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Gravitational Insanity

I’m introducing a new category of brain scramblings, Physics Insanity. I’m not sure how much I’ll actually post in it, but I have on several occasions composed my thoughts about matters relating to fascinating physical properties of the universe, so I might as well share them here. My point of view is less mathematical and more of a layman seeking a broader, deeper understanding of whatever this thing that we’re part of is. Expect to see the phrase “my mind was just blown” frequently.

My mind was just blown. It turns out that if a body (like the Sun) is in motion, its gravity doesn’t pull directly back toward the originating body, but toward a point ahead of the body. The further away from the body, the further ahead of it its gravity pulls.

Background: I think at least for this concept, it’s best to think of gravity not as a field surrounding a body (like the Sun), but as a type of radiation that continuously emanates from a body. As this radiation passes through surrounding objects, they are pulled in a certain direction by it. Two things are interesting about this. First, gravitational interaction between two bodies happens indirectly through “gravitational radiation”. Second, gravity is not an instantaneous effect, but one that travels through space, just like radiation from a nuclear meltdown or light from a bulb. Gravitational radiation travels at the speed of light, and when the mass/weight or location of a body changes, the corresponding changes to its gravity also travel away from the body at the speed of light. For example, if the Sun were to suddenly double in mass, the increased gravitational pull is not instantaneously felt by the various planets in the Solar System. Instead, Mercury would feel the corresponding increase in gravitational pull 3 minutes later (the same time Mercurians would first see the Sun double in size), and the Earth 8 minutes later, because that’s how long it takes for the corresponding in increase gravitational radiation to reach them.

Ok that’s neat, gravity radiates. Makes sense given what we’ve told about nothing (matter, energy, and mere information about physical changes) being able to travel faster than the speed of light. But there’s more to the story, so much more, dear reader. In the mundane, normal scenario of the Sun just drifting along at constant speed and direction, the direction of the gravitational pull that is radiating from it changes as it travels further away from the Sun! When you’re really close to the Sun, the gravitational radiation that passes through you pulls you straight back toward the Sun. But further away, for example at 92 million miles where Earth orbits, the gravitational radiation will have gradually changed so that its not pulling you straight back, but rather at a slight angle, toward the point in space the Sun will have drifted to during that 8 minutes! After traveling away from the Sun for another 22 minutes, the same gravitational radiation will reach Jupiter and will have changed more, such that it pulls toward wherever the Sun should have drifted by that time. As gravitational radiation travels away from a constantly moving body, the direction that it pulls in continuously changes so that it’s always pulling toward that body’s present expected location, not simply straight back to the location it was in when the radiation left it!!!!!1111!!1 o_O

If I explained that well, our minds are now in equivalent states of blown-ness. It’s awesome to think gravity has this dynamic nature. A crazy way to think about it is that even though we see the Sun in one location (where it was 8 minutes ago), we’re pulled by its gravity toward a different location (where it actually is right now). Apparently, the law of conservation of momentum requires that gravity behaves this way (I don’t understand this point conceptually yet). Also apparently, if the Earth were always being pulled toward the straight back to the old location of the Sun, and likewise all inter-planetary gravitational interactions were pulling straight back to “outdated” locations, their orbits wouldn’t be stable and the planets would fling each other out of the Solar System (assuming they would ever be able to assume an orbit, or even coalesce into existence, in the first place). Another crazy thing, each of us radiates weak gravity that, when it leaves us, is imbued with a sense of what direction we were heading in at that moment!

(I should note that gravitational radiation pulls toward wherever a body should be as long as its speed and direction remained unchanged. If either abruptly changes, the gravitational radiation already traveling through space does not magically “know” that it needs to recalibrate to the body’s new heading. Also, what I dub “gravitational radiation” is just my way of capturing the essence of its nature linguistically. I don’t understand the intricacies of quantum gravity or anything else that attempt to explain gravity more deeply.)

A cheesy visualization. At a given moment, gravity can be thought of as spherical arrangements of miniature electric fans floating away from a moving body, capable of blowing on anything they pass by. From a massive stationary rock, the set of fans “launched” at any given moment will always be pointed straight back at the rock, no matter how far away they float. But if the rock was drifting at a constant speed/direction when the fans were launched, each one of them will slowly change the direction they’re blowing in as they travel away from the rock, such that they’re always pointing to where the rock has drifted. The rock’s very motion puts a slight spin on the fans when they launch. The faster the rock is travelling, the stronger that initial spin; the slower its travelling, the weaker the spin. Such is gravity.


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