Why Gravity is NOT a Force

Here's a question I've seen a lot in comments: OK, I'm accelerating up but then shouldn't someone on the other side of the globe fall off? No, here's why: Either watch again from 8:28 or read what I've written below... Spacetime is curved - it curves the opposite direction on the other side of the Earth.  Neither us on this side of the Earth nor they on the other side are changing our spacial coordinates - we're not moving up, they're not moving down - Earth isn't flying into one of us. BUT we both ARE accelerating. In curved spacetime you have to accelerate just to remain stationary. The traditional definition of acceleration is something changing its velocity. In general relativity you have to embrace a new definition of acceleration: it means deviating from a geodesic - not going on a straight line path through spacetime. Near the Earth a geodesic is a parabola so unless you're moving in a parabolic arc (like on a zero-g plane) you are accelerating. This definition is the same as the old one  so if you're accelerating in deep space then your velocity is changing. *BUT*... if you are near a large mass you are in curved spacetime, now acceleration  your velocity is changing. You can stay stationary relative to Earth's surface and still be accelerating. This is because your acceleration should be measured not relative to the Earth's surface but relative to free-falling objects - they are inertial observers. Imagine this - I'm in deep space and I make horizontal rows and rows of stationary golf balls. Then I hop in my rocket and accelerate up through them. Just think about what that looks like. Now my rocket is back on Earth just sitting there. I freeze time for a sec and make horizontal rows and rows of golf balls up into the atmosphere. Now unfreeze time. What do you see? If you just look at the golf balls and the rocket ship it looks the same as the situation in space where the golf balls were stationary and the rocket was accelerating. Einstein's point was the golf balls have the better claim as the "stationary" thing since their experience is just like the golf balls in deep space - no forces experienced. The rocket on Earth is just like the rocket in space. It feels a force and hence an acceleration.

So Newton actually rammed his head into that apple. Rude.

This video is a masterpiece. The best explanation of gravity on the internet currently.

Love how Derek is blasting himself off into outer space so that we understand gravity better, he always works so hard for his audience 😢❤

Been through college in physics and physical engineering and honestly no professor would explain in such an interesting and somehow profound way. Thank you

Mom: "son did you fall down?" Son: "No mom, you fell up!"

As long as I've known Derek he's consistently asked the difficult questions. This video challenged me, and taught me many things. I want to try the eclipse photo now. Impressed Eddington did it in 1919.

Here I am watching this video for the 4th time trying to better understand the concept, while Einstein thought of this in 1915 with only a fraction of the technology available today. It is mind boggling how smart he was

you make learning so very much fun. Thank you so much. You never fail to blow my mind 🙏🏼

At this point, Newton throws his apple at Einstein.

"Gravity is an illusion" flat earthers: "WRITE THAT DOWN WRITE THAT DOWN"

wow..just speechless.. You are a great scientist and even more great a storyteller ... Absolutely loved it :)

Till now i saw many videos ..they just said spacetime mesh and curve and all but they could make me visualise like you did... best explained!!

"Gravity is an illusion." Eugh, FINALLY! [floats off, to get groceries]

When mountain climbing, try not to become an inertial observer.

My first time commenting ever. Veritasium's videos are so significant, I like to think of it as the way Einstein and others published their papers, Veritasium actually makes those papers worth 10x by explaining it intuitively. Its incredibly humbling.

This video explains space/time and gravity better than any textbook, lecture or video I've ever seen

The fact that you can make videos on topics that are so out of the ordinary, and most people would never be able understand it without years of education, into a short video that is free to watch and actually understandable is amazing

Good animations ❤

I love this video. I've come back to it several times. When I first saw it I had to think differently about not just gravity, but spacetime as well. This video helps by putting the pieces together. And his charged particle question at the end was the icing on the cake. That's when I knew that I understood him correctly.