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Comment by bluetwo | Hacker News Reader

Gravity is the most romantic force in all of nature.

The idea that everything in the universe is connected and pulling is mind blowing.

I'm a physics teacher, and we just did Universal Gravitation last week. I whimsically explained it as "the longing of everything for everything". (They liked it, but devolved into a giggle fest about being attracted to each other.)

Other easily romantic notions in physics:

* we are all born of star stuff

* every breath we take has elements of the same breath as every human who ever lived (excluding some currently living ones, due to diffusion rates, but eventually...)

* (The current grav force between my partner and me is 1.9159965e-15 N)

> every breath we take has elements of the same breath as every human who ever lived (excluding some currently living ones, due to diffusion rates, but eventually...)

Is it conceivable that some of the oxygen and nitrogen, etc, in the atmosphere today wasn’t present in the atmosphere, say, 40,000 years ago due to it being bound up in minerals or biological matter?

That's a great lyric - I will have to put it into a song. Credit goes to pugio.

I know gravity has a speed limit, but does it have a upper distance limit or lower magnitude limit?

As in, are we really feeling gravitational effects of mass that is 14 light years away, even if they are insignificantly tiny and we probably can't measure it, or is there a point where a force is so miniscule that the universe truncates the force to 0?

If one arbitrarily defines "zero force" as less than the force sufficient to accelerate one proton at one planck length per age-of-universe^2, I come up with a distance of 10^12 meters between two protons (+/- an order of magnitude).

However, (and if I didn't screw up the math) since gravity scales linearly with mass, and the size of the visible universe is about 10^27 meters, then anything with mass greater than 10^15 protons -- or about the mass of a human cell -- has a "non zero" gravitational effect on you.

Ah, one little slip of the pen and everything changes. It's more like 3.6 * 10^16 meters.

Didn't check the math but I enjoyed your "zero force" definition!

As far as we can tell no. Of course, what we know with certainty is bounded by a nonzero experimental resolution.

Something is happening to hold galaxies and galaxy clusters together.

No, but neither does the EM force, it all just asymptotically approaches zero. Don’t let that fool you though, in that it really approaches zero to a high degree.

Well.... I'm pretty sure the field interpretation of EM also applies across the universe. So you could, on a dark night, with the electrons in the atoms of your body, feel the movement of electrons in a star, millions of light years away in space and time...

Yeah, but the electrons in the star are pushing on your electrons, not pulling. I think bluetwo called gravity romantic because it is bringing together, not driving apart.

I hesitate to say this because I dont entirely grasp it at the moment but I think when electrons are travelling parallel to each other they can stop repelling... their repulsive force is overtly effected by their relative movement, as well as their position and charge - due to how charges in motion modify and follow the magnetic field.

I believe that if we were just dealing with electrons, the repulsion from the electric field would be much much stronger than the attraction from the magnetic field.

We only see a net attraction between current carrying parallel wires because the wires contain both electrons and protons in almost perfect balance, so that the electron/electron repulsions and proton/proton repulsions between the wires are almost perfectly canceled by the electron/proton attractions between the wires, leaving no net electrical attraction or repulsion.

That lets the weak magnetic attractions (or repulsions, depending on the directions of the currents) dominate.

That interpretation is entirely valid, but the fact that objects can be electrically neutral mitigates the magnitude of that effect substantially.

Neutral is a tricky thing. Composite particles aren't truly neutral..just like gravity isn't really from point sources but can be considered as such.

https://en.wikipedia.org/wiki/Truly_neutral_particle

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