I've been reading up on the  of Quantum Mechanics, and there is one thing (among many) that I really don't understand. How many worlds are 'created' by an 'observation' or 'event'?  For example, in the traditional Schrödinger's Cat experiment a cat is put into a box with a poison flask that has a chance of breaking, with the cat dying if it does. But how many worlds are created by this? If the probabilities are even I could understand two worlds being created. But what if the flask has a 70% chance of breaking? Are ten worlds created, in seven of which the cat is dead? Or are an infinite number created, but in 70% of which the cat is dead? Or are just two created, but you somehow have a 70% chance of ending up in the world with the dead cat? Or is it something entirely different?
122320
When I read descriptions of the many-worlds interpretation of quantum mechanics, they say things like "every possible outcome of every event defines or exists in its own history or world", but is this really accurate? This seems to imply that the universe only split at particular moments when "events" happen. This also seems to imply that the universe only splits into a finite number of "every possible outcome".   I imagine things differently. Rather than splitting into a finite number of universes at discrete times, I imagine that at every moment the universe splits into an uncountably infinite number of universes, perhaps as described by the Schrödinger equation.  Which interpretation is right? (Or otherwise, what is the right interpretation?) If I'm right, how does one describe such a vast space mathematically? Is this a Hilbert space? If so, is it a particular subset of Hilbert space?
109510
I'm fascinated by the fundamental questions raised by the Double Slit Experiment at the quantum level. I found this  which seems like a great explanation. But is it scientifically accurate?
104200
There's an 80% probability of a certain outcome, we get some new information that means that outcome is 4 times more likely to occur.  What's the new probability as a percentage and how do you work it out?  As I remember it the question was posed like so:     Suppose there's a student, Tom W, if you were asked to estimate the   probability that Tom is a student of computer science. Without any   other information you would only have the base rate to go by   (percentage of total students enrolled on computer science) suppose   this base rate is 80%.      Then you are given a description of Tom W's personality, suppose from   this description you estimate that Tom W is 4 times more likely to be   enrolled on computer science.      What is the new probability that Tom W is enrolled on computer   science.   The answer given in the book is 94.1% but I couldn't work out how to calculate it!  Another example in the book is with a base rate of 3%, 4 times more likely than this is stated as 11%.
63732
I am reading Berkeley Physics Course vol. 4 (Quantum Mechanics) , chapter 4 (photons). (1)  Section 46: book says: consider a typical photon emitted by  the source. It can be regarded as a a wave train of finite duration, spreading out in all directions in space and carrying a total energy $\hbar\omega$. If this is true, a photon emitted by a star which is few light years away, would have spread over a sphere of radius of few light years. Then it is not possible to &quot;detect&quot; that photon using a detector.  Then  why does the book say like this?  In section 47, the book gives same example of photon spreading over light years. But in this section, it says that how the people can use this as a paradox; and clarifies that paradox by saying that classical expression for &quot;energy density&quot; refers to a large number of photons, and not for single photon.  So does photon really spread in all directions?   (2)  In Section 38 book says: almost monochromatic photons can not be split into 2 photons of the same frequency which carry only a fraction of the energy of the original photon. While in section 48, it says (with reference to double-slit diffraction experiment) photon came through BOTH the slits.  Does a photon REALLY goes through both the slits? Doesn't it indicate that a single photon can be split into two photons of same frequency ?   Any help will be appreciated. Thanks
94504
It is my understanding of the double-slit experiment that when a measuring device is activated, to measure which of the two slits a particle is travelling through, this measuring is responsible for a different outcome than when the device is left off. When the device is left off particles leave an interference pattern, suggesting a single particle (or its wave) interferes with itself; when the device is activated no interference pattern is left, and particles are observed to travel through a single slit.  Now, does all this imply that this experiment is typically carried out in a vacuum? If not, how is it that interaction with other particles in the environment of the experiment's set-up are not already influencing the outcome, regardless of whether the measuring device is on or left off?  This question may very well be a very naive one, due to my very limited understanding of particle interaction, vacuums and/or physics in general. If so, I would appreciate a brief, layman's, overview of where my understanding of the different subjects at play here is likely to be lacking.
74910
I was looking at a website that claims to generate random numbers from observation of quantum effects. This lead me to question how we know that the numbers are truly random.  When we observe a probability wave and it collapses in one place into a particle, how do we know that the location of the particle is really random?  Do we have any evidence of the randomness, or is it just that no one can predict the location right now?
13338
Fundamental notions of QM have to do with observation, a major example being The Uncertainty Principle.    What is the technical definition of an observation/?  If I look at a QM system, it will collapse. But how is that any different from a bunch of matter "looking" at the same system?  Can the system tell the difference between a person's eyes and the bunch of matter?  If not, how can the system remain QM?  Am I on the right track?
105701
I have heard a few times that one way of describing quantum computers is that they essentially use the computing power of their counterparts in alternate realities that they access through superposition. My first question is, of course,    is this actually an accurate description of how quantum computers work, or just a misrepresentation?   Also, if it were to be assumed true and taken literally, then presumably all the possible outcomes of any given computation done would be experienced by each of these alternate realities. I have a few questions regarding the implications of this:   Would it be correct to say that our universe essentially "spawns" these alternate realities to use for each computation, or that they all exist simultaneously in a higher dimension and only come into contact via the initiation of superposition by each computer at the same time? Would it be possible to create contact between universes via this connection? I'd expect that, if it was, it would be entirely random, and so not only would not cause any information transfer, it also would be completely undetectable, because the deviations would be within the expected probabilities of error rates - but is there any way to circumvent this?
93821
PART 1: I was reading the article  Wikipedia. I couldn't understand this line:       "if the two events are causally connected ("event A causes event B"), the causal order is preserved (i.e., "event A precedes event B") in all frames of reference."     Is this an assumption or a consequence of STR? Please explain.       Note: My question consists of 2 parts this is the 2nd part. Below is a genral version of my previous question question:.   PART 2: Let there be three events $A$,$B$ and $C$ s.t: $C$ is the result of Simultaneous occurrence of $A$ and $B$. In other words $C$ occurs iff $A$ and $B$ are simultaneous. Now as we know in STR any two events separated in space are not simultaneous in different frames. So In some frames $C$ will occur and in some $C$ will not occur which will cause paradox. I tried many thought experiments to make such a paradox but i failed. In all the experiments that i thought i could not break the causality even by breaking the simultaneity because everytime the fact: "all signals move slower than light" preserved the causality.    So why causlity remains preserved always? Is it due to the fact that nothing can move faster than light?
32375
I haven't yet gotten a good answer to this: If you have two rays of light of the same wavelength and polarization (just to make it simple for now, but it easily generalizes to any range and all polarizations) meet at a point such that they're 180 degrees out of phase (due to path length difference, or whatever), we all know they interfere destructively, and a detector at exactly that point wouldn't read anything.  So my question is, since such an insanely huge number of photons are coming out of the sun constantly, why isn't any photon hitting a detector matched up with another photon that happens to be exactly out of phase with it? If you have an enormous number of randomly produced photons traveling random distances (with respect to their wavelength, anyway), that seems like it would happen, similar to the way that the sum of a huge number of randomly selected 1's and -1's would never stray far from 0. Mathematically, it would be:  $$\int_0 ^{2\pi} e^{i \phi} d\phi = 0$$  Of course, the same would happen for a given polarization, and any given wavelength.  I'm pretty sure I see the sun though, so I suspect something with my assumption that there are effectively an infinite number of photons hitting a given spot is flawed... are they locally in phase or something?
121972
It was mentioned to me that it can be shown that there is no classical explanation for the uncertainty in Quantum Mechanics -- i.e. that there are no hidden workings that we have just not yet seen, which could be explained classically and would explain the probabilistic nature of Quantum events in a 'deterministic' fashion.  Can someone explain how this is known please?
36330
I've been reading about different ideas of getting the observer out of quantum theory. One Sean Carroll seems to like is Everett's many worlds concept.  I had a thought that I need someone here to shoot down. It could be something as basic as the double slit experiment that proves me to be wrong. Here's my thought:  As I see it, we never really "measure" the position of something. We sense a photon that is emitted by what we are "looking" at. Why couldn't the wave function of a particle just break down at the moment it emits the photon at a specific point in space-time regardless of whether there is an observer or not. After the emission, the particle location would go back to being a probability function.  What am I missing?
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?
53530
I often think about problems that require an understanding of the very essence of computation and its inherent limitations. So, my questions are as followed:   Is the universe isomorphic to a universal turing machine? Is the universe isomorphic to a universal quantum computer? What evidence do we have within our universe to support an answer to such a question? What corollaries follow from a universe that is or is not isomorphic to a universal turing machine (or universal quantum turing machine)? Is this question really answerable, and what is the current literature on this subject?   I'm mostly interested in the implications and corollaries that follow from a universe that is or is not isomorphic to a turing machine (or universal quantum computer) and the insights that it would provide us with, about the nature of computation.
94690
If I have $1$ fish in the beginning, and each fish can do the following things:   Dies with a probability of $1/3$. Gives a birth to $1$ fish and dies with a probability of $1/3$. Gives birth to $2$ fishes and dies with a probability of $1/3$.   The question is:&nbsp;The population goes extinct if none remains in the $i$ generation.  What is the  exact probability that the population goes extinct&nbsp;?.    So I thought about the following: in the first step, if the fish dies, then its over. $1/3$ probability  if it dies in the second step, then the first fish dies and gave a birth to 1 fish so its $1/3$ * $1/3$  if it dies in the third then we have a couple of options so i got confused here.. it can either give birth to 1 each time and then die or can give birth to 2 fishes and then in the second step 1 dies and 1 gives birth to 1 and dies so we will have 1 fish, and then it will die. but it can also be couple of other options (such as we have 2 fishes and all of them just die)  Would like to get some help with that because i feel like i got lost. Thanks!
111757
I just finished the book of Lawrence Krauss "Universe from nothing" and he said that virtual particle can pop to existance from nothing by quantum fluctuations of the field from the vacuum and in the same way our universe can pop to existence, but my question is..  How this quantum fluctuations works? If the space-time began to exist with universe how can this quantum fluctuations generate universe? Doesn t them requer space-time in the first place?? The book doesn t answer that..  I dont get a answer also on Yahoo
78160
I just recently watched  where they were explaining the double slit experiment.  I understand that the interference pattern would be observed if you send photons or electrons one after the other. The conclusion that the author seems to make is that it is as though the particle/packet of energy travels through both the slits after somehow splitting into two.  Has anyone done an experiment where they send just one particle? (And end the experiment) Do you observe two points on the screen in that case or just one?