Gravity


Isaac Newton's actual apple tree, the tree under which he was sitting when an apple fell on his head. (Actually that never happened, he was inside watching through a window.)

Did anybody else learn the story at school about Newton sitting under his apple tree in the garden, and an apple falling on his head, prompting a eureka moment about gravity? As is often the case, it turns out this is a bit of an embellishment of what went on, but he did observe apples falling from his tree and derive ideas about gravity from it.

Once you do even just a bit more physics, which most people don't now, you suddenly realise how profoundly misleading that story is as a teaching tool. It has led generations of students to think of gravity as some sort of force that makes things fall down, towards the Earth. "What goes up must come down", "Newton would think he had made a mistake, to see those young men and the chances they take", etc. This isn't what Newton was saying at all, and it isn't how gravity works.

Yes things do seem to fall towards the Earth. But falling is the wrong word. They don't fall, they move towards the Earth. The problem we have with understanding gravity is a bit like the one a yacht finds itself in when a supertanker appears out of the fog. We're stuck next to this planet that is so monumentally big compared to us as human beings that gravity manifests itself in ways that don't really show what it's all about.

For example unless we're told otherwise, we tend to experience the Earth as just there. As a big lump of static stuff under us, that is passive and inert. In truth what Newton was saying about gravity is that every body or mass attracts every other body or mass. When that apple fell down he didn't mean that gravity was a force that made things fall onto the Earth, he meant that the Earth and the apple were both pulling on each other. Gravity is not a separate thing to the masses or bodies themselves - it is a property of the mass itself, to attract every other mass. All forces come from interactions. It's only because the Earth is so much bigger than the apple that the apple moves towards the Earth - if you sit two equal apples on the table in front of you they don't move towards each other, because there's no real difference in mass between them - there is a gravitational attraction between them, but it's tiny and because they're about the same mass, neither moves strongly towards the other. (A bit like how you pushing a trolley with nothing on it and a trolley fully loaded, with the same pushing force, will move the unloaded trolley a lot further than the loaded trolley.) There's also gravitational attraction between those apples and the ceiling, and the walls off to the side of them, and your body sitting near them, etc.

If we grew up in outer space or underwater I suspect gravity would be a much simpler idea for us to grasp. Those environments remove us more or less from the gravitational influence of that huge planet of ours. Say you're floating underwater, and you want to move in a particular direction. You can't push off the water as you would on the ground, your foot or arm just moves through the water and doesn't come to rest anywhere you can push off from. (In practice we do get a bit of resistance from water, which allows us to thrash about and move, but the basic point should be clear.)

To be 'weightless', as astronauts have made famous, is to experience gravity without the distorting influence of a massive neighbour. They're not weightless because they've moved out of some gravity force-field, but because they've moved away from the Earth (Newton showed that gravitational force varies with the distance between two bodies). An astronaut in space is utterly powerless to move anywhere unless they throw some mass off in the opposite direction, like gases from a rocket, or a spanner or other tool (which leads to other physics ideas like momentum, which won't be gone into here).

I'm more interested in what this proper understanding of gravity means for the average person going about their daily lives. Firstly it means what they already know it means, namely that there's no getting away from that Earth under their feet. You're stuck on that ground. But this is where another of Newton's discoveries is vital to understand what's going on. Newton's 3rd law of motion basically says for every action there is an equal and opposite reaction. Which means that forces always come in pairs, so that if I push on the wall here a certain amount, the wall actually pushes back on me the same amount. If it didn't it would fall over.

We treat things like walls like we treat the Earth, as just stuff that magically stands there inert, but a wall actively pushes back on us in this sort of situation, it doesn't just lie there and think of England. Doesn't mean it's a living being or anything silly like that, it just means that all stability and strength is relational - it's the result of interactions, it's not an inert property of anything. It might make more sense to think of the same experiment but with you pushing on another person's hands - if they don't push back (even sub-consciously) then obviously you're going to push them over. Their stability or solidity is all based on the force they're applying on you, not on any innate inertia they have.

This is where it suddenly becomes interesting. If I stand on the ground, not only am I (all of me) 'pushing' down on the ground, but the ground is pushing right back up through me, all of me. And those two forces are exactly the same size. (When you first learn the 3rd law, at this point you often think that this must mean you weigh nothing, because you have a force going down balanced by an equal and opposite force going back up. But the forces are on different objects - the down force is on the Earth, and the up force is on you.) It's like a Carrollian through the looking glass thing, before you and the Earth come into contact, you are attracted to each other, and then when you do meet and can't move, this mutual attraction passes through to the other side and becomes a pushing away.

This is interesting because normally when we're standing there we think our 'weight' is all heading downwards, towards the ground. But that's backwards - the forces on us from the Earth, from gravity, are up. The same applies to sitting or anything else we're doing, gravity is actually pushing us up, not pulling us down. And yet we go through every day feeling as if gravity is this constant drag on us, pulling us towards the ground. Slouching in our chairs because (we think) our weight is something that wants to head downwards, and we get tired trying to pull it all upwards again. But in truth gravity will suspend us effortlessly in space because at every moment it's coming right up through us, because we're on that massive Earth. That's why astronauts struggle to stay healthy in space, because without that upward force from the Earth their bodies have nothing to erect them in space.

Most physicists and engineers would argue that this isn't completely true, because the human body isn't (they think) a balanced object. So it will tend to topple over even with that force coming up through it. But that's why I started last time with the muscle suit, because once you realise that we're completely wrapped in that elastic muscle suit, you realise that we are in fact perfectly balanced. When you combine the upward force of gravity and that elastic suit stopping you from falling backwards or forwards or sideways, you have the recipe for a completely and sublimely effortless being in space.

Once you really experience that, directly, it's as beautiful as life can get, I promise. Effortless being, no feeling of weight or effort or tension or stress. As if you were a leaf being carried by a breeze.


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