Gravity is not a force. But what does that mean?

this video is all about Albert Einstein yes that guy again Einstein is famous for his theory of general relativity it's taught us that gravity is not a force and that's probably the most misunderstood physics fact ever like even physicists get it wrong all the time gravity is not a force and since there's no Force pushing down on you that means you're currently accelerating upwards unless you're watching this from the ISS in which case please say High to the Moon how does gravity work if it's not a force that's what we'll talk about today an accelerometer

is a device that measures acceleration if you'd hold an accelerometer steadily in your hand it'll tell you it's accelerated upwards this isn't hypothetical where theoretical physics talk you can buy one and just see what it says this is an example of of how the reading of an accelerometer looks like when it's lying still on a table it tells you it's accelerated upwards and this is entirely correct if you hold an accelerometer still on the surface of Earth it's indeed accelerated upward why does that surprise you if you hold something on your hand you feel that

it's pressing against your hand downward it's like when you're trying to accelerate a car with your hand it'll push against it if you hold something in your hand that's not the gravitational force pushing it down because gravity isn't a force it's pushing on your hand because you are accelerating it upward why is the thing in your hand accelerated if you don't move your hand it's because you are accelerated and you are accelerated because there's a force pushing on you from below that's the floor or rather the atomic Bonds in the floor and in the ground

below you see the entire Earth has internal pressure otherwise it collapse that pressure generates a force on everything on the surface and as Newton taught us a force creates acceleration so that pressure of Earth creates a force which accelerates you on the surface but wait you may say there's also the gravitational force pushing down so the two forces balance to zero right no because gravity is not a force gravity is a consequence of moving in A Spacetime that's been curved by the presence of masses if there are no masses nearby to curve SpaceTime it's flat

so long as there's no force acting on an object in this flat SpaceTime the object will move in a straight line if there are masses nearby SpaceTime will curve and the object no longer moves in a straight line even in the absence of a force the usual way to explain this is with a large marble on a rubber sheet that deforms the rubber this is an analogy for how say a planet Cur SpaceTime around it and then another smaller marble which seems to be attracted to the bigger one it's a pretty good visualization it's just

that you shouldn't ask why the big marble bends the rubber then it gets really confusing so in this curved SpaceTime around a planet if there's no force acting on you you fall towards the planet that's not an acceleration but if something gets in your way like the surface of the planet then that creates a force so now you're accelerated this is why there's no Force pushing down on you right now there's only a force pushing up if you took away the surface of Earth you'd fall and if you fell there' be again no force acting

on you this is also why the Earth would collapse if it had no internal pressure it's not because there's a gravitational force acting on the matter because gravity isn't a force it's because without internal pressure stuff would just freely fall in the curvature that it itself creates without internal pressure Earth would form a tiny black hole about 3 mm in diameter and the real estate market would go to hell but let's go back to the accelerometers to see what else they can tell us the easiest example of an accelerometer is a spring yes a plain

boring spring suppose you put a spring into outer space empty space flat space how does it get there I don't know maybe Bob the astronaut lost it on a spacewalk I hear these things happen okay there's a spring in outer space for whatever reason it'll have some extension now if you tie that spring to a rocket and that rocket blasts off what's going to happen with the spring it'll expand so long as the velocity of the rocket changes why is that well as Newton taught us Force equals mass time acceleration so if you accelerate one

end of the spring that creates a force and that Force stretches the spring you can measure how much it stretches and from that you can calculate how much it was accelerated yeah I know this isn't exactly rocket science even if it is but bear with me for a moment this example tells us three very important things first you can use a spring to measure acceleration it's not the only way to do it and maybe not even a particularly good one but it works and acceleration is measurable second you cannot of course use the spring to

measure the velocity of the rocket because velocity in and of itself is a meaningless concept you can only talk about relative velocities not absolute ones this is why Einstein's theories have the word relative in them this brings us to the third and most important point that we learn from the spring acceleration is not relative it's absolute you can measure how much you're accelerated for example with that spring and you can tell when you're not accelerated so we have a spring on a rocket in outer space all right what's this got to do with gravity well

this is where Einstein's genius comes in Einstein said let's put the spring in a box from which you can't look out you have no idea what's going on outside we tie the box with you and the spring to a rocket the rocket blasts off what happens well the spring expands again Einstein now said that if you're in the Box you can't tell whether you're being accelerated or whether you're sitting still on the surface of a planet he then said if there isn't any measurement that you can do to tell these situations apart they're physically indis

distinguishable that is in a small box the effect of gravity on the surface of a planet is indistinguishable from acceleration in the absence of gravity this is what's called the equivalence principle the point where people usually go wrong is to conclude that because gravity is indistinguishable from acceleration that means gravity accelerates you but that isn't so because to accelerate something you need a force and gravity is is not a force Einstein's equivalence principle says that you are accelerated if you sit still in a gravitational field this is because the only way to sit still in

a gravitational field is to have a force acting on you that prevents you from falling for example because that floor is pushing on you but you can't te this apart from being accelerated in the absence of gravity in a flat SpaceTime imagine for example you're falling into a black hole no I don't mean YouTube recommendations I mean a literal black hole you're in a box with your spring and without any rocket boosters and so on the black hole curve SpaceTime and you can use the equations of Einstein's general relativity to calculate what happens they'll tell

you that if you get too close to the event horizon you'll inevitably cross it and then you'll end up in the singularity which might not be there but that's a different story the point for to day is that your spring won't budge because there's no force acting on you because gravity is not a force this is what physicists mean by free fall in a gravitational field it means there's no force acting on you so you're not accelerated a zero g flight is the closest you'll get to that on Earth at least if you want to

survive the idea of a zero g flight is exactly Einstein's elevator experiment that if you're in a box whose floor is not pushing on you that's the plane then you can't tell whether you're in outer space far away from any heavy objects where SpaceTime is flat or whether you're freely falling in a curved SpaceTime physically it's exactly the same so the zero g flight is indeed a good simulation for outer space and why is that it's because gravity is not a force in the plane you're falling in a gravitational field but there's no force acting

on you now I sometimes people say that z g is a bad term because it suggests that there's no gravity but of course the curvature of SpaceTime doesn't go away just because you sit in a plane and that's right but Zer G doesn't mean zero gravity the G is the local acceleration that you experience if you are at rest in the gravitational field if you are on the Earth's surface that acceleration is roughly 9.8 m per square second as you probably all learned in kindergarten though strictly speaking this gravitational acceleration isn't constant the acceleration that

you experience on the surface of Earth drops if you go to higher altitudes and it also slightly depends on what the ground is made of this is why measuring this acceleration tells you something about the composition of Earth's mantle but I digress by the way this video comes with a quiz on quiz withth it.com that lets you check how much you understood to come back to zero g flights on a zero GF flight your acceleration is indeed almost zero because you're not addressed with the surface of Earth that's why it fails like free fall of

course it's not exactly zero because you always move around somewhat that way or this way or if you're rohin because you're doing CPR but it's pretty close to zero so I'd say z g is a good description same thing on the ISS okay but I can see you're still confused because you learned that acceleration is a change in velocity and if you're not changing velocity you're just standing on Earth how can you be accelerated well the entire point of Einstein's theory of relativity is that velocity is not absolute what you mean if you say you're

not changing velocity is that you're not moving relative to the surface of Earth but this doesn't mean you have zero velocity because that's a meaningless statement it just means you're not moving relative to something else that accelerated the same way which is the surface of Earth you can however return to talking about Newtonian Gravity by redefining what you mean by no acceleration in general relativity as we've just seen you are accelerated if you don't move relative to the surface of Earth say because you stand on top of a building if you jump and fall you're

not accelerated because there's no force acting on you if you hit the floor you're very suddenly accelerated again which has well measurable consequences in Newtonian gravity you introduce this new force that you call the gravitational force and say if you're on top of the building that force of gravity cancels that from the building pushing on you so now there's no net force acting on you and Newton says you're not accelerated if you jump off the building Newton would say the force from below disappears and there's only the gravitational force acting on you and you're accelerated

and if you hit the ground you suddenly return to zero acceleration the Newtonian picture is the one we learn at school and it's useful so long as you are on Earth but strictly speaking it's wrong because you can't just redefine what you mean by zero acceleration you can't do that because acceler eration is not a relative quantity if you're staying in place in a gravitational field so you aren't falling then you are not at zero acceleration as any accelerometer will tell you now of course physicists are no linguistic Saints and they are sometimes refer to

gravity as a force even though it isn't a typical example is when they speak of the four fundamental forces of nature if they're being careful they refer to it as the gravitational interaction and the four fundamental interactions but often they don't I think they really just call it a force because that starts with an F in a nutshell why is gravity not a force because if you are only exposed to the gravitational interaction you fall and that is measurably not an acceleration if you need to freshen up your knowledge of forces and Newton's laws before

taking on Einstein's ideas the best place to start is brilliant.org they have lovely courses on Newtonian gravity and classical mechanics and even on special relativity and not just that brilliant offers courses on a large variety of topics in science Computer Science and Mathematics if you want to learn something new or brush up your knowledge on something you learned long ago brilliant is an easy and fun way to do it all their courses are interactive with follow-up questions they come with Graphics that you can play with and some have little executable Python scripts or videos with

demonstration experiments I like learning things on brilliant because I can do it everywhere and whenever I have the time and they're adding new content each month on timely topics such as solar panels Quantum Computing neural networks or computational biology I even have my own course on brilliant which is a beginner's course to Quantum Mechanics this will introduce you to the basics like what super positions and entanglement are and how interference works and after that you can continue with the other Quantum courses on brilliant if you want to give it a try sign up for free

using our link brilliant.org Saina you'll get to try out everything for the first 30 days and the first 200 people will get 20% off their premium subscription so go and check it out if it works for me maybe it'll work for you too thanks for watching see you next week