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!"

At this point, Newton throws his apple at Einstein.

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 🙏🏼

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.

On a lighter note, this means that the apple didn’t fall on Newton’s head. He accelerated right into it.

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

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

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

Officer I can't walk in a straight line because we all walk in geodesics.

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!!

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.

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

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

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 is so good and well explained that it was shown to us in our hsc physics classes

“Do you feel weightless? No of course not” me falling off a roof: that’s what you think science boi

I’m starting to realize if any of these videos were elaborate April fools jokes, I would never be able to tell

Watching this video made me what an amazing physics teacher i had during my O levels and A levels. Even though it wasn't in our syllabus, our sir rly took the time explain this topic in great detail. Truly the best teacher I've ever had Ps the vdo is spectacular, as always

i didnt start to understand it till 9:32 in when you said the observers in the building were being pushed up. this was really fun to watch

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

my physics teacher: Draw an arrow showing the direction of gravity. me: [draws nothing] my physics teacher: What video did you see this time?

Great condensed delivery of information, as always. A few points of clarification that might be of help to viewers of this great video (and great channel): 1.) Based on general relativity, the question isn't one of whether or not gravity is an illusion. Rather, what we have, as you pointed out, is an exact one-to-one isomorphism between gravity and acceleration. So acceleration is just as much an illusion as gravity. We have to approach this from both directions. That is, bodies in a "gravitational field" can be thought of as accelerating bodies, as you've illustrated so well here, but accelerating bodies can also be thought of as bodies within a gravitational field. It works both ways. Acceleration and gravity are two useful ways of thinking that Einstein's theory reveals as equivalent. We could say that when we think we're experiencing gravity, we're actually experiencing acceleration, but we could just as easily say the opposite. The true illusion isn't one or the other -- gravity or acceleration; rather, the true illusion is the notion that gravity and acceleration are two different phenomena. Einstein shows us that the same thing is happening in both cases. 2.) Still, the claim that gravity is not a force isn't so easily made. Based on general relativity, it is a true statement, yes. General relativity shows us that gravity is not actually a force at all, but rather the motion of bodies through the curvature of spacetime. However, general relativity is not our only valid theory of reality. In quantum mechanics -- or, more specifically, in the standard model of particle physics, based on quantum field theory -- gravity is, indeed, a force. And we first detected gravitational waves in 2016, I believe -- several years before the release of this video, showing us pretty conclusively that gravity is, after all, a force (though no one has detected a graviton as yet, and likely never will). This is, of course, one of the many, many, many contradictions between quantum mechanics and general relativity tormenting physicists. Einstein's model is so aesthetically pleasing -- compelling, even -- it's only natural to want to believe it. But quantum mechanics tells us something else. As we know, the search for a super theory that unifies these two theories -- that is, the search for the theory of quantum gravity (or, at this point, I should say, the search for the correct theory of quantum gravity) -- is ongoing and should provide us with an answer to which of these theories is correct on the issue of gravity as a force. That said, that's probably not the right way to think about the problem. With acceleration and gravity, Einstein showed us that it wasn't about one being right and the other being wrong; it was about an equivalency -- a synthesis that allowed us to see both phenomena as facets of a larger conceptual structure, a structure in which the two phenomena are actually one. We should expect something similar from quantum gravity. The history of science is, if nothing else, a history of these kinds of syntheses of what previously appeared to be opposing, or at least separate, ideas, going back to electricity and magnetism and beyond. The fact that quantum mechanics is telling us that gravity is a force and general relativity is telling us that gravity is not a force should be a smoking gun that the entire question of whether or not gravity is a force is simply not the right question to ask. Quantum gravity, whenever it is established, will likely offer us a larger conceptual structure through which to understand these ideas, so that this apparent contradiction is resolved, much in the same way general relativity itself presented us with a larger conceptual structure through which we could understand the equivalence of gravity and acceleration, or the way special relativity presented us with a larger structure through which we could understand the equivalence of mass and energy, or the way Newton's theory of gravity gave us a larger structure through which we came to understand the equivalence of the heavenly and earthly realms -- that is, that the same laws were governing both -- an idea that would have been beyond foreign to Aristotle, and many who came long after him. In short, we can't simply state that gravity isn't a force, because quantum mechanics refutes that idea. Nor can we simply state that gravity is a force, because general relativity refutes that idea. And, of course, quantum mechanics and general relativity refute each other. And so we await the synthesis. It is one of a thousand key places in contemporary physics/cosmology where we hope quantum gravity will give us answers. Or, more realistically, lead us to ask better questions.

I can confidently say that i have watched the majority of Derek's videos... but this one has me stumped, i can't wrap my head around it at all 😭

7:58 "You are not an inertial observer." Me, watching this while skydiving: You sure about that?

Good animations ❤

Some issues with the use of language in this video. 5:31 It is stated that "gravity does not exist" which is simplified too much. It may not exist in the way we think it does, and it may depend upon or be conditioned by an observer, but to say it does "not exist" is to misunderstand what is meant when we talk about existence. Things can exist in an illusory way, in fact the more we learn about the universe the more we learn about the illusory nature of the universe. Also a 'force' is simply something that has an effect and therefore even if the 'effect' we experience is caused by something like acceleration through space-time it is still a 'force' we experience

Your way of explaining things is remarkable and highly addictive. Great job!

Props for the camera man who went through space to film this video

I dont understand the example with the light beam. If I'm in the frame of reference outside the rocket and see the rocket being accelerated, I unnderstand that I would be "seeing" a single photon traveling in a straight line. But as the light source is accelerated, I should still see the beam as curved. The depiction in the video seems incorrect. The light being emitted at time t1 at height x1 would not be at height h2>h1 at time t2 but still at h1.

Just found this video. Thanks, this might be the best explanation for why there is no finding of a graviton I have ever seen.

"Come on, Doc, I can't be accelerating if my spatial coordinates don't change." "You're not thinking fourth-dimensionally, Marty!"

Now every time that someone mentions gravity as a force, I'll open my mouth in preparation to correct them but then remember that I hardly understood this video and smile instead.

This video still blew my mind today, watching it the second time, after watching it when it first came out three years ago. Ive finally started my first actual physics class and we have been applying the newtonian equations for gravity, i now know what they are and how they work and why. one day, I hope to know Einstein’s equations as well and watch this video again. See if it still makes me rethink my perspective.

17:33 I love this part. It's so hilarious because Derek is acting as if people had any idea of what the equation even means.

Veritasium: You can accelerate even if your spatial co-ordinates do not change. Me: Say what now?

Rocket Man Derek just floating around with a single molecule of ethanol.

Sorry for my simple question: Why mass curves spacetime? Regards!

No officer. I wasn't accelerating unnecessarily. I was trying to stay still at the light.

" acceleration is a deviation from a geodesic " - core point

even the visual example of planets on a trampoline type test bed that is supposed to represent the curvature of space time are both supported by a material that is consistent in its elasticity so if a helium filled balloon were resting on the test bed even if its size were the same size of either of the example planets the curvature of space would have to curve the same exact amount as either of the two test planets on the test bed however there would be a massive difference in the density between the helium filled balloon and the 2 example planets.

Speaking about gravity is a simplification of a more complex explanation. It is much more convenient to speak about "gravitational pull" than to go with more lengthy explanation and get the same result.

“In curved space-time, you have to accelerate to stand still” Mind. Blown.

Imagine having a physics class where you learn gravitation fields and forces then go on to watch this video right after...

I like the idea of geodesics. The example of 2 people walking in a straight line and meeting because they were walking on a curved surface was very good. But if spacetime is a single thing (not 2 separate phenomena) then are we also moving on a geodesic through time? If we're moving through time, but it always feels like "now", is that the temporal equivalent of a spatial geodesic? Are we in the temporal equivalent of free fall?

The budget for this video most have sky rocketed

jokes on you, i watched this in a crashing airplane so yes. i am a inertial observer

I'm an apple 🍎 ... I was just chilling the other day and suddenly my tree detached and started moving up and before I knew it a freakin scientist just came straight at me from the bottom and rammed me with his gigantic head.... Like what is wrong with that dude... Thanks to him now no one will eat me

Hi Veritasium, great work, i've just discovered your channel and it's a pleasure to watch your videos and learning by feeling and picturing things. I have a question though, about what you are saying at time 15:40. You say that if GR view is more valid, we should see radiations emitted from the stationnary particles but bot from the free falling ones. That's the part i don't get. Since a charged particle emits radiations when accelerating with respect to an observer, a free falling particle will be accelerating with respect to the observers on earth, and therefore radiates in our referential, with no possibility to distinguish if either Newton or Einstein theory is more valid at that level. That's what i understand about the situation. I hope you'll get the time to answer and explain me what i miss, or anyone else reading me and having better understanding than me.

I watch this video every couple months and think of it often. Definitely more than the Roman Empire.

@EVERYONE: He's NOT saying gravity doesn't exist... he's saying it's not, technically, a force. EDIT: Ok he does actually say it doesn't exist a few times. *shrug* More importantly: Don't mess with the wiring in your house unless you know what you're doing and you're not breaking any laws/regulations.

Veritasium: *"gravity is just an illusion"* Flat Earthers: Our Time Has Come.

Finally got to know why I get so tired while lying on bed all day. I am working(accelerating) so hard to stay still!!

Im just flabergasted. Every question i ask myself during this video gets ansert within a few minetes. Thats how you can regognise a great video!

So newton wasn’t observing the apple, the apple was observing him

As a stem cell researcher I've recently read about studies investigating why organisms need gravity to develop and would have great difficulties in spaceships. Early attempts of growing plants in spaceships failed since plants need gravity for their root development. However, even single cells need gravity for molecular processes. For example, the cytoskeleton (which help cells to maintain their shapes) and several protein families have been shown to be affected by the absence of gravity (it would actually be funny to make a video about that myself). Great video as always!

7:57 Little does he know i'm falling of a building while watching this

This blew my mind @Veritasium ! and is the clearest explanation of (the consequences of) general relativity i have ever seen...(This video should be part of every middle school curriculum on the planet 🧑‍🚀🚀🌎)

As a Harvard Neuroscientist, here's another mind-bending gravity-related thought. When you rotate your head, you are NOT in control of the primary directionality of your eye movements under most circumstances. Try this. Keep your head still and look at your finger while you shake your finger left and right; your finger will look blurry due to motion blur. Now, instead, keep your finger still and look at it while you simultaneously shake your head left and right, which will demonstrate - in this situation - that you are still able to maintain a stable image of your finger. This is because your gravitationally-influenced sense of your head in space - your vestibular system - has a hardwired reflex to move your eyes in the opposite direction of your rotational head movements. This vestibular-ocular reflex (VOR) ensures automatic dynamic stabilization of images during head movements. You are not consciously doing this, and you can not consciously override it. The VOR is truly a reflex and, interestingly, does not depend on vision. During head movements, you will still have a strong VOR if you are in the dark or if you have your eyes closed. Your VOR originates from your vestibular system, which is located in your inner ear right next to your cochlea - your organ of hearing. Your vestibular system for rotational head movements consists of a series of three fluid-filled semicircular canals that have little "hair" cells inside the canals. When your head moves up-down (pitch), left-right (yaw), or shoulder-to-shoulder (roll), fluid inside the corresponding semicircular canal flows. This head motion-induced flow bends the hair cells within the canal to ultimately send electrical impulses to your hindbrain. These electrical impulses - encoding head movement - then talk to parts of your brain controlling eye movements, which connect to your eye muscles to reflexively move your eyes in the opposite direction of your head movement. So be thankful that your sense of your head in space - your vestibular system - takes care of maintaining a stable image of the world even when your head bobs around as you walk. Otherwise, walking would be a blurry journey of craziness. Videos coming soon.

*As a Harvard Neuroscientist, here's another mind-bending thought. When you rotate your head, you are NOT in control of the primary directionality of your eye movements.* Try this. Keep your head still and look at your finger while you shake your finger left and right; your finger will look blurry due to motion blur. Now, instead, keep your finger still and look at it while you simultaneously shake your head left and right, which will demonstrate - in this situation - that you are still able to maintain a stable image of your finger. This is because your sense of your head in space - your vestibular system - has a hardwired reflex to move your eyes in the opposite direction of your rotational head movements. This vestibular-ocular reflex (VOR) ensures automatic dynamic stabilization of images during head movements. You are not consciously doing this, and you can not consciously override it. The VOR is truly a reflex and, interestingly, does not depend on vision. During head movements, you will still have a strong VOR if you are in the dark or if you have your eyes closed. Your VOR originates from your vestibular system, which is located in your inner ear right next to your cochlea - your organ of hearing. Your vestibular system for rotational head movements consists of a series of three fluid-filled semicircular canals that have little "hair" cells inside the canals. When your head moves up-down (pitch), left-right (yaw), or shoulder-to-shoulder (roll), fluid inside the corresponding semicircular canal flows. This head motion-induced flow bends the hair cells within the canal to ultimately send electrical impulses to your hindbrain. These electrical impulses - encoding head movement - then talk to parts of your brain controlling eye movements, which connect to your eye muscles to reflexively move your eyes in the opposite direction of your head movement. So be thankful that your sense of your head in space - your vestibular system - takes care of maintaining a stable image of the world even when your head bobs around as you walk. Otherwise, walking would be a blurry journey of craziness. Videos coming soon.

We need a video on how the earth doesn’t expand along with everything else. Another thing I’d like to say is that, let’s say matter does bend spacetime. Wouldn’t we not be able to tell that that was the case since particles would just move as they normally would but instead through bent space. Like they would move the same way but instead just bent but everything else would be bent too so how could they even tell gravity was going on?

Props to Cameraman keeping up with Derek with Derek in space and Earth at the same time..

I’ve never understood the concept of bending space time around masses until this video. And now it makes sense how light gets trapped in black holes despite having essentially no mass

Whenever my mom says I am being lazy and not doing anything I will tell her that I am accelerating.

I'm still a bit confused but this video is really amazing! I never understood it better.

I'm so glad you talked about this 9:00 Everyone always says the same thing about things going up but nobady ever addesses it any further. I always thought the reason for it was the earth expanding which in my mind seemed to align perfectly with the idea that the whole universe is expanding

Veritasium changing video thumbnail 4th times. bro we watch video whatever thumbnail it is

The only thing that remains stationary is my understanding

I fall off the ladder. Gravity pull me down. Gravity did not pull me up. Gravity is a pain in the neck.

Your experimental test segment made me start to think about Maxwell and the electrons in a copper wire.

“...and I will prove it to you by blasting off into space.” *Reaches for giant bong*

"Do you feel weightless?" Me, looking in a mirror: ...no

Love the fact all of a sudden there are 2 Veritasium’s, when there is only one.

What a great Veritasium video!

Trying to explain this to a friend is a crazy challenge. I've tried. I think it's more confusing, this is such a great explanation

"Gravity doesn't exist!" Flat earthers: I knew it!

Even if stationary (to observer on earth) magnetic charge does radiate something, it may just mean, that it is under the influence of gravitational field of the Earth (which flows through it, for example), while free-falling one is somehow not under the influence of this same field or just in equilibrium with it. Pluse, first we need to conduct this experiment to proof that falling one is not emitting something, or that "standing" one is actually emitting.

please expand on gravity not being a field and therefore no field carriers but why the theory of the graviton exists and how this all parties with the quantum gravity theories especially when it comes to gravity around and inside black holes!

*"You are not an inertial observer"* Astronauts watching this: Am I a joke to you?

“Are you in an inertial frame of reference? No!” *The people on the ISS watching this video:*

11:05 - this moment reminded me of a quote from the book "Alice in Wonderland" by Lewis Carroll “My dear, here we must run as fast as we can, just to stay in place”

I came to learn a little about gravity, and I’m leaving with a headache, is a very fun video.

Newton: apple fall from a tree Einstein: man fall from a roof

"a falling man appreciates the gravity of the situation"

Thanks for the video, I will be showing it to my boss tomorrow to convince him we don’t actually need to take gravity into account in our next engineering project 😅

Great vid thank you! Now i'm trying to figure out why time is the same in space and on earth in a bent time space. How can it be c² at both the inner and outer diameter at the same time? Seems to me time would have to move ever do slightly slower down here (or in the videos case the sun) to make the bend.

i love this video, it really made me think about how i view gravity. Would a different way of putting this be that the Earth curves 9.8m/s of space time towards itself every second? Then if a person was "falling" they would just be moving with space time as it curved towards the Earth. The surface wouldn't be accelerating upward so much as space time is accelerating toward the surface.

"Einstein tells us one thing: focus on the experience of the observer" He really was a genius Now, all marketing is based on this

"In curved space-time, you need to accelerate just to stand still". Mind. Blown.

You should do a video on the Lagrange points!

It seems that Lewis Carroll (Reverend C.L. Dodgson) knew it long before Einstein published his paper: "... all she remembers is, that they were running hand in hand, and the Queen went so fast that it was all she could do to keep up with her: and still the Queen kept crying “Faster! Faster!” but Alice felt she could not go faster, though she had not breath left to say so. The most curious part of the thing was, that the trees and the other things round them never changed their places at all: however fast they went, they never seemed to pass anything. ... and still the Queen cried “Faster! Faster!” and dragged her along. “Are we nearly there?” Alice managed to pant out at last. “Nearly there!” the Queen repeated. “Why, we passed it ten minutes ago! Faster!” ... “Now! Now!” cried the Queen. “Faster! Faster!” And they went so fast that at last they seemed to skim through the air, hardly touching the ground with their feet, till suddenly, just as Alice was getting quite exhausted, they stopped, and she found herself sitting on the ground, breathless and giddy. The Queen propped her up against a tree, and said kindly, “You may rest a little now.” Alice looked round her in great surprise. “Why, I do believe we’ve been under this tree the whole time! Everything’s just as it was!” “Of course it is,” said the Queen, “what would you have it?” “Well, in our country,” said Alice, still panting a little, “you’d generally get to somewhere else-if you ran very fast for a long time, as we’ve been doing.” “A slow sort of country!” said the Queen. “Now, here, you see, it takes all the running you can do, ...” ("Alice through the looking-glass" by Lewis Carroll, 1871)

"There is no gravity" Flat-earthers: "write that down, write that down"

When I was a kid I fell off a wall and injured myself. I can remember the adults laughing at me when I described the experience as seeming like the ground came up and hit _me._ I'm the one laughing now.

Love the content and quality of videos

After watching this video I couldn't stop shaking my head due to the fact that I just realized Einstein was way ahead of his time like imagine if he was here today. Legend says that I am still shaking my head from amazement.