How Gravity Actually Works 

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.

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

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

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!!

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

At this point, Newton throws his apple at Einstein.

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

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

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.

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

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

My brain is short circuiting, how this guy understands what he is saying and teach it is beyond me.. your'e genius

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

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

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 😭

Thank you so much for these videos. they are opening my mind. I worked for a influencer marketing agency years ago and tried to get you to do a sponsorhip for the dumbest brand of all time. You were so kind in letting me know you would never do that. haha.

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

*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.

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.

... excellent cinematography ! love the rocket !

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

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

What a great Veritasium video!

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

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

This guy's dedication for his work is pretty breathtaking ❤

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.

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

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

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

The budget for this video most have sky rocketed

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

Good animations ❤

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

Reading the comments of the *youtube scientist* idiots putting down Newton, its amazing how advanced he was centuries ago to ask 'Why'.

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

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

I watched this long ago, but i forgot the answer. So i'm back! And once again I am amazed with the answer

The way you explain it is divine!

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

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

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.

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

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!

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

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've seen other excellent videos on this topic. This one is the best.

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

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

Time moves more slowly when relative density is higher. The pulling down sensation you feel is your particles literally being dragged by slower time. Like when your tires hit the gravel on the side of the road and your car lurches towards the shoulder.

For so long I've found it hard to have an intuition for how we are accelerating upwards while standing on earth, but not apparently moving. This video sure helps me understand that. Mostly anyway

The only thing that remains stationary is my understanding

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.

At this point Newtown throws his 🍎 at Einstein.

You should do a video on the Lagrange points!

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

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.

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!

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?

7:50 Ha! Joke's on you, because I'm watching this while falling from a

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

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

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.

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

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

Visiting this old video after the greatest discovery of Gravitational waves is so satisfying😄

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

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!

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

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

9:53 OMG THE VIDEO IS ACCELERATING UP, ohwait its my hand

Thanks you! This cake recipe will make my son happy for sure!

Saying that gravity is an illusion and that it is simply a symptom of curvature of spacetime is a bit of an oversimplification. The truth is that we do not understand gravity. We can observe and predict the effects of gravity but we do not fully understand it which is why there is such a problem as soon as we go from macro to microcosmos of quantum physics and quantum gravity and why the idea of gravitons was proposed in the first place. Also, there is no such thing as being at rest outside of the effects of gravity. There is always some gravity well you would be in. Even if there was only one star in the entire universe and you were on the other side of the observable Universe, you would still be in the gravity well of that star, no matter how shallow it would be. The only reason we talk about escape velocity is that you manage to escape from a deeper gravity well into a more shallow one.

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

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 🧑‍🚀🚀🌎)

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.

"a falling man appreciates the gravity of the situation"

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

You couldn’t have found a cooler rocket to pretend-launch into space?

*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 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 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.

@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.

Beautifully explained

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.

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

Best presentation ever

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

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

I have a question ! At 14:55 He showed that light bent twice than Newtonian Model predicted, So Why did light bend in Newtonian Model ??

I got it: curved space is like packing foam that keeps me pinned down on the Earth’s surface. Amazing that old Albert managed to sell this over Newton’s falling apple.

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

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

bro your informations are superb and mind thriilling your explanation is too good

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

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

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

Didnt know english speakers use the word „schadenfreude“😂😂

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

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

Just imagine a German man giving all these Mind blowing ideas 100years ago.