Prime Eval, Part 17 – Heavy


Gravity is a weird thing. As Walter Lewis says in For the Love of Physics, it’s not something that most people think about. If we do think about it, we limit ourselves to the pull of the Earth on us, and possibly the Moon’s pull on the tides and the Sun’s pull on earth. Very rarely, someone will talk about gravity wells, as if there’s a “wall” around the planet that a rocket has to climb out of to reach outer space. But, as Einstein put it, gravity warps space-time, so it’s not so much that we have an attractive force that must be surmounted, as it is that space is more concentrated around larger clumps of matter, and we have to work harder to get past that space to escape that pull.

But, that pull goes on to infinity. Think about it – entire solar systems spiral around a galactic center. The gravity of the Milky Way as a collection of masses is so great that it’s pulling on our sun, and it’s not even like the center of the Milky Way is all that close to us. And then we have clusters of galaxies that rotate around each other, or in which the pull of masses from one galaxy is acting on the masses of another one. Einstein predicted gravitational lensing, where light from one galaxy is bent by the gravity of another galaxy it passes by, and the lensing can act like a gigantic focuser that we can use to view things that otherwise may be too far away to observe conventionally. And it is true, we’ve discovered examples of this phenomenon happening. That gravitational warping of space is all-pervasive.

The thing is, we can’t sense smaller variations in gravity normally, and it’s not like any changes are going to occur quickly. Gravity as we perceive it is a smooth collection of additive forces. And, we are going to be influenced more by the Earth, Moon and Sun than by Jupiter and Mercury, even though all of the planets are pulling and tugging on each other and the Sun to produce chaotic orbits. We know that Jupiter, Uranus and Saturn are making the Sun wobble as they go around, but we don’t see that as it’s happening.

Which brings me to my point – Have you ever wondered if you’re heavier in the middle of the night than you are during the middle of the day?

Gravitational forces are vectors, and we can do vector math with them. The combined vectors for gravity from the Sun and the Earth as they work on us as humans will be at a maximum when the Sun and the Earth are in a straight line “under” us, and a minimum when the Sun is directly over head (the Sun’s gravitational pull will be fighting against the Earth’s from our viewpoint). So, if sunset is at 8 PM and sunrise is at 6 AM, then “mid-night” would be at 1 AM, and that’s when you would weigh the most (the Earth and Sun are both pulling on you in the same direction). “Mid-day” would be 1 PM and you would weigh the least (the Sun is pulling “up” on you as the Earth is pulling “down”). Couple this with the Moon, and the biggest impacts would be during the lunar and solar eclipses, when everything is in more or less straight lines. If you’re in the U.S., you’d weigh the most when Japan experiences a solar eclipse, and the least when you see one.

And then, when you get a cosmic alignment of all the planets…!

Anyway, the next time you complain of having a heavy gravity day, check to see where they put the moon.

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