Gravity is not a force

Sabine Hossenfelder chimed in [0] on this discussion a couple days ago. I generally find her to be trustworthy on topics related to physics:<p>&gt; The easiest way to see that gravity is not a force is to note that a force causes acceleration, but gravity does not.<p>&gt; Acceleration is measurable with a device called an accelerometer. Acceleration is not relative (like velocity), it&#x27;s absolute.<p>&gt; If you are standing on the surface of Earth, an accelerometer will show that you are accelerated in the upward direction. That&#x27;s because a force is acting on you from below, it&#x27;s the solidity of Earth&#x27;s crust (or whatever you are standing on), going back to a combination of electromagnetic forces and the Pauli principle.<p>&gt; If you take away that support from Earth, eg by jumping off a plane, you are not accelerated. You are freely falling. Since you are not accelerated, there is no force acting on you. You experience gravity but no force, hence gravity is not a force.<p>&gt; We can assign a pseudo-force to gravity by defining it as acceleration relative to the surface of Earth. This is how Newtonian gravity works. One can derive it from general relativity as an approximation.<p>&gt; Physicists frequently do refer to gravity as a force anyway -- even I do -- because that&#x27;s linguistically simpler. But it&#x27;s like we say &quot;internet&quot; rather than &quot;world wide web&quot; even though we know that the two aren&#x27;t the same, just because &quot;internet&quot; is simpler.<p>&gt; So I usually don&#x27;t pick on this. But strictly speaking, gravity is indeed not a force. If you have doubts about it, buy an accelerometer and do your own research...<p>[0] <a href="https:&#x2F;&#x2F;x.com&#x2F;skdh&#x2F;status&#x2F;1850120005070799153" rel="nofollow">https:&#x2F;&#x2F;x.com&#x2F;skdh&#x2F;status&#x2F;1850120005070799153</a>

First off, the author doesn&#x27;t appear to be a crank.<p>Here&#x27;s the Wikipedia entry: <a href="https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Jonathan_Oppenheim" rel="nofollow">https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Jonathan_Oppenheim</a><p>Second, we should note that even Einstein himself cautioned against believing spacetime was actually curved. His writings inform us he didn&#x27;t believe it. I don&#x27;t want to appeal to authority, that&#x27;s just to say smart people, including the main developer of general relativity, didn&#x27;t believe it. But he didn&#x27;t believe in the non-local nature of quantum theory either, which we have now, since Einstein&#x27;s death, <i>proven</i> to be true.<p>Third, the claim that only gravity can be described using geometry is false, which the author himself notes later in this article. The stress-energy-momentum tensor simply makes gravity <i>universal</i>, unlike the other forces. I don&#x27;t see any reason why that universality confers something special to gravity with regards to interpreting it as geometry. Just because we <i>can</i> model gravity as geometry, doesn&#x27;t make gravity a result of geometry, and the author notes that modeling gravity that way makes it so we can&#x27;t unify the forces.<p>Finally, as the old saying goes, if you think gravity isn&#x27;t a force, drop a brick on your toe! :)<p>I&#x27;ll also point out that singularities are generally considered to be a sign of issues with a model. GR has singularities. Maybe that should tell us something.

I like the model of an insulated tunnel bored through the Earth from the north to the south pole and filled with a vacuum (technologically implausible, yes). If we drop a steel ball into the tunnel at the north pole, what forces does it experience?<p>From Newton&#x27;s perspective, F = ma and the ball accelerates towards the center of the Earth. The value of g diminishes to zero at the center, the ball is at its maximum velocity, and then enters the negative acceleration regime until it just reaches the surface of the Earth at the south pole. This will continue indefinitely in harmonic motion. It&#x27;s not a perpetual motion machine because machines do work and we&#x27;re not doing any work on the ball; it&#x27;s similar to an orbiting sphere. (ChatGPT-o1 claims the period is 84.4 minutes, assuming uniform density)<p>The general relativity perspective seems to be the ball is just rolling up and down a bowl of spacetime, without any friction or drag, which isn&#x27;t all that satisfying a picture, since it implies a restorative force being involved to keep the ball from escaping the bowl. It helps to consider the state of the ball right before it is kicked into the tunnel - it is being <i>prevented</i> from following its natural geodesic trajectory by the electromagnetic forces of the rocks of the Earth&#x27;s crust upon which it is being held up - that&#x27;s the only relevant force in this picture.

Accelerometers don&#x27;t measure acceleration, they measure the difference between the force on a mass on springs and the body of your phone. If you drilled a tiny hole and pulled the mass up, the sensor would think the phone was being pulled down.<p>Acceleration due to gravity has an important property of acceleration in general: radiation. Spiraling black holes emit gravitational waves, in the same way that an accelerating electric charge emits light. There are free falling reference frames where uniform gravitational fields can go away, but the gravitation of a massive body isn&#x27;t uniform and can&#x27;t be eliminated by changing the coordinates.<p>The relationship between gravity and fictitious forces is an important stepping stone, but it does not have all the properties of a fictious force, only some of them.

He lost me there:<p>&gt; Is this purely a semantic difference? You could argue that it doesn&#x27;t really matter whether we describe gravity as a force or through geometry, and we should conflate these two concepts. But I think this distinction is important to make because it has predictive power. If you believe that gravity is manifest through spacetime bending, then you will never find two different test particles that follow different geodesics.<p>Don&#x27;t we get the same conclusion if we believe gravity is a force _and_ equivalence principle is true?

Question to the physicists out there: When an electron gets accelerated it emits &quot;Bremsstrahlung&quot; because it radiates away photons when it changes its velocity vector.<p>So for an electron on a circular path in a magnetic field, we know that it emits this radiation because this is the synchrotron radiation.<p>Now what happens to an electron on a circular path around a black hole? Does it emit synchrotron radiation or not?

“If you accept that we live in spacetime, and it can be curved, then I think you should accept that gravity cannot be a force.”<p>I am on the opposite time of thinking — where spacetime itself emerges from particle interaction events. Pairwise distance (i.e. metric) is just yet another interaction parameter in the world graph.

&quot;is to say that when no force acts on a test particle in curved space, it should move along a geodesic&quot;<p>Every single author who calls gravity not a force, just hand waves right past this: why should the particle move at all?<p>Sure if the particle is moving it will follow a curved path thinking the path is straight.<p>But if the particle just sits there, okay the path is curved, but it just sits there.<p>I&#x27;ve heard explanations having to do with the fact that particles move through time, that doesn&#x27;t really answer the question because it can continue moving through time while just sitting there.<p>&quot;in the absence of being pushed or pulled, test particles in a curved spacetime will free fall&quot;<p>Really? Why exactly will they free fall? Why can&#x27;t they just stay exactly where they are?<p>I&#x27;ve also never heard anyone explain what the issue is with calling gravity a force. One person said it can&#x27;t be a force because it makes light move, and light is massless.<p>However gravity does not act on mass it acts on energy, and mass is just a form of energy. Since photons have energy obviously they gravitate.

The author is 50% correct. As John Wheeler stated &quot;Spacetime tells matter how to move; matter tells spacetime how to curve.&quot;<p>The author is unfortunately forgetting the second part. If matter is quantized spacetime curvature must also be quantized, because matter defines our spacetime.<p>In principle one can even shorten the sentence and say: matter tells matter how to move. Spacetime appears only to be a convenient calculation tool. And in that sense spacetime=gravity isn&#x27;t a force. It actually doesn&#x27;t even exist.

I am sympathetic to the author&#x27;s thesis. I favor the idea that gravity is a different thing from the other fundamental forces, and possibly an emergent phenomenon rather than a fundamental thing in its own right.<p>But, I don&#x27;t buy the argument made here:<p>&gt; To call gravity a force, is to privilege flat space as somehow being special.<p>Flat space is special, and we didn&#x27;t make it special.<p>This is taking an important aspect of known physics---that there exist various symmetries and all elements of the corresponding group are equal players (there is no privileged reference frame, positive charge and negative charge are indistinguishable save for their oppositeness, etc.)---and attempting to apply this principle to spacetime curvature. But the zero curvature state is a unique one that is differentiatable from the others. It&#x27;s the only one where a circle is perfect, having circumference 2 * pi * r. And pi is a fundamental invariant of geometry, curved or otherwise. The mathematics privileges flat space. Further, experiments can be constructed to detected whether we are in flat space or not [1]. That wouldn&#x27;t be possible if the whole concept of flat were only relative to an arbitrary frame.<p>[1] <a href="https:&#x2F;&#x2F;en.m.wikipedia.org&#x2F;wiki&#x2F;BOOMERanG_experiment" rel="nofollow">https:&#x2F;&#x2F;en.m.wikipedia.org&#x2F;wiki&#x2F;BOOMERanG_experiment</a>

This is not article-content-related but article-presentation-related: the trouble with 3-pixel-wide scrollbars is that you have to be very exact when you try to use them. Yes, there are scroll wheels and keyboard buttons, but forcing the mobile experience (on a mobile I probably wouldn&#x27;t notice) on non-mobile setups is at least annoying.

Actually even Newtonian gravity is no force, because Newtonian spacetime is curved:<p><a href="https:&#x2F;&#x2F;youtu.be&#x2F;IBlCu1zgD4Y?feature=shared" rel="nofollow">https:&#x2F;&#x2F;youtu.be&#x2F;IBlCu1zgD4Y?feature=shared</a><p>It must be so, because it is just a approximation to general relativity.

Whatever comes out of this kerfuffle, the based dial will be set to 11.<p>It’s unfortunate that we can’t monetize this stuff in the same vein as crypto and ChatGPT because I’d love to hear YouTube grifters telling you how you can make money overnight from quarks and neutrinos.