# Tag Info

1 vote

### Do waves really exist?

The problem here is confusing physics with reality. Physics models reality. It aims to produce models which are accurate "enough" for a given purpose. A water wave is a mathematical ...
1 vote

### Do waves really exist?

A mathematical wave is an abstract notion, a mathematical solution to a wave equation. In that sense, "waves" do not exist, because there are no systems that perfectly follow any ...
• 2,087

### GW luminosity depends on the 3rd time derivative but quadrupole formula depends on the 2nd time derivative?

The quadrupole formula is an approximate result which is valid at distances $r \gg c \tau \gg d$, where $\tau$ is the timescale of variation and $d$ is the size of the system. So, it works when you're ...
• 103k
1 vote

### GW luminosity depends on the 3rd time derivative but quadrupole formula depends on the 2nd time derivative?

So if I have 2 masses ($m=1$) at $\pm R=\pm 1$ rotating at $\omega=1$ in the $x-y$ plane, then: $$\vec r_1(t) = (\cos t)\hat x +(\sin t)\hat y$$ $$\vec r_2(t) = -\vec r_1(t)$$ so  \vec r_1\vec ...
• 36.1k
1 vote

### GW luminosity depends on the 3rd time derivative but quadrupole formula depends on the 2nd time derivative?

Unless the strain has a time derivative then the metric ($\eta_{\mu \nu} + h_{\mu \nu}$) is time-independent. But for the strain to have a time derivative requires $\ddot{I_{ij}}$ to have a time ...
• 133k

### Do gravitational waves cause matter to radiate?

Gravitational waves passing through the Earth cause strains on the order of $h\approx10^{-20}$ - see here and here. This means that a gravitational wave changes the length of a meter stick by about ...
• 784

### Can perfectly stable orbits exist in GR?

In the 1979 paper "Time without end", Freeman Dyson calculates a time in the order of 1020 years until the earth would fall into the sun due to gravitational decay alone, based on the ...
• 365