this post was submitted on 03 Sep 2024

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Today I Learned

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342

TIL Gravity is not about mass pulling other mass. It's mass bending ~~time~~ spacetime that makes other mass get closer (www.youtube.com)

submitted 2 months ago* (last edited 2 months ago) by [email protected] to c/[email protected]

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[–] [email protected] 27 points 2 months ago (6 children)

Oh yes let's talk about my favourite subject ever!

The coolest thing I know of comes from wondering why bent spacetime makes you move at all. The answer is that you always move through time and the bending of spacetime actually turns a bit of time into space and vice versa.

Unnecessary tangent

For a horrible but intuitive explanation of how this works, time is kinda just a direction and bending sorta rotates things so that time looks like it's one of the space directions. Just like turning to the left makes what was your left look like it's straight ahead.

This leads to my favourite saying about black holes, once you enter them you can no more escape falling to the singularity than you can escape tomorrow.

[–] [email protected] 5 points 2 months ago (2 children)

I've never heard that take on it before (the idea of turning time into space), but it sounds fascinating. Do you have any links I could follow up on?

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[–] [email protected] 14 points 2 months ago (1 children)

An object will always follow the shortest path between two points in spacetime.

When it's sitting alone in the universe, the shortest path is to move through time from A to B.

When other things are present to also curve spacetime the shortest path can entail accelerating in space and slowing in time (from the viewpoint of us, the omniscient massless observer floating nearby pointedly not having any casual interactions).

[–] [email protected] 2 points 2 months ago

Variational principle goes brrrrr

[–] [email protected] 64 points 2 months ago* (last edited 2 months ago) (13 children)

They call it the "fabric of reality" because that's a good metaphor to describe how gravity works. (Or at least I assume that's where it came from, I could very well be wrong.)

When you stretch a fabric thin, and place something heavy in it, it's going to sink and stretch the fabric down with it. Then, if you place a smaller object next to the larger one, it's going to roll around the larger one, gradually moving closer as it goes down the slope created by the larger object.

That might be hard to visualize, so here's a neat video I found.

Edit: guys I think you're reading too much into this I wasn't trying to provide a foolproof explanation of how gravity works I was just trying to relate an interesting metaphor to a piece of linguistics.

And I wasn't even right, a quick google search says the term predates our understanding of the universe. Its probably a coincidence.

[–] [email protected] 13 points 2 months ago (3 children)

It's worth noting that spacetime isn't static. Space "flows" into mass. It's akin to a treadmill, you need to constantly move "upwards" to stay in place.

This is also the reason that uniform gravity, and acceleration are identical. With acceleration, the "ground" is constantly moving upwards into new space, pushing you along. With gravity, space is constantly moving down through the floor, trying to push you into the floor. It's functionally the same thing.

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[–] [email protected] 6 points 2 months ago (2 children)

I think that explains the "how" more than the "why".

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[–] [email protected] 4 points 2 months ago (1 children)

Or, if you prefer, here is "Einstein" explaining it.

https://www.youtube.com/watch?v=duW_CPDrUpE

[–] [email protected] 2 points 2 months ago

Not gonna lie I really thought it was gonna be Neil DeGrasse Tyson

[–] [email protected] 23 points 2 months ago (3 children)

One could be picky and say you're explaining gravity with gravity. But for the sake of simplicity that's OK.
I've once read an article where someone complained about that and tried to explain it with the actual cause, curvature of space time, like using a model car with glue attached to the wheels. But that was not really intuitive and simple to understand.

[–] [email protected] 6 points 2 months ago (1 children)

I think of it as a 2d cross section of the experiment (it’s happening in every direction possible tangent to the ball), which necessarily breaks into a third dimension. In our 3-spatial-dimension reality that’s the best we can do.

[–] [email protected] 3 points 2 months ago* (last edited 2 months ago) (1 children)

Yes, but the smaller object is dragged into the valley formed by a heavier object due to gravity (of the earth), not due to following the curvature of the blanket.

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[–] [email protected] 3 points 2 months ago (1 children)

I find the metaphor frustrating because of that.

[–] [email protected] 6 points 2 months ago (1 children)

https://youtu.be/DYq774z4dws

This video includes a much better representation of what gravity is IMO. The creator had the same issue with people describing gravity with gravity.

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[–] [email protected] 7 points 2 months ago (2 children)

I found a video once where the guy built a device to demonstrate curving based on mass, to avoid the gravity simulating gravity problem, but I failed to find it again when searching. It was something he'd bend to show larger mass, and you'd see the effect with the bands along it or something. Even that isn't accurate, but visualizing 4D can be challenging, especially if you then have to put it in 2D media.

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[–] [email protected] 7 points 2 months ago (1 children)

This is a pretty accessible visualization of gravity.

https://youtu.be/b9XhexlQMZw

[–] [email protected] 1 points 2 months ago (2 children)

But it's kind of a terrible one, because it uses gravity to explain gravity.

[–] [email protected] 4 points 2 months ago

It's just meant as a physical analogue to demonstrate some features (namely how the shape of the sheet/gravity affects things that travel on/through it) in a way that people can understand easily.

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[–] [email protected] 39 points 2 months ago (1 children)

Bending spacetime.

[–] [email protected] 13 points 2 months ago (2 children)

True. But that correction still doesn't unbend my mind

[–] [email protected] 3 points 2 months ago

Nor mine!!

[–] [email protected] 7 points 2 months ago (1 children)

What's really freaky is one of the theories we can't see other life, is to our perspective (due to expansion of the universe) their rate of expansion is faster than the speed of light, and we won't know till they're pretty much at our doorstep.

There might be a bunch of them all over, jumping around the universe and claiming everything they see like old school European countries.

If that happens the best result is they make us a "colony" but honestly a civilization that scale doesn't need resources or anything. The most likely explanation for that behavior would be just to eliminate any future competition if we advance unchecked.

Rather than colonize, they might just want to wipe out our planet, which would be practically effortless for a civilization that advanced. If something crashed into Earth at those speeds, the planet itself would be gone, no way for us to survive or even stop it.

The only defense is to spread out from Earth, which makes the hypothetical advanced space race afraid of our expansion kind of right

Any civilization in a universe like that needs to constantly expand just to ensure it's survival. Because the only reason we'd be preserved was if they cared about the novelty of life.

If they've spent millions of years expanding their space, alien life probably isn't that novel to them anymore. So really, our only bet is we're entertaining, like a zoo.

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