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

Check out my quantum mechanics course on Brilliant! First 200 to use our link brilliant.org/sabine will get 20% off the annual premium subscription.
Just exactly what does it mean that gravity is not a force? In this video I will revisit the question and explain why you are currently accelerating upwards, and how Einstein's equivalence principle works.
The quiz for this video is here: quizwithit.com/start_thequiz/...
Rohin's zero-g video is here: • Doing Real Science (an...
00:00 Intro
00:42 Acceleration is absolute
02:17 How gravity works in general relativity
04:21 Einstein's Equivalence principle
11:39 From Einstein back to Newton
13:48 Learn Science with Brilliant
🤓 Check out our new quiz app ➜ quizwithit.com/
💌 Support us on Donatebox ➜ donorbox.org/swtg
📝 Transcripts and written news on Substack ➜ sciencewtg.substack.com/
👉 Transcript with links to references on Patreon ➜ / sabine
📩 Free weekly science newsletter ➜ sabinehossenfelder.com/newsle...
👂 Audio only podcast ➜ open.spotify.com/show/0MkNfXl...
🔗 Join this channel to get access to perks ➜
/ @sabinehossenfelder
🖼️ On instagram ➜ / sciencewtg
#science #physics

[@SabineHossenfelder]

That was a tough one! The quiz for this video is here: quizwithit.com/start_thequiz/1702972458163x675901602454850000

[@smlanka4u]

Gravity is a force, and space doesn't bend due to gravity. The density of space increases near massive object due to the gravitational force. The existence of Dark Matter shows that space is material. General relativity is not a quantum theory, and it doesn't explain the high gravitational force that make Black Holes during the supernova explosion. Neutrinos can be the cause of gravity because stars emit neutrinos from their 99% of energy of the supernova explosion, making pressure to make a small Black Hole.

[@javahaxxor]

​@@smlanka4uinteresting statements care to explain that to us chimps?

[@_John_P]

You left out tidal forces, which break the premise in the video's title.

[@emifro]

Quizwithit asks for registration to see the correct answers :/

[@davidmaxwaterman]

You should have given the correct answers 😜

[@user-bi7nq4nj7q]

I tried to tell my wife this the other day... she just pretended to care and nodded her head in approval. The life of a physicist :-/

[@user-hk8yp7cw1v]

I tell this to both family and friends and they tend to do the same so don't feel alone 😅

[@nicklacelle]

That's just the life of a husband.

[@dtibor5903]

Well, don't try to explain this to regular people. For regular people and for practical purposes gravity is a force.

[@TransdermalCelebrate]

Very Funny, I’d wished to of been there 😄👍

[@josir1994]

She cared enough to pretend to care, that's a good start

[@richtheobald4390]

"9.8 m/s/s as you were probably taught in kindergarten" Maybe in Germany but I grew up in Canada and was still figuring out that plasticene wasn't a food group. I think you're right though: never too young to learn that thing that holds you down is not holding you down.

[@MrKotBonifacy]

"PLASTICINE", perhaps...? ;-)

[@hooked4215]

Say pleistocene better@@MrKotBonifacy

[@milanstevic8424]

@@MrKotBonifacy no he likely means Plasticene, as an informal "geological" epoch nomenclature, as the last part of the current age called Holocene, which is further subdivided to Anthropocene, an epoch in which all humans tend to be terminally guilty for existing. Needless to say these are all unofficial addendums, and are mostly there for rhetorical and socioeconomical purposes, of which Canada is a prime consumer.

[@AlexAnteroLammikko]

@@milanstevic8424 Wonderful, but definitely wrong. OP obviously meant Plasticine because thats putty and thats what children tend to eat, and its not a food group. So your Chat GPT/wikipedia blurb doesn't add much to that.

[@c.augustin]

Well, it was a joke. As Sabine likes to do. I can assure you that we don't have physics in Kindergarten here in Germany.

[@BarryPiper]

I have to say that gravity is a force because I teach high school physics and not university-level relativistic physics. The same reason I tell middle schoolers that there are three phases of matter and that electrons orbit the nucleus in a nice, neat circle. We can't jump right into relativistic physics on day 1, so we have to use the best working equivalent that students might have a chance of wrapping their brains around.

[@Feroand]

But, this approach mess up with the minds of your students. İt's impossible to "unlearn" something unless you lose your memory. There should be a better approach.

[@Donate_Please]

The gravitational force is constant. The value of G is approximately 6.67430 × 10^-11 N·m²/kg². This constant is fundamental to the understanding of gravitational interactions in physics. General Relativity is built upon that fundamental aspect of gravity. Gravity is a force as it defines the Plank length. There is a "G" in Einsteins field equations as well, which describes how mass and energy curve space. They are two different uses of the letter "G" where in the context of Einsteins field equations G is used to describe the curvature of space by mass and energy. In that context G is not a force but an actual curve in the geometry of space. However, at low relative velocities, Einsteins field equations reduce to Newtonian Mechanics which define G as a force between objects. So therein you can deduce that there are two different kinds of gravity. There is "G" gravity as a force and "G" gravity as the curvature of space. But gravity does exist as a force in both contexts of GR and Newtonian physics as well as the definition of the Plank length.

[@lordgarion514]

When they say gravity isn't a force, what they mean is gravity isn't a *fundamental force* of the universe.

[@Donate_Please]

@@lordgarion514 Well they aren't aware that GR's field equations reduce to Newtonian mechanics where gravity is a force in small velocities or gravitational fields. Something like buoyancy would demonstrate that force.

[@lordgarion514]

@@Donate_Please Who is they?

[@Donate_Please]

I guess I should add that while the gravitational force is constant. It's value of G is different depending on where you are in relation to massive objects. For example, the value of G the gravitational constant is different on the moon than it is here on earth. It's kind of a subtle but important nuance.

[@alonamaloh]

Perhaps Sabine didn't want to introduce reference frames, and there are good reasons for that, but for some people it might help to think about this by talking about different types of reference frames. The whole thing can be summarized by saying that the usual reference frame, where the floor is not moving, is not inertial. The force of gravity is then a 'pseudo-force', an illusion that appears because we chose a non-inertial reference frame, similar to the centrifugal force or the Coriolis effect in a rotating reference frame. In general relativity, inertial reference frames follow geodesics of space-time, which implies that the origin must be in free fall.

[@Earthstein]

So much of a construct; right?

[@Matthew-by2xx]

@@kleinerprinz99 Statements that start with “It’s vert simple,” and then simply miss the nuances are always fun.

[@NewNecro]

This so much. I think it'd have been much more helpful to better explain the spacetime model with geodesics, worldline and gravity's role within it rather than vaguely affirm what gravity is not. For most layman Newtonian gravity is the standard which makes special and general relativity particularly unintuitive. The fundamental differences between inertial and non-inertial reference frames are very important distinctions to explain Fictitious Forces you mentioned.

[@ObjectsInMotion]

There’s no reason to not consider pseudo-forces to be as “real” as a “real” force. “Real” forces are mediated by virtual particles, which are themselves not “real”, so why do those forces get special consideration? They shouldn’t. A pseudo-vector is just as “real” as a normal vector. This entire video is just pedantry.

[@gramail2009]

I have a vague sense you might be able to explain this better than Sabine does. It makes no sense to me yet. Maybe it is just a matter of language. Seems to work quite well for me (and most of the world's scientists too!|) to think in terms of the 'force of gravity 'pulling me onto this chair! Will I really benefit by pretending there is no such force??! Or calling it something else. First I guess I will have to find out what people mean by an inertial frame of reference as opposed to any other kind...

[@DruMusica]

The fact that you use several examples makes room for different brain wirings to link in. At each step in this video, I felt a little closer to getting this right. It was extremely satisfying and educative. Well done and thank you!

[@chrisstevens-xq2vb]

Pffft this is beyond stupid. If gravity wasn’t a force it wouldn’t do anything.

[@andrewjoliver82]

​@@chrisstevens-xq2vb just because you're incapable of understanding does not make a complex set of ideas stupid. The stupid is you 🤷

[@bartsanders1553]

​@@chrisstevens-xq2vbIt's just another lie from big globe. Stay strong, brother.

[@CSpottsGaming]

​@@chrisstevens-xq2vbIf you don't understand, you can say that instead of being rude.

[@thenonsequitur]

@@chrisstevens-xq2vb Gravity isn't "doing anything". Gravity is the natural fall of mass toward other mass due to the curvature of space-time. It's a description of the structure of space-time, not "doing something".

[@Zandaarl]

As a layperson in physics, I consider myself to be fairly educated. But this was a wild ride. I went from "Wait, what?!" to "That can't be right but Sabine wouldn't tell us something incorrect." to "Oh, now I get it!" to "I'm just slightly confused but I get it but I'm not trying to explain it to my friends." Thank you Sabine for expanding our understanding and knowledge with every video! 🎉

[@thehadster7043]

I usually can at least grasp the content of your videos. But... I gotta say, this had my head spinning. I eventually got it, but it was difficult. Thanks for the mental gymnastics!

[@Slitter_the_Dubstep]

Every time she says "Gravity is not a Force!" I feel like she got me. Its like a punchline that doesnt grownold and messes you up no matter how often you hear it, just because most of our lives weve been learning something different that we adapted into our Framework of reality

[@andrew3203]

Not something different, simply wrong. If you teach wrong things in school, you shouldn't be surprised when people say those things.

[@biopsiesbeanieboos55]

I agree. It’s like an unripe plum. No matter which direction you approach it from, it doesn’t become any more palatable.

[@robert-wr9xt]

Thankfully the phone didn’t ring.

[@Slitter_the_Dubstep]

@@robert-wr9xt huh :D

[@robert-wr9xt]

@@Slitter_the_Dubstep New to the channel? Sometimes she has a red phone on a desk. It rings and she answers it. Charlie Brown adult voice talks on other end. She makes comments and hangs up. You’ll laugh. Have a nice week.

[@juzoli]

Here is my favorite analogy which helped me understand the concept: Imagine you and your friend are standing at the equator, and start walking towards north, parallel to each other. But as you walk, you notice that you start to get closer to each other, and would collide by the time you reach north pole. Some mysterious “force” is pulling you together. You have to physically accelerate to keep your paths parallel. Is it a force pulling you together? Of course not. The Earth’s surface is curved.

[@Markielee72]

I like that. 👌🏻

[@audience2]

​@@harmless6813Lines that intersect are not parallel by definition.

[@acebulletman7389]

It seems that latitude lines are parallel, but longitude lines are not since they intersect.

[@TBJ1118]

Nah it's the force of love 'cause we gay for each other

[@ak74udieby]

@@acebulletman7389a latitude doesnt have a "line" besides the equator

[@noobsauce]

Thanks for drilling in the phrase "because gravity is not a force", it really does beg for repetition haha. I love this topic. I originally came across it while watching a simulation of the universe expanding, through the perspective of what our solar system looks like as its moving away from the center of the galaxy. Coincidentally, the planets' orbits in tandem with the whole system moving across space time simultaneously follow the shape of a 4D spring. Thought that was a fun little fractal coincidence when you used a spring as a measuring unit for acceleration.

[@simonbowden8408]

But Sabine isn't acceleration also defined as rate of change of velocity (I know that velocity is relative to something)? Can one separate acceleration from force? If you're in a black box and it accelerates then you can't tell the difference between gravity & acceleration? Which means that gravity is equivalent to a force?

[@ionsilver557]

One of my favorite explanations of gravity is a quote from John Wheeler, which interestingly, doesn't include the word "gravity" at all: "Space-time tells matter how to move; matter tells space-time how to curve."

[@Nocholas]

yes, but maybe not.

[@samibraheem1579]

I think this ia why we haven't and will not see a subatomic particle for gravity since it's a force like nuclear and electromagnetic

[@juliavixen176]

The thing about General Relativity, is that this _is_ all that it says about gravity. It exactly describes how gravity works... but not _why_ Why does mass and energy curve space? Yeah, it just does, and we can calculate exactly how much and stuff... but what's the actual mechanism? Why should geodesic worldlines converge towards the largest pile of confined energy, and curve away from a vacuum. What is the mass (or vacuum) actually *doing* ? General Relativity just says that the spatial distance between two points shrinks as the time distance increases... that's it, that's all it says. It's not very satisfying. It really is just pure geometry.

[@magicmulder]

Nobody puts gravity in a corner! 😂

[@patinho5589]

So the matter matters. It makes a curvature within which lifeforms like us do our stuff. This mean planets matter.

[@dougjamesberwick2625]

Absolutely fascinating as always - most accessible explanation I've ever heard!

[@schweinehundbullshit9176]

As Absolutely fascinating as absolutely meaningless.

[@AH-jt6wc]

you comparison between newtons law and how we applied it up to now and general relativity point of view is amazing. First time I understand this difference and I have seen many videos on that...

[@peterturner6497]

Yeah she certainly proved beyond doubt that Einstein was indeed a fraud and his "theory" is a worthless hunk of junk didn't she? Garbage is garbage no matter whey you try to spin it.

[@galenbarnaby7272]

I read Calder's book Einstein's Universe in the 1980s. I bought the book Gravitation by Thorn, Archibald, and Wheeler because Einstein said gravity is "not a force". Whatever Calder claimed, I look up. If f=ma and f is zero, then a is zero. I had to find the perspective where when I dropped my pen, it didn't accelerate. Since speed of light was constant, distance and time would need to be variable. My conclusion was the shells of time mentioned in the book showed the pen was traveling at a constant velocity through changing space and time. With e=hv, Doppler shift became important to see what was going on. When Hawking said the event horizon was where time stopped and space stretched to infinity, I couldn't accept that. How much stretching of space and slowing of time before space becomes infinite and time stopped. I believe as you approached an event horizon because your space and time become closer to the space and time of the event horizon, so it moves away from you. Maybe that is where Planck comes in. Since I haven't had the physics and the math, I don't know what I don't know. But I have believed for years what you said here. I wasn't biased by being taught Newton's perspective. I recommend teaching Einstein first, then teach Newton as a kludge. When Newton is taught first you have a formidable barrier to overcome for most students. They are taught gravity is a force, then taught it is not. How many students never get to gravity is not a force. The are stuck with bad consensus.

[@jonathandavid3298]

Fantastic video! Please do a video covering Mark Kasevich's experiment demonstrating the Aharonov Bohm effect for gravity. I don't know why this is never mentioned in physics when it seems to be one of the greatest findings in decades. Your take would help naive science hobbyists like me who don't know if this finding is significant or why nobody covers it.

[@ChaoticNeutralMatt]

Effect.. for gravity? I'm not familiar with that effect in that context.

[@lfelype.azevedo]

Thanks for the awesome video about the matter (or the space-time curvature in this case). As much as we study it, having a graphical and very well done explanations is good to cement the ideas, and this one was a blast to watch.

[@undercoveragent9889]

If 'mass' does not exert a force on spacetime then why should spacetime experience any warping?

[@jeremypearson9019]

The problem that people have with this is that they have a hard time accepting that there is positive net acceleration when there is no apparent movement. We're trained to think that if an object appears to be at rest, then all of the forces are balanced and there is no net acceleration. The key is to understand what Sabine is trying to explain is that gravity interacts in 4D SPACETIME, not just 3D space. In 3D space, gravity appears to be a force pulling massive objects together, but in the 4D spacetime equations the objects are simply at "rest" (no acceleration). In the 4D General Relativity equations, gravity never accelerates any object--they will always move at a constant "4D velocity" until they interact with an outside force. A rock that appears to be at rest on the 3D surface of the earth is actually accelerating in 4D spacetime. 🤯

[@vonmagiernemcunddemkosmos2290]

Physicists still don't understand the difference between space and surface. A spherical surface is curved, but the space inside is not, nor is the space around it. Gravity and electrical charges are determined using the Heaviside torsion balance. The torsional force of a wire is used. Coulomb determined this power. Consequently, gravity and electric charge have the same origin in atoms.

[@klauswassermann8054]

This seems profound. Still wrapping my head around it. Great way to launch the New Year. Heartfelt thanks Sabine, brilliant food for thought as always :)

[@davidmudry5622]

NIST FAQ 31 - "the top of the WTC north tower came down essentially in free fall" - "as the floors fell more and more weight fell on each floor below" - in free fall? www.nist.gov/world-trade-center-investigation/study-faqs/wtc-towers-investigation

[@ivoryas1696]

klauswassermann8054 It is, and back in 1915 it was a _such_ a big deal for a reason. 🧠

[@davidmudry5622]

Things on earth fall only when Nothing Is Pushing Them Up... As we speak, do you feel a force on top of your head and shoulders, or do you feel a force under your feet and butt? The only way you can fall is when the force you can feel...underneath you...is removed. When sitting in a car that is accelerating forward you will feel a force of being pushed on your back, not on your chest. You will always feel the pushing force of acceleration on the opposite side of your body to the direction of the force causing the acceleration. When a force pushes on your body your body pushes back on the force, what you feel is a resistance to being pushed. In free fall there is no pushing back, you feel no forces on your body, therefore there is no force in free fall. Einstein would call the acceleration one sees in free fall apparent acceleration. Velocity is the speed that is relative to your surroundings, whereas acceleration is not relative to your surroundings. Acceleration is absolute. F = ma...and real acceleration gives mass weight, where weight is the mass resisting the acceleration.

[@RobertStCyr-zh1tw]

Is gravity a force? Now my answer will depend on why you want to know. Lol.

[@davidmudry5622]

@@RobertStCyr-zh1tw Gravity is not a force unless it's the year is 2001, especially September, and especially in NYC.

[@user-bq4zk7fh1s]

Nice video Sabine! It's so interesting that completely different views can describe facts from different perspectives. I like the beauty of the underlying mathematics and its symmetries. A paradox glimpse what space and time really are. Some facts always connecting and some doesn't fit together. So sad we will never completely understand a fractal universe.

[@-danR]

Here's another perspective. I turn a teeter-totter on its side and apply pressure on one end and someone is resisting on the other end. Am I applying a force? Does the resisting party experience a force? The commonplace language of levers would say "yes". They are forces and can demonstrate acceleration, if the resistance is removed. Put the teeter-totter in its proper configuration and put a fat kid half way down and a little kid on the other end. And hold up the fat kid's end. Is the fat kid causing a force against my hands? No. Because of the Sabine youtube video effect: I'm accelerating the fat kid. I get tired of the Sabine effect and let go of the fat kid's end. Does the light kid accelerate upward? No he doesn't. Because there are no forces involved here; gravity is not a force You sit and scratch your head and say, but wait a minute... . But I've gone from the park to watch a Veritassium video: "Energy doesn't flow in wires", or something equally confusing.

[@andrewmosse6544]

wow! your explanation is the clearest I heard so far and I checked a lot of videos on you tube. thank you!

[@landonian1223]

this is my favorite thing to teach about relativity because you can get people to really think about what gravity feels like, which is nothing. i always start with the question, "can you actually FEEL gravity?" basically same as sabine's accelerometer example

[@thisuserhasaname]

Here's what I don't get: If the argument is that a spring in free fall does not experience acceleration because it doesn't change shape, then would the same not also be true if we swapped the gravitational field for a magnetic one? Since magnetism also works on the entire spring at once (rather than just on contact area), the observed effect would be the same: The spring keeps its shape and therefore is not accelerated. So therefore magnetism should also not be considered a force? Same with an electric field.

[@pasqgrasso]

@thisuserhasaname, yes your argument is valid. Electric and electromagnetic forces are recognised as forces, but due to a lack of understanding, gravity is not seen as a force by some (which it is of course, sorry Sabine). The wider community of Physicists STILL haven't got a clue what gravity is. They must discard Einstein's theory in order to move forward. He was very good at describing effects, but he was not good at identifying causes. This is a major issue with General Relativity and Quantum Mechanics - cause and effect are divorced, which has led to misunderstanding. We will never make leaps forward if we do not get past this paralysis.

[@187nemesis3]

Who says a spring has to be made of a material that can be affected by magentism?

[@DanielSamaniego-of5xl]

Gravity is the bending of spacetime in a 4th conceptual medium per Albert Einstein it's an effect not a force. (Pseudo math formula for a conceptual medium) This replaced Newton for mass does not attract mass i.e. 🎈 ☁ Not 1 single scientific (natural phenomenon independent variable and dependent variable) experiment has even been conducted to prove Gravity!

[@jasonsutton4415]

If you were to experience being pulled by a magnetic field (say you were wearing a suit of steel armour) you would feel the force, when in free fall you feel nothing. Oh and electric and magnetic fields are the same thing.

[@QuasiRandomViewer]

@@187nemesis3 Who says Dr. Hossenfelder's spring has to be made of a material that can be affected by gravitation? ;^)

[@ExplicitPublishing]

I concentrated very hard and still have so many questions. It seems more like a semantic trap than an actual Physics problem.

[@vix86]

Glad I'm not the only one that feels that way. It also feels like this whole video would fall apart when discussed in the sphere of quantum mechanics.

[@garymarkowitz5059]

"Don't ask what is holding down the ball in the middle of the trampoline. It's too confusing."

[@garymarkowitz5059]

Irene is great but this is ridiculous dogma period it's a mathematical analogy. Her tortured attempts to avoid using the words push or pull. Gravity is a pushing force. Force. See LeSage theory of gravity and think about how that relates to dark matter

[@voltydequa845]

It is not a semantic trap, it is just "instilling cognitive confusion". All this mess doesn't have to do with basic logic, let's not talk about Physics. It is implicit that Physics is about representing and understanding our world / reality. A thoughtful person cannot be but relativistic. But being relativistic such a person discards the Relativity Model because he/she would apply the Occam's Razor - choosing the simplest from the possible models (of "reality"). Between "when down to earth we are in reality accelerating" and "we are down to earth due to a gravitational force", we choose the second one since it is the simpler representation of (whatever could be) reality. Reality is what/how we perceive it, and we perceive it in a way that is easier to think about and infer laws and rules. Try to ask her to explain the acceleration and curving of space when it comes to tide moving due to the influence of the moon. Even if she / they succeed in giving an inevitable abstruse relativity curving-space explanation, they will implicitly give a proof that their relativistic model is counterproductive because excessively abstruse. The curvature of space is utter nonsense. Curved compared to what? Anyway you cannot perceive the eventual curvature. Immagine beings living in two dimensions. For them it is flat. They can live on a curved bidimensional surface seen from a 3d, but for them that curvature cannot be pertinent in any way. The same holds for the so called "curvature of spacetime" - it can eventually be seen from extraterrestrials that live in superior dimensions, but for us it is just a sophistry nonsense.

[@phenanrithe]

In the video, the question about a = dv/dt is quickly discarded "because it's another referential", which doesn't help if you don't know about general relativity. The spatial position of a free-falling object doesn't change in freefall because it's not simply dv/dt = a in space. There's an additional term cancelling out the acceleration upward, which comes from spacetime distortion. That's what explains that Earth's surface is accelerating upward without Earth expanding. The geodesic equation shows that d²z/dt²=a - Γ (dz/dt)². If a, which is F/m, equals the gamma term, the position remains constant: the ground pushes the object upward but spacetime distortion compensates it. Anyway, it's only one theory, so saying gravity's not a force is only true in that theory. Don't try to give it any meaning.

[@kylelochlann5053]

Gravity is not a force by direct measurement, and has nothing necessarily to do with relativity.

[@zappababe8577]

Dr Rohin Francis demonstrates a very good point here - it would be extremely difficult to administer CPR to a patient in zero-G. Best not to take any risks whilst you're in zero-G, like doing flips or somersaults...oh, dear...

[@todddembsky8321]

Wonderful Channel, Incredible Host, Makes learning fun again. Thank you Sabine for a wonderful channel. Wishing you and yours a wonderful Holiday Season.

[@Earwaxfire909]

It would be helpful to explain why charge interactions are driven by a force and the differences with gravity.

[@drgetwrekt869]

Maxwell equations are linear, and thats why they can be represented as a field of vector """particles""" (photons) that interacts with electrons and so on. Gravity apparently doesn't fit in this formalism because it is inherently non-linear and defines the same coordinates that are used for the calculations. Edit: actually even "non-linear" fields can be quantized without issues, for example Higgs or phi^4 terms. But as far as I know that's it ? Not sure tho

[@josefpharma4714]

@@drgetwrekt869 I'd say linearity (none-linearity) should not make any difference. But AFAIK: If gravitation is not a force, electro magnetic interactions are no force, too. (But this is a kind of definition only?)

[@dannydetonator]

@josefpharma57.. Difference is in the origins: electro/magnetic forces have quantised matter-energy as a direct cause for forces exerted. Gravity is causing forces, but itself it's just a constant of spacetime bending per general relativity. The latter have no particles or known fields carrying or causing the forces created. It's like acceleration without an engine doing the work, while still carrying the accumulated potential energy.

[@PrivateSi]

Spacetime is not a technically not a force, but gravity could be, and the cause/bits of space time could/SHOULD exert a force. Unless you believe space is empty or some nonsense like that..

[@Dom-Nom-Nom]

⁠​⁠@@dannydetonatorbut if EM forces require work, like an engine, why don't I quickly run out of energy from all the EM acceleration from sitting on top of the Earth?

[@MarkusVeller]

Okay but the gag at 3:47 had absolutely perfect delivery

[@weylinstoeppelmann9858]

This gives me a headache because if a rocket is trying to accelerate but is anchored in place you're not gonna be, like, pinned against it, so the idea that the earth is accelerating in every direction yet motionless is weird. If we were standing on the surface of an infinitely expanding bubble, THAT applying a force makes sense, but if it's being held back then it's not applying a force.........

[@randelbrooks]

Wonderfully spoken and difficult to comprehend. Merry Christmas Sabina and everyone else!

[@harrykirk7415]

I used to say that gravity was a force, but that was back before I started describing everything in terms of curved space time coordinates. Before when I did something like building a wooden shed at my job I would say crazy stuff like " this shed must be built strong to resist the force of gravity acting on the building materials and potential occupants". It was so confusing!!! Now I just layout the whole building in curved space time coordinates, and all the confusion just disappears!!! All the workers on the job site can clearly see that the building is accelerating upwards and there are no gravitational forces at all. This is fantastic!!!! Thanks, Einstein and Sabine!!!

[@every1665]

If the shed collapses soon after being finished, most builders will gently point out that you accelerated it upwards too fast. Nothing to do with inferior materials or construction methods.

[@mikegale9757]

@@every1665 Not sure if that argument would stand up in court. Engineers are supposed to anticipate the unexpected and build in some safety margins. Nobody expects the Spanish Inquisition of course, but come on. Look at my shed. It's in ruins!

[@mikegale9757]

Quite. There's no force pushing you down, but that doesn't mean you're not going to fall when you remove the force which is pushing you up. It's not entirely incorrect to refer to the latter as the force of gravity. It's just semantics. The force due to gravity would be more accurate.

[@Krokodil986]

​@@every1665 that's what I tried telling them 😢 Just like that time when they accused me of punching that kid. Little do they know, atoms never touch So no I didn't punch him

[@Krokodil986]

​@@mikegale9757 it would be as correct as saying that the centrifugal effect is a force, which can simplify things a lot in certain cases

[@wu1908]

Love the lesson with the quizwithit! added level of fun~

[@WillisLinn]

Oh I love knowing new things I listen to you a lot and your smarts makes me wish that when I was younger I could have learned from you. Thank You fo promoting understanding in science!

[@dougdupont6134]

As a programmer making a hard sci fi game and not a physicist, it's a little scary trying to advance a theory of gravity without knowing what I'm talking about. A character in the game says that if you only perceived in 2D but approached a 3D hill, you would experience it's effects as a mysterious pull (or push as the case may be). I was especially concerned that I was only moving the goal posts on this one. Nice to see I might not be so far off. Thanks for the great explanation!

[@juliavixen176]

Well in the case of a two (space-like) dimensional manifold with intrinsic curvature... or extrinsic curvature as a hill in a three dimensional embedding space (with no time-like coordinate) What happens to two 2D creatures walking in straight parallel lines a constant distance apart from each other, when they encounter the hill, is that even as they continue to walk straight, the distance between them will change. The 2D creatures might interpret this as a mysterious force that is moving them either closer or further away from each other... but there is no force... they are not actually accelerating... they are still on straight line inertial paths and feel no force... but the distance between them is changing because the space between them is curved. This is General Relativity... it's just like this except in a 4D Spacetime (so the time interval between events can also stretch and shrink, and it will look like things are mysteriously changing velocity without accelerating, but it's actually just spacetime curving).

[@dougdupont6134]

@@juliavixen176 Yeah, I'm a programmer and writer of fiction trained academically as a philosopher, so I want to write stories and craft games with a meaningful and accurate portrayal of science on characters that are digestible to regular people. My limited understanding of physics can be frustrating in that endeavor, especially since I know enough to know that I don't know anything (as Plato would say). It seems like what you wrote essentially confirms that my example might be a meaningful and accurate portrayal. I appreciate you taking the time to explain it better than I can. I hope you don't mind that I might borrow some of it.

[@MrGemaxos]

@@dougdupont6134 Dont be frustrated, if you go down the rabbithole its like a Hydra. Every answer makes a few new questions and in the end you are rarely understanding, but you are still just realizing that there is more and more that you dont understand. (youd still be in platos place) In my Opinion its a good thing, it leaves more room for the fiction :3 if not, wouldnt it be just science? I have read so many good books with physic that dont work out. But without the "wrong" physics you couldnt tell the story. Jules Verne for example. With correct physics as Dogma most of his storys dont work out and you would have a very hard time to find a possibility to tell a similar story.

[@eVill420]

That's really cool! So it can be imagined as falling into a Whirlpool and streching like spaghetti

[@voltydequa845]

@@juliavixen176 I liked you other comment (though I do not remember what it was about). you say «when they encounter the hill, is that even as they continue to walk straight,» It is a hill for you, looking from outside, from a superior dimension, their "walk straight" from from 3d pov is not "walk straight" from their 2d pov. Their "walk straight" would put them to walk with constant distance between them, but could present some other "irregularities", like the impossibility to maintain the same distance while walking at the same speed. I usually use the example of 2d to try to show that there's no way 2d's can imagine seeing them from a 3d, or that they should be that conformist to buy into an abstruse 3d model if they already have some another explanation that is simpler. The main point being "Man is a measure of all things". What "exists" is the representation of the "reality". While the abstruse and overcomplicated curvature of the "reality" should be left to parrots.

[@junaidsajid8867]

Another highly inspired video. Thank you for teaching us how to think scientifically :) peace and love

[@kurtn4819]

I missed 2 out of 12. I LOVE the quiz after the “lecture” because I often wonder how much I retained and this is a good way to gauge that. Thanks Sabine. Only one suggestion: We don’t know which ones we got wrong, or am I missing something?

[@DazzaOnGoogle]

Thanks you so much for this Sabine. One of your earlier videos about this topic, which had a simple "gravity is not a force" and the raw explanation has been used by flat earthers as evidence of their position. This fully qualifies what you meant, and removes one more "justification" they can pull out of the bag

[@alikifahfneich]

Thank you for making such great videos about Important and debated Topics!

[@LuvHrtZ]

This concept is one that I still can't get my head around. As always, love your stuff, Sabine.

[@vibaj16]

Everything moves in a straight line when under no force. Since gravity is not a force, the Earth is under no force. So why does it orbit the Sun? That's not a straight line, right? Actually, it is. The sun's mass warps spacetime's geometry such that a straight line gets bent around the sun. Geometry itself is warped.

[@rockovahsacralonte570]

I can't get my head around 1+1=3, mainly because it's not true!

[@fewwiggle]

@@vibaj16 OK, but why does the floor push on me?

[@D1N02]

Think of it as deceleration instead of acceleration and the quarter will fall (decelerate :p)

[@D1N02]

@@fewwiggleit doesn't you push on it because you want to free fall the the center of the earth, but the floor is in your way. Your atoms do not want to be in the same spot as the floor atoms, so you are stuck in the cosmic water slide because a fat kid called "the floor" is blocking it.

[@lisalesinszki7536]

My daughter and I like to share interesting facts with each other every day. I will send her the link to this video because I never knew that I was accelerating upward. 🤓 Thanks, Sabine! New subscriber.

[@dr.danielmckeownastrophysics]

The equivalence principle only applies locally, its actually possible to see the difference between a person standing in a gravitational field and a person standing in a box with a rocket because when you look at the 2nd derivative and compare the fact that the person in a gravitational field will experience differing ("non-uniform") accelerations at their feet vs. their head while a person standing in a box with a rocket accelerating will experience uniform acceleration, you can see that the gravitational field can be distinguished. So while the two are close, they actually are very different and cannot be said to be physically the same. One could be treated as essentially a uniform field, while the other is non uniform when you compare it at different regions of spacetime.

[@dimitardonev4507]

If the rocket could pull on every atom of the spring simultaneously (instead only on the contact patch), as the gravity does, the spring won’t stretch either, i.e. it will shown zero acceleration. The equivalency principle has a major flow - a rocket pushes (or pulls) only on the contact patch, the gravity acts on all the atoms simultaneously.

[@soopergoof232]

Under the flowing-space model, 'space' is accelerating through the spring's ENTIRE atomic lattice. So the spring will stretch equally in both scenarios. In both situations, the lattice comes under tension, resisting being 'stretched' in the axis of acceleration. The same mechanism causes both gravity and inertia, i.e., the equivalence of gravitational and inertial mass.

[@user-ut2mk6fm4y]

@@soopergoof232 "So the spring will stretch equally in both scenarios." Doubt. I am pretty sure you can test the opposite by following easy approximation of the given thought experiment: Push the spring from one side. Pull the spring with the same force from the other side. I am certain that the spring won't change its overall length. Then try with 3 points to apply the force to (start, middle, end of spring). Then with 4 etc. The result always will be no overall change in spring length.

[@williamtait3700]

A uniform force on individual atoms is an acceleration? An external push on the outside only is a force? If so centrifugal force is not a force either only the reactive centripetal force on the string is valid??? We just need to agree on our terms.

[@dimitardonev4507]

@@williamtait3700 all of the above are forces, the problem is that the accelerometer only works if a force is applied on the contact patch. So just because your instrument has limitations in measuring a force you cannot declare that such doesn’t exist.

[@williamtait3700]

@@dimitardonev4507 Agreed the force of gravity is a FORCE. The accelerometer is just not appropriate to measure it.

[@cavesalamander6308]

1:00 These graphs only show that all three sensors are not calibrated to '0' (have offsets typical of electronics). Sorry, this is not theoretical physics, it's engineering.

[@declanwk1]

if you allowed the accelerometer sensors to freely fall, then they would read zero during the free fall, so they are calibrated.

[@zbaktube]

Thanks for teaching us the Spring Theory! 🙂

[@MrKelaher]

The whole "gravity is a force" thing confused my intuition for years. It took me ages to see why the Einstein box thing is not metaphorically equivalent, but literally so, because the "floor" of the box is the key thing. Should not be taught to kids - why is science taught from the more incorrect oldest ideas to the more correct ones only in later years ? I think THIS is why quantum mechanics is still hard, not because it is, but because we have to undo all the incorrect intuitions we learn in the name of "false simplicity". It makes ZERO sense to me, we mathematicians do NOT teach calculus from fluxions to modem analysis, that would be insane, as much as I loved learning about fluxions and reading Newtons works at uni later and got some interesting understandings from it.

[@peterromero284]

This was great, Sabine. Another thing that would be interesting to address would be, why does curved space time cause objects to move?

[@soyosunset]

Same problem. The bowling ball on the trampoline illustration is used to explain the reality behind our naive notions of how gravity works. But the illustration makes sense to this naive person only because it implicitly shows a world with an up and a down and a bowling ball that goes down, just like our naive ideas about gravity say it should. This seems circular and evasive. I am very willing to accept that there is no way of explaining physics to ordinary naive people such as me. You can't teach even Aristotelian physics to dogs or goldfish -- why should we imagine that all people can understand Einstein? If something can't be explained, that's the end of it -- a pretense of explanation accomplishes nothing.

[@Gingnose]

Because mass also causes the time of curvature not only space curvature. Every object in this universe is moving with 'a speed of light' as GR says and that makes the object move towards mass as if there's a force but this is just a visual illusion. Since we can only visualize 3D space, we cannot recognize the axis of time dimension. But it is still there although we can't see. The Earth causes the time curvature and time moves slowly as you get closer to the Earth. Since we're all moving in the time dimension with a 'speed of light', the delay of time which is closer to the Earth side causes you to move towards Earth. Space curvature works likewise but it is only relevant when the two objects have the motion vector that is different from the axis between the two objects (if two objects aren't just free falling to each other but moving to other direction as well).

[@ValeriePallaoro]

it doesn't. I think is the answer. f=ma is the math to explain the movement, gravity is the explanation for how they move.

[@douginorlando6260]

Maybe the only thing moving is space, not the object …🤯

[@calinculianu]

Something something rotates you with respect to time but not space or something. The Science Asylum guy did a good video on this.

[@metube6859]

Thank you so much for finally making this clear to me! I've spent years trying to understand why the upward acceleration of the Earth did not allow me to spring into the air and fly!

[@Bob4golf1]

I've had a lot of exposure to Einstein's work but this particular one violates my physical experience and teachings. At 73 I've had a lot of experience with being in touch with mother earth and this view requires a significant adjustment to ones thinking. Thanks for this interesting lesson.

[@user-og4fk6os1r]

On the question of whether you're "accelerating" while in free fall vs. resting on the Earth's surface- Too many people say "space" is curved by gravity. That's wrong. If it were just space being curved it wouldn't take any more energy to move away from a gravitational field than to move into it - any more so than it requires more energy to move north on the earth than to move south. Nor would there be gravitational time dilation. Spacetime is what's curved by gravity in the GR model. The time part of that is what makes the model work. It therefore doesn't make sense to directly compare GR's four dimensional "spacetime" model of motion with Newtonian mechanics' 3D model where time is absolute and acceleration is *defined* as the second derivative of distance with respect to absolute time. In GR, the Newtonian definition of acceleration doesn't even make sense because the absolute magnitude of any object's 4D velocity vector is a constant (spoiler alert - it's always c); only the direction can change, which is of course not a constraint of 3D velocity vectors in classical mechanics. So any statement that you "are" or "are not" accelerating in GR has to be heavily qualified as to whether you're talking about a 4D velocity vector or a 3D classical velocity. When you are being acted upon by no non-gravitational influences, it is true that your 4D velocity vector doesn't change as you follow a 4D geodesic - because that vector is *defined* relative to a 4D geodesic! If it makes you happy, you can say you are not "accelerating in 4 dimensions." When you are being acted upon by a non-gravitational influence, on the other hand, your 4D velocity components DO change relative to a geodesic, for as long as that influence is acting on you. If it makes you happy, you can say that you are "accelerating in 4 dimensions". When you're standing on Earth's surface, the electromagnetic repulsion from the surface is pushing you away from the 4D geodesic you would otherwise be following, and therefore, if it makes you happy, you can likewise say you are "accelerating in 4 dimensions". But if you drop the "in 4 dimensions" part, then you're mixing apples and oranges - taking a statement that's true for a particular model and applying it to concepts from the prior model, which have no applicability in the new model, as if they prove the prior model wrong. The ugly truth is that all models are wrong, especially when it comes to spacetime. Some just make better predictions than others. No one has any clue what space or time even are. And the fact that GR doesn't work at the quantum level, and vice versa, ought to make us even more humble about making sweeping claims such as "gravity is not a force." The most common sin physicists commit in my opinion is confusing models for reality. This video, I think, is such an example.

[@bluesque9687]

This is brilliantly explained! Very lucid; however, for a layman like me this is mind shattering!! I can appreciate that you have done your best to make it clear but I am just so confused now!! I will have to rework my ideas in my head and find some answers!! Thanks!! I can't believe the ease of access to the privilege of these things being explained by a physicist of your caliber!! Love you, and love KZbin!! ❤

[@Markielee72]

I feel the same. I am beyond grateful to people like Sabine, who attempt to convey complex physics to the layperson. But videos like this just remind me how little I know. 🤯

[@VolodymyrLisivka]

I like how gravitational force is used to demonstrate that gravitational force is not a force because of geomethry of nothing. It's like 1 apple and 1 bannana: 1 = 1.

[@BooksRebound]

Just wait until you realize that the reason things fall is because your head is moving throught time slightly (like 0.00001 nanoseconds or something ridiculously small) faster than your feet, which basically takes your flat horizontal floating line and starts curving it downward (falling) to the ground. Time passes at different speeds depending on the curvature of space time, so that's further away from the planet move through time slightly faster.

[@antonystringfellow5152]

I find it helps to think of space and time as part of the same thing... spacetime. After all, that's how causality works (faster through space = slower through time and vice versa). When you take time into account, everything travels at the same speed, the speed of causality (cause and effect). From there, understand that time passes slower nearer a massive object, such as the Earth. Therefore, in order to maintain the same speed through spacetime, your path must be in the direction of the slower time... towards the object (or down). An object in orbit is not travelling a curve, it is travelling a straight path through spacetime. The difficulty comes from starting off with simple analogies that are very different from the reality. At the heart of space, time, speed and the gravitational effect is one single thing; causality. It is constant everywhere and for eveything.

[@ChristopherCurtis]

I'm not a physicist and I've seen too many videos to recommend one, but a moment that "clicked" for me was the realization that if you see someone throw a basketball and watch it curve up and back down into a net, you are not observing gravity, but are watching the ball travel in a straight line through a curvature in time (mostly in time; space itself is "flat"). For more related videos/channels, check out PBS Spacetime, especially "Does time cause gravity". Sabine has another video titled "You move through time at the speed of light". Science Asylum has "The REAL source of Gravity may surprise you". And then, to confuse everything, Fermilab has "Is gravity a force?". Have fun!

[@alexneigh7089]

4:57 Unless the spring itself has zero mass. It would be a clearer illustration if a weight is attached to the other end of the spring, and the spring's mass is assumed to be zero.

[@HughCStevenson1]

And for a non-massless spring the extension proportion (strain) of the spring is not uniform. The end that is attached extends more and the free end extends 0, proportionally, right at the end, because there is no mass attached to the end. It seems to be shown as uniform in the animation...

[@andrewm9425]

Why would you assume a massless spring for this discussion?

[@alexneigh7089]

@@HughCStevenson1 Yes. In effect, it measures (the manifestation of) weight, and it would be more intuitively clear if weight is attached to the spring rather than the non-uniformly distributed weight of the spring is measured with an additional complication of changing distribution when the spring extends/contracts. In simple terms, when you use scales, you do not determine the weight of the scales, but the weight of the item whose weight you try to determine.

[@e1123581321345589144]

Fyi: Every phone nowadays has an accelerometer on it. It's how it determines orientation

[@robertbrown1778]

This was so much clearer to me than Veritasium's attempt on the same subject which left me confused and with a reduced will to live.

[@danielbateman6518]

I think when we imagine (or even physically hold) a spring or slinky it becomes easy to see. We just have to change our perspective of what is acting our applying force. When the slinky sits in a pile on my hand with no gaps between the coils, because my hand is applying acceleration pushing it up. If I hold it from the top coil and it opens up into loose coils, it's because of my hand applying acceleration holding it up. If I drop it (and we imagine wind resistance and air turbulence having no effect) it will go to a position where it's neither fully compressed, nor under tension being stretched out. Additionally, it would be straight, the coils would all be in line, whereas if I went back to holding it, with one end in each hand, the middle coils would sag, because of the acceleration applied by my hands. If I apply more acceleration by lifting it above my head, the sag would be greater while I moved it. And if I suddenly pushed it towards the ground, the bend would be upwards rather than downwards. Simply put, the sag/bend in the coils is opposite to the direction of acceleration.

[@rustyspygoat4089]

I love my wife, but she is so bored of this. She rather watch tick tock and facebook shorts .. women ☕

[@Soulshine77]

brilliant. thank yo Sabine. They say that satellites are always "falling toward the earth" but are "moving too fast to hit the earth". I'm gonna rewatch this and try to map out a more correct and precise explanation.

[@lordkancer6962]

I absolutely love your videos. I am interacting in the comments section specifically so that it gets pushed out to more people.

[@JoachimJacob]

Finally, I was always wondering the acceleration i felt on this earth, without things moving. Thanks.

[@MrClivesinger]

Nice video! I teach this material to undergraduate students, and must admit i sometimes use the wrong terminology in the moment. I would say that I think your black hole animation may confuse some viewers who have heard of spaghettification - though i understand why you might have wanted to avoid the complication that two ends of the same object can be travelling along diverging spacetime paths and get stretched. Perhaps that's another video for another time!

[@oddarneroll]

Good point, idk if i understand your point about spagettification, maby you can help me underastand this betterr? What about equliliance principle in a box resting on a massive small object, in my head you chould measure a change in direction of accelleration across your box? And what about the theorised graviton particle, can you have a graviton particle but no force? All suggest gravity being a force, no? If quantum gravity theorems look at gravity as a force, is not quantum theroy really more fundamental (and therefore, closser to truth) than general relativity? If gravity is a force in quantum gravity, is it correct to say we know gravity to not be a force? im confused.

[@KaiVieira-jj7di]

@@oddarneroll Gravity is not force by direct measurement and measurements of other parameters that determine if the gravitational field cannot/(can) exert a force (the Local Lorentz violating parameter, β, for example). To date all measurement confirms that absence of a gravitational force, and this is independent of any theory. The graviton further demonstrates that the gravitational field cannot produce a force as the massless spin-2 field we associate with it reproduces the Einstein-Hilbert action and the Einstein field equations in the appropriate limits. What the virtual graviton field does (assuming it exists for the moment) is communicate the curvature, the curvature over which particles would travel along their geodesic paths.

[@raymondlines5404]

What book do you use for undergraduates? And does it go into simple dynamics examples like Sabine did in this video?

[@MrClivesinger]

@@raymondlines5404 It's quite a surface level introduction in a UK University 2nd year module called "Fundamental Forces" (and yes I do stress the irony regarding gravity not being a force!), where the majority of the year long module is spent on electromagnetism and the nuclear fundamental interactions. On the quick intro to gravity I go through a number of thought experiments involving a little doll of Einstein in a little elevator flying through space. My favourite part is explaining gravitational redshift :)

[@darkpheonix77]

What is the definition of "force" then? Its never stated in the video. I was actually thinking about spaghettification, or conversely an object long enough to mesure the difference in "gravitational interaction" on each end. Would that not show an acceleration?

[@treahblade]

This is a great video as you see gravitational force everywhere in science media all the time. I think people just have a hard time wrapping there head around the fact that normal space is not curved but space near earth is and how that would actually look like. The marble on rubber is a good visual aid for 2d space but then trying to translate that into 3d space can give people trouble. Another interesting thing is when you think about when spacecraft use a planet's gravity well for slingshot maneuvers. Where does the energy come from if gravity is not a force that can give the spacecraft more energy then when it started with? I know the answer to that one but curious if others here do :)

[@darkgreenmeme]

It seems like a semantics argument. As a human observer, we usually experience a "force" transmitted through the electromagnetic force, which keeps up from passing through other objects, as you point out. Hopefully physics will come up with a better description of gravity, but it appears to me that the "force" of gravity is caused by entities with mass tend to want to go towards each other such that "time" (another nebulous concept not fully explained by physics) passes at a slower rate compared to space further away from other entities with mass. The ultimate goal of any mass is to move towards the event horizon of a black hole, where time appears to stop in this universe. The Cavendish experiment does measure something that appears to be a force.

[@northvegassailrabbit3642]

Very informational, I'll need to watch this one a couple of times for it to sink in and I've had undergraduate level Physics, although I was not that good at it.

[@briancrowther3272]

Me too. I ended up teaching it at high school pre uni level when earth science is my natural go to ( a geology, physics, physical geography comboned hons degree). All high schools find it hard to get physics teachers. I eneded up doing it for 35 years and got better and better at it. It is a great topic am still learning, both the earth science and physics (a politics BA as well).

[@Thomas-gk42]

Fabulous explanation, you´re an extraordinary teacher. Peace and love for you.

[@SabineHossenfelder]

Thanks! Wish you happy holidays 🎄🎅

[@keithscott1957]

Yes, Sabine is quite a … um … force.

[@Thomas-gk42]

@@keithscott1957😉

[@VolodymyrLisivka]

She make me a lot of laugh this time..

[@danielstan2301]

Although a nice explanation I feel it is incomplete. For example it doesn't explain why gravity still accelerates mass while is not a force, what happens with the body once the whole earth suddenly disappears(will it continue moving towards where it was the center of the mass, stay still or will it go towards the direction where it was pushed by the force of the surface and why is that) and a few more questions that really makes gravity seem to behave like a force. On another note, can we consider gravity as a "force" that pushed against spacetime fabric causing its curvature? 😊

[@dealwolfstriked272]

Thank you Sabine for bringing science into youtube so we can try to become smarter.

[@mt7able]

If you are perpetually “free-falling” into the black hole and there is no force acting… what causes the probable spaghettification? Falling towards earth only hurts/is fatal once you collide with the accelerating earth. So is it actually possible to survive falling into a black hole? This is a legitimate, searching question. Thank you Sabine!

[@kurtwinslow2670]

Not a physicist by any means and I'm only repeating what I heard. As space\time gets squeezed the matter is said to be a singularity. Now I haven't a clue as to what this means. They use the term singularity at the big bang. Also, as much as I understand things, the Plank length is the smallest that energy\matter can go with any meaning.

[@declanwk1]

Sabine was describing falling into a very large black hole, for which the gravitational field is uniform over short distances. You could cross the event horizon without realising it. If you fall in a strong gradient, different parts of you fall at different rates which causes the spaghettification.

[@hamzahbakouni6208]

Wonderful analogy and presentation. As a fan of physics I may please ask whether the illustrated example of falling into a blackhole without noticing anything, may apply specifically to smaller objects and maybe in context of bigger blackholes in order to limit the tidal effects, as spacetime curvature may vary between adjacent points. Thanks. 🙏

[@hu5116]

Great video Sabine! Two comments. First, I’m with you on the whole gravity is not a force. BUT, then there are really only 3 fundamental “forces” (interactions if that is the preferred term), and then there is no need to quantize gravity, because gravity is not a force. This would explain also why it has been so hard to do. Second comment, it would be very good to get your take on the time causes gravity (or visa versa) discussion in many KZbin videos. There have been counter videos on this as well, which is why I think you weighing in would be a great arbiter. Thanks!

[@S.L.S-407]

@hu5116-Sabine already did a video on does time cause gravity.

[@dhruvvikrant]

Does time cause gravity? Need a video on this pls!😅

[@hu5116]

@@S.L.S-407ok thanks! I guess missed that one so need to track it down.

[@michele3900]

Floatheadphysics channel has a video to help visualise this rather neatly, he uses paper cutouts to show how it's the bending of time that causes gravity

[@MA-iridium]

Hello, greetings from a layman here who admires you and your videos, thank you for your work!

[@pon1]

What I think most people have trouble with, and me too, is the idea that earth is accelerating upwards towards you, it's a strange concept because obviously the earth isn't expanding yet you feel the earth pushing against you and you can indeed measure the acceleration upwards. Very hard to wrap my head around that part. Maybe it could be explained by the spacetime that earth is in is making it seem like it expands outwards, while it's actually the spacetime that bends inwards (gets "smaller"/"thinner" making the earth for all intents and purposes expand outwards, at least as defined by the spacetime it is in).

[@user-tq2no2wn9o]

Can an analogy be a gym treadmill ? We are running or accelerating on a gym treadmill though we are are not changing position as the treadmill moving underneath us on same velocity on opposite direction . So earth is us running on treadmill and the curved spacetime is the treadmill. The curved spacetime equalises earth expanding . I don't know if this I am saying it makes sense , sorry for my english

[@pon1]

@@user-tq2no2wn9o Yeah, maybe something like that.

[@simba995]

Sabine…one of a kind. Another great tutorial.

[@PedroPedruzzi]

Thanks, Sabine! You know, I've seen a bunch of intro GR videos and I got everything that's explained. However I still miss a graphical exemple showing how the curved spacetime causes mass to fall. I know non accelerated matter follows geodesic paths but how geodesics can be free-falling ones?

[@Gunni1972]

The "Gravity is not a force" theorem(eme). Acelleration is only absolute to it's starting point. NOT TO CURVED SPACETIME EITHER.

[@random_Person347]

As an eager physics undergraduate 50 years ago I struggled more than I ever expected with the explanations of relativity theory I was given, and this was the final straw that convinced me I did not want to ever be a physicist after all. I kind of understand it now, but I still can't quite get my head around the idea that the earth is making me accelerate upwards even though I'm motionless and the earth is not visibly expanding. The explanation, so I understand, is that the earth, due to its internal pressure, is expanding into space at the same rate at which space-time is collapsing inwards due to gravity, but why are these two things happening at the same rate? Why should there be an equivalence between the earth's internal pressure and the curvature of space-time?

[@narfwhals7843]

Because the earth's pressure and the "infalling space" have an equilibrium. That equilibrium defines the earth's size. If gravity were stronger, the earth would shrink until the pressure increases enough to balance it again. If it were weaker, the earth would grow until the pressure decreases enough.

[@montyburnham7704]

@Sabine I wish you would have used geodesics as part of your explanation to show how the object existing across converging spacetime geodesics must flow in the direction that the parallel lines get closer together

[@lrvogt1257]

One of the great things about the TV series "The Expanse" is how important acceleration, deceleration, and rotational simulated gravity are to the entire series. Spaceships are built like skyscrapers rather than ocean liners. They accelerate to keep everyone on the floor for half of a journey then flip the ship 180º around and decelerate for the second half so we see the rocket's engines firing towards the destination. Too rapid a change has obvious dire consequences. Spin gravity on larger ships usually provide 1/3 G. In one scenario people injured in a sudden deceleration had to get to the spin gravity ship so the simulated gravity would allow their wounds to heal. Very smart stuff.

[@georgeholloway3981]

I think Sabine has either redefined what acceleration means, or she is explaining to us that the common use of the word "acceleration" is the wrong one. Either way, she should explain this directly at the start (or middle, or anywhere for that matter). She does not seem to do this, however.

[@JT-sv9bi]

Exactly. Thank you. It is arguably somewhat addressed near the end, but indeed one should lead with that.

[@rivergladesgardenrailroad8834]

Acceleration is relative, my friend.

[@woobilicious.]

Acceleration is a change in velocity, there's nothing different about how she explains it here. I'm not sure where your confusion is coming from, maybe it's because you're still assuming distances and time are constant (newtons model), but the reality is that the speed of light is the only constant, and acceleration is absolute, and distances and time are relative.

[@rukidding7588]

@@rivergladesgardenrailroad8834 I would love to have a cousin named Acceleration, so I could truly say Acceleration is relative.

[@matsogren7143]

In general relativity there are only local inertial systems, that is, inertial systems that are (approximately) valid in the vicinity of a point in space-time. An inertial system is by definition a coordinate system in which Newton's laws of motion holds. Thus, these are the coordinate systems that do not accelerate. In Newtonian gravitation, there are inertial systems that cover the whole universe. For example, this means that an object in free fall towards the earth will have an acceleration with respect to such an inertial system. However it will not have have an acceleration with respect to a local inertial system that follows the falling object, and that is was is dealt with in general relativity. In Newtonian gravitation, a freely falling system will experience a cancellation of the gravitational force by a so called fictitious force that arises because the system is accelerating with respect to a global inertial system. For example, a local inertial system could be attached to a space station orbiting the earth, since the gravitational force is cancelled by a centrifugal force. An observer in the space station that does not look out, will not be aware of either force, though, and will not detect any acceleration or any gravitational force from external bodies; a fundament of general relativity is that gravitation and acceleration are equivalent. In Einstein's general relativity, both the Newtonian gravitational forces and the fictitious forces can be thought of as being absorbed into the space-time geometry. Still, the claim that gravity is not a force is rather pointless if you ask me, since you cannot describe gravitational interaction using only local inertial systems, but chacun à son goût.

[@JuanPabloCarbajal]

My kinematic picture gets confused. Isn't acceleration the second derivative of the position coordinates w.r.t. time anymore?

[@random_Person347]

Yes, but only in flat space-time, not curved. Also there is no fixed point in space that you can measure your position from anyway.

[@daviddayag]

it is so hard to explain this to people, i've been trying for years.. everytime im trying people think im either wrong or dont understand physics

[@edwardlulofs444]

Yes, exactly. But after 300 years of Newton saying gravity is a force, and only 100 years of a deeper understanding from Einstein, it’s still difficult to understand and believe. But I know it’s true. This might be the best video you have made this year.

[@cherubin7th]

Also in school you learn Newton's gravity, not General Relativity.

[@DanielCheng]

I don't know, but it's Einstein redefining things without giving it a new name..

[@brothermine2292]

Also, just because acceleration can be measured (with an accelerometer), that doesn't imply it can't be deduced by observing its (relative) velocity and applying Newton's equation for acceleration ( a = dv/dt ) as we were all taught to do in high school.

[@pootthatbak2578]

Sabine shut me up..i cant seem to absorb any of this lesson

[@edwardlulofs444]

@@DanielCheng yes that does happen. Sometimes I wish life was more simple. But I want the truth.

[@michaelaxton5005]

I'm confused. When you are falling above the earth, according to Einstein and an accelerometer, you are not being accelerated. But acceleration means change in velocity, which is definitely occurring relative to the earth. Also, I was looking for what would be different in our world if gravity _was_ a force. I don't think either were addressed in the video.

[@EdwinMartin]

Sabine explained why acceleration is absolute and not “relative to some other thing (earth)”.

[@forsakenquery]

@@EdwinMartin she did, and it made no sense. The definition of acceleration is "change of relative velocity with respect to time". If acceleration is absolute but the thing that defines it and it derives from (both conceptually and mathematically, acceleration is the literal derivative of velocity) isn't, then logic is broken. You can't just make words mean different things. Use a new word if you want for this idea of "acceleration without experiencing force".

[@EdwinMartin]

@@forsakenquery In our Newtonian world it doesn't make sense, in Einstein's spacetime it does make sense. It's fascinating! 🙂

[@forsakenquery]

@@EdwinMartin no, it doesn't make sense as the words don't have variable meanings in science.

[@michaelaxton5005]

@@forsakenqueryExactly. It makes sense to me only if acceleration is defined to be "what an accelerometer measures." Otherwise, if two objects leave the earth, moving with equal velocity and equal and positive increase in velocity with respect to (WRT) the earth then their acceleration WRT each other is 0 while their acceleration WRT the earth is positive. So is that what Einstein did? Redefine acceleration? Or is it more subtle than that?

[@icaronunes4074]

Great video! I have one question. In the example of the person jumping from the top of a building, around 12:00, why does the person suffer an impact, a larger acceleration or something, if it wasnt accelerating in the first place. In general, how can free falls cause collisions with consequences if the bodies did not gain velocity or energy (if energy or velocity even matter in this context) ?

[@user-np7ic2dh3n]

And I would have thought his speed relative to the building would change ie he would be experiencing accelerations ... ?!?

[@sergueigoussev491]

Great indeed. Now the famous "Black square" by K. Malevich could be surely considered as illustration of the scientific equivalence principle in Fine Arts. The part about pressure from inside the Earth is my favorite.😂.

[@Shamanicus]

Thank you so much for this video and explanation. Many times I told people that gravity isn't a force, but a consequence. I just never could explain what gravity is a consequence of. I'm not a scientist, there's more that I don't understand than that I do understand, I try to understand things though. People said to me that I didn't understand gravity and I simply replied that maybe I didn't, but that the other person didn't understand gravity as well and I was laughed about. Now I can address them to your video 🙏

[@jarirepo1172]

We have dark matter to explain how galaxies stay together, dark matter to explain why galaxies move apart from each other on large scale.... does anyone REALLY understand gravity?

[@paweljakubas7186]

Hi Sabine! Any chance you could make a video with book recommendations for beginners, medium and advanced physicist enthusiasts, amateur that want to follow your content with some fundamentals understood, that you found particularly worthy and didactic? Thanks in advance and best regards!

[@laestrella9727]

Seconded

[@JohnsOnStrings]

My main take-awsy: "acceleration is not a relative concept." While the equivalence principle may tempt us to think acceleration "might as well be due to a force" it's actually going the other way: that thing we thought was a force might as well be something else. (Granted, for earthbound Newtonian physics, the old way works fine.)

[@dr.python]

"Teachers use the term force because it starts with an F", simply brilliant.

[@kabongpope]

General Relativity was such a breakthrough, it's quite amazing after all these years

[@pholdway5801]

General Relativity is much more fun than Corporal Punishment

[@kabongpope]

@@pholdway5801 both can lead to Major Issues!

[@petergroves3153]

@@pholdway5801 Chacun à son goût.

[@silvergreylion]

None of the theories of relativity define an absolute rest frame. WIthout that, how do you apply the light speed limit to any inertial frame?

[@BosqueProfundo]

I think I undestood pretty much everything Sabine said in this video, but I still don't get the most important part: The space is curved because of mass, but why would you follow the path of that curvature (towards the center of Earth) instead of remaining on the spot you are? Why follow that direction of the curve specifically? Is it because you have to assume a pre-existing movement of the object relative to (towards) the other bodies (eg. the Earth)?

[@streettrialsandstuff]

It's not the space alone that is curved, but the space-time. As time passes, you are moved in space in a direction of a nearby object with a large mass.

[@lorscarbonferrite6964]

It's because that apparently curved path is actually a straight line (or sort of one, the search term you want to look up is a geodesic) in 4d space. Imagine a 2d being walking around on a 3d curved object, like a sphere. If they plot their coordinates in a 2d grid and move around, they'll notice some really weird things about their movements. For instance, if they were to try to walk in an equiangular triangle by moving in a straight line for a fixed distance then turning 60 degrees (both measured according to their 2d grid) 3 times in a row, they won't end up where they started, as on a curved surface the angles of a triangle don't add up to 180. But to the being that only knows 2d space, there will appear to be something weird deflecting their path. Similarly, assume two of these beings standing at the equator of a sphere. They move in opposite directions along the equator at the same speed, and then, at the same time, both turn 90 degrees towards the north and starting moving north at the same speed. In flat 2d space, their lines are parallel, so they should never meet, and yet they both meet at the north pole. To them, it looks like something is dragging them towards the north pole.

[@richarquis]

"And the real estate market would go to hell." That made me just about spit out a mouthful of wine. Wonderful. Thank you for making me laugh. I am constantly amazed at how people think science videos are boring. They have no curiosity and tiny attention spans. I'm an English Major, and I minored in European languages. I never studied Science formally. But... I watch a lot of the World Science Festival videos. The people involved (yourself included) are full of wit and enthusiasm. Those panels are constantly full of you guys cracking jokes and teasing each other, and it's so much fun to watch. Stephen Hawking's "Brief History of Time" is littered with jokes. Einstein made so many sarcastic and cynical remarks that he was almost up there with Mark Twain. Show me a business mogul with even a fraction of the wit and intelligence that you guys show, and I'll show you a pumpkin seed that can play the piano. I also go to sleep with the WSF videos and other related links playing beside my bed. The monologues or dialogues in the videos shape my dream narratives, in real time. It's a wonderful exercise in lucid dreaming. If I hear a dream character say something, then wake up, I only need to hit the back arrow once or twice to hear the exact same words on the video. I've had many dreams where I'm at university and Brian Greene was my professor. I was looking at purchasing an oscilloscope in real life, and he was talking about light rays in the video, so my dream had a bunch of oscilloscopes on the bench, and I was watching the screens change while he talked about the different wavelengths. Science videos on YT have opened my mind in ways I'd never imagined, and it's because of them that my interest in science has grown so much over the last few years. I'm also a kindergarten teacher, but I don't believe in talking to my students like babies. They know I am interested in Science, so when we do science class, I talk to them genuinely, and try to impart my enthusiasm to them. They're 6 years old (Korean age is measured differently to Western standards, so they're really just 5) but they know the difference between latent energy and kinetic energy. They understand that a rainbow is not just a pretty picture, but a combination of different wavelengths of light rays. They know that they can never actually walk towards the rainbow and touch it because of focal length. They know why Earth has a magnetic field, because of the revolving core, and how that deflects solar winds. They know that static electricity is the result of loosely bound electrons jumping from one medium to another, and that protons and neutrons do not come unbound in the same way. They know about the tectonic plates and how their movements create earthquakes and tsunamis. They understand the phases of the moon, and why the seasons change because of Earth's axial tilt. They know the difference between solar systems, constellations, galaxies, and nebulas. They can tell you that Venus is hotter than Mercury, or that Uranus is colder than Neptune, because of the atmosphere, not the distance from the Sun. They know what supernovas are, and how they become black holes. And yes, they even know the basics of Einstein's theories of General and Special relativity - They know gravity is not a force, but the result of space bending. If another teacher tells them that "Gravity makes things fall down" they will negate it, and say "No, gravity is not a force, and things don't fall down, they just move towards the centre." They know that a clock that is sitting still registers time faster than a clock in motion. They also know a lot about biology, they know the difference between producers, consumers (primary, secondary, and tertiary) and decomposers, they can answer all about today's lunch, which foods have carbohydrates, vitamins, protein, calcium, or acids, and what they do for our bodies. They know the difference between simple and complex carbohydrates. They know that our eyes project what they see as an inverted image on our retinas, but our brain inverts the image again. They know how our lungs combine oxygen with our blood and send it to our muscles. They know how photosynthesis works so that the plants can function as producers. We don't really do much in the way of Chemistry - Just the baking soda/vinegar volcano thing, but we also don't want them opening the cupboard under the sink and mixing household cleaners, so that's fine. And while we're on safety, they also know that it's not the volts that hurt you, it's the amps, so that's why they mustn't play with electricity, even though their body is already full of it. Next week, we will be making an electromagnet using a nail and some copper wire, so you bet we'll be talking about the speed of light. I'm the only teacher in my school who goes this route, but it's wonderful to see the results. I have a class full of kids who are totally into it, and can recall what we learned months ago, and answer correctly. They're already more scientifically literate than other kids twice their age. I couldn't be prouder. I watch these videos by you and other scientists, and I learn and grow from them. As a teacher myself, I pass that knowledge on as best I can.

[@MadAtheist]

I have ordered your book ... Lost in Math.... Looking forward to read it . I'm so happy to discover you.

[@__christopher__]

Gravitational force is a force in the same way that centrifugal force and Coriolis force are forces. All three appear because you are describing movement in an accelerated frame of reference.

[@jacksons1010]

We need to understand the word "force" has different meanings in context. For practical engineering, gravity is considered a force. The English language is full of such words with multiple definitions.

[@__christopher__]

@@viktorm3840 Yes, the (usually unspoken) assumption is that the box is small enough that all tidal forces/effects of curvature are too small to be measured. Otherwise you can just let two objects fall side by side and notice that they don't fall exactly parallel. And when falling into a black hole, the eventual spaghettification will quite violently tell you that you are not just floating in free, flat space.

[@DrDeuteron]

The reverse is true too. I.e., the Coriolis force is in fact gravitation.

[@DrDeuteron]

@@viktorm3840the vomit comet chooses a frame with no gravity. And no one cares about tidal forces, which is why the word local is used.

[@DrDeuteron]

@@__christopher__ or you could but an earth mass black hole in your elevator and then even your though experiment doesn't work.

[@tcl5853]

Sabine forced me to have an interaction with something or another relative to something else. The gravity of her excellent discourse about the myth of the force of gravity has left me wanting a half gallon of chocolate ice cream and another look at the video! And she’s one of the few reasons the internet and KZbin are worthwhile. ❤

[@undercoveragent9889]

A word salad is not the same as an explanation. And Sabine is very good at producing word salads that explain nothing.

[@tcl5853]

@@Benevezzioficial Relax! Did you read the entire post, the last sentence?

[@BeachHunter-ky6qn]

If you are accelerated , isn't any object held in your hand also subject to acceleration? Why then do we feel the weight of the object? Are we being accelerated at a greater rate? I watched this video last night and am watching again.Very interesting. Thank you for sharing your knowledge. I love science.

[@rolandlastname5532]

great talk! One of the pitfalls is to think all the time relative to our earth. Just imagine being "in space", far from a reference object like earth. It would be hard to measure your velocity. Velocity relative to what? Earth? But earth itself is running fast around the sun. And our solar system is in orbit through the Milky Way, etc. This way I am starting to get the picture of relativity

[@SALESENGLISH2020]

This is the most useful 15 minutes I have spent on KZbin. Thanks, Sabine.

[@nikinnorway]

I watched the Veritasium episode on gravity is not a force and it was really good. This one is even better. I love this channel.

[@governmentis-watching3303]

Yes, but Sabine said Acceleration is Absolut, but I dispute it is vodka.

[@mschwaller3371]

Thanks Sabine, great as always :) I've got this lingering question of how we know things are rotating or not. I mean especially in the universe scope. If a rocket would fly from one galaxy to another which has a much different rotation to the other, would that cause the rocket to get ripped apart? The example may not be very good but i hope you get my point. For example im quite confused by statements like the "sun is rotating". Please help :)

[@chrisbaker2903]

If you stand in one spot for a long enough time and you see the same distant objects come into view regularly, then you are rotating. That's different from revolving wherein you move around another object. So the Earth rotates on it's axis about every 24 hours and you know this because the sun comes into view again (sunrise) in that time. So does the moon although it revolves around the Earth while the Earth rotates under it. So it takes a bit longer to come back into view again because it has completed part of it's revolving in it's orbit around the Earth. Your question about flying to another galaxy is not something anyone would need to worry about because if you have a means to get there, the energy involved in doing so would be enormous, I mean really stupendously enormous, you would certainly have the ability to match velocity with wherever you entered the destination galaxy.

[@mschwaller3371]

@@chrisbaker2903 Thank you for taking the time. It is interesting that by just observing the sky we can determine those things. However, i'm aiming at a different aspect. When we rotate ourselves, we intuitively feel our arms being pulled outwards. The rotation seems clear here, its in regard to the ground or earth. But if earth rotates and the solar system rotates and the galaxy rotates ... what is the "base system" for this pulling force? Is it the nearest planet? Or solar system? Or maybe even a whole galaxy?

[@lew_ism]

8:37 The only thing I don't get about this is that you can only "feel" force when it's unevenly distributed. You can feel force/acceleration sitting in a car because the seat is only pushing on your back and the rest of your body resists that acceleration. Even if gravity were technically a force you still wouldn't be able to feel it in freefall because your entire body is being accelerated at the same time. With the "spring being pulled by an accelerating spaceship" example the spring only stretches out because it's only being pulled at one end. Say you had a negatively-charged spring (made of some hypothetical material where the charge is evenly distributed) floating in space and accelerating towards a large positive charge. It would stretch slightly because the end that's closer to the positive charge experiences more acceleration, but apart from that there wouldn't be any stretching because the electromagnetic force is affecting the entire length of the spring at the same time - i.e. it doesn't appear as though there's a force acting on the spring. Wouldn't that be the same effect as a spring in freefall towards a large mass, even though electromagnetism is a "real" force unlike gravity?