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Feb
10
comment Why do we theorize that the Big Bang created space?
thrown ball is an an analogy, pointing to a common cause: gravity. Of course gravity is not Newtonian, so ballistics can't be taken true, but what they have in common is that they're both solutions to the equations of a theory of gravity: given an initial condition, all else on the large scale follows from gravity. (In the case of cosmic expansion, the FLRW family of GTR solutions.)
Feb
9
comment Why do we theorize that the Big Bang created space?
if you think it's "without any cause", you're mistaken. There's as much cause as in the trajectory of a throwing object making an arc (etc.). What's actually not understood is the origin of the initial conditions, i.e. for what reasons (if any) was it initially thrown as it was.
Feb
9
comment Why do we theorize that the Big Bang created space?
There is of course no observational difference between cosmological expansion and everything else shrinking. But it's the everything part that's important: not just atoms, but particle Compton wavelengths, classical electron radius, etc. Everything one could ever use as a ruler should shrink the same, so it's much simpler to say space expands. ... It's no different than, say, time dilation: does time 'really' run slow for moving clocks or is time the same and the moving clocks just wrong? Since every clock, no matter its make, would be affected the same way, time dilation is much simpler.
Feb
9
comment Is Energy attracted to Energy?
Yes, the gravitational charge is energy, and the source of for the gravitational field is a more complicated stress-energy-momentum tensor. This is (edit: related to) this question,
Feb
4
answered Is the polar coordinate system non-inertial or inertial?
Feb
4
answered What information do $|\psi(0)\rangle$ and $|\psi(t)\rangle$ represent?
Feb
1
awarded  Nice Answer
Feb
1
comment Should all theories of gravity have Schwarzschild solution?
If we're talking about the static case specifically, all solar system tests that can be modeled by Schwarzschild should be compatible with any geometry that can be approximated by $\mathrm{d}s^2$ $=$ $-(1+2\Phi+2\Phi^2)\mathrm{d}t^2$ $+$ $(1-2\Phi)\mathrm{d}S_\text{Euclid}^2$, which also matches Schwarzschild in isotropic coordinates through $\Phi = -M/r$. In principle, that's a lot of freedom available to deviate from Schwarzschild at higher orders, but of course not all of them 'reasonable'.
Feb
1
answered Why does non-commutativity in quantum mechanics require us to use Hilbert spaces?
Feb
1
comment Can a metric in General Relativity, Supergravity, String Theory, etc., be asymmetric?
+1 (fake edit: cosine of the) angle being independent of order is a reasonable and intuitive physical requirement.
Feb
1
answered Can a metric in General Relativity, Supergravity, String Theory, etc., be asymmetric?
Feb
1
comment Can a metric in General Relativity, Supergravity, String Theory, etc., be asymmetric?
@innisfree there's nothing to reconcile because asymmetry in the metric tensor doesn't imply that distance between points is asymmetrical.
Jan
31
answered Is it correct to say that falling object are standing still?
Jan
31
comment Can orbital energy be a source of perpetual power?
This argument is a bit strangely phrased because according to the usual conventions, orbital energy is already negative for many situations. Perhaps an explicit consideration of a lower bound would be appropriate.
Jan
27
comment Is $∣1 \rangle$ an abuse of notation?
@ChrisWhite From Conway and Guy's Book of Numbers: Wacław Sierpiński, the great Polish mathematician... was worried that he'd lost one piece of his luggage. "No, dear!" said his wife. "All six pieces are here." "That can't be true," said Sierpiński, "I've counted them several times: zero, one, two, three, four, five."
Jan
27
answered Is $∣1 \rangle$ an abuse of notation?
Jan
27
comment Does bra-ket always assume all space?
One thing I dislike about such motivations is that it encourages one to think of vectors as lists of numbers, which is just plain wrong, and already shows up in multiple comments by the OP in a follow-up question. The distinction between a vector and its basis representation is quite important. Less important, though somewhat relevant, in that in QM we routinely use some bras/kets that do not actually correspond to a vector.
Jan
23
reviewed Edit Does the magnetic anisotropy state only have two possible directions?
Jan
23
revised Does the magnetic anisotropy state only have two possible directions?
fix grammar
Jan
23
comment Do we need an orthonormal basis in Quantum Mechanics?
(1) This is mistaken: "Non-hermitian operators is operators that have at least one complex eigenvalue." Its converse is true, but this statement is not. (2) This is not quite true: "We cannot measure quantities that could assume complex values." There's no problem with generalizing hermitian operators to normal operators, thus allowing complex-valued observables (spectral theorem works, etc.). It's just not conventionally done because it's redundant and not useful, being equivalent to a simultaneous measurement of two commuting real-valued observables.