Hacker News Reader

Comment by xenophonf | Hacker News Reader

Are you indirectly asking whether planets can be dark matter candidates? If so, those are called MACHOs (massive compact halo objects). They have been studied via the gravitational microlensing effect. The presence of MACHOs can't account for most dark matter.

https://arxiv.org/pdf/astro-ph/0001272.pdf

https://arxiv.org/pdf/astro-ph/9904401.pdf

When you see figures like "1E-7 to 1E-2 M⊙ mass range" (from the above paper's introduction), note that a Jupiter mass is about 1E-3 M⊙---one thousandth or 0.1% the mass of the Sun. Earth is 3E-6 M⊙, the Moon is 3.7E-8 M⊙, and Pluto is 6.6E-9 M⊙; or 0.00037%, 0.000004%, and 0.00000066% the mass of the Sun respectively. Compared to stars, planets comprise a tiny fraction of the mass of a galaxy.

TL;DR - No, that's not how gravity works, but you can still ignore the planets in between because they're tiny.

View on HN

I think the question is: given a 3-body system with objects A, B, and C, is C's gravitational effect on A altered in any way by the position of B, or are B's and C's effects on A independent?

The reason for the question, I think, is because of an intuition built up around EM. In the system

A B C

B can block and otherwise interfere with C's EM radiation so that its effect on A is different depending on B's location.

No, if you want to think of gravity in analogy to light, you can see every mass as analog to a fully transparent light source. There is nothing in this universe that can absorb gravity.

LIGO was able to detect gravitational waves, which I'm pretty sure must require non-zero transfer of energy in one form or another. (I'm hedging a little only because I know LIGO was measuring relative changes in metric distance, which might possibly not require absorbing energy... but just intuitively it's hard for me to fathom any transfer of information without a transfer of energy: it simply shouldn't be possible.) But 1) it's a remarkably small loss of energy, since the gravitational coupling to matter is so weak, and 2) LIGO is very specifically reacting to gravitational waves rather than to static (or quasi-static) gravitational fields, and the absorptive properties there will certainly be different (just as they are for electromagnetism).

General relativity will probably add some effects like that, but then quantum electrodynamics adds light scattering off of light, so the analogy is still ok as far as analogies go.

> fully transparent light source

Thanks. So, in the analogy between gravity and light, you have to change all "massive bodies" to "fully transparent light sources" to make the analogy complete.

A nontransparent light source would not translate as a (straightforward) massive body.

Reply on news.ycombinator.com