Veritasium
7 HN recommenders · recommended in 9 months, 2020–2025
17:34 · 16M views · Oct 9, 2020
Veritasium explains, with expert input, that in general relativity gravity is not a force and objects follow straight paths through curved spacetime, including why standing on the ground means you are accelerating.
HN links it as a go-to intuitive explanation whenever gravity as spacetime curvature or free fall comes up.
13 comments from Hacker News
I think he is easily one of the smartest people on YouTube and is producing some of the very best videos. He is also a brilliant educator. For examples, his videos on special [1] and general relativity [2] were real eye-openers for me, even though I have studied physics and have seen this stuff explained a million times before. What I really don't like are the new video titles. At some point he started A/B testing them to maximize his revenue. He even made a video about it [3]. Of course, this selects for the most outrageous clickbait in order to get that sweet engagement. I am starting to wonder if he took into account, how much this is starting to alienate long-term followers. [1] [LINK] [2] this video [3] [LINK]
Respin of a comment I posted a while back: This is what helped me wrap ahead around the fact that gravity "is not really a force": when you are in free fall you feel 0 acceleration. You appear to be accelerating relative to the ground-- but you're actually motionless in an "inertial reference frame". (Similar to how the astronauts on the ISS don't "feel" acceleration despite accelerating rapidly relative to the earth.) The "force" of gravity is often modeled as "gravity pulling you down" and the ground "pushing you back up". This works mathematically, but isn't quite logically consistent. In reality, on the ground you're in a region of warped spacetime, so you feel constant upward acceleration despite not actually accelerating. (Thinking of this another way, standing on earth feels identical to being in a far away spaceship accelerating at 9.8 m/s².) This is also why time "speeds up" near more massive objects. (Separate from "acceleration".) We're so used to gravity this it doesn't seem weird. But when you consider the fact that free-fall is when you truly do not experience acceleration... well pondering that from many angles is what ultimately led Einstein to his model of relativity. (This is me trying to condense what could be a 10 minute explanation into a few sentences, so apologies if it's not particularly clear.) Veritasium had a good video on this topic: this video (Thanks to u/badocr for commenting with this link last time.)
Ah yeah there are multiple definitions of 'acceleration' here. Unruh radiation occurs when you're not 'in an inertial reference frame,' loosely meaning that you feel acceleration. So in a rocket in space or (presumably) standing on Earth's surface. What you say makes intuitive sense, but it was actually the opposite logic that lead Einstein to his general theory of relativity. Here's a slightly dorky but very good Veritasium video that explains this issue and general relativity this video
While falling, the cat is in an inertial reference frame, so it is not accelerating. The ground is actually accelerating upward at 9.8 m/s2, counteracting the flow of spacetime. This Veritasium video gives an intuitive explanation: this video
Here's [0] a bit more thorough explanation of this concept for those who are interested. [0] this video (Veritasium on Youtube: Why Gravity is NOT a Force @ 8:24)