PBS Space Time
3 HN recommenders · recommended in 4 months, 2018–2020
15:48 · 1.6M views · Oct 3, 2018
A PBS Space Time episode on how gravitational wave observations of a neutron star merger, GW170817, can limit the number of spacetime dimensions. It is based on a 2018 paper.
HN points to it as a clear explanation of the evidence that gravity does not propagate into extra dimensions.
6 comments from Hacker News
Well, it has a gravitational effect, so it definitely exists inside of our normal spacetime. Scientists have explored alternative models of gravity or spacetime with additional spatial dimensions and things like that. As far as I know, none of them are very promising. There was an interesting PBS space time video about one example recently: this video Of course, it is definitely interesting to think about :)
The problem is that gravity, like EM, follows an inverse-square rule. If gravity had a 4th dimensional component, we would expect to see an inverse-cube situation. The equation is: 1/(1-dimensions) Youtube video that explains this in depth: this video
For what it’s worth, evidence has come out (explained well by this PBS Space Time episode [1]) that makes the “gravity propagates over more than three spatial dimensions” theory a bit less likely. [1] this video
Yes. In fact that this is the exact (very exciting and somewhat recent) experiment that proved that if gravity does propagate in higher dimensions, then the dimensions must be very thin. this video Edit: To be clear: the experiment I’m referring to was the observation of two neutron stars merging. Since there was no event horizon prior to the merger, we’d expect a lot of EM energy to be radiated away. This new observation is neat because we’d expect much less EM energy out of a black hole merger.
The 5th dimension have already been experimentally conducted. this video I won't give away the spoiler but this article is written within a few months of the above video (oct vs dec). The paper in reference is: "Limits on the number of spacetime dimensions from GW170817" Pardo, Fishbach, Holz & Spergel 2018 [LINK]