What Everyone Gets Wrong About Gravity

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

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

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.)

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

When you are at rest on the surface of the earth, you’re actually accelerating. I barely understand it but veritasium explains it in the newest vid ▶ this video

Original page kinda doesn’t explain the idea in simple terms. This Veritasium video gives detailed “popsci” 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)

First, the idea of describing it that way comes from Veritasium. Take complaints to him. See ▶ this video for the video where he does it. Yes I know it is handwavy and misleading. But I consider it less misleading than most attempts at visualizing it. > Ok, how does this sketch work for a low-ellipticity eccentric orbit? At what point in the orbit does it not work as a description of what's going on locally where the orbiting body is? > Sure, it's a set of events in a region of the whole spacetime. If we take "Big Bang" colloquially enough to include the inflationary epoch, always assuming GR is correct, then at every point in that "Big Bang" region of the whole spacetime there is a small patch -- a subregion -- of exactly flat spacetime. However, these small patches must be small because most choices of initially-close pairs of test objects can only couple to timelike curves that wildly spread in one direction (and focus in the other). No, there is no requirement of any region of locally flat spacetime existing. It is required (outside of singularities) that, when measuring things to first order, things are flat. However in curved space-time, the curvature can be theoretically revealed in any region, no matter how small, by measurements that are sufficiently precise to show the second order deviations from flatness that we call curvature. > I don't know how to understand your two final sentences: how do you connect the period just before the end of inflation and the expansion his

> See ▶ this video for the video where he does it. I'd be grateful if you take me to the time stamp, because in a casual watch of the video there was nothing like your: Gravity pulls things in by causing space-time to accelerate in a particular direction. In other words we accelerate towards the Earth at 9.8 meters per second per second because that is what space-time itself does. The closest thing I noticed was just after the 10 minute mark, where he points at Christoffel symbols and essentially says that you can choose a set of accelerated coordinates such that to remain at the spatial origin you have to undergo proper acceleration. "Your acceleration must be equal to this curvature term ... in curved spacetime you have to accelerate just to stand still". Which is totally fine, and even finer if he made it clear that you are "standing still" at some spatial coordinate after an arbitrary splitting of spacetime into space + time. But I don't know how or if those approx 90 seconds connect to what I quoted from you above. (Even finer still if he removed the coordinates and completed the equation: see e.g. slide 20/50 at link - his whiteboard is the term labelled in cyan, adapted. approx 10m45s "You don't have to worry about the details here. The point is...". The point is that I'm not his intended audience, and his presentation is fine enough, so there's no good reason for me to take you up on your suggestion to contact him.) > At what point in the orbit does it not work as a desc

Albert Einstein said, “Intellectuals solve problems, geniuses prevent them”; he also likely was the main reason the US became aware of the need to enter the race to create nuclear weapons. [1] At some point problems become impossible to avoid, especially if you’re biased [2] — though in the end, at a given scale, most problems (and their solutions) become as predictable as watching a man falling through the air. [3] - Second-order thinking is not result of being smart, but not being lazy, biased, egotistical, etc. _________ [1] link [2] link [3] ▶ this video

> Considering any disagreement with the idea that gravity is a force would end your academic career Gravity is not a force. So thinking it is a force would end one’s academic career. See, even scientists can live in their own “bubbles” once things get complicated enough :-) Gravity is the effect of masses moving along geodesic lines in curved space-time. Edit: Since you probably don’t believe me ▶ this video

Light is not keeping the Earth and Moon "in sync". Gravitational force is.[1] That force propogates at the speed of light. It isn't light itself. Light does play a role in illuminating the system, rendering it observable. It's a signalling component. But again, that's exogenous to the system itself. (See discussion of Maxwell's Daemon and the resolution of that paradox for the role of light in that specific thermodynamic process.) But, and this is key: ANY clock needs some way of passing information on to the observer. From TFA, that might be light, or sound, or chemical potential (received as the smell of rotting garbage). But that is a common element, part of the signalling mechanism, but not of the timekeeping mechanism itself . For an astronomical clock, in most cases, the regular rotational or orbital motion is the timekeeping mechanism. The signalling mechanism is entirely separate. E.g., in the Earth-Moon system, timekeeping is noted by both the daily rotation of the Earth, and the monthly orbit of the Moon. The kinetic energy of both those regular motions is not driven in any sense, nor is it measuring a decay.[2] The signalling comes from an exogenous light source (the Sun) which illuminates both bodies. But the Sun isn't powering the timekeeping mechanism in question. And is equally significant in providing the light necessary for you to read a wristwatch or wall clock (of a non-illuminated variety). ________________________________ Notes: 1. I'm going to leave disc