Reputation
Next tag badge:
100/100 score
24/20 answers
Badges
7 206 374
Newest
 fermions
Impact
~3.6m people reached

Apr
25
comment In the many worlds interpretation of q.m. what makes a superposition af states split in two sepearate ones?
So there can't exist any "universal observer" in quantum mechanics - that would be equivalent to making quantum mechanics independent of observers but quantum mechanics simply cannot be formulated independently of observers. These are the basic features that Heisenberg and pals had to realize in the early 1920s, the true heart of the new theory, the reason why they (especially Heisenberg and Born and partly Dirac) got their Nobel prizes and why we celebrate them. To deny these things means to deny quantum mechanics. Someone calls this denial "XY interpretation of QM" but it's still the denial.
Apr
25
comment In the many worlds interpretation of q.m. what makes a superposition af states split in two sepearate ones?
The split of the world into two or several worlds with different "classical" outcomes of measurements is always just an approximation, and it's a conscious observer - whether or not we use these spiritually sounding words or any words that just mask it into a more boring language - who must make the decision that he really perceives something, and therefore the approximation may be done. But a more rigorous external observer is always possible who allows all the "blobs" to reinterfere in the future and who can study their interference.
Apr
25
comment In the many worlds interpretation of q.m. what makes a superposition af states split in two sepearate ones?
I thought that you wrote the answer in order to obfuscate the vagueness and pretend that it makes sense! ;-) You know, this idea about their life in different Universes is exactly the feature that quantum mechanics forbids. The point is that the two "blobs" of the wave function may always return to each other and interfere. The double slit experiment makes this point obvious. So every "apparent split" into several blobs may always be reversed in principle. That's why the worlds can't ever exactly split. That's really the basic point of quantum mechanics - interference is possible.
Apr
25
comment In the many worlds interpretation of q.m. what makes a superposition af states split in two sepearate ones?
There exists and there can exist no derivation of the Born rule from something that is fundamentally inequivalent to the Born rule. In particular, all papers talking about many worlds and claiming to have "derived the Born rule" are either demonstrably wrong or demonstrably circular. Observations have to be connected to the maths in some way for a physical theory to exist and the connection saying that (only) probabilities are calculable and how is clearly the most direct, irreducible way how to connect the mathematics with the natural science and it's the way labeled as correct by QM (theory)
Apr
25
comment In the many worlds interpretation of q.m. what makes a superposition af states split in two sepearate ones?
No, quantum mechanics is a physical theory, not (just) a mathematical one. It is primarily the set of rules connecting the mathematical objects to the physical concepts (from the experiments). In particular, the Born rule is an inseparable part of quantum mechanics. It says that probabilities of observations may be predicted and computed as $|c_i|^2$ from amplitudes. No physical theory would exist and no application of the mathematics of QM would be possible without this rule, Born got a Nobel, it is absolutely fundamental and the talk that it may be reintepreted or interpreted away is silly
Apr
25
comment What is the expectation value of the number operator when the vacuum has a VEV?
Dear @NameYYY - the dispersion relation $p^2=m_h^2$ is a relationship that holds for the quantum oscillations around the $h=v$ vacuum expectation value. In other words, the potential $V(h)$ may be approximated by $V(h)=m^2(h-v)^2/2$ for small $h-v$ (near the vacuum). The vacuum expectation value is dictated (or quantified) by the value of $v$ and the mass dictates the value of $m$. These two parameters are completely independent of each other. The vacuum classically has $h(x,y,z,t)=v$ which is translationally invariant and obeys equations of motion. All excitations are made of $p^2=m_h^2$.
Apr
25
comment In the many worlds interpretation of q.m. what makes a superposition af states split in two sepearate ones?
@DirkBruere - again, quantum mechanics does clearly say what "really" happens. It says that nothing "really happens" before the experiment and only probabilities of outcomes may be predicted by physical theories. To contradict these basic claims means to deny quantum mechanics. The word "Copenhagen interpretation" was introduced by Heisenberg in the 1950s (original "Copenhagen spirit" around 1930) and he was sorry about the choice of words already before the book with this phrase came out exactly because he would know it would be abused by deniers of quantum mechanics to claim they can choose
Apr
25
awarded  fermions
Apr
24
comment In the many worlds interpretation of q.m. what makes a superposition af states split in two sepearate ones?
It is easy to see that there can't be any universal well-defined yet experimentally viable rule for your vague words such as "blobs" and "splitting the blobs". Quantum mechanics is important especially in the situations in which the wave function isn't separated to blobs. For example, a brain is a bound state of many nuclei and electrons and its phase space is totally contiguous - there's no "spare room" and there are no blobs in it. But quantum mechanics has to describe what's observed by neurons etc.
Apr
24
comment In the many worlds interpretation of q.m. what makes a superposition af states split in two sepearate ones?
To modify any statement about the probabilistic character of the predictions, the formulae for them, the dependence of the predictions on the observers or observations, the change of the state induced by an observation, or any of these things is proposing a different theory, and no theory different from the actual correct quantum mechanical theory is compatible with the body of empirical evidence that are relevant for these questions.
Apr
24
comment In the many worlds interpretation of q.m. what makes a superposition af states split in two sepearate ones?
@Dirk, what you wrote is a deep misunderstanding. One can't take quantum mechanics and change its "interpretation". Instead, quantum mechanics is the interpretation. Quantum mechanical theories share some basic rules, the so-called universal postulates of quantum mechanics. They were defined by Heisenberg, Born, Jordan, Bohr, Dirac, and others, and deniers of quantum mechanics like to dismiss them as "just an interpretation". But these postulates aren't "just an interpretation". They're the very heart of the theory and it cannot be changed.
Apr
23
comment Is the Wave Function a Unitary Operator?
The wave function isn't any operator let alone a unitary operator. An operator is something that acts on wave functions.
Apr
23
comment In the many worlds interpretation of q.m. what makes a superposition af states split in two sepearate ones?
There is no well-defined theory that answers these questions and the founders of the "many worlds" philosophy - in particular, Everett and DeWitt, a key early champion - disagreed about all the details that could be a part of the answer, just like most of the current "many worlds" partisans disagree. Don't expect any sensible answer to any of these questions, the theory just doesn't make sense to the extent that it could be quantatively discussed by actual physicists.
Apr
22
comment How many percent of the visible light reaching the Earth are from other stars than the Sun?
There are so many factors over there that I am not surprised by a factor of 50. Probably the comparison of the Sun with the average star is the greatest source of an error. Well,if you're really interested, you may want to check every individual step, there are too many of them and I don't think that too many people would be interested in the refinements of the argument.
Apr
22
comment How does QFT help with entanglement?
QFT is perfectly local - because the commutators of fields are zero at spacelike separation. But even non-relativistic QM has interactions vanishing quickly with distance, so the interactions are negligible at macroscopic separation. However, QM is by definition non-realist because statements about observables (e.g. $x$ is between 2 and 5) only make sense relatively to observers who observe the observables, and whether someone observes it is ultimately subjective, observer-dependent, so there's exact no way to "objectify" this information.That's what we call non-realist - it's not classical.
Apr
22
comment How does QFT help with entanglement?
Dear Nir, your question is basically equivalent to "Can you explain quantum mechanics to me?". I don't know how to shorten the explanation. You need to know the general rules of QM. Possible states form a linear space. Composite systems are described by tensor-product Hilbert spaces. Most states in these tensor product spaces are not tensor products of two vectors describing subsystems, but general superpositions of such tensor products - and those are entangled states. Probabilities are predicted from Born's rule and they may be seen to have all the entanglement-like correlations.
Apr
22
awarded  Nice Answer
Apr
15
awarded  Enlightened
Apr
15
awarded  Nice Answer
Apr
13
revised Quantum non-locality with commuting measurements?
added 389 characters in body