Is Gravity a Force?

Hi everybody. I was sick last week and lost like 10 pounds
in 3 days, which brings up the question, what is weight? Weight is actually the force that acts on
your body due to the pull of gravity. Now, the gravitational force depends on the
mass of the object that is generating the force, in this case, planet Earth. So you can lose weight by simply moving to the moon. Technically, therefore, I should have said
I lost mass, not weight. Why do we normally not make this distinction? That’s because in practice it doesn’t
matter. Mass just a number – a “scalar” – as
physicists say, but weight, since it is a force, has a direction. So if you wanted to be very annoying, I mean
very accurate, then whenever you’d refer to weight you’d have to say which direction
you are talking about. The weight in East direction? The weight in North direction? Why doesn’t anyone ever mention this? We don’t usually mention this because we
all agree that we mean the force pulling down, and since we all know what we are talking
about, we treat weight as if it was a scalar, omitting the direction. Moreover, the gravitational attraction downwards
is pretty much the same everywhere on our planet, which means it is unnecessary to distinguishing
between weight and mass in everyday life. Technically, it’s correct: mass and weight
are not the same thing. Practically, the difference doesn’t matter. But wait. Didn’t Einstein say that gravity is not
a force to begin with? Ah, yes, there’s that. Einstein’s theory of general relativity
tells us that the effect we call gravity is different from normal forces. In General Relativity, space and time are
not flat, like a sheet of paper, but curved, like the often-named rubber sheet. This curvature is caused by all types of mass
and energy, and the motion of mass and energy is in return affected by the curvature. This gives you a self-consistent, closed,
set of equations know as Einstein’s Field Equations In Einstein’s theory, then, there is no
force acting on masses. The masses are just navigating the curved
space-time. We cannot see the curvature directly. We only see its effects. And those effects are what we call gravity. Now, Einstein’s theory of General Relativity
rests on the equivalence principle. The equivalence principle says that locally
the effects of gravity are the same as the effects of acceleration in flat space. “Locally” here roughly means “nearby”. And acceleration in flat space is described
by Einstein’s theory of Special Relativity, so with the equivalence principle, you can
generalize special relativity to general relativity. Special relativity is the special case in
which space-time is flat, and there is no gravity. The equivalence principle was well illustrated
by Einstein himself. He said, let us consider you are in an elevator
that is being pulled up at constant acceleration. There is one force acting on you, which is
the floor pushing up. Now Einstein says, gravity has the very same
effect without something pulling up the elevator. And again, there is only one force acting
on you, which is the floor pushing up. If there was nothing pulling the elevator,
so if there was no acceleration, you would feel no force at all. In General Relativity, this corresponds to
freely falling in a gravitational field. That’s the key point of Einstein’s insight:
If you freely fall, there is no force acting on you. And in that Einstein and Newton differ. Newton would say, if you jump off a roof,
the force of gravity is pulling you down. Einstein says, nope, if you jump off a roof,
you take away the force that was pushing you up. Again, however, the distinction between the
two cases is rather technical and one we do not have to bother with in daily life. That is because in daily life we do not need
to use the full blown apparatus of General Relativity. Newton’s theory works just fine, for all practical
purposes, unless possibly, you plan to visit a black hole. Thanks for watching, see you next week.

100 thoughts on “Is Gravity a Force?

  1. I once read Mass is a property of Energy. It would be cool to hear your interpretation of that statement.

  2. Nice content! Though I had to laugh…
    Hehe you said you lost 10 pounds? Nooooooo kg???
    Isn't the SI system the all mighty system at least for non theoretical physicists? XD

    Glad you got better 🙂

  3. I don't know, is it really correct to say we can't see spacetime curvature? Isn't gravity lensing exactly that?

  4. If gravity is actually just curvature of space, then search for graviton (and theory of everything, which joins all forces together) is meaningless.

  5. Very nice indeed ! I have been interested in this matter, although I am not a scientist. So, because the moon has caused less curvature in space than Earth has caused, a body that falls freely to the moon will drop at a slower rate than the corresponding same case on Earth. I hope this is correct. As if changing the angle of an invisible slide ! The "slide" is the curved space and the angle of the slide depends on the mass/density that caused the curvature.
    We say that light cannot escape a black hole. Since gravity is not in essence a traction force , light should once have escaped the black holes. Since this is not the case, I assume that the curvature of space is so "big" that forces the light to move only inside the black hole (from inside to inside).

  6. I think people speak of weight in directions other than down as pulling g's. I'm not a physicist, though.

  7. Can't you say that it is both? That the force is proportional to the curvature, and is normal to the surface? But then I think of the infinite plane, whose surface is flat, but there is still a force. Oh, it can't look flat to an observing standing on it – you would be in the center of a parabola (or some-such) of revolution, so it appears that the time component is necessary.

    If I sound confused, it's because I studied Einstein's equation(s) years ago, and only the spherically symmetric solution of Schwarzschild.

  8. …if gravity is not force then a falling object is not increasing its kinetic energy ergo its mass is being converted to energy which is different from an accelerated rocket adding, mass-energy…go back to 4th grade and make sure you understand that much instead of precocious-Einsteinian-conclusion-jumping…

  9. So, can we say that this force that pushes us "upwards" is somehow connected to the expansion of the universe?

  10. Have you ever heard about the theory that dark matter/energy could be explained by gravitational time dilation?

  11. One of the things that is always difficult to me when the idea of 'spacetime' is presented is the fact that the 'time' concept itself seems to be very elusive. What is the difference of the 'time' from spacetime and the 'time' concept from the ever-increasing entropy of the universe that determines which direction systems evolve? If there's any connection, how is the entropic 'time' then so intimately entangled with space?

  12. When you say that something, like space, is curved it must be curved relative to something else which is flat. What is space curved relative to?

  13. Have you done any work with/investigation into CCC proposed by Penrose? If so, do you have any insights to share with those of us laymen who are interested in it?

  14. It's comforting to know there is someone who is more intelligent than me out there and still sane.

  15. hope you feel better! don't want you to lose more weight…er.. mass, that is without moving to the moon.

  16. You look great, Sabine, and your intelligence makes you even more foxy. I'm just being accurate, not annoying. Oh, by the way, Einschtein was wrong about the whole curved space-time thang. Acceleration does not curve space or time, and neither does gravity. #SlapMeSabine.

  17. The really big and fundamental question is why do two objects warp the region of space time around them and why is it that being inside that region where space time is warped do the two objects accelerate towards each other. Consider an empty universe where nothing exists except two identical spheres. Each sphere warps the region of space time around it. As such each sphere exists inside the region of warped space time created by the other sphere. The two spheres will then accelerate in a straight line towards each other. Figure out exactly why that happens and bingo – your Nobel Prize will be in the mail!

  18. Einstein used to be my hero in physics. Finally, I realized how many bad things he did to physics with his theory of relativity. Cosmology is trapped in field equations and can't get out of it. That's why we are in 'dark ages' in mainstream astrophysics.

  19. Never got why Einstein says there is no forces acting on you in the free fall example. My feet are more attracted than my head and even if the force is tiny, it is still there.

  20. But, isn't the "force" in Einstein's version coming from deflecting motion through time into motion through space? This assumption of motion through time is not the same as ordinary motion is it?

  21. comedic gold right there.. now I need a bit of flat mass with cheese stuck to the top side due to gravity

  22. I wonder if things really get pulled apart by a black hole?
    Perhaps they just stretch and from the locality of the black hole everything appears as normal as ever?

  23. Loss of 10 lbs over 3 days? Wow, 11,550 calories burned per day. Whatever she had was way better than seeing Jenny.

  24. 3:20 The elevator would have to keep accelerating faster and faster in order to maintain "gravity".

  25. When you say gravity is different from normal force, you have to define what is a normal force in the first place. You don't, Einstein didn't. But when we investigate what is a normal force, we get to the conclusion that it is a force like gravity. How comes? Locally, the potential of any gauge theory can be set to zero, the motion there is then a "free fall." It can't be set to zero in a neighbourhood so that the field strength cancel there, but so can't gravity when it is a true field, that is, like in the vicinity of a compact body. Then the equivalence principle breaks down, that's why general relativity isn't the equivalence principle, but has only been inspired by it, in addition to the Mach principle.
    Now please, please, let down the trampoline analogy that is not only oversimplified, but above all plain wrong. Granted, it is easier to explain something simpler because different, but that is not what is expected from you.

  26. Nice Segway to gravity. Glad you feel better.

    Einstein didn't quite get it. Gravity is the accelerating Force. When you jump off a cliff, it is the accelerating Force of gravity which propels you downward.

    Not having a quantum theory of gravity or not understanding that it is both a particle and a force is what leads to the inherent flaws with general relativity.

    All known forces (there are more than the usual 4, gravity, electromagnetism [which is 2 separate forces] nuclear weak and nuclear strong) come from the Graight (not a typo) force.

    After the Graight force broke apart in the big crunch/bang reality now has 2 basic types of building blocks; gravity and power. Each of those shows up as 3 forces.

    Power: electric, nuclear weak and nuclear strong

    Gravity: magnetism, dark gravity and acceleration.

    Of course, being human, I could be wrong. Unfortunately I don't think I am.

  27. If gravity is not a force then why bother with unified theory of wonder quantum forces do not match with is like mixing apples and oranges.

  28. I have a hypothesis in the realm of Physics and Astronomy; but, I have no occupational or educational highlights in either field.
    Should I share my ideas with the world, or should I just keep them to myself?

  29. "Wait" is what I am doing to meet the woman I love. Something to do with time. BTW I am losing weight. ;-).

  30. Imagine an empty universe, place only two motionless objects near each other, how can they start to move with no force?

  31. On a quantum wave scale there is only one time cycle. Only their infinite links on our scale hinder the clock. Large masses with gravity and the natural constant of the speed of light result in only one local valid time for each location.

    The Newton gravity as the limiting case of Einstein's field equation is obtained without curvature of space in the stationary case. ds²=-c²*t²

    ⇒∆Φ = 4πGρ

    Gravity is the effort of nature to capture energy. The pulse (time cycle) of the larger mass / energy forces the smaller mass / energy to sink in the gravitational field and adopt the slower time cycle.

  32. If you find the force of gravity at the top of the elevator is slightly less than at the floor, then you know you're not smoothly accelerating.

  33. So then, dark matter doesn't pull a force? But may I ask, what happen when dark matter, dark energy, and non empty vacuum are in a black hole? Does the climate change? The answer to this question would give several videos in one stroke.

  34. Newton had no notion of gravitational force, it is only an empirical formula that, along with the laws of dynamics, allow to calculate the motion of the planets and their satellites. It is not "subsumed" in general relativity, it is not even a theory. General relativity necessarily give this formula as an approximation, otherwise it wouldn't agree with experiment, but that doesn't mean the law of universal gravity is a limiting case of general relativity, they are of a very different nature, namely the space is absolutely flat for Newton, not approximately, and the prediction for the deflection of a photon by a mass is half in value. That was precisely the triumph of general relativity on the solar eclipse of 1929.

  35. I always thought it was a field, and it seems like this curvature is more or less a field as well. Not that I think it is true ultimately, but it seems to be the theory, even for Newton. Its seen as a force if you don't take the mass of the object being accelerated into account. Anyway, I could be totally off. 🙂

  36. Have you ever looked into the whole "electric universe" thing? I know it's pretty out there, but some of the theories about gravity and spacetime are fascinating to me and seem at least as plausible as the existence of dark matter. Of course I think all this stuff is fascinating and I'm definitely not a physicist.

  37. Ok, I will hazard to make a guess before I watch the vid and see if I;m right. I hope that you'll indulge me (to any kind passer-by who might be curious anyway). Gravity can be thought as a fictitious force, since even in GR objects in free-fall will follow geodesics. But there also is a set of equations, namely Neutonian gravity, which is an excellent effective theory for low fields (but will fail at strong fields or for sufficiently long times, when relativistic corrections become significant… and of course chaotic systems, I will not talk about this). In a similar way the Standard Model is seen as an effective quantum field theory for pretty much every high-energy physics experiment, but might fail at some energy which is yet not accessed (at least not easily) by experimentalists.
    Apologies for all that guesswork

  38. Frank DiMeglio is the world authority regarding physics/physical experience. ELECTROMAGNETISM/ENERGY IS GRAVITY.

  39. Sabine's body (matter) isolated in a cosmic void would bend space.
    Matter and space are conterminous everywhere in the Universe from the quantum level to the relativistic level.
    Matter and space comprise the dualistic fabric of the Universe.
    Matter's force of gravity and space's force of dark energy are negative reciprocals of each other.

  40. Danke Frau Hossenfelder, ich liebe einfach Ihre Videos, da Sie es nicht nur schaffen Dinge gut und klar darzustellen, sondern auch immer sehr wissenschaftskritisch sind ohne dabei eine Extremposition einzunehmen, sondern wirklich daran interessiert scheinen Wissenschaft voranzutreiben. Das muss man Ihnen mehr als hoch anrechnen!! Danke auf jeden Fall für Ihren Beitrag zur guten Wissenschaftlichkeit. Beste Grüße RD

  41. I am not educated in physics or astronomy. Hell, I don’t even have a bachelors degree. I wonder why those who are SME (subject matter experts) use theories but present them as facts. I do have a blunt understanding of the theory of relativity. I don’t understand why it is used as fact. Here is where my ignorance shows. Why don’t the SME’s use the theory but leave the door open as to the results of its effect on the subject at hand e.g. gravity as a force and many more.

    In advance, I am fascinated by these videos and I watch them often. Thank you both for the videos and hopefully for an explanation of an answer to my question.

  42. our body doesn't 'feel' a force. our body simply feels being constricted under pressure. so we feel a force with touching (in our back when accelerating a car). we don't feel a force acting on our whole body (a gravitational field). if our cells were negative we would not feel an electromagnetic field either though attracted by a force. so i don't agree with "i don't feel, so gravity is not a force".

  43. I believe it is possible that gravity really is a force produced by quantum level particles. Yes, GR produces accurate "gravity" but I believe GR is more like a mathematical regression fit.

  44. The mass of a particle or body stretches inifinitely. It fades, but never to zero. Not space is curved, but the mass. If space contains neither mass nor energy, it cannot be curved. But mass is filling the universe entirely. What we regard as the surface of a body (like that Hossenfelder body) is not the surface of a mass, but a strong change of mass density. If we touch it (i.e. that body…), what resists it not the mass, but electrical charge. Mass cannot resist, it can only attract.

    Could this be true or am I just talking BS?

  45. Can you write a song about this please… maybe from the point of view of a raindrop or everyday objects. 🌹🍄🍁🎄

  46. How do you explain then, when Wile E. Coyote runs off a cliff, he only falls when he realizes there's nothing holding him up?

  47. As usual, you presented a Great content, Sabine. But You look way more comfortable in front the camera . Congrats! You’re challenging yourself and helping a lot of people to understand some very complex theories.

  48. The problem with the closed lift, constant acceleration, analogy is that you relatively quickly bump up against the limiting speed of light. Therefore there is a way that you could distinguish between constant acceleration and gravity. The effect in the lift would diminish over time.

  49. you talked about the images that have been used by newton and einstein to describe gravity. but you never went further. What other concepts have we for gravity that work mathematically? It seems to me that one of the problems maybe is that we do not have a comprehensive description for what gravity really is.

  50. Are sub-atomic particles in free fall? Meaning, no force. Do electrons fall like big objects do? Maybe at some point in scale the universe is flat. You zoom enough into any smooth curve and it approaches being a straight line the closer you look. Does quantum mechanics handle acceleration?

  51. Sabine, hope you are fully recovered. You say you were sick and lost like 10 pounds in three days. I am interested in knowing which disease it was because I will need to have it for some 45 days…

  52. At the risk of sounding dumb: I think there's a difference between a force and space curvature. Since a force has a direction, then actions at a right-angle are immune to the force.
    However, if space is curved near an object, actions at a right-angle to the object causing the disturbance may also be affected.
    When would this be evident? When you are measuring the gravitational constant.
    The method originally designed to measure this force was – 2 masses, on vertical rods. As they approach each other, the force they exert is measured. BUT – this ASSUMES that, since they rotate around a vertical axis, then the force of gravity has been removed from the experiment. However, if gravity is a curvature of space, this assumption may be wrong.

  53. Equivalence: Move one meter in one second or in one second move one meter? 1 meter / 1 second vs. 1 sec / 1 meter. In the latter, time as the numerator becomes the variable. Time absolutely is a variable. Additionally in the first, 1m / 1 sec with only meters as a variable leaves 1 second or one unit of time as a mistake waiting to happen! Have we been been dividing by 1 in equations, therefore inadvertently removing time from the equation! Anything divided by 1 is itself so lets remove time from space-time and make space curved. Visit time-factor-theory com for more. Please use the contact form at the bottom of the main page if anyone is interested in helping advance this theory. Any contributions will be posted! (This theory also proposes that matter falling in a black hole will slowly be forever lost in the past and does not evaporate!)

  54. I was sick too recently and the curvature of space-time was pushing me down more than usual – no wait or was it pushing me up?

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