Comment by nomel | Hacker News Reader

Comment by nomel | Hacker News Reader

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My high school self thought that "dark matter" was just two three-dimensional objects/galaxies with some offset in a four dimensional cartesian space, influencing each other with gravity.

Galaxies would attract each other and "pool" to the same 3d space. This one would have an associated galaxy with less mass or at a greater distance. For the opposite case (galaxy present there, but not here), it would mean that we should see areas in our space with more gravitational lensing than is warranted by the local matter.

I made a simulator (dumb 4d cartesian stuff) back then, and have been waiting for someone to find gravitational lensing out in the middle of nowhere, so I could see if it matches the expected shape.

I would post links to my cool geocities page containing the source/graphic I made in QBasic, but it's been dead for a couple decades now. Maybe I can find my palm pilot that has ranting notes about it.

edit: Why downvote. I'm fully aware it was just a dumb idea from high school. I really enjoyed physics at the time, and really enjoyed making simulations. I'm don't honestly believe this is how the universe works.

Related to your idea,

- modified gravity (MOND), which accurately models the centripetal acceleration of stars in hundreds of galaxies, models acceleration as sqrt(GM/R^2 * cH/(2pi))

- cH/(2pi) = c^2 / (2pi * 13.8 bly), which looks like some super extreme v^2/r centripetal acceleration

- like something moving at c, from a point 2pi * 13.8 bly away = 87 bly away

- maybe if ones goes straight for 87 bly, one ends up at the "same point" in abstract space

- maybe the universe has the shape of a hypersphere. maybe the radius of the hypersphere is 13.8 bly.

This sounds similar to the idea of branes [https://en.wikipedia.org/wiki/Brane_cosmology] which were a proposal to explain why gravity is "weak" relative to other forces - the idea being that gravity expands through extra dimensions.

Yeah, that was the motivation. I had just leaned about the concepts of higher dimensional spaces, and learned about that from some history channel space documentary, and wanted to see what a gravitational field would look like at a 4d offset, naively assuming that it would follow the same strength vs distance relation.

Isn’t mass the 4th dimension?

It deforms spacetime, but I remember reading the other day that the deformation can be permanent. For a layperson (me) that looks like a plausible idea for what dark matter could be - space that's permanently deformed, but there's nothing there to collide with.

I'm referring to plain old four dimensional space [1], which is just 3d space with another orthogonal axis. Same as 3d is to 2d space. If a creature existed in a 2d world, say on a piece of paper, they would never perceive another 2d world, another piece of paper, one inch above theirs, in the direction of the third third dimension. The same rules apply for two 3d spaces separated by a distance in the fourth dimension. The thought was that gravity bleeds through this higher-dimensional separation.

1. https://en.wikipedia.org/wiki/Four-dimensional_space

In this case we could verify if the ratio of dark matter is always the same. The article would suggest that it isn't. So this might lend possibility to the idea. There was a video a while back about what can happen in higher dimensional space when a volume would pass through a "fold" for lack of a better word. It would be right there next to you but not visible. I wonder why this can't be a more popular hypothesis or maybe we're missing something.

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