winning the election, death of sun, end of the universe), has already happened (from the frame of a photon). We're just too "slow" to experience this.  Is this correct? Because that would bring some weird implications about free will, and the very nature of our existence. If everything has already happened, then we're just following a predetermined path.  Edit 1: I'm genuinely fascinated by this topic, however, I am also an idiot with very superficial knowledge, gained mostly from youtube videos and Wikipedia. Bear that in mind :)  Edit 2: The nature of my question seems to leave the realm of physics and cross into a philosophical issue. I was seeking physical explanation, to help answer something of philosophical nature, as both these realms seem to coincide for this particular topic.   Some have pointed out that thinking about the frame of a photon doesn't make sense. Instead, one could assume a spaceship traveling at $0.9999c$.
4725
According to the special theory of relativity, distant simultaneity depends on the observer's reference frame.  And, according to the quantum theory, in the case of two entangled particles, a measure on one of the particles simultaneously affects the second one. Under which reference frame is this simultaneous?
65189
discusses the recent invention of a "quantum radar" that works by bouncing one photon off of something, storing a photon that's entangled with it, and watching for interference on the stored photon that indicates the radar photon has bounced off an object.  Most of that makes sense, (about as much as anything in quantum physics makes sense, at least,) but... how exactly do you "store" a photon?  To my mind at least, the concept of storage involves putting something in a location and keeping it there, unmoving, but photons are not capable of not moving at the speed of light.  So what's going on in this scenario?
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?
11374
Thought experiment:   Entangle a pair of particles.  Package one half of each pair in a special package, and send it out in a nano-spaceship keep the other half of the pair of entangled particles here on earth accelerate nano-spaceship to 99.99999999999999% of the speed of light using a number of    Due to Lorentz ,  $$ L = L _\text{0} \sqrt {1 - \frac {v^{2}}{c^{2}}} $$  In one years time, the spaceship would have traveled 93B ly away from us (in the spaceships relative frame)  After one year of travel, the spaceship could be preconfigured to detect the CMB in the direction of travel, and then trigger their quantum entangled particle to send a '0' or a '1' depending on whether the CMB was detected.  Repeat this process using any number of spaceships in any number of directions, and soon you would experimentally show how big the universe really is.  Any problems?
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?
13660
I've heard this pop up in a discussion with my physicist/engineer roommates, but didn't care to ask at the time.  Now I'm mighty curious about it.   doesn't really seem to say much on this issue.  From what I understand about the Uncertainty Principle, it says that there are certain properties of electrons and stuff that cannot be measured, and are therefore uncertain.  Then Wikipedia (under ) states that Sir Arthur Eddington says that the Uncertainty Principle isn't really so because we can't measure these properties, but because turns out nature is indeterministic.  At least that's what I took from those paragraphs.  Even without my biased wording, it sounds more like an assertion than evidence.  I've also read a few things about how other scientific conventions perceive the issue, like how a ball on the peak of a perfect mound might randomly roll down in any direction, and I'm still unconvinced.  My belief of determinism is generally that if you knew every single variable that existed as a factor at the very beginning and birth of the universe, you could correctly determine all properties of any individual particle at any point in time.  Could anyone provide some more background about this?  Especially regarding quantum mechanics?
42729
We've learned that the wave function of a particle collapses when we measure a particle's location. If it is found, it becomes more probable to find it a again in the same area, and if not the probability to finding it in the place that was checked decreases dramatically.  My question is about the definition of measurement. What makes a measurement different from any other interaction between two particles (gravity and EM fields for example)?  In reality, almost every particle interacts with any other particle, so shouldn't there be constant collapse of the wave function all the time? If this happens we're right back in classical mechanics, aren't we?
69377
I am currently trying to learn some basic quantum mechanics and I am a bit confused. Wikipedia defines a  as a quantum of light, which it further explains as some kind of a .     What exactly is a quantum of light?   More precisely, is a quantum of light meant to be just a certain number of wavelengths of light (something like "1 quantum = a single period of a sine wave" perhaps?), or is the concept completely unrelated to wavelengths? In other words, how much is a single quantum?
76290
My confusion about quantum theory is twofold:    I lack an adequate understanding of how the mathematics of quantum theory is supposed to correspond to phenomena in the physical world I still have an incomplete picture in my mind of how cause and effect relationships occur at the quantum level of reality.    This is why phenomena such as "entanglement" make absolutely no sense to me. So, in an attempt to come to some understanding of all of this, I would like to know that if what we conceptualize as a "field" is merely an interaction among particles, and particles themselves are actually fluctuations in "fields", then which comes first, particles or fields?
36793
Consider that we have two balls, one white and one black, and two distant observers A and B with closed eyes. We give the first ball to the observer A and the second ball to the observer B. The observers don't know the exact color (state) of their balls, they know only the probability of having one or another color, until they look at them (measure). If the observer A looks at his ball he will see its color, which is white, so he immediately knows the color of the second ball. Lets call this “classical entanglement”.  My question is: What is the difference between this “classical entanglement” and the quantum entanglement, for example, of two entangled electrons with opposite spins states? Can this analogy be used to explain the quantum entanglement?