Gravity is not a force. But what does that mean?
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- čas přidán 17. 05. 2024
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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
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That was a tough one! The quiz for this video is here: quizwithit.com/start_thequiz/1702972458163x675901602454850000
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.
@@smlanka4uinteresting statements care to explain that to us chimps?
You left out tidal forces, which break the premise in the video's title.
Quizwithit asks for registration to see the correct answers :/
You should have given the correct answers 😜
The fact that Einstein married his first cousin Elsa, means even he didn’t understand relativity….
😂😂😂
Lol! But it is “relative” easy to understand reading books of G. Gamow (Mr. Thompkins…)
Btw: People, who say, they understand quantum theory, don’t understood it.
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 :-/
I tell this to both family and friends and they tend to do the same so don't feel alone 😅
That's just the life of a husband.
Well, don't try to explain this to regular people. For regular people and for practical purposes gravity is a force.
Very Funny, I’d wished to of been there 😄👍
She cared enough to pretend to care, that's a good start
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! 🎉
By C300
Veritasium has a video on the same topic years ago, I think he did a pretty good explanation
As a Star Trek fan , this is very disturbing. How can we travel without gravity, our bodies were designed to work with gravity. I can not accept the ability to create artificial gravity. ✌️
@@rayRay-pw6gz Speaking as a professional SF editor, while artificial gravity has been a common feature in SF stories and novels for decades, the writers have fewer ideas about how it might work than they do about warp drives or FTL drives in general. As SF ideas go, it's certainly one of the most unlikely. But we keep using it anyway because it's so convenient. This kind of winking compromise with physics is why the genre is called “science _fiction_”!
@@MosheFeder 😀. Reality sucks ! Thanks . 👍✌️
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...
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.
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.
So much of a construct; right?
@@kleinerprinz99 Statements that start with “It’s vert simple,” and then simply miss the nuances are always fun.
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.
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.
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...
"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.
"PLASTICINE", perhaps...? ;-)
Say pleistocene better@@MrKotBonifacy
@@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.
@@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.
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.
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.
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.
🤯
You lost me at " 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." How do objects interact with an "outside force" ? The ball rolling around on a rubber sheet, "captured" by a mass sitting on the sheet is NOT interacting with an outside force but it is changing its relative velocity and is therefore being accelerated.
Or is that the wrong way to understand this ?
@@onedaya_martian1238 A ball rolling on a rubber sheet is touching the rubber sheet. The atoms in the rubber and the atoms in the ball are repelling each other by the electromagnetic force. The ball travels in a circle because the sheet is pushing it that way.
If the sheet (and the air) weren't there, then General Relativity would say that the ball would travel in a non-accelerating trajectory through spacetime, which is curved by the strong gravitational influence of the nearby Earth. To our perception, the ball would seem to accelerate because it increases its speed with respect to the dimension of altitude. But, in General Relativity, it's not accelerating when you analyze it in the spacetime equations. Space and spacetime are not the same thing.
I had a hard time with this concept when I was younger. People would usually describe relativistic gravity by explaining that an object in orbit travels on a "straight line in curved spacetime". That kind of made sense to me, but what about if a metal ball were to fall straight down, starting at rest, from 1000 kilometers above the earth? That doesn't seem like a "constant spacetime curve". The ball starts at rest, then is accelerated to hundreds or even thousands of km/hour before it hits the atmosphere. Well, I had a breakthrough in understanding when I studied the General Relativity equations and realized that their definition of "non-accelerating" is in 4 dimensions. An object can accelerate in 3 spatial dimensions but be non-accelerating in the 4D spacetime equations.
I get a little irritated when people use the rubber sheet analogy to explain Relativity. The only way to really understand it is in the 4D equations. Gravity doesn't curve space, it curves spacetime, which is a mathematical concept.
@@onedaya_martian1238 The rubber ball contacts the rubber sheet. The atoms in the rubber and the atoms in the ball repel each other with the electromagnetic force.
The point I was trying to make is that General Relativity uses 4 dimensional math. People say that gravity "curves" space like a rubber sheet. It's much more than that. Gravity curves *spacetime* (there's a difference between spacetime and space). If gravity only curved 3D space, not 4D spacetime, then I think that it could explain how moving objects could orbit the planet, but I don't think that it would explain why stationary objects fall straight down. The fact that they are 4D equations enables gravity to actively morph 3D space over time. The altitude dimension of 3D space around a planet is constantly shrinking. According to General Relativity, a ball dropped from a tower doesn't fall because of gravitational "acceleration", it falls because the space underneath them is actively contracting. That morphing of space isn't considered to be acceleration. In spacetime coordinates, the object isn't moving. Once the ball contacts the ground, it does accelerate due to the contact force. After bouncing for a while, the ball settles on the ground. Gravity is contracting the space under the ball, but the earth is accelerating the ball upward. To us, the ball seems like it is at rest, but it's actually under constant upward acceleration that counteracts the shrinkage of the altitude dimension.
@@jeremypearson9019Interesting. It kind of reminds me of a flat earth theory which states that gravity is just the earth moving/accelerating upwards at 9.8m/s/s. But what i wanted to actually ask was about the seemingly perceived acceleration of free fall, or rather in this case according to your explanation, the increase of rate contraction over time. Like it's possible I'm missing something really simple which explains it but i dont really see why it should be the case that we "accelerate" in free fall
The flat earthers seem to have borrowed the acceleration/gravitation equivalence to make their ideas seem more scientific. But Relativity actually matches with observation and is mathematically sound and the flat earth theory fails miserably.
Your comment got me thinking: imagine you had two balls. You drop one of them from the top of the Tower of Pisa, then, when that ball reaches the middle of the tower, you drop the second one alongside it. The first ball will have a much higher velocity because it has already had time to accelerate, so it will speed past the second ball and strike the ground first. However, the two balls travel along the same path at the same time--straight down. If their motion is explained solely by the curvature of spacetime and not by acceleration due to gravity, then how could they move differently while occupying nearly the same space?
Well, the short answer is that 1. the math is very complicated and 2. I don't actually have a sound enough understanding of this particular case to give a satisfactory explanation. It just goes to show that when we talk about Relativity in layman's terms, the analogies that we use don't adequately explain straight, vertical falling. It's been a long time since I studied it. The bottom line is that the 4D math is complicated and the analogies we use (like the ball rolling on the rubber sheet) don't really do it justice.
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
Not something different, simply wrong. If you teach wrong things in school, you shouldn't be surprised when people say those things.
I agree. It’s like an unripe plum. No matter which direction you approach it from, it doesn’t become any more palatable.
Thankfully the phone didn’t ring.
@@robert-wr9xt huh :D
@@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.
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!
Pffft this is beyond stupid. If gravity wasn’t a force it wouldn’t do anything.
@@chrisstevens-xq2vb just because you're incapable of understanding does not make a complex set of ideas stupid. The stupid is you 🤷
@@chrisstevens-xq2vbIt's just another lie from big globe. Stay strong, brother.
@@chrisstevens-xq2vbIf you don't understand, you can say that instead of being rude.
@@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".
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.
"The most common sin physicists commit in my opinion is confusing models for reality." is probably the truest statement to be said about modern science.
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?
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.
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."
yes, but maybe not.
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
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.
Nobody puts gravity in a corner! 😂
So the matter matters. It makes a curvature within which lifeforms like us do our stuff. This mean planets matter.
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.
I like that. 👌🏻
@@harmless6813Lines that intersect are not parallel by definition.
It seems that latitude lines are parallel, but longitude lines are not since they intersect.
Nah it's the force of love 'cause we gay for each other
@@acebulletman7389a latitude doesnt have a "line" besides the equator
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.
wow! your explanation is the clearest I heard so far and I checked a lot of videos on you tube. thank you!
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.
if you allowed the accelerometer sensors to freely fall, then they would read zero during the free fall, so they are calibrated.
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.
Gravity is not a force by direct measurement, and has nothing necessarily to do with relativity.
The title confused me a lot at first, since we even call gravity "weight force" in Dutch (zwaartekracht). Wonderfully explained, very intriguing video!
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!
Absolutely fascinating as always - most accessible explanation I've ever heard!
As Absolutely fascinating as absolutely meaningless.
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...
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?
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.
Effect.. for gravity? I'm not familiar with that effect in that context.
I concentrated very hard and still have so many questions. It seems more like a semantic trap than an actual Physics problem.
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.
"Don't ask what is holding down the ball in the middle of the trampoline. It's too confusing."
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
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.
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.
Love the lesson with the quizwithit! added level of fun~
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.
If 'mass' does not exert a force on spacetime then why should spacetime experience any warping?
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 :)
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
klauswassermann8054
It is, and back in 1915 it was a _such_ a big deal for a reason. 🧠
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.
Is gravity a force? Now my answer will depend on why you want to know. Lol.
@@RobertStCyr-zh1tw Gravity is not a force unless it's the year is 2001, especially September, and especially in NYC.
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
That's a false analysis. No you cannot feel gravity. But that doesn't mean it's not there. You don't feel air around you when it's still. If your submerged in the sea you don't feel the sea around you. But in fact as you decend to deeper depths your body is compressed. Equally on the moon where gravity is much this has an effect.
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.
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)?
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.
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.
It would be helpful to explain why charge interactions are driven by a force and the differences with gravity.
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
@@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?)
@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.
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..
@@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?
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!
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.
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.
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.
Merry Christmas Sabine! 🎄🎅🌌🌟
Merry Christmas to you too!
I find it most intuitive to START by thinking of a leaf in a stream. That is like us in a gravitational field. :)
The stream (gravitational field), will have us float effortlessly downward.
If we get stuck against the rock in the stream the rock will impede our “natural” flow, and push against us. Then, we’ll feel like we’re accelerating against the water flowing across us.
If this analogy is helpful for anyone, give me a thumbs up, please. :)
The issue with this analogy is the issue with sabines video. It's just wrong to say you don't accelerate in free-fall. You keep accelerating (unlike the leaf, which will reach the speed of the water and stop getting faster) - as in, your relative velocity to objects resisting gravity continues to increase. To say you aren't accelerating at G is to redefine the terminology - the unit of measure of G is it itself metres per second per second - acceleration. It may be the accelerometer is seeing past the curtain of apparent acceleration but it requires us to redefine our terms so as to make them meaningless.
@@forsakenquery Thanks for your note. I think what you are describing is exactly what the Einsteinian revolution is about; it does involve redefining terms. :)
@@michaelyaziji but...that's not a revolution. That's just semantics. Einstein offered a different view of reality. Either it's a flawed view, or these science communicators (of which Sabine is usually one of the better ones) are failing. Because you can't say "you aren't accelerating" while ignoring the acceleration we observe without explaining what you mean. Acceleration means "change in relative velocity with respect to time". It doesn't mean anything else. The idea that it is absolute while velocity is relative is circular nonsense.
>> "It's just wrong to say you don't accelerate in free-fall."
If gravity is the accelerating flow *of "space" itself*, you are weightless in freefall, simply "going with the fllow".
@@soopergoof232 I'm not making an argument about weight. I'm saying your relative velocity changes.
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.
Exactly. Thank you. It is arguably somewhat addressed near the end, but indeed one should lead with that.
Acceleration is relative, my friend.
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.
@@rivergladesgardenrailroad8834 I would love to have a cousin named Acceleration, so I could truly say Acceleration is relative.
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.
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?
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.
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.
@@Feroand No need to "unlearn" anything. Just add to current understanding. We teach three states of matter in early science education because children don't have the capacity to understand plasma and theoretical states of matter. We teach Newtonian physics because kids don't have the capacity or math education to learn relativistic physics. The omissions and "corrections" can come later, when students have the capacity to understand them.
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.
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.
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.
@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.
Who says a spring has to be made of a material that can be affected by magentism?
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!
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.
@@187nemesis3 Who says Dr. Hossenfelder's spring has to be made of a material that can be affected by gravitation?
;^)
Before high school physics I thought Albert E adapted his math because spent a lot of time and energy walking around hilly terrain. He just naturally thought in terms of curved landscape. Then since inertia and gravity seemed so similar I thought everything must be expanding. Now this!
@sabine if an object is in free fall and if we track its motion at 1sec, 2sec etc. then how is the position of the object explained without acceleration?
Wonderfully spoken and difficult to comprehend. Merry Christmas Sabina and everyone else!
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.
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...
Why would you assume a massless spring for this discussion?
@@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.
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).
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
@@user-tq2no2wn9o Yeah, maybe something like that.
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.........
This concept is one that I still can't get my head around. As always, love your stuff, Sabine.
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.
I can't get my head around 1+1=3, mainly because it's not true!
@@vibaj16 OK, but why does the floor push on me?
Think of it as deceleration instead of acceleration and the quarter will fall (decelerate :p)
@@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.
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!
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).
@@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.
@@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.
That's really cool! So it can be imagined as falling into a Whirlpool and streching like spaghetti
@@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.
We are nowhere near as intelligent as we think we are.
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!
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.
So the mystery of gravity as a force between masses is replaced with curved space-time between masses. Ah, so clear now.
Well, it leads to equations that work for more scenarios than the older concept. That's progress.
@@harmless6813 ... but harder to solve so that's half a step back.
@@harmless6813 No it doesn't, relativity gives wrong results for a lot of anomalies in our solar system. It is a crap theory.
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.
Whether something is a force depends on your reference frame... If your reference frame is something relative to which you are accelerating, then you are experiencing a force. If your reference frame is such that you are *not* accelerating, then it is not a force within that frame.
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 CZcams videos. There have been counter videos on this as well, which is why I think you weighing in would be a great arbiter. Thanks!
@hu5116-Sabine already did a video on does time cause gravity.
Does time cause gravity? Need a video on this pls!😅
@@S.L.S-407ok thanks! I guess missed that one so need to track it down.
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
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 CZcams!!
❤
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. 🤯
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.
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.
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.
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!
Fall off the building and the fall won't kill you, rather, it's the sudden stop at the end of the fall that will kill you. That said, you probably wouldn't give a fig about gravity. Well done Sabine, keep em coming.
I absolutely love your videos. I am interacting in the comments section specifically so that it gets pushed out to more people.
Wonderful Channel, Incredible Host, Makes learning fun again.
Thank you Sabine for a wonderful channel.
Wishing you and yours a wonderful Holiday Season.
This was great, Sabine. Another thing that would be interesting to address would be, why does curved space time cause objects to move?
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.
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).
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.
Maybe the only thing moving is space, not the object …🤯
Something something rotates you with respect to time but not space or something. The Science Asylum guy did a good video on this.
I once understood gravity from a unique perspective. That understanding is long gone.
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) ?
And I would have thought his speed relative to the building would change ie he would be experiencing accelerations ... ?!?
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.
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.
@@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.
The reverse is true too. I.e., the Coriolis force is in fact gravitation.
@@viktorm3840the vomit comet chooses a frame with no gravity. And no one cares about tidal forces, which is why the word local is used.
@@__christopher__ or you could but an earth mass black hole in your elevator and then even your though experiment doesn't work.
Another highly inspired video. Thank you for teaching us how to think scientifically :) peace and love
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
Hello, greetings from a layman here who admires you and your videos, thank you for your work!
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!!!
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.
@@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!
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.
@@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
@@mikegale9757 it would be as correct as saying that the centrifugal effect is a force, which can simplify things a lot in certain cases
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.
Also in school you learn Newton's gravity, not General Relativity.
I don't know, but it's Einstein redefining things without giving it a new name..
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.
Sabine shut me up..i cant seem to absorb any of this lesson
@@DanielCheng yes that does happen. Sometimes I wish life was more simple. But I want the truth.
Thanks Sabine. I have a number of your books. I'm a physics graduate so maybe I can understand most of your concepts.
The notion of force itself is an abstraction that is arbitrarily created to measure changes in states of motion. What happened with Einstein is just that he saw behind this construct in relation to gravity. I wouldn't be surprised if the same twist would turn out with the other three fundamental interactions, meaning that there are no forces at all.
This is the most useful 15 minutes I have spent on CZcams. Thanks, Sabine.
General Relativity was such a breakthrough, it's quite amazing after all these years
General Relativity is much more fun than Corporal Punishment
@@pholdway5801 both can lead to Major Issues!
@@pholdway5801 Chacun à son goût.
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?
Fyi: Every phone nowadays has an accelerometer on it. It's how it determines orientation
Hi Sabine
Could you tell me where I have gone wrong….Let’s say you take the case of the train and flashing lights and put the flashing lights one up high and one on the floor in a rocket ship with Alice. Have the rocket ship accelerate upwards constantly the light basically blue shifts from bottom to top which is opposite of what the gravitational potential does. Now as the rocket ship comes into a landing back on earth it now shifts as the gravitational potential does. Would this not indicate a difference to be noticed between acceleration and gravity thus a flaw in the equivalence principle in this scenario? Try drawing the different scenarios out as I did one just considering acceleration up and down and another scenario of just using gravitational potential (which would not matter if going up or down as it is always red shifted upwards) curious where I have gone wrong.
Love your videos
"the light basically blue shifts from bottom to top" That is not correct. Light sent from the bottom to the top will arrive red shifted as the rocket accelerates away from it while light sent from the top to the bottom will arrive blue shifted as the rocket accelerates into it. Just like in a gravitational potential.
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.
Sabine explained why acceleration is absolute and not “relative to some other thing (earth)”.
@@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".
@@forsakenquery In our Newtonian world it doesn't make sense, in Einstein's spacetime it does make sense. It's fascinating! 🙂
@@EdwinMartin no, it doesn't make sense as the words don't have variable meanings in science.
@@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?
The equivalence principle is a simplification. You can distinguish between forces due to gravity and a constant acceleration because gravity is inhomogeneous. If you are a 1D point equivalence holds, but a 3D object experiences tidal forces.
Doesn’t the box and spring stretch when they enter the black hole as space is getting bigger one end in comparison to the other ? Or does this depend on the frame of reference you observe it from ?
In using words such as "Force" which Gravity is not, it would be good to define what a "Force" is. It is also good to know that we won't accelerate towards a black hole, when that time comes (or the space-time exists)
"How am I accelerating if I am standing still on the surface of earth". Your answer to that seems too simplistic and can be easily challenged. You say "All you do is not moving relative to the surface of earth, which itself is accelerating by the same amount." My complaint however is: I am also not moving relative to a chunk of earth 10m below the ground. This chunk however has a different acceleration than I have, and still, we stay at constant distance. I guess this comes down to the tricky definition of "not moving relative to each other" when spatially separated. I just want to emphasize that it cannot be brushed off as easily as done in the video.
Good point.
My views:
There are some things we simply do not have a mental model for, so, therefore, we must make up some explanation that fits into our world view. Middle World is a term coined by evolutionary biologist Richard Dawkins. This is the realm generally experienced by humans that lies between the microscopic world of quarks and atoms and the cosmic world of stars and galaxies.
We know how things 'work' in the realm, so we build models out of the things in that world.
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For example, photons sometimes act like particles and sometimes waves.
We understand particles by comparing them to balls. We understand balls This is the best analogy we are familiar with so we use it.
On the other hand, in other circumstances they act like waves, so we have a mixed explanation to fit our 'middle world' view.
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So, we come up with the rubber sheet model .. HOWEVER, please note that she totally avoids the question of WHY mass curves space-time. . .
She also says that a planet surface changes the physics without an explanation, again.
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First she says: Somehow the surface of Earth causes a force.
Then: "if you took away the surface of Earth, you'd fall." With no explanation of why.
Contradiction.
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This is a self contradictory story, not an explanation.
It's just a different model that is shoe-horned in to sound like it is an explanation.
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This totally ignores an important concept that what Newton's First law was supposed to teach you. WHY does the spring "measure" acceleration"?
Why would Earth "collapse" with out a force?
What causes Earth's "internal" pressure"?
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Velocity is not meaningless. It has meaning relative to other things - terrible justification.
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Yes, this thing we call gravity is indistinguishable from acceleration, so is acceleration real, or is it all gravity.
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Then she says you need a force to a accelerate something; so she does go back to Newton's First law.
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Also the spring would change going into the Black Hole, depending on the gradient of the space-time curvature being different on the two ends.
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Falling from the building you ARE accelerating in the absolute sense. yet she says you're not - DIRECT contradiction. It doesn't appear she understands the words she speaks, but isd merely parroting a script. .
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Sorry. Doesn't work. Too much contradiction.
I ask why it can't it be modeled as an attractive phenomenon and call it an force between all atoms.
Why does the chunk of earth have a different acceleration?
@@maddi62 Roughly, because the acceleration depends on the gravitational attraction which depends on the distance from the center of earth and the amount of earth above and below you.
Sabine, everyone is repeating that there is no way to distinguish free fall in a gravitational field from absence of any gravitational field. But recently I found some papers about the velocity dependence of the free fall "acceleration" (name it as you like, you know what I mean). This velocity dependence would allow experiments with high speed particles in a closed cabin which would definitely enable the observer to distinguish these two situations ...
Name the paper(s).
You mean like the Coriolis force is velocity dependent? So how would the experimenter determine that it's not just that the cabin is rotating?
If two bodies are falling in a gravitational field won't the distance between them reduce as they get closer to the centre of the earth? That wouldn't be the same as an absence of gravitational force where the distance would remain constant.
>distance between them reduce
No, it won't.
@@mikenewey3949 Yes, it would. But if the box is small enough, and the time of measurement is small enough, the relative movement will be too small to be measured.
Such a nice explanation of a complicated (well not anymore) concept. I remember seeing Veritasium's video on this topic but I remember I understood much less.
I figured, an accellerometer registers change in motion.
You're moving at 1000mph at the equator, along the surface
and your motion changes direction downwards, 15 degrees per hour.
I was thinking, this is why an accellerometer registers an accelleration upwards.
Finally, I was always wondering the acceleration i felt on this earth, without things moving. Thanks.
Sabine…one of a kind. Another great tutorial.
Thank you Sabine for bringing science into youtube so we can try to become smarter.
So I must've missed it, as I always seem to, every time someone explains how gravity is a geodesic distortion of space, not a force...
but by what physical [phenomenon; effect] does aggregate matter/mass accrue the *shear influence* to cause space to distort as described?
Thanks in advance.
Exactly where I get hung up, and why I end up thinking it’s a force after all.
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. 🙏
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!
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
Thanks for teaching us the Spring Theory! 🙂
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Although this explanation is perfectly valid, I always have trouble visualizing how all of us standing on different points of a sphere feel a similar acceleration in different directions.
From my understanding since retiring from teaching physics high school pre uni level, is this. Read a great book, why e equals m c squared by brian cox and a mate, I think that is where I got this.
IN general relatively, the clocks run slower (ill need to check, faster or slower the point is the same though) as you move further away from a mass. With big ones like Earth. That gives the illusion of acceration as the relative velocity at the mass is different than away from it. So this is a bit like say travelling from the equator to the north pole (in my case south as I am in Australia), but you want to go in a straight line (no warped sapce time due to no mass), but space time is curved due to the mass and that bends the line to the pole, as on a globe. The effect is you are made to conintually change direction (or it feels like it) as your path is being constantly corrected or resisted by the curved line between the pole and the equator. This gives the effect of acceleration but is not a force, it is like centripetal or centrifugal force by analogy. This makes the clocks run at different speeds at different disatnces from the mass. SO cool.
I really want to re read all that but cant find the book in a local bookstore ie Sydney CBD rats. Ill need to try harder, it has disappeared from my library.
@@briancrowther3272 I guess one can use a clock inside "Einsteins elevator" to tell wether one is beeing at rest on a planetary surface or accelrated in space then? If it ticks differently when at the bottom compared to ceiling, one is at rest on a planet, and if there is no difference, one is beeing accelrated somewhere in space?
@@heisag You're right, if you can detect a non-uniformity in the acceleration then you must be in a gravitational field. This doesn't contradict the equivalence principle, because the equivalence principle is only talking about a uniform gravitational field. BTW, you don't need a clock to detect non-uniformity, just make measurements with the accelerometer in different places.
You could try imagining a rocket hovering 1 meter above earth's surface. It's rather obvious that the rocket needs 1g of thrust to keep hovering above the surface and avoid falling down. If the rocket needs 1g of acceleration to maintain the same distance to earth's surface then earth's surface must be experiencing exactly the same acceleration as the rocket does.
@@JerehmiaBoaz Okay, except the rocket isn't accelerating. It's hovering, like you said. Now I get that's what this entire video is about, but if we're going to change the definition of something so fundamental, we should really lead with that, and replace the old meaning with something else. If the earth was accelerating upwards in all directions in a Newtonian sense, it would constantly expand. That's why people, myself included, still don't understand this. Pressure -> force -> acceleration, sure, if there is movement. If I press my hand on the wall, I'm exerting a force, but nothing is moving. No kinetic energy, no motion, no acceleration.
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 CZcams are worthwhile. ❤
A word salad is not the same as an explanation. And Sabine is very good at producing word salads that explain nothing.
@@Benevezzioficial Relax! Did you read the entire post, the last sentence?
"If you're in the enclosed box, you can't tell if you are accelerated or not." Are you sure? Because in an elevator, I sure can tell when the lift starts accelerating upwards, and when it slows down (As a little child, I used to to jump up when the elevator was about to reach the destination floor; and I feel that could jump higher when the lift was decelerating.
That's the thing. Are you in empty space accelerating up, or on Earth simply experiencing gravity? The point of Einstein's thought experiment is to demonstrate that you can't tell which situation is which from the inside.
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 :)
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?
The "Gravity is not a force" theorem(eme). Acelleration is only absolute to it's starting point. NOT TO CURVED SPACETIME EITHER.
Thank you for making such great videos about Important and debated Topics!
My comprehensive college physics book put it this way: "There is mass and spacetime fabric, mass tells spacetime how to curve, spacetime tells mass how to move under its curvature". A subjective interpretation of the cosmos in modern physics.
I should replace "mass" with "matter" above to be precise.