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Gravity is not a force – free-fall parabolas are straight lines in spacetime

• pininja 6y ago
  The video for this page provides great context https://youtu.be/XRr1kaXKBsU https://youtu.be/XRr1kaXKBsU
  • umvi 6y ago
    Great Veritasium video. So some flat earth arguments are actually correct if general relativity is correct, namely that gravity is an illusion and that the real reason we are stuck to the earth is that the earth is accelerating toward us at 9.8 m/s^2
    • vbezhenar 6y ago
      I don't understand it. How can it accelerate towards anyone on its surface? Where does it get energy to accelerate? We generally need to burn fuel to accelerate something in the space.
      • umvi 6y ago
        "in curved spacetime, an object needs to accelerate just to stay still" I don't really understand why, that was just the explanation Derek gave.
        • Smaug123 6y ago
          This amounts to a confusion over notation: "proper acceleration" (e.g. as measured by an accelerometer) vs "coordinate acceleration" (the acceleration an observer observes an object to be undergoing). The acceleration an observer sees you undergoing is the same as the inherent "proper" acceleration you're undergoing, minus the acceleration of their coordinate frame with respect to yours. For me to stay still with respect to you, if you're in a frame that is accelerating away from me, I need some proper acceleration to catch up and counteract the fact that our frames are diverging. But if spacetime is curved, your frame probably is accelerating relative to mine - c.f. the example on the Earth's surface, where our frames inexorably accelerate towards each other as we move parallel to each other. So for me to stay still with respect to you, I need to have some proper acceleration to balance out the coordinate acceleration derived from the fact that our frames are moving in a curved space.
      • nobody9999 6y ago
        My understanding of General Relativity[1] is that mass distorts space-time, so an object traveling in a "straight line" through distorted space-time will curve with that distortion. If the object's velocity isn't enough to traverse the curved space-time, it will move toward the center of the mass generating the distortion and fall out of the sky. If the object is traveling quickly enough, it can continue traversing the distorted space-time and orbit that mass. If the object is traveling even more quickly, it will traverse the distorted space-time and continue on without orbiting the mass. In all three cases, from the perspective of the object traversing the distorted space-time, it continues to travel in a straight line, as it's the space-time that's distorted. A (flawed) analogy would be riding a bicycle between the peaks of two identically sized hills. Starting at the top of the first hill, you coast down increasing your velocity. Once you reach the bottom of the first hill and head up the second, your velocity decreases. If your velocity at the bottom of the first hill is too small, you'll go up the second hill and as your velocity reaches zero, you roll back toward the bottom of the hill. You will pick up velocity and then roll back up the first hill, then down again, then back up the second, etc. until you end up stopped at the bottom of the hill. This is akin to falling to the center of the distorting mass. If your velocity is high enough to carry you back up to the top of the second hill and then stop, you'll roll back down and get to the bottom with the same velocity you had coming down the first hill. You'll then oscillate between the tops of both hills. This is akin to orbiting the mass. If your velocity at the bottom of the first hill is enough to carry you past the top of the second hill, you'll just keep going after reaching the top of the second hill. That's akin to flying by the mass. It's a flawed analogy, because in a curved space-time the directional portion of the motion vector doesn't change. As John Wheeler[0] simplified it: "Mass tells space-time how to curve, and space-time tells mass how to move." [0] https://en.wikipedia.org/wiki/John_Archibald_Wheeler https://en.wikipedia.org/wiki/John_Archibald_Wheeler [1] https://en.wikipedia.org/wiki/General_relativity https://en.wikipedia.org/wiki/General_relativity
      • manigandham 6y ago
        This is about relativity. We're going straight in space-time, but space-time itself is curved because of the heavy mass nearby (the Earth). This is visualized by the rocket curving towards the planet in the video, and the bent sheet experiment where the balls spiral towards the center. So we're curving in towards the center of the mass of the Earth, but the reason we don't end up in the core of the Earth is because the surface stops us. The Earth is "pushing" us away from the center, and that's the acceleration. It's accelerating you off your straight line path, and this is the deviation from the geodesic.
        • strken 6y ago
          I thought that the Earth curved spacetime, and since an inertial observer approaching the earth would follow a straight line through curved spacetime, they'd appear to be following a curved line through space towards the earth. What's confusing me here is the notion that when two objects collide, they accelerate into each other. Why and how is force constantly applied after the collision? My intuition is falling down here, and none of the resources I've looked at so far have explained why the acceleration happens.
          • manigandham 6y ago
            1st sentence is correct and is the same as what I said, sorry if I wasn't clear. Remember that spacetime = space + time dimensions. The object is always travelling through time, and the curvature of spacetime is converting some of that speed through time into speed through space. That's what you perceive as motion (caused by gravity). Time and space are linked together. The faster you go through space, the "slower" you go through time (as in you experience it slower). This is very measurable and even used to alter timings for satellites GPS readings. You can take an atomic clock on a plane and age slower than someone who just stayed on the ground. So the spacetime curvature is continuously converting some of your temporal motion into spatial motion, until that's stopped by the surface of the Earth which is constantly "accelerating" to stop you from going further. As to why we always move through time, that's beyond my understanding at this point but it's a fundamental axiom of physics.
  • c06n 6y ago
    I do not understand how you can have acceleration without changing position (at 10:06). Acceleration is the derivative of speed, which is the derivative of position change. If the position change is zero, how can the acceleration be non-zero?
    • saagarjha 6y ago
      If you watch a bit more, there's another term that is added.
    • Smaug123 6y ago
      Position is not an absolute notion: you need to answer "position with respect to what?". If the thing you're measuring position against is also accelerating, then you need to apply some acceleration of your own to stay still with respect to it. The terms you want to look up are "proper acceleration" and "coordinate acceleration". The curvature of spacetime means the thing I'm measuring position against is moving relative to me (c.f. the example of two people walking in parallel across the Earth, nevertheless eventually meeting: the curvature means that even though neither of them is measuring an acceleration, nevertheless they are accelerating towards each other), so I need to have some internal ("proper") acceleration of my own to counteract the fact that our geodesics are moving away from each other.
    • manigandham 6y ago
      Your position in spacetime is changing. You're going straight in spacetime, but spacetime is curved by the mass of the Earth so you're following that curve into the center. The surface of the Earth keeps you from actually falling in, and is therefore pushing you away or upwards from the center. This is the acceleration acting on your straight line path through curved spacetime. This is the deviation from your geodesic.
• layoutIfNeeded 6y ago
  How would periodic “free-fall” motion look in this setup? E.g. a point mass orbiting around a body, or a point mass oscillating back and forth in a 1D gravity well.
  • jdmichal 6y ago
    In the left-hand image, an orbit would be a horizontal line, because it's a constant distance. So it's a mirror of the time axis, but translated upwards in space. It would be exactly the same axis-mirroring translation in the other images. So, importantly, it would not be a line in the right-hand image.
    • bweitzman 6y ago
      The concept of a constant distance orbit doesn't make much sense in 1D. A horizontal line in this model wouldn't indicate an orbit, but rather a completely stationary object. It would then make sense for the world line to curved because it must be accelerating in order to resist the attraction of the body it's near. By definition, the world line of a stable orbit would be a line in curved spacetime.
  • xaedes 6y ago
    This has some visualizations of space time curvature where you can get a sense of the lines particles take in different circumstances: http://www.relativitet.se/Webtheses/lic.pdf http://www.relativitet.se/Webtheses/lic.pdf I tried to explain it with words, but I guess the images are worth more than I could write..
  • tim_hutton 6y ago
    With two space dimensions and one time dimension (2+1), an orbit in Newtonian physics is a helix. In general relativity that same helical path would be a straight line in an interestingly curved spacetime. Likewise I guess for the 1D gravity well case, where a sine wave would become a straight line in spacetime.
• frutiger 6y ago
  True gravitational force is something that can’t be transformed away by an arbitrary choice of frame (even an accelerating one). As a brief example, consider two objects in downwards free fall toward the centre of some massive object. Since they head towards the centre, in a free falling frame the two objects actually get closer to each other until they collide as they reach the centre. This is known as the tidal effect of gravity and is the actual physical content of general relativity. This effect can be shown to be obtained by an appropriate curvature of spacetime which itself can be shown to be related to the stress-energy of matter inhabiting spacetime.
  • freshhawk 6y ago
    "If you and a friend started walking straight north, both at the equator but a long distance apart, you would gradually get closer to each other until you collided at the north pole." I always thought that was a nice way to drop one dimension down to get the intuition. To the metaphorical 2D ant they see two friends attracted to/falling towards each other, but they are going in a straight line on a curved surface and there are no forces at play.
  • rokobobo 6y ago
    It's been some time since I studied these things, but I believe the post was trying to illustrate that the geodesic lines in spacetime created by Earth's gravity field can be visualized as straight lines after a nonlinear change of the coordinates system. [1] To simplify, these are the lines along which particles move when no outside force is exerted on them--again, considering gravity to be a distortion of spacetime and not a force, very much like the well-known metaphor of steel ball on a rubber sheet, which would curve marbles towards the "well" it's created in the sheet. But you're right that once the marbles and the steel ball get to a point where one of the gravitational fields cannot be ignored, this framework becomes less useful. [1] https://en.wikipedia.org/wiki/Geodesic https://en.wikipedia.org/wiki/Geodesic
  • dreamcompiler 6y ago
    I take your point, but I think the author's main point was that because objects in free-fall merely follow spacetime geodesics, it makes calling gravity a "force" a little bogus, at least compared to the other forces. Tidal effects don't change that; tidal effects mean the spacetime curvature "over here" is different than the spacetime curvature "over there", which means the principle of equivalence isn't true in a global sense. But objects still follow spacetime geodesics, which is a concept that's hard to reconcile with the notion of a "force."
• shireboy 6y ago
  I saw this and the related Veritasium video, but am still scratching my head about something. Does this mean that away from all gravity a body does not experience any time? ie a person on a spacecraft stopped in intergalactic space would not age relative to persons on planets?
  • chrisseaton 6y ago
    How do you get away from all gravity? Aren't you subject to gravity from all other matter in the universe at all times?
    • nobody9999 6y ago
      >How do you get away from all gravity? Aren't you subject to gravity from all other matter in the universe at all times? Yes. But the effect of the distortion of space-time that we call "gravity" is subject to the inverse square law[0]. This means that, as Newton described: "The gravitational attraction force between two point masses is directly proportional to the product of their masses and inversely proportional to the square of their separation distance. The force is always attractive and acts along the line joining them" As such, while objects are affected by the distortion of space-time, the effect is diminished (but not eliminated) by increased distance from the mass that's distorting space-time. Or at least that's how I understand it. [0] https://en.wikipedia.org/wiki/Inverse-square_law https://en.wikipedia.org/wiki/Inverse-square_law
      • chrisseaton 6y ago
        > but not eliminated So... you can't get away from it. That's what I said isn't it?
        • nobody9999 6y ago
          Yes, you did. However, it depends on how you define "getting away" from it. At some point, the effect is so small that it either can't be measured or even if it can, the effect is so small that any impact is irrelevant in practical terms. If that's the case, you're effectively "getting away" from it. I guess it's a matter of perspective.
• a3w 6y ago
  Somewhere I read that a free fall parabola does not even take into account earth's curvature. Although I cannot remember, what kind of function describes the reference system specific ``path, as a function of time''. Could this be named more correctly: Gravity is not a force – free-fall hyperbolas are straight lines in spacetime (timhutton.github.io) ?
  • kubanczyk 6y ago
    The planet's curvature is not very relevant to the article, but, yes, if an object is in a free fall at a slow speed (I mean non-orbital) its trajectory is: - parabola if you assume flat&infinite ground, - ellipsis if you assume a spherical planet (an ellipsis is crossing the planet's surface). This is Newton, not GR.
    • dreamcompiler 6y ago
      Yep. Orbits are always conic sections. Which sometimes inconveniently intersect with the surface of the thing being orbited.
      • dreamcompiler 6y ago
        More precisely, 2-body orbits are always conic sections. Once you add more bodies, orbits usually become chaotic.
    • skykooler 6y ago
      Of course, a very very eccentric ellipse approximates a parabola quite well. You only start to notice errors when dealing with trajectories dozens of miles long (on Earth).
• Smaug123 6y ago
  I heard an interesting question at one point: "how come, when you throw a ball up on Earth, the parabola is so strongly curved? Spacetime is nearly flat, so how can a straight line become such a steep parabola?" I'll answer this question as I understand it, but I only took four lectures of General Relativity before I gave it up in favour of computability and logic, so if there is a more intuitive and/or less wrong answer out there, please correct me. Intuitive answer: the curve is indeed very gentle, and (e.g.) light will be deflected only very slightly by the curvature; but the ball is moving for a couple of seconds, and that's an eternity. On human scales, the time dimension is much "bigger" than the space dimensions (we're quite big in the time dimension and quite small in the spatial dimensions); the ball moves only a small distance through space but a very large distance through time, amounting to a big distance in spacetime, and so the slight curvature has a bigger effect than you might expect.
  • atoav 6y ago
    This is in tune with what happens when you change the timescale in a game engine with proper physics.
    • grenoire 6y ago
      I think this is more of an integration method error than an analogous case.
    • hunterloftis 6y ago
      I'm not aware of any game engines that simulate general relativity / 4D spacetime. I think what you may be noticing is that, as you reduce the tick frequency of a newtonian physics simulation, parabolas become less accurate as integration error accumulates.
      • Agentlien 6y ago
        Along those lines: during my university years, this is what I saw in my homemade engine before I had a good understanding of numerical stability.
      • 6y ago
• willswire 6y ago
  Veritasium just put out a great video on this: https://www.youtube.com/watch?v=XRr1kaXKBsU https://www.youtube.com/watch?v=XRr1kaXKBsU
  • avmich 6y ago
    A great video :) . Makes you wish he'd include more explanations of mathematical details...
    • lucb1e 6y ago
      I'm happy they didn't, making it a great video for me as well :) Not that I'm afraid of math, I just don't feel like I need to see equations when it's a concept that is being explained rather than how to numerically calculate something. Math helps me as much as a code implementation does to explain the concept of a variable: I can observe its behavior but it doesn't necessarily teach me the concept.
  • seaish 6y ago
    It's linked at the bottom of this article.
  • trungdq88 6y ago
    This, and the one by Vsauce [1] are the 2 best videos explain spacetime on Youtube I have seen. [1] https://youtu.be/Xc4xYacTu-E https://youtu.be/Xc4xYacTu-E
• ars 6y ago
  Would not an electron and a positron trace out an identical line/parabola? Would that mean electromagnetism is not a force either? What is the difference between the two that makes one a force and one not a force?
  • tim_hutton 6y ago
    This is a great question!
  • caf 6y ago
    You can't create a similar transformation for an electric field under which the motion of a proton, electron, muon etc are all straight lines. The indicator that gravity is special is the fact that 'inertial mass' and 'gravitational mass' are the same thing.
    • nsonha 6y ago
      > gravity is special 'inertial mass' and 'gravitational mass' are the same thing. That's undecidable and gravity is not special in that way https://physics.stackexchange.com/questions/390540/what-is-the-relationship-between-gravity-and-inertia https://physics.stackexchange.com/questions/390540/what-is-t... > the gravitational field and acceleration are inductive pairs (similar to the electromagnetic field and electric current.)
      • caf 6y ago
        That's undecidable and gravity is not special in that way How so? You can have different electric charge to inertial mass ratios, but that is not so for gravitational mass to inertial mass.
        • ars 6y ago
          Gravitational attraction is a function of both objects, so the force between them is not a constant, same as for the electron. Also, what if you have a charge so high that the electrical-escape velocity exceeds the speed of light? A strange kind of black hole? I ask these things because I do not understand why gravity is called "not a force", while electromagnetism is, when I see no real difference between how particles act.
• mycommenthe 6y ago
  if there is no force as gravity why does the curvature shape cause movement? why foes curve imply a movement at all?
• didibus 6y ago
  I'm a bit confused, how does a 2D graph with a curved X axis work? Does this just mean to say that if we saw things distorted with an outward curve, then something that is moving in a curve would look to be moving in a straight line? And what's the point of such an observation?
  • aeternum 6y ago
    The polar or cylindrical coordinate systems are examples of graphs that can be though of as having a curved x-axis.
    • bawolff 6y ago
      Sure, but that is irrelavent since the topic at hand is neither a polar nor cylindrical graph.
• deleted 6y ago
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• j1vms 6y ago
  Gravity is not a force. The surface of the Earth is moving up to the object in free-fall at an acceleration of 9.8 m/s^2. The force pushing the surface, and the pressurized atmospheric shell, upward is a result of the processes occurring within the Earth (likely, in particular, those within the the core).
  • lopmotr 6y ago
    What direction is the earth accelerating in Australia? It's not deforming.
  • j1vms 6y ago
    Here's [0] a bit more thorough explanation of this concept for those who are interested. [0] https://www.youtube.com/watch?v=XRr1kaXKBsU&t=504 https://www.youtube.com/watch?v=XRr1kaXKBsU&t=504 (Veritasium on Youtube: Why Gravity is NOT a Force @ 8:24)
• colordrops 6y ago
  How is it not a force though? Regardless of curvature, a ball starts moving if you let it go without applying any force. Curvature alone can't account for that could it?
  • doomrobo 6y ago
    When you are at rest on the surface of the earth, you’re actually accelerating. I barely understand it but veritasium explains it in the newest vid https://youtu.be/XRr1kaXKBsU https://youtu.be/XRr1kaXKBsU
  • saagarjha 6y ago
    In general, it is convenient to assume that forces exist. However, the model presented here shows that you can instead dispense of the need for there to be a force and show that the apparent acceleration is just the object moving along its curved path in spacetime. You could consider it as if the curvature itself making it look like there are forces.
  • tim_hutton 6y ago
    An object at rest is still travelling through time. In fact it continues to travel at the same speed through spacetime when you let it go. It's just that spacetime is distorted such that the straight line takes it across space too, as seen in the inertial frame on the right.
  • gilgoomesh 6y ago
    The falling object doesn't accelerate. You, standing on the ground are the one that's accelerating. You see the object as accelerating but that's an illusion due to frames of reference. As evidence: which object feels a force on it? You can feel the force the ground continually pushes up at you. The ground is accelerating you up. The falling object is completely idle in its inertial frame and feels nothing.
    • colordrops 6y ago
      Ok so that makes sense, but if we are accelerating by standing on the ground, isn't that implying that we are continuously increasing in energy? Couldn't that be harvested for a perpetual motion machine of some sort?
• deleted 6y ago
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• nimish 6y ago
  Gravity is a force in the same way the centrifugal force is a force An artifact of rotating coordinate frames in one, the curvature of spacetime in the other
  • tiffanyh 6y ago
    I think you mean centripetal force. https://www.diffen.com/difference/Centrifugal_Force_vs_Centripetal_Force https://www.diffen.com/difference/Centrifugal_Force_vs_Centr...
    • nimish 6y ago
      No I mean centrifugal. It's a fictitious force, an artifact of choosing a rotating non inertial coordinate frame. Gravity arises similarly, by being in a non inertial frame induced by mass energy https://en.m.wikipedia.org/wiki/Fictitious_force https://en.m.wikipedia.org/wiki/Fictitious_force This was the key insight behind Einstein attempting to get general relativity working.
  • yunruse 6y ago
    Mm, just like we have wave-particle duality, one can also speak of some sort of “force-coordinate duality”, as it were. Just as we let the sine function be linear for small angles, quasiparticles with forces in Cartesian space are wonderful abstractions. Good physics – especially applied physics – is about handling this balance between precision and mathematical/computational simplicity. Mind, research physics can be the opposite; a lot of nuclear involves (often special) relativity as combined with QM. Until we have a unifird theory, however, the graviton and its force ain’t dead yet.
    • nimish 6y ago
      I'm unconvinced that treating spacetime as a quantum field is the right approach -- just a gut feeling -- given gravity "feels" so much different than the quantum theories and general relativity is just so beautifully geometric. I'd be very sad if we had to lose that and deal with gravitons.
• thesz 6y ago
  That brought up the verse from Pushkin [1]: One bearded sage concluded: there's no motion. Without a word, another walked before him. He couldn’t answer better; all adored him And all agreed that he disproved that notion. But one can see it all in a different light, For me, another funny thought comes into play: We watch the sun move all throughout the day And yet the stubborn Galileo had it right. [1] https://ruverses.com/alexander-pushkin/motion/ https://ruverses.com/alexander-pushkin/motion/ Both are right, in short. We observe parabolas and we are right counting on them - we have great successful experience using these prediction. GR theory predicts lines, they are right too, but in different context.
• abeppu 6y ago
  I've seen this explained elsewhere, and it does look very cool when displayed this way. But perhaps someone with more background can explain to a lay person -- what even is a force? Why does the existence of a transformation that makes movement under a supposed force actually follow a straight line mean it's not really a force? For the other forces (e.g. electromagnetism) can we say that there's _no way_ to exhibit a transformation that causes charged particles travel on "straight" lines?
  • ncallaway 6y ago
    Newton's first law provides a guide: "Every object persists in its state of rest or uniform motion in a straight line unless it is compelled to change that state by forces impressed on it." So, the definition of a force as something that causes an object to change its movement from a "straight line" comes to us from Newton's laws. > Why does the existence of a transformation that makes movement under a supposed force actually follow a straight line mean it's not really a force? It's not just the fact that the transformation exists that means we don't really consider gravity to be a force. It's that the transformation exists, and provides useful predictions about the universe that turn out to be backed by experiment. Under the theory of general relativity gravity isn't a force, because the fundamental premises of general relativity assume that gravity is actually a distortion of spacetime caused by mass. We say that "gravity" isn't a force simply because the predictions made by general relativity have been validated by a number of experimental observations—at least on the macro scale. > For the other forces (e.g. electromagnetism) can we say that there's _no way_ to exhibit a transformation that causes charged particles travel on "straight" lines? I'm certainly not an expert, so I can't really comment on this. My best guess is that there doesn't exist any such transformation that causes charged particles to travel on "straight" lines that makes experimental predictions as well as our current scientific theories.
    • marta_morena_28 6y ago
      So what you are saying is that if we assumed that all particles, including light are magnetic, and everything that has a mass, emits a corresponding magnetic field with a strength relative to its mass, we could not form a similar theory of "general magnetic relativity" in which the frame of reference under magnetic fields would behave in a similar way it does for gravity? That seems kinda odd. What exactly would provide this difference? How is magnetism different from gravity? Is it that gravity doesn't have a mass, but magnetic fields do?
• nnx 6y ago
  Veritasium recently posted an excellent video on the subject: https://youtu.be/XRr1kaXKBsU https://youtu.be/XRr1kaXKBsU
  • manigandham 6y ago
    That's linked at the bottom of the article. Another good video to visualize GR is this by ScienceClic: https://www.youtube.com/watch?v=wrwgIjBUYVc https://www.youtube.com/watch?v=wrwgIjBUYVc
• mrwnmonm 6y ago
  I can't imagine how this applies to the whole earth. Assume there is 'top' and 'bottom' of the sphere, don't know how they do navigation in space, but assume it is like the usual earth maps. So how the things on the bottom get drawn to the earth, while they should go into the other side. Clearly there is something I don't understand, but I don't know what it is.
• sfink 6y ago
  Ok, great opportunity for me to ask a dumb question that's been bothering me for a while, for practical reasons I won't go into. How is gravity like a force at all, even in Newtonian physics? It seems like mismatched units. Gravity is an acceleration, not a force. F=ma, right? So if gravity were a force, it would produce an acceleration that was dependent on the mass, and it doesn't, so it seems to me like the only sense in which gravity is a force is if you define force as "something you can't see that makes things move", which is a pretty useless definition.
  • bananabreakfast 6y ago
    Gravity is a force that acts due to mass just as the electromagnetic force acts due to charge. Since gravity acts in a magnitude directly proportional to the mass being acted upon, it just so happens that it manifests as an acceleration since the mass cancels out, unlike EM where acceleration due to charge does not cancel out the mass.
  • aflag 6y ago
    According to Newton's equation, gravity's force is F=(GmM) / r^2. Those are the gravitational masses. The mass in F = ma is the inertial mass. The fact that they are the same is remarkable. If it weren't for that, different mass objects would feel different accelerations due to gravity.
  • caf 6y ago
    In Newtonian physics it's a force with a magnitude that is proportional to the mass of the body. For two different objects at the same point in the same gravitational field, the difference in gravitational force due to their differing mass exactly cancels out the difference in acceleration due to their differing mass, so they both accelerate at the same rate.
    • sfink 6y ago
      Sure, but the phrase "it's a force with a magnitude that is proportional to the mass" is like me asking "how heavy are you?" and you replying "75 liters", expecting me to know that humans are basically the density of water. My point is that it really seems like "gravity" is not a force, it's an acceleration, but there is something you could call "force due to gravity" that you reverse-engineer from the known acceleration, and that means you need to multiply by the mass. Clearly different masses will just cancel, so the resulting acceleration is the same. I'm fine with saying "force due to gravity" or even "gravitational force". Which is what you're describing in your first sentence, and I have no disagreement with that. "Force of gravity" starts to sound a little off, and I bet if I ask "what is gravity at Earth's surface?" I'll get back "9.8 m/s^2", which is an acceleration not a force.
• smt1 6y ago
  What's more deep IMHO, is say, generaling things so you don't think so rigidly. Remember, just by naming things, it creates a bias. It's an implicit bias but a biased behavior to be sure. It creates a cultural bias just by knowing certain "groups" of people who know that name and associate with a sensation cause stuff to happen. A lot of modern day cognitive scientists and neurosciencts know about this. It causes a cultural bias. If you drop the rigidity of the "names necessity" semantic bias (just google or whatever thsa). Then it can lead to much better innovation and software/hardware sensing. Remember in physics, despite Yangs-Mills theory, we still haven't been able to use deformation mechanics and some unitary renormalization group but can apparently explain a lot of physical phenomoninon, but still can't do a full integration despite flavors of the standard model. So local gauge theories which respect various local/global laws that can easily encode symmetries in constructive D-G symmetries/anti-symmetries to make things commute (reunify math) across boundaries to attach to enough new constructive cohomologies in comonadic space (you get full stack frames back cause a page fault if you imagine the second dimension as breaking the riemann hypothesis, a lot of problems are isomorphic to it). This basically allows you to perform reverse mathmatics in some eigen computation problem to solve decision problems reflexively by using any old applicative language. Just turn stuff and look the self-adjoint properties of any arbitrary graph and look at the compositional structure. I learned most of this stuff by just re-reading Edwin Thompson Jaynes ideas on mathmatical physics and probability theory. He clearly talks about joins and meets (lattices are just combinatorial 2-ary lattice structures) but you can generalize on arbitary distributive lattices these days and generalize the creation of artifical neural networks. You can just point to how to apply it to any higher order language like swift that has a SIL where differentiate could be defined to define big step small step semantics and then use say a language like rust to create a series of n-ary modules to define n-ary 1 order prepositional substructures with 2-ary levels of static type checking about memory and type safety. Remember, names are arbitrary and prevent creativity, make things you are talking in the right semantic codecs (shared language), once you can understand the digital (unicode) spectrum, you can just abstract stuff. It's easier for me to talk in differential geometry rather than "trivialized linear algebra" which has been trivialized by non-linear optimization. You can talk about much larger invariants of spaces that can triangulate two indeo-differential systems of equations easily back to a simple 1-transversal span folded space bit which is too regular these days. Remember all the world sheets that physicists used to scan worldsheets over? The same thing applies in theory of computation. Remember convex optimization? Same things. In linear non-linear functions it's all trivialized. You don't even need FPGAs or ASICs anymore once you learn about discrete C-algebras where can be thought about gossip in cryptographically encrypted protocols.
• avmich 6y ago
  A pretty good model. Thank you. The model shown is for 1D space and 1D time, and this 2D spacetime is curved, remaining 2D. Can you though show us a 3D model of curved 2D spacetime? Maybe as a rotatable, scaleable 3D scene, where more complex phenomena, like planetary motion around a star, would be possible to show?
  • avmich 6y ago
    Read TODO list on Github :) . Looks like it's harder than appears...
• GuB-42 6y ago
  My enlightening moment about general relativity: apples do not fall on the ground, instead, the earth is inflating, and the inflation of the earth is accelerating at 9.8 m/s^2. Eventually, the ground catches the apple. Of course, you are going to tell me that the earth is not inflating, obviously, because it is still the same size after so many years. But here is the trick: the earth is inflating at the same rate as spacetime contracts. If the earth didn't inflate, the contraction of spacetime would have collapsed it into a black hole. Note: It is related to Einstein's elevator thought experiment. Here, the inflating earth replaces the rocket powered elevator. Note 2: If the idea of an inflating earth bothers you, I suggest you start considering that the earth is flat, seriously! Flat Earthers took Einstein's thought experiment quite literally and consider the Earth to be a disk that is continuously accelerated upwards. And in fact, if free fall trajectories were parabolic, that would be the correct explanations. In reality, because the earth is not flat, free fall trajectories are elliptic, though it is only apparent on a large scale.
  • k__ 6y ago
    How would spacetime know to contract right to the center of earth?!
    • manigandham 6y ago
      The mass of the Earth curves space-time.
    • GuB-42 6y ago
      That the big idea behind Einstein field equations: energy (and mass because E=mc2) curve spacetime and spacetime curvature affects the energy fluctuations (and the way things move). Because both terms of the equation affect one another, solving it is complicated but here, the result is that spacetime contracts to the center of the earth.
  • jiggawatts 6y ago
    Some interesting alternative viewpoints I've seen: Matter continuously destroys spacetime at its location, sucking in the fabric of the universe. The Universe isn't expanding; matter is shrinking. Light isn't redshifted on the way to us, it's just that our sensors are getting smaller relative to the unmodified wavelength of light.
• crazygringo 6y ago
  I'm curious, how does the "gravity is not a force" viewpoint relate to the hypothesized graviton particle [1]? Are they incompatible viewpoints, or just different perspectives on the same thing? (E.g. are gravitons hypothesized to disappear depending on frame of reference?) I'm assuming they're incompatible (that we need the theory of everything [2] to reconcile them) but would love to know if there's something I'm missing. [1] https://en.wikipedia.org/wiki/Graviton https://en.wikipedia.org/wiki/Graviton [2] https://en.wikipedia.org/wiki/Theory_of_everything https://en.wikipedia.org/wiki/Theory_of_everything
  • seaish 6y ago
    It is likely that both the "fabric of spacetime" idea and the Graviton are parts of the whole picture, like with particle-wave duality. In the end, particles, waves, and spacetime are really just abstractions on top of the true, possibly unknowable rules of the universe.
  • amelius 6y ago
    As you may know, one of the great problems in physics is to unite relativity and quantum mechanics. It happens to be that the graviton is a concept from QM and "gravity is not a force" is a concept that lives in the theory of relativity.
    • BlackFly 6y ago
      Yeah, I always found the assumption that QM was more correct than GR to be a bit odd. One of the outcomes of the geometric approach to gravity is the total lack of anti-gravity: there is simply no such thing as an anti-geodesic. The lesson of quantum field theory to me was that observables are operators on fields and not simple scalars evolving in time. General relativity is quite similar in that respect.
  • contravariant 6y ago
    In quantum physics the particles are excitations of a particular field. The gravitons correspond to (excitations in) the curvature of space-time. The other force bosons correspond to (excitations in) the curvature of other fields, such as the electromagnetic field. Interestingly the Yang-Mills equations (used in quantum mechanics) and the Einstein-Hilbert action (used in general relativity) are pretty much identical if you use general enough mathematics.
• xixixao 6y ago
  You might enjoy my special relativity interactive illustration: https://xixixao.github.io/cheetah-paradox/ https://xixixao.github.io/cheetah-paradox/ Play with the accelaration to see some spooky seemingly faster-than-light movement.
• TheRealPomax 6y ago
  Someone watched Veritasium... but the real question is: can you counter your own arguments? Because that's where absolutely fascinating science happens.
• vinceguidry 6y ago
  Speaking of reducing the dimensions of physical reality, a physics professor shared a very interesting video saying we should be thinking of photons and other massless objects as existing in 2d "areatime" as opposed to 3d "spacetime." I'm wondering if a revolution in simplifying the math behind physics is brewing. https://www.youtube.com/watch?v=cb_PCuv0vcY&feature=emb_logo https://www.youtube.com/watch?v=cb_PCuv0vcY&feature=emb_logo
• pmcollins 6y ago
  Wonderful lecture series on Audible by Benjamin Schumacher: https://www.audible.com/pd/Black-Holes-Tides-and-Curved-Spacetime-Audiobook/1682769992 https://www.audible.com/pd/Black-Holes-Tides-and-Curved-Spac...
  • freedomben 6y ago
    Can second this. I was worried that course would be over my head since I never took physics in college, but he does a great job prepping you to understand the end of the course.
• kwargs 6y ago
  How come on half of the earth's sphere people don't get ejected out into outer space while the other half of the sphere it pushes onto the people giving them sense of acceleration? If earth is pushing equally on all of the people around the globe how come it doesn't swell up and burst? And how do you explain force of attraction weakening the higher in altitude you go away from earth?
• deleted 6y ago
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• person_of_color 6y ago
  How can we manipulate gravity?
  • lopmotr 6y ago
    By arranging mass. We can also generate propagating gravity waves by accelerating mass.
• zestyping 6y ago
  This video really cleared up a lot of misunderstandings for me: https://www.youtube.com/watch?v=wrwgIjBUYVc https://www.youtube.com/watch?v=wrwgIjBUYVc I had never visualized it this way in my head before, and it all makes a lot more sense now. Highly recommend!
  • xt00 6y ago
    Thats a great video
  • elteto 6y ago
    What a fantastic video.
  • mensetmanusman 6y ago
    Great video. Reminded me of the arguments still going on about ‘whether this is true or not’ in the sense that the mathematics is painting this more accurate picture than what Newton’s math painted, but the math can’t explain most of the universe’s lack of observable mass/energy, so there might be some higher level of mathematics that describes a different but ‘more true’ state of events.
    • anomaloustho 6y ago
      I think you might be getting at MOND here, but so far, some other observations seem to indicate that the lack of observable mass are actually clumps of some type of matter. Because not all galaxies diverge from the math. Many do, but not all. The inconsistency points towards an actual type of matter as opposed to systematic error.
• logicallee 6y ago
  I was taught that gravity is a force (like electromagnetism, or the strong and weak nuclear forces.) The title says "gravity is not a force". In the article's perspective, why does a kitchen scale report a higher number when I place an object on it? (In my perspective there is gravitational attraction between the object on the scale and the rest of the Earth, and the the scale, assuming it is "level" (set perpendicular to the direction of gravitation toward the center of the Earth) reports the magnitude of that attraction.) If gravity isn't a force, then when the object and Earth are not in freefall or moving, what does the kitchen scale measure?
• babesh 6y ago
  Gravity creates the stage (spacetime) upon which other forces (electromagnetism, strong and weak nuclear forces) can play out?
• the_arun 6y ago
  Don't we always consider gravity as "acceleration"? In f = m * a, for objects falling from sky, it will be f = m * g where g is gravity, m is mass & f is force Sorry, I think I am missing the point of the article.
  • derex 6y ago
    g is not gravity, it’s a constant that quantifies the force that gravity impresses on a mass in Newtonian mechanics
    • the_arun 6y ago
      https://en.wikipedia.org/wiki/Gravitational_acceleration https://en.wikipedia.org/wiki/Gravitational_acceleration Isn't "G" (uppercase G) is gravitational constant and Lower case "g" is acceleration?
      • nsonha 6y ago
        yes but they have nothing to do with your assessment "gravity is acceleration". "Forces cause acceleration" is more accurate, there are 4 kinds of forces so gravity is just one cause.
  • nsonha 6y ago
    in Newtonian physics only a force can cause acceleration
    • the_arun 6y ago
      Something I learnt in 8th grade - if an object is free falling from sky of mass "m" the force it applies when it hits on the ground is m * g, where g is the gravitation.
      • nsonha 6y ago
        I think you are mashing up different lessons: - when you stand on the ground you are the subject of 2 forces: gravitational force m * g and the opposite reaction force of the ground surface, they cancel each other so you stand still - when you free fall your acceleration is g and the gravitational force is m * g, this is according to Newton's 2nd law when you hit the ground depending on your velocity and... deformation, the force of impact will be different because you will de-accelerate to 0 or even bounce back very quickly much faster than g. It's gonna be much stronger than g * m
• perryizgr8 6y ago
  If I throw a ball straight up, it loses speed. The speed at the top of the parabola is exactly zero. So how is the change in speed explained if there is no force involved? Even in a straight path, a change in speed implies a force acting on the object.
  • Jweb_Guru 6y ago
    My understanding here is that the ball (like everything else) is moving at a constant "speed" through spacetime. From the Earth's reference frame, when the ball is frozen in space, all of that speed is in the time component (as is yours). The reason its path is parabolic is due to the very slight curvature in spacetime induced by the Earth; if you instead stand in a "flat" reference frame (e.g. freefall) and watch the ball, it will indeed look like it's going in a straight line (but the Earth will follow a curved path in spacetime, allowing it to "catch" the ball). You can verify this using the link to https://timhutton.github.io/GravityIsNotAForce/ https://timhutton.github.io/GravityIsNotAForce/. I won't push my luck by pretending to understand how this changes when your spatial velocity is nontrivial (intuitively I feel like it should not even possible to differentiate between spatial and temporal components except relative to your own reference frame, but I could easily be wrong there). But in the case you describe it seems pretty clear.
• WClayFerguson 6y ago
  Here's another 'though experiment' I like which some people disagree with, by not understand reference frames: Light always travels in straight lines. Even when light is experiencing a gravitational lensing and looks to us from earth that it's bending around a star or whatever, from the perspective of the light beam itself, it's moving in a straight line. It's entire reference frame is bent compared to ours (relativity) but nonetheless the correct view is that the light is still moving 'straight' in it's own reference frame. Also if the light wasn't moving straight that would mean it's changing direction, which is the same as an acceleration, and a beam of light traveling thru a gravitational field feels no acceleration, because it's not accelerating. Again from this view you can say light is moving straight and experiencing no acceleration, just like an object in free-fall doesn't 'feel' any acceleration, even though they are accelerating from the perspective of some other reference frame other than it's own.
  • Ace17 6y ago
    > Also if the light wasn't moving straight that would mean it's changing direction, which is the same as an acceleration, and a beam of light traveling thru a gravitational field feels no acceleration, because it's not accelerating. You could have both a deviation (i.e tangential acceleration) and a constant speed.
    • WClayFerguson 6y ago
      Any change in direction is an acceleration (by definition). Even an object moving in a perfect circle at constant radians per second is nonetheless undergoing a constant non-zero acceleration just due to change in direction. Acceleration is any change in a velocity vector, including simply a change in direction, and requires a force (if the object has mass)
      • Ace17 6y ago
        Yes, obviously. But the message seemed to say that the constant speed of light implied that the derivative of the velocity with respect to time had to be zero.
• jamemuraca 6y ago
  Has any papers been written describing/modelling spacetime as a fluid? You also see diagrams of space time as a plane with the gravity coming from the dip in that plane. But that model would hold in a 360 degree view, so I think we should model spacetime as a fluid with the density of that fluid going rise to drag and therefore gravity effects
• fouc 6y ago
  Seems rather tricky to invent anti-gravity when gravity is just an emergent phenomenon of distortions in the spacetime fabric.
• drran 6y ago
  Can anybody sane tell us what gravity is in the case of flat space-time?
• yarnichts 6y ago
  F = ma F means force. Gravity is a type of acceleration, not a force without being multiplied by a mass!
• hhjinks 6y ago
  If gravity is not a force, how is spaghettification a thing? If you are stationary when you're weightless, how can different stationary parts of your stationary body move at the different speeds required for spaghettification to take place.
  • fosspowered 6y ago
    The way I try to understand this is that the stationary body is not a single particle but a collection of particles. Since the black hole distorts the space-time gravity in a humongous manner each of the particles in the body at different points in space-time have significant different curvature in space-time and hence would experience the different tidal "forces" which would account for spaghettification.
  • skykooler 6y ago
    It's only a thing in an extremely high gravity gradient, such that one end of you experiences far higher gravitational forces than the other. This means that while you are in freefall "on average", overall there is a stretching effect as parts of you closer to the source of gravity try to accelerate faster and parts farther away are lagging behind.
• r3trohack3r 6y ago
  This is my favorite description of curved space time: bugs realizing the geometry of their space is not Euclidean. They discover their triangles are wrong. https://www.feynmanlectures.caltech.edu/II_42.html https://www.feynmanlectures.caltech.edu/II_42.html
• causality0 6y ago
  Always seemed pretty wild to me that for the tiny moment of time photons from my computer screen travel from the LEDs to my eyes, they're being dragged toward the floor at the same 9.8m/s^2 as everything else.
  • Craighead 6y ago
    Aren't they being dragged by the Sun, Jupiter, Milky Way Black Hole, Alpha Centauris Black Hole, Sirius, etc also just at an exceptionally small rate?
• gokulkrishh09 6y ago
  Nice!
• bawana 6y ago
  but the calculation of time dilation due to a gravitational force results in a DIFFERENT amount than time dilation due to accelerating in a spaceship at one G. So the equivalence principle would fail- I can tell if I am accelerating due to gravity or if I am accelerating due to motion by measuring time dilation. Without doing the math you can know this is true by looking at the age of a person on the ground on earth and the age of a person in a spaceship accelerating at 1 G. The spaceman will experience greater time dilation (age slower) as his speed approaches that of light. In a spaceship people will age slower. But both people will only feel 1 G.
• pier25 6y ago
  Sooooo planet Earth is actually flat? :)
• ezconnect 6y ago
  The way I understand it. We are moving towards the ball the ball dis not fall back it just accelerated away from us for a while.
• nsonha 6y ago
  Is there such thing as a force then, I heard they invented some model in which fundamental forces are just curves in folded dimensions or something.
• Yuioup 6y ago
  So ... what is terminal velocity then?
  • Jweb_Guru 6y ago
    Terminal velocity is entirely a product of air resistance (or in general, resistance from any fluid medium). That's mostly caused by electromagnetic interactions (AFAIK) which despite physicists' best efforts is still best explained as a "real" force. Hence why objects with different shapes or masses can have very different terminal velocities on Earth (and the same is true if you vary the atmospheric composition, or consider objects falling in water). But they will all fall at the same rate in a vacuum, unless the object is massive enough for gravity to warp the trajectory of the object onto which it's falling.
• abbiya 6y ago
  Given the small 'g' acceleration due to gravity, can we calculate the mass and dimensions of the object that is the reason for that value of g ?
• yukijilokulo 6y ago
  if it is not force how come there is movement? where is the force coming from?
• raverbashing 6y ago
  General Relativity may be a theory with an excellent prediction power but to me it is the one that's lacking in realism (as in the philosophical definition) It's not a fundamental problem, nobody has abandoned Maxwell's equations for EM except for the most specific cases, but it's a similar case. There's probably a better explanation than just "distortion of space time" which is a great way of viewing it, but it's a bit of a stretch (pun intended)
• peter_retief 6y ago
  So gravity is not a force even though it acts on mass as a force? This is really confusing.
• edem 6y ago
  Not just free-fall parabolas. Think about the path airplanes traverse when they go to another continent. Still a parabola, but flattened.
• imvetri 6y ago
  How would you define it ?
• klunger 6y ago
  I recently finished "We Have No Idea" by Jorge Cham and Daniel Whiteson. Its a masterpiece of science communication and I cannot recommend it enough as a way for non-physicists to get a grasp on this concept, as well as related ones.
• brianpan 6y ago
  The scrollbars in the demo feel backwards to me. Like Mac trackpad scrolling vs mouse scrollwheels. I want left to be 0 frame acceleration and left to be backwards in time. I kind of also want the graphs to be reverse order too, but I get why it's all presented this way. :)
• slmjkdbtl 6y ago
  (off topic) i thought the title is "gravity is not free", hmm thanks that's an interesting topic for a story..
• immmmmm 6y ago
  Physicist here, gravity is a force, just a different one. Also, like everything else in physics: it depends how you observe it. For instance, electromagnetism comes from the curvature of a U(1) bundle over space time, the (local) U(1) symmetry yields electromagnetic interactions. For gravity the symmetry is the (local) Pointcaré (SO(1,3) + translations) symmetry and curvature of spacetime itself. Also gravity on Earth (weak gravitational field) is mostly curvature of time, namely the spacelike curvature can be ignored, and the g_{00} component of the metric can be seen a a gravitational potential. see p 80 of this: http://www.blau.itp.unibe.ch/newlecturesGR.pdf http://www.blau.itp.unibe.ch/newlecturesGR.pdf
  • klunger 6y ago
    "This ultimately led [Einstein] to the realisation that gravity is best described and understood not as a physical external force like the other forces of nature but rather as a manifestation of the geometry and curvature of space-timeitself." This is in the introduction of the paper you shared...
    • jb1991 6y ago
      Give the commentor some credit -- I doubt they'd link a paper they are not familiar with. Also in the paper: "In a certain sense the main effect of curvature (or gravity) is that initially paralleltrajectories of freely falling non-interacting particles(dust, pebbles,. . . ) do not remainparallel, i.e. that gravity is an attractive force that has the tendency to focus matter." Maybe don't immediately look for ways to discredit someone's contribution to the discourse without examining all of the content they have shared?
    • SideburnsOfDoom 6y ago
      > best described and understood not as a physical external force "best described as not a force" is not the same statement as "is not a force". In physics and mathematics there can be multiple different, complementarity descriptions of the same thing. In fact, if you can reach the same result by two very different routes that seems to make it more robust.
    • 6y ago
• Synaesthesia 6y ago
  relevant XKCD https://xkcd.com/123/ https://xkcd.com/123/
• deleted 6y ago
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• Gormisdomai 6y ago
  my physics teacher would always say that "Gravity is not a force, but we experience a force due to gravity and call it weight"
• anotheryou 6y ago
  The first explanation that actually worked for me: https://youtu.be/jlTVIMOix3I?t=55 https://youtu.be/jlTVIMOix3I?t=55 (also nice engineering on the physical "space time stretcher" he built) I understood it rationally before, but only with this video it also feels right.
• NewEntryHN 6y ago
  You could call inertia a force then.
• rsp1984 6y ago
  When standing on Earth we experience a frame of reference that is accelerating upwards, causing objects in free-fall to move along parabolas, as seen in the accelerating frame of reference on the left. I'm trying to understand what is meant by this. When I drop an item on the floor it goes there in a straight line, not a parabola. Same if I drop something from a helicopter. Obviously I'm missing something here. Can someone with more insight ELI5 this to me please?
  • meditative 6y ago
    it will look like a parabola (or ellipsis if you read elsewhere in this thread) if you add another dimension to your graph of the balls position. That could be a physical dimension, such as throwing a ball to your friend across the room and plotting x vs y. Or even plotting the height of the ball against time. https://imgur.com/a/ccBDZf6 https://imgur.com/a/ccBDZf6 consider the x-axis of this image the distance between you and your friend, or time.
• PavleMiha 6y ago
  What I don't get is when I'm standing on the Earth, I'm in an accelerating frame, but what direction am I accelerating in?
• Communitivity 6y ago
  Gravity is a force, because if it is not for this reason then probably neither are the other forces. Many physicists believe we will achieve grand unification of the forces, which means that in some way they are all related to the curvature of spacetime and each force can be viewed as applying acceleration in a straight line. It's just that the geometry defining that line, the frame of reference, is different for each force.
  • koheripbal 6y ago
    Yeah, Tim's argument is fundamentally semantic one.
• deleted 6y ago
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• johndoe42377 6y ago
  Bullshit. Space-time is just an abstract concept.
• 734129837261 6y ago
  From the future here, gravity is just light reflecting off of stellar bodies, which causes the universe to "bend" space/time in all directions, just different strengths based on the reflectiveness of whatever it hit and the strength of the light source. The reason that a whole bunch of nothingness between planets keeps them spinning around one another is just light and the sheer force it applies by bumping into objects and then reflecting into the next one. Don't think of just one beam of light, think of a cacophony of endless bounces of light in varying degrees of strength. Light is without mass, but not without energy. That energy causes what you could describe as "propulsion". Even the slightest bit of propulsion in the emptiness of space will cause matter to move around. It's like light being a big ladle. And using that ladle to spin the water + tiny floating objects in a bath. They will all affect one another until the heat death of the bath occurs and all life grinds to a halt.
• deleted 6y ago
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• politician 6y ago
  Hasn't it been shown that mass originates from coupling with the Higgs field and it's mass that deforms spacetime? Given the large size of the Higgs boson, I don't understood why the lack of gravity in QM is viewed as a problem. Why does gravity have to be fundamental? What if it just shows up right around the same time as electroweak symmetry breaks down?
• TheMightyLlama 6y ago
  Does this mean that space time is ‘bent’ and objects follow an apparent parabola along that bend, or that space time is flowing towards a mass and the object is following a straight line on a ‘moving’ space time?