Slightly related: my favorite video on gravity and the first one that made me truly grasp it: https://youtu.be/jlTVIMOix3I?t=60

My favorite gravity explanation video:

https://youtu.be/wrwgIjBUYVc

Also good :)

With a slingshot maneuver, why does the the direction of the inertia change though? If you zoom out far enough the lines are mostly parallel with a "dip" where there as mass. Why can an object enter parallel to a line, be dragged with the line towards the mass, miss it and exit on a path not aligned with a line? https://i.imgur.com/qj8fafd.png

That's a screenshot of an animation where the grid is moving. The lines don't capture the object. They just nudge it a bit. Kind of like a ball rolling across a treadmill.

Also that image doesn't show the slight twist in the grid caused by the earth's rotation. Another part of the video does though.

You can only slingshot in a direction another body is orbiting (to gain velocity). You're harvesting it's momentum, basically!

Not sure if the term itself restricts it to an accelerating maneuver, but of course you can do the reverse and slow down.

The more I try to read about this the more unsure I am I truly understand it, so perhaps not...

I’m confused by the question, possibly due to misunderstanding something fundamental in the basic physics. But I’m gonna explain my confusion anyway because it might be an opportunity for me (and maybe others) to learn.

I would assume that all of the forces have to be delayed. Being a force, inherently in relation to some other thing, there’s a transfer that has to occur for the force to interact. That has to travel through space. Even in my (admittedly lay) understanding that we don’t have a full explanation of gravity, I would assume it’s subject to those same basic (spatial) principles.

Is that assumption way off base?

Yes. As far as we know gravitational influences are locally limited by the speed of light.

For me, the more amazing thing is that we are able to detect 2 neutron stars merging from 130 million light years away. Just curious how sensitive these instruments need to be and our ability pick signals from the noise.

Sensitive enough that the randomness in the photon pressure on the 40kg mirrors is a source of noise they have to contend with[1]:

First, we observe that [quantum radiation pressure noise] contributes to the motion of the kilogram-scale mirrors of LIGO. This observation is also made in the Advanced Virgo GW detector. It is remarkable that quantum vacuum fluctuations can influence the motion of these macroscopic, human-scale objects, and that the effect is measured. This is quantum mechanics at its experimentally most macroscopic scale.

There's some great info in this[2] paper about the detector, sensitivity and performance of Advanced LIGO. And yeah it blows my mind every time I think about it.

[1]: https://arxiv.org/abs/2002.01519

[2]: https://journals.aps.org/prd/abstract/10.1103/PhysRevD.102.0...

Sensitive enough to detect a change in the relative length of two 4km-long arms equivalent to less than 1/10,000th the width of a proton.

And we distinguish signals from noise via a few different approaches, but our flagship search relies on matched filtering against simulated waveforms, and requiring the same signal to be coincident in time across multiple, geographically-distant detectors for which very few noise sources are correlated.

A somewhat unrelated note: the proportion between Earth and Milky Way diameters is (100 trillions) the same as the proportion between a human body and a large atom nucleus. We are galaxy sized objects in some sense.

Great article. Not sure they ever come back to the early suggestion that orbits would be wrong if gravity was delayed.

> Not sure they ever come back to the early suggestion that orbits would be wrong if gravity was delayed.

They do, at least obliquely. In summary: the issue is that if everything else about Newton’s model was right except that gravity was delayed, orbits would be wrong. But, actually, it turns out, as Einstein theorized and later evidence supported, everything (well, enough) else about Newton's model turns out to be wrong, too.

the speed of light is perhaps a convenient benchmark.

light has no resting mass thus is not limited to sublight speed due to this lack of mass.

in this scenario the limitation is about the maximum deflection/distortion rate of spacetime, thus spacetime distortion can not occur at a rate faster than light speed.

if spacetime is distorted beyond this curvature you get a black hole, and if a blackhole is smaller than the schwarzschild radius it spontaneously collapses and backlashes into ultrahigh energy photons[gamma rays]

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

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

But spacetime can expand at a rate that makes two points recess from each other faster than the speed of light, so still weird.

i love to read more about this do you have any sources?

Here: http://www.mso.anu.edu.au/~charley/papers/DavisLineweaver04....

The weirdest part is that there are photons that are currently receding away from us faster than the speed of light but they will still be able to reach us eventually anyway.