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comment How can laser interferometry be used to measure path difference smaller than wavelength of laser light?
not $10^{-22}$ meters, but $10^{-22} L$ with $L$ total arm length, including all round trips
Dec
29
awarded  Popular Question
Dec
24
comment Proof that fixed points of a null field are zero
I should add that the DC term of the Fourier expansion must be zero, since to synthetise a finite waveform with the Kirchhoff theorem, you need nonzero frequencies
Dec
22
comment Proof that fixed points of a null field are zero
I thought about it, but since it is related to holophony and wavefield synthesis, I thought it would find more expertise on Physics.SE
Dec
19
revised Proof that fixed points of a null field are zero
added 226 characters in body
Dec
19
revised Proof that fixed points of a null field are zero
added 65 characters in body
Dec
19
asked Proof that fixed points of a null field are zero
Dec
16
awarded  Popular Question
Dec
8
comment Why does no physical energy-momentum tensor exist for the gravitational field?
put in other words, to compute the energy carried by a gravitational wave, the energy depends on the shape of the gravitational wave in the whole compact support of the metric perturbation. However, this dependence cannot be put as a simple spacetime integral over a density. The question would be: over what variables must be integrated what, in order to obtain the non-local energy of the wave?
Dec
6
accepted Null geodesic equations
Dec
5
comment Null geodesic equations
I made an edit, please review. Also, when you say "Moreover the null vector $\dot{\gamma}$ must be proportional to either $n_+$ or $n_-$." I think this is the part that I cannot follow. In the $xt$ plane, the $n_+$ would be $(-1,1,0,0)$ and $n_-$ would be $(-1,-1,0,0)$. The step (B) does not seem to be giving much information, since both the colineal component along $\dot{\gamma}$ and the orthogonal component $n_-$ will satisfy $g(n_- , \dot{\gamma})=0$. So is still possible for $\nabla_{\dot{\gamma}}\dot{\gamma}$ to have non-zero components along both directions
Dec
5
suggested rejected edit on Null geodesic equations
Dec
5
comment Null geodesic equations
if it is colinearity, then what does the $~\neq~ 0$ means? if that stands for a inner vector product, then the question is, how do we know that the component of $\nabla_{\dot{\gamma}} \dot{\gamma}$ orthogonal to $\dot{\gamma}$ is zero?
Dec
5
comment Plasma wakefield acceleration for Protons
"I am not aware of a way to utilize this for accelerating protons." What about using a heavy atom gas that have a single electron in the outer layer as a 'wakefield plasma' for accelerating protons? as long as the atoms have an extra electron and their mass ratios are big enough, you should be able to create 'some' sort of acceleration
Dec
4
comment Null geodesic equations
I have the impression that the $||$ symbol in (E) is not actually an 'either A or B', but a colinearity assertion, correct?
Dec
4
comment Null geodesic equations
so, the expression in (B) is actually equivalent to $g_{ij} (\ddot{\gamma}^i + \Gamma^i_{mn} \dot{\gamma}^m \dot{\gamma}^n)( \dot{\gamma}^j) = 0$ in coordinates. Correct?
Dec
4
comment Null geodesic equations
what does it mean to be "compatible"?
Dec
4
comment Null geodesic equations
where does the $\nabla_{\dot{\gamma}}\dot{\gamma}$ symbol come from in (B)? If I derive $g_{ij}\dot{\gamma}^i \dot{\gamma}^j$ wrt $\lambda$ I just get $2 g_{ij}\dot{\gamma}^i \ddot{\gamma}^j$
Dec
4
asked Null geodesic equations
Dec
1
comment Why is the space-time interval squared?
you need the square root so that when you integrate proper distances, the integral is dimensionally consistent